Investigation Of Friction Stir Welded Al Alloy To Steel Dissimilar Joints: Interface Characterization And Mechanical Behavior Estimation

Joining aluminum to steel is of great application for weight reduction in automotive manufacture.However,due to poor weldability,it is difficult to obtain good joints by conventional fusion welding.In this paper,friction stir welding?FSW?was used to join6000 series aluminum alloys to advanced high strength steels in lap configuration.The aluminum sheet was placed on the steel sheet,and the pin tip was slightly inserted into the steel sheet to join 1.5-mm AA6082-T6 with 2.0-mm zinc-coated DP800-galvanized.The pin tip was not inserted into the steel sheet,and 1.35-mm AA6022-T43 was welded with 2.0-mm Zn-coated DP800-galvanized or a 2.0-mm aluminized Usibor?1500-galvanized.Interfacial characterization and mechanical properties of three kinds of joints were studied.Iso-strain and Iso-work assumptions were used to model the mechanical behavior of the stir zone of joints.Conclusions could be drawn as followed:Shear strength of the AA6082/DP800 FSW joint reaches the maximum at 1250 rpm and 1250 mm/min,which is 71%of the base aluminum alloy metal.The AA6022/DP800joints with the zinc coating as the middle layer were obtained,the optimized parameters are 2000 rpm,100 mm/min with the shear strength of the joints up to 87%of the base metal.The AA6022/Usibor?1500 joints with the aluminum coating as the middle layer were obtained,the travel speed could reach 4000 mm/min at a rotation speed of 2000 rpm with the shear strength of the joints up to 96%of the base metal.For the AA6082/DP800 FSW joints,in the stir zone,the thickness of the interfacial IMC layer gradually increased with decreasing revolutionary pitch?travel speed/rotation speed?,Al_3.2Fe formed at the Al-rich side and Al₅Fe₂ formed at the Fe-rich side.When revolutionary pitches are 1.0 and 0.5 mm/rev,the average thicknesses of the interfacial IMCs layers are 0.89?m and 1.12?m,respectively.However,in the hook zone,the ones are 0.99?m and 0.98?m,respectively.When the revolutionary pitch is 1.0 mm/rev,the zinc content in the interfacial region is light,Al_3.2Fe formed at the Fe-rich side with a small amount of Al₆Fe formed at the Al-rich side.When 0.5 mm/rev,there is an intermittent but high content of zinc at the interface,which should be the original zinc layer residue.In addition,continuous silicon segregation was found at the Al-rich side interface.The presence of zinc and silicon reduced the thickness of the IMC layer,leading to the formation of an unknown Al-Fe-Si phase.The AA6022/Zn-coated DP800 joints exhibit friction stir brazing characteristics with free of Al-Fe-based IMC at the interface.The original zinc coating below the stir zone entered into the stir zone and reacted eutectic with the Al matrix.An Al_4.01MnSi_0.74IMC layer with an average thickness of 200 nm formed at the interface.The original Zn coating melted below the tool shoulder region as a filler metal.The brazing layer consists of aluminum-zinc eutectic with a thickness of about 10?m.The AA6022/aluminized Usibor?1500 joints exhibit friction stir diffusion characteristics with a 10-nm Al/Fe diffusion layer at the interface.No IMC was found.For the original aluminized layer,there are many microvoids between the particles and the interface.After welding,the thickness of the aluminized layer was thinned by 15?m,the voids between the particles were reduced,the interface defects were eliminated.With no consideration of the microscale or nanoscale IMC layer,the mechanical behaviors of the stir zone modeled by the two assumptions are in good agreement with the experimental results.The difference between two modelings is small,but the Iso-strain assumption gives a closer result,which shows the trend of strain development of aluminum and steel in the stir zone is similar when shear testing.