Deliverables Coating of light weight valves Functional surfaces designed to withstand severe working conditions are produced by means of laser surface treatments using additive materials on lightweight metals selected for car engine valves (aluminium alloys for intake valves -hypereutectic Al-Si made by various methods, such as spray forming-, and intermetallic Ti-Al for the exhaust valves). The coating systems were developed to improve the material properties that are fundamental in engine valves: good wear resistance, high superficial hardness, good corrosion resistance and adequate thermal stability. Different materials were selected for the coatings. They are deposited using a high power diode laser on targets of the aluminium and Ti-Al intermetallic alloys. In automotive applications, these coatings could allow the use of lightweight alloys instead of traditional steels while maintaining good material properties. The coatings have been characterised and tested in order to evaluate their properties for the engine valve application. Results have been transferred to prototypes of new valves. The prototypes have undergone testing to validate the introduction of these metal-coating pairs in engine valves. The results obtained could be applied to other fields of industry, mainly: aerospace, rail and transport vehicles (other automotive components), machine tools, forging, injection and casting tools, shipbuilding, energy generation, packaging. Ceramic engine valves The interest for ceramic materials for high temperature automotive engine applications is due to their excellent thermo-mechanical characteristics in comparison with traditional metallic materials. The objective of the work was to develop a low-cost forming and sintering process to produce near-net shape ceramic valves, thus requiring very low finishing operations and significantly minimizing material waste. Ceracom activity was devoted to the main issues as follows: 1. Optimize the process for forming near net shaped valves; 2. Characterize microstructural and mechanical properties of these valves in cooperation with CRF; and 3. Produce 200 valve mock-ups/rods for grinding and evaluation at TRW. Process Optimization and Characterization consisted of: 1. Selection of the best starting materials(taking into account the requirements of a cost effective and high volume production); 2. Development of an innovative pressure-injection molding process to produce near-net shape parts via a thermoseting feedstock; 3. Optimization of a proper pressure-less sintering route to obtain cost-competitive, real scale components with adequate final density and mechanical properties. Material Selection: Silicon Nitride (Si3N4) offers the best mechanical and thermal properties for engine applications. Si3N4 has in fact excellent thermal shock resistance and relatively high fracture toughness; in the last decade a lot of development work has been conducted in order to improve the strength and the resilience and different industrial applications have been successful, especially for gas turbines applications. Other materials, such as silicon carbide (SiC)exhibit excellent strength and creep resistance, but its fracture toughness is still so low that it cannot be used in dynamically loaded parts. Another promising ceramic, partially stabilized zirconia, has high strength at room temperature as well as excellent fracture toughness, but its density is too high and it losses its strength at moderate temperatures, so that its application for rotating and moving engine parts is not feasible. Finally, SiAlON, a promising ceramic material with good toughness and strength, was investigated, but its availability and price are still fairly lacking in comparison with more conventional silicon nitride. Between available technical ceramic materials, silicon nitride was thus chosen to replace conventional steels and Ni-based alloys for the exhaust valves application. Process Selection: The most promising forming process for ceramic parts is injection molding; in this process the ceramic powder is mixed with a thermoplastic polymer to obtain a feedstock mixture. Such feedstock is injected into a mold through a typical plastic injection molding equipment. After the production of a "green" part, the parts are debinded and then sintered. Conventionally, these are very time-consuming processes. In order to avoid such long and complicated debinding and firing cycles and at the same time guarantee a high green density, a new feedstock was developed based on a thermosetting binder, and combined with a special injection molding process. Such type of binder, even if in low content can provide green bodies with relatively high mechanical strength and high green density. A rapid debinding process in air was also developed, resulting in a simpler and considerably shorter process than that using conventional injection molding binders. This is an important, exclusive advantage for mass production of cost-effective and near-net-shape ceramic parts. Repeated iterations of this process resulted in numerous improvements to avoid discontinuities in the "greenbody" valves, leading to cracking. Feedstock homogeneity, tape casting improvements, feedstock drying and mold design all contributed to the goal of crack-free valves. The sintering process was carried out in pressure-less conditions, under inert (N2) atmosphere, without using expensive HIP equipment. Different temperature programs have been tested, in order to improve the final density and the mechanical properties of silicon nitride as function of its sintering cycle. Prototype manufacturing and testing: Ceracom produced 406 valves, 234 of which were sintered at CRF and machined at TRW. Conclusion: Silicon Nitride valve samples were produced using low cost forming and sintering process. These samples have been characterized in order to asses their reliability; in terms of final density and microstructure the pressure-less silicon nitride valves seem to be promising. CAE methodology for the design of new materials A complete CAE methodology for the design of ceramic and intermetallic structural components was developed, mainly targeting light weight valves, with the prevision of the operative life according to the real loadings and to the specific thermo mechanical materials properties of the materials. On the basis of the specific materials properties (measured on hypereutectic aluminium alloys for intake valves, gamma-TiAl intermetallics and Si3N4 ceramics for exhaust ones), the