Thermal-Stress Distribution Characteristics of V-Shaped Coupling Bilateral Heat Source Autogenous Welding
With the demand of car lightweight,aluminium alloy T-joint was widely applied in the automotive industry.Different from laser beam welding and friction stir welding on T-joint,which required precise assembly and back support of workpiece,the arc weld-ing technology possessed the advantages of flexibility and convenience.Using conventional arc welding for T-joint needed a series of processes,including groove processing,one side welding,back gouging,nondestructive test,the other welding,etc.Therefore,a V-shaped coupling bilateral heat source welding(VCBHSW)process was proposed,referring to a welding method in which two gas tung-sten arc welding arcs were symmetrically arranged on both sides of the T-joint with a certain angle between the heat source axes,and the dual heat sources were welded in the same direction at the same speed.The angle between the heat source axis and the web was 42°,and the autogenous welding without wire filling was adopted.VCBHSW process could reduce processes and improve efficiency compared with conventional arc welding.Numerical simulation and experiment were carried out on VCBHSW process to investigate the thermal,force field characteristics,morphology,and microstructure of T-joint assembled by 5083 aluminum alloy plate with thickness of 8 mm.Pure argon protection was adopted in welding experiment,and the gas flow was 11 L·min-1.Four welding parameters were em-ployed with a series of welding current(205,207,210 and 220 A)under other fixed parameters.The rationality of heat source model and parameter setting was verified by fusion line contour method.Stereo microscope was used to observe the cross-section morphology of T-joint,and the geometric characteristic parameters of joint were quantitated.The optical microscope was applied to observe the mi-crostructure and pore defects of the T-joint.To illustrate the characteristics of VCBHSW,the single heat source sequential welding(SHSSW)was used as a control group.The results showed that the thermal cycle curve of VCBHSW process was unimodal,and the temperature field was symmetrically distributed on both sides of the web due to the equal heat input of the bilateral heat source.Howev-er,the thermal cycle curve of SHSSW process was bimodal since the heat source needed to approach to the test position twice during SHSSW process,and the maximum temperature of the test position was 16 ℃ lower than that of VCBHSW.Since the test point was lo-cated on the first welding side of the flange,the temperature of the first peak was higher than that of the second peak.The temperature field was asymmetrically distributed on both sides of the web during SHSSW.The cooling rate(0.23 ℃·s-1)of VCBHSW from maxi-mum temperature to 25 ℃ was higher than that of SHSSW(0.21 ℃·s 1),which was ascribed to the preheating and post heating effects of the previous and subsequent heat sources on the workpiece.Through the calculation of the longitudinal residual stress of T-joint by both VCBHSW and SHSSW,the tensile stress was distributed near the weld with a symmetrical M-shape,and the maximum stress was 175.1 MPa.The compressive stress was distributed in the far seam area,with a peak value of 56.9 MPa,and the farther away from the weld,the greater the compressive stress was.At the top of T-joint web,the longitudinal residual stress was expressed as the compres-sive stress,and the tensile stress was at the bottom.The maximum compressive stress was 24.1 MPa and the maximum tensile stress was 139.5 MPa.The residual stress distribution of SHSSW joint was similar to that of VCBHSW joint,but the maximum stress was dif-ferent.The peak tensile stress of SHSSW flange was 118.8 MPa,the peak compressive stress was 53.2 MPa,the peak tensile stress of SHSSW web was 183.2 MPa,and the peak compressive stress was 24.2 MPa.The tensile stress curve of SHSSW was asymmetrically M-shaped distributed owing to the stress relieving effect of heat treatment of the rear weld bead on the first bead.VCBHSW experiment showed that the joints with welding current of 205,207,210 and 220 A were all successfully obtained.The geometric characteristic parameters of VCBHSW T-joint were positively correlated with the welding current when other parameters were fixed.When the current was 220 A,the geometric characteristic parameters of the joint section reached the maximum,the maximum weld leg height of T-joint was 9.88 mm,the maximum depression depth at the top of the joint was 2.66 mm,the maximum melting depth of the flange was 3.93 mm,and the maximum melting area was 109 mm2.Pores were generated in the weld,which were judged as hydrogen because of the moisture absorbed by oxide film on the surface of aluminium alloy.The dense hydrogen pores were located near the fusion line at the top of the T-joint,which was related to the escape direction of less dense hydrogen in the liquid molten pool.The pores could be re-strained by strengthening the cleaning of oxide film before welding,preheating before welding or greater heat input.T-joint microstruc-ture of VCBHSW was mainly composed of α-Al,β(Al3Mg2)and Mg2Si.The research of VCBHSW process enriched the multi heat source cooperative welding theories and provided a high-quality and efficient welding method for the welding of moderately thick alumi-num alloy car body.
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