Controlling grain size, morphology and texture in additively manufactured β-titanium alloy with super transus hot isostatic pressing

Abstract

Titanium alloys produced by Additive Manufacturing (AM) are often characterised as having coarse columnar grain structures on account of the prevailing thermal conditions at the time of solidification. This work investigates whether thermal post processing treatments above the β-transus temperature at both standard atmospheric pressure and 1500 bar pressure (Hot Isostatic Pressing – HIPing) can refine the columnar morphology in the high strength Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C/ASTM Grade 19) alloy produced by AM. Heat treatment at standard atmospheric pressure was ineffective in modifying the coarse columnar microstructure but HIPing was found to promote new equiaxed grains and significantly refine the grain size by up to 72.5% compared to the as-built component. The strong dominant <001> texture characteristic of the as-built (and heat treated) alloy was weakened and shifted towards <103> with a 5-fold reduction in texture intensity after the HIPing process. EBSD analysis reveals evidence of grain recrystallisation processes including but not limited to discontinuous dynamic recrystallisation (DDRX) which may be associated with pore collapse. Primitive columnar grains were also shown to contain numerous substructures which may have formed through dynamic recovery (DRV).