| Aluminum alloy plays an important role in the lightweight process of high-speed train body structure,and the butt-lap structure is an important joining structure in train body.Therefore,it is great significant that study on the welding aluminum alloy butt-lap joints.In this study,7N01 aluminum alloy was selected as research object,and butt-lap structure was welded by laser-melt inert-gas welding(MIG)hybrid welding and friction stir welding(FSW).The study focused on effect of welding parameters on surface forming,the relationship between the internal defect and the quality of the joint,and the microstructure and mechanical properties of the 7N01 aluminum alloy butt-lap joints was analyzed.The results showed that for laser-MIG hybrid welding,the good butt-lap joint can be obtained under the laser power of 2.8 k W,wire feed speed of 10.0 m/min and welding speed of 1.5 m/min.By increasing the wire feed speed and the MIG arc heat input,the number of pores in the joint can be effectively reduced.Thus the weld formation was improved,the quality and properties of joint was increased.The value of tensile strength and elongation of joint was 286.2 MPa and 6.96%.The joint consisted of the basal zone(BZ),the heat affected zone(HAZ),the fusion zone(FZ)and the weld zone(WZ).The BZ structure presented rolled fibrous,and the value of hardness in BZ was about 114 HV.The grains in HAZ was coarsening,the value of hardness in HAZ was 85 HV～105 HV.The grains in WZ was equiaxed,the value of hardness was 75 HV～100 HV,and was slightly lower than that in HAZ.The grain of the FZ was fine equiaxed and columnar near WZ,the value of hardness was the lowest.The hardness distribution curve was "W " type in the vertical direction,and the value of hardness at the bottom of joint was higher than that at the top of joint in the horizontal direction.For the FSW,by placing the surface of lap joint on the RS and using the method of the newly stirring pin inward biased,the good butt-lap joint can be obtained under the welding speed of 0.2 m/min and the rotational speed of 1400 r/min.By stirring pin inward biased and increasing the rotational speed,it can reduce the adverse effect of "S" wire on the joint under the condition of reducing the interface distortion.thus improving the tensile strength of the joint but limited.By adopting the newly designed stirring pin,the adverse effect of the interface distortion on the joint can be further reduced effectively,and a high quality welded joint can be obtained.The maximum tensile strength and the elongation of the joint is up to369.5 MPa and 9.39%.The joint consisted of four zones: BZ,HAZ,the thermo-mechanically affected zone(TMAZ)and the nugget zone(NZ).The grains in NZ was equiaxed,the value of hardness was 106 HV～110 HV.The grains in HAZ was coarsening and the value of hardness decreased slightly,was 100 HV～106 HV.The grains in HAZ was thermoplastic deformation,and the value of hardness was slightly higher than NZ,was 110 HV and 112 HV.In addition,through the optimization of the process parameters,both welding techniques can achieve good welding.The good weld of the aluminum alloy butt-lap joint structure can be achieved by the optimization of the process parameters.However,it is necessary to pay attention to the porosity defects in the laser-MIG hybrid welding and the interface distortion existing in the FSW joint,which can affect the performance of the joint partly. |
PDF Full Text Request