Final Report Summary - HYMACER (HYbrid sintering and advanced Machining of technical CERamics)
Executive Summary:
The development of advanced technical ceramics is continuously improving leading to new potential applications and markets in industrial applications by increased product development/design capabilities, high added value products and sustainable processing. At this point, the current processing technologies form a limitation to manufacture and convert these promising materials into complex custom-made components. The development of innovative hybrid pressure assisted sintering technologies to densify difficult-to-sinter ceramics would allow economic production of large ceramic blanks that could be shaped by cost effective non-conventional machining processes into complex 3D shapes. Machining processes such as Ultrasonic Assisted Machining (UAM), Electrolytic In-process Dressing (ELID) grinding or Electrical Discharge Machining (EDM) overcome several limitations of conventional processes, like diamond tool grinding and sawing, allowing more accurate, flexible and cost effective shaping.
The Hymacer project focussed on the production of cylindrical blanks with a diameter up to 40 cm and a height up to 32 mm, by means of hybrid spark plasma sintering. Hybrid heating was realised by the incorporation of an induction heating coil in a conventional spark plasma sintering set-up. The ceramic composites focussed upon were ZrO2-based composites with TiN, WC or NbC addition and Al2O3-based composites with TiC and SiCW addition. A thermo-electrical finite element model, allowing to combine Joule and induction heating was developed to assist the hybrid sintering experiments and design of the set-up. Ultrasonic assisted (UAM) and electrical discharge (EDM) machining strategies were developed and optimised for both the ZrO2 and Al2O3-based composites. The ZrO2-based composites were machined into demonstration components, i.e. pump gear components and stamping dies and punches, by means of electrical discharge machining and ultrasonic assisted machining. The Al2O3-TiC-SiCw composites were converted into cutting tool inserts.
Project Context and Objectives:
Project context
The current production, supply and machining technologies for advanced and technical ceramics have a number of shortcomings, which upon resolving will improve the competitive position of the SME participants.
• During the past 10 years, new innovative difficult-to-densify ceramics with an excellent set of properties and multifunctional applicability have been developed on a lab-scale but not yet commercialised. This is due insufficient technological know-how by industries to upscale, use and commercialise these innovative ceramics.
• The market is continuously demanding from larger ceramic blanks (e.g. diameter larger than 200mm) to machine niche SME-specific components, e.g.: cutting tools, wear parts, customized accessory .... In order to produce large difficult-to-densify ceramic blanks, innovative pressure assisted hybrid heating sintering needs to be applied. The SMEs assessed and explored the potential of a recently installed large scale hybrid spark plasma + inductive sintering equipment.
• Technical ceramics are difficult to machine with close tolerances and a high surface finish in a sintered state. Even more so for larger size 3D components and complex shapes. To achieve highly-efficiency machining of advanced technical ceramics, the employment of alternative technological solutions has become evident. In this context, SME partners exploited the potential of non-conventional machining technologies.
In this prospective, the HYMaCER project, driven by 4 dedicated complementary Hi-Tech SMEs, aimed at developing a complete supply chain for producing and machining large size technical ceramic blanks, able to fulfil the current need of cost effective, feasible to machine and highly-innovative ceramics components, all three parameters which are currently limiting the SMEs’ ceramic market expansion and their use.
The SMEs joined their knowledge on spark plasma sintering (NANOKER & FCT) and machining of ceramics (Ceratec & ARTOOLING) to produce innovative solutions with an already proven market potential for dedicated niche markets (wear parts, tooling, machinery components...). In order to realise this, the SMEs were guided by 3 RTD centres with a long standing expertise in ceramic material development and characterisation (KU Leuven and CSIC), Hybrid spark plasma sintering (CSIC and KU Leuven) and non-conventional machining (KU Leuven and IK4-TEKNIKER).
Overall project objectives
The research was focussed on industrial up-scaling and machining technologies for electrically conductive ZrO2- and Al2O3-based composites, with main focus on ZrO2-WC, -TiN or -NbC and Al2O3-TiC-SiCw. Those innovative ceramics were already developed within earlier EU projects, demonstrating huge potential for engineering applications, such as industrial machinery components, cutting tools, wear parts... but not yet commercialised, mainly due to the lack of an economical production route. The focus in HYMaCER was not on material development as such, but on the economical up-scaling of the selected compositions, as requested by the SME participants.
Up-scaling of sintering was conducted on a large-scale continuous hybrid spark plasma sintering equipment, of FCT origin, acquired by CSIC and installed at Nanoker. This device implements a hybrid SPS technology, where Joule heating of the ceramic powder compact is combined with induction heating of the graphite die, as supplementary way to provide the necessary heat to achieve a homogeneous temperature distribution in the large scale mechanically loaded punch/powder/die set-up. The equipment is designed to produce disks up to 400 mm in diameter. In order to exploit economical production routes, a minimum part diameter of 150 mm was targeted. Numerical modelling and simulations of the temperature distribution within the punch/powder/die set-up were essential to achieve a profound knowledge of the hybrid technology, and to provide the SMEs (NANOKER and FCT) the necessary know-how for an optimised exploitation of the equipment and further development.
For each ceramic composition, machining procedures and strategies were developed and delivered to the SMEs partners for the cost effective manufacturing of prototypes and ceramic components. Fabrication technologies of main interest were electrical discharge machining, advanced cutting technologies (e.g. via PCB, CBN or diamond coated tools), ELID grinding and ultrasonic assisted processes. Those technologies were found to overcome current limitations, such as constrained shaping flexibility and surface damage.
Finally, the impact of the new materials and developed strategies was industrially assessed in specific case studies. According to the main core business of the end-users, the ceramic blanks produced were used by Ceratec for the manufacturing of gears and by ARTOOLING for the fabrication of stamping tools and dies. NANOKER exploited the Al2O3-TiC-SiCw ceramics as indexable cutting tool inserts.
Detailed project objectives
• A complete SME-scale processing route for innovative difficult-to-densify ceramic composites targeting engineering applications (WP1).
• Fully-dense large ceramic blanks sintered by means of hybrid spark plasma sintering in combination with inductive heating. Targeted sizes were diameters up to 400 mm and heights up to 32 mm (WP2).
• Thermo-electrical finite element modelling of the temperature distribution during hybrid sintering. (WP2)
• High-efficiency machining strategies to enable flexible, efficient, reliable and accurate machining of fully sintered large ceramic blank. Machining strategies included ultrasonic assisted machining, electric discharge machining and ELID grinding.(WP3)
• New and enhanced ceramic engineering applications, such as pump components, cutting tools, dies and stamping tools (WP4)
WP1: Material processing and characterisation: context and objectives
• The HYMaCER project proposed the preparation of large blanks by SPS. In the case of small samples, problems due to deficient arrangement of powders during pressing can be easily overcome during the sintering process. Nevertheless, in the case of large blanks deficient arrangement can become a strong problem limiting the final material properties due the generation of critical defects in the green body that cannot be removed during the sintering at high temperature. Then, conditioning of powders appeared as a critical step in order to facilitate the preparation of large discs by SPS that reproduce the properties obtained at lab scale. Moreover, large components required large powder batches preparation and this implied mixing, milling and/or granulation methods that may lead to the production of dense and homogenous composite materials that can fulfil the mechanical, electrical and thermal properties that had been proposed in the following objectives:
• Preparation of large composite powder batches that can be used as starting mixtures to load and fill the hybrid SPS moulds and that will be densified in WP2.
• Characterization of the as-densified ceramic blanks from WP2, focussing on the evaluation of microstructural, mechanical, thermal and electrical properties in different regions of the blanks in order to evaluate the homogeneity of the sintered disks.
• Information obtained at this point is used to optimise the temperature and pressure cycles during sintering, to support and improve the finite elements simulation and to improve the mould design.
WP 2: Hybrid SPS technology context and objectives
The overall objective of this workpackage (WP) was to upscale the processing of selected ceramic composites, shifting from laboratory scale SPS technology to industrial scale hybrid SPS technology. During upscaling, the excellent mechanical and microstructural features, such as high bending strength, high hardness, high fracture toughness and submicrometer sized grains, that could be obtained in laboratory size specimens (Ø 40 mm, height: 4 mm), was to be maintained. Finally, ceramic blanks (Ø 150-400 mm, height: 10-40 mm) with homogeneous microstructures and properties were to be made within this WP. The as-prepared blanks would be further machined by KUL-PMA and IK4-TEKNIKER within WP 3. The developed material processing (WP1-WP2) and machining (WP3) strategies would be implemented in WP4 for the realization of ceramics blanks and ceramic components for demonstration purposes.
In order to successfully upscale the SPS technology, especially when combined with an additional external inductive heating system, numerical modelling was required to a) understand the current density profiles inside the tool/sample set-up, b) calculate the concomitant generated temperature field and c) understand the interaction between the temperature field generated by inductive and Joule heating. The independently generated temperature fields, either by Joule heating using a pulsed DC current or by local Joule heating using inductively generated skin current, defined the overall temperature distribution inside the heated and densifying powder compact that was loaded in the SPS tool set-up. The applied FE modelling strategy was aiming at homogenizing the temperature field inside the sintering powder compact.
A final objective was be to offer a finite element (FE) model that could adequately describe the interaction of the two independent temperature fields and that would be available as a user-tool for the hybrid SPS operator.
WP 3: Hard machining: context and objectives
All ceramics are difficult to be machined in the sintered state with close tolerances and superior surface quality. This challenge is even more pronounced for larger size 3D components and complex shapes. In order to achieve highly-efficiency machining of advanced technical ceramics, the employment of alternative technological solutions has become evident. In this context, SME partners aim at exploiting the potential of non-conventional machining technologies, such as electrical discharge machining and advanced cutting technologies, also including ELID grinding and ultrasonic assisted processes.
The main objective of this WP is therefore the development of high-efficiency machining strategies to enable flexible, efficient, reliable and accurate machining of fully sintered large ceramic blanks (based on ZrO2 and Al2O3) obtained from WP2.
For the development of technological settings and machining strategies, two different approaches will be taken with different objectives:
Roughing/pre-finishing machining strategies that focus on the development of process settings to fabricate pre-finished parts starting from large blanks. In this task, the main aim is to obtain a high productivity and high machining speed, while surface quality is of less importance (although the crack formation should still be limited). This will be done in task 3.1.
Finishing/super finishing machining strategies, aims to investigate processes to finish the ZrO2 and Al2O3 based ceramic parts within the specified tolerances and surface quality. This will be done in task 3.2.
WP 4: Case studies demonstration: context and objectives
To fully explore the usage of ZrO2 and Al2O3 composites developed within the project. Industrial demonstrator cases coming from the industrial SME’s within the project will be manufactured and tested. By applying the large scale blanks developed within the project directly within the industry, the consortium is aiming ad an acceleration in usage of these materials in the industry.
Besides showing the technological benefits of hybrid SPS ceramic composites, the economic benefit will also be investigated within the project. SME’s providing demonstrator cases are also requested to estimate the cost of the entire production chain and retail prices for these components.
