| 7075-T651 aluminum alloy has been widely used in aerospace,automotive and transportation for its great properties of high specific strength,high specific stiffness.Traditional fusion welding methods were difficult to apply to the joining of aluminum alloys.As a new solid state welding technique,Friction stir spot welding(FSSW)presented a great potential for the elimination of solidification crack,weld porosity and other weld defects.However,a defect named "keyhole" always appeared in the center of the joint welded with traditional FSSW method,which narrowed the effective area of the joint and weaken the joint strength.Probeless friction stir spot welding(P-FSSW)was a variant of FSSW,which used a tool without probe on shoulder.P-FSSW can eliminate the "keyhole" with simple equipment,and presented great potential in industrial application.In order to improve the performance of the P-FSSWed joint of 7075 aluminum alloy,the influence of welding parameters and shoulder geometry on mechanical properties and microstructures was studied.The mechanism of welding process,such as heat generation and material interaction,was researched by both experiment and simulation.Finally,the technology of electrically assisted P-FSSW was carried out.The main contents and conclusions of this work were presented as follows:(1)The effects of welding parameters on joint strength were investigated with response surface method and self-designed experimental equipment.It was found that the parameters have great effects on the fractured load,and plunge depth was the most important parameter,while the plunge rate has little influence on joint strength.By analyzing the statistics,the equation was proposed to predict the joint strength,and the optimized parameters was obtained.(2)For the research of the influence of shoulder geometry,three types of probeless tools were designed and applied for welding.The mechanical properties and microstructures of the joints were tested.The temperature and material flow during welding process were studied.It was found that the shoulder geometry presented little influence on heat generation,while great effects on material flow.Archimedes grooves was more effective on the promotion of the material flow and produced more serious "Hook" defect.The concave tool decreased the escape of plasticized material,improved the forge force,and optimized the metallurgical bond at the interface.(3)Numerical model was created for the simulation of the P-FSSW process With the solver of ABAQUS,the evolution and distribution of temperature,equivalent plastic strain and Mises stress were obtained,and the material flow action was calculated.The results showed that the temperature,stress and strain of the were distributed symmetrically,and the temperature,stress and strain in the weld core area are obviously higher than those in other areas.During the welding process,the material particles at the center of the joint move most violently.With the effects of the tool,the plasticized material rotated around the center,meanwhile,moved outward and downward along the edge of stir zone.(4)The effectiveness of electro-plastic effect on 7075-T651 aluminum alloy softening was evaluated by electro-plastic tensile experiment.The research of electrically assisted probeless friction stir spot welding was conducted on self-designed equipment.It was found that the pulse current can improve the welding temperature of P-FSSW process,enhance the material interaction,and facilitate the improvement of the tensile shear strength of the joint. |
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