Thermal cycles, microstructures and mechanical properties of AA7075-T6 ultrathin sheet joints produced by high speed friction stir welding

Abstract

0.5 mm thick AA 7075-T6 ultrathin sheets were butt jointed by conventional friction stir welding (C-FSW) and high speed friction stir welding (HS-FSW) under a constant rotational speed ( ω )/welding speed ( v ) ratio of 6.67 rad/mm using a pinless tool. In this study, the conventional ω of 2000 rpm and conventional v of 300 mm/min were adopted by C-FSW, and the high ω of 8000 rpm and high v of 1200 mm/min were adopted by HS-FSW. Compared with C-FSW, HS-FSW presented a fast heating and cooling rate thermal characteristic with a slightly higher peak temperature and significantly shorter elevated-temperature exposure time. Kissing bond defect was appeared in the weld bottom for C-FSW. While, no obvious defect was found in the weld obtained by HS-FSW. Compared with C-FSW, the proportion of deformed grains in the center of nugget zone (NZ) fabricated by HS-FSW was increased. For the joint fabricated by HS-FSW, the upper NZ was dominated by recrystallized grains, and the lower NZ was dominated by deformed grains. The ultimate tensile strength of the joint fabricated by HS-FSW can reach 87% of base metal (BM), and its elongation, yield strength and ultimate tensile strength were increased by 58.8%, 23.3% and 22.8% respectively compared with C-FSW. Both the joints fractured in the center of the NZ. The joint fabricated by C-FSW broke due to the kissing bond defect. While the joint fabricated by HS-FSW may be broken due to the weld thickness reduction. • The thermal cycle characteristics of HS-FSW were revealed. • The effect of the thermal cycle on microstructure was revealed. • The microstructures of the joint produced by HS-FSW were investigated by EBSD. • The tensile properties of the joint produced by HS-FSW were superior to by C-FSW.