Investigation on solid-state phase transformation in a Ti-47Al-2Cr-2V alloy due to thermal cycling during laser additive manufacturing process

Laser additive manufacturing (LAM) technique always gives a directional temperature gradient with fast cooling rate, which enhances the formation of columnar grains. However, the LAMed titanium aluminides usually present an alternatively ranged columnar and equiaxed grains with patterned microstructures. This study tries to understand formation mechanisms of grain morphology in a Ti-47Al-2Cr-2V alloy and also explain evolution of microstructures and hardness variation. Results indicate that the last deposition layer consists of a fully lamellar microstructure, and equiaxed grains are formed due to the columnar-to-equiaxed transition (CET) at the very top of layer. At the stable zone, alternatively arranged columnar and equiaxed grains are observed. Columnar grain region maintains columnar lamellae but with massive γ (γ_m) phases appearing. For the equiaxed grains, a duplex microstructure of (α₂+γ) lamellae and γ_m phase are observed. During thermal cycles, columnar grains transform to equiaxed grains through solid-state transformation, and microstructures of alternatively ranged columnar and equiaxed grains are finally established along deposition direction. The change of microstructures also correspondingly changes microhardness of the alloy, and which decreases from 410 HV at the top part to about 310 HV at the middle of the plate. Nanohardness of (α₂+γ) lamellae is higher than γ_m, and nanohardness of (α₂+γ) lamellae in columnar grains is higher than (α₂+γ) lamellae in equiaxed grains. The variation in microhardness is dependent mainly on lamellar spacing, precipitation of γ_m and size of grains.