Final Report Summary - TAILORTOOL (Development of new Tool materials with Tailored thermomechanical properties)
Executive summary:
TailorTool, "Development of New Tool Materials with Tailored Thermomechanical Properties", is a FP7 project within the funding scheme Collaborative Project Small of medium-scale focused research project with the participation of the following companies, universities and research centres: Fundació CTM Centre Tecnològic (Spain) as project coordinator, VW AG (Germany), Gestamp Hardtech (Sweden), Rovalma (Spain), Oerlikon Balzers (Liechtenstein), Tecnalia (Spain), Armines (France), Technische Universität München (Germany), Technical University of Lulea (Sweden) and Metakus (Germany).TAILORTOOL has been conceived to give response to the limited life of the tools used in different manufacturing processes within the automotive industry. Materials were also specially developed to build functionally graded tools that allow producing components with graded mechanical properties, as an innovative procedure to get high crashworthiness and weight savings.
According to the expertise of the industrial partners as well as the research capabilities of the consortium the following forming processes with severe thermomechanical requirements are chosen apply the developed materials: hot stamping of B steels, hot forging, high pressure die casting (HPDC) of light alloys (aluminium and magnesium) and cold forming of Ultra High Strength Steels (UHSS) and hardened B steels sheets. Accordingly, different families of materials and tools have been developed: (a) Tools with tailored thermal conductivity and high mechanical performance for forming processes at high temperature, incorporating materials with high thermal conductivity (b) Materials with a combination of hardness and toughness for cold forming processes, (c) FGM as a base material for hard coatings, with tailored surfaces to increase the coating performance in cold forming.
One of the main project objectives is the identification and measurement of the thermomechanical solicitations acting on industrial tools during service which up to now has not been possible to be accurately measured except for HPDC. For attaining this goal, special sensors with fast time response have been developed. Industrial tools have been instrumented for accurate measurements of heat flux and local temperature at the contact zone, values which are very important for validation of FE simulations and tool material design. Based on this information, and the results from an exhaustive study of the damage mechanisms acting on industrial tools during service and the FE simulations of the different processes, the main properties required for new tool materials have been identified.
Another main objective of the project is the development of a new generation of materials specially tailored to enhance tool performance, process efficiency and component quality. Materials have been tailored according to the requirements needed for each manufacturing process and have been designed based on an optimization of:
- Thermal properties to improve resistance to thermal fatigue or to produce components by hot stamping with functionally graded properties.
- Mechanical properties (optimization of microstructure and hard particles to improve resistance to different mechanical solicitations).
Project Context and Objectives:
SUMMARY DESCRIPTION OF THE PROJECT CONTEXT AND OBJECTIVES
TAILORTOOL, "Development of New Tool Materials with Tailored Thermomechanical Properties", is a FP7 project within the funding scheme Collaborative Project Small of medium-scale focused research project with the participation of the following companies, universities and research centres: Fundació CTM Centre Tecnològic (Spain) as project coordinator, VW AG (Germany), Gestamp Hardtech (Sweden), Rovalma (Spain), Oerlikon Balzers (Liechtenstein), Tecnalia (Spain), Armines (France), Technische Universität München (Germany), Technical University of Lulea (Sweden) and Metakus (Germany).TAILORTOOL has been conceived to give response to the limited life of the tools used in different manufacturing processes within the automotive industry. Materials were also specially developed to build functionally graded tools that allow producing components with graded mechanical properties, as an innovative procedure to get high crashworthiness and weight savings.
According to the expertise of the industrial partners as well as the research capabilities of the consortium the following forming processes with severe thermomechanical requirements are chosen apply the developed materials: hot stamping of B steels, hot forging, high pressure die casting (HPDC) of light alloys (aluminium and magnesium) and cold forming of Ultra High Strength Steels (UHSS) and hardened B steels sheets. Accordingly, different families of materials and tools have been developed: (a) Tools with tailored thermal conductivity and high mechanical performance for forming processes at high temperature, incorporating materials with high thermal conductivity (b) Materials with a combination of hardness and toughness for cold forming processes, (c) FGM as a base material for hard coatings, with tailored surfaces to increase the coating performance in cold forming.
One of the main project objectives is the identification and measurement of the thermomechanical solicitations acting on industrial tools during service which up to now has not been possible to be accurately measured except for HPDC. For attaining this goal, special sensors with fast time response have been developed. Industrial tools have been instrumented for accurate measurements of heat flux and local temperature at the contact zone, values which are very important for validation of FE simulations and tool material design. Based on this information, and the results from an exhaustive study of the damage mechanisms acting on industrial tools during service and the FE simulations of the different processes, the main properties required for new tool materials have been identified.
Another main objective of the project is the development of a new generation of materials specially tailored to enhance tool performance, process efficiency and component quality.
Materials have been tailored according to the requirements needed for each manufacturing process and have been designed based on an optimization of:
- Thermal properties to improve resistance to thermal fatigue or to produce components by hot stamping with functionally graded properties.
- Mechanical properties (optimization of microstructure and hard particles to improve resistance to different mechanical solicitations).
As a final project objective, microstructural and mechanical characterization of the new materials have been carried out. Special attention was place in developing laboratory tests and experimental methodologies that reproduces tool working conditions, aimed at discerning the industrial applicability of the developed materials.
The first part of the project has been focused on the identification of damage mechanisms, evaluation thermomechanical solicitation acting on tools and FE simulations of the process, aiming at gaining full knowledge of the forming process which up to now is not completely possible, specially for hot forming operations where temperature at the tool surface is still very difficult to be measured with accuracy. For such, new sensors have been developed and afterwards successfully implemented in serial industrial tools in hot forging and HPDC. For hot forging, the temperature at the contact zone and the heat flux has successfully been determined which is an important breakthrough due to the short contact times (less than 0.1 second) and the severe mechanical solicitations acting on tools. These results have been used to validate the FE simulation models developed for each forming process.
Furthermore, failed tools and tools in service for relatively short periods of time have been studied for damage evolution studies. Also, a new methodology for damage evolution studies has been optimized for hot stamping tools. Polymeric replicas have been applied on tool surface after different number of strokes. Replicas allow to obtain exact copies of a surface, and after microscopy inspection, the surface roughness can be quantitatively evaluated, i.e. evolution of adhesive or abrasive wear can be quantified.
The main approach to increase tool performance while also increasing the component quality is the design of materials with improved thermal conductivity and wear resistance at the regions of interest, aimed at minimizing the extension of the different damage mechanisms acting on tools surface. Therefore, the correlation of the instrumentation and damage analysis studies have been used as a basis for the definition of the requirements of the new materials properties. Materials with improved thermal conductivity and/or wear resistance have been developed. Also, the optimization of substrate materials for better sustaining PVD coatings has been carried out.
The materials developed were tested in laboratory aimed at reproducing real working conditions. Tools with the new materials have been designed and constructed with the objective of validate their response under industrial conditions. An experimental tool with geometry similar to a B-pillar has been designed in order to check the feasibility of manufacturing components with graded mechanical properties by hot stamping. The tool consists of two symmetric parts made of materials with marked differences in thermal conductivity providing different cooling velocities of the hot sheet which allows obtaining two regions with different mechanical properties. The transition zone and the different regions of the final product have been characterized in terms of microstructure, hardness, tensile and fatigue tests. Furthermore, this tool has allowed carrying out a fast screening of the influence of the different process parameters on the final product properties.
Forging tools suffer different damaging mechanisms at different locations. High wear resistance together with high thermal conductivity are required at the red regions in order to increase the resistance to abrasive wear and thermal cracking. However, the remaining part of the tool where no surface damage has been detected only high toughness while keeping high thermal conductivity is needed. Taking into account these die requirements, a functionally graded material was designed to cope with the thermo-mechanical requirements acting on forging tools. A bevel gear die (almost final shape) was manufactured by Nanocasting®, obtaining a material with functionally graded properties: high wear resistance and high thermal conductivity at the dent surface and high thermal conductivity and toughness at the base material.
Tools for cold forming UHSS mainly suffer from premature failure by chipping and accelerated surface wear. Tool steels present microstructures consisting of primary alloy carbides embedded in a tempered martensitic matrix. Primary carbides are aimed at bringing hardness and load bearing capabilities to the metallic matrix (especially to avoid plastic deformation), as well as increased wear resistance. Hard particles, as primary carbide characteristics dictate the mechanical and tribological response of the material and consequently, the performance of tools. In this sense, the Nanocasting® technology has allowed the introduction of selected hard particles with an optimised dispersion into an existing matrix, what is of less effort than trying to reach the same goal by controlling complex precipitation and growth processes of primary carbides within tool steels. Thus, individual hard particles have been studied at laboratory scale to assess their micro-mechanical properties by means of nanoindentation. The properties of hard particles have important effects on the stresses generated in the surrounding metallic matrix. Under a same external applied pressure, particles with higher Young modulus (E) have been proved to increase stresses in the microstructure. Thus, special attention must be paid in choosing particles with very high E values even if in most cases, they also present higher fracture toughness (KC). The effects of the increased stresses generated to their surrounding can lead to more detrimental consequences than what can be achieved by improving their KC. A compromise between E and KC must be found together with particle shape and geometries to increase the material performance. Primary carbides have been shown to present lower mechanical properties than the aforementioned particles, but they still have reasonably good results. Such results are especially relevant when developing tool steels substrates for hard coatings. These steel substrates should present microstructures preferentially with round, and small particles as well to help reducing coating crack phenomena, as determined by FE simulation and laboratory tests.
Expected final results and potential impact and use
The new materials and know-how developed within the project will contribute in introducing the forming processes of new and advanced knowledge-based materials and lead to greater productivity at a lower economic and energy cost. Consequently, TAILORTOOL impact will provide the market with higher quality and more innovative products along with the perspectives to increase of productivity and cost effectiveness of the investigated processes in one hand. On the other hand, the understanding developed provides new tools and investigation orientations to the partners involved towards more innovation and creation. The resulting new technology, know-how and manufacturing practices will contribute to strengthening the relationship between the automotive components sector and the major motor manufacturers.
The address of the project public website: http://tailortool.ctm.com.es/
Project Results:
SCIENTIFIC AND TECHNOLOGICAL RESULTS
The material development in the TAILORTOOL project was based on the understanding of the real thermo-mechanical requirements and the main tool damaging mechanisms acting in hot stamping, die casting forging and cold sheet forming. The main challenge was to identify them in industrial tools and replicate them at the laboratory scale to properly tailor the material properties to the forming tool needs and process parameters. The scientific and technological results obtained in TAILORTOOL are described below in terms of: (a) Identification of thermo-mechanical solicitations and damaging mechanisms acting on tools, (b) Development of materials and functionally graded tools, (c) Properties and performance of the developed materials and tools.
(a) Thermo-mechanical solicitations and damaging mechanisms acting on tools
The thermo-mechanical solicitations on tools in hot stamping, casting and forging, were mainly obtained by finite elements (FE) simulations. Even if this approach gives good results, the temperature and pressure values in real forming tools depend on experimental parameters that are difficult to fully incorporate in the computer simulation. Currently, the tool surface temperature can be measured via infrared cameras but these cannot provide accurate tool surface temperature values or heat flux due to the high speed of the process (in forging contact time is less than 0.1 seconds). In the TAILORTOOL project, some sensors have been developed to evaluate the thermo-mechanical history of hot stamping, die casting and hot forging tools. Such direct measurements on instrumented industrial tools give valuable results from a material and process design point of view. Regarding to wear and tool inspection a novel experimental techniques was developed. Mechanical solicitations of cold forming tools were determined by means of FE simulation and results were experimentally validated through tests with an instrumented cutting tool.
