Development of permanent-mold cast TiAl automotive valves

This paper presents the results of an EMTEC funded project to develop low-cost TIAl automotive valves. The alloy studied was Ti-47Al-2Nb-1.75Cr (at %). Over 800 valves were cast, using several variations of the permanent-mold process, in a multi-cavity steel mold. Applying pressure during solidification improved the casting fill. However, none of the permanent-mold casting methods produced pore free as-cast valves. The as-cast microstructures of valves produced by permanent-mold casting were much finer than investment castings of similar section sizes. Of the permanent-mold casting methods, the injection cast method exhibited the finest as-cast structure showing a potential for a new high output method for producing fine-grained TiAl components. Room temperature tensile properties of the permanent-mold material were superior to those of investment castings with a similar microstructure. Two sets of valves were road tested for a total of 50 000 km with average 2% fuel savings and no valve damage. There is a need to overcome few challenges before this technology can be implemented in the automotive industry. (C) 1998 Elsevier Science Limited. All rights reserved.