The SME within the HYMaCER project will cover the entire process chain from blank production, to manufacturing of components, up to usage of end-products. Nanoker and FCT have knowledge on hybrid SPS sintering of ceramic composites. ARTooling and Ceratec will gain knowledge on hard machining of these blanks, finally Nanoker, ARTooling and Ceratec will sell finished products and ARTooling is also end users of the finished products.
Specific objectives for this work package:
• Fully-dense large ZrO2- and Al2O3-based ceramic blanks
• New and enhanced ceramic engineering applications
For this latter Nanoker, ARTooling and Ceratec each provided a demonstrator case.
• Nanoker provided a cutting insert for cutting of hardened steel.
• ARTooling provided stamping tools for high impact stainless steel.
• Ceratec provided a component for a gear pump, for abrasive mediums.
Project Results:
Overall project achievements
• Free-flowing powder batches of ZrO2-TiN, -WC, -NbC and Al2O3-SiCw-TiC have been prepared on an industrial scale. The lab scale production by means of multidirectional mixing of typically 500 g batches has been up-scaled to 2.7 kg batches by means of wet attritor milling for 4 hrs in isopropyl alcohol, drying and sieve granulation.
• SPS-only discs (diameter = 150 mm, height = 10-32 mm) have been processed from all material classes. A height of 32 mm was found to be too challenging due to the > 15 cm initial powder height in combination with the limited height of the graphite die available. 22 mm blanks however were feasible. Due to the lower thermal conductivity of the ZrO2-based ceramics (especially the ZrO2-WC material), the ZrO2-based ceramics were found to be much more challenging than the Al2O3-TiC-SiCw ceramics. Hybrid heating and controlled cooling turned out to be crucial for the ZrO2-based ceramics, and more work is needed to provide high quality blanks. Although the targeted hardness and fracture toughness could be reached, the blanks revealed a strength gradient over the diameter of the blanks, most probably due to thermal gradients or an imperfect compaction behaviour. A few blanks of acceptable quality all material grades were forwarded for demonstration component manufacturing.
• Hybrid-SPS blanks with a diameter of 250 mm and a height up to 20 mm were produced, and a pure WC blank with a diameter of 400 mm and a height of 5 mm was manufactured as demonstration part. The hybrid sintered blanks were less prone to fracture during extraction from the set-up. The edges however were systematically overheated, requiring a more accurate control of the hybrid heating contribution.
• A Thermo-electrical finite element modelling for the temperature distribution simulation during hybrid (= SPS + inductive) sintering has been realised and was experimentally evaluated.
• Machining strategies for ultrasonic assisted machining, electric discharge machining and ELID grinding for all electrically conductive ceramic grades were developed.
• Al2O3-TiC-SiCw cutting tool inserts, ZrO2-WC, -TiN and -NbC stamping tools and ZrO2-TiN gears were realised as demonstration components.
WP1: Material processing and characterisation: summary of the main S&T results and foreground
1. PROCESSING AND CHARACTERIZATION OF ALUMINA BASED MATERIALS
In this section it is described the work involved in the preparation and characterization for the alumina based materials. First a summary of the results obtained during the first period of the project will be presented; to end with the final results obtained in this work package that will be presented in more detail.
1.1. 1st period result review
The Raw materials used in this project were high purity powders. XRD analyses were performed in order to show that no secondary phases were observed in any of the starting powders.
SEM analysis was also performed on the different starting materials in order to measure the size of the particles. The average particle size of alumina and TiC powders are < 500 nm and < 50 nm respectively. SiCw have an average diameter < 500nm and an aspect ratio over 10:1.
As the alumina based materials are composites formed by three components, firstly it was studied the rheological behaviour of each starting powder separately. The purpose was to find the maximum solid content that can be reached with each component and the dispersant needed to get a suitable viscosity. With this information, the final formulation of the three components slurry was adjusted. The results showed that the viscosity of the three components slurry is a slightly high to use it in a process of atomization. After modification of slurry formulation it was observed that spherical granulates without SiCw segregation were obtained.
After the sintering process of these materials several problems were observed. First, it was noted that the use of water as liquid medium for the slurries formulation led to detrimental effects derived from the reactivity promoted by this component. It also was observed that the formulation of slurries for atomization processes involves the use of organic compounds, as binders or deflocculants. In order to avoid impurities and pores formation during sintering, these components needed to be removed after the atomization process (debinding), but the temperatures required for this process must be above 300 ° C. This implied an increased reactivity of the ceramic components of the slurry with air, because it was observed that during the sintering process, mullite formed by the reaction between alumina and silicon oxide formed from silicon carbide previously oxidized during the debinding process. Furthermore it was noted that the materials densification was quite low during the firsts sintering tests and this led to results that did not meet the minimum requirements proposed in this project. This problem was supposed to be associated with the small TiC grain size.
After that, three solutions were proposed and adopted in order to solve these problems. First the elimination of water in the processing of slurries. Secondly the substitution of the atomization process for the attrition milling without using any organic additive, and finally the substitution of the nano-sized TiC for a micro-sized TiC.
1.2. 2nd Period results
Task 1.1. Ceramic composite powder preparation
During the final part of this project CINN started to process also the ZrO2-WC and ZrO2-TiN materials that had been processed at KUL so far. After adopting the previous considerations the processing of large batches of all the ceramic powder materials was performed by attrition milling.
For the processing of large batches of the Al2O3-SiC-TiC, ZrO2-WC and ZrO2-TiN materials, the milling conditions were the following:
• Amount of powder per batch: 2.7 Kg
• Liquid media: Isopropanol
• Milling media: 3 mm diameter alumina balls
• Milling time: 4 hours
• After that powders were dried at 100 °C during 72 hours and sieved by 180 microns.
For the processing of lab scale batches of the Al2O3-SiC-TiC, ZrO2-WC and ZrO2-TiN materials, the milling conditions were performed in PET containers using isopropanol as liquid media and alumina balls as milling media. The containers were placed in a rotatory mill at 150 rpm during 48 hours. After that the powders were dried at 100 °C during 72 hours and sieved by 180 microns.
The amount of powder from every raw material was adjusted in order to obtain a concentration of 52.0 % vol. Al2O3, 26.0 % vol. SiC and 22.0 % vol. TiC for the Al2O3 based composite, 0.7 % vol. Al2O3, 60 % vol. ZrO2 and 39.3 % vol. WC for the ZrO2-WC composite, and 0.7 % vol. Al2O3, 60 % vol. ZrO2 and 39.3 % vol. TiN for the ZrO2-TiN composite.
Task 1.2. Characterization of sintered composites
The disks of the Al2O3-SiC-nTiC and Al2O3-SiC-µTiC materials were characterized by terms of density, hardness, flexural strength, microstructure, electrical conductivity and thermal conductivity. During the development of this project, mechanical properties and the microstructure were evaluated in three different regions of the samples for the large scale disks (150 mm diameter): centre (CE), centre of the radius (CR) and edge of the disk (ED). Herein only mean values are shown in order to be more comparable with the results obtained on laboratory scale.
Density of the samples was measured by the Archimedes’ method using water as immersion media. In the center of the disk the Al2O3-SiC-TiC had a density of 3.97 +/- 0.02 g/cm3 (97.63 +/- 0.37 % relative density), in between the center and edge 3.97 +/- 0.02 g/cm3 (97.56 +/- 0.49 % relative density) and at the edge 3.96 +/- 0.01 g/cm3 (97.32 +/- 0.27 % relative density). As it can be observed the results are very homogeneous throughout the sample with values higher than 97 % of the theoretical density in all regions.
Hardness measurements were made using a micro indenter by applying 1 Kgf during 10 seconds on each sample zone. Results show the mean and standard deviation values corresponding to 20 measurements on each sample region. In the center of the disk a hardness of 22.01 +/- 1.45 GPa was observed, in between the center and edge 21.60 +/- 0.83 GPa and at the edge 21.06 +/- 1.29 GPa. As it can be observed hardness is also fairly consistent across the sample. As a mean value, 21.56 GPa may be adopted which is approximately 91 % of the required value.
The flexural strength was determined by a three point bending test using 3 prismatic bars cut from each region of the sintered disks to 4 mm width and 3 mm thickness. The tensile surface was polished down to 1 µm. The tests were performed at room temperature using a universal testing machine. The specimens were loaded to failure with a cross-head speed of 0.5 mm min–1 and a span of 16 mm. Results show the mean and standard deviation values corresponding to 3 measurements.
Fracture toughness was measured using a Vickers diamond indenter on polished surfaces, with an applied load of 300 N.
The substitution of nano TiC for the µ-TiC promoted results in an improvement on all the measured mechanical properties. Besides, the increment of the TiC content up to a 40 vol. % led to a fully dense material. While the hardness values are very close to the expected values, the other properties exceed the target values proposed in the project.
The microstructural characterization of the polished surfaces with diamond to1 µm roughness and thermally etched (1300°C, 5 min, vacuum atmosphere), was performed by scanning electron microscopy. All phases are uniformly dispersed. In the case of the nano-TiC material some porosity may be observed but in the material with µ-TiC interfaces are well bonded and no microcracks or porosity are observed.
WP 2: Hybrid SPS technology: summary of the main S&T results and foreground
The following list provides a summary of the main S&T results and foreground generated during the project with respect to understanding and simulation of the hybrid spark plasma sintering process.
• Thermo-electrical FE model enabling the simulation of the temperature distribution during hybrid SPS sintering.
• Experimental verification of the developed FE model based on comparison between simulated temperature distributions inside sintering ZrO2-TiN (60/40) (vol%), Al2O3-SiCw-TiC and binder less WC compacts.
• A user-friendly FE model, using a COMSOL code, that allows industrial hybrid SPS practitioners to predict the temperature distributions during hybrid SPS.
In the following a more detailed description of each of this item is provided.
1. Thermo-electrical FE model enabling the simulation of temperature distributions during hybrid SPS
A fully coupled thermal-electromagnetic finite element model was developed using Comsol® software. Gradually the complexity of the model was increased. Throughout different optimization procedures the following aspects were implemented:
• A thermal electrical FE model enabling the calculation of current density and temperature distributions during SPS (Joule effect only).
• A thermal-electrical model (SPS only – Joule heating) taking into account the contact resistances induced between the different parts of the SPS tool set-up.
• A thermal-electrical model (SPS only) with a moving mesh feature at the position where the powder compact is located so that shrinkage effects can be taken into account.
• A thermal-electromagnetic model enabling the simulation of the hybrid SPS process, taking into account both the Joule heating aspect (SPS only) as well as the inductive heating aspect (hybrid SPS).
• Inside the hybrid SPS FE model two independent P controllers, separately controlling the Joule and inductive heat power input, were implemented while aiming at minimizing the radial temperature gradient inside the sample.
• A user friendly MS Excel interface was built allow industrial hybrid SPS practitioners to perform FE simulations using the finalized hybrid SPS model.
2. Experimental verification of the developed FE model
The previously developed hybrid SPS model was used to modify the temperature profile and hybrid SPS tool set-up in case of hybrid SPS processing of electrically conductive ZrO2-TiN (60/40) (vol%) composites as well as binderless WC materials.