Determination of wear mechanisms in industrial tools is complicated; techniques with the precision required are largely restricted to laboratory scale, requiring specific equipment and long inspection time. Their use on industrial tools would demand complex logistics and severely affect productivity, and thus they appear unattractive from an industrial point of view. In this project, a novel inspection technique has been developed using surface replication to study industrial tools without removing them from the production line. This technique consists in the application on the surfaces to be inspected of a viscous compound which cures in a few minutes into a high precision thermoset polymer replica of the surface topography. Replicas can be cast during natural stops of the production line, with no need to schedule additional inspection stops. Afterwards, replicas were inspected in the laboratory by means of different microscopy techniques.
Wear mechanisms in hot stamping of boron steel. Replicas were obtained from two sets of tools; one used to form Al-Si coated B steel and one used for hot stamping of uncoated B steel. In the case of hot stamping of Al-Si coated boron steel, the most obvious feature found on the tool surface was the presence of irregular lumps of adhered material, which were accurately reproduced in the replicas. These lumps were up to more than 50 µm high and several millimetres across. Another feature was a homogeneous, continuous material layer that formed on certain parts of the die. The surface of this layer is mostly flat and regular, although lumps can also be found. The existence of such layer can only be detected by observing points where the original tool surface is exposed. Layers of adhered material regularly reached thicknesses in excess of 50 µm. EDX analysis was carried out on particles extracted from the adhered layer; results showed that adhered material matched the composition of the Al-Si coating after heat treatment.
Wear mechanisms observed in hot stamping of uncoated B steel were completely different, due to the very different tribosystem. In this case, the main mechanisms observed were gradual abrasive wear and galling. Abrasive wear corresponds to the loss of material from the tool. The tool surface consists in a pattern of peaks and valleys. The most intense contact takes place on the peaks of this pattern, which are flattened by a combination of material removal and plastic deformation. As cycles progress, this mechanism removes these peaks until the whole surface has been flattened and only the deepest valleys remain. Galling is a mechanism involving solid state welding between tool and sheet metal asperities. A small volume of material remains soldered on the tool, which acts as nucleation point for further growth. These lumps may attain sizes in excess of 50 µm; a trench of sunken material usually surrounds large lumps, generated through substrate plastic deformation under and around the protruding feature.
Damage mechanisms in HPDC. The same characterization technique was applied on aluminium and magnesium HPDC dies. As in the case of hot stamping, replicas were applied during production stops, and inspected in the laboratory afterwards; allowing obtaining information from surface features and damage micro-mechanisms without taking the tool from production. In both cases, the observed damage phenomena were equivalent. The main wear mechanisms observed were washout (removal of die material by the impinging jet of molten metal), thermal fatigue (cracks generated on the surface by the alternating temperature cycles), soldering or die sticking (adhesion of material on the tool by a combination of chemical and mechanical means), and oriented drag marks (lines in the direction of die closing generated by interaction with the component during extraction). All these mechanisms leave marks on the component, and therefore tools need to be refurbished once these damages attain an unacceptable severity.
Damage mechanisms in hot forging. In the hot forging process, tools are subjected to high thermo-mechanical loads which are the responsible for the severe damaging mechanisms acting on them during service. Forging tools have been investigated and three main damage mechanisms have been identified: abrasive wear, thermal fatigue and plastic deformation. Usually they act simultaneously even at early stages of the tool life, what explains why this uses to be extremely short. Damage is located at the tool surface and subsurface, which means that a local characterization approach is needed. Mechanical damage was estimated at the subsurface of the most damaged areas by extracting local properties using nanoindentation. Damage was quantified through the correlation of nanoindentation results to plastic deformation of the material subsurface due to the high thermo-mechanical solicitations acting on tools during service. The developed methodology based on nanoindentation has probe to be a valuable tool to identify and quantify damage in forming tools.
The knowledge of the thermal history at which tools are subjected is very crucial for understanding the damage mechanisms and set the basis for the new material development. Therefore, the successful instrumentation of a bevel gear die has supposed an important breakthrough since new sensors with very fast acquisition time had to be developed within this project, and afterwards they had to be implemented in serial industrial tools where heavy loads are applied. These results have allowed determining the temperature at different depths from the tool surface and the heat flux. These values have been used to validate the FE simulation models developed for this forging process.
Analogously to hot forging, for the hot stamping and HPDC processes instrumentation studies have been carried out, and the results have been used to validate the FE models. For the hot stamping, a semi-industrial tool designed and constructed within this project has been instrumented, whereas for HPDC an industrial clutchbox die (Mg casting) has been selected. In both processes, the instrumentation campaign gave very accurate temperature and heat flux data.
Mechanical requirements acting on tools in cutting and drawing of UHSS. They have been determined by means of FE-simulations which have been later validated in test facilities specifically developed to carry out cutting and drawing of high strength steel sheets. In cutting applications, the influence of the process parameters on tool life defining factors such as cutting load and stress distribution within the cutting knives was assessed. The maximum von Mises stress appears at the beginning of plastic deformation of the sheet, whereas the maximum cutting force is reached shortly before sheet fracture.
Process Parameters
- Punch velocity [mm/s]: 90
- Blankholder force FBh [N/mm]: 480
- Tool radius rp [µm]: 50, 100,
150, 200
- Blank thickness t [mm]: 1.5 1.75
- Cutting Clearance C [%]: 5, 10, 15
An increase of cutting clearance leads to an increase of maximum von Mises stress, while the maximum cutting force is reduced. This is due to the blank contact length, which is smaller for 15 % cutting clearance than for 5 %. As the maximum stresses are very high, larger punch radii have been analyzed in order to reduce the stresses. It is known that by increasing the punch radius the required cutting force increases as well, whereas the stresses inside the punch are reduced. In this investigation it has been shown that the maximum stress value could be reduced by approximately 300 MPa by increasing the punch radius to 200 µm. Furthermore fracture of the sheet material takes place at increased punch displacements by increasing the punch radius.
In forming applications of high strength steels, galling, abrasive wear and plastic deformation as well as eggshell effect of coatings are the most common failure mechanisms encountered. When forming UHSS higher press forces are necessary due to the higher yield stress. In general a high strength material also requires higher blank holder force to prevent wrinkling at the flange region. High localized surface pressure at the forming tools makes high demands on the tool material and on the tool surface properties, respectively. Therefore, too high contact pressure between the die and the sheet is one of the main reasons that cause the failure of tools. Contact pressures have been determined by means of FE-simulation for a given combination of tool and sheet material according to punch and die radii.
Depending on the blank holder pressure, drawing force and contact pressure are distributed differently for the combination of DP1000 and uncoated conventional tool material such as AISI D2. In this case, when applying the blank holder pressure of 3, 5 and 7 MPa, the maximum contact pressure of each die amounts to about 1050, 1280 and 1800 MPa respectively. Furthermore, the higher blank holder pressure, the larger the drawing force becomes. The blank holder pressure also affects the distribution of contact pressure at the die inlet radius. During the deep drawing process the thickness becomes particularly higher. Therefore, higher contact pressure appears at the die surface which may lead to the eggshell effect.
(b) Development of materials and functionally graded tools
Hot stamping. Nowadays, there is an increasing demand for functionally graded components, especially within the automotive industry (eg. B-pillars with two zones (hard and soft) for improved performance in case of car crash). There are several ways to manufacture these graded components at once, most of them playing with different cooling and heating strategies of the die. However, at first, a complete new approach was considered in this project, i.e. the development of tool materials with a horizontal variation of the thermal conductivity. However, microstructural FE simulations have shown that for the hot stamping process, and due to thermal diffusion, the transition zone width in the final component would be far too large, and as a consequence, the use of these materials would not bring any benefits. Instead, a functionally graded die was designed. It consists of two blocks manufactured with materials developed in the frame of the project with different thermal properties and separated by an air gap. A semi-industrial tool following this concept was designed and constructed, with a shape similar to a B-pillar, an industrial part that can directly benefit from the tailored properties obtained. The tool was manufactured with a High Thermal Conductivity Steel (HTCS) and a low thermal conductivity one (GTCS). The part reached the targeted values of hardness at the soft part and the transition zone width. Furthermore, the use of this semi-industrial tool has allowed a deep study of the different process parameters, and a reduction in the cycle time when using these two material configurations is also foreseen.
High Pressure Die Casting (HPDC). FE-simulations regarding the final microstructure of a Mg clutchbox have shown that the use of a complete die made of HTCS steel, does not give any improvements on the final casting properties since porosity levels are not reduced in comparison with conventional tool steels (H11). However, if a functionally graded die with material optimization at different die locations is approached, very promising results can be obtained. Thermal fatigue is also expected to be delayed by the decrease of the die surface temperature.
Moreover, microstructural refinement especially at the thicker regions of the final component is predicted with this new die configuration. The HTCS materials developed in TAILORTOOL (thermal conductivity values between 40 and 60 W/mK) have allowed the design of functionally graded dies, fact that suppose an important breakthrough for the HPDC development. Commercially available tool steels have thermal conductivities of 25-30 W/mK that do not allow improving the solidification rate (i.e. reduce porosity in the final cast component).
Hot forging. Forging tools suffer different damaging mechanisms at different locations. High wear resistance together with high thermal conductivity are required at the red regions in order to increase the resistance to abrasive wear and thermal cracking. However, the remaining part of the tool where no surface damage has been detected only high toughness while keeping high thermal conductivity is needed. Taking into account these die requirements, a functionally graded material was designed to cope with the thermo-mechanical requirements acting on forging tools. A bevel gear die (almost final shape) was manufactured by Nanocasting® technology, obtaining a material with functionally graded properties: high wear resistance and high thermal conductivity at the dent surface and high thermal conductivity and toughness at the base material.
Cutting and drawing of UHSS. Tools for cold forming UHSS mainly suffer from premature failure by chipping and accelerated surface wear. Tool steels present microstructures consisting of primary alloy carbides embedded in a tempered martensitic matrix. Primary carbides are aimed at bringing hardness and load bearing capabilities to the metallic matrix (especially to avoid plastic deformation), as well as increased wear resistance. Carbide characteristics such as chemical composition and structure, micro-mechanical properties, size, shape, arrangement in the matrix, etc., mainly dictate the mechanical and tribological response of the material and consequently, the performance of tools. In this sense, the Nanocasting® technology has allowed the introduction of selected hard particles with an optimised dispersion into an existing matrix, what is of less effort than trying to reach the same goal by controlling complex precipitation and growth processes of primary carbides within tool steels. Thus, individual hard particles have been studied at laboratory scale to assess their micro-mechanical properties by means of instrumented nanoindentation techniques, and the influence of these properties on the stresses generated in the metallic matrix has been asserted by means of FE simulation.
The micro-mechanical properties of hard particles have important effects on the stresses generated in the microstructure, and particularly, in the surrounding matrix. Under a same external applied pressure, particles with higher Young modulus (E) have been proved to increase stresses in the microstructure compared to particles whose E is lower. Thus, special attention must be paid in choosing particles with very high E values even if in most cases, they also present higher fracture toughness values (KC). The effects of the increased stresses generated to their surrounding can lead to more detrimental consequences than what can be achieved improving their KC. A compromise between particles E and KC must be found which, together with their shape and geometries, help decreasing the amount of stresses generated in the surrounding matrix to increase the performance of the overall material. Primary carbides, in turn, have been shown to present lower mechanical properties than the aforementioned particles, but they still have reasonably good results.
