Friction stir welding of AA6063 pipe: Effect of double pass on microstructure, strength and defects

AA6063 pipes were welded by single and double pass friction stir welding at 162 mm/min speed, 560 rpm and 0.25 mm offset. Microstructural examination was performed for analysis of various zones. Tensile tests were performed to test the tensile strength. The presence of a minute and discontinuous kissing bond at the 40° weld location of the single pass pipe weld. Double pass weld did not have any defect. The average of tensile strength of samples taken from the three locations in single and double pass pipe weld gave the joint efficiency of single pass weld as 101% and double pass as 117%. The grain sizes in all zones of the double pass FSW pipe welds were smaller than the respective zones in the single pass welds. The microhardness of the double pass weld was found to be higher in the double pass weld in comparison to the single pass weld.