Study on the effects of ultrasonic assistance and external heat dissipation on friction stir welded 7075 aluminum alloy joints

In this paper, conventional friction stir welding (C-FSW), ultrasonic assisted friction stir welding (Ua-FSW) and heat pipe assisted friction stir welding (Ha-FSW) were performed on 4 mm thick 7075 aluminum alloy. The effects of the two different types of assisted technologies on C-FSW were qualitatively compared and analyzed. The microstructure of the joints in the NZ, HAZ and other regions were primarily characterized via SEM-EBSD and TEM, and the strength of the joints were evaluated via the room-temperature tensile properties and microhardness tests. The test results showed that both Ua-FSW and Ha-FSW effectively improved the weld forming, while the surface quality of the joints fabricated by Ha-FSW was better. The acoustic flow effect from ultrasound reduced the temperature gradient, and the heat pipe provided excellent heat dissipation, which well controlled the peak heat and the rate of heat loss throughout FSW, resulting in finer grain sizes after both the Ua-FSW and Ha-FSW processes. EBSD analysis showed that the assistance of two technologies provided a greater effect on TMAZ and HAZ, the stress field generated by ultrasonic vibration favored the formation of subgrains, the heat dissipation of the heat pipe also inhibited the recrystallization, and more subgrains were retained. The assistance of ultrasonic vibration and the heat pipe was mainly in the elongation, especially the joints prepared by the Ha-FSW showed an improvement in elongation by 16.7 % and 33.3 % compared to the Ua-FSW and C-FSW, respectively. This work provides insight into improving welding efficiency and joint performance through modified FSW processes.