Influence of process parameters on the microstructural evolution and mechanical characterisations of friction stir welded Al-Mg-Si alloy

Abstract

Heat generation and plastic deformation during Friction Stir Welding (FSW) produce profound changes in the microstructure and structural properties of welded joints. Strengthening precipitate, grain size and crystallographic texture evolution are the most important microstructural changes in the case of welding aluminium alloys. An interaction relationship has been developed in this study to understand the evolution of microstructure during FSW of Al-Mg-Si alloy for a wide range of welding temperatures and plastic deformations by controlling two important process parameters (tool rotation and welding speeds). Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) have been used for microstructural characterisation. The mechanical properties were evaluated using microhardness, tensile and low cycle fatigue tests. In all cases, fine recrystallised equiaxed grains with a partial <111> fibre texture was evolved in the nugget zone. Microstructural development was found to be significantly influenced by the weld pitch (welding speed/rotation speed), as it is controlled the heat input, cooling rate, exposure time and plastic deformations. The strength of the FSW joints is improved by increasing the welding speed because of grain refinement, incomplete dissolution or re-precipitate of strengthening precipitates Mg5Si6(β)´´ and the introduction of edge dislocations. The fatigue performance of FSW joints is dependent on grain size with a remarkable improvement in the fatigue life observed for smaller grain sizes.