FE simulations show that stresses barely affect an area of the cutting edge higher than 500x500 µm. Ideally, as in case of FE-simulations where the tool steel is assumed as continuous linear elastic material, such stresses are compressive during the loading stage and when the sheet fractures, they are completely released. However, in real tools and with such complex microstructures, compressive fields may lead to situations for which nucleation of cracks is possible; i.e. large and irregular hard particles induce local tensile stresses at their neighbourhood, and compressive stresses beyond the elastic limit of the matrix generate tensile residual stresses at the unloading stage, as well. In both cases fracture of particles is prone to occur, and provided that the acting stresses are high enough, fatigue cracks may grow from them and propagate through the metallic matrix. Therefore, materials with small, smooth and circular particles are desired at the cutting edge, since local stresses around particles are not so magnified, as may be discerned. It will increase the resistance to the nucleation of natural cracks in the microstructure, and thus the tool performance will be enhanced.
However, at the surface and at narrow depths below, other mechanisms leading to crack nucleation and propagation are active due to very high sliding and friction conditions, in addition to plastic deformation caused by the high contact pressures. Surface roughness is then a key factor, driving local tensile stresses in small asperities, protuberances, or even accumulations of plastically deformed material at the surface. Such "defects" at the surface act as "notches" from which cracks can nucleate and propagate under the repetitively applied stresses. Even if such mechanisms only apply until narrow depths below the surface, they may affect the whole distance in which the sheet is in contact with the tool (which can be more than 1 mm from the tool edge). In this case, materials with very low surface roughness, high surface hardness and even with a hard coating and optimal nitriding treatment are desired. Such narrow layers directly below a coating should present microstructures preferentially with round, and small particles as well to help reducing coating crack phenomena.
When cracks nucleated at the surface grow perpendicular inwards the tool, they may attain rather long sizes for which materials with high resistance to crack propagation and toughness are desired. Then microstructures free of hard particles or with very marked matrix bands would be applied inside the tool material where stresses are lower and the problematic of surface wear would be avoided.
Finally, far from the most requested zones in tools, where the microstructure does not need to present such high mechanical or tribological properties, the selected material even if satisfying minimal hardness and stiffness properties should prioritise a budget cost.
However, in the end FGM for tools will have to be individually designed depending on the final application and the types of acting damaging mechanisms, in view of the characteristics of different microstructures. Some cutting tools may present chipping elongated parallel to the stroke direction, while others can present it elongated perpendicular to the stroke direction. Drawing tools, in turn, may basically suffer from wear at the surface with minimal fracture phenomena. Sheet thickness and strength may dictate the stress level and distribution in tools, as well as contact pressures and friction coefficients. And finally, process parameters such as tool radii, shearing clearances, lubrications, tool rigidity, etc. may also affect the choice of the final microstructure.
(c) Properties and performance of the developed materials and tools
Materials and tools for hot stamping and HPDC. High wear resistance and tailored thermal diffusivity are the design properties for materials to be applied in hot stamping. The developed materials (named HTS-1 and HTS-2) show different values of thermal diffusivity that allow the construction of functionally grades dies. Custom laboratory wear tests have been designed which reproduce the conditions and wear mechanisms observed in hot forming processes: one elevated temperature abrasive wear test, in which materials are tested against a hard counterpart at a temperature of 250 ºC and one high temperature abrasive wear test, where the resistance of die materials to adhesion and soldering is tested at 450ºC against an aluminium counterpart. By means of these tests, and posterior microscopy analysis, the performance of new die materials in conditions relevant to their final application has been characterized. These tests have been applied on various materials for hot stamping. Materials already widespread on the industry, including reference tool steel DIN 1.2344 and high thermal diffusivity steels HTCS 130 and HTCS 150, have been compared to the newly developed HTS-1 and four coated systems combining high thermal conductivity substrates with hard PVD coatings LUMENA and ALCRONA PRO from Oerlikon Balzers.
In particular, the new HTS-1 shows outstanding resistance to adhesion while showing improved wear resistance relative to 1.2344 (but not as good as HTCS 130 and 150). Coated systems show exceptional resistance to abrasive wear, while performing slightly better than the reference in terms of adhesion. Performance of LUMENA and ALCRONA coatings is similar, and both coatings show good adhesion on all high thermal conductivity substrates.
Die cooling strategy is ones of the keys for hot stamping and HPDC efficiency. Such strategy includes the thermal properties of the tool material and the design of cooling channels. Therefore, when water circulates at the cooling channels in hot stamping and HPDC dies, there is a certain risk for stress corrosion cracking (SCC) to occur (Note that the thermal loads generate tensile stresses at the cooling channels, that if they are so close to the die surface will be high enough for initiating the SCC). Thus aimed at optimizing the properties of the developed materials for hot stamping and HPDC applications, their susceptibility to SCC has been evaluated following the ASTM G-159 procedure. Slow strain rate tests have been carried out in air and in water. The differences in elongation determine the susceptibility of the material to SCC in water. Also, the fracture surfaces have been investigated since brittle fractures characteristics are clear indications that SCC is active when compared to the ductile response of the samples tested in air. Results have shown that the susceptibility to SCC increases with the hardness of the tool steel, as expected. At 33 HRC no SCC occurs, whereas at 52 HRC, the reduction in elongation is around 30% (at 80ºC). Therefore, strategies to minimize the risk of SCC have been assessed, and very good results have been found when using some corrosion inhibitors, when even a better response than in air has been found. Furthermore, the fracture surfaces show a ductile response in air and in the solution with corrosion inhibitors, whereas the samples tested in water showed a more brittle aspect confirming the SCC only under these tested experimental conditions.
The knowledge gathered in the TAILORTOOL project permits to build a functionally graded hot stamping die to produce tailored components, with soft and hard areas that optimized crash behaviour. A semi-industrial tool, a split Omega tool, was designed and constructed, following two strategies: the combination of two steels with very different thermal diffusivity, or the combination of two HTCS steels with one segment of the die heated up. The tool permits to obtain parts with functionally graded mechanical properties that met the crash requirements of the automotive industry and is a source for further weight reduction.
Materials and tools for hot forging. As stated before the occurrence of several damaging mechanisms simultaneously in bevel gears and their severity, can be overcome by a functionally graded material. A vast trial campaign has been conducted to validate the material design and tool performance.
Materials and tools for cold forming. Tool behaviour in industrial conditions is complex and different material properties interact to give rise to the final performance. No single standard mechanical test allows discerning the applicability of different tool materials and coatings. Under this regard, special efforts have been devoted in TAILORTOOL project to develop a laboratory test that reproduces as close as possible the working conditions of cold forming tools. Among the different approaches followed, the contact fatigue test arises as a promising laboratory test to evaluate the performance of coated tool materials (the extension to uncoated materials has also been explored). Contact fatigue resistance is one of mechanical requirements that coated materials must fulfil to show high performance in tooling applications. Indentation is one of the traditional methods to quantify the mechanical properties of materials, and during the last decades it has also been advocated as a tool to characterize the properties of thin films or coatings. At the same time, for example for hard wear-resistant coatings, indentation can be viewed as an elementary step of concentrated loading. For these reasons, many experimental as well as theoretical studies have been devoted to indentation of coated systems during recent years. However, only few of them are focused on indentation test under cyclic loading conditions, but any of these studies analyzed the influence of substrate microstructure.
Aimed at developing high performance tool materials it is necessary to understand and document the contact fatigue behaviour of the overall coated set: thin film as well as substrate properties. In Tailor Tool project a systematic spherical indentation test procedure was applied to simulate a typical "blunt" in-service condition and to identify damage evolution associated with increasing load or number of cycles using a small dimension size of samples. This procedure was based on determining critical applied loads, under both monotonic and cyclic loading condition, for appearance and evolution of distinct damage modes in residual imprints, such as: circumferential and radial cracking, cohesive spallation and thin film delamination. Final aim was to ensure their effective usage in complex service conditions, where wear is accompanied by contact loads of cyclic nature.
Circumferential cracks
Radial + circumferential cracks
Cohesive damage
Delamination
Contact fatigue behaviour of coated FGM substrates were summarized as follows:
- Plasma nitride process seems to have a detrimental effect on those FGM materials with high hardness values (between 66-68 HRC) and high density and elongated hard particles. In this specific case, detachment of coating deposited on nitrided tool steel was achieved at lower contact pressures than unnitrided substrates. This phenomenon could be attributed to high residual compression stress inside diffusion zone, which induce premature failure of irregular shape hard particles and as a consequence crack propagation in metal matrix substrate during cyclic loading test.
- Contact fatigue behaviour of FGM substrates with high hardness value (between 66 and 68 HRC) is improved when a high density of small and rounded carbides is used, and nitriding plasma process is avoided.
- Nitriding process tends to improve contact fatigue behaviour of FGM substrates with hardness value of around 60-62 HRC, as well as hard particles with more regular shapes and distributed homogenously in a metal matrix. However, it is important to control the thickness of diffusion zone, in order to avoid thicker layers with high nitrogen content that induce cracks inside primary carbides and reduce the adhesion of coating under cyclic loading conditions.
Under conventional standard adhesion any difference in coating detachment was discerned when different microstructure configuration of substrate was used. Even more, Scratch resistance and Rockwell adhesion of coating were not influenced by broken primary carbides in some nitrided FGM samples. Although these standardized tests permit to evaluate the coating integrity under monotonic and sliding load, they do not give specific information of whole coated system behaviour under complex loading conditions, similar to those observed in forming tools. In this regard, a systematic procedure based on spherical indentation test allowed to discerned clear differences between different microstructure configurations in those coated FGM with similar hardness substrates.
Two different processes were carried out regarding shear cutting of plane UHSS sheets (Process 1 using a tool with lower rigidity and Process 2 with a tool with very high rigidity) and one for deep-drawing and subsequent cutting of the deformed material (Process 3). In process 1 and 2 sheared edges of the cut components were analysed according to the most important cutting edge properties. The most important component property for shear cutting of ultra high strength steel is burr height. Industrial standards say that burr height that reaches 0.3 mm makes re-machining of the tool necessary.
In process 1, all tests were carried out using a UHSS with an ultimate tensile strength of 1200 MPa. The sheet thickness was 2 mm. In both series 100,000 strokes were carried out. For DIN 1.2379 sheared edges did not change from 0 to 100,000 strokes. However, chipping, which resulted in inadmissible burr height, appeared. It started between 50,000 and 75,000 strokes and grew in size until 100,000 strokes were reached. Chipping did not appear with reference tool material HWS. But sheared edges changed due to abrasive wear. Clean-shear increased from 0.46 mm to 0.63 mm. As a consequence fracture was reduced from 1.41 mm to 1.24 mm. Burr size did not increase.
In process 2 the used sheet material was AlSi coated 22MnB5 with a sheet thickness of 1.5 mm. It has an ultimate tensile strength of 1550 MPa. Large burr height and an increase of roll-over occurred at 60,000 strokes for DIN 1.2379 because chipping along the cutting edge appeared. There are no significant changes of the cutting edges for tool material HWS, except a small increase of clean-shear.
Sheared edges of components, however, changed, especially at the larger shearing angle of 58°. Therefore chipping at this place of the cutting punch appeared and bad component properties were the consequence. In the beginning clean-shear and fracture were distributed homogeneously along the cutting edge. After 7000 strokes an interruption of clean-shear on the front side and high burr at the back side could be seen. These were the reason why tests had to be terminated at 7000 strokes.
