Elucidation of microstructural evolution of beta-type titanium alloy joint during friction stir welding using liquid CO2 cooling

摘要

Normal friction stir welding (FSW) and liquid CO2 cooling FSW were conducted on the beta-type Ti-15V-3Cr-3Al-3Sn (Ti-15-3, mass%) alloy plates under a processing condition. Since liquid CO2 cooling with high cooling efficiency can freeze the microstructure formed during the high-temperature stirring stage of FSW, it is possible to separately elucidate the microstructural evolution of the Ti-15-3 alloy during the high-temperature stirring stage and the post-stirring cooling stage in FSW. The deformation temperature during the normal and the CO2 cooling FSW was similar in the top and the bottom of stir zone, which is considered to be owing to that the top and bottom are located adjacent to the heat sources, i.e., the shoulder and probe surfaces. In the high-temperature stirring stage of FSW, discontinuous dynamic recrystallization predominantly occurred, and a simple shear texture generated in an early stage of the material flow around the tool, then the microstructure started to evolve steadily after a steady-state dynamic recrystallization. In the post-stirring cooling stage of FSW, static recovery and grain growth were found to mainly occur within the stir zone.