Friction stir welding of similar and dissimilar AA7075 and AA5083

Abstract

Aluminum alloys AA7075-T6 and AA5083-H111 were friction stir welded at constant rotation rate of 300rpm and different traverse speeds of 50,100, 150, and 200mm/min in similar and dissimilar joints. The microstructures and crystallographic textures of base materials (BM) and the welds were investigated using electron backscatter diffraction (EBSD) technique. The mechanical properties were evaluated using hardness and tensile testing. The fracture surface of the tensile tested samples was examined using scanning electron microscope (SEM). Microstructural examination of AA7075 and AA5083 BMs showed significant difference in the grain structure of the two alloys with average grain size of 40μm that was free of substructure in case of AA7075. While AA5083 BM revealed an average grain size of 25μm with high density of substructure. In addition the type precipitates also are different in the two alloys. Although, the two alloys welded using the same FSW parameters the two alloys exhibited different response in terms of the recrystallized fine grains after FSW. Significant grain refining occurred in the NG zone of AA7075 with average grain size of 6μm at 50mm/min welding speed that was reduced to 2μm by increasing the welding speed to 200mm/min. A relatively recrystallized coarser grain structure obtained in AA5083 joints NG zone with average grain size of 9μm at 50mm/min that is reduced to 3μm at 200mm/min. This implies that the starting material characteristics have a significant impact on the final grain structure after FSW. The crystallographic texture in the nugget zone revealed a simple shear texture without a significant influence of varying the welding speed. The dissimilar joints exhibited ultimate tensile strength ranged between 245 and 267MPa and fracture strain ranged between 3 and 5.6%.A brittle/ductile fracture mode was dominating in the examined fracture surface of the dissimilar joints.