Influence of Rotation Speed on Microstructure and Mechanical Properties of Friction Stir Lap Welded Joints of AA 6061 and Ti6Al4V Alloys

Three-mm-thick AA 6061 plate and 2-mm-thick Ti6Al4V plate were friction stir lap welded using a W-Re pin tool. Defect-free joints were obtained at various rotation speeds ranging from 800 to 1400rpm with a constant welding speed of 100mm/min. The influence of rotation speed on the microstructure and mechanical properties of the Al/Ti friction stir lap welding (FSLW) joints was investigated. At low rotation speed within 1000rpm, the obtained joint has no obvious mechanical Al/Ti intermixing at the interface. However, a significant mechanical mixture of Al and Ti alloys accompanied by hooks could be found in the joint welded at a rotation speed over 1000rpm. Only diffusion layer was observed in the joint obtained at low rotation speed. Further, IMCs could be found in the Al/Ti mixture and hook when the rotation speed increased to 1400rpm. The highest microhardness was measured at the interface due to the formation of IMCs. The tensile shear load of the welded joints reached the maximum at 1000rpm and the specimens failed along the interface during the test.