Study On Resistance Spot Welding Of Steel/aluminum Dissimilar Materials

With the increasing of serious problems such as environmental pollution and energy crisis,the challenges faced by the automobile and railway vehicle industry are intensifying day by day.At present,the lightweight of automobiles and railway vehicles is the main countermeasures of energy saving and emission reduction,so the most direct and effective way to achieve vehicle lightweight is to increase the use of lightweight materials(aluminum alloy,etc.).The welding of steel/aluminum dissimilar materials has become an urgent problem to be solved.In physical and chemical aspects,there are great differences between steel and aluminum,which make the weldability of steel/aluminum dissimilar materials extremely poor,and the mechanical properties of welded joints are difficult to meet the actual use standards.The problem of steel/aluminum weldability is one of the scientific and technological problems that restrict the progress of vehicle lightweight technology.Resistance spot welding is a widely used welding technology.Therefore,the research on resistance spot welding of steel/aluminum dissimilar materials has important practical application value and theoretical significance.Firstly,resistance spot welding joints of SUS301L stainless steel(16Mn low alloy steel)/6063-T6 aluminum alloy dissimilar materials were studied in this paper.According to their mechanical behaviors and microstructures,resistance spot welding joints of stainless steel(16Mn steel)/aluminum alloy were mostly made up of aluminum alloy nugget,stainless steel(or 16Mn steel)nugget and steel/aluminum interface layer,which belonged to fusion-brazing joint.Cellular crystal,cellular dendrite,dendrite and a small amount of equiaxed dendrite were the main crystal structures of aluminum alloy nugget.The stainless steel nugget was mainly composed of columnar austenite.And 16Mn steel nugget was mainly composed of martensite,pearlite and bainite.During resistance spot welding,liquid aluminum alloy wet and spread on the surface of solid stainless steel(or 16Mn steel),and the inter-diffusion of Fe and Al and interfacial reaction occurred.Then Fe-Al intermetallic compound(IMC)layer was formed at the steel/aluminum interface.The steel/aluminum interface layer had a double-layer structure:tongue-shaped Fe₂Al₅layer(near stainless steel or 16mn steel);needle-like Fe Al₃layer(aluminum alloy side).There were two fracture modes(interface fracture and button fracture)in stainless steel(16Mn steel)/aluminum alloy spot welding joints under tensile shear load.Under the test conditions,the crack of stainless steel/aluminum alloy spot welding joints mainly initiated and propagated in the interface IMC layers(interface fracture mode).On the other hand,when the diameter of aluminum nugget was smaller than 5.8 mm,16Mn steel/aluminum alloy joint was the interface fracture mode;when the diameter of aluminum nugget was bigger than 5.8 mm,the crack mainly initiated and propagated along the aluminum nugget and its heat affected zone(button fracture mode).Aluminum alloy nugget and brittle IMC layer were the main reasons for the deterioration of mechanical properties of steel/aluminum joint.The influence of welding technology factors(welding parameters,electrode morphology)on stainless steel/aluminum alloy spot welding joint was studied.When F-type electrode was used,the nugget diameter,indentation ration and IMC layer thickness increased with the increasing of welding current(or welding time).However,with the increase of joint tensile shear load,it first increased and then decreased.In the range of 4 k A-7 k A in welding current or 100 ms-200 ms at welding time,the increase of joint tension shear load was mainly attributed to the increase of nugget diameter.And continuing to increase welding current or welding time led to the decrease of tension shear load,which was mostly owing to the thicker IMC layers and the shrinkage in aluminum nugget.When welding current was 7 k A,welding time was 200 ms and electrode force was 2 k N,the joint nugget diameter,indentation ratio,IMC thickness and joint tensile shear load were 5.4 mm,30.1%,2.3?m and 1.8 k N,respectively.The research results showed that the optimized electrode(round flat end electrode with diameter 10 mm on steel side and spherical face electrode with radius35 mm on aluminum side)was more beneficial to improve the microstructures,properties and surface quality of steel/aluminum joint.When welding current was 13k A,welding time was 300 ms and electrode force was 3 k N,the steel/aluminum spot welding joint with nugget diameter 7.2 mm,indentation ratio 10.9%,IMC layers thickness 1.4?m and tension shear load 3.6 k N was obtained.Compared with the joint using F electrode,the nugget diameter and tensile shear load increased by33.3%and 100.0%respectively,and the indentation ratio decreased by 63.8%.The effect of metal or nonmetal(Cu,Si,Zn,Ti)on stainless steel/aluminum alloy spot welding joints was studied by nano-powder addition method with F electrode.Cu,Si,Zn and Ti all had great influences on mechanical properties and microstructures of joints.Cu and Si not only promoted the wettability of liquid aluminum on the surface of solid steel,but also inhibited the growth of intermetallic compounds at the interface so as to increase the tensile shear load of joints.Zn not only restrained the interfacial reaction,but also improved the properties of the intermetallic layers(forming Fe₂Al₅Zn_0.4),resulted in the improvement of the mechanical properties of joints.Ti refined the grains and formed a new phase(Fe₂Ti)with Fe,which inhibited the formation of IMCs,and improved the mechanical properties of joints.With the addition of 1.51 mg Cu,5.78 mg Si,0.97 mg Zn or 0.62mg Ti powder,the tensile shear load of steel/aluminum joint could reach 3.07 k N,3.55 k N,2.74 k N and 2.68 k N respectively,which was 70.56%,97.22%,52.22%and 44.40%higher than that of the joint without alloying element(1.80 k N).Therefore,the addition of metal(nonmetal)was an effective way to improve the tensile shear load of resistance spot welding joints of stainless steel/aluminum alloy dissimilar materials.The axisymmetric finite element analysis model of stainless steel/aluminum alloy spot welding process was established by ANSYS,and the thermal process of dissimilar materials of steel/aluminum resistance spot welding was studied.The results showed that when welding current was 7 k A and electrode force was 2 k N with F electrode,the temperature at the interface of steel/aluminum spot welding joints reached the maximum(913?)at 200 ms,and the nugget diameter reached the maximum value(5.5 mm),which was consistent with the experimental results(5.4mm).Based on the thermal cycle curve,the reaction mechanism of steel/aluminum interface in stainless steel/aluminum alloy spot welding process was studied.Through the thermodynamic analysis of the growth of intermetallic compounds at the interface,it was found that Fe₂Al₅was formed first,and then Fe Al₃was formed.Finally,the growth mechanism of steel/aluminum interface was discussed.And the action mode of Cu,Si,Zn and Ti and the metallurgical reaction process of steel/aluminum interface layers were discussed.Cu,Si,Zn and Ti effecting the growth of the interface layers should be divided into four stages:melting,dissolution and diffusion,growth and solidification.