Effect of tool pin profiles on defects formation during friction stir welding of AA 6082-T6 straight-pipes-butt-weld joints

Friction stir welding (FSW) has emerged as a solid-state bonding technique widely used due to the efficacy in welding challenging materials and various joint configurations. However, welding of the pipes present unique challenges due to the limited contact surface area between the tool and the pipes. In the current study, FSW was employed on the AA 6082-T6 aluminium (Al) straight-pipe-butt-weld (SPBW) joints using high-speed steel tools with different tool pin profiles (i.e., plane cylindrical, tapered cylindrical, threaded cylindrical, and square pin profiles) in flat position (i.e. workpiece rotates). A series of single-pass and two-pass FSW runs were performed to weld the AA 6082-T6 pipes, and their effects were systematically investigated on defect formation. Microstructural examination of samples welded with threaded cylindrical pin revealed larger tunnel defect compared to other pin profiles. Additionally, defects such as kissing bonds and lack of fusion were also found in the samples. Interestingly, a very small tunnel defect was observed in the sample welded with square pin tool. Consequently, the tensile test of this sample exhibited superior tensile strength and weld joint efficiency compared to other pin profiles. Microhardness testing revealed that the sample welded with a tapered cylindrical tool exhibited highest microhardness, while the two-pass sample using a threaded pin exhibited the lowest hardness.