Numerical Simulations Of Dissimilar TIG Welding-brazing Of Titanium And Magnesium Alloy AZ31B Lap Joints

The application of light materials is an important way for weight reduction ofengineering structures. Titanium and Magnesium alloys which represent the family oflight metals are increasingly getting widespread use, due to the advantages of highspecific strength, comprehensive mechanical performance, good machiningperformance and recyclability. There is no much researches on the dissimilar weldingof TA2, a widely used commercial pure titanium, and magnesium alloy AZ31B reportedso far. These researches mostly focus on the welding process and mechanicalperformance of the joints, barely on the temperature, residual stress and distortionduring and after welding with numerical simulation methods. In the present study, theTA2/AZ31B dissimilar joints were achieved using TIG welding-brazing method, andnumerical simulation method combined with welding and stress measurementexperiments were applied to carefully analysis the welding temperature fields, residualstresses and distortion after welding, and the effect of welding processing parameters onthem.①Comparisons of the features of temperature fields and thermal cycles using twokinds of heat source models show that the double ellipsoid heat source model cancapture the welding thermal process of TA2/AZ31B TIG welding-brazing. Thesignificant difference of thermal conductivity coefficient between AZ31B and TA2results in the discriminating distribution of isotherm in base metals. To be specific, theisotherm in titanium is more intensive that that of titanium. The heat affected zonelocated in the narrow fusion area between weld bead and AZ31B base metal, which is inagreement with the location of fusion line in simulation results. This is because heat isnot easy to accumulate due to the rapid heat transfer in magnesium alloy.②Both of the increase of welding current and decrease of welding speed, willraise the maximum temperature during welding, which can be ascribed to the increaseof heat input that allows more heat input within unit length of weld bead. On the otherhand, when heat input keeps constant, the effect of welding current and speed ontemperature field distribution is different. That is, the increase of welding current playsa dominant role in the increase of maximum temperature, and the reduction of weldingspeed mainly affect the distribution of isotherm which becomes narrower.③Due to the asymmetry of overlap joints and dissimilar base metals, the stress distribution during and after welding is complicated. During welding process, both ofthe value and amplitude of variation of stress are small, and residual stress eventuallyforms in the cooling stage of welding. The longitudinal residual stress is obviouslyhigher than transverse residual stress and stress in thickness direction. The peak residualstress value appears in the heat affected zone on both sides of weld bead, and theresidual stress shows a sharp change in the edge of weld bead on the titanium side, fromtensile stress status to compressive status. The location of large residual stress, bothtensile and compressive, is in good agreement with the fracture locations in tensile test.In addition, angular distortion and transverse shrinkage appeared after welding, and themagnitude of both is larger in titanium than that of magnesium alloy. The difference ofthermal physical and mechanical properties between dissimilar base metals imposesignificant influence on the distribution and magnitude of both residual stress anddistortion of weldments.④The distribution of residual stress and magnitude of compressive residual stresswithin weld-bead of weldments of different weld-bead length is similar, while thecompressive residual stress on both base metal rise with the increase of weld-beadlength. In addition, the deformation in thickness is larger,which means the bendingdeflection is more serious. With the increase of welding current, magnesium alloyseems to be influenced more than titanium. To be specific, the location of peak tensileresidual stress slightly moves away from the weld-bead and peak value reduces to somedegree, and the distribution area of tensile stress expands. The maximum compressivevalue in weld-bead goes up from120.5MPa to135.3MPa.⑤By adjusting the processing parameters of TIG welding-brazing, the tensilestrength of tested joints can reach193.5MPa. Comparing the calculated results andexperimental measurements of residual stress, it can be found that calculation results inthe tested areas are in good agreement with experimental values, in the distribution andmagnitude of residual stress and deformation. It can be concluded the finite elementmodel established in the present study in effective in prediction of residual stress anddeformation of TIG welding-brazing and can be useful reference when choosingwelding process parameters.