| Magnesium alloys have the advantages of light weight,high specific stiffness and strength.Steel is the basic material for the manufacture of automobiles and aerospace vehicles.Magnesium/steel joints are widely used in the manufacture of automobiles and aerospace vehicles.Magnesium instead of steel can reduce the weight of the machine in some non-loadbearing parts,thus achieving the goal of energy saving and emission reduction.The study of magnesium/steel connections is of great significance.Magnesium and steel have great differences in physical and chemical properties.It is difficult to dissolve or produce intermetallic compounds,They unable to choose a traditional fusion welded joint because of the difference between the melting point and the boiling point.At present,the most reasonable method for magnesium/steel connection is fusion brazing.In this paper,laser-arc hybrid welding is used to connect AZ31 B magnesium alloy with Q235 steel to study joint morphology,microstructure and mechanics under different laser power,welding current and welding speed.After the Cu and Ni elements were electroplated on the steel surface,changes in microstructure and mechanical properties were investigated.The welding process parameters have a crucial influence on the microstructure and mechanical properties of the magnesium/steel joint.The test results show that when the laser power is 1200 W,the current is 140 A,and the welding speed is 8mm/s,the surface forming and mechanical properties are optimal.The highest tensile strength is 117.04 MPa.The magnesium/steel laser-arc hybrid welding head is a typical fusion welded joint,which is divided into steel side heat affected zone HAZ1,steel side brazed zone IZ,weld zone WZ,magnesium side weld zone FZ,and magnesium side heat affected zone.HAZ2.The grain in the weld zone is significantly thinner than the magnesium alloy base material,and the main structure is ?-Mg solid solution and A112Mg17 intermetallic compound.Through the tensile test,it is found that the weak connection zone of the magnesium/steel laser-arc hybrid welding head lies in the steel side brazing zone.XRD inspection of the fracture shows that the fracture interface has the existence of Fe-Al phase,and the nature of the brittle hard phase deteriorates.Connector performance.After pre-plating the Cu layer on the steel surface,the mechanical properties of the welded joint are improved,the tensile strength reaches 188.97 MPa,and the fracture mode is ductile-brittle mixed fracture.XRD shows the steel side fracture.After adding Cu element,there are a small amount of Fe-Al and Al16CuMg4 compounds in the interface reaction layer,indicating that Cu element inhibits the formation of Fe-Al phase,and the Cu element diffused into the weld reacts with Mg element.Mg2 Cu,Cu2Mg compounds were produced.After pre-plating the Ni layer on the steel surface,the fracture mode of the joint is similar to that of the copper-plated joint.The tensile strength reaches 176.39 MPa.Ni and Fe form a(Fe,Ni)solid solution,which reacts with Al to form AlNi phase and Al0.The presence of.42Ni0.58 phase,Al-Ni phase and(Fe,Ni)solid solution greatly improved the mechanical properties of the welded joint.The influence of pre-plated metal elements on the steel surface on the welded joints mainly lies in the formation of different interface reaction layers.After the addition of Cu and Ni elements,the formation of the interface reaction layer is mainly divided into four stages: composite heat source melting coating and welding of laser and MIG arc.The Fe element in the steel and the plating elements on the surface(Cu,Ni)and the weld metal interdiffusion reaction;the interface reaction product is produced and grown;the interface reaction layer and the weld metal are solidified. |
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