Abstract
Induction skull melting (ISM) and vacuum induction rollover (VIR) furnaces have been used to melt a γ-TiAl alloy prior to casting into ceramic shell moulds. Castings produced by ISM contained only a slightly higher oxygen content than the charge material, but the melt was cast at a relatively low superheat resulting in numerous turbulence-induced gas bubbles in the castings. Although higher superheat could be achieved by VIR, the use of refractory crucibles resulted in castings with a high oxygen content. Castings poured turbulently under an argon atmosphere were liable to misrun and contained gas bubble defects. These defects were eliminated by pouring the melt into the moulds in a quiescent manner. Casting in a vacuum helped to reduce misrun even when the mould was filled quickly in a turbulent manner. Zirconia cannot be used as a crucible material for melting γ-TiAl alloys, whereas high purity (>98%) calcia is more suitable.
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