| It has an important impact on energy conservation and environmental protection by lightweight design in green development.The goal of lightweight is to minimize the selfweight of the structure under given boundary conditions,while satisfying certain life and reliability requirements.To achieve this goal,some methods,such as appropriate construction,lightweight materials,joining techniques,as accurate a design as possible and an achievable manufacturing process need to be selected.As one of the light metals,7075 aluminum alloy is widely used due to its excellent mechanical properties.Polyimide is also one of the polymer materials with the best comprehensive properties.In this paper,the metal/polymer composite structure was fabricated by friction stir spot welding with 7075 aluminum alloy and polyimide as matrix,and the effective welding joint of aluminum alloy/polyimide was obtained.Firstly,the polyimide joint was carried out via friction stir spot welding,and the effects of different shape FSSW tool on the forming and properties of polyimide joints were compared.It is impossible to obtain effective welding joint using type a FSSW tool,while type b FSSW tool is more conducive to flow and mixing of polyimide materials,resulting in a joint with stronger binding force.The FSSW parameters of the rotate speed,plunge depth,dwell time and plunge rate were optimized based on the tension-shear load.The optimal process parameters was obtained:rotate speed 1000 r/min,plunge depth 1.2 mm,dwell time 15 s and plunge depth 15 mm/min.The order of major and minor effects of each parameter on tensile shear load of the joints is:rotate speed>dwell time>plunge rate>plunge depth.The bonding surface of the welding joint is located at the bottom and side of the stir pin,and the binding force of the side is lower than that of the bottom due to the existence of pores.The thermal performance test results show that there is no large amount of decomposition of polyimide in different regions of the polyimide joint under the optimal process parameter combination,and the thermal performance of the materials in different regions is not different.Secondly,aluminum alloy and polyimide were directly welded by friction stir spot welding.The FSSW parameters of the rotate speed,plunge depth,dwell time and plunge rate were optimized,and the maximum tensile shear load of the joint was 3556 N.The optimal process parameters is rotate speed 2400 r/min,plunge depth 0.4 mm,dwell time 20 s,plunge rate 10 mm/min.The order of major and minor effects of each parameter on the joint performance is:rotate speed>plunge depth>dwell time>plunge rate.Mechanical interlocking and element diffusion by mechanical interlocking of aluminum alloy and polyimide are the main joining mechanisms.The mechanical interlocking at the joint interface at high tensile shear loads was stronger,the number of interlocking was more,and the peel area on the polyimide side was larger.Finally,in order to improve the strength of the aluminum alloy/polyimide joint,the welding tests were carried out by three interfacial control processes:laser texture on the aluminum alloy surface,nylon interlayer added on the welding interface and silane coupling agent treatment on the aluminum alloy surface.The laser texture treatment of aluminum alloy surface formed more and more effective mechanical occlusion mode,which improved the mechanical interlocking effect of the interface between aluminum alloy and polyimide.Under the parameter of 60 scanning times and 0.1 mm scanning interval,the joint with the highest tensile shear load of 6087 N was formed,which was 71%higher than that of conventional welded joint.The fracture site is located in the polyimide matrix.The tensile shear load of the aluminum alloy/polyimide joint with nylon interlayer increased by 7.3%compared with that of the control joint,and the mechanical interlocking at the interface did not change much.The XPS analysis of the polyimide fracture showed the formation of Al-O-C and Al-C chemical bonds,and it was concluded that the new mechanism of chemical bonding at the interface led to the improvement of the joint strength.The Al-O-C chemical bond was formed at the interface of the aluminum alloy surface after the treatment of silane coupling agent.The tensile shear test results showed that the tensile shear load of the aluminum alloy was lower than that of the nylon intermediate layer joint,and the tensile shear load was 3%higher than that of the control group.The chemical bonding effect was lower than that of the nylon intermediate layer joint. |
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