Research On Process And Mechanism Of Laser Wire Welding-brazing For Al/Steel Dissimilar Metals Based On Interface Control

Facing the increasingly prominent energy problems,making out reliable connection of aluminum/steel dissimilar metal can greatly reduce the weight of parts in the automotive,industrial and other fields to achieve the goal of saving energy.However,the physical and chemical properties of the two kind of alloys differ considerably,and it is easy to produce brittle intermetallic compounds by reaction of them,which deteriorate the joint properties.Therefore,how to control the formation and growth of intermetallic compounds has been one of the focuses of the research.Scholars at home and abroad have shown that controlling the heat input during the welding process and adding alloy elements are effective ways to control the microstructure of the interfacial reaction layer.To some extent,laser welding brazing can control the production of intermetallic compounds during the connection of dissimilar materials for its characteristics of high energy density,small heat input,small heat affected zone and precise control.The 1.5mm thickness of 6061 aluminum alloy and 1.2mm thickness of DP590 dual phase steel were welded by laser melting brazing in butt joint by using different kinds of flux cored wire including pure aluminum wire,Al-Si5 wire,Al-Si12 wire,Zn-Al2 wire and Zn-Al15 wire.The microstructure features of the joint,structure of the interfacial reaction layer and the corresponding strength were researched to elucidate the mechanism of alloying elements in laser brazing of dissimilar metals of aluminum and steel.At first,the Al-Si5 wire was chosen as the filling material of aluminum/steel dissimilar metal laser welding brazing in butt joints and the process parameters were explored.After ideal joints were obtained with good forming performance and mechanical property,the effect of heat input on the interfacial microstructure and mechanical properties was studied by changing the laser power.The result shows that a typical reaction layer is composed of Fe2Al5 layer near the steel side and thin FeAl3 layer near the seam.A small amount of Al8Fe2 Si exists on the edge of FeAl3 layer.With the increase of heat input,the thickness of Fe2Al5 layer increases remarkably,and the volume of FeAl3 phase increases,while the intermetallic layer increases from 2.2?m to 10.9?m.The mechanical properties of the joints has a certain relationship with the average thickness of intermetallic compound layer and the maximum tensile strength is 209.39 MPa with the thickness is about 6.5?m,achieving 68% of the Aluminum Alloy parent material(307MPa).The brittle fracture always occurred at the interface in the joints of different power.The fracture surface is smooth and the main component is Fe2Al5 phase with a small amount of FeAl3 phase distributed on it.Furthermore,the influence of two elements,Si and Zn,on the microstructure of the joint interface was systematically analyzed by using different welding wires with different alloy composition as filling materials.Results show that when using the aluminum wire,the interface reaction layer is made up of two layers,one is tongue-like Fe2Al5 layer,and the other is thin FeAl3 layer.With the content of Si element increases,the structure of the interface reaction layer changes to be consisted of a serrated Al8Fe2 Si layer and a fine acicular FeAl3 layer enclosed in it,while the intermetallic layer thickness reduces from 17.1?m down to 5?m.There is a big difference in the microstructure of interface layer with the zinc based wire.The reaction layer is composed of Fe2(Al,Zn)5 layer and dispersed FeZn10 phases which can form a continuous layer near the steel.When the content of Al elements increased,the thickness of Fe2(Al,Zn)5 layer increased slightly,and the number of Fe Zn10 phases decreased.When the content of Al elements increased,the thickness of Fe2(Al,Zn)5 layer increased slightly,and the number of Fe Zn10 phases decreased,and only dispersed in the Fe2(Al,Zn)5 layer.Especially,the joint with the zinc based wire has the best mechanical properties,and the tensile strength reaches 249.43 MPa,which is 81.24% of the aluminum alloy base metal.Finally,a prediction model for the chemical potential of ternary alloy system is established.The reason for the enrichment of Si in Si-Al-Fe ternary system during interfacial reaction and its influence on the diffusion of Al and Fe elements are explained by thermodynamic calculation.The reason for the enrichment of Al elements near the interface in Zn-Al-Fe ternary system has also been explained with the sequence of the reaction products during welding was confirmed.The influence of element diffusion on the structure and phase composition of the reaction layer was explained.In the Si-Al-Fe ternary system,the Si element changes the position and extent of the reaction by influencing the diffusion behavior of Al and Fe elements,and then controls the phase composition and thickness of the interfacial intermetallic compound layer.In the Zn-Al-Fe ternary system,the Al element promotes the diffusion of Fe elements into the weld and reacts to form the Fe2(Al,Zn)5 phase,while the Zn element reacts with the Fe element to form the FeZn10 phase,which is distributed in the of Fe2(Al,Zn)5 layer.