Influence of water on microstructure and mechanical properties of a friction stir spot welded 7075-T651 Al alloy

Abstract

Abstract 7075-T651 Al alloy sheets were welded by friction stir spot welding (FSSW) method without and with tap water. Conducting FSSW with water reduced the weld microstructure grain size and the hook defect formation at the weld, resulting in an increase in the hardness and tensile strength of the weld. The joint made with water showed an average tensile load of 4237 N, whereas the joint made without water exhibited 3690 N. Using water enhanced the strength of the joint, but slightly decreased its ductility. The hardness of the joint made with water presented slightly higher, probably due to the formation of smaller grains in its microstructure as a result of the water leading to a decrease in the welding process temperature. In both joints, the highest hardness measurements in the stir zones (SZs) and the lowest ones in the heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ) were found as the SZs had a finer-grained microstructure. Furthermore, the application of water noticeably, though not completely, inhibited the occurrence of the hook defect in the joint and also reduced the sizes of HAZ and TMAZ. According to the weld fracture surface analysis, both joints failed in a ductile manner.