In case of the ZrO2-TiN composites, it was shown that a) the outer die temperature should be lower than the central pyrometer temperature in order to avoid overheating of the sample edge and to homogenize the radial temperature field inside the hybrid SPS sample, b) the presence of CFC plates will influence the temperature difference between the sample centre and the temperature measurement by the central pyrometer, c) the die height is extremely important when axial temperature gradients are to be avoided, especially when the axial height of the samples is increased and d) modified punch designs can help to improve the temperature distribution inside large diameter electrically insulating powders compacts.
In case of the binderless WC materials (Ø 400 mm, thickness 10 mm), it was clearly shown that thermal insulation is absolutely required when the machine is operated in the SPS only mode. Besides a more homogeneous temperature distribution inside the sintering powder compact, this would result in a lower power consumption during the SPS process. The use of a hybrid heating device, however, is suggested based on the performed FE simulations, indicating that both a more homogeneous temperature distribution and lower power consumption can be achieved when operating the machine in the hybrid SPS mode.
3. User-friendly FE code that allows industrial hybrid SPS practicioners to predict temperature distributions
Based on the previously developed model, a user-friendly MS Excel interface was built to allow industrial practitioners of hybrid SPS devices to simulate the temperature and current distributions inside their industrial devices. The interface was tested for the binderless WC case described in the previous paragraph.
WP 3: Hard machining: summary of the main S&T results and foreground
In this WP, the development of process settings and machining strategies to manufacture parts starting from sintered blanks has been done. To do so, experiments have been done either on small scale SPS samples and large scale samples manufactured under WP2. The technologies that have been evaluated are: Wire Electric Discharge Machining (EDM); Ultrasonic grinding (Rotary Ultrasonic Machining, RUM); ELID grinding and Laser machining. The selection of hard machining technologies has been made according to the main requirements and interests of the end-users (ARTooling, Ceratec and Nanoker). Based on the requested demonstrator cases machining process chains could be setup and according to the requested requirements being: desired shapes, dimensional accuracy and surface finish. After machining samples have been assessed for the desired quality aspects.
The tested samples have been characterized and the effect of the different process parameters on surface quality, material removal rate, and damaged layer thickness, etc. has been studied. This has been done for roughing and finishing operations (Task 3.1 and Task 3.2 respectively).
The details and results of the knowledge obtained during the project are reported below for each task. Additionally, all the details on the work carried out in hard machining are gathered in deliverables D3.1 D3.2 and D3.3.
Task 3.1 Roughing/pre-finishing processes
In this task, it has been carried out the experimentation to evaluate the machinability of the developed materials in the roughing and pre-finishing regime, that is a surface roughness above 0.8 micrometres, but still limited (e.g. below 2-3 microns) to avoid in depth material damage. Operations within this task are used to create the rough shape of the components, this is needed as finishing operations do not allow material removal in cost effective way. Finishing operations are used to create the final dimensional accuracy and surface finish. That is why in this step machining efficiency in terms of machining speed is of mayor importance.
Experiments were conducted on SPS lab scale samples and also in samples taken from large scale discs manufactured in WP2. Tests were conducted by wire-EDM and RUM technology on different alumina and zirconia based compositions.
For the wire-EDM technology rectangular samples have been cut from the blanks. In order to find a suitable process window. To find this suitable process window roughness, sparking gap, cutting speed, material removal rate (MRR), damaged out layer (heat affected and recast layers) and material removal mechanism have been measured on the manufactured samples and the influence of generator settings, e.g.: peak current, pulse duration, delay time and reference servo voltage has been studied.
In RUM, different grinding diamond tools have been used for machining of the ceramics composite blanks. For the RUM process a large number of machining strategies can be followed but only three have been evaluated. In order to find suitable process windows, the influence of cutting speed (directly related to the revolutions per minute of the cutting tool), feed rate and depth of cut has been studied on different quality aspect parameters of the process and finished samples such as cutting force, surface roughness and surface integrity.
This WP3 contains three deliverables but only D3.1: Initial knowledge of roughing/pre-finishing machining technologies and D3.3: Final report on roughing/pre-finishing and finishing technologies, are related to the activities described in Task 3.1.
1st period result review
In general, the ZrO2-composite blanks (ZrO2-TiN & ZrO2-WC), were faster to process using wire EDM. Furthermore the surface roughness was much less (down to about 1 microns) and the sparking gap is smaller. From a processing point of view this material is easier to process than the Al2O3-SiCw-TiC blanks, most likely because of the different material removal mechanism (MRM) demonstrated by SEM imaging of the processed and cross sectional surfaces. When Al2O3-SiCw-TiC is processed chemical decomposition (mainly of the SiCw and TiC) is the dominating MRM, while melting and evaporation (similar as for metals) dominates as MRM for ZrO2 composites. Furthermore the conductive phase (TiC) of the Al2O3-composite is a mere 22 vol%, during the second year of the project the TiC concentration was increased also showing better results for W-EDM of Al2O3-SiCw-TiC.
Except for the MRM, wire EDM settings seem to have a similar result on the processing of ZrO2-TiN and Al2O3-SiCw-TiC. The MRR during the processing of the Al2O3-SiC-TiC samples did not remain stable, most likely due to a tendency of spalling (i.e. break off of small particles due to brittle fracture). This behavior is probably caused by the lower toughness of the alumina composites. Furthermore SEM imaging of the cross section of the samples shows that the damaged layer of the Al2O3-SiC-TiC is larger than (above 10μm) the ZrO2-TiN (below 5μm) samples.
Regarding ultrasonic grinding (or RUM technology) three different machining operations have been evaluated: face milling, shallow slot milling and contour milling. The first results indicate that Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach higher removal rates in safe conditions for tool and workpiece. This is again probably because the higher hardness and lower toughness of the material, thus providing more favorable mechanical properties for the type of machining working principle.
It has been found that the use of 10 mm diameter diamond tools are not suitable for contour milling due to an excessive bending of the tool. Therefore, for this machining operation big diameter tools or end milling instead of side milling is recommended.
From the results on residual stresses it can be drawn that RUM technology induces more compressive residual stresses on the workpiece, which might be of further benefit for the material during service.
Most important results related to electrical discharge machining (EDM) and ultrasonically assisted machining (RUM)
Important remarks:
a) MRR [mm3/min] for Wire-EDM vs. RUM cannot be compared, both technologies have different strategies for removing/cutting material.
b) mm2/min is commonly used to describe the MRR for wire-EDM.
The roughing opperations in the EDM process resulted in surfaces finishing of 1.30 to 1.86 micrometer Ra roughness, 18.1 to 32.2 mm3/min. MRR (23 to 41 mm2/min.) and sparking gaps between 17 and 46 micrometer for ZrO2-TiN material. Rough EDM opperations of Al2O3-SiC-TiC material resulted surfaces finishing of 7.08 to 13.70 micrometer Ra roughness, 1.8 to 6.7 mm3/min. MRR (2.3 to 8.5 mm2/min.) and sparking gaps between 23 and 64 micrometer.
The roughing opperations for RUM resulted in surfaces finishing of 0.22 to 0.92 micrometer Ra roughness and 20 to 850 mm3/min. for ZrO2-TiN material, and surfaces finishing of 0.13 to 0.79 micrometer Ra roughness, 25 to 1200 mm3/min. MRR for Al2O3-SiC-TiC.
2nd period overview
All ZrO2 composites within the project can easily be processed using W-EDM. The most dominant MRM is melting and evaporation. This allows fast, accurate and stable processing with a minimum on wire breakage. A rectangle has also been cut from a large scale ZrO2-TiN blank and put upright in order to also manufacture higher parts. These tests were conducted in preparation of one of the case studies, however also in general they show that cutting of higher parts is possible on an industrial scale.
For the processing of Al2O3-SiCw-TiC tests have been done on micro and nano graded samples. Tests do not show a difference in machining performance between the two grades. Most likely because the conductive TiC phase is too low for efficient W-EDM operations. In the second year of the project the TiC content was increased from 22 to 40 vol%. The higher TiC content allowed more efficient machining of Al2O3-SiCw-TiC and Ra surface roughness below 2 μm could be obtained for roughing operations. The advantage of using W-EDM is that complex geometries are easily created in these hard metals, furthermore high parts can be manufactured.
Contour milling leads to low dimensional accuracy and bad control of the cutting parameters. Therefore, this type of tool should be avoided and end milling or slot milling strategies should be chosen to perform the geometry on the workpiece. This strategies reduce to the minimum the radial force in the machining so that the bending of the tool is also minimum.
The RUM technology, even using big grit size diamond tools (D151), shows quite good finishing results on the ceramics tested. The measured roughness Ra in any case is lower than 2 µm. This defines the process as a pre-finishing/finishing method.
Two material removal modes co-exist: a brittle fracture mode and plastic flow (ductile fracture mode). In Al2O3-TiC-SiC the presence of the ductile fracture mode is more evident than in ZrO2-TiN, where the brittle fracture mode seems to be dominant.
The RUM machining of water or ethanol based ZrO2-TiN does not show any significant difference. However, in the case of Al2O3-SiC-µTiC higher cutting forces are obtained compared to the machining of Al2O3-SiC-nTiC, what leads to lower MRR both in face and slot milling.
When RUM machining ZrO2-WC ceramics, in roughing operations, lower cutting forces are obtained. However higher material removal rates cannot be achieved because of unstable cutting and risk of tool damage.
Task 3.2 Finishing/super-finishing processes
Experiments were conducted to evaluate the machinability of the materials developed in the finishing and super finishing regime, that is a surface roughness (Ra) between 0.8 and 0.2 micrometres, as according to the ISO standard. Of mayor importance in this range is indeed surface quality, in terms of low surface roughness and integrity (limited surface/sub-surface damage).
Experiments were conducted on SPS lab scale samples and samples taken from large scale discs of alumina and zirconia based compositions. Machining technologies which were investigated are Wire EDM, RUM technology, laser machining and ELID grinding.
For Wire EDM up to 7 finishing steps were investigated on the ZrO2 composites, using a water submerged W-EDM machine. Surface quality needed for the industrial case studies could be obtained, even though some finishing steps could be improved and with some further optimization a higher level of surface finishing (Ra below 0.3 μm) could probably be obtained. The Al2O3 composites proved to be very difficult to process using W-EDM, more optimization steps are still needed to firstly optimize the roughing and pre-finishing operations. Finishing operations were not performed on this material. Although a higher TiC content showed promising results for this material.
In RUM, different grinding diamond tools and several diamond grain sizes have been used for the machining of the ceramics. The same machining strategies studied in task 3.1 have been evaluated in order to find suitable process windows, the influence of cutting speed (directly related to the revolutions per minute of the cutting tool), feed rate and depth of cut has been studied on different quality aspect parameters of the process and finished samples such as cutting force, surface roughness and surface integrity.
For ELID grinding, different metal-bonded grinding wheels, grit size of 46 µm, 10 µm and 2,5 µm, have been used. The influence of process parameters on grinding force, surface roughness and surface integrity has been studied.
In the first period laser machining was evaluated . The laser source was defined and a preliminary study on the machinability of ZrO2-TiN ceramic was done. Further work could be done depending on the real needs of the SMEs involved in the project. This technology is suitable for the machining of small details or features.