Potential Impact:
POTENTIAL IMPACT AND MAIN DISSEMINATION ACTIVITIES
The materials and technologies investigated in TAILORTOOL have been specially tailored to enhance the tool performance in forming processes with severe thermo-mechanical loading, that are currently used in the automotive industry for lightweight construction of safety-related components: hot stamping, high pressure die casting (HPDC) of light alloys (Al and Mg), forging and cold forming of Ultra High Strength Steels (UHSS). The European automotive industry is facing increasingly tough competition from low cost and emerging countries particularly China and India. There is general agreement that the best way for European industry to successfully compete is through the creation and development of value added products with high knowledge contents that fit the perspective of the automotive industry. The materials and know-how developed within the project will contribute in introducing the forming processes of new and advanced knowledge-based materials and lead to greater productivity at a lower economic and energy cost. Producing high-quality products at a competitive price is becoming an ever-tougher challenge as product life-cycles are getting shorter all the time and new products must be brought to market more rapidly. The need for new tools in response to these requirements is becoming more and more challenging. As a result OEMs now expect to make money at 100,000 units, not 300,000 or 400,000. Decreasing margins mean that the life of capital equipment such as presses and metal cutting machines is now expected to be as long as possible and the focus is on changes to what actually comes in contact with the finished part (i.e. the insert die or fixture). The developed tool materials can be employed to attain different produced part material responses and adapt installations to the same without high investments in processing equipment. Consequently TAILORTOOL impact will provide the market with higher quality and more innovative products along with the perspectives to increase of productivity and cost effectiveness of the investigated processes in one hand. On the other hand, the understanding developed provides new tools and investigation orientations to the partners involved towards more innovation and creation. The resulting new technology, know-how and manufacturing practices will contribute to strengthening the relationship between the automotive components sector and the major motor manufacturers.
Furthermore, European best practice will play a role in avoiding the displacement of automotive component manufacture to third countries and keeping Europe in a privileged position over its competitors. European cars need to be more economical and more environmentally friendly than those of global competitors to gain competitive advantage. This competitive advantage will lead to a global increase in the sale of European cars, resulting in an increase of production and a consequent increase of employment in Europe. Europe is the world's largest motor vehicle producer. More than 250 automobile manufacturing plants in the EU are supporting over 12 million families directly employing 2.3 million Europeans (and indirectly supporting a further 10 million jobs in related sectors). Europe produced one third of the world's passenger cars in 2006. TAILORTOOL results will help to strengthen European position and leadership in the global market.
According to the CBI market survey [CBI10], total EU imports of castings and forgings grew by 11% per year to 318 billion EUROS in 2008 and by 0.4% in 2009. Imports from Developing Countries (DC) increased the fastest, which resulted in an increasing share of Developing Countries in EU imports. China represented 41% of all imports coming from Developing Countries, followed by Turkey (14%), Ukraine (9.6%), India (8.1%) and Brazil (3.8%). The general pattern is that the more sophisticated the casting, the larger the labor factor in the landed cost price and the larger the interest of EU companies to outsource to DCs. The tool solutions provided by TAILORTOOL will contribute to fight against this effect by increasing the productivity of the hot stamping, casting and forging and cold forming, hence thus reducing the impact of labor costs in the total cost of the component.
The project was initially directed to automotive industry demands of light components with higher performance with lower costs. But, obviously the generated knowledge will facilitate the rapid generation and prototyping of tool materials tailored to the needs of other industrial sectors that uses forming processes with severe thermo-mechanical solicitations on tools as cutting and forming of thick metal plates, cutting of metal sheet at very low clearance, cold forging, plastic moulding, different conventional die casting processes and cast alloys, hot stamping of brass and other metallic alloys, extrusion, thixocasting and reocasting of several metallic alloys, etc. Besides the automotive sector, other sectors will benefit from TAILORTOOL achievements in the future, as surface transportation, agriculture and mining machine building, white goods manufacturing, etc. For instance, previously experiences prove the use of UHSS in locomotives and railways components can lead to more than 25% weight reduction. Also the use of these materials in containers for road transport can lead to reductions of 10.000 l of fuel and 26 Tons of CO2 emissions yearly.
TAILORTOOL results have a strong and direct impact on tooling performance, process efficiency and the properties of the formed components. It will contribute towards the flexible production of knowledge-based products. On the other hand, indirect impact will be related to the use of this knowledge to produce new products and substantial improvements of industrial processes. TAILORTOOL concepts and results can be applied to a smart design and rapid prototyping in tooling, which will allow obtaining components with tailored properties or with graded mechanical properties. The possibility of having tools with functionally graded thermo-mechanical properties will allow to obtain cost-effectiveness components with forming processes investigated that have been previously dealt with challenge of the competitively. The main possible impact of TAILORTOOL solutions in the efficiency and flexibility of the investigated forming processes is summarised below.
Hot stamping of steel sheets. This technology allows obtaining tailored components with a unique combination of mechanical properties that give high crashworthiness together with weight savings. Crashworthiness ratings measure the relative safety of vehicles in preventing severe injury to their own drivers in crashes and in material terms can be measured in maximum energy absorption at mixed failure modes and great energy absorption in bending and axial collapse.It is roughly schematized the load transmission path in a modern vehicle structure in different crash situation. Looking at the B-pillar it is clear that locally varying properties tailored to the component specific load profile would be of great benefit regarding passenger safety. It implies the combination of high strength (to avoid intrusion into the vehicle body during crash) and high deformability (in regions where high plasticity is needed to absorb energy). These requirements cannot be met at the same time with a single material, since hard materials present low ductility. One solution is to join metal sheets with different composition and properties. Components with functionally grades properties produced with tailored tools as those developed in TAILORTOOL allows lightweight construction by reducing the component thickness, due to the improved mechanical properties, and by reducing the number of metal sheets to be added. It will also give lower production costs by reducing the joining operations.
Within the growing field of hot-stamping major challenges for future serial production will constitute the further integration of functionally graded material properties in order to tackle the demands regarding energy absorption and structural integrity within the different part zones. The desired areas of soft and hard material will not be limited towards areas of big extend, which can already be achieved with the technology at hand, but will include a multitude of small areas within the tool.
Within TAILORTOOL, the possibility of utilizing tool steels with tailored thermal conductivities has been researched in order to achieve tailored properties through transient temperature phenomena and hence tailored temperature fields within the tool in order to achieve a reduction of the quench rate and hence tailored microstructural properties. The utilization of transient temperature phenomena through low thermal conductivity material allows the achievement of tailored soft zones without the necessity of implementing a complex control set-up and additional heating paraphernalia. Moreover, the implementation of multiple soft zones within on tool is feasible, allowing serving the demand for an additional surplus in functionality. In combination with the newly developed very high thermal conductivity materials an additional requirement regarding very sharp transition areas, from hard to soft, can be achieved.
Nevertheless, it is mentioned that the production of these tailored properties involves an additional use of production time within the tool and thus the press, due to the fact that a decreased quench rate below the critical cooling temperature is required for ferritic-pearlitic microstructures. Hence, the advantage gained from the implementation of these production strategies arises from the benefit in performance rather than mere economic aspects.
In some applications strength is the main designing factor, and no graded properties are needed. Fully hardened components obtained by hot stamping are then a competitive option, with also possibilities for further weight reduction. In that case or when a productivity gain is required, the entire tool may be built with the very high thermal conductivity material developed in TAILORTOOL. It was reported that the high thermal conductivity tool allows to increase the sheet cooling rate during stamping, hence reducing the holding time. The achieved improvements in process cycle and tool life are confidential.
Light alloy die casting: The introduction of the developed technology and understanding in both tool material and process point of view will result in improvement the cost- effectiveness of the process and pave the way toward new tool and part design . The improved tool materials that present very high thermal conductivity will result in reducing the cycle time for a given application compared to the conventional tool materials. The cycle time reduction depends on the application configuration. For instant for produced part with raw weight of more than 4 Kg, a reduction of approximately 15% - 20%. The reduction would be higher if the part geometry presents thick thicknesses. Within this project much higher thermal conductivity tool steel has been improved and challenged for tooling in die casting. Reduction of the cycle time impacts directly the production and investment cost for a given application. On the other hand, the new High Thermal Conductivity Tool Steels challenged through this project help in reducing the scraps through smart application of the same in a FGM conception approach that allow reducing the porosity, soldering and hot spot generations. The achieved improvements in process cycle and tool life are confidential.
Furthermore, in the recent years the automotive industry put a lot of effort in order to replace the structural parts of automobiles, previously built with steels, by the light alloys in order to reduce the weight, energy consumption and CO2 emissions. The High Thermal Conductivity Steel challenged through this project will contribute in achieving this objective as they produce a higher cast solidification rate when they are applied as dies. The higher solidification rate results in a finer microstructure of the cast products which enhance the mechanical properties of the same.
Considering the fact that Europe constitutes the single largest market for Al die-castings, accounting for an estimated 29% of global demand in 2010 (3.4 billion pounds), the technologies and know how developed through this project contribute in maintaining the European leader ship and the innovation perspectives for this process.
Cold cutting of UHSS, hardened B steels and UHSS: TAILORTOOL improvements in tool performance through the application of the developed materials are expected to increase tool in tooling durability under the highest demand. The quality of the final product will be improved by preventing early wear of forming tools that result in a loss in tool accuracy. The results of TAILORTOOL will also lead to more efficient and energy-efficient cutting techniques. Post cutting of hot stamped components can either be done with laser or by blanking. The main advantage with blanking is a higher productivity. On the other hand there are high demands on the tool steel since the quality of the component, mainly burr height, is dependent of the status of the cut edges. The application of increased wear resistance materials (with optimized distribution of hard particles and surface treatments) increase the maintenance interval and hence the process efficiency. It is envisaged to save up to 40% in tooling maintenance costs and tool reparation or substitution.
Forging: In TAILORTOOL project, the main requirement of forging process, tool durability and process productivity has been challenged. Within this objective, the technology has been developed that allow measuring, in production conditions, the thermo-mechanical loading acting on the tool. This was an important achievement as the reliability of the related design (tool geometry, tool material, process parameters, etc.) by simulations depends strongly on the data which was not previously measurable in industrial conditions. Thus, understanding of the thermo-mechanical solicitations and cycles in forging has seen an important progress in this project that can be applied to every type hot forging application. The achieved improvements in process cycle and tool life are confidential.
From this understanding new tool material has been challenged for the investigated tools. Especially high thermal conductivity and high hardenability tool material have been improved for this application. The high Thermal conductivity is to response to the forging process technology progress, especially, very fast machines that could make the tool thermal properties one of the limiting factor of the process productivity, especially for the forged parts that require large forging time (tool-part effective contact time). The high thermal conductivity helps also in reducing the maximum surface temperature reached during each forging cycle, hence the temperature gradient responsible for the premature heat checking of the die surface. The high hardenability tool material with required surface treatability has also been approved in response to the hot forging tool requirement to increase the resistance to surface wear, hence the durability
The main dissemination activities have been mainly focused on the publication of papers in relevant scientific Journals (Steel research international, Production Engineering and Journal of Heat Treatment and Materials (formerly Zeitschrift für Werkstoffe, Wärmebehandlung, Fertigung)) and on the participation to conferences where results from the project have been presented. A total of 10 papers have been published in the conference proceedings. Also, one PhD thesis and two Diploma thesis have been carried out within this project framework. Moreover, three PhD thesis and two more Diploma thesis are still undergoing and they will be finished in 2013.
Also, several press releases via paper or electronic format have been carried out, and a final project brochure has been edited and distributed among project partners for further dissemination. Also, the project website (see http://tailortool.ctm.com.es online) has been periodically updated.
Regarding to the exploitation results, a patent has already been applied by Oerlikon Balzers. Also, possibilities of patenting the different materials and coatings developed within this project are under consideration by Rovalma and Oerlikon, respectively.
