OBRABOTKAMETALLOV technology Vol. 26 No. 2 2024 A Field Emission Scanning Electron Microscope (FESEM) was used at several magnifications to examine the material flow in the WN and the joint microstructure at various welding zones. The specimens were cut in the perpendicular direction to the weld contour by wire electric discharge machining. Results and Discussion This section discusses the performance of shot-peened UVaFSWed AA7075-T651 joints. Tensile strength, microhardness, fracture behavior, and microstructure of welded joints are assessed considering the effect of welding speed and tool rotation. The UTS of the BM attained after the tensile test is 550 MPa. For the shot-peened UVaFSWed (Run P1 to P9) the experimental matrix and mechanical properties are presented in Table 3. Ta b l e 3 Experimental matrix with mechanical properties for shot-peened UVaFSWed AA7075 joints Run Tool rotation (rpm) Welding speed (mm/min) UTS (MPa) Joint efficiency (%) Microhardness (HV) Surface roughness (µm) WN TMAZ HAZ P1 1,000 20 301.98 54.91 148 129 119 15.350 P2 1,000 28 294.57 53.56 152 133 125 15.480 P3 1,000 40 292.32 53.15 154 143 130 16.341 P4 1,400 20 281.88 51.25 150 138 127 15.976 P5 1,400 28 304.20 55.31 158 145 129 18.277 P6 1,400 40 312.95 56.90 149 141 132 15.918 P7 2,000 20 345.73 62.86 155 145 135 17.672 P8 2,000 28 362.95 65.99 160 144 132 15.169 P9 2,000 40 373.43 67.90 161 145 136 15.651 Mechanical properties of shot peened UVaFSWed joints Stress-strain curves for AA7075-T651 shot-peened UVaFSWed joints (Run P1 to P9) are obtained. For the shot-peened UVaFSWed AA7075-T651 aluminum alloy joints, the maximum UTS of 373.43 MPa (run P9) is obtained at tool rotation of 2,000 rpm and welding speed of 40 mm/min, whereas the minimum UTS of 281.88 MPa (Run P4) is obtained at tool rotation of 1,400 rpm and welding speed of 20 mm/min. The UTS for the shot-peened UVaFSWed AA7075 joints is compared to that obtained using traditional FSW with conical and conical threaded tool pin profiles [22–25]. This study found higher UTS for joints obtained using UVaFSW, followed by the shot-peening process. In the shot peening, steel balls acted at high velocity on the UVaFSWed joint. This high velocity induces compressive residual stresses on the fabricated joint. This effect improves the UTS as well as the microhardness of the joint. Better performance, almost more than two times higher values of tensile strength, and joint efficiency can be seen for the shot-peened UVaFSWed joints compared to the FSWed joints with conical threaded tool pins [22–25]. The higher mechanical properties of the shot-peened UVaFSWed joints could be attributed to an increased strain rate leading to higher plastic deformation and better material flow around the tool pin due to the ultrasonic vibrations. A group of researchers observed better mechanical properties of the UVaFSWed joints made of similar-dissimilar aluminum alloy joints [7–9]. Higher values of UTS are obtained at a higher tool rotation speed. The higher the tool rotation speed, the greater the frictional heat between the tool shoulder and the surface of the workpiece. The increased
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