Improving Forging Outcomes of Cast Titanium Aluminide Alloy via Cyclic Induction Heat Treatment

Abstract

The objective of this research was to improve the forging outcome of peritectic solidifying cast titanium aluminide (TiAl) 4822 alloy (Ti-48Al-2Nb-2Cr at.%) in hot isostatic pressed and homogenised (HH) condition using cyclic induction heat treatment (CHT). This study adds to research around CHT for TiAl alloys by applying industrially relevant induction heating to conduct five heating cycles at the single αphase temperatures (1370 °C) necessary for grain refinement. Two cooling rates were explored in each cycle, air cooling (ACCHT) and controlled furnace-like cooling (FCCHT), without returning to room temperature. Samples were assessed at each stage in terms of their morphologies, lamellar grain size and content, as well as phase and dynamic recrystallised fraction, and subsequent primary and secondary compression behaviour with uniaxial isothermal compression. The FCCHT process resulted in a homogeneously refined fully lamellar microstructure, and ACCHT, in a heterogeneous microstructure consisting of lamellar and feathery γ (γf) at differing fractions across the piece, depending on the cooling rate compared with HH. The results show that CHT improved forging outcomes for both compression stages investigated, resulting in uniform compression samples with higher volumes of dynamic recrystallised material compared with the instability seen with the compression of HH material.

Keywords

• intermetallics
• titanium aluminides
• heat treatment
• induction heating