List of Websites:
http://tailortool.ctm.com.es/
TailorTool, "Development of New Tool Materials with Tailored Thermomechanical Properties", is a FP7 project within the funding scheme Collaborative Project Small of medium-scale focused research project with the participation of the following companies, universities and research centres: Fundació CTM Centre Tecnològic (Spain) as project coordinator, VW AG (Germany), Gestamp Hardtech (Sweden), Rovalma (Spain), Oerlikon Balzers (Liechtenstein), Tecnalia (Spain), Armines (France), Technische Universität München (Germany), Technical University of Lulea (Sweden) and Metakus (Germany).TAILORTOOL has been conceived to give response to the limited life of the tools used in different manufacturing processes within the automotive industry. Materials were also specially developed to build functionally graded tools that allow producing components with graded mechanical properties, as an innovative procedure to get high crashworthiness and weight savings.
According to the expertise of the industrial partners as well as the research capabilities of the consortium the following forming processes with severe thermomechanical requirements are chosen apply the developed materials: hot stamping of B steels, hot forging, high pressure die casting (HPDC) of light alloys (aluminium and magnesium) and cold forming of Ultra High Strength Steels (UHSS) and hardened B steels sheets. Accordingly, different families of materials and tools have been developed: (a) Tools with tailored thermal conductivity and high mechanical performance for forming processes at high temperature, incorporating materials with high thermal conductivity (b) Materials with a combination of hardness and toughness for cold forming processes, (c) FGM as a base material for hard coatings, with tailored surfaces to increase the coating performance in cold forming.
One of the main project objectives is the identification and measurement of the thermomechanical solicitations acting on industrial tools during service which up to now has not been possible to be accurately measured except for HPDC. For attaining this goal, special sensors with fast time response have been developed. Industrial tools have been instrumented for accurate measurements of heat flux and local temperature at the contact zone, values which are very important for validation of FE simulations and tool material design. Based on this information, and the results from an exhaustive study of the damage mechanisms acting on industrial tools during service and the FE simulations of the different processes, the main properties required for new tool materials have been identified.
Another main objective of the project is the development of a new generation of materials specially tailored to enhance tool performance, process efficiency and component quality. Materials have been tailored according to the requirements needed for each manufacturing process and have been designed based on an optimization of:
- Thermal properties to improve resistance to thermal fatigue or to produce components by hot stamping with functionally graded properties.
- Mechanical properties (optimization of microstructure and hard particles to improve resistance to different mechanical solicitations).
Project Context and Objectives:
SUMMARY DESCRIPTION OF THE PROJECT CONTEXT AND OBJECTIVES
TAILORTOOL, "Development of New Tool Materials with Tailored Thermomechanical Properties", is a FP7 project within the funding scheme Collaborative Project Small of medium-scale focused research project with the participation of the following companies, universities and research centres: Fundació CTM Centre Tecnològic (Spain) as project coordinator, VW AG (Germany), Gestamp Hardtech (Sweden), Rovalma (Spain), Oerlikon Balzers (Liechtenstein), Tecnalia (Spain), Armines (France), Technische Universität München (Germany), Technical University of Lulea (Sweden) and Metakus (Germany).TAILORTOOL has been conceived to give response to the limited life of the tools used in different manufacturing processes within the automotive industry. Materials were also specially developed to build functionally graded tools that allow producing components with graded mechanical properties, as an innovative procedure to get high crashworthiness and weight savings.
According to the expertise of the industrial partners as well as the research capabilities of the consortium the following forming processes with severe thermomechanical requirements are chosen apply the developed materials: hot stamping of B steels, hot forging, high pressure die casting (HPDC) of light alloys (aluminium and magnesium) and cold forming of Ultra High Strength Steels (UHSS) and hardened B steels sheets. Accordingly, different families of materials and tools have been developed: (a) Tools with tailored thermal conductivity and high mechanical performance for forming processes at high temperature, incorporating materials with high thermal conductivity (b) Materials with a combination of hardness and toughness for cold forming processes, (c) FGM as a base material for hard coatings, with tailored surfaces to increase the coating performance in cold forming.
One of the main project objectives is the identification and measurement of the thermomechanical solicitations acting on industrial tools during service which up to now has not been possible to be accurately measured except for HPDC. For attaining this goal, special sensors with fast time response have been developed. Industrial tools have been instrumented for accurate measurements of heat flux and local temperature at the contact zone, values which are very important for validation of FE simulations and tool material design. Based on this information, and the results from an exhaustive study of the damage mechanisms acting on industrial tools during service and the FE simulations of the different processes, the main properties required for new tool materials have been identified.
Another main objective of the project is the development of a new generation of materials specially tailored to enhance tool performance, process efficiency and component quality.
Materials have been tailored according to the requirements needed for each manufacturing process and have been designed based on an optimization of:
- Thermal properties to improve resistance to thermal fatigue or to produce components by hot stamping with functionally graded properties.
- Mechanical properties (optimization of microstructure and hard particles to improve resistance to different mechanical solicitations).
As a final project objective, microstructural and mechanical characterization of the new materials have been carried out. Special attention was place in developing laboratory tests and experimental methodologies that reproduces tool working conditions, aimed at discerning the industrial applicability of the developed materials.
The first part of the project has been focused on the identification of damage mechanisms, evaluation thermomechanical solicitation acting on tools and FE simulations of the process, aiming at gaining full knowledge of the forming process which up to now is not completely possible, specially for hot forming operations where temperature at the tool surface is still very difficult to be measured with accuracy. For such, new sensors have been developed and afterwards successfully implemented in serial industrial tools in hot forging and HPDC. For hot forging, the temperature at the contact zone and the heat flux has successfully been determined which is an important breakthrough due to the short contact times (less than 0.1 second) and the severe mechanical solicitations acting on tools. These results have been used to validate the FE simulation models developed for each forming process.
Furthermore, failed tools and tools in service for relatively short periods of time have been studied for damage evolution studies. Also, a new methodology for damage evolution studies has been optimized for hot stamping tools. Polymeric replicas have been applied on tool surface after different number of strokes. Replicas allow to obtain exact copies of a surface, and after microscopy inspection, the surface roughness can be quantitatively evaluated, i.e. evolution of adhesive or abrasive wear can be quantified.
The main approach to increase tool performance while also increasing the component quality is the design of materials with improved thermal conductivity and wear resistance at the regions of interest, aimed at minimizing the extension of the different damage mechanisms acting on tools surface. Therefore, the correlation of the instrumentation and damage analysis studies have been used as a basis for the definition of the requirements of the new materials properties. Materials with improved thermal conductivity and/or wear resistance have been developed. Also, the optimization of substrate materials for better sustaining PVD coatings has been carried out.
The materials developed were tested in laboratory aimed at reproducing real working conditions. Tools with the new materials have been designed and constructed with the objective of validate their response under industrial conditions. An experimental tool with geometry similar to a B-pillar has been designed in order to check the feasibility of manufacturing components with graded mechanical properties by hot stamping. The tool consists of two symmetric parts made of materials with marked differences in thermal conductivity providing different cooling velocities of the hot sheet which allows obtaining two regions with different mechanical properties. The transition zone and the different regions of the final product have been characterized in terms of microstructure, hardness, tensile and fatigue tests. Furthermore, this tool has allowed carrying out a fast screening of the influence of the different process parameters on the final product properties.
Forging tools suffer different damaging mechanisms at different locations. High wear resistance together with high thermal conductivity are required at the red regions in order to increase the resistance to abrasive wear and thermal cracking. However, the remaining part of the tool where no surface damage has been detected only high toughness while keeping high thermal conductivity is needed. Taking into account these die requirements, a functionally graded material was designed to cope with the thermo-mechanical requirements acting on forging tools. A bevel gear die (almost final shape) was manufactured by Nanocasting®, obtaining a material with functionally graded properties: high wear resistance and high thermal conductivity at the dent surface and high thermal conductivity and toughness at the base material.
Tools for cold forming UHSS mainly suffer from premature failure by chipping and accelerated surface wear. Tool steels present microstructures consisting of primary alloy carbides embedded in a tempered martensitic matrix. Primary carbides are aimed at bringing hardness and load bearing capabilities to the metallic matrix (especially to avoid plastic deformation), as well as increased wear resistance. Hard particles, as primary carbide characteristics dictate the mechanical and tribological response of the material and consequently, the performance of tools. In this sense, the Nanocasting® technology has allowed the introduction of selected hard particles with an optimised dispersion into an existing matrix, what is of less effort than trying to reach the same goal by controlling complex precipitation and growth processes of primary carbides within tool steels. Thus, individual hard particles have been studied at laboratory scale to assess their micro-mechanical properties by means of nanoindentation. The properties of hard particles have important effects on the stresses generated in the surrounding metallic matrix. Under a same external applied pressure, particles with higher Young modulus (E) have been proved to increase stresses in the microstructure. Thus, special attention must be paid in choosing particles with very high E values even if in most cases, they also present higher fracture toughness (KC). The effects of the increased stresses generated to their surrounding can lead to more detrimental consequences than what can be achieved by improving their KC. A compromise between E and KC must be found together with particle shape and geometries to increase the material performance. Primary carbides have been shown to present lower mechanical properties than the aforementioned particles, but they still have reasonably good results. Such results are especially relevant when developing tool steels substrates for hard coatings. These steel substrates should present microstructures preferentially with round, and small particles as well to help reducing coating crack phenomena, as determined by FE simulation and laboratory tests.
Expected final results and potential impact and use
The new materials and know-how developed within the project will contribute in introducing the forming processes of new and advanced knowledge-based materials and lead to greater productivity at a lower economic and energy cost. Consequently, TAILORTOOL impact will provide the market with higher quality and more innovative products along with the perspectives to increase of productivity and cost effectiveness of the investigated processes in one hand. On the other hand, the understanding developed provides new tools and investigation orientations to the partners involved towards more innovation and creation. The resulting new technology, know-how and manufacturing practices will contribute to strengthening the relationship between the automotive components sector and the major motor manufacturers.
The address of the project public website: http://tailortool.ctm.com.es/
Project Results:
SCIENTIFIC AND TECHNOLOGICAL RESULTS
The material development in the TAILORTOOL project was based on the understanding of the real thermo-mechanical requirements and the main tool damaging mechanisms acting in hot stamping, die casting forging and cold sheet forming. The main challenge was to identify them in industrial tools and replicate them at the laboratory scale to properly tailor the material properties to the forming tool needs and process parameters. The scientific and technological results obtained in TAILORTOOL are described below in terms of: (a) Identification of thermo-mechanical solicitations and damaging mechanisms acting on tools, (b) Development of materials and functionally graded tools, (c) Properties and performance of the developed materials and tools.
(a) Thermo-mechanical solicitations and damaging mechanisms acting on tools
The thermo-mechanical solicitations on tools in hot stamping, casting and forging, were mainly obtained by finite elements (FE) simulations. Even if this approach gives good results, the temperature and pressure values in real forming tools depend on experimental parameters that are difficult to fully incorporate in the computer simulation. Currently, the tool surface temperature can be measured via infrared cameras but these cannot provide accurate tool surface temperature values or heat flux due to the high speed of the process (in forging contact time is less than 0.1 seconds). In the TAILORTOOL project, some sensors have been developed to evaluate the thermo-mechanical history of hot stamping, die casting and hot forging tools. Such direct measurements on instrumented industrial tools give valuable results from a material and process design point of view. Regarding to wear and tool inspection a novel experimental techniques was developed. Mechanical solicitations of cold forming tools were determined by means of FE simulation and results were experimentally validated through tests with an instrumented cutting tool.
