Study On CMT Welding Technology Between Q235 Steel And 5052 Aluminum Alloy

Currently,with the development of aluminum and its alloy that applied in the automotive industry,the connection of heterogeneous metal of aluminum and steel has become a hot issue in the field of welding.In this thesis,based on the lightweight structure manufacture of aluminum alloy and Q235 steel in automotive industry,the 1mm thin plate was lap jointed with cold metal transfer（CMT）fusion-brazing method.The influences of welding speed,wire feeding speed,gas type and flux on the macroscopic morphology,microstructure and mechanical property of joints were investigated.Experimental results showed that,under the Ar atmosphere,the best joint of Al/Fe can be obtained with a welding input of the welding speed 0.45m/min,the wire feeding speed4.5m/min,the welding voltage 12.5V,as well as the welding current 71A,respectively.The performance of the joint achieved its maximum tensile strength of 128MPa,with the hardness of the metal transition layer was 179.4HV and the diffusion layer thickness was about 5.04μm.Under the CO₂ atmosphere,the best joint of Al/Fe can be obtained with a welding input of the welding speed 0.45m/min,the wire feeding speed 5.3m/min,the welding voltage 12.7V,as well as the welding current 81A,respectively.The performance of the joint achieved its maximum tensile strength of 133MPa and microhardness of194.6HV,and the diffusion layer thickness was about 5.68μm.Under the CO₂ protective gas,the fusion zone of the weld was mainly a small amount of Mg₂Al₃ and Al,α-Al,and Zn element was not completely evaporative burning,a small amount of Zn existed in the brazing interface beside the Al area.The coarse Fe₂Al₅ phase was observed in the brazing interface along the aluminum side.When CO₂ was used as the protective gas,the tensile strength and hardness of welded joints will be enhanced and the softening problem of aluminum joints will be improved.In the welding process of non-galvanized steel Q235 and 5052 aluminum alloy,under the standard parameters,the solder had the better spreadability under the action of the flux KAlF₄+Zn+Sn,wetting angle was smaller.Using flux KAlF₄+Zn can inhibit the growth of Fe-Al intermetallic compounds and reduce the content of the hard brittle phase and improve the toughness and plasticity of welded joints.The hardness of welded joint was the highest when using the flux KAlF₄+K₂SiF₆,the maximum hardness can reach 211.4HV.When the flux KAlF₄+Zn+Sn was used,weld the welding joints by adjusting the welding parameters,the maximum tensile strength can reach 134.77MPa when the welding parameters were current 71A,voltage 12.5V,welding speed 0.3m/min,wire feeding speed 4.5m/min.The hardness of the joint was also significantly higher than that of other welding parameters,the maximum hardness of the transition layer can reach 454.5HV.Through the microstructure of the brazing area,it was observed that some intermetallic compounds were serrated in the weld,theα-Al matrix was interlaced with intermetallic compounds,sawtooth crisscross combined structure can enhance the interface bonding force of aluminum and steel and reduce the tendency of crack propagation.