This WP3 contains three deliverables but only D3.2: Initial knowledge of finishing/super-finishing machining technologies and D3.3: Final report on roughing/pre-finishing and finishing technologies, are related to the activities described in Task 3.2.
1st period result review
At the first stage of the project W-EDM finishing operations were not executed on the materials. Because firstly roughing and pre-finishing operations needed to be optimized and fully explored, before finishing operations can be examined efficiently.
From the initial experimentation with RUM technology on finishing operations, it can be stated that Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach lower roughness values than on ZrO2-TiN ceramic. It has also been found that the use of 10 mm diameter tools for contour milling is not recommended even at low machining parameters.
The RUM technology demonstrates that can achieve surface roughness values on the threshold of super-finishing (~0.2 µm Ra, optical surface quality). Compressive residual stresses are induced also at finishing cutting conditions.
Regarding ELID grinding, it has been found that when a wheel of small grit size is used for finishing grinding, ELID-grinding does demonstrate advantage over normal grinding.
In the perspective of surface roughness, ELID-grinding shows obvious benefits and mirror quality surface (below 0.2 µm Ra) can be achieved through ductile mode material removal on ceramic materials. Additionally, ELID-grinding can be implemented and carried out on a retrofitted surface grinder.
Finishing tests on wire-EDM technology have started, but are still ongoing. Since there is only a limited amount of data available, no results on wire-EDM have been reported for task 3.2.
Comparison of surface roughness values Ra [µm] obtained after RUM and ELID grinding
ZrO2-TiN: RUM: 0.22 µm vs ELID: 0.06 µm / Al2O3-SiC-TiC: RUM: 0.13 µm
2nd period detailed review
An overview of findings for the Wire-EDM processing on different ceramic composites is here given. For the ZrO2-TiN and ZrO2-WC the targeted 0.5μm Ra roughness has been achieved. It must be noticed that ZrO2-NbC material has only recently become available and has only been tested in roughing and pre-finishing. Further finishing will most likely decrease the roughness below the targeted 0.5μm Ra.
The Al2O3-SiCw-TiC material is more difficult to process with wire-EDM. Besides the poor surface quality, this material also causes wire breakage and has a low MRR. A possible solution for this problem would be the increase of electrically conductive TiC phase. That is why new blanks are being produced with 40 vol% TiC content. These blank will also be tested within the project.
For Wire-EDM melting & evaporation was concidered the main material removal mechanism for the ZrO2 grades and oxidation & decomposition for the Al2O3 grades.
Below an overview of conclusions for the Wire-EDM tests on different ceramic composite materials is given. The table shows the obtained surface quality and gives recommendations towards the industry on how to set W-EDM machine parameters. For ZrO2-TiN Ra surface roughness up to 0.30μm could be obtained, 0.45μm for ZrO2-WC and 0.56μm for ZrO2-NbC. This latter was only processed under pre-finishing conditions, higher surface finishing could probably be obtained if also processed with finishing conditions. For the Al2O3 grades with 22 % vol. TiC Ra surface roughness up to 5 μm coulde be obtained and 1.50μm for the 30 % vol. TiC Al2O3 grade. To process the Al2O3 grades with EDM high open gap voltage is required with a positive polarity short pulses with long off times. A higher TiC content would be needed to obtain good EDM quality.
Regarding the RUM technology, Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach lower roughness values than on ZrO2-TiN ceramic. The final roughness is lower in those cases where the presence of ductile machining mode is more clearly observed and in the material with finer microstructure.
Two different grit sizes have been used for finishing operations. The obtained results show a slight improvement of the surface finish when using finer diamond tools, but the difference is not significant.
The RUM technology demonstrates that can achieve surface roughness values on the threshold of super-finishing (~0.2 µm Ra).
Regarding the surface integrity, it has been shown that residual stress state is homogeneous and similar in different samples and at different points of the same sample, both for ZrO2-TiN and Al2O3-SiC-TiC samples.
In the case of ZrO2-TiN ceramics the volume fraction of ZrO2 monoclinic phase has been calculated. Volume fraction of ZrO2 monoclinic phase seems to be slightly lower after the cleaning and roughing processes in comparison to as-sintered state. RUM of ZrO2-TiN does not induce any phase transformation of desirable stable tetragonal ZrO2 to detrimental monoclinic ZrO2.
From the results on residual stresses it can be drawn that RUM technology induces more compressive residual stresses on the workpiece.
In finishing operations, in face milling, lower Ra values are obtained for ZrO2-WC while in slot milling higher Ra values are obtained due to a change in the removal mode from ductile to brittle fracture mode.
Concerning ELID grinding, alumina based pieces have been ELID-grinded but the results were not so good (visual observation) compared to the Zirconia +TiN.
WP 4: Case studies demonstration: summary of the main S&T results and foreground
The demo part of Nanoker consists of a cutting tool insert made of Al2O3-SiCw-TiC, to turn hardened steel (HRc > 50). According to the ISO designation, a SNGN 120712 insert with a tolerance of 0.025 mm will be machined out of a hybrid SPS sintered disc. The entire process chain for producing these cutting inserts has 5 steps:
• Powder processing (attrition milling, drying, sieving),
• Blank manufacturing (150 mm – pure SPS ; next step 400 mm – hybrid)
• Blank grinding and cutting into dices
• Cutting tool finishing
• Validation
This process chain has been executed on a 150 mm pure SPS Al2O3-SiCw-TiC composite blank. Blocks were cut from a 150 mm diameter blank using diamond saw. The cutting of the blank into smaller blocks was outsourced. For the cutting tool finishing a prototype cutting insert of the case demonstrator of Nanoker has been produced by IK4TEKNIKER in Al2O3-SiCw-TiC (micro graded TiC). The rounding at the corners of the cutting insert were produced by RUM using a cylindrical tool of a diameter of 10 mm with a grid size of D46. The chamfers at the top of the insert are produced on a conventional CNC milling machine using a Spherical diamond tool with a diameter of 10 mm and a grid size of D126. An industrial manufacturer has been contacted to produce medium sized batches of the cutting insert. This cycle includes positioning of the cut blocks in a prism, grinding of the blocks to the wanted thickness, repositioning the blocks and grinding the rounding at the corners. A PVD coating is still under consideration, depending on cutting tests with the demonstrator case.
ARTooling introduced a ceramic stamping tools as demonstrator case. These tools consisting of a die and a punch, to stamp a high strength stainless steel automotive part with a thickness of 0.38 mm and an elastic limit of 1850 MPa. The first complex shaped parts were manufactured by EDM from a hybrid SPS sintered ZrO2-TiN (60/40) (vol%) disc and requires a surface quality of 0.3-0.4 µm Ra and a tolerance of 0.005 mm. 2 Large scale (Ø 150 mm) of ZrO2-TiN and ZrO2-WC were provided by Nanoker and were available to ARTooling for producing the punch and die. From these disks 2 punches in ZrO2-TiN, 2 punches in ZrO2-WC, 1 die in ZrO2-TiN and 1 die in ZrO2-WC were created on a Sodick AP200L W-EDM, this machine uses oil as dielectric. Ra surface roughness down to 0.3 could be achieved and all dimensions were within specifications (tolerance below 0.005mm). These punches and dies were tested at customer facilities. One ZrO2-TiN and one ZrO2-WC punch broke off at the clamping after 5 strokes, one die on ZrO2-TiN lasted up to 212 000 strokes. Punches are catapulted after passing the material, this can be resolved by having a flexible punch holder or adjusting the cutting edge. These broken punches could not be reused. New punches have been made in the same materials. This time the cutting edge on the punches was adjusted, the punches were also tested and did not break-off at the clamping, Instead the punches chipped-off at the cutting edge.
Ceratec is aiming at the pump market with their case study comprises of a gear pump component in ZrO2-TiN and Al2O3-SiCw-TiC. They request corrosion-wear resistance, fracture toughness, surface finish of Rz below 4 μm on W-EDM processed surfaces and Rz below 1μm and dimensional tolerance of ±0.008 mm. For this case study KU Leuven –PMA assisted in providing a proper manufacturing strategy and aiding in CAD/CAM programming of the part. Except for some flat surfaces which can easily be created with flat grinding, W-EDM is the needed to create the complex shapes of the part. Ceratec has no W-EDM facilities, so the final shape was created by ARTooling. This component was not tested, since the thicker blanks for producing this part were only available towards the end of the project.
Potential Impact:
Even though the Hymacer project is a highly industrially orientated project where the main expected impact is the commercialization of innovative products based on cutting edge sintering and machining technologies a very intense dissemination programme has been accomplished during the whole lifetime of the project targeting three main publics: General public, the scientific community and industries within the markets addressed by the project (Automotive, Technical ceramics, Production technologies, cutting tool industry, mechanical engineering).
A total of 46 outreach actions have been performed so far either by a joint initiative of the consortium or by single partners and a wide range of communication channels both online and off-line (Internet, TV, Press, Events...) thus contributing to a broad dissemination of the project goals and achievements. Following amount of dissemination actions have been achieved.
- Website publications: 4
- Brochures/Leaflets/Magazines: 4
- Presentations: 1
- Press releases: 5
- TV Reports: 2
- Multimedia materials: 2
- Scientific journal papers: 1 (1 more is foreseen to be published in the coming months)
- Contributions in congresses: 16
- Fairs attended with stands and Hymacer materials: 11
- Guided visits : 3
During the first project year the activity mainly focused on the design of materials aimed at the presentation of the project as a whole. In this context several initiatives were launched at the very beginning of the project such as the publication of the project website, the project brochure (first version) and a project presentation in powerpoint format. Hymacer partners also took individually an active role and contributed to the creation of materials and their dissemination by means of their own channels. In this way, several press releases were drafted and sent to the Media and also specific webpages were published within the partners’ own websites.
The dissemination activities for the promotion of the project among the general public continued during the second project year mainly by means of press releases and the periodical updates of the project website but the scope was widen to cover also the first project results and thus the focused moved to the other two target publics: scientific and industrial communities. In this way during the second reporting period of the project a strong effort has been put on the dissemination of the scientific and technical results obtained in congresses (11 lectures given in 7 countries i.e Germany, France, Belgium, South Africa, Spain, Italy and Russia) and fairs attended by project partners (participation in 10 industrial events in Spain, Germany, Sweden and The Netherlands).
All the objectives targeted in terms of publication initiatives can be considered achieved and both the number of publicity materials released and the dissemination channels used (Press, videos, websites, TV, Radio, Presentations...) have contributed to the successful widespread of project’s information among the three targeted publics.
It’s important to highlight the active involvement of the Hymacer partners in the dissemination activities during the entire project lifetime. All partners supported not only to the design of the joint promotional materials such as the website or the brochures with contents but also took an active role in the promotion of the project sometimes by means of individual initiatives such as the participation in industrial fairs where the project brochures were distributed or by collaborative activities involving 2 or more partners such as the writing of scientific papers and communications to congresses.
The list with all the dissemination actions performed in the context of Hymacer is shown in the following section and further details are included in Deliverable D5.8.
