Grain refinement and improved tensile properties of Ti5Al2Sn2Zr4Mo4Cr titanium alloy fabricated by laser solid forming

Abstract

Laser solid forming (LSF) Ti5Al2Sn2Zr4Mo4Cr titanium alloy usually exhibits brittle fracture with a poor elongation when the tensile direction is perpendicular to the growth direction of columnar β grains. In the present study, the effect of powder feed rate on the columnar to equiaxed transition (CET) and grain refinement of Ti5Al2Sn2Zr4Mo4Cr titanium alloy during the LSF process was investigated. The results showed that increasing in powder feed rate could effectively promote CET during LSF process. However, keeping increasing in the powder feed rate did not further refine the β grains effectively, especially in the regions near the fusion lines. The pauses of the LSF process aimed to obtain equiaxed β grains also lead to the precipitation of very fine α laths inside the β grains since the heat accumulation was avoided. The microstructure containing equiaxed β grains and homogeneous α laths was obtained after heat treatment. The tensile test was performed both in the vertical (parallel to the build direction) and horizontal (perpendicular to the build direction) directions under the as-deposited and heat treatment conditions. The tensile properties, especially the ductility, were significantly improved compared to that in the previous studies. It is partly because the equiaxed β grains effectively prevented the crack propagation.