Study On High Entropy Welding Of Steel/Copper Dissimilar Metal

The composite structure composed of copper and steel can not only meet the requirements of material properties in different environments,but also reduce the use of copper material and greatly reduce the cost of components.However,the compatibility between Cu and Fe in solid state is very low.No matter what welding method is used to directly weld steel/copper,there will always be liquid phase separation or welding defects.The high entropy alloy has high mixing entropy,so it has strong solid solution ability for components,and the structure of the solid solution will not change when the components change within a certain range.Therefore,the application of high entropy alloy as an intermediate layer in the field of copper-steel dissimilar metals can solve the metallurgical incompatibility problem of copper-steel welding by using the super solid solubility of high entropy alloy.In this study,the microstructure and mechanical properties of three kinds of high entropy alloy Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅,Fe₁₀Co₂₅Cr₃₀Ni₃₀Cu₅ and Fe₁₅Co₂₀Cr₃₀Ni₃₀Cu₅ with different components were studied.The results show that the three kinds of high entropy alloy are all simple FCC solid solution structure,and the microstructure is typical dendritic structure.After compression test,all the samples turn into drum shape without fracturing phenomenon,indicating that the three kinds of high entropy alloy have good plasticity and toughness.Then the high entropy alloy was used as the interlayer of steel-copper dissimilar welding to obtain the optimal welding process parameters through experiments.And to change the proportion of elements in the middle layer of the high entropy alloy,to deeply study the influence of the composition of the high entropy alloy in the middle layer on the microstructure and mechanical properties of the welded joint.The main research results are as follows:（1）Using Fe₅Co₃₀Cr₃₀Ni₃₀Cu₅ as the middle tier of T2 copper and Q235 docking TIG welding experiment,the welding parameters for welding by one side and the influence of the double sided weld forming,optimizing the welding parameters of the single side welding for the welding current of 200 A,the welding voltage of 13.7 V,the welding speed of 1.5 mm/s,and double-sided welding parameters for welding current of 140 A,voltage of 11.7 V,welding speed of 1.5mm/s.The effect of arc offset distance on weld microstructure is studied.It is found that it is difficult to obtain a well-formed welded joint when the arc is tilted to the side of Q235.When the arc is biased to the T2 copper side,the weld is well formed;when the arc offset distance is 0.5 mm,the weld microstructure is compact equiaxed crystals and dendrites,which is the most appropriate arc offset distance.（2）The microstructure and properties of the welded joints obtained by two different forming methods of single-side welding and double-side welding were compared.It was found that the microstructure of the welded joints with two different processes was columnar grain on the side of T2 copper base metal,and the center area of the weld was equiaxed grain and dendritic grain.Due to the high heat input of single-side welding,the microstructure of single-side welding is columnar crystal,while that of double-side welding is dendritic crystal.Both kinds of welded joints are well formed,without macroscopic and microscopic defects,and the fracture locations occur in the heat-affected zone on the copper side.The tensile strength of single-side welding is 204 MPa,and the tensile strength of double-side welding is 227 MPa.The welded joints with double-side welding have better performance.The phase structure of the welded joint is simple FCC structure,which realizes the purpose of high entropy of the weld and solves the problem of poor compatibility between steel and copper metallurgy.（3）There is little difference in the welded joint microstructure obtained by using high entropy alloy of three different components as the intermediate layer,and the side of T2 copper base material is composed of columnar crystal.In morphology of weld center area organization,with the increase of Fe element and the decrease of Co element,the number of equiaxed grains decreased and the number of dendrites increased.The side microstructure of Q235 is composed of dendrites,but with the increase of Fe element and the decrease of Co element,the grain size becomes larger.When Fe element increases to 15%and Co element decreases to 20%in the interlayer of high entropy alloy,a high entropy solid solution rich in Fe and Cu is formed in the intergranular interlayer.The weld microstructure of the welded joint obtained by the three intermediate layers is FCC solid solution structure,and the tensile strength reaches more than218 MPa,which is about 88%of the T2 copper base metal.From the economic point of view,it is a good choice to choose Fe₁₅Co₂₀Cr₃₀Ni₃₀Cu₅.