Effects of post-weld heat treatment on microstructures and properties of laser welded joints of new high-strength Ti-55531 alloys

This study investigated the effects of post-weld heat treatment (PWHT) on microstructures and properties of a laser welded joint of high-strength and high-toughness titanium alloy, namely Ti-5Al-5V-5Mo-3Cr-1Zr (Ti-55531). The experimental results demonstrate that PWHT can improve the tensile strength of the joint by 47.8 %, which reaches 92.4 % of base metal (BM). Average microhardness of a weld metal (WM) of the joint increases from 309 HV in the as-welded state to 371 HV. The analysis shows that PWHT can significantly improve microstructures of the joint, which is an important reason for rising strength of the joint. The BM of the as-welded joint presents a typical duplex microstructure and 10 similar to 20 % of equiaxed alpha(p) phases are inserted into fine transformed beta microstructures. Equiaxed grains are retained in a heat affected zone (HAZ) and the WM is composed of coarse columnar grains. After PWHT, there is no obvious change in microstructures of the BM of the joint. While maintaining beta phases in BM, the edge of equiaxed a phases in the HAZ gradually dissolves and change to the WM microstructure. In the WM, microstructures change the most significantly and a lot of acicular alpha phases appear.