| The present study is supported by the National Natural Science Foundation Emphases Item of China(51075413/E050803). Since friction stir welding technique can avoid some welding defect such as pore, inclusion and crack, it has become one of the welding techniques that experience rapid development and have wide application during the last twenty years. However, the applications of conventional friction stir welding on high-melting-point alloys are impeded by its low welding heat input. The electric current aided friction stir welding(EFSW) technique in the present study combines friction heat in conventional friction stir welding with resistance heat, and generates hybrid heat source during welding to provide heat input for the weld. This technique could reduce welding defects, improve welding efficiency and expand the field of application of friction stir welding.However, presently there is a lack of studies concerning the numerical simulation for the thermal-mechanical- electrical coupling process of EFSW. Besides, the previous studies failed to consider the effects of workpiece deformation on the contact state between the tool and the workpiece, electric current density distribution and temperature fields. In the present study, a thermal-mechanical- electrical coupling model for the EFSW using finite element method was established. The obtained temperature fields, stress field and electric current field of the workpiece was analyzed. The validation of the numerical model was verified through an experiment. The stability of heat generated by the hybrid heat sourse of electric current aided firction stir welding was evaluated using the maximum temperature curve of the workpiece during welding stage. The results show that the interface between the workpiece and the tool cannot be in complete contact during welding. Electric current concentration will happen when electric current pass through the small contact area between the workpiece/tool interface. This will cause a significant increase in the local electric current density. When the total heat generation during the welding stage is similar, comparing with voltage controlled EFSW, extremely high temperature is more likely to appear in current controlled EFSW. The mean square error obtained in the linear fitting of maximum temperature on the workpiece during welding stage can be employed to quantitively indicate the overall fluctuation level of workpiece temperature during welding stage.In addition, the present study predicted tool wear using Archard wear model and Usui wear model, and evaluated the accuracy of the two models through comparison of experimental results with simulation results. The wear of cold worked tool and DMLS tool was compared on both experimental and simulation results. The wear resistance difference of tools obtained by the two processing methods was explicated using their microstructural and harness characteristics. The results show that the Archard wear model can accurately represent the influence of tool’s hardness on its wear. The maximum wear depth of cold worked tools is notably larger than DMLS tools. The higher wear resistance of DMLS tools may be attributed to the constraint effect of solidification boundaries, which will reduce grain boundary sliding in tool materials. |
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