Study On MIG Welding Of Dissimilar Metals Of SYG960E Ultra-high Strength Steel And 6082 Aluminum Alloy

To deal with the world energy crisis more effectively,and to achieve the goal of energy-saving and emission reduction,the lightweight of automotive and other transportations has become an inevitable trend.High strength steels（ultra-high strength steel）are major auto lightweight material,followed by the aluminum alloy,magnesium alloy,composite material and plastic,etc.With low density,high specific strength,excellent thermal conductivity and electrical conductivity,strong corrosion resistance,good processability and easy to recycle,the aluminum alloy was regarded as the more promising automotive lightweight material.It is the most effective and directive way to make introduction of aluminum alloy parts into automotive with steel structures for reducing the weight of the automotive body.However,due to the large differences of physical and chemical properties between ultra-high strength steel and aluminum alloy,their joinability became very poor.Therefore,it has important theoretical significance and application value to conduct the study of MIG welding of dissimilar metals between ultra-high strength steel and aluminum alloy.In this paper,the microstructures and mechanical properties of MIG welded joints of SYG960 E ultra-high strength steel and 6082 aluminum alloy,and effects of welding parameters and alloying elements on them were studied.The results show that the MIG welded joints with Al-Mg and Al-Cu welding wires had the welding-brazing characteristics.The joints are mainly composed of interface area（IZ）,weld zone（WZ）,bond zone（BZ）.The interface zone of steel/Al joint has a double-layer structure consisting of needle-like Fe4Al13 layer at aluminum side and lath-shaped Fe2Al5 layer at steel side.Compared with that with Al-Mg welding wire,the interface intermetallic compound（IMC）layer thickness with Al-Cu welding wire decreased and interface IMC layer was uniform and compact without obvious cracks,which is related to the Cu restraining the growth of interface IMC layer,lowering its hardness and brittleness and improving its crack resistance.The weld zones with Al-Mg and Al-Cu welding wires consist mainly of α-Al and β-Al3Mg2,and α-Al and Al2 Cu,respectively.Compared with the aluminum alloy base metal,the α-Al grain of the welds was refined.The microhardness distribution of the steel/Al joints is not uniform,and the highest hardness values of the joints appeared in the interface zones due to containing brittle and hard intermetallic compounds.The interface zone hardness（424.2 HV）with Al-Cu welding wire is lower than that（447.1 HV）with Al-Mg welding wire,being mainly attributed to the Cu lowering the hardness and brittleness of Fe-Al IMCs.The joints fractured at the interface between steel and aluminum during the tensile test.Thus,the interface zone was the weakest zone of MIG welded joints of ultra-high strength steel and aluminum alloy.The welding process factors have significant effects on the weld appearance,microstructure and mechanical properties of the MIG welded joints of ultra-high strength steel and aluminum alloy.It is beneficial to select the appropriate welding direction,the angle of the torch and the groove as well as the position of the welding wire for improving the weld appearance and the weld quality of MIG welded steel/Al joints.The influences of welding parameters（welding current and welding speed）on the interface zone structure and the joint strength of steel/Al joints are mainly due to the changes of welding line energy and microstructure of the interface zone,respectively.When the welding line energy was low,the interface layer was thin and easy to generate incompleted joining defects,which can obviously reduce the tensile strength of the joint.With the increase of the welding line energy,the thickness of interface zone IMC layer increased,incompleted joining defects of the interface reduced,and the tensile strength of steel/Al joint was improved.However,the Fe2Al5 and Fe4Al13 intermetallic compounds will increase obviously due to the excessive welding line energy,which can also largely reduce the tensile strength of the steel/Al joint.The interface zone thickness and cracking sensibility of joints decreased and the tensile strength of joints was improved with Al-Cu welding wire compared with those with Al-Mg welding wire,which is related to the Cu restraining interdiffusion of Fe and Al atoms and lowering hardness and brittleness of Fe-Al IMCs.The tensile strength of MIG welded steel/Al joints can reach 128 MPa under the condition of welding current of 100 A,welding speed of 50cm/min and E=1520J/cm.The interface intermetallic compound（IMC）layer growth mechanism of steel/Al joints and the growth model of the interface IMC layer under the effect of alloy elements were studied,and the effect of alloy element Cu on the interface IMCs layer growth mechanism of steel/Al joints was revealed.With the Cu element dissolving into the phases of Fe2Al5 and Fe4Al13,the Cu atoms partialy substituted of Fe atoms in the Fe-Al binding,and a small amount of Al-Cu binding and（Fe,Cu）4 Al13 and（Fe,Cu）2Al5 formed,lowering the thickness of the interface IMCs layer,the brittleness of the intermetallic compound and the crack sensitivity,which improves the performance of the steel/Al joints.Through further quantitative analysis of the Cu element,when the content of Cu maintained at 6wt%,the steel/Al joints had higher mechanical properties.Continuing to increase the content of Cu,the steel/Al joints will not improve performance,and inversely the joints will get embrittlement due to excessive Al₂Cu.