List of Websites:
https://www.hymacer.eu/
The development of advanced technical ceramics is continuously improving leading to new potential applications and markets in industrial applications by increased product development/design capabilities, high added value products and sustainable processing. At this point, the current processing technologies form a limitation to manufacture and convert these promising materials into complex custom-made components. The development of innovative hybrid pressure assisted sintering technologies to densify difficult-to-sinter ceramics would allow economic production of large ceramic blanks that could be shaped by cost effective non-conventional machining processes into complex 3D shapes. Machining processes such as Ultrasonic Assisted Machining (UAM), Electrolytic In-process Dressing (ELID) grinding or Electrical Discharge Machining (EDM) overcome several limitations of conventional processes, like diamond tool grinding and sawing, allowing more accurate, flexible and cost effective shaping.
The Hymacer project focussed on the production of cylindrical blanks with a diameter up to 40 cm and a height up to 32 mm, by means of hybrid spark plasma sintering. Hybrid heating was realised by the incorporation of an induction heating coil in a conventional spark plasma sintering set-up. The ceramic composites focussed upon were ZrO2-based composites with TiN, WC or NbC addition and Al2O3-based composites with TiC and SiCW addition. A thermo-electrical finite element model, allowing to combine Joule and induction heating was developed to assist the hybrid sintering experiments and design of the set-up. Ultrasonic assisted (UAM) and electrical discharge (EDM) machining strategies were developed and optimised for both the ZrO2 and Al2O3-based composites. The ZrO2-based composites were machined into demonstration components, i.e. pump gear components and stamping dies and punches, by means of electrical discharge machining and ultrasonic assisted machining. The Al2O3-TiC-SiCw composites were converted into cutting tool inserts.
Project Context and Objectives:
Project context
The current production, supply and machining technologies for advanced and technical ceramics have a number of shortcomings, which upon resolving will improve the competitive position of the SME participants.
• During the past 10 years, new innovative difficult-to-densify ceramics with an excellent set of properties and multifunctional applicability have been developed on a lab-scale but not yet commercialised. This is due insufficient technological know-how by industries to upscale, use and commercialise these innovative ceramics.
• The market is continuously demanding from larger ceramic blanks (e.g. diameter larger than 200mm) to machine niche SME-specific components, e.g.: cutting tools, wear parts, customized accessory .... In order to produce large difficult-to-densify ceramic blanks, innovative pressure assisted hybrid heating sintering needs to be applied. The SMEs assessed and explored the potential of a recently installed large scale hybrid spark plasma + inductive sintering equipment.
• Technical ceramics are difficult to machine with close tolerances and a high surface finish in a sintered state. Even more so for larger size 3D components and complex shapes. To achieve highly-efficiency machining of advanced technical ceramics, the employment of alternative technological solutions has become evident. In this context, SME partners exploited the potential of non-conventional machining technologies.
In this prospective, the HYMaCER project, driven by 4 dedicated complementary Hi-Tech SMEs, aimed at developing a complete supply chain for producing and machining large size technical ceramic blanks, able to fulfil the current need of cost effective, feasible to machine and highly-innovative ceramics components, all three parameters which are currently limiting the SMEs’ ceramic market expansion and their use.
The SMEs joined their knowledge on spark plasma sintering (NANOKER & FCT) and machining of ceramics (Ceratec & ARTOOLING) to produce innovative solutions with an already proven market potential for dedicated niche markets (wear parts, tooling, machinery components...). In order to realise this, the SMEs were guided by 3 RTD centres with a long standing expertise in ceramic material development and characterisation (KU Leuven and CSIC), Hybrid spark plasma sintering (CSIC and KU Leuven) and non-conventional machining (KU Leuven and IK4-TEKNIKER).
Overall project objectives
The research was focussed on industrial up-scaling and machining technologies for electrically conductive ZrO2- and Al2O3-based composites, with main focus on ZrO2-WC, -TiN or -NbC and Al2O3-TiC-SiCw. Those innovative ceramics were already developed within earlier EU projects, demonstrating huge potential for engineering applications, such as industrial machinery components, cutting tools, wear parts... but not yet commercialised, mainly due to the lack of an economical production route. The focus in HYMaCER was not on material development as such, but on the economical up-scaling of the selected compositions, as requested by the SME participants.
Up-scaling of sintering was conducted on a large-scale continuous hybrid spark plasma sintering equipment, of FCT origin, acquired by CSIC and installed at Nanoker. This device implements a hybrid SPS technology, where Joule heating of the ceramic powder compact is combined with induction heating of the graphite die, as supplementary way to provide the necessary heat to achieve a homogeneous temperature distribution in the large scale mechanically loaded punch/powder/die set-up. The equipment is designed to produce disks up to 400 mm in diameter. In order to exploit economical production routes, a minimum part diameter of 150 mm was targeted. Numerical modelling and simulations of the temperature distribution within the punch/powder/die set-up were essential to achieve a profound knowledge of the hybrid technology, and to provide the SMEs (NANOKER and FCT) the necessary know-how for an optimised exploitation of the equipment and further development.
For each ceramic composition, machining procedures and strategies were developed and delivered to the SMEs partners for the cost effective manufacturing of prototypes and ceramic components. Fabrication technologies of main interest were electrical discharge machining, advanced cutting technologies (e.g. via PCB, CBN or diamond coated tools), ELID grinding and ultrasonic assisted processes. Those technologies were found to overcome current limitations, such as constrained shaping flexibility and surface damage.
Finally, the impact of the new materials and developed strategies was industrially assessed in specific case studies. According to the main core business of the end-users, the ceramic blanks produced were used by Ceratec for the manufacturing of gears and by ARTOOLING for the fabrication of stamping tools and dies. NANOKER exploited the Al2O3-TiC-SiCw ceramics as indexable cutting tool inserts.
Detailed project objectives
• A complete SME-scale processing route for innovative difficult-to-densify ceramic composites targeting engineering applications (WP1).
• Fully-dense large ceramic blanks sintered by means of hybrid spark plasma sintering in combination with inductive heating. Targeted sizes were diameters up to 400 mm and heights up to 32 mm (WP2).
• Thermo-electrical finite element modelling of the temperature distribution during hybrid sintering. (WP2)
• High-efficiency machining strategies to enable flexible, efficient, reliable and accurate machining of fully sintered large ceramic blank. Machining strategies included ultrasonic assisted machining, electric discharge machining and ELID grinding.(WP3)
• New and enhanced ceramic engineering applications, such as pump components, cutting tools, dies and stamping tools (WP4)
WP1: Material processing and characterisation: context and objectives
• The HYMaCER project proposed the preparation of large blanks by SPS. In the case of small samples, problems due to deficient arrangement of powders during pressing can be easily overcome during the sintering process. Nevertheless, in the case of large blanks deficient arrangement can become a strong problem limiting the final material properties due the generation of critical defects in the green body that cannot be removed during the sintering at high temperature. Then, conditioning of powders appeared as a critical step in order to facilitate the preparation of large discs by SPS that reproduce the properties obtained at lab scale. Moreover, large components required large powder batches preparation and this implied mixing, milling and/or granulation methods that may lead to the production of dense and homogenous composite materials that can fulfil the mechanical, electrical and thermal properties that had been proposed in the following objectives:
• Preparation of large composite powder batches that can be used as starting mixtures to load and fill the hybrid SPS moulds and that will be densified in WP2.
• Characterization of the as-densified ceramic blanks from WP2, focussing on the evaluation of microstructural, mechanical, thermal and electrical properties in different regions of the blanks in order to evaluate the homogeneity of the sintered disks.
• Information obtained at this point is used to optimise the temperature and pressure cycles during sintering, to support and improve the finite elements simulation and to improve the mould design.
WP 2: Hybrid SPS technology context and objectives
The overall objective of this workpackage (WP) was to upscale the processing of selected ceramic composites, shifting from laboratory scale SPS technology to industrial scale hybrid SPS technology. During upscaling, the excellent mechanical and microstructural features, such as high bending strength, high hardness, high fracture toughness and submicrometer sized grains, that could be obtained in laboratory size specimens (Ø 40 mm, height: 4 mm), was to be maintained. Finally, ceramic blanks (Ø 150-400 mm, height: 10-40 mm) with homogeneous microstructures and properties were to be made within this WP. The as-prepared blanks would be further machined by KUL-PMA and IK4-TEKNIKER within WP 3. The developed material processing (WP1-WP2) and machining (WP3) strategies would be implemented in WP4 for the realization of ceramics blanks and ceramic components for demonstration purposes.
In order to successfully upscale the SPS technology, especially when combined with an additional external inductive heating system, numerical modelling was required to a) understand the current density profiles inside the tool/sample set-up, b) calculate the concomitant generated temperature field and c) understand the interaction between the temperature field generated by inductive and Joule heating. The independently generated temperature fields, either by Joule heating using a pulsed DC current or by local Joule heating using inductively generated skin current, defined the overall temperature distribution inside the heated and densifying powder compact that was loaded in the SPS tool set-up. The applied FE modelling strategy was aiming at homogenizing the temperature field inside the sintering powder compact.
A final objective was be to offer a finite element (FE) model that could adequately describe the interaction of the two independent temperature fields and that would be available as a user-tool for the hybrid SPS operator.
WP 3: Hard machining: context and objectives
All ceramics are difficult to be machined in the sintered state with close tolerances and superior surface quality. This challenge is even more pronounced for larger size 3D components and complex shapes. In order to achieve highly-efficiency machining of advanced technical ceramics, the employment of alternative technological solutions has become evident. In this context, SME partners aim at exploiting the potential of non-conventional machining technologies, such as electrical discharge machining and advanced cutting technologies, also including ELID grinding and ultrasonic assisted processes.
The main objective of this WP is therefore the development of high-efficiency machining strategies to enable flexible, efficient, reliable and accurate machining of fully sintered large ceramic blanks (based on ZrO2 and Al2O3) obtained from WP2.
For the development of technological settings and machining strategies, two different approaches will be taken with different objectives:
Roughing/pre-finishing machining strategies that focus on the development of process settings to fabricate pre-finished parts starting from large blanks. In this task, the main aim is to obtain a high productivity and high machining speed, while surface quality is of less importance (although the crack formation should still be limited). This will be done in task 3.1.
Finishing/super finishing machining strategies, aims to investigate processes to finish the ZrO2 and Al2O3 based ceramic parts within the specified tolerances and surface quality. This will be done in task 3.2.
WP 4: Case studies demonstration: context and objectives
To fully explore the usage of ZrO2 and Al2O3 composites developed within the project. Industrial demonstrator cases coming from the industrial SME’s within the project will be manufactured and tested. By applying the large scale blanks developed within the project directly within the industry, the consortium is aiming ad an acceleration in usage of these materials in the industry.
Besides showing the technological benefits of hybrid SPS ceramic composites, the economic benefit will also be investigated within the project. SME’s providing demonstrator cases are also requested to estimate the cost of the entire production chain and retail prices for these components.
The SME within the HYMaCER project will cover the entire process chain from blank production, to manufacturing of components, up to usage of end-products. Nanoker and FCT have knowledge on hybrid SPS sintering of ceramic composites. ARTooling and Ceratec will gain knowledge on hard machining of these blanks, finally Nanoker, ARTooling and Ceratec will sell finished products and ARTooling is also end users of the finished products.
