Effects of pin morphology on the interface defects of the FSWed lap joints of 2A12 aluminum alloy

The current work aims to study the effects of stirring pin morphology on the FSWed lap joints defects and mechanical properties. The 2A12 aluminum alloy friction stir welding lap joints were obtained using two shapes and three thread lengths of stirring pins by experiments. Besides the interface defects forming mechanism were studied by joints plastic metal flow numerical simulation. The results show that the conical pin can lead to uniform and continuous plastic metal velocity field which forms a Hook defect with sharp end, smaller deflection angle (minimum 8 degrees) and larger size (maximum 0.18mm). It also will lead to a larger failure load (maximum is 9.5kN), and the fracture position easily occurs at the Advancing side (AS) of lower plate. The three section pin has a periodic shearing effect causing velocity difference between the arc edge and cutting section of the pin which forms a Hook defect with thicker end, larger deflection angle (maximum 42 degrees) and smaller size (minimum 0.06mm). It also causes discontinuous wavy debris forms in the AS. The three section pin leads to a lower failure load (maximum is just 7.6kN), and the fracture position turns to occur at the lap interface. The increase of the thread length can promote the exchange of plastic metal in the thickness direction and interface materials flow velocity. So the angle (from 20 degrees to 150 degrees) and size (from 0.1mm to 0.6mm) of Hook increases but the height of Cold Lap reduces (increasing the effective sheet thickness of retreating side from 0.69mm to 0.95mm). In addition, the increase of the thread length can make the periodic shear effect of three section pin gradually weak. It is also found that the fracture position is closely related to the shape and the thread length of pins. When using conical pin, the fracture occurs at AS of lower plate or Retreating Side of Nugget Zone (NZ-RS) of upper plate; When using three section pin, the fracture occurs at the bonding interface. And with the increase of the thread length, the fracture position changes from the bonding interface to the NZ-RS of upper plate, and finally to the AS of lower plate.