| Summary: | This study investigates the effects of superimposed ultrasonic vibration on resistance spot welding (RSW) of aluminum alloy AA6061-T6. A novel ultrasonically assisted resistance spot welding process, referred to as ultrasonic resistance welding (URW), is applied to weld AA6061 at various conditions and compared with the results from RSW. Lap shear tensile tests of URW welds show significantly improved mechanical properties, including higher strength, better ductility and higher amount of energy absorbed prior to failure compared with RSW welds. Optical micrographs of the weld cross section show that the ultrasonic vibration enhances nugget formation and minimizes porosities defects. Besides, equiaxed crystal structure is generally observed in the nugget center of URW welds whereas the majority of RSW fusion zone is dominated with columnar grain structure. In situ thermal imaging of the two processes shows a lower temperature distribution, smaller heating and cooling rates during URW. This is attributed to the ultrasonically assisted fracture and breakdown of surface oxides and contaminates, which reduces the contact resistance and accordingly heat generation rate.
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