Investigation On Interfacial Control Of Laser Welded-Brazed Al/Steel Dissimilar Lap Joint

Energy saving and emission reduction is the trend of today’s social development.In the field of automobiles and high-speed trains,aluminum alloys are used to replace some steel materials to form aluminum/steel composite car body structures,which reduce the weight and cost of structural parts,and meet the requirements of energy conservation and environmental protection.However,the thermophysical properties of aluminum and steel are significantly different.During the welding heating process,a large number of brittle IMCs with uneven distribution and excessive thickness are easily generated at the interface,which reduces the mechanical properties of the joint,resulting in the aluminum/steel welded joints not reaching the industrial level.In this paper,2mm thick 6061 aluminum alloy/304 stainless steel lap joints were welded by laser self-fusion brazing.Based on the optimization of fixed laser welding joints,a rotating laser was introduced to control the homogenization of IMCs,and Cu was further added to control the Fe element diffusion in the welding process.thereby reducing the Fe atoms required for the growth of the interfacial Fe-Al intermetallic compound,reduces the thickness of the interfacial IMCs,and improves the tensile properties of the joint.Taking the joint wetting and spreading properties,tensile properties and weld forming as the evaluation criteria,four basic process intervals of welding parameters that have a great influence on the heat source energy distribution are obtained.The evolution law of the interface microstructure of aluminum/steel lap joints under different welding line energies is analyzed.The results show that two kinds of IMCs,η phase and θ phase,mainly appear in the aluminum/steel laser self-fusion brazing joint.,the interface IMCs are thick,the hard and brittle layered η phase is too thick,and cracks are generated at the layered η phase position;when the welding line energy is small,the interface IMCs are thin,but the thickness of the IMCs at the root of the weld toe is less than 1 μm,There is no reliable connection between aluminum and steel;when the welding line energy is 554J/mm,there are no defects such as cracks at the interface,and the tensile properties of the joint are the best.The breaking line load of the joint is 243N/mm.Aiming at the problem of uneven thickness and phase composition of the intermetallic compound layer at the aluminum/steel interface after parameter optimization,a rotating laser was introduced to control the homogenization of IMCs.The temperature field of the aluminum/steel interface was calculated by finite element simulation,and different laser rotating diameters were obtained.The results show that the uneven distribution of interfacial layer thickness and phase composition is mainly caused by the difference of peak temperature in different regions of the interface.The increase of the laser rotation diameter will lead to the decrease of the interface temperature and the existence time of the liquid aluminum alloy,which will reduce the interface reaction rate and time,thereby reducing the thickness of the interface IMCs.If the temperature is higher than the generation temperature of η phase,the composition of the interface phase can be uniform.When the laser rotation diameter is 1.0 mm,the temperature difference at the interface decreases,and the temperatures in different regions of the interface are in the range of η phase generation.Therefore,an interface layer with small difference in thickness and consistent phase composition is obtained,and the fracture line load of the joint is increased to 273N/mm,which is 12.35% higher than that of laser fixed welding.In order to reduce the formation of hard and brittle Fe-Al intermetallic compounds,copper foil was added to the aluminum/steel interface for element control.Combining the interface micro-morphology and element distribution,it is found that the addition of Cu element reduces the aggregation of Fe elements at the interface and reduces the thickness of the interface layer.The solid solution in the IMCs reduces the hardness of the interfacial layer.The chemical potential and Gibbs free energy of the Al-Fe-Cu ternary system were calculated,and the regulation mechanism of Cu on the diffusion of interfacial elements and the growth of IMCs was clarified.It is found that the chemical potential of Fe element is higher in the region with higher Cu concentration,so it promotes the diffusion of Fe atoms to the aluminum alloy molten pool,reduces the aggregation of Fe atoms at the interface,reduces the generation of Fe-Al IMCs at the interface,and fixes the thickness of the copper interlayer for laser welding.When the thickness of the copper interlayer is 10 μm,the line load of the welded joint increases to 297 N/mm,which is 22.22% higher than that of the fixed laser al/steel laser brazing joint.