| Friction Stir Welding(FSW)joints of 6 series aluminum(Al)alloys have excellent mechanical properties compared to fusion welded joints and are widely used in aerospace and high-speed train industries.However,the softening issue in the heat affected zone(HAZ)leads to a certain gap in the mechanical properties of the joint compared with the original base metal.In order to further optimize the joint mechanical properties,post-weld heat treatment technology has become the key to improve the softening zone of the joint.However,the traditional post-weld heat treatment methods suffer from high energy consumption and low efficiency.Therefore,it is of great significance to study an efficient post-welding treatment technology.In this thesis,taking the 6N01 Al alloy FSW joint as the research object,a novel electrical current treatment technology is proposed.The solid solution treatment and aging treatment of the joints were carried out by pulse current and direct current(DC)respectively.The evolution of precipitates under electric current treatment is investigated at the nanoscale level,and the effects of electric current treatment on the microstructure and mechanical properties of the joint are explored from morphology observation,phase analysis,and theoretical research perspectives.Firstly,the duty cycle and pulse time of the pulse current were optimized.It was found that after 3 s of pulse current treatment with a duty cycle of 25%,the tensile stress-strain curve of the joint exhibited a significant Portevin-Le Chatelier(PLC)phenomenon,which determined the optimal electropulsing solution parameters.Secondly,the effects of electropulsing solution+artificial aging and electropulsing solution+DC aging on the mechanical properties of the joints were analyzed.The results showed that electropulsing solution(3 s)+artificial aging(6 h)could optimize the mechanical properties of the joint,with an efficiency improvement of about 50%compared to traditional heat treatment.When the current density was 225 A/mm~2,the hardness and strength of the joint reached the optimum state,and its tensile strength increased by 14%compared to untreated samples.Electropulsing solution(3 s)+DC aging(5 h)further improved the efficiency,with an efficiency improvement of about 58%compared to traditional heat treatment.After electropulsing solution+DC two-stage aging,the tensile strength and HAZ hardness of the joint increased by 15.9%and 21.5%,respectively.Compared with artificial aging,DC aging had higher efficiency(increased by 16.7%),better strengthening effect(increased by 13.6%),and lower treatment temperature(reduced by more than 30℃).Finally,the evolution of precipitates under electric current treatment was studied.The rapid dissolution of the rod-likeβ′can be completed within 3 s by the electropulsing solution,which is due to the thermal effect and the athermal effect of the pulse current.The thermal effect provides the temperature basis for the atomic diffusion,and the athermal effect can significantly reduce the energy barrier and accelerate the phase transition process.DC aging could control the precipitation process of the precipitates.Compared with DC single-stage aging,DC two-stage aging was more conducive to promoting the diffusion of Mg and Si atoms,thereby accelerating the nucleation and growth of precipitates.After electropulsing solution+DC two-stage aging,the precipitates in the HAZ of the 6N01 Al alloy FSW joint were all dispersed needle-likeβ".In summary,the electric current treatment technology can improve the microstructure and mechanical properties of joints,solve the problems of high energy consumption and low efficiency of traditional heat treatment of 6 series Al alloy,and provide new methods and ideas for metal heat treatment process. |
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