OBRABOTKAMETALLOV Vol. 26 No. 2 2024 technology other hand, cryogenic cooling led to an increase in the ductility of the joint by 80 % compared to the base metal (BM), whereas the addition of element metal powder led to an increase in the joint ductility by up to 60 %. Hatamleh et al. in [11] examined the effect of laser-peening, shot-peening, and cryogenic cooling on fatigue crack development and residual stresses of friction stir welded (FSWed) AA2195 aluminum alloy joints. Their study found that fatigue crack development for the specimen treated with laser peening was the same as that of shot-peening and as-welding at ambient temperature. In addition, it was difficult to distinguish crack growth from residual stresses during cryogenic treatment. Hatamleh et al. in [12] investigated the effect of laser shock peening and shot peening on the FSWed joint of the AA2195 aluminum alloy. Their study observed improved mechanical properties with laser peening compared to shot peening. They observed an increase in the yield strength in the weld nugget (WN) by about 38 % when laser welding was used as post-welding treatment, compared with an increase in the yield strength in the weld nugget (WN) by 8 % observed during shot peening. Khorrami et al. in [13] investigated the effect of cryogenic and ambient temperature on the FSW of severely deformed AA1050 aluminum alloy with SiC nanoparticles. Their work observed bimodal and finer grain sizes when using FSW joints with cryogenic cooling treatment as a measure against abnormal grain growth during the FSW. Singh et al. in [14] performed the cryogenic treatment after the FSW on a joint of AA7075 aluminum alloy. Their experimental study showed that post-weld cryogenic treatment led to a slight increase in the hardness of the joint and tensile strength. Wang et al. [15] inspected the effect of low-temperature aging and cryogenic treatment on the mechanical properties of the FSWed AA2024-T351 aluminum alloy. The elimination of softened zones near the HAZ was noted due to a single low-temperature aging. However, due to a single low-temperature aging, a decrease in the strength of the joint was noted. Wang et al. [16] performed the cryogenic treatment during FSW of the Cu joint. Their experimental work showed that grain refinement in the WN increased initially with increase in the rotational speed. However, it was observed to decrease with further increases in rotational speed. Zhemchuzhnikova et al. [17] observed extensive grain refinement and increase in the tensile strength of cryogenically treated AlMg-Sc-Zr FSW joints. Ferreira et al. [18] examined the effect of the glass and steel beads in shot peening on the welded joint. They noticed better results in fatigue and tensile strength with glass beads than with steel beads. Also, higher surface roughness was observed when using the steel beads as compared to glass beads. A group of researchers [19–21] investigated the effect of laser shock peening on the microstructural properties, fatigue properties, and corrosion resistance of FSWed aluminum alloy joints. They observed a finer grain size, better corrosion resistance, and higher fatigue strength with laser shock peening-treated joints as compared to joints without laser shock peening as a post-welding treatment. However, more studies are required in post-weld treatments to obtain better mechanical properties for the welded joint. The literature reviewed shows that the UVaFSW and post-weld treatment improved the mechanical properties and material flow. However, limited studies have been conducted on the UVaFSW joints of AA7075-T651, considering the consequence of welding speed, tool rotation, and post-weld shot peeing treatment. With this interpretation, the current work comparatively evaluates the performance of untreated and post-weld shot-peened ultrasonic vibration-assisted friction stir welded (UVaFSWed) AA7075-T651 joints, taking into account the effect of welding speed and tool rotation. The performance is evaluated in terms of microhardness in the different regions of the weld, ultimate tensile strength (UTS), surface roughness, microstructure evaluation, and fracture analysis using scanning electron microscopy (SEM) images. The experiments were performed with conical threaded tool pin-type. The results are compared with the available literature on FSW of AA7075-T651 joints produced by means of conical threaded and conical tool pin profiles. Experimental Design In the present study, the square butt joint of AA7075-T651 was produced using UVaFSW. The UVaFSW experimental setup is depicted in Fig. 2. Experiments were carried out with welding speeds of 20, 28, and 40 mm/min and tool rotations of 1,000; 1,400 and 2,000 rpm. The experiments were carried out using conical
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