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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.

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