Determination of wear mechanisms in industrial tools is complicated; techniques with the precision required are largely restricted to laboratory scale, requiring specific equipment and long inspection time. Their use on industrial tools would demand complex logistics and severely affect productivity, and thus they appear unattractive from an industrial point of view. In this project, a novel inspection technique has been developed using surface replication to study industrial tools without removing them from the production line. This technique consists in the application on the surfaces to be inspected of a viscous compound which cures in a few minutes into a high precision thermoset polymer replica of the surface topography. Replicas can be cast during natural stops of the production line, with no need to schedule additional inspection stops. Afterwards, replicas were inspected in the laboratory by means of different microscopy techniques.
Wear mechanisms in hot stamping of boron steel. Replicas were obtained from two sets of tools; one used to form Al-Si coated B steel and one used for hot stamping of uncoated B steel. In the case of hot stamping of Al-Si coated boron steel, the most obvious feature found on the tool surface was the presence of irregular lumps of adhered material, which were accurately reproduced in the replicas. These lumps were up to more than 50 µm high and several millimetres across. Another feature was a homogeneous, continuous material layer that formed on certain parts of the die. The surface of this layer is mostly flat and regular, although lumps can also be found. The existence of such layer can only be detected by observing points where the original tool surface is exposed. Layers of adhered material regularly reached thicknesses in excess of 50 µm. EDX analysis was carried out on particles extracted from the adhered layer; results showed that adhered material matched the composition of the Al-Si coating after heat treatment.
Wear mechanisms observed in hot stamping of uncoated B steel were completely different, due to the very different tribosystem. In this case, the main mechanisms observed were gradual abrasive wear and galling. Abrasive wear corresponds to the loss of material from the tool. The tool surface consists in a pattern of peaks and valleys. The most intense contact takes place on the peaks of this pattern, which are flattened by a combination of material removal and plastic deformation. As cycles progress, this mechanism removes these peaks until the whole surface has been flattened and only the deepest valleys remain. Galling is a mechanism involving solid state welding between tool and sheet metal asperities. A small volume of material remains soldered on the tool, which acts as nucleation point for further growth. These lumps may attain sizes in excess of 50 µm; a trench of sunken material usually surrounds large lumps, generated through substrate plastic deformation under and around the protruding feature.
Damage mechanisms in HPDC. The same characterization technique was applied on aluminium and magnesium HPDC dies. As in the case of hot stamping, replicas were applied during production stops, and inspected in the laboratory afterwards; allowing obtaining information from surface features and damage micro-mechanisms without taking the tool from production. In both cases, the observed damage phenomena were equivalent. The main wear mechanisms observed were washout (removal of die material by the impinging jet of molten metal), thermal fatigue (cracks generated on the surface by the alternating temperature cycles), soldering or die sticking (adhesion of material on the tool by a combination of chemical and mechanical means), and oriented drag marks (lines in the direction of die closing generated by interaction with the component during extraction). All these mechanisms leave marks on the component, and therefore tools need to be refurbished once these damages attain an unacceptable severity.
Damage mechanisms in hot forging. In the hot forging process, tools are subjected to high thermo-mechanical loads which are the responsible for the severe damaging mechanisms acting on them during service. Forging tools have been investigated and three main damage mechanisms have been identified: abrasive wear, thermal fatigue and plastic deformation. Usually they act simultaneously even at early stages of the tool life, what explains why this uses to be extremely short. Damage is located at the tool surface and subsurface, which means that a local characterization approach is needed. Mechanical damage was estimated at the subsurface of the most damaged areas by extracting local properties using nanoindentation. Damage was quantified through the correlation of nanoindentation results to plastic deformation of the material subsurface due to the high thermo-mechanical solicitations acting on tools during service. The developed methodology based on nanoindentation has probe to be a valuable tool to identify and quantify damage in forming tools.
The knowledge of the thermal history at which tools are subjected is very crucial for understanding the damage mechanisms and set the basis for the new material development. Therefore, the successful instrumentation of a bevel gear die has supposed an important breakthrough since new sensors with very fast acquisition time had to be developed within this project, and afterwards they had to be implemented in serial industrial tools where heavy loads are applied. These results have allowed determining the temperature at different depths from the tool surface and the heat flux. These values have been used to validate the FE simulation models developed for this forging process.
Analogously to hot forging, for the hot stamping and HPDC processes instrumentation studies have been carried out, and the results have been used to validate the FE models. For the hot stamping, a semi-industrial tool designed and constructed within this project has been instrumented, whereas for HPDC an industrial clutchbox die (Mg casting) has been selected. In both processes, the instrumentation campaign gave very accurate temperature and heat flux data.
Mechanical requirements acting on tools in cutting and drawing of UHSS. They have been determined by means of FE-simulations which have been later validated in test facilities specifically developed to carry out cutting and drawing of high strength steel sheets. In cutting applications, the influence of the process parameters on tool life defining factors such as cutting load and stress distribution within the cutting knives was assessed. The maximum von Mises stress appears at the beginning of plastic deformation of the sheet, whereas the maximum cutting force is reached shortly before sheet fracture.
Process Parameters
- Punch velocity [mm/s]: 90
- Blankholder force FBh [N/mm]: 480
- Tool radius rp [µm]: 50, 100,
150, 200
- Blank thickness t [mm]: 1.5 1.75
- Cutting Clearance C [%]: 5, 10, 15
An increase of cutting clearance leads to an increase of maximum von Mises stress, while the maximum cutting force is reduced. This is due to the blank contact length, which is smaller for 15 % cutting clearance than for 5 %. As the maximum stresses are very high, larger punch radii have been analyzed in order to reduce the stresses. It is known that by increasing the punch radius the required cutting force increases as well, whereas the stresses inside the punch are reduced. In this investigation it has been shown that the maximum stress value could be reduced by approximately 300 MPa by increasing the punch radius to 200 µm. Furthermore fracture of the sheet material takes place at increased punch displacements by increasing the punch radius.
In forming applications of high strength steels, galling, abrasive wear and plastic deformation as well as eggshell effect of coatings are the most common failure mechanisms encountered. When forming UHSS higher press forces are necessary due to the higher yield stress. In general a high strength material also requires higher blank holder force to prevent wrinkling at the flange region. High localized surface pressure at the forming tools makes high demands on the tool material and on the tool surface properties, respectively. Therefore, too high contact pressure between the die and the sheet is one of the main reasons that cause the failure of tools. Contact pressures have been determined by means of FE-simulation for a given combination of tool and sheet material according to punch and die radii.
Depending on the blank holder pressure, drawing force and contact pressure are distributed differently for the combination of DP1000 and uncoated conventional tool material such as AISI D2. In this case, when applying the blank holder pressure of 3, 5 and 7 MPa, the maximum contact pressure of each die amounts to about 1050, 1280 and 1800 MPa respectively. Furthermore, the higher blank holder pressure, the larger the drawing force becomes. The blank holder pressure also affects the distribution of contact pressure at the die inlet radius. During the deep drawing process the thickness becomes particularly higher. Therefore, higher contact pressure appears at the die surface which may lead to the eggshell effect.
(b) Development of materials and functionally graded tools
Hot stamping. Nowadays, there is an increasing demand for functionally graded components, especially within the automotive industry (eg. B-pillars with two zones (hard and soft) for improved performance in case of car crash). There are several ways to manufacture these graded components at once, most of them playing with different cooling and heating strategies of the die. However, at first, a complete new approach was considered in this project, i.e. the development of tool materials with a horizontal variation of the thermal conductivity. However, microstructural FE simulations have shown that for the hot stamping process, and due to thermal diffusion, the transition zone width in the final component would be far too large, and as a consequence, the use of these materials would not bring any benefits. Instead, a functionally graded die was designed. It consists of two blocks manufactured with materials developed in the frame of the project with different thermal properties and separated by an air gap. A semi-industrial tool following this concept was designed and constructed, with a shape similar to a B-pillar, an industrial part that can directly benefit from the tailored properties obtained. The tool was manufactured with a High Thermal Conductivity Steel (HTCS) and a low thermal conductivity one (GTCS). The part reached the targeted values of hardness at the soft part and the transition zone width. Furthermore, the use of this semi-industrial tool has allowed a deep study of the different process parameters, and a reduction in the cycle time when using these two material configurations is also foreseen.
High Pressure Die Casting (HPDC). FE-simulations regarding the final microstructure of a Mg clutchbox have shown that the use of a complete die made of HTCS steel, does not give any improvements on the final casting properties since porosity levels are not reduced in comparison with conventional tool steels (H11). However, if a functionally graded die with material optimization at different die locations is approached, very promising results can be obtained. Thermal fatigue is also expected to be delayed by the decrease of the die surface temperature.
Moreover, microstructural refinement especially at the thicker regions of the final component is predicted with this new die configuration. The HTCS materials developed in TAILORTOOL (thermal conductivity values between 40 and 60 W/mK) have allowed the design of functionally graded dies, fact that suppose an important breakthrough for the HPDC development. Commercially available tool steels have thermal conductivities of 25-30 W/mK that do not allow improving the solidification rate (i.e. reduce porosity in the final cast component).
Hot forging. Forging tools suffer different damaging mechanisms at different locations. High wear resistance together with high thermal conductivity are required at the red regions in order to increase the resistance to abrasive wear and thermal cracking. However, the remaining part of the tool where no surface damage has been detected only high toughness while keeping high thermal conductivity is needed. Taking into account these die requirements, a functionally graded material was designed to cope with the thermo-mechanical requirements acting on forging tools. A bevel gear die (almost final shape) was manufactured by Nanocasting® technology, obtaining a material with functionally graded properties: high wear resistance and high thermal conductivity at the dent surface and high thermal conductivity and toughness at the base material.
Cutting and drawing of UHSS. Tools for cold forming UHSS mainly suffer from premature failure by chipping and accelerated surface wear. Tool steels present microstructures consisting of primary alloy carbides embedded in a tempered martensitic matrix. Primary carbides are aimed at bringing hardness and load bearing capabilities to the metallic matrix (especially to avoid plastic deformation), as well as increased wear resistance. Carbide characteristics such as chemical composition and structure, micro-mechanical properties, size, shape, arrangement in the matrix, etc., mainly dictate the mechanical and tribological response of the material and consequently, the performance of tools. In this sense, the Nanocasting® technology has allowed the introduction of selected hard particles with an optimised dispersion into an existing matrix, what is of less effort than trying to reach the same goal by controlling complex precipitation and growth processes of primary carbides within tool steels. Thus, individual hard particles have been studied at laboratory scale to assess their micro-mechanical properties by means of instrumented nanoindentation techniques, and the influence of these properties on the stresses generated in the metallic matrix has been asserted by means of FE simulation.
The micro-mechanical properties of hard particles have important effects on the stresses generated in the microstructure, and particularly, in the surrounding matrix. Under a same external applied pressure, particles with higher Young modulus (E) have been proved to increase stresses in the microstructure compared to particles whose E is lower. Thus, special attention must be paid in choosing particles with very high E values even if in most cases, they also present higher fracture toughness values (KC). The effects of the increased stresses generated to their surrounding can lead to more detrimental consequences than what can be achieved improving their KC. A compromise between particles E and KC must be found which, together with their shape and geometries, help decreasing the amount of stresses generated in the surrounding matrix to increase the performance of the overall material. Primary carbides, in turn, have been shown to present lower mechanical properties than the aforementioned particles, but they still have reasonably good results.