Specific objectives for this work package:
• Fully-dense large ZrO2- and Al2O3-based ceramic blanks
• New and enhanced ceramic engineering applications
For this latter Nanoker, ARTooling and Ceratec each provided a demonstrator case.
• Nanoker provided a cutting insert for cutting of hardened steel.
• ARTooling provided stamping tools for high impact stainless steel.
• Ceratec provided a component for a gear pump, for abrasive mediums.
Project Results:
Overall project achievements
• Free-flowing powder batches of ZrO2-TiN, -WC, -NbC and Al2O3-SiCw-TiC have been prepared on an industrial scale. The lab scale production by means of multidirectional mixing of typically 500 g batches has been up-scaled to 2.7 kg batches by means of wet attritor milling for 4 hrs in isopropyl alcohol, drying and sieve granulation.
• SPS-only discs (diameter = 150 mm, height = 10-32 mm) have been processed from all material classes. A height of 32 mm was found to be too challenging due to the > 15 cm initial powder height in combination with the limited height of the graphite die available. 22 mm blanks however were feasible. Due to the lower thermal conductivity of the ZrO2-based ceramics (especially the ZrO2-WC material), the ZrO2-based ceramics were found to be much more challenging than the Al2O3-TiC-SiCw ceramics. Hybrid heating and controlled cooling turned out to be crucial for the ZrO2-based ceramics, and more work is needed to provide high quality blanks. Although the targeted hardness and fracture toughness could be reached, the blanks revealed a strength gradient over the diameter of the blanks, most probably due to thermal gradients or an imperfect compaction behaviour. A few blanks of acceptable quality all material grades were forwarded for demonstration component manufacturing.
• Hybrid-SPS blanks with a diameter of 250 mm and a height up to 20 mm were produced, and a pure WC blank with a diameter of 400 mm and a height of 5 mm was manufactured as demonstration part. The hybrid sintered blanks were less prone to fracture during extraction from the set-up. The edges however were systematically overheated, requiring a more accurate control of the hybrid heating contribution.
• A Thermo-electrical finite element modelling for the temperature distribution simulation during hybrid (= SPS + inductive) sintering has been realised and was experimentally evaluated.
• Machining strategies for ultrasonic assisted machining, electric discharge machining and ELID grinding for all electrically conductive ceramic grades were developed.
• Al2O3-TiC-SiCw cutting tool inserts, ZrO2-WC, -TiN and -NbC stamping tools and ZrO2-TiN gears were realised as demonstration components.
WP1: Material processing and characterisation: summary of the main S&T results and foreground
1. PROCESSING AND CHARACTERIZATION OF ALUMINA BASED MATERIALS
In this section it is described the work involved in the preparation and characterization for the alumina based materials. First a summary of the results obtained during the first period of the project will be presented; to end with the final results obtained in this work package that will be presented in more detail.
1.1. 1st period result review
The Raw materials used in this project were high purity powders. XRD analyses were performed in order to show that no secondary phases were observed in any of the starting powders.
SEM analysis was also performed on the different starting materials in order to measure the size of the particles. The average particle size of alumina and TiC powders are < 500 nm and < 50 nm respectively. SiCw have an average diameter < 500nm and an aspect ratio over 10:1.
As the alumina based materials are composites formed by three components, firstly it was studied the rheological behaviour of each starting powder separately. The purpose was to find the maximum solid content that can be reached with each component and the dispersant needed to get a suitable viscosity. With this information, the final formulation of the three components slurry was adjusted. The results showed that the viscosity of the three components slurry is a slightly high to use it in a process of atomization. After modification of slurry formulation it was observed that spherical granulates without SiCw segregation were obtained.
After the sintering process of these materials several problems were observed. First, it was noted that the use of water as liquid medium for the slurries formulation led to detrimental effects derived from the reactivity promoted by this component. It also was observed that the formulation of slurries for atomization processes involves the use of organic compounds, as binders or deflocculants. In order to avoid impurities and pores formation during sintering, these components needed to be removed after the atomization process (debinding), but the temperatures required for this process must be above 300 ° C. This implied an increased reactivity of the ceramic components of the slurry with air, because it was observed that during the sintering process, mullite formed by the reaction between alumina and silicon oxide formed from silicon carbide previously oxidized during the debinding process. Furthermore it was noted that the materials densification was quite low during the firsts sintering tests and this led to results that did not meet the minimum requirements proposed in this project. This problem was supposed to be associated with the small TiC grain size.
After that, three solutions were proposed and adopted in order to solve these problems. First the elimination of water in the processing of slurries. Secondly the substitution of the atomization process for the attrition milling without using any organic additive, and finally the substitution of the nano-sized TiC for a micro-sized TiC.
1.2. 2nd Period results
Task 1.1. Ceramic composite powder preparation
During the final part of this project CINN started to process also the ZrO2-WC and ZrO2-TiN materials that had been processed at KUL so far. After adopting the previous considerations the processing of large batches of all the ceramic powder materials was performed by attrition milling.
For the processing of large batches of the Al2O3-SiC-TiC, ZrO2-WC and ZrO2-TiN materials, the milling conditions were the following:
• Amount of powder per batch: 2.7 Kg
• Liquid media: Isopropanol
• Milling media: 3 mm diameter alumina balls
• Milling time: 4 hours
• After that powders were dried at 100 °C during 72 hours and sieved by 180 microns.
For the processing of lab scale batches of the Al2O3-SiC-TiC, ZrO2-WC and ZrO2-TiN materials, the milling conditions were performed in PET containers using isopropanol as liquid media and alumina balls as milling media. The containers were placed in a rotatory mill at 150 rpm during 48 hours. After that the powders were dried at 100 °C during 72 hours and sieved by 180 microns.
The amount of powder from every raw material was adjusted in order to obtain a concentration of 52.0 % vol. Al2O3, 26.0 % vol. SiC and 22.0 % vol. TiC for the Al2O3 based composite, 0.7 % vol. Al2O3, 60 % vol. ZrO2 and 39.3 % vol. WC for the ZrO2-WC composite, and 0.7 % vol. Al2O3, 60 % vol. ZrO2 and 39.3 % vol. TiN for the ZrO2-TiN composite.
Task 1.2. Characterization of sintered composites
The disks of the Al2O3-SiC-nTiC and Al2O3-SiC-µTiC materials were characterized by terms of density, hardness, flexural strength, microstructure, electrical conductivity and thermal conductivity. During the development of this project, mechanical properties and the microstructure were evaluated in three different regions of the samples for the large scale disks (150 mm diameter): centre (CE), centre of the radius (CR) and edge of the disk (ED). Herein only mean values are shown in order to be more comparable with the results obtained on laboratory scale.
Density of the samples was measured by the Archimedes’ method using water as immersion media. In the center of the disk the Al2O3-SiC-TiC had a density of 3.97 +/- 0.02 g/cm3 (97.63 +/- 0.37 % relative density), in between the center and edge 3.97 +/- 0.02 g/cm3 (97.56 +/- 0.49 % relative density) and at the edge 3.96 +/- 0.01 g/cm3 (97.32 +/- 0.27 % relative density). As it can be observed the results are very homogeneous throughout the sample with values higher than 97 % of the theoretical density in all regions.
Hardness measurements were made using a micro indenter by applying 1 Kgf during 10 seconds on each sample zone. Results show the mean and standard deviation values corresponding to 20 measurements on each sample region. In the center of the disk a hardness of 22.01 +/- 1.45 GPa was observed, in between the center and edge 21.60 +/- 0.83 GPa and at the edge 21.06 +/- 1.29 GPa. As it can be observed hardness is also fairly consistent across the sample. As a mean value, 21.56 GPa may be adopted which is approximately 91 % of the required value.
The flexural strength was determined by a three point bending test using 3 prismatic bars cut from each region of the sintered disks to 4 mm width and 3 mm thickness. The tensile surface was polished down to 1 µm. The tests were performed at room temperature using a universal testing machine. The specimens were loaded to failure with a cross-head speed of 0.5 mm min–1 and a span of 16 mm. Results show the mean and standard deviation values corresponding to 3 measurements.
Fracture toughness was measured using a Vickers diamond indenter on polished surfaces, with an applied load of 300 N.
The substitution of nano TiC for the µ-TiC promoted results in an improvement on all the measured mechanical properties. Besides, the increment of the TiC content up to a 40 vol. % led to a fully dense material. While the hardness values are very close to the expected values, the other properties exceed the target values proposed in the project.
The microstructural characterization of the polished surfaces with diamond to1 µm roughness and thermally etched (1300°C, 5 min, vacuum atmosphere), was performed by scanning electron microscopy. All phases are uniformly dispersed. In the case of the nano-TiC material some porosity may be observed but in the material with µ-TiC interfaces are well bonded and no microcracks or porosity are observed.
WP 2: Hybrid SPS technology: summary of the main S&T results and foreground
The following list provides a summary of the main S&T results and foreground generated during the project with respect to understanding and simulation of the hybrid spark plasma sintering process.
• Thermo-electrical FE model enabling the simulation of the temperature distribution during hybrid SPS sintering.
• Experimental verification of the developed FE model based on comparison between simulated temperature distributions inside sintering ZrO2-TiN (60/40) (vol%), Al2O3-SiCw-TiC and binder less WC compacts.
• A user-friendly FE model, using a COMSOL code, that allows industrial hybrid SPS practitioners to predict the temperature distributions during hybrid SPS.
In the following a more detailed description of each of this item is provided.
1. Thermo-electrical FE model enabling the simulation of temperature distributions during hybrid SPS
A fully coupled thermal-electromagnetic finite element model was developed using Comsol® software. Gradually the complexity of the model was increased. Throughout different optimization procedures the following aspects were implemented:
• A thermal electrical FE model enabling the calculation of current density and temperature distributions during SPS (Joule effect only).
• A thermal-electrical model (SPS only – Joule heating) taking into account the contact resistances induced between the different parts of the SPS tool set-up.
• A thermal-electrical model (SPS only) with a moving mesh feature at the position where the powder compact is located so that shrinkage effects can be taken into account.
• A thermal-electromagnetic model enabling the simulation of the hybrid SPS process, taking into account both the Joule heating aspect (SPS only) as well as the inductive heating aspect (hybrid SPS).
• Inside the hybrid SPS FE model two independent P controllers, separately controlling the Joule and inductive heat power input, were implemented while aiming at minimizing the radial temperature gradient inside the sample.
• A user friendly MS Excel interface was built allow industrial hybrid SPS practitioners to perform FE simulations using the finalized hybrid SPS model.
2. Experimental verification of the developed FE model
The previously developed hybrid SPS model was used to modify the temperature profile and hybrid SPS tool set-up in case of hybrid SPS processing of electrically conductive ZrO2-TiN (60/40) (vol%) composites as well as binderless WC materials.
