Columnar to equiaxed transition during β heat treatment in a near β alloy by laser additive manufacture

Abstract

Columnar β grains are widely present in titanium alloys fabricated by Additive Manufacturing (AM), which leads to significant anisotropy in mechanical properties. An outstanding challenge is to facilitate the columnar to equiaxed transition (CET) of the prior β grains. In this research, CET and the underlying grain growth mechanism during β heat treatment in Ti–5Al–5Mo–5V–1Cr–1Fe (Ti55511) alloy manufactured by selected laser melting (SLM) and laser metal deposition (LMD) were investigated. For the first time, the quasi-in-situ observation was employed in the AM near β titanium alloys by the electron backscatter diffraction technique to analyze morphology, crystal orientation and substructure evolution, especially in the situation of bamboo-like microstructure with columnar grains and equiaxed grains distributed layer by layer. It was found that the prior β grains enriched low-angle grain boundaries (LAGB) are in a high stored strain energy state in as-deposited SLM Ti55511 leading to a recrystallized nucleation-and-growth process during heat treatment, while the prior β grains containing a small amount or even no LAGB are in a relatively lower stored strain energy state in as-deposited LMD Ti55511 resulting in recrystallization and grain growth processes.

Keywords

• Titanium alloy
• Selective laser melting
• Laser metal deposition
• Post heat-treatment
• Recrystallization
• Grain growth