Study On The Joining Process And Mechanical Properties Of Pinless Friction Stir Spot Weld-bonding Hybrid Joints

Aluminum alloy is an ideal material for reducing the weight of automobiles.It is used more and more in the automobile industry.However,traditional welding methods are difficult to adapt to the connection requirements of new materials such as aluminum alloys due to their defects.Friction stir spot welding has become the preferred technology for welding aluminum alloys and dissimilar materials due to its advantages of solid phase welding.However,due to the existence of stirring needle,it can not weld thin plates below 1.5 mm,but pinless friction stir spot welding has great advantages in thin plate welding,but spot welded joints can create stress concentrations.Therefore,this paper uses aluminum alloy 6061-T6 sheet and adhesive Aralidte 2015 as experimental materials,and adds glue on the basis of pinless friction stir spot welding,and proposes a pinless friction stir spot welding-bonding hybrid connection process.The disadvantages of these two types of joints can be fully utilized to form a hybrid joint with uniform stress distribution and excellent mechanical properties.At this stage,there are few studies on the hybrid joining process and the mechanical properties of its hybrid joints.Therefore,the mechanical properties and fracture process of the hybrid joining process and its hybrid joints are deeply studied in this paper.The specific research contents are as follows:Firstly,the mechanical properties of the aluminum alloy 6061-T6 sheet were tested,the displacement was measured by the DIC system,and the Johnson-Cook constitutive equation and material performance parameters at the reference strain rate and reference temperature were obtained through the relevant parameters obtained from the quasi-static tensile test,which provides modeling data for material properties in subsequent tensile-shear simulations of hybrid joints.Then the range of process parameters of pinless friction stir spot welding is determined through orthogonal experiment.The selected process parameters are stirring head speed of 2500 r/min,the pressing amount 0.5 mm and the residence time6 s.This parameter is used as the process parameters of all subsequent welding experiments.Then the welding process of pinless friction stir spot welding is numerically simulated.Based on Abaqus platform,ALE thermal coupling method is used to simulate the real welding process,and the temperature field of the welding process is obtained.Compared with the experimental temperature field results,the simulation effect is good,which verifies the accuracy of the simulation model.Through the welding temperature field obtained by numerical simulation,it can be seen that the solder joint is well formed,and the whole lap area will be in a high-temperature state for a long time during welding.Therefore,it can be inferred that the adhesive layer in the whole lap area will evaporate and fail under the welding heat,and the mechanical properties of the hybrid joint of the penetration glue welding process will be lower than that of capillary action glue welding hybrid joint.Further,a comparative study on the mechanical properties of hybrid connection joints with different process sequences was carried out.The optimal process sequence was determined by preparing the capillary action glue-welding joints that were spot-welded first and then glue-injected,and the glue-penetrating glue-welding joints that were glued first and then spot-welded,and their tensile and shear mechanical properties were tested.The macroscopic and microscopic morphology of the fracture of the sequential mixed joint was studied,and it was found that there were a large number of adhesive compounds and carbides at the fracture of the mixed joint of the penetration glue welding method,which affected the metallurgical bond between the metals,and a large number of evaporation failures of the adhesive layer led to the failure of this process.Compared with the spot welded joint,the mechanical properties of the hybrid joint decreased.However,the capillary action bonding method is effective in bonding and spot welding,and there are obvious dimples at the fracture,which have both plastic and brittle fractures,which further explains the superiority of the mechanical properties of the capillary action bonding method.Finally,based on the multi-angle tensile test and the tensile-shear test,the cohesive force finite element model of the spot-welded joint and the hybrid joint in the tensile-shear process was established respectively.The tensile shear fracture process was compared and analyzed,and the results showed that: 1)the maximum tensile shear load of the hybrid joint with solder joints connected obliquely is the largest,followed by series,and the lowest in parallel.The tensile shear load has no significant effect.2)The main load of the hybrid joint is the adhesive layer.The existence of solder joints will hinder the expansion of cracks in the adhesive layer.The arrangement of the solder joints will affect the cracking behavior of the hybrid joint.The double solder joints arranged in series will hinder the expansion of the cracks in the adhesive layer to the inside of the joint,while the parallel arrangement or oblique arrangement of solder joints will not produce this phenomenon.3)The maximum tensile and shear load that can be endured by using the flexible glue 2015 hybrid joint is higher than that of the brittle glue AV138 hybrid joint,and the flexible glue 2015 hybrid joint is more resistant to deformation and can absorb more energy when damaged.