In case of the ZrO2-TiN composites, it was shown that a) the outer die temperature should be lower than the central pyrometer temperature in order to avoid overheating of the sample edge and to homogenize the radial temperature field inside the hybrid SPS sample, b) the presence of CFC plates will influence the temperature difference between the sample centre and the temperature measurement by the central pyrometer, c) the die height is extremely important when axial temperature gradients are to be avoided, especially when the axial height of the samples is increased and d) modified punch designs can help to improve the temperature distribution inside large diameter electrically insulating powders compacts.
In case of the binderless WC materials (Ø 400 mm, thickness 10 mm), it was clearly shown that thermal insulation is absolutely required when the machine is operated in the SPS only mode. Besides a more homogeneous temperature distribution inside the sintering powder compact, this would result in a lower power consumption during the SPS process. The use of a hybrid heating device, however, is suggested based on the performed FE simulations, indicating that both a more homogeneous temperature distribution and lower power consumption can be achieved when operating the machine in the hybrid SPS mode.
3. User-friendly FE code that allows industrial hybrid SPS practicioners to predict temperature distributions
Based on the previously developed model, a user-friendly MS Excel interface was built to allow industrial practitioners of hybrid SPS devices to simulate the temperature and current distributions inside their industrial devices. The interface was tested for the binderless WC case described in the previous paragraph.
WP 3: Hard machining: summary of the main S&T results and foreground
In this WP, the development of process settings and machining strategies to manufacture parts starting from sintered blanks has been done. To do so, experiments have been done either on small scale SPS samples and large scale samples manufactured under WP2. The technologies that have been evaluated are: Wire Electric Discharge Machining (EDM); Ultrasonic grinding (Rotary Ultrasonic Machining, RUM); ELID grinding and Laser machining. The selection of hard machining technologies has been made according to the main requirements and interests of the end-users (ARTooling, Ceratec and Nanoker). Based on the requested demonstrator cases machining process chains could be setup and according to the requested requirements being: desired shapes, dimensional accuracy and surface finish. After machining samples have been assessed for the desired quality aspects.
The tested samples have been characterized and the effect of the different process parameters on surface quality, material removal rate, and damaged layer thickness, etc. has been studied. This has been done for roughing and finishing operations (Task 3.1 and Task 3.2 respectively).
The details and results of the knowledge obtained during the project are reported below for each task. Additionally, all the details on the work carried out in hard machining are gathered in deliverables D3.1 D3.2 and D3.3.
Task 3.1 Roughing/pre-finishing processes
In this task, it has been carried out the experimentation to evaluate the machinability of the developed materials in the roughing and pre-finishing regime, that is a surface roughness above 0.8 micrometres, but still limited (e.g. below 2-3 microns) to avoid in depth material damage. Operations within this task are used to create the rough shape of the components, this is needed as finishing operations do not allow material removal in cost effective way. Finishing operations are used to create the final dimensional accuracy and surface finish. That is why in this step machining efficiency in terms of machining speed is of mayor importance.
Experiments were conducted on SPS lab scale samples and also in samples taken from large scale discs manufactured in WP2. Tests were conducted by wire-EDM and RUM technology on different alumina and zirconia based compositions.
For the wire-EDM technology rectangular samples have been cut from the blanks. In order to find a suitable process window. To find this suitable process window roughness, sparking gap, cutting speed, material removal rate (MRR), damaged out layer (heat affected and recast layers) and material removal mechanism have been measured on the manufactured samples and the influence of generator settings, e.g.: peak current, pulse duration, delay time and reference servo voltage has been studied.
In RUM, different grinding diamond tools have been used for machining of the ceramics composite blanks. For the RUM process a large number of machining strategies can be followed but only three have been evaluated. In order to find suitable process windows, the influence of cutting speed (directly related to the revolutions per minute of the cutting tool), feed rate and depth of cut has been studied on different quality aspect parameters of the process and finished samples such as cutting force, surface roughness and surface integrity.
This WP3 contains three deliverables but only D3.1: Initial knowledge of roughing/pre-finishing machining technologies and D3.3: Final report on roughing/pre-finishing and finishing technologies, are related to the activities described in Task 3.1.
1st period result review
In general, the ZrO2-composite blanks (ZrO2-TiN & ZrO2-WC), were faster to process using wire EDM. Furthermore the surface roughness was much less (down to about 1 microns) and the sparking gap is smaller. From a processing point of view this material is easier to process than the Al2O3-SiCw-TiC blanks, most likely because of the different material removal mechanism (MRM) demonstrated by SEM imaging of the processed and cross sectional surfaces. When Al2O3-SiCw-TiC is processed chemical decomposition (mainly of the SiCw and TiC) is the dominating MRM, while melting and evaporation (similar as for metals) dominates as MRM for ZrO2 composites. Furthermore the conductive phase (TiC) of the Al2O3-composite is a mere 22 vol%, during the second year of the project the TiC concentration was increased also showing better results for W-EDM of Al2O3-SiCw-TiC.
Except for the MRM, wire EDM settings seem to have a similar result on the processing of ZrO2-TiN and Al2O3-SiCw-TiC. The MRR during the processing of the Al2O3-SiC-TiC samples did not remain stable, most likely due to a tendency of spalling (i.e. break off of small particles due to brittle fracture). This behavior is probably caused by the lower toughness of the alumina composites. Furthermore SEM imaging of the cross section of the samples shows that the damaged layer of the Al2O3-SiC-TiC is larger than (above 10μm) the ZrO2-TiN (below 5μm) samples.
Regarding ultrasonic grinding (or RUM technology) three different machining operations have been evaluated: face milling, shallow slot milling and contour milling. The first results indicate that Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach higher removal rates in safe conditions for tool and workpiece. This is again probably because the higher hardness and lower toughness of the material, thus providing more favorable mechanical properties for the type of machining working principle.
It has been found that the use of 10 mm diameter diamond tools are not suitable for contour milling due to an excessive bending of the tool. Therefore, for this machining operation big diameter tools or end milling instead of side milling is recommended.
From the results on residual stresses it can be drawn that RUM technology induces more compressive residual stresses on the workpiece, which might be of further benefit for the material during service.
Most important results related to electrical discharge machining (EDM) and ultrasonically assisted machining (RUM)
Important remarks:
a) MRR [mm3/min] for Wire-EDM vs. RUM cannot be compared, both technologies have different strategies for removing/cutting material.
b) mm2/min is commonly used to describe the MRR for wire-EDM.
The roughing opperations in the EDM process resulted in surfaces finishing of 1.30 to 1.86 micrometer Ra roughness, 18.1 to 32.2 mm3/min. MRR (23 to 41 mm2/min.) and sparking gaps between 17 and 46 micrometer for ZrO2-TiN material. Rough EDM opperations of Al2O3-SiC-TiC material resulted surfaces finishing of 7.08 to 13.70 micrometer Ra roughness, 1.8 to 6.7 mm3/min. MRR (2.3 to 8.5 mm2/min.) and sparking gaps between 23 and 64 micrometer.
The roughing opperations for RUM resulted in surfaces finishing of 0.22 to 0.92 micrometer Ra roughness and 20 to 850 mm3/min. for ZrO2-TiN material, and surfaces finishing of 0.13 to 0.79 micrometer Ra roughness, 25 to 1200 mm3/min. MRR for Al2O3-SiC-TiC.
2nd period overview
All ZrO2 composites within the project can easily be processed using W-EDM. The most dominant MRM is melting and evaporation. This allows fast, accurate and stable processing with a minimum on wire breakage. A rectangle has also been cut from a large scale ZrO2-TiN blank and put upright in order to also manufacture higher parts. These tests were conducted in preparation of one of the case studies, however also in general they show that cutting of higher parts is possible on an industrial scale.
For the processing of Al2O3-SiCw-TiC tests have been done on micro and nano graded samples. Tests do not show a difference in machining performance between the two grades. Most likely because the conductive TiC phase is too low for efficient W-EDM operations. In the second year of the project the TiC content was increased from 22 to 40 vol%. The higher TiC content allowed more efficient machining of Al2O3-SiCw-TiC and Ra surface roughness below 2 μm could be obtained for roughing operations. The advantage of using W-EDM is that complex geometries are easily created in these hard metals, furthermore high parts can be manufactured.
Contour milling leads to low dimensional accuracy and bad control of the cutting parameters. Therefore, this type of tool should be avoided and end milling or slot milling strategies should be chosen to perform the geometry on the workpiece. This strategies reduce to the minimum the radial force in the machining so that the bending of the tool is also minimum.
The RUM technology, even using big grit size diamond tools (D151), shows quite good finishing results on the ceramics tested. The measured roughness Ra in any case is lower than 2 µm. This defines the process as a pre-finishing/finishing method.
Two material removal modes co-exist: a brittle fracture mode and plastic flow (ductile fracture mode). In Al2O3-TiC-SiC the presence of the ductile fracture mode is more evident than in ZrO2-TiN, where the brittle fracture mode seems to be dominant.
The RUM machining of water or ethanol based ZrO2-TiN does not show any significant difference. However, in the case of Al2O3-SiC-µTiC higher cutting forces are obtained compared to the machining of Al2O3-SiC-nTiC, what leads to lower MRR both in face and slot milling.
When RUM machining ZrO2-WC ceramics, in roughing operations, lower cutting forces are obtained. However higher material removal rates cannot be achieved because of unstable cutting and risk of tool damage.
Task 3.2 Finishing/super-finishing processes
Experiments were conducted to evaluate the machinability of the materials developed in the finishing and super finishing regime, that is a surface roughness (Ra) between 0.8 and 0.2 micrometres, as according to the ISO standard. Of mayor importance in this range is indeed surface quality, in terms of low surface roughness and integrity (limited surface/sub-surface damage).
Experiments were conducted on SPS lab scale samples and samples taken from large scale discs of alumina and zirconia based compositions. Machining technologies which were investigated are Wire EDM, RUM technology, laser machining and ELID grinding.
For Wire EDM up to 7 finishing steps were investigated on the ZrO2 composites, using a water submerged W-EDM machine. Surface quality needed for the industrial case studies could be obtained, even though some finishing steps could be improved and with some further optimization a higher level of surface finishing (Ra below 0.3 μm) could probably be obtained. The Al2O3 composites proved to be very difficult to process using W-EDM, more optimization steps are still needed to firstly optimize the roughing and pre-finishing operations. Finishing operations were not performed on this material. Although a higher TiC content showed promising results for this material.
In RUM, different grinding diamond tools and several diamond grain sizes have been used for the machining of the ceramics. The same machining strategies studied in task 3.1 have been evaluated in order to find suitable process windows, the influence of cutting speed (directly related to the revolutions per minute of the cutting tool), feed rate and depth of cut has been studied on different quality aspect parameters of the process and finished samples such as cutting force, surface roughness and surface integrity.
For ELID grinding, different metal-bonded grinding wheels, grit size of 46 µm, 10 µm and 2,5 µm, have been used. The influence of process parameters on grinding force, surface roughness and surface integrity has been studied.
In the first period laser machining was evaluated . The laser source was defined and a preliminary study on the machinability of ZrO2-TiN ceramic was done. Further work could be done depending on the real needs of the SMEs involved in the project. This technology is suitable for the machining of small details or features.
