Study On Sc Microalloying In Al-Zn-Mg-Cu Alloys And The Properties Of Resistance Spot Welding

In light alloy research field,the development of new high-strength aluminum alloy is a frontier innovative technology in the world.There is quite a broad prospect to study the rare earth element microalloying in Al alloys.In this paper,the microalloying of scandium?Sc?in 7075 Al alloy was set as the main content by producing 7075 alloys containing certain scandium content.Metallographic and scanning electron microscopy,room temperature tensile test,hardness and electrochemical test was adopted to systematically investigate the mechanism of Sc in 7075 alloys.Also,the effect of heat-treatment and thermal deformation on microstructure and properties of Sc-containing7075 alloys was revealed.On this basis,the resistance spot welding?RSW?was conduct using the prepared Sc-containing 7075 alloys.The evolution mechanism of different Sc contents on microstructure and mechanical properties of 7075 alloys RSW joints was proved.Furthermore,a new three-sheet RSW process of aluminum alloy with Al-Sc intermediate alloy as the intermediate layer was developed innovatively,and the microstructure of joints were analyzed in depth.The main conclusions are as follows:The microstructure segregation in 7075 alloy casting ingot could be eliminated by homogenization,moreover,the coarse secondary phase at the grain boundary was dissolved,and the microhardness of the alloy was reduced.After thermal deformation,solid solution and aging treatment,a large number of fine and dispersed aging phases were precipitated in the grains and grain boundaries,which greatly improves the mechanical property of the alloy.The addition of Sc can significantly reduce the grain size of 7075 alloy,stabilize the substructure and restrain the recrystallization after hot deformation,thus further improving the mechanical properties of the alloy.In addition,Sc addition in 7075 alloy can not only reduce the corrosion current density,but also improve the impedance of the alloy in the corrosion environment,led to an improvement of the corrosion resistance of 7075 alloy.The difference of Sc content had a great influence on the grain type and size of RSW joints of 7075 alloys.The grains in the nugget of Sc-free 7075 alloy was composed of thick and interlacing dendrites,and the grain size was 51.51±5.74?m.With the increase of Sc content,a sharp decline in the grain size to 5.13±0.77?m was observed.Because of the dissolution of the strengthening phases,the microhardness of the nugget is the lowest in the RSW joint.Meanwhile,the amount of brittle phase was reduced due to the addition of Sc,and the tensile shear strength of the joints was increased from 2816.82±302.89 N when without Sc to 3641.69±99.52 N when with 0.4 wt.%Sc.The RSW joints mainly underwent a ductile–brittle mixed fracture mode.All the fracture locations were in the nuggets.Due to the oxide film generated on the surface and the wide solidification temperature range of Al alloy,splash,shrinkage hole and thermal crack were observed in the RSW joints of Sc-containing 7075 alloys.Three-sheet RSW process of 7075 alloy with an Al-Sc intermediate alloy as intermediate layer was studied by means of range analysis and variance analysis of orthogonal test.The effects of parameters on the joints were studied.In order to obtain a larger nugget size,higher shear strength and smaller warpage angle as the optimization target,the optimal parameters of three-sheet RSW were 24 kA as welding current,6 cyc as welding time,and 2.4 kN as electrode pressure.Through the microscopic analysis to the optimized spot welding samples,it was found that after the use of Al-Sc alloy as the intermediate layer,the RSW fusion zone was filled with a large amount of Al₃Sc particles,and many grains grew with Al₃Sc particles as nucleating particles,resulting in smaller grain size.However,when a larger nugget was obtained,the overall composition of the fusion zone is not uniform,resulting in microstructure differences between the center and the edge of the RSW nuggets.