Effect of synchronous gas cooling method on microstructure and mechanical properties of laser welded joint for Ti6Al4V titanium alloy

The utilization of argon cooled by liquid nitrogen during the laser welding process shows great promise to enhance the welding quality. In this paper, the synchronous gas cooling method (SGCM) was designed to investigate its effect on the microstructure evolution and mechanical properties of laser welded joint for Ti6Al4V titanium alloy. A laser welding simulation model, incorporating the use of cooling argon, was developed to elucidate the welding thermal process. The simulation results indicate that the introduction of cooled argon gas primarily enhances the heat exchange between the surface of the melt pool and the external environment, resulting in improved temperature distribution and an accelerated cooling rate of the melt pool. Furthermore, through observations of the surface temperature field of the weld seam at different time points, it was discovered that the utilization of cooled argon gas significantly reduces the occurrence of melt pool trailing during the welding process. The improvement in temperature distribution results in grain refinement and a reduction in the size of α’-martensite, and confirms the growth pattern of columnar grains along the direction of the maximum temperature gradient. Further, the mechanical properties of the laser welded joint exhibit significant improvement. The enhanced tensile strength of welded joint can be attributed to the formation of acicular α’-martensite and the appearance of basket-weave structure. Additionally, the microhardness value of the weld metal is increased from 423.7 to 453.7 HV1.0 due to the decrease in the size of acicular α’-martensite. Compared without SGCM, the deformation of welded joints during the welding process is reduced by 28.5%. The study shows that the contribution of cooling argon is sufficient to produce welded joints with optimized microstructure alteration. The comprehensive description of mechanical performance enhanced mechanism caused by cooling argon was presented based on thermal cycle analysis on laser welding with SGCM.