Controlling Interface Products During Friction Stir Welding Steel/Aluminum And Its Effect On Mechanical Properties

Steel/aluminum joints are widely used in the automotive industry.However,as huge difference in physical and chemical performance between steel and aluminum,and the hard and brittle intermetallic compound in welded joint,welding between the steel and aluminum was very hard to be carried out.In this paper,friction stir lap welding(FSLW)of 6061 Al alloy and IF steel was successfully achieved,and the morphology,interface products and mechanical properties of the lap joints were discussed in detail.Furthermore,this paper reveals the relationship between the formation of intermetallic compounds and mechanical properties under different welding process conditions.This study includes following four parts:(1)Under various welding parameters FSLW Al/steel was conducted.At the rotating speed of 800rpm and the traverse speed of 100mm/min,pin plunge depth of 0.1mm,defectfree FSLW joints were obtained.The average fracture load is 3.40kN,and the fracture interface is cleavage fracture with fracting at the steel/aluminum interface.The main intermetallic compounds were confirmed to be Al13Fe4 and Fe2Al5.With the increase of heat input,the thickness of the bonding layer increasd and the mechanical properties of the joint increases and then decreases.The hardness distribution of the joint correspond to the microstructure of the joint.Mixed stir zone(MSZ)consists of thin Fe-Al intermetallic layers and fine recrystallized ?-Fe grains.(2)High melting point metal of Ni-foil was added as a interlayer between steel/aluminum.Broken Ni inhibits Fe and Al atom diffusion in each other,generating Al3Ni2.The main products of the combing layer is still Fe-Al intermetallic compound.The inhibition of Ni Larger thined the hard and brittle intermetallic compound layer and generated FeAl.Fe-Al intermetallic compounds disappeard in the MSZ instead of(Fe,Ni)solid solution.The hardness value of the combing area was significantly reduced,and the tensile fracture position was in the steel matrix,with the average fracture load being 4.27kN.The peeled fracture belongs ductile and brittle mixed fracture mode.(3)Nb-foil with high melting point metal was added as a interlayer into steel and aluminum.Compared with the addition of Ni foil,the thickness of combing layer increased,and the tensile fracture is in steel matrix.XRD analysis fracture surface shows the new product of NbFe2 and NbAl3 at the interface,and MSZ consists of a strip of(Fe,Nb)solid solution/NbFe2 and crystalline ferrite.A small amount of Al atoms are dissolved in the MSZ,which has a solid solution strengthening effect.(4)Adding lower melting zinc foil as the transition layer into steel and aluminum FSLW joint would significantly improve the properties of the joint,and tensile shear fracture located in the steel matrix.During welding process,the Zn layer is heated and melted,the liquid flow of the zinc is pushed out carring with the surface oxide film and surfaces inclusions of steel plate and the aluminum plate,which makes the steel/aluminum bonding more intimate,improveing the mechanical properties of the joint.The best transition layer material for FS W steel/aluminum is Zn foil.