Study On CMT Arc Melting Brazing Technology And Interface Microstructure Of Aluminum Alloy And Steel

Aluminum alloy is being used in automobile manufacturing because of its high specific strength,good corrosion resistance and plasticity.But for safety reasons,aluminum is unlikely to completely replace steel in automotive components.Therefore,it is of great significance to introduce a certain amount of aluminum alloy with steel as the main body in automobile manufacturing.CMT welding is beneficial to the realization of high quality aluminum/steel welding due to its advantages of low heat input and stable welding process.In this thesis,2mm thick 6082 aluminum alloy and Q235B steel were selected,and Al Si12 flux cored wire was used as filler metal for CMT fusion brazing test.Explore the influence of preset clearance,heat input and Ni element on joint formation,microstructure and properties,and determine the best range of welding parameters.The effects of holding time and annealing temperature on the microstructure and mechanical properties of the joint were studied after heat treatment.Finally,the electrochemical corrosion test of the welded joint was carried out to study the influence of heat input,heat treatment process and Ni element on the corrosion resistance of the jointWhen the preset clearance is controlled at 0.1mm,the wire feeding speed is controlled at 5.5m/min,and the welding speed is controlled between 300 and 350 mm/min,the welded joint with good forming and mechanical properties can be obtained.The weld microstructure is composed of dense and continuous?-Al dendrites and network Al-Si eutectic between dendrites.When the heat input is large,disordered elongated and needle-like Al-Fe-Si compounds will be formed in the weld.When the heat input is small,a single?₅-Al_7.2Fe_1.8Si compound is formed at the interfacial layer.When the heat input is large,the interfacial layer is composed of the serrated?-Fe(Al,Si)₃ phase and?₅-Al_7.2Fe_1.8Si phase.Post-weld heat treatment can refine the grain,promote the uniform diffusion of elements and improve the joint performance.However,too high temperature or too long holding time will increase the thickness of the intermetallic compound layer,prone to crack and other defects,reduce the performance of the joint.The Ni coating can effectively improve the wetting and spreading ability of the molten welding wire and aluminum base metal,and the diffusion of Ni into the weld can avoid the formation of Al-Fe-Si compounds.The intermetallic compound layer consisting of(Fe,Ni)Al₃and(Fe,Ni)₂Al₃ was formed at the groove interface by the 10?m nickel plating layer,and there was a small amount of(Fe,Ni)Al₃ intermetallic compound at the weld toe interface.However,the thin nickel coating at 5?m has no effect on the intermetallic compounds.only a few Ni atoms occupy the position of Fe atoms.The nickel plating layer can reduce the formation of intermetallic compounds in the weld and improve the mechanical properties of the joint.The increase of welding heat input leads to the increase of the thickness of Fe-Al compound layer in the interface layer and the formation of Fe-Al-Si intermetallic ternary compounds in the weld seam,which reduces the corrosion resistance of the joint.After annealing at 300?and holding for 0.5h,the corrosion resistance of the weld is obviously improved.If the holding time is too long or the annealing temperature is too high,the Fe element diffused into the weld will increase,forming the iron-rich phase and reducing the corrosion resistance of the joint.The formation of intermetallic compounds can be reduced and the corrosion resistance of the joint can be improved by 10?m nickel plating.The thin nickel coating at 5?m has limited inhibition on the formation of Fe-Al compounds,and the electronegativity of Ni and Fe is similar,which reduces the corrosion resistance of the joint when it diffuses into the welding seam.