Variant selection in laser melting deposited α + β titanium alloy

Abstract

Variant selection in laser melting deposited (LMD) α + β titanium alloy (TC21) was investigated using linear thermal expansion and EBSD test. At micro level, volume change of lattice is anisotropic during solid phase transformation in titanium alloy. If several variants are more abundant than others, the anisotropic volume changes could be observed at macro level. In linear thermal expansion test, the growth directions of as-deposited and α + β treated LMD TC21 alloy expanded during α→β phase transformation while at the transverse direction, a contraction was observed. The β treated alloy showed isotropic expansion in the same temperature region. Anisotropic volume changes of as-deposited and α + β treated LMD TC21 alloy confirmed the variant selection happened in deposition process. During depositions, the anisotropic internal thermal stress generated in the as-solidified metal before solid phase transformations started. The existed thermal stress made the lattice spacing a little distorted. The anisotropic distortion of lattice spacing led to the variant selection. The theoretical analysis was supported by EBSD results.