Effect of overlapping friction stir welding passes in the quality of welds of aluminium alloys

The aim of this investigation is to study the effect of overlapping friction stir welding passes on the soundness and the change in the microstructure and mechanical properties of welds in two aluminium alloys. Overlapping passes were made in 3 mm thick plates of 5083-O and 6063-T6 alloys. Tunnel defects were detected in the first and second passes of welds in the 5083-O alloy but not in the 6063-T6 alloy. The first pass resulted in a small increase in hardness in welds in the 5083-O alloy. However, in the 6063-T6 alloy, there was a substantial decrease in hardness and strength. The subsequent overlapping passes produced a modest increase in hardness and strength in both alloys as well the elimination of tunnel defects in welds in 5083-O alloy. The mechanical efficiency of welds in alloy 5083-O is equal to one as opposed to welds in alloy 6063-T6 that show an efficiency around 0.47. An increase in efficiency of 15% was reached if tensile specimens were polished, in order to remove weld roughness.