| Lightweight technology is particularly important to solve the problem of resource shortage and environmental pollution in future automotive industry.The application of lightweight materials is a more direct way to realize the lightweight of automobile.Due to its advantages of high strength/weight ratio,good formability and corrosion resistance,and relatively low cost,aluminum alloy is expected to be applied to automobile engine,gearbox,wheel hub and body parts.However,it also brings the joining problem of aluminum alloy.Magnetic pulse welding(MPW)is an eco-friendly and efficient solid-state welding technique,which can reduce the generation of brittle intermediate compounds and improve the strength of aluminum alloy joint.Meanwhile,adhesive bonding(AB)can evenly distribute the stress of joint and improve its energy absorption and corrosion resistance.In this paper,a magnetic pulse weld bonding(MPWB)joining technology based on spot-welded connection structure was put forward for AA5052 material.The weldability conditions of magnetic pulse welding,the coupling mechanism of weld and adhesive layer,the mechanical properties of MPWB hybrid joint at different high-speed tensile shear tests,and the performance degradation in neutral salt spray corrosion were investigated by means of numerical simulation,process test and microscopic observation.Firstly,a magnetic pulse spot welding(MPSW)connection structure was proposed for AA5052 sheet.The simulation model of magnetic pulse welding process was established by the commercial finite element software Ls-dyna.The actual impact velocity of flyer plate was measured by all-fiber laser doppler velocimeter(PDV),which verified the reliability of numerical simulation.According to the simulation results,the distribution of current and magnetic field in the process of magnetic pulse welding,as well as the deformation and impact of the flyer plate were analyzed.The results showed that in the process of magnetic pulse welding,the induced current first formed a symmetrical elliptic circuit on the single lap area of flyer plate near the coil,and then gradually formed eddy current along plate thickness.The induced current density first increased and then decreased.The current in the coil was concentrated on the surface near flyer plate.According to the flow limit condition of explosive welding,it was found that the required minimum impact velocity increased linearly with impact angle to achieve magnetic pulse welding.Since impact angle at the center of flyer plate was too small,and impact velocity far away was insufficient.Therefore,only two weld lines could form at a suitable distance to the center of flyer plate,which was consistent with the numerical simulation results.Secondly,combined with adhesive bonding and magnetic pulse welding technologies,the hybrid magnetic pulse weld bonding structure and process were proposed.Considering the joint strength and process cost,the suitable welding gap and adhesive area were determined.Compared with control groups of MPSW and AB joints,ARAMIS strain measurement system was used to show the strain state of MPWB joint during shearing process,and coupling mechanism between weld and adhesive layer was revealed by combining with microscopic morphology of fracture.It was found that the weld could delay the initiation and propagation of crack in adhesive layer.Meanwhile,the adhesive layer could significantly reduce the average shear stress around the weld,contributing to larger peak load and failure displacement of MPWB joint.In aspect of fracture morphology,there were both equal-axis and shear dimples in MPSW joint,whereas only shear dimple was found in MPWB hybrid joint,indicating that there were different stress states in MPSW and MPWB joints.Finally,the mechanical properties of MPWB joint under high-speed tensile shear and neutral salt spray corrosion tests were investigated.In the high-speed tensile shear test,with the increase of shearing speed,the peak load and absorbed energy increased,while the cracking degree of adhesive layer reduced.The failure mode changed from the weld and adhesive fracture to parent material fracture.In the neutral salt spray corrosion test,compared with MPSW joint,the adhesive layer in MPWB hybrid joint could absorb salt spray droplets and prevent them from penetrating the weld,thus delaying the deterioration of mechanical properties of the joint.By studying the corrosion path of salt spray droplets,it was found that most of the salt spray droplets in MPSW joint penetrated into the hump of deformation zone,while some of the salt spray droplets in MPWB hybrid joint were absorbed by the adhesive layer.However,as the corrosion continued,the salt spray droplets gradually penetrated the adhesive layer,reducing the bonding force between adhesive and base material and the cohesion between adhesive layers.A small number of salt spray droplets flowed to outside of the weld,and gradually penetrated the weld and dissolved the metal around,resulting in local pitting connection phenomenon. |
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