OBRABOTKAMETALLOV technology Vol. 26 No. 2 2024 seen in Fig. 7, a. These attributes resulted in a higher UTS for the shot-peened UVaFSWed joint compared to the conventional FSWed joint. Fine, equiaxed, and uniformly distributed grains of ~630 nm-5 µm in the WN of the shot-peened UVaFSWed joint can be seen in Fig. 7, a. UVaFSW improves the grain size in WN, and the material flows inside the weld bead compared to the conventional FSW. Fig. 7, b depicts the SEM image of TMAZ of the shot-peened UVaFSWed joint. Homogeneous mixing of the material with equiaxed grains can be seen. The coarser distributed grains than those in the WN differentiate this weld region. This weld region is adjacent to the WN. The grains having coarser sizes of ~ 5–7 µm can be seen in TMAZ. This could be due to lower heat distribution from the WN to TMAZ. Fig. 7, c depicts a SEM image of HAZ of a shot-peened UVaFSWed joint. Elongated grains with sizes varying in the range of ~8–11 µm can be seen. This weld zone is between TMAZ and BM and is designated as HAZ. The reduction in microhardness with an increase in grain size can be seen from the WN to HAZ. The highest microhardness observed in the WN was 161 HV, followed by 145 HV in TMAZ and 136 HV in HAZ. SEM images show that shot-peened UVaFSW improves the grain size in the WN and the material flow inside the weld bead compared to the conventional FSW [21–25]. Fine, equiaxed, and uniformly distributed grains at the WN can be seen for the shot-peened UVaFSWed joint. The higher mechanical properties obtained for the shot-peened UVaFSWed joints can be confirmed from SEM images showing higher plastic deformation, dynamic recrystallization, and better material flow toward the weld bead. Fracture behavior of UVaFSWed AA7075-T651 joints Fig. 8 depicts the fractured specimens of shot-peened UVaFSWed joints of AA7075-T651 that were obtained during Runs P1–P9. These specimens were subjected to a tensile test, and the results were compared side by side. The study found that all the test specimens had fractured in the HAZ and exhibited ductile behavior during the fracture. Fig. 9 shows SEM images of a fracture surface of specimen of the shotc Fig. 7. SEM images of the shot-peened UVaFSWed joints at WN (a), TMAZ (b), and HAZ (c) a b
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