Effect of tool pin-tip profiles on material flow and mechanical properties of friction stir welding thick AA7075-T6 alloy joints

Five tools with different pin-tip profiles like thread taper, triangular, square, three grooves, and conical platform were designed to friction stir weld 20-mm-thick AA7075 alloy plates. The effects of pin-tip profiles on thermal cycles, material flow, and mechanical properties of the joints were studied. The results show that the temperature greatly decreases from the top to the bottom of the weld, and higher peak temperatures along the thickness are obtained using three grooves tool. Due to poor plastic flow reflected by local transfer behavior of material, thread taper and conical platform tools yield macro cross-sectional level pore defect on advancing side in nugget zone. However, triangular, square, and three grooves tools produce defect-free joints due to improving plastic flow resulted from stronger pulsating action and stirring power, leading to higher transfer displacements of local material. The tensile test results show that the joints fabricated by three grooves tool have better mechanical properties including strength and ductility owing to better material flow. A higher ultimate tensile strength of 391 +/- 10 MPa, yield strength of 310 +/- 12 MPa, and elongation of 8.3 % are obtained, respectively. This can be attributed to finer grains, greater natural aging response and optimum hardness.