| Light-weighting of automobile is an important measure for energy conservation and emissions reduction, it has a significant meaning for sustainable development of automotive industry. Magnesium alloy is a very competitive material in the light-weighting process of automobile. However, because the chemical properties of magnesium alloy are lively and the thermal conductivity is very strong, high quality welding joint cannot be get easily by means of ordinary methods of arc welding and resistance spot welding. Friction stir welding is a kind of high efficient solid phase welding method, it has been proved to weld magnesium alloy and get high quality welding joint. During the welding process, it is inevitably to produce residual stress because of the non-uniform temperature field, and the microstructure and mechanical properties of the welding joint will change compared to base material. Therefore, it is very necessary to research the microstructure, mechanical properties and residual stress of friction stir welded joints.As a fundamental research, an advanced computational approach based on ABAQUS code was developed to investigate residual stress distribution in AZ31 magnesium alloy joint with a thickness of 8mm produced by friction stir welding. The transient temperature field and residual stress state during the welding process were simulated. In addition, the temperature cycles at several typical locations and residual stress distribution were measured through using thermos-couples and hole-drilling method, respectively. Microstructures and mechanical properties of AZ31 magnesium alloy joint by FSW are investigated by room-temperature tensile, impact, hardness test and metallographic analyze. Moreover, this paper studied the weld-ability of dissimilar friction stir welding between magnesium alloy and aluminum alloy.(1) The comparison between numerical results and the measured data suggests that the thermal cycles and residual stress distribution predicted by finite element model are in good agreement with the measurements. The numerical results indicate that the longitudinal residual stress distributions on both the top surface and the center surface presents as "M" shape, while the transverse residual stress distributions through plate thickness are uneven, and the transverse residual stress distributions on the top surface presents as ―W‖ shape.(2) The grains in the WNZ are uniform and equiaxed because of the occurrence of dynamic recrystallization, and the grains in the HAZ grow up due to affected by the friction heat. In addition, the grain size increases with increasing heat input.(3) The research results show that the tensile and impact properties of friction stir welded joints are reduced compared to those of the BM. The highest joint efficiency obtained in this study is about 90% when the tool rotation rate and welding speed are 1200r/min and 240mm/min, respectively. In addition, almost all FSW tensile specimens failed along the boundary between WNZ and TMAZ, the fracture surfaces of the FSW samples are made up of tear ridges, which show the quasi-cleavage cracking feature.(4) The experiment results show that the connection between magnesium alloy and aluminum alloy with 8mm thickness can be achieved by FSW, however, the range of technological parameter of FSW Al alloy and Mg alloy is very narrow and the highest ultimate strength obtained in this study is just 81 Mpa. |
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