This WP3 contains three deliverables but only D3.2: Initial knowledge of finishing/super-finishing machining technologies and D3.3: Final report on roughing/pre-finishing and finishing technologies, are related to the activities described in Task 3.2.
1st period result review
At the first stage of the project W-EDM finishing operations were not executed on the materials. Because firstly roughing and pre-finishing operations needed to be optimized and fully explored, before finishing operations can be examined efficiently.
From the initial experimentation with RUM technology on finishing operations, it can be stated that Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach lower roughness values than on ZrO2-TiN ceramic. It has also been found that the use of 10 mm diameter tools for contour milling is not recommended even at low machining parameters.
The RUM technology demonstrates that can achieve surface roughness values on the threshold of super-finishing (~0.2 µm Ra, optical surface quality). Compressive residual stresses are induced also at finishing cutting conditions.
Regarding ELID grinding, it has been found that when a wheel of small grit size is used for finishing grinding, ELID-grinding does demonstrate advantage over normal grinding.
In the perspective of surface roughness, ELID-grinding shows obvious benefits and mirror quality surface (below 0.2 µm Ra) can be achieved through ductile mode material removal on ceramic materials. Additionally, ELID-grinding can be implemented and carried out on a retrofitted surface grinder.
Finishing tests on wire-EDM technology have started, but are still ongoing. Since there is only a limited amount of data available, no results on wire-EDM have been reported for task 3.2.
Comparison of surface roughness values Ra [µm] obtained after RUM and ELID grinding
ZrO2-TiN: RUM: 0.22 µm vs ELID: 0.06 µm / Al2O3-SiC-TiC: RUM: 0.13 µm
2nd period detailed review
An overview of findings for the Wire-EDM processing on different ceramic composites is here given. For the ZrO2-TiN and ZrO2-WC the targeted 0.5μm Ra roughness has been achieved. It must be noticed that ZrO2-NbC material has only recently become available and has only been tested in roughing and pre-finishing. Further finishing will most likely decrease the roughness below the targeted 0.5μm Ra.
The Al2O3-SiCw-TiC material is more difficult to process with wire-EDM. Besides the poor surface quality, this material also causes wire breakage and has a low MRR. A possible solution for this problem would be the increase of electrically conductive TiC phase. That is why new blanks are being produced with 40 vol% TiC content. These blank will also be tested within the project.
For Wire-EDM melting & evaporation was concidered the main material removal mechanism for the ZrO2 grades and oxidation & decomposition for the Al2O3 grades.
Below an overview of conclusions for the Wire-EDM tests on different ceramic composite materials is given. The table shows the obtained surface quality and gives recommendations towards the industry on how to set W-EDM machine parameters. For ZrO2-TiN Ra surface roughness up to 0.30μm could be obtained, 0.45μm for ZrO2-WC and 0.56μm for ZrO2-NbC. This latter was only processed under pre-finishing conditions, higher surface finishing could probably be obtained if also processed with finishing conditions. For the Al2O3 grades with 22 % vol. TiC Ra surface roughness up to 5 μm coulde be obtained and 1.50μm for the 30 % vol. TiC Al2O3 grade. To process the Al2O3 grades with EDM high open gap voltage is required with a positive polarity short pulses with long off times. A higher TiC content would be needed to obtain good EDM quality.
Regarding the RUM technology, Al2O3-SiC-TiC ceramic shows a better machinability making it possible to reach lower roughness values than on ZrO2-TiN ceramic. The final roughness is lower in those cases where the presence of ductile machining mode is more clearly observed and in the material with finer microstructure.
Two different grit sizes have been used for finishing operations. The obtained results show a slight improvement of the surface finish when using finer diamond tools, but the difference is not significant.
The RUM technology demonstrates that can achieve surface roughness values on the threshold of super-finishing (~0.2 µm Ra).
Regarding the surface integrity, it has been shown that residual stress state is homogeneous and similar in different samples and at different points of the same sample, both for ZrO2-TiN and Al2O3-SiC-TiC samples.
In the case of ZrO2-TiN ceramics the volume fraction of ZrO2 monoclinic phase has been calculated. Volume fraction of ZrO2 monoclinic phase seems to be slightly lower after the cleaning and roughing processes in comparison to as-sintered state. RUM of ZrO2-TiN does not induce any phase transformation of desirable stable tetragonal ZrO2 to detrimental monoclinic ZrO2.
From the results on residual stresses it can be drawn that RUM technology induces more compressive residual stresses on the workpiece.
In finishing operations, in face milling, lower Ra values are obtained for ZrO2-WC while in slot milling higher Ra values are obtained due to a change in the removal mode from ductile to brittle fracture mode.
Concerning ELID grinding, alumina based pieces have been ELID-grinded but the results were not so good (visual observation) compared to the Zirconia +TiN.
WP 4: Case studies demonstration: summary of the main S&T results and foreground
The demo part of Nanoker consists of a cutting tool insert made of Al2O3-SiCw-TiC, to turn hardened steel (HRc > 50). According to the ISO designation, a SNGN 120712 insert with a tolerance of 0.025 mm will be machined out of a hybrid SPS sintered disc. The entire process chain for producing these cutting inserts has 5 steps:
• Powder processing (attrition milling, drying, sieving),
• Blank manufacturing (150 mm – pure SPS ; next step 400 mm – hybrid)
• Blank grinding and cutting into dices
• Cutting tool finishing
• Validation
This process chain has been executed on a 150 mm pure SPS Al2O3-SiCw-TiC composite blank. Blocks were cut from a 150 mm diameter blank using diamond saw. The cutting of the blank into smaller blocks was outsourced. For the cutting tool finishing a prototype cutting insert of the case demonstrator of Nanoker has been produced by IK4TEKNIKER in Al2O3-SiCw-TiC (micro graded TiC). The rounding at the corners of the cutting insert were produced by RUM using a cylindrical tool of a diameter of 10 mm with a grid size of D46. The chamfers at the top of the insert are produced on a conventional CNC milling machine using a Spherical diamond tool with a diameter of 10 mm and a grid size of D126. An industrial manufacturer has been contacted to produce medium sized batches of the cutting insert. This cycle includes positioning of the cut blocks in a prism, grinding of the blocks to the wanted thickness, repositioning the blocks and grinding the rounding at the corners. A PVD coating is still under consideration, depending on cutting tests with the demonstrator case.
ARTooling introduced a ceramic stamping tools as demonstrator case. These tools consisting of a die and a punch, to stamp a high strength stainless steel automotive part with a thickness of 0.38 mm and an elastic limit of 1850 MPa. The first complex shaped parts were manufactured by EDM from a hybrid SPS sintered ZrO2-TiN (60/40) (vol%) disc and requires a surface quality of 0.3-0.4 µm Ra and a tolerance of 0.005 mm. 2 Large scale (Ø 150 mm) of ZrO2-TiN and ZrO2-WC were provided by Nanoker and were available to ARTooling for producing the punch and die. From these disks 2 punches in ZrO2-TiN, 2 punches in ZrO2-WC, 1 die in ZrO2-TiN and 1 die in ZrO2-WC were created on a Sodick AP200L W-EDM, this machine uses oil as dielectric. Ra surface roughness down to 0.3 could be achieved and all dimensions were within specifications (tolerance below 0.005mm). These punches and dies were tested at customer facilities. One ZrO2-TiN and one ZrO2-WC punch broke off at the clamping after 5 strokes, one die on ZrO2-TiN lasted up to 212 000 strokes. Punches are catapulted after passing the material, this can be resolved by having a flexible punch holder or adjusting the cutting edge. These broken punches could not be reused. New punches have been made in the same materials. This time the cutting edge on the punches was adjusted, the punches were also tested and did not break-off at the clamping, Instead the punches chipped-off at the cutting edge.
Ceratec is aiming at the pump market with their case study comprises of a gear pump component in ZrO2-TiN and Al2O3-SiCw-TiC. They request corrosion-wear resistance, fracture toughness, surface finish of Rz below 4 μm on W-EDM processed surfaces and Rz below 1μm and dimensional tolerance of ±0.008 mm. For this case study KU Leuven –PMA assisted in providing a proper manufacturing strategy and aiding in CAD/CAM programming of the part. Except for some flat surfaces which can easily be created with flat grinding, W-EDM is the needed to create the complex shapes of the part. Ceratec has no W-EDM facilities, so the final shape was created by ARTooling. This component was not tested, since the thicker blanks for producing this part were only available towards the end of the project.
Potential Impact:
Even though the Hymacer project is a highly industrially orientated project where the main expected impact is the commercialization of innovative products based on cutting edge sintering and machining technologies a very intense dissemination programme has been accomplished during the whole lifetime of the project targeting three main publics: General public, the scientific community and industries within the markets addressed by the project (Automotive, Technical ceramics, Production technologies, cutting tool industry, mechanical engineering).
A total of 46 outreach actions have been performed so far either by a joint initiative of the consortium or by single partners and a wide range of communication channels both online and off-line (Internet, TV, Press, Events...) thus contributing to a broad dissemination of the project goals and achievements. Following amount of dissemination actions have been achieved.
- Website publications: 4
- Brochures/Leaflets/Magazines: 4
- Presentations: 1
- Press releases: 5
- TV Reports: 2
- Multimedia materials: 2
- Scientific journal papers: 1 (1 more is foreseen to be published in the coming months)
- Contributions in congresses: 16
- Fairs attended with stands and Hymacer materials: 11
- Guided visits : 3
During the first project year the activity mainly focused on the design of materials aimed at the presentation of the project as a whole. In this context several initiatives were launched at the very beginning of the project such as the publication of the project website, the project brochure (first version) and a project presentation in powerpoint format. Hymacer partners also took individually an active role and contributed to the creation of materials and their dissemination by means of their own channels. In this way, several press releases were drafted and sent to the Media and also specific webpages were published within the partners’ own websites.
The dissemination activities for the promotion of the project among the general public continued during the second project year mainly by means of press releases and the periodical updates of the project website but the scope was widen to cover also the first project results and thus the focused moved to the other two target publics: scientific and industrial communities. In this way during the second reporting period of the project a strong effort has been put on the dissemination of the scientific and technical results obtained in congresses (11 lectures given in 7 countries i.e Germany, France, Belgium, South Africa, Spain, Italy and Russia) and fairs attended by project partners (participation in 10 industrial events in Spain, Germany, Sweden and The Netherlands).
All the objectives targeted in terms of publication initiatives can be considered achieved and both the number of publicity materials released and the dissemination channels used (Press, videos, websites, TV, Radio, Presentations...) have contributed to the successful widespread of project’s information among the three targeted publics.
It’s important to highlight the active involvement of the Hymacer partners in the dissemination activities during the entire project lifetime. All partners supported not only to the design of the joint promotional materials such as the website or the brochures with contents but also took an active role in the promotion of the project sometimes by means of individual initiatives such as the participation in industrial fairs where the project brochures were distributed or by collaborative activities involving 2 or more partners such as the writing of scientific papers and communications to congresses.
The list with all the dissemination actions performed in the context of Hymacer is shown in the following section and further details are included in Deliverable D5.8.
List of Websites:
https://www.hymacer.eu/
