Control of temperature field, microstructure and mechanical properties of variable rotation speed refill friction stir spot welded high strength aluminum alloys

For 7B04-T74 aluminum alloy, a new method of variable rotation speed refill friction stir spot welding (V-RFSSW) was used. Numerical simulation and experiments were conducted. The results showed that the V-RFSSW temperature field was symmetrically distributed around the axis of the welding tool, and the high temperature area was concentrated in the cavity of the sleeve. Compared with the conventional refill friction stir spot welding (RFSSW) with the same rotation speed during the plunging stage, the new V-RFSSW method can fully plasticize the materials during the plunging stage to ensure the weld formation. At the same time, by reducing the rotation speed of the welding tool during the refilling stage, the welding peak temperature and high temperature dwelling time can be reduced, the occurrence of component liquefaction can be restrained and the formation of eutectic phase can be avoided. The microhardness distribution of the V-RFSSW joint and conventional RFSSW joint was "W" shape The average hardness of V-RFSSW joint was higher at the condition of the rotation speed iduring the plunging stage was the same. Under the tensile shear load, the two kinds of joints fractured in the form of "button", in which the tensile shear failure load of V-RFSSW joint was 8 835 N, which was higher than that of RFSSW joint (8 162 N).