valves? design optimisation by means of Finite Element Modelling and appropriate simulation, based on the statistic failure prediction according the up to date methodologies for brittle materials, was accomplished. The development of low cost, reliable and light weight valves with innovative, less ductile materials required the integration of various disciplines, from materials sciences to mechanical engineering. Fundamental research on the advanced light weight materials considered in the Project and the basic system requirements have been considered as starting points. Then the peculiar mechanical behaviour of brittle ceramic materials has required the development of a different design procedure than the standard methodologies applied with conventional metallic alloys. The methodology to overcome this lack of design procedures has been developed: after the selection of the material on the basis of the operating conditions, by means of FEA (Finite element Analysis) simulation tools, the stress distribution within the operating valves and in the contact area between valve and valve seat has been modelled and, as a result, the design is modified in order to possibly reduce the peak stress concentrations and to improve the material utilisation. Then the failure probability for the valves has been calculated taking into account the real mechanical characterisation of the considered brittle material (for example bending tests for silicon nitride) and the calculated stress distribution, utilising the well-established concepts of linear fracture mechanics and of Weibull statistics. After this process is accomplished, the prediction of the operating life of valves can be evaluated using both dynamic and static fatigue of the material: if the calculated results cannot meet the required conditions, then design and/or material have to be opportunely modified. High efficient production of TiAl valve blanks The IRC's contribution to the Livalve programme has been to select and to process to near net shape a TiAl-based alloy that would have the appropriate balance of mechanical properties and sufficient oxidation resistance for automotive exhaust valves. Final machining and development of a coating for wear resistance was to be done by other partners. One of the concerns has been to reduce the cost of the valves to a minimum. This concern with cost has meant that the whole effort has been directed to developing a casting route. On that basis a grain-refined alloy was selected and in order to obtain sufficient oxidation resistance and high temperature strength it was essential that Nb content was high and the Al content low. With these considerations in mind the alloys which have been used for all casting work during Livalve have contained 8at%Nb, 1at%B and Al contents between 44 and 46at% i.e. with compositions between Ti44Al8Nb1B and Ti46Al8Nb1B (at %). Progress in casting valves: Melting of sections of ingots has been carried out exclusively using a high power (350kW) cold wall induction furnace, which has a capacity of about 5kg of TiAl. Casting has been carried out into pre-heated investment moulds, manufactured in the IRC, using face coats developed during this work to minimise interaction with the molten TiAl alloys. The most successful face coats contain yttria. Mould filling was done simply by pouring the molten TiAl alloy into the pre-heated moulds as quickly as possible because the superheat is limited to about 60 degrees Celsius when using a cold wall furnace. This casting technique is not ideal (it is turbulent and thus increases the chance of trapping argon (the melting atmosphere) during solidification) but early experiments showed that if gravity casting were used this approach gave the fewest failures. When all conditions were optimised (mould pre-heating temperature, argon pressure, mould material, filling and feeding systems) using this approach the success rate for the production of valves with only minimal porosity was typically around 50%. This minimal porosity cannot always be closed on HIPping and the overall yield of good valves is below 50%. Conclusions from casting work: The fact that some valves could be cast successfully has allowed valves to be supplied for machining tests, for coating and these valves will be used for engine tests. It is clear however from the poor yield from the casting process used in this project (which is however as good as any yields which have been reported in the literature) that the potential advantage of the casting route (its relative cheapness) has not been realised. This is a major problem which has bedevilled casting of TiAl-based alloys for the aerospace industry and appears to be associated with the fact that superheat obtainable under clean melting conditions (i.e. using a water-cooled cold wall furnace) is extremely low and never exceeds 60 degrees Celsius. Thus the major conclusion from the work focussed on TiAl, which has been funded through this programme, is that current casting technologies are not cost-effective and that other approaches should be investigated. Additional (independently funded) work: The IRC has been involved in two other casting programmes and these have yielded a significant improvement in the quality of the cast valves. The programme with China makes use of CaO crucibles to melt the alloy to superheat the molten alloy to over 160 degrees Celsius above its melting point. This melting technology, which it must be emphasised is not a clean melting technique (since CaO is in direct contact with the molten alloy) used in conjunction with centrifugal casting has the potential to produce reliable valves, relatively cheaply, because the yield is high. The improvement obtained in the IRC is associated with the development of anti-gravity casting, whilst still using a cold wall furnace. This results in casting far less turbulently so that the probability of entrapping argon is much reduced. This technique is not as fully developed as is the technique used in China and figures for the yield are not yet available. The important conclusion from the viewpoint of Livalve is that excellent valves can now be produced using a CaO crucible with centrifugal casting and that counter gravity casting used with a cold wall furnace shows potential. Future work within Livalve: The IRC will continue to arrange the supply of valves from China on a semi-commercial basis to TRW for testing and assessment within Livalve and for subsequent programmes. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
