| Rare earth magnesium alloy retains the excellent properties of magnesium alloy such as low density,high specific stiffness,specific strength,good electromagnetic shielding,easy recovery,and its room temperature mechanical properties and heat resistance are greatly improved due to the addition of rare earth elements,making it more widely used.In this paper,the connection of Mg-10Gd-0.4Zr rare earth magnesium alloy was realized by vacuum electron beam welding.By optimizing the welding parameters,the electron beam welded joints with excellent surface forming and higher mechanical properties were obtained.At the same time,on the basis of the optimal parameters,the effects of ultrasonic-assisted on the microstructure and porosity defects of the weld were compared and analyzed.Finally,the influences of welding processes on the corrosion resistance of the joints were explored,and the mechanism of the difference in corrosion resistance of the joints was clarified.Firstly,the welding process parameters were optimized.The weld forming was significantly affected by the optimization of welding process parameters.The joint porosity defects can be reduced significantly by low welding speed and the application of scanning electron beam on the basis of ensuring the weld forming.When the optimal process parameters of electron beam welding were as follows: the focusing current was600 m A,the scanning diameter was 1mm;the scanning frequency was 500 Hz,the welding speed was 300mm/min,and the welding beam was 8.5m A,the joints were broken at the base metals and the tensile strength was 227 MPa.The transverse microhardness of the weld had little change,and the microhardness was increased slightly by the grain refinement.During the tensile test,the strain of rare earth magnesium alloy joints was small,and the samples were suddenly broken,showing the characteristics of brittle fracture.The cleavage fracture was presented in the fracture of the base materials and the heat-affected zones.The fracture surface of the weld zone was filled with a large number of bremsstrahlung,indicating that the joint transform from brittle fracture to ductile fracture was made by the grain refinement of magnesium alloy.The transgranular fracture was characterized by the crack passing through the grain interior.Based on the optimization of welding parameters,ultrasonic assistance was applied,and the effects of ultrasonic assistance on the microstructure and properties of the weld were compared and analyzed.The weld zones of rare earth magnesium alloy electron beam welding were uniform and fine equiaxed grains,and the black Mg-Gd phases were precipitated at the grain boundaries.The formation of the back side of the weld was significantly improved by the ultrasonic assistance.With the increase of ultrasonic power,the grains were refined,and the average diameter of the grains after refinement was4.05?m.As the ultrasonic power increased,the pore defects were significantly reduced,the porosity of the weld was reduced from 0.168% to 0.064%.Finally,the influences of the electron beam welding process on the corrosion resistance of the joints were investigated.The results of hydrogen evolution test and polarization curve analysis in Na Cl solution showed that the corrosion resistance of the base material was better than that of the electron beam welding joint.The corrosion resistance of the ultrasonic assisted electron beam welding joint was worst.The corrosion resistance of magnesium alloy samples was decreased and the corrosion rate was increased with the increase of immersion corrosion time.The grain boundaries were increased caused by the grain refinement,which severely reduced the corrosion resistance of the joints.The Mg-Gd phases continuously distributed at the grain boundaries produced by electron beam welding could delay the corrosion process,making the corrosion resistance of the joints higher than that of the ultrasonic-assisted electron beam welding joints. |
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