OBRABOTKAMETALLOV Vol. 26 No. 2 2024 technology frictional heat softens the material and enhances its movement to the weld pool, resulting in uniform mixing of the material. The microhardness of joints was measured at several points from the weld center on both sides of the joint. The dynamic recrystallization of grains and higher plastic deformation causes variations in the microhardness in the welded region. The microhardness of the shot peened UVaFSWed joints showed variation in the welding zones, mostly following the distribution of a letter ‘W’-shape, and found maximum at the WN and minimum at the HAZ. A higher microhardness was obtained in all the zones of the weld: WN, TMAZ, and HAZ for the shot peened UVaFSWed joints (Run P1-P9) as compared to that obtained using traditional FSW with conical and conical threaded tool pin profiles [22–25]. This study found higher microhardness for joints obtained using UVaFSW, followed by the shot-peening process. A higher microhardness in WN is obtained for shot-peened UVaFSWed joints at higher tool rotation speeds. The shot peening process imparts compressive residual stress on the fabricated joints, resulting in higher microhardness in the WN. The maximum microhardness in WN was 161 HV and was obtained at the higher tool rotation of 2,000 rpm for the shot-peened UVaFSWed (Runs P7-P9) AA7075-T651 aluminum alloy joints. The shot-peened UVaFSWed joints showed better mechanical properties compared to FSWed joints. The shot-peened UVaFSWed joints sustained higher tensile loads, as shot-peening induces compressive stresses in the workpiece. UVaFSWed joints sustained higher tensile loads, which could be attributed to higher heat input due to the application of ultrasonic vibrations on the weld bead. Moreover, ultrasonic vibrations acting on the weld bead contributed to dynamic recrystallization and improved the material movement towards the weld bead. Further, the assistance of ultrasonic vibrations also reduced weld defects/flaws in the WN and its interfaces with TMAZ compared with the conventional FSWed joints. The joint quality was assessed by obtaining the surface roughness. An average surface roughness was 15–18 µm for shot-peened UVaFSWed AA7075-T651 aluminum alloy joints. Lower surface roughness values were obtained at a lower tool rotation speed of 1,000 rpm, regardless of the welding speed (Runs P1-P3). Fig. 6 represents the top surface appearance of the shot-peened UVaFSWed AA7075-T651 aluminum alloy joints. The surface modifications and filling of the weld bead can be seen. The “onion rings” are formed, and the tool shoulder marks can be seen. Microstructure of shot-peened UVaFSWed joints Fig. 7, a, c shows SEM images of WN, TMAZ, and HAZ of the shot-peened UVaFSWed joints obtained at Run P9, respectively. The homogeneous grain distribution in WN and the absence of tunnel defects can be Fig. 6. Weld top surface of AA7075 UVaFSWed shot-peened joints
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