Study On Microstructure And Properties Of Friction Stir Welding Interface Of Steel And Aluminum Dissimilar Materials

Aluminum alloy has the characteristics of high specific strength,good corrosion resistance and low density,and has partially replaced steel for automobile manufacturing.In order to meet the requirements of lightweighting new energy vehicles,aluminum alloys are increasingly used in automobiles,and welding of steel/aluminum dissimilar materials has become one of the key technologies to be solved.However,due to the huge differences in the physical properties and chemical compositions of steel and aluminum,the traditional arc welding technology has been difficult to meet the performance requirements of steel-aluminum welded joints.In recent years,Friotion Stir Welding?FSW?has shown unique advantages in aluminum alloy welding.The use of FSW technology to solve the welding problem of steel/aluminum dissimilar metals has become a hot and difficult point in the field.In this paper,3.0 mm thick 6005A-T6 aluminum alloy and 3.0 mm thick automobile sub-frame are lap jointed with S420MC steel plate by friction stir welding technology,and the weld microstructure,joint microstructure and mechanical properties are analyzed and tested.The effects of different agitating needle lengths,welding parameters?rotation speed and steel side pressing amount?on the strength of steel-aluminum lap joints were studied,and a method of plasma spraying Zn powder at the steel-aluminum lap joint was proposed to suppress the interface.A method of forming an intermetallic compound to improve the performance of a steel/aluminum stir welding joint.The experiment shows Large and deep holes are produced at the cross-section of the joint with a needle length of 2.7 mm,and the cross-section of the joint with a needle length of 3.0 mm is well formed,and no hole defects are generated.At the steel/aluminum interface of the two needle length welds,the steel side metal"flash"appears and extends into the aluminum matrix to form a mechanically snapped joint.Fe-based particles of different sizes in the aluminum side base material are dispersed in the aluminum matrix.At the interface,the mixed microstructure of steel and aluminum is laminated.Through the analysis of the interdiffusion results of the two elements at the steel-aluminum interface,the metallurgical reaction of the two elements of steel and aluminum occurred at the interface,forming a joint of"metallurgy+machinery".As the length of the mixing pin increases,the average tensile shear load of the corresponding joint increases.When the length of the stirring needle is 3.0 mm,the average tensile shear load of the joint is the largest,the maximum tensile shear load reaches 4023 N,and the joint fracture position is at At the corresponding interface,the aluminum side fracture has an upward convex shape,and the steel side has a concave shape.With the increase of welding feed rate,almost no Fe-based particles appear on the aluminum side,and the steel side metal"flash"on the advancing side and the retreating side at the interface becomes larger,and there is a flow line formed by plastic deformation of the aluminum alloy;when the feeding speed is At 150 mm/min,the average maximum shear load reaches 4307 N.When the feed speed is 150 mm/min and the rotation speed is1400r/min,controlling the amount of the stirring needle to the steel side??h?will also affect the load of the joint.When the?h is increased from 0.1 mm to 0.2 mm,the joint is The load increased from 4639 N to 4826 N.Under different welding parameters,Al,Fe,AlFe₃,AlFe and Al₁₃Fe₄ intermetallic compounds were produced at the joint interface.According to the experimentally confirmed optimum welding process parameters,the effect of spraying different thickness of Zn layer on the microstructure and properties of the joint at the steel-aluminum lap joint was studied.When the thickness of the sprayed Zn layer is 0.4 mm,the steel-aluminum joint can be effective.For metallurgical connection,the average thickness of the interface reaction layer is 88?m,and the maximum average tensile load of the joint is 3539N.Sprayed Zn layer steel/aluminum"metallurgy+mechanical"combined friction stir welded joint,the average thickness of the reaction layer at the joint interface is reduced by 12?m compared with the joint without Zn element,and the average maximum shear load reaches 5628 N,which is the same process.The joint without spraying Zn under the parameter is increased by 17%,indicating that the addition of Zn element can greatly improve the joint strength.The same intermetallic compound is produced at the interface of the two joints,but the content of intermetallic compounds with Zn layer is significantly less than that of the non-Zn layer.In summary,the intermetallic compounds formed during the welding of steel and aluminum dissimilar metals will affect the mechanical properties of the joint.Too much intermetallic compounds will affect the strength of the joint.On the contrary,the joints cannot achieve effective metallurgical bonding and reduce the strength.