Effect of rotating laser welding on microstructure and mechanical properties of Ti6Al4V alloy butt-welded joint

Since the high-density energy, the laser welding could realize the joining of Ti6Al4V alloy with high depth-width ratio welding zone (WZ), but the rapid solidification of thin WZ and low fluidity of alloy always leads to many metallurgical defects. Then, it becomes a critical issue to obtain a welding with fewer defects. In the present research, the linear laser welding (LLW) and rotating laser welding (RLW) with low laser power were applied to weld thin Ti6Al4V alloy plates. The surface topography, welding defects, microstructure evolution and mechanical properties of the welding joints were investigated. The results reveal that the LLW and RLW joints possess the similar widths of welding track with the size of 3.69 mm and 3.85 mm, respectively. For the LLW joint, the insufficient heat input and rapid solidification lead to the initiation of cracks in bottom of WZ, which propagates toward the surface and has the total length about 1.8 mm, accompanying with lots of pores in sides. The top area of LLW joint is dominated by the coarse grains with basket-weave microstructure, while some equiaxed grains with finer size form in the central area. For the RLW joint, the stronger convection in molten pool promotes the homogeneous temperature distribution and handicaps the rapid solidification in local region, which contributes to the sequential solidification and restricts the initiation of crack, even though some pores form inside. Compared with the high strain concentration along prior beta grain boundary in LLW joint, the RLW weakens the strain concentration along prior beta grains in the top area of joint, which should be ascribed to the strengthened convection. The ultimate tensile strength (UTS) and elongation (EL) of LLW joint are just 428 MPa and 1.15%, respectively. Comparatively, the UTS and EL of the RLW joint are significantly increased to 860 MPa and 8.94%, which are 100.93% and 677.39% higher than LLW, respectively.