Vacuum Induction Melting and Investment Casting Technologies Tailored to Near-Gamma TiAl Alloys

摘要

The present study focuses on vacuum induction melting and investment casting of near-gamma TiAl intermetallic alloys. The attention is mainly given to a cost-effective melting process in which a primary alloy ingot is re-melted in a ceramic crucible and cast into a ceramic shell mould. Two types of crucibles (based on Al_2O_3 and Y_2O_3) are considered. The most detrimental reactions that govern the contamination of the molten alloy with ceramic particles were determined. Results suggest that the crucible wall attack can be considerably limited by using either the Y_2O_3 (with no SiO_2-type binder) or Al_2O_3 crucibles with a suitable coating. After pouring, a mechanical interaction associated with different thermal expansions of TiAl casts and ceramic shell moulds can result in serious product damage. A simple 1D-1D model of the cooling process was formulated and the heat flow as well as stress states in the cast-mould system were numerically solved. Process parameters (melt superheat, initial mould temperature, cooling kinetics and mould composition) were optimized in order to reduce the stress in the casts. The optimized parameters delimited a processing window in which complex-shaped TiAl castings like turbocharger wheels can be fabricated.