FE simulations show that stresses barely affect an area of the cutting edge higher than 500x500 µm. Ideally, as in case of FE-simulations where the tool steel is assumed as continuous linear elastic material, such stresses are compressive during the loading stage and when the sheet fractures, they are completely released. However, in real tools and with such complex microstructures, compressive fields may lead to situations for which nucleation of cracks is possible; i.e. large and irregular hard particles induce local tensile stresses at their neighbourhood, and compressive stresses beyond the elastic limit of the matrix generate tensile residual stresses at the unloading stage, as well. In both cases fracture of particles is prone to occur, and provided that the acting stresses are high enough, fatigue cracks may grow from them and propagate through the metallic matrix. Therefore, materials with small, smooth and circular particles are desired at the cutting edge, since local stresses around particles are not so magnified, as may be discerned. It will increase the resistance to the nucleation of natural cracks in the microstructure, and thus the tool performance will be enhanced.
However, at the surface and at narrow depths below, other mechanisms leading to crack nucleation and propagation are active due to very high sliding and friction conditions, in addition to plastic deformation caused by the high contact pressures. Surface roughness is then a key factor, driving local tensile stresses in small asperities, protuberances, or even accumulations of plastically deformed material at the surface. Such "defects" at the surface act as "notches" from which cracks can nucleate and propagate under the repetitively applied stresses. Even if such mechanisms only apply until narrow depths below the surface, they may affect the whole distance in which the sheet is in contact with the tool (which can be more than 1 mm from the tool edge). In this case, materials with very low surface roughness, high surface hardness and even with a hard coating and optimal nitriding treatment are desired. Such narrow layers directly below a coating should present microstructures preferentially with round, and small particles as well to help reducing coating crack phenomena.
When cracks nucleated at the surface grow perpendicular inwards the tool, they may attain rather long sizes for which materials with high resistance to crack propagation and toughness are desired. Then microstructures free of hard particles or with very marked matrix bands would be applied inside the tool material where stresses are lower and the problematic of surface wear would be avoided.
Finally, far from the most requested zones in tools, where the microstructure does not need to present such high mechanical or tribological properties, the selected material even if satisfying minimal hardness and stiffness properties should prioritise a budget cost.
However, in the end FGM for tools will have to be individually designed depending on the final application and the types of acting damaging mechanisms, in view of the characteristics of different microstructures. Some cutting tools may present chipping elongated parallel to the stroke direction, while others can present it elongated perpendicular to the stroke direction. Drawing tools, in turn, may basically suffer from wear at the surface with minimal fracture phenomena. Sheet thickness and strength may dictate the stress level and distribution in tools, as well as contact pressures and friction coefficients. And finally, process parameters such as tool radii, shearing clearances, lubrications, tool rigidity, etc. may also affect the choice of the final microstructure.
(c) Properties and performance of the developed materials and tools
Materials and tools for hot stamping and HPDC. High wear resistance and tailored thermal diffusivity are the design properties for materials to be applied in hot stamping. The developed materials (named HTS-1 and HTS-2) show different values of thermal diffusivity that allow the construction of functionally grades dies. Custom laboratory wear tests have been designed which reproduce the conditions and wear mechanisms observed in hot forming processes: one elevated temperature abrasive wear test, in which materials are tested against a hard counterpart at a temperature of 250 ºC and one high temperature abrasive wear test, where the resistance of die materials to adhesion and soldering is tested at 450ºC against an aluminium counterpart. By means of these tests, and posterior microscopy analysis, the performance of new die materials in conditions relevant to their final application has been characterized. These tests have been applied on various materials for hot stamping. Materials already widespread on the industry, including reference tool steel DIN 1.2344 and high thermal diffusivity steels HTCS 130 and HTCS 150, have been compared to the newly developed HTS-1 and four coated systems combining high thermal conductivity substrates with hard PVD coatings LUMENA and ALCRONA PRO from Oerlikon Balzers.
In particular, the new HTS-1 shows outstanding resistance to adhesion while showing improved wear resistance relative to 1.2344 (but not as good as HTCS 130 and 150). Coated systems show exceptional resistance to abrasive wear, while performing slightly better than the reference in terms of adhesion. Performance of LUMENA and ALCRONA coatings is similar, and both coatings show good adhesion on all high thermal conductivity substrates.
Die cooling strategy is ones of the keys for hot stamping and HPDC efficiency. Such strategy includes the thermal properties of the tool material and the design of cooling channels. Therefore, when water circulates at the cooling channels in hot stamping and HPDC dies, there is a certain risk for stress corrosion cracking (SCC) to occur (Note that the thermal loads generate tensile stresses at the cooling channels, that if they are so close to the die surface will be high enough for initiating the SCC). Thus aimed at optimizing the properties of the developed materials for hot stamping and HPDC applications, their susceptibility to SCC has been evaluated following the ASTM G-159 procedure. Slow strain rate tests have been carried out in air and in water. The differences in elongation determine the susceptibility of the material to SCC in water. Also, the fracture surfaces have been investigated since brittle fractures characteristics are clear indications that SCC is active when compared to the ductile response of the samples tested in air. Results have shown that the susceptibility to SCC increases with the hardness of the tool steel, as expected. At 33 HRC no SCC occurs, whereas at 52 HRC, the reduction in elongation is around 30% (at 80ºC). Therefore, strategies to minimize the risk of SCC have been assessed, and very good results have been found when using some corrosion inhibitors, when even a better response than in air has been found. Furthermore, the fracture surfaces show a ductile response in air and in the solution with corrosion inhibitors, whereas the samples tested in water showed a more brittle aspect confirming the SCC only under these tested experimental conditions.
The knowledge gathered in the TAILORTOOL project permits to build a functionally graded hot stamping die to produce tailored components, with soft and hard areas that optimized crash behaviour. A semi-industrial tool, a split Omega tool, was designed and constructed, following two strategies: the combination of two steels with very different thermal diffusivity, or the combination of two HTCS steels with one segment of the die heated up. The tool permits to obtain parts with functionally graded mechanical properties that met the crash requirements of the automotive industry and is a source for further weight reduction.
Materials and tools for hot forging. As stated before the occurrence of several damaging mechanisms simultaneously in bevel gears and their severity, can be overcome by a functionally graded material. A vast trial campaign has been conducted to validate the material design and tool performance.
Materials and tools for cold forming. Tool behaviour in industrial conditions is complex and different material properties interact to give rise to the final performance. No single standard mechanical test allows discerning the applicability of different tool materials and coatings. Under this regard, special efforts have been devoted in TAILORTOOL project to develop a laboratory test that reproduces as close as possible the working conditions of cold forming tools. Among the different approaches followed, the contact fatigue test arises as a promising laboratory test to evaluate the performance of coated tool materials (the extension to uncoated materials has also been explored). Contact fatigue resistance is one of mechanical requirements that coated materials must fulfil to show high performance in tooling applications. Indentation is one of the traditional methods to quantify the mechanical properties of materials, and during the last decades it has also been advocated as a tool to characterize the properties of thin films or coatings. At the same time, for example for hard wear-resistant coatings, indentation can be viewed as an elementary step of concentrated loading. For these reasons, many experimental as well as theoretical studies have been devoted to indentation of coated systems during recent years. However, only few of them are focused on indentation test under cyclic loading conditions, but any of these studies analyzed the influence of substrate microstructure.
Aimed at developing high performance tool materials it is necessary to understand and document the contact fatigue behaviour of the overall coated set: thin film as well as substrate properties. In Tailor Tool project a systematic spherical indentation test procedure was applied to simulate a typical "blunt" in-service condition and to identify damage evolution associated with increasing load or number of cycles using a small dimension size of samples. This procedure was based on determining critical applied loads, under both monotonic and cyclic loading condition, for appearance and evolution of distinct damage modes in residual imprints, such as: circumferential and radial cracking, cohesive spallation and thin film delamination. Final aim was to ensure their effective usage in complex service conditions, where wear is accompanied by contact loads of cyclic nature.
Circumferential cracks
Radial + circumferential cracks
Cohesive damage
Delamination
Contact fatigue behaviour of coated FGM substrates were summarized as follows:
- Plasma nitride process seems to have a detrimental effect on those FGM materials with high hardness values (between 66-68 HRC) and high density and elongated hard particles. In this specific case, detachment of coating deposited on nitrided tool steel was achieved at lower contact pressures than unnitrided substrates. This phenomenon could be attributed to high residual compression stress inside diffusion zone, which induce premature failure of irregular shape hard particles and as a consequence crack propagation in metal matrix substrate during cyclic loading test.
- Contact fatigue behaviour of FGM substrates with high hardness value (between 66 and 68 HRC) is improved when a high density of small and rounded carbides is used, and nitriding plasma process is avoided.
- Nitriding process tends to improve contact fatigue behaviour of FGM substrates with hardness value of around 60-62 HRC, as well as hard particles with more regular shapes and distributed homogenously in a metal matrix. However, it is important to control the thickness of diffusion zone, in order to avoid thicker layers with high nitrogen content that induce cracks inside primary carbides and reduce the adhesion of coating under cyclic loading conditions.
Under conventional standard adhesion any difference in coating detachment was discerned when different microstructure configuration of substrate was used. Even more, Scratch resistance and Rockwell adhesion of coating were not influenced by broken primary carbides in some nitrided FGM samples. Although these standardized tests permit to evaluate the coating integrity under monotonic and sliding load, they do not give specific information of whole coated system behaviour under complex loading conditions, similar to those observed in forming tools. In this regard, a systematic procedure based on spherical indentation test allowed to discerned clear differences between different microstructure configurations in those coated FGM with similar hardness substrates.
Two different processes were carried out regarding shear cutting of plane UHSS sheets (Process 1 using a tool with lower rigidity and Process 2 with a tool with very high rigidity) and one for deep-drawing and subsequent cutting of the deformed material (Process 3). In process 1 and 2 sheared edges of the cut components were analysed according to the most important cutting edge properties. The most important component property for shear cutting of ultra high strength steel is burr height. Industrial standards say that burr height that reaches 0.3 mm makes re-machining of the tool necessary.
In process 1, all tests were carried out using a UHSS with an ultimate tensile strength of 1200 MPa. The sheet thickness was 2 mm. In both series 100,000 strokes were carried out. For DIN 1.2379 sheared edges did not change from 0 to 100,000 strokes. However, chipping, which resulted in inadmissible burr height, appeared. It started between 50,000 and 75,000 strokes and grew in size until 100,000 strokes were reached. Chipping did not appear with reference tool material HWS. But sheared edges changed due to abrasive wear. Clean-shear increased from 0.46 mm to 0.63 mm. As a consequence fracture was reduced from 1.41 mm to 1.24 mm. Burr size did not increase.
In process 2 the used sheet material was AlSi coated 22MnB5 with a sheet thickness of 1.5 mm. It has an ultimate tensile strength of 1550 MPa. Large burr height and an increase of roll-over occurred at 60,000 strokes for DIN 1.2379 because chipping along the cutting edge appeared. There are no significant changes of the cutting edges for tool material HWS, except a small increase of clean-shear.
Sheared edges of components, however, changed, especially at the larger shearing angle of 58°. Therefore chipping at this place of the cutting punch appeared and bad component properties were the consequence. In the beginning clean-shear and fracture were distributed homogeneously along the cutting edge. After 7000 strokes an interruption of clean-shear on the front side and high burr at the back side could be seen. These were the reason why tests had to be terminated at 7000 strokes.
