Numerical Simulation And Experimental Study On VPPA-MIG Hybrid Welding Of 7A52 Aluminum Alloy

Variable polarity plasma arc(VPPA)-MIG is a high-quality,high-efficiency and deep penetration welding technology for thick aluminum alloy plates.It combines the advantages of high energy density,high penetration depth,and cleaning effect of oxide film from VPPA welding with those of high welding efficiency and easy filler metal from MIG welding.At the same time,it also makes up for the shortage of MIG welding penetration and VPPA can only be formed by vertical welding.Thus,it is of significant potential in industrial applications.At present,the research on the welding process was mainly focused on the process,but the numerical simulation of its physical phenomena has not been reported.Moreover,the hybrid welding process has many parameters and complex physical process.It is not conducive to the promotion and application of the welding process only through the test guidance.In this paper,the temperature field and stress field in VPPA-MIG hybrid welding of aluminum alloy were analyzed by using numerical simulation technology and developing accurate finite element model.This will be helpful to the study of the process mechanism of hybrid welding and the optimization of process parameters and welding structures,which is of great significance and application value.In this study,the thermal physical properties of 7A52 aluminum alloys were analyzed and measured by a laser flash technique(LAF)and differential scanning calorimeter(DSC).A material thermophysical property parameter database considering the phase transition of 7A52 aluminum alloy was established and used in the numerical calculation of VPPA-MIG hybrid welding.A combined volume heat source model considering the interaction of two heat sources was developed for VPPA-MIG hybrid welding.According to the heat source characteristics of VPPA in different phases,a variable polarity heat source model was developed.Two curve-rotated body heat sources with different power and different heat source distribution parameters were loaded periodically to express the heat input of VPPA.On the basis of the characteristics of weld shape and heat transfer of MIG welding,a combined volume heat source model considering heat and kinetic energy of droplets was developed for MIG welding.The heat source characteristic of VPPA-MIG hybrid welding was analyzed by high speed camera.On the basis of integrating the above VPPA and MIG heat source models,the heat source model of hybrid welding was further optimized to realize the variations of the distance between the hybrid heat sources and the MIG heat flux distribution in different polarity phase.The temperature fields in MIG welding and VPPA-MIG hybrid welding of 7A52 aluminum alloys with different thickness and different process conditions were calculated by using the established database of thermo-physical properties parameter and the developed VPPA-MIG hybrid heat source model.The calculated weld bead geometry,weld pool shape and thermal cycles showed excellent agreement with the experimental results.Through the studies of the calculation results and the test results,it is found that the penetration depth increased and the penetration ability enhanced with increasing VPPA power and decreasing MIG power.The weld width of VPPA-MIG hybrid welding was much narrower than that of MIG welding.The weld width of the hybrid welding was mainly determined by MIG,and the VPPA was the main factor in determining the penetration depth.The hybrid welding processes of aluminum alloys with different thickness were optimized to get better weld seam by appropriately adjusting the power ratio of VPPA to MIG.The mechanical properties of 7A52 aluminum alloys in original state and after softening were researched at different temperatures by high temperature tensile test,Thus,the database of mechanical properties parameters of each phase of aluminum alloy was established.Dased on the classical LSW theory,a material softening model of aluminum alloy was developed to calculate the softening process of aluminum alloy in hybrid welding.The effect of softening behavior on welding stress was considered.The stress field in the hybrid welding of 7A52 aluminum alloy was calculated by using the material softening model and the traditional material model,respectively.The calculated results showed that the residual stress calculated by the material softening model was lower than that calculated by the traditional material model in the heat affected zone near the weld seam.The residual stress distributions of the two were the same outside the heat affected zone.The residual stress was measured by X-ray diffraction method,and the measured results were in good agreement with the calculated results by using the material softening model.It was proved that the material softening model could improve the calculation accuracy of hybrid welding stress field.The difference of residual stresses was numerically analyzed in the hybrid welds of 7A52 aluminum alloy filled with different metals.The results showed that the maximum longitudinal residual tensile stress filled with 5-series aluminum alloys was reduced by 122.1 MPa and the maximum transverse residual stress was reduced by 15 MPa compared with that filled with 7-series aluminum alloys.Filling 5-series aluminum alloy could reduce the stress in the weld bead and reduce the driving force of crack propagation,so it can reduce the tendency to produce cracks.The residual stress distributions were compared and studied under the different welding conditions.It was found that the maximum longitudinal residual stress of VPPA-MIG hybrid welding was 25.5 MPa higher than that of traditional double-layer MIG welding,while the maximum transverse residual stress was 11.6 MPa lower.The hybrid welding showed a decrease of 26.97 mm in the tensile stress region compared to the MIG welding.When ensuring the weld forming,the longitudinal and transverse residual stresses and the area of tensile stress decreased with increasing VPPA power and decreasing MIG power.Under the optimal hybrid welding process,the peak value of longitudinal residual tensile stress was 276.8 MPa and the peak value of transverse residual tensile stress was 32.9 MPa.