Research On CMT Brazing-welding Technology Of Aluminum Alloy And Steel

In response to the call for energy conservation,emission reduction and environmental protection,the lightweight production of vehicles and ships has become a development trend.The aluminum/steel composite structure has the advantages of high strength and low density,which can lead to weight loss.CMT arc fusion brazing technology can accurately control heat input,which can effectively improve the metallurgical bonding performance of aluminum/steel.In this thesis,2mm thick 6082 aluminum alloy and Q235B steel plate are selected,and Al Si12 flux welding wire is used as the filler material to carry out the butt CMT arc welding test of aluminum/steel.After obtaining the optimum parameters,the specimens were subjected to post-weld heat treatment at 300°C.The effect of different annealing holding time on the microstructure and properties of the joint was studied.The butt welding of Q235B steel plate and 6082 aluminum alloy with different thicknesses of copper plating layer was carried out.Zr powder was pre-coated between aluminum and steel to study the effect of Zr element on the macro-forming,microstructure and properties of lap joints.The research results show that in the CMT arc fusion brazing test of aluminum alloy and copper-plated steel,the aluminum alloy is in the copper-plated steel.The wetting spread on both sides is good,and the welding seam is beautifully formed.Excessive welding heat input will promote the formation of fine needle-like Al-Fe-Si ternary compounds in the weld,and at the same time make Fe and Cu elements diffuse into the weld,increasing the thickness of the Fe-Al intermetallic compound layer.With the increase of heat input,the Al-Cu and Al-Fe compounds in the weld increase.The copper plating layer improves the wettability of aluminum on the steel surface,and the weld is formed beautifully.The copper plating layer reduces the formation of Al-Fe-Si phase in the weld.The copper plating layer can inhibit the formation ofθ-Fe（Al,Si）₃phase and reduce the thickness of the intermetallic compound layer.At the same time,it will be solid-dissolved into the interface layer to generateτ₅-Al_7.2（Fe,Cu）_1.8Si phase.If the thickness of the copper plating layer is too large,a large amount of Al₂Cu phase will be formed in the weld.An appropriate thickness of copper plating is beneficial to improve the corrosion resistance of aluminum/steel joints.The copper plating layer can hinder the diffusion of Fe atoms,so that the high-potential Al-Fe-Si ternary compound in the weld is reduced until it disappears,and the microgalvanic corrosion effect in the weld is weakened.If the thickness of the copper plating layer is too large,the Al-Cu compound in the weld will increase,the potential difference of the micro-battery composed of various substrates in the weld will increase,and the corrosion resistance of the joint will decrease.When the thickness of the copper plating layer is 10μm,the corrosion resistance of the joint optimal.In the CMT arc welding test of lap joint of aluminum alloy and steel.After the addition of Zr powder,the grains of the weld are significantly refined.Zr powder promotes the fluidity of the molten pool,reduces the generation of cracks and slag inclusions in the weld,and inhibits the growth of the intermetallic compound layer.The topography of the layer changes from the original blocky prism shape to a small wave-like protrusion,which increases the pinning effect on the weld.The addition of Zr powder transforms the brittle Fe-Al intermetallic compound layer into a ductile Fe-Al-Zr intermetallic compound layer,which improves the toughness of the interface layer.The joints were fractured at the interface layer before and after adding Zr powder.The maximum tensile force of the joints without Zr powder was 3244N,and the maximum tensile force of the joints after adding Zr powder reached 3882N,both of which were brittle fractures.When Zr powder is not added,the fracture is mainly composed of Fe,Al,Al_0.5Fe₃Si_0.5phases,and after Zr powder is added,the fracture is composed of Fe,Al,Al_0.5Fe₃Si_0.5,Zr Fe_3.3Al_1.3phases.