Microstructure And Properties Of 5052 Aluminum Alloy/304 Stainless Steel Rotary Friction Welded Joint

Aluminum/steel composite structures have a promising application in lightweight structures.However,joining aluminum to steel has been a difficult and hot spot in the welding filed due to the large difference in their thermo-mechanical properties and the lower solid solubility of Fe in Al.Rotary friction welding,a solid-state welding method,has outstanding advantages of high welding quality and welding efficient,which is especially suitable for the welding of rotary structures.However,the unique rotary motion of rotary friction welding would result in the inhomogeneity of friction heat along the radius direction,and further prompts the inhomogeneous distribution of microstructure and mechanical properties.This inhomogeneity has become one of bottlenecks restricting the application in key filed.Thus,systematically studying and optimizing the microstructure and mechnical properties of aluminum/steel rotary friction welded joints has an important significance in engineering application.In this paper,5052 aluminum alloy?AA5052?and 304 stainless steel?304SS?were applied as base metal,and rotary friction welding was used to achieve high quality connection of aluminum to steel.The effect of welding process?friction pressure,burn-off length and upset pressure?and heat treatment process?temperature and time?on microstructure and joint properties were systematically studied to obtain the high-quality joints.In addition,the potential defects of joints in edge location were removed and four zones?center,R/4,R/2 and 3R/4?were divided by decreasing the diameter of tensile samples.The inhomogeneous microstructure and properties of joints were further investigated,and the main results were as follows.When friction pressure was 90 MPa,burn-off length was 1 mm and upset pressure was250 MPa,the ultimate tensile strength and elongation of entire joints were 153 MPa and 6.06%,respectively.After tensile test,the tensile samples fractured along the interface and predominantly presented brittle fracture feature.During tensile test,the propagation of cracks exhibited three different ways as follows,i)along the IMCs/AA5052 substrate interface in center and R/4 locations,ii)between IMCs and the IMCs/AA5052 substrate interface alternately in R/2 and partial 3R/4 locations,and iii)along IMCs/304SS substrate interface in partial 3R/4 and edge locations.The residual stress and storage energy in joints were released after post-weld heat treatment?PWHT?at 250?/20 min,and the ultimate tensile strength and elongation were improved to 161 MPa and 7.69%,respectively.When heat treatment temperature was above350?,the IMC layer thickened and tensile properties decreased with the increase of heat treatment time.By analyzing the inhomogeneous microstructure and properties of joints,the thinner and discontinuous IMC layer distributed in center location,while thicker and continuous IMC layer distributed in R/4,R/2 and 3R/4 locations.The ultimate tensile strength and elongation first increased and then decreased with decreasing the diameter of tensile samples.For tensile samples with?2.875 mm?D/8?,the as-welded joints had 99.51%joint efficiency,and fractured in the HAZ of AA5052.The ultimate tensile strength of as-welded joints in center location was higher than that in R/4,R/2 and 3R/4 locations,and that in R/4 location was minimum.After PWHT at 250?/20 min,the ultimate tensile strength of joint in R/4 location was greatly improved,and the difference of the ultimate tensile strength along the radius direction decreased,which was beneficial for engineering application.After PWHT at 450?,the thickness of IMC layer increased with the increase of heat treatment time.However,the growth of IMC layer,Fe₂Al₅ and Fe₄Al₁₃ deviated from parabolic law,which was attributed to?i?a large number of dislocations,grain boundaries and storage energy in the interface,?ii?linear growth of Fe₄Al₁₃ phase,and?iii?effect of Cr from 304SS.