Investigations On Stir Friction Welding Of 304 Stainless Steel/6061 Aluminum Alloy With Al-Si Intermediate Layer

In this experiment,the 3mm thick 6061 aluminum alloy and 304 stainless steel were joined by friction stir welding(FSW)under the Al Si5% intermediate layer.The effect of experiment parameters on the morphology of weld,the microstructure of interface,the tensile and shear strength of the joint and the distribution of elements were investigated.And,the temperature and residual stress distributions of joint in different thickness of AlSi5% intermediate layer were also measured.At the same time,the interfacial connection mechanism of aluminum/stainless steel friction stir welding with AlSi5%intermediate layer was explored through the analyze of joint fracture behavior.The equipment of friction stir welding was reformed by ourself.The mechanical properties,the characteristics interface and the element distribution of joints in different process parameters(thickness of intermediate layer,rotation speed,welding speed,pin offset)were investigated in this paper.The study shows that with the thickness of AlSi5%intermediate layer increased,the cracks and unbounded defects at interface were reduced,and the number and size of steel cuttings in aluminum alloy weld seam increased at the same time,the strength of joints was first increased and tend to stabilize.When the thickness of the AlSi5% intermediate layer is 0.3mm,the joint strength is highest the joint breaks in the heat affected zone of the aluminum alloy.With the increasing of rotating speed,the bending degree of the aluminum/stainless steel interface was increased,and the number and size of the steel cuttings in aluminum alloy weld seam were decrease simultaneously,the strength of joint was rapidly rises firstly,then falls rapidly and keep stabilizes at last.The joint strength reaches a maximum value at rotating speed 660 rpm.With the increasing of welding speed,the welding heat input were decreased,the number and size of steel cuttings in aluminum alloy weld seam were increased.The tensile strength of welded joints increases first and then stabilizes,the strength of joints first increase rapidly,then tend to stabilize.The maximum tensile strength of joint was obtained at the welding speed of 44 mm/min.With the increase of the offset of the stirring needle,the number and size of the steel cuttings in the aluminum alloy welds were increased at the same time.When the pin offset is 0.4 mm,the strength of joints have little fluctuation.The joint strength reaches a maximum value.Suitable process parameters were obtained through the experiment: stainless steel is located on the advance side,the thickness ofAlSi5% intermediate layer is 0.3mm,the rotating speed is 660 rpm,the welding speed is44mm/min,the offset is 0.4mm,and the penetration depth is 0.3mm.The joint tensile strength was 37.2MPa increased compared with the joint tensile strength without AlSi5%intermediate layer.Afterwards,the FSW joints were heat-treated at 525°C for 20 min and aged at 160°C for 8 hours.The joint tensile strength increased to 151.4 MPa and fractured at the aluminum alloy/stainless steel interface,reaching 73.7% of the aluminum alloy base metal tensile strength.Based on the process study of FSW,the temperature field and residual stress of the FSW joints with different thicknesses were investigated.With the thickness of AlSi5%intermediate increased,the peak temperature of the welding thermal cycle at the joints was reduced significantly.The increase of Si content in the weld would inhibited the formation of Fe-Al intermetallic compounds.The bending degree of the interface,the amount and the size of steel grains in the weld also increased with the welding temperature decreased.A composite weld with a coefficient of thermal expansion between the aluminum alloy and the stainless steel was formed as the steel particles friction into the aluminum alloy weld.This phenomenon slows down the transitional gradient of the thermal expansion coefficient between the dissimilar materials,and the residual stress of the joints is significantly reduced during the cooling process.As the thickness of the AlSi5% intermediate layer increases from 0mm to 1mm,the peak temperature at joint interface decreases 28°C,and the residual stress at the interface decreases 53 MPa.However,when the intermediate layer is too thick,large steel appear in the aluminum alloy weld near the interface.The cracks would like to appered at the bounday of the large steel grain,which can leading to reduce the joint strength.