Potential Impact:
POTENTIAL IMPACT AND MAIN DISSEMINATION ACTIVITIES
The materials and technologies investigated in TAILORTOOL have been specially tailored to enhance the tool performance in forming processes with severe thermo-mechanical loading, that are currently used in the automotive industry for lightweight construction of safety-related components: hot stamping, high pressure die casting (HPDC) of light alloys (Al and Mg), forging and cold forming of Ultra High Strength Steels (UHSS). The European automotive industry is facing increasingly tough competition from low cost and emerging countries particularly China and India. There is general agreement that the best way for European industry to successfully compete is through the creation and development of value added products with high knowledge contents that fit the perspective of the automotive industry. The materials and know-how developed within the project will contribute in introducing the forming processes of new and advanced knowledge-based materials and lead to greater productivity at a lower economic and energy cost. Producing high-quality products at a competitive price is becoming an ever-tougher challenge as product life-cycles are getting shorter all the time and new products must be brought to market more rapidly. The need for new tools in response to these requirements is becoming more and more challenging. As a result OEMs now expect to make money at 100,000 units, not 300,000 or 400,000. Decreasing margins mean that the life of capital equipment such as presses and metal cutting machines is now expected to be as long as possible and the focus is on changes to what actually comes in contact with the finished part (i.e. the insert die or fixture). The developed tool materials can be employed to attain different produced part material responses and adapt installations to the same without high investments in processing equipment. Consequently TAILORTOOL impact will provide the market with higher quality and more innovative products along with the perspectives to increase of productivity and cost effectiveness of the investigated processes in one hand. On the other hand, the understanding developed provides new tools and investigation orientations to the partners involved towards more innovation and creation. The resulting new technology, know-how and manufacturing practices will contribute to strengthening the relationship between the automotive components sector and the major motor manufacturers.
Furthermore, European best practice will play a role in avoiding the displacement of automotive component manufacture to third countries and keeping Europe in a privileged position over its competitors. European cars need to be more economical and more environmentally friendly than those of global competitors to gain competitive advantage. This competitive advantage will lead to a global increase in the sale of European cars, resulting in an increase of production and a consequent increase of employment in Europe. Europe is the world's largest motor vehicle producer. More than 250 automobile manufacturing plants in the EU are supporting over 12 million families directly employing 2.3 million Europeans (and indirectly supporting a further 10 million jobs in related sectors). Europe produced one third of the world's passenger cars in 2006. TAILORTOOL results will help to strengthen European position and leadership in the global market.
According to the CBI market survey [CBI10], total EU imports of castings and forgings grew by 11% per year to 318 billion EUROS in 2008 and by 0.4% in 2009. Imports from Developing Countries (DC) increased the fastest, which resulted in an increasing share of Developing Countries in EU imports. China represented 41% of all imports coming from Developing Countries, followed by Turkey (14%), Ukraine (9.6%), India (8.1%) and Brazil (3.8%). The general pattern is that the more sophisticated the casting, the larger the labor factor in the landed cost price and the larger the interest of EU companies to outsource to DCs. The tool solutions provided by TAILORTOOL will contribute to fight against this effect by increasing the productivity of the hot stamping, casting and forging and cold forming, hence thus reducing the impact of labor costs in the total cost of the component.
The project was initially directed to automotive industry demands of light components with higher performance with lower costs. But, obviously the generated knowledge will facilitate the rapid generation and prototyping of tool materials tailored to the needs of other industrial sectors that uses forming processes with severe thermo-mechanical solicitations on tools as cutting and forming of thick metal plates, cutting of metal sheet at very low clearance, cold forging, plastic moulding, different conventional die casting processes and cast alloys, hot stamping of brass and other metallic alloys, extrusion, thixocasting and reocasting of several metallic alloys, etc. Besides the automotive sector, other sectors will benefit from TAILORTOOL achievements in the future, as surface transportation, agriculture and mining machine building, white goods manufacturing, etc. For instance, previously experiences prove the use of UHSS in locomotives and railways components can lead to more than 25% weight reduction. Also the use of these materials in containers for road transport can lead to reductions of 10.000 l of fuel and 26 Tons of CO2 emissions yearly.
TAILORTOOL results have a strong and direct impact on tooling performance, process efficiency and the properties of the formed components. It will contribute towards the flexible production of knowledge-based products. On the other hand, indirect impact will be related to the use of this knowledge to produce new products and substantial improvements of industrial processes. TAILORTOOL concepts and results can be applied to a smart design and rapid prototyping in tooling, which will allow obtaining components with tailored properties or with graded mechanical properties. The possibility of having tools with functionally graded thermo-mechanical properties will allow to obtain cost-effectiveness components with forming processes investigated that have been previously dealt with challenge of the competitively. The main possible impact of TAILORTOOL solutions in the efficiency and flexibility of the investigated forming processes is summarised below.
Hot stamping of steel sheets. This technology allows obtaining tailored components with a unique combination of mechanical properties that give high crashworthiness together with weight savings. Crashworthiness ratings measure the relative safety of vehicles in preventing severe injury to their own drivers in crashes and in material terms can be measured in maximum energy absorption at mixed failure modes and great energy absorption in bending and axial collapse.It is roughly schematized the load transmission path in a modern vehicle structure in different crash situation. Looking at the B-pillar it is clear that locally varying properties tailored to the component specific load profile would be of great benefit regarding passenger safety. It implies the combination of high strength (to avoid intrusion into the vehicle body during crash) and high deformability (in regions where high plasticity is needed to absorb energy). These requirements cannot be met at the same time with a single material, since hard materials present low ductility. One solution is to join metal sheets with different composition and properties. Components with functionally grades properties produced with tailored tools as those developed in TAILORTOOL allows lightweight construction by reducing the component thickness, due to the improved mechanical properties, and by reducing the number of metal sheets to be added. It will also give lower production costs by reducing the joining operations.
Within the growing field of hot-stamping major challenges for future serial production will constitute the further integration of functionally graded material properties in order to tackle the demands regarding energy absorption and structural integrity within the different part zones. The desired areas of soft and hard material will not be limited towards areas of big extend, which can already be achieved with the technology at hand, but will include a multitude of small areas within the tool.
Within TAILORTOOL, the possibility of utilizing tool steels with tailored thermal conductivities has been researched in order to achieve tailored properties through transient temperature phenomena and hence tailored temperature fields within the tool in order to achieve a reduction of the quench rate and hence tailored microstructural properties. The utilization of transient temperature phenomena through low thermal conductivity material allows the achievement of tailored soft zones without the necessity of implementing a complex control set-up and additional heating paraphernalia. Moreover, the implementation of multiple soft zones within on tool is feasible, allowing serving the demand for an additional surplus in functionality. In combination with the newly developed very high thermal conductivity materials an additional requirement regarding very sharp transition areas, from hard to soft, can be achieved.
Nevertheless, it is mentioned that the production of these tailored properties involves an additional use of production time within the tool and thus the press, due to the fact that a decreased quench rate below the critical cooling temperature is required for ferritic-pearlitic microstructures. Hence, the advantage gained from the implementation of these production strategies arises from the benefit in performance rather than mere economic aspects.
In some applications strength is the main designing factor, and no graded properties are needed. Fully hardened components obtained by hot stamping are then a competitive option, with also possibilities for further weight reduction. In that case or when a productivity gain is required, the entire tool may be built with the very high thermal conductivity material developed in TAILORTOOL. It was reported that the high thermal conductivity tool allows to increase the sheet cooling rate during stamping, hence reducing the holding time. The achieved improvements in process cycle and tool life are confidential.
Light alloy die casting: The introduction of the developed technology and understanding in both tool material and process point of view will result in improvement the cost- effectiveness of the process and pave the way toward new tool and part design . The improved tool materials that present very high thermal conductivity will result in reducing the cycle time for a given application compared to the conventional tool materials. The cycle time reduction depends on the application configuration. For instant for produced part with raw weight of more than 4 Kg, a reduction of approximately 15% - 20%. The reduction would be higher if the part geometry presents thick thicknesses. Within this project much higher thermal conductivity tool steel has been improved and challenged for tooling in die casting. Reduction of the cycle time impacts directly the production and investment cost for a given application. On the other hand, the new High Thermal Conductivity Tool Steels challenged through this project help in reducing the scraps through smart application of the same in a FGM conception approach that allow reducing the porosity, soldering and hot spot generations. The achieved improvements in process cycle and tool life are confidential.
Furthermore, in the recent years the automotive industry put a lot of effort in order to replace the structural parts of automobiles, previously built with steels, by the light alloys in order to reduce the weight, energy consumption and CO2 emissions. The High Thermal Conductivity Steel challenged through this project will contribute in achieving this objective as they produce a higher cast solidification rate when they are applied as dies. The higher solidification rate results in a finer microstructure of the cast products which enhance the mechanical properties of the same.
Considering the fact that Europe constitutes the single largest market for Al die-castings, accounting for an estimated 29% of global demand in 2010 (3.4 billion pounds), the technologies and know how developed through this project contribute in maintaining the European leader ship and the innovation perspectives for this process.
Cold cutting of UHSS, hardened B steels and UHSS: TAILORTOOL improvements in tool performance through the application of the developed materials are expected to increase tool in tooling durability under the highest demand. The quality of the final product will be improved by preventing early wear of forming tools that result in a loss in tool accuracy. The results of TAILORTOOL will also lead to more efficient and energy-efficient cutting techniques. Post cutting of hot stamped components can either be done with laser or by blanking. The main advantage with blanking is a higher productivity. On the other hand there are high demands on the tool steel since the quality of the component, mainly burr height, is dependent of the status of the cut edges. The application of increased wear resistance materials (with optimized distribution of hard particles and surface treatments) increase the maintenance interval and hence the process efficiency. It is envisaged to save up to 40% in tooling maintenance costs and tool reparation or substitution.
Forging: In TAILORTOOL project, the main requirement of forging process, tool durability and process productivity has been challenged. Within this objective, the technology has been developed that allow measuring, in production conditions, the thermo-mechanical loading acting on the tool. This was an important achievement as the reliability of the related design (tool geometry, tool material, process parameters, etc.) by simulations depends strongly on the data which was not previously measurable in industrial conditions. Thus, understanding of the thermo-mechanical solicitations and cycles in forging has seen an important progress in this project that can be applied to every type hot forging application. The achieved improvements in process cycle and tool life are confidential.
From this understanding new tool material has been challenged for the investigated tools. Especially high thermal conductivity and high hardenability tool material have been improved for this application. The high Thermal conductivity is to response to the forging process technology progress, especially, very fast machines that could make the tool thermal properties one of the limiting factor of the process productivity, especially for the forged parts that require large forging time (tool-part effective contact time). The high thermal conductivity helps also in reducing the maximum surface temperature reached during each forging cycle, hence the temperature gradient responsible for the premature heat checking of the die surface. The high hardenability tool material with required surface treatability has also been approved in response to the hot forging tool requirement to increase the resistance to surface wear, hence the durability
The main dissemination activities have been mainly focused on the publication of papers in relevant scientific Journals (Steel research international, Production Engineering and Journal of Heat Treatment and Materials (formerly Zeitschrift für Werkstoffe, Wärmebehandlung, Fertigung)) and on the participation to conferences where results from the project have been presented. A total of 10 papers have been published in the conference proceedings. Also, one PhD thesis and two Diploma thesis have been carried out within this project framework. Moreover, three PhD thesis and two more Diploma thesis are still undergoing and they will be finished in 2013.
Also, several press releases via paper or electronic format have been carried out, and a final project brochure has been edited and distributed among project partners for further dissemination. Also, the project website (see http://tailortool.ctm.com.es online) has been periodically updated.
Regarding to the exploitation results, a patent has already been applied by Oerlikon Balzers. Also, possibilities of patenting the different materials and coatings developed within this project are under consideration by Rovalma and Oerlikon, respectively.
List of Websites:
http://tailortool.ctm.com.es/