| With the growing demand for lightweight designs in automobiles,particularly in the field of new energy vehicles,the research on body lightweight has become a crucial research focus.The use of lightweight materials,such as aluminum alloys,is a practical approach to achieving this goal.However,the successful application of aluminum alloy materials in the vehicle body structure not only depends on advanced forming processes but also relies on reliable joining techniques.Friction stir spot welding is a new solid phase welding technique that overcomes the welding defects associated with melt welding of aluminum alloys.Despite its benefits,the conventional friction stir spot welding process has some limitations,such as keyhole formation and small effective connecting areas,which can weaken the mechanical properties of the joint.To address these issues,the weld bonding technique has been developed,which combines adhesive bonding and spot welding processes to compensate for the shortcomings of each method.However,the thermal welding effect near the weld nugget leads to adhesive carbonization,and gas impact reduces adhesive denseness.Additionally,adhesive impurities involved in the weld nugget can decrease the mechanical properties of the weld joint.Therefore,this study proposed a temperature-controlled friction stir spot weld bonding technique in a welding-through manner to address these challenges,and the influence mechanism of adhesive on the joint formation was investigated.Furthermore,the effect of interfacial treatment processes on the tensile-shear mechanical properties of the hybrid joint was also examined for lightweight aluminum alloys for automotive applications in a flow-in manner.The details are as follows:(1)Mechanical performance analysis and service evaluation of friction stir spot welds.For the three typical processes: conventional friction stir spot welding(CFSSW),probeless friction stir spot welding(PFSSW),and swept friction stir spot welding(SFSSW),the influence of welding parameters on the mechanical properties of the joint was investigated,and the joint properties were optimized to obtain the optimal processing parameters.Moreover,the joint macrostructure of the three spot welding processes was analyzed,and the connection mechanism was investigated.The connection mechanism of PFSSW is the surface connection between sheets in the connection area.The tool pin increases the material flow causing the CFSSW and SFSSW welds with a body connection.In addition,for a small all-aluminum electric vehicle body structure,the evaluation of the service performance of welded joints in multiple conditions was proposed,and the service performance of the welded joints in typical positions was compared.The results indicate that the service performance of CFSSW joints was slightly lower than that of SFSSW,and significantly higher than PFSSW joints.However,considering the process complexity,CFSSW joints have a greater advantage in the structural connection of thin aluminum sheets for automotive applications.(2)Analysis of joint formation and tensile-shear mechanical performance of friction stir spot weld bonding joint in flow-in and weld-through manners.A comparison of the microstructure,mechanical properties,and internal stress distribution between the weldthrough and flow-in joint was conducted using a simple CFSSW structure.The results revealed that the flow-in joints exhibited shear dimples in the weld region,indicating a sound metallurgical connection.Cohesive failure was observed in the adhesive region,demonstrating the adhesive’s excellent load-bearing capacity.However,the weld-through joint experienced adhesive carbonization near the weld nugget and adhesive impurities within the weld nugget,resulting in decreased load capacity.Moreover,the stress distribution analysis in the flow-in joints revealed that the adhesive layer reduced stress concentrations at the hook tip,enhancing the joint’s load-carrying capacity.Compared to spot welds,the flow-in joints experienced a68.29% reduction in hook tip stress and 1.1 times increase in tensile-shear failure load.(3)Tensile-shear performance and failure analysis of modified weld-through friction stir spot weld bonding joint.The temperature field distributions in the welding process were analyzed using the DFLUX subroutine.To address issues such as adhesive layer carbonization and impurities in the weld-through process,a temperature-controlled connection technology was proposed that enables precise control of the adhesive layer thickness and separation of the welded joints from the adhesive layer.Compared with the traditional weld-through process,the tensile-shear failure load of the pre-cured and post-cured temperature-controlled friction stir spot weld bonding joints increased by 42.56% and 14.53%,respectively.The effect mechanism of pre-cured and post-cured adhesives on the joint formation and mechanical performance in friction stir spot weld bonding joint was analyzed using the proposed joint structure.The findings revealed that prior to adhesive curing,the adhesive layer component was vaporized,resulting in a dendritic cohesive fracture morphology in the adhesive region.Moreover,the fracture mode in the adhesive region was identified as an interfacial failure due to the welding heat and forging pressure of the welding tool,leading to the adhesive layer cracking in the welding joint after adhesive curing.Additionally,the adhesive layer thickness led to mismatches between the top and bottom surfaces of the weld joint,resulting in inadequate stirring in the weld region and cleavage failure in the weld region.Therefore,epoxy-based adhesives cannot directly participate in the friction stir spot welding formation process.Friction stir spot weld bonding in a flow-in manner is the optimal combination of bonding and friction stir spot welding technology.(4)Improvements in tensile-shear performance of the flow-in friction stir spot weld bonding joint.The typical brittle and plastic adhesives were employed to investigate the effect of adhesive plasticity on the load-bearing properties of weld bonding joints,and the influence of laser and surface sandblasting on the tensile-shear properties of the bonded region was also analyzed.The results indicated that the failure loads of brittle,double-mixed adhesive,and plastic friction stir spot weld bonding joints were successively enhanced,demonstrating the reliability of the friction stir spot weld bonding process in aluminum alloys.Additionally,the shear strength in the bonding region of laser and surface blast joints was affected by both interfacial wettability and surface roughness.Moreover,the interface treatment could augment the failure load in bonded and spot-welded regions.The results showed that the failure loads of weld bonding joints with sandblasted and laser surface treatment were increased by 66.30%and 72.92%,compared to bonding joints with the same bonding area.(5)Exploratory study of friction stir spot weld bonding connection in joints and structures.Based on the previous study,this research compared the basic mechanical properties of spot weld bonding joints and structures in a flow-in manner.The application of adhesive to spot welds significantly improves their mechanical properties.The shear impact mechanical properties of welded,weld bonding,and adhesive bonding joints were compared at three strain rates(25 s-1,125 s-1,and 250 s-1).The results indicate that the mechanical properties of the spot weld bonding joints were similar to bonding joints at high strain rates.Compared to spot welds,the failure loads of the spot weld bonding joint were increased by 221.17%,229.61%,and 224.87% at the three strain rate levels.In addition,the static mechanical performance analysis of the friction stir spot weld bonding joint structure showed that the use of friction stir spot weld bonding could significantly improve the compression,torsion,bending stiffness,and NVH performance of the structure.In summary,this research systematically investigates connection technology for lightweight vehicle bodies.In particular,the temperature-controlled weld-through friction stir spot weld bonding technology effectively addresses adhesive layer thickness control and adhesive impurities.The effective combination of spot welding and adhesive bonding for mechanical property regulation has been achieved through systematic evaluation of spot welding technology,optimization of adhesive processes,and mechanical evaluations of preweld treatments.This study provides favorable technical support and theoretical foundations for promoting the lightweight design of high-quality automotive aluminum alloy sheet connections. |
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