Effects Of Sc And Zr Addition On The Microstructure And Mechanical Properties Of Al-Mg Alloy Welded Joint

Aluminum alloy has great potential and advantages in the lightweight research of automobile and other fields.Welding is a common material connection technology for aluminum alloy,but the performance of welded joint needs to be improved due to the surface oxidation and high coefficient of linear expansion of aluminum alloy.It is a very effective method to improve the welding performance of aluminum alloy by adding Sc and Zr in the welded joint.In this paper,Al Mg alloy,which is widely used in automobile field,is taken as the research object.Melt inert-gas arc welding(MIG)and friction stir welding(FSW)are used in experiment.To study the effect of the compound addition of Sc and Zr on the microstructure and mechanical properties of MIG and FSW welded joints.Finally,the two welding methods are compared.The results show that:(1)The basic morphology of MIG welded joint is symmetrical on both sides.The weld zone is typical as cast structure,which is the weakest part of mechanical properties of MIG welded joint.The range of heat affected zone of MIG welded joint decreases with the increase of Sc content.When 0.05%Sc is added into the alloy,the softening zone has been reduced obviously.Due to the high average temperature of MIG weld pool,Al₃(Sc,Zr)is dissolved into the aluminum matrix and difficult to precipitate,so that the MIG weld zone does not have obvious dispersion strengthening effect due to the composite addition of Sc and Zr.The MIG welded joint was kept at 250°C and 300°C for 24 hours.The TEM analysis of the weld zone showed that the precipitation degree of Al₃(Sc,Zr)was improved,and the precipitation could improve the hardness of the weld zone.(2)FSW welded joint is funnel-shaped,and the left and right sides are asymmetric.The nugget zone is a fine equiaxed grain structure,and the thermo mechanical affected zone is the weakest part of the mechanical properties of FSW welded joint.The compound addition of Sc and Zr can restrain the heat affected zone of FSW and greatly reduce the softening zone.Because the peak temperature of FSW nugget zone is 0.7-0.9times of the melting point of the alloy,Al₃(Sc,Zr)in the weld zone can be better precipitated without post weld heat treatment,which plays the role of dispersion strengthening,so as to improve the strength and hardness of FSW joint.(3)In the heat affected zone,the effect of Sc and Zr restraining recrystallization of FSW welding is better than that of MIG welding,so that the softening zone of FSW welding is narrower than that of MIG welding.The average grain size of FSW nugget zone with the same Sc content is much smaller than that of MIG weld zone,and the hardness,strength,elongation and welding coefficient of FSW nugget zone are higher than those of MIG weld.The higher the Sc content,the higher the hardness,tensile strength and yield strength of FSW nugget zone compared with MIG weld zone.