Fatigue Response of Aluminum 7075-T6 Joints through Inclusion of Al₂O₃ Particles to the Weld Nugget Zone during Friction Stir Spot Welding

This study intends to evaluate tensile and fatigue responses of aluminum 7075-T6 joints in the absence and presence of Al₂O₃ particles introduced into the nugget zone (NZ) during friction stir spot welding (FSSW) operation. Alumina particles were dispensed into the pre-existed small hole on the top of the lower plates; then, the upper plates were placed on the lower plates to form localized composite in the stirred zone of the weld joints. Tensile and fatigue strengths of the localized composite zone were then compared with those of as-welded joints in the absence of alumina particles. Stirred zone of joints consists of 1wt.% Al₂O₃ particles that slightly improved tensile strength while fatigue lives of composite joints were enhanced as high as 2.5 times as compared to as-welded joints. Joint samples with dispersed particles of 2.5wt.% in the nugget zone, however, resulted in no improvements in tensile and fatigue strengths. Different cracking modes were observed at various applied load levels and dispersed amounts of alumina particles in weld nuggets. Joint specimens with 1wt.% alumina resulted in the formation of circumferential cracks followed by lower plate fracture at low loading levels, and joints tested at higher loading magnitudes promoted pull-out fracture mode in nugget zones. In opposite, specimens containing 2.5wt.% alumina particles revealed annular cracks and lap-shear failure modes at the high loading level of 6kN, and nugget pull-out mode in upper plates at the low loading level of 2kN.