Study On Electron Beam Welding Technology And Blowhole Defect Control Of Mg-10Gd Magnesium Alloy

Mg-10Gd magnesium alloy is a typical aging strengthened rare earth magnesium alloy,which has good heat resistance,high creep resistance and good corrosion resistance.In the fields of aviation,aerospace and automobile industry,magnesium alloy can replace other alloys to reduce the structural weight.In this paper,the electron beam welding(EBW)of 6.5 mm thick Mg-10Gd magnesium alloy was carried out,and the weld formation,microstructure and mechanical properties of the joint under different heat input were studied.The problem of porosity defect in the joint was solved,and the reliable connection of rare earth magnesium alloy was realized.Mg-10Gd magnesium alloy has poor weldability,easy to form collapse after welding,and there are serious blowhole defect in the welding seam.Under the condition of ensuring weld penetration,