Study On Interfacial Microstructure And Process Of Aluminum Alloy/Galvanized Steel Welded By CMT

Automobile lightweight is one of the effective measures for energy conservation and emission reduction.Adopting aluminum alloy materials to substitute for some steel components can effectively lighten the weight of automobiles.However,aluminum alloys are not complete replacement for steel materials due to the thermal expansion coefficient,forming and cost,etc.It made sense to weld the aluminum alloy with the characteristics of low density,corrosion resistance and the steel material which is easy to form.Steel/aluminum dissimilar welding is of great significance for achieving light weight vehicles.This paper is a systematic research on the microstructure and mechanical properties of the joints of 5052 aluminum alloy and CR340LA galvanized steel sheet with CMT welding-brazing method under different process conditions.Trough metallographic observation,X-ray diffraction,energy dispersive spectrum and fracture analysis,the effects of welding parameters and flux component on joint properties and interface morphology were proved;the formation mechanism of intermetallic compounds and the influence factors of intermetallic compounds thickness were analyzed.The results show that the joint of 5052 aluminum alloy and CR340LA galvanized steel welded by CMT fusion-brazing method is well formed.The aluminum alloy can be fully wet and spread on the surface of the galvanized steel sheet,and perform the typical characteristics of the welding-brazing joint.Studies have shown that the joint is mainly divided into four parts:weld zone,interface layer,Zn-rich zone and heat affected zone?HAZ?,respectively.The intermetallic compound in the interfacial layer has a stratification phenomenon,and an intermetallic compound FeAl is formed on the steel side,Fe₂Al₅ and FeAl₃ are produced on the aluminum side.The microhardness of the various parts of the welding-brazing joints are obviously different.There is softening phenomenon in heat affected zone with low microhardness.There are many brittle intermetallic compounds in the interface layer,and the microhardness is the highest.The microhardness of weld is higher than that of the aluminum alloy base material.In the tensile test,the joint fracture occurred in the heat affected zone on the aluminum alloy side.Under the experimental conditions,the maximum tensile strength can reach 213MPa,and the fracture form is ductile fracture.The welding current and welding speed have a significant influence on the macroscopic forming,wettability,microstructure and mechanical properties of the 5052aluminum alloy/CR340LA galvanized steel joint welded by CMT.The research shows that with the increase of welding current,the weld width increases continuously,the?-Al in the weld zone changes from equiaxed crystal to dendrite,the grain in the heat affected zone is coarse,and the thickness of the intermetallic compound in the interface layer becomes larger and larger.The mechanical properties are gradually reduced,and the fractures occur in the heat affected zone.As the welding speed increasing,the microstructure of the weld zone gradually changes from slender dendrites to elliptical equiaxed crystals.The coarseness of the grain in the heat-affected zone is reduced,and the thickness of the intermetallic compound layer in the interface layer is constantly narrowed.The fracture occurs in the heat affected zone.Through studying and analyzing the process parameters of5052 aluminum alloy/CR340LA galvanized steel sheet joint welded by CMT,it has found that the weld formation,wettability,microstructure,the thickness of intermetallic compound layer and mechanical properties of the joint reached the optimum under the experimental conditions.The addition of alloying element Ni has a certain influence on the macroscopic morphology,wettability,microstructure and mechanical properties of 5052 aluminum alloy/CR340LA galvanized steel sheet joint in CMT.After the addition of Ni element,the weld is uniformly formed and the wettability is improved.It is considered that Ni element hinders the diffusion of Fe atoms,thereby restrain the growth of Fe-Al intermetallic compounds.The thickness of the intermetallic compound layer is remarkably lowered,and two intermetallic compounds of flocculent?Fe,Ni?₂Al₅ and?Fe,Ni?Al₃ strips are formed.With the addition of Ni element,the mechanical properties of the fusion-brazing joint also changed,and the microhardness of the interface layer and part of the weld zone increased.