Study On Microstructures And Properties Of Plasma Arc Welded Joints Of Aluminum Alloy/High-Strength Steel Dissimilar Materials

It is an effective way to realize lightweight in automobile,railway vehicle and other walking machinery by introducing a certain amount of aluminum alloy based on high-strength steel as the main body to form a steel-aluminum composite structure.However,due to the significant differences in chemical and physical properties between aluminum alloy and high-strength steel,the weldability and joint mechanical properties of aluminum/steel are seriously deteriorated.The reliable connection of aluminum/steel is the key to ensuring the implementation of lightweight solutions.Firstly,the microstructure and mechanical behavior of 5A06 aluminum alloy/DP1180 high strength steel plasma arc welded joint obtained using Al-Si5 and Al-Cu6 welding wires were studied.The results showed that the aluminum/steel joint had characteristics of welding-brazing.The weld zone was mainly composed ofα-Al cellular crystal,cellular dendritic crystal and dendritic crystal,andα-Al+Si eutectic mixture（or CuAl₂ compound）was precipitated atα-Al grain boundaries.During the welding process,the interdiffusion of Fe and Al atoms and the interfacial reaction occurred at the aluminum/steel interface,and the Fe-Al intermetallic compounds（IMC）layer with double structure was formed(the needle-like Fe₄Al₁₃ layer was close to the aluminum weld and the tongue-shaped Fe₂Al₅ layer was close to the steel base metal).The test results of mechanical properties for the aluminum/steel joint showed that the microhardness of aluminum/steel interface zone increased significantly（>464 HV）and the nano-hardness value of the interface zone was Fe₄Al₁₃（12.8 GPa）>Fe₂Al₅（12.0GPa）.Under the tensile stress action,the aluminum/steel joints all fractured in the interface zones and the fracture surfaces had obvious brittle fracture characteristics.Therefore,the interface zone was the weakest area of the Al/steel plasma arc welded joint,and the brittle and hard Fe-Al IMC was the main factor affecting the mechanical properties of the joint.The influences of welding current and welding speed on the thermal cycle at aluminum/steel interface and joint microstructures and properties were studied.The IMC layer thickness was obviously increased and cracks were generated due to the excessive E value,hence the tensile strength of the joint was obviously reduced.The increase of welding current or the decrease of welding speed led to an increase in welding line energy（E）,thermal cycling peak temperature（T_M）and high-temperature residence time(t₆₆₀),resulting in the increase of the thickness of the interface IMC layer.Excessive heat input can cause stress cracks in the IMC layer of the interface zone significantly reducing the tensile strength of the joint;however,too less heat input can generate discontinuous IMC layers and also reduce the tensile strength of the joint.Under the condition of this experiment,the optimized welding parameters obtained were the welding current of 90 A and welding speed of 320 mm/min,and the tensile strength of aluminum/steel joint reached 88.5 MPa（Al-Si5 wire）and 118.0 MPa（Al-Cu6 wire）,respectively.It was more beneficial to use Al-Cu6 wire for improving the aluminum/steel joint strength,which was mainly related to the metallurgical behavior of Cu in the interface reaction process.Cu dissolved in Fe-Al IMC to form（Fe,Cu）₄Al₁₃and（Fe,Cu）₂Al₅ solid solutions,reducing the brittleness and the generating crack tendency of the interface zone,thus increasing the tensile strength of the joint.Based on above research and thermodynamic and kinetic analysis results,the IMC growth model of aluminum/steel interface was established,and its growth mechanism was revealed.In order to prevent the generation of Fe-Al IMC and further improve the mechanical properties of aluminum/steel joint,the design idea and welding process scheme of aluminum/steel joint assisted by metal transition layer were proposed.The results showed that the Cu transition layer assistance aluminum/steel plasma arc welded joint was composed of the steel/Cu weld zone,Cu transition layer and Cu/Al jointed zone.The weld zone mainly containedα-Fe andα-Cu solid solutions.The jointed zone consisted of four interfacial reaction layers:the reaction layerⅠwas mainly fine Cu₉Al₄ phase,reaction layerⅡwas mainly serrated CuAl₂,reaction layerⅢwas CuAl₂+α-Al eutectic structure and reaction layerⅣwas mainlyα-Al solid solution.Based on above results,the formation process model of Cu/Al jointed zone was established.The Schmidt factors of Cu-Al intermetallic compound Cu₉Al₄ and CuAl₂ were 0.350 and 0.366,respectively.The maximum microhardness（420 HV）of the aluminum/steel joint assisted by the Cu transition layer appeared in the Cu-Al intermetallic compound layers of the jointed zone,and the nano-hardness values of the jointed zone were Cu₉Al₄（10.1GPa）>CuAl₂（8.8 GPa）.Since the Cu transition layer prevented the combination of Fe and Al atoms to produce brittle Fe-Al IMC,the tensile strength of the aluminum/steel joint assisted by the Cu transition layer increased to 147.5 MPa.Cu-Al intermetallic compounds were the main factor affecting the mechanical properties of the joint.It was found that Cu+Ag composite transition layer was more beneficial to improve structures and properties of aluminum/steel joint.During the interfacial reaction process,Ag atoms diffused into the jointed zone to form Cu₉（Al,Ag）₄ and Cu（Al,Ag）₂ solid solutions.The microhardness of aluminum/steel joint decreased（387 HV）,and the nano-hardness values of Cu₉（Al,Ag）₄ and Cu（Al,Ag）₂ were 9.4GPa and 8.3GPa,respectively.The tensile strength of the joint increased significantly（165.0 MPa）.Compared with the joint strength with Al-Cu6 welding wire（118.0 MPa）,the tensile strength of aluminum/steel joint assisted by Cu+Ag transition layer was increased by about 40%.It was mainly attributed to improving the toughness of Cu-Al intermetallic compounds by Ag atoms dissolving in Cu₉Al₄ and CuAl₂.In order to investigate and discuss the physical nature of effects of intermetallic compounds on the aluminum/steel joint properties,the lattice constants,thermodynamic properties,mechanical properties and electronic properties of four typical IMC were calculated by first principles calculation method based on density functional theory.The results showed that the calculated lattice constants of Fe₄Al₁₃,Fe₂Al₅,Cu₉Al₄ and CuAl₂ were close to the measured values.The formation enthalpic（ΔH）and binding energy（Ecoh）of the compounds were negative,indicating that they could form spontaneously and had thermodynamic stability.The Poisson’s ratio and Pugh modulus,which represented the brittleness/plasticity of the material,were Fe₄Al₁₃<Fe₂Al₅<Cu₉Al₄<CuAl₂.The hardness value was Fe₄Al₁₃>Fe₂Al₅>Cu₉Al₄>CuAl₂.The chemical bonds of Fe₄Al₁₃,Fe₂Al₅,Cu₉Al₄ and CuAl₂ were mainly the covalent bond,with a certain metal bond characteristic.The covalent bond characteristics were Fe₄Al₁₃>Fe₂Al₅>Cu₉Al₄>CuAl₂.Compared with Cu₉Al₄ and CuAl₂,the Poisson’s ratio and Pugh modulus of Ag atom doping Cu₉Al₄ and CuAl₂ were increased,and the metal bond characteristics were strengthened,indicating that Ag doping was beneficial to improve the toughness and reduce the brittleness of Cu-Al intermetallic compounds.The calculation results based on first principles were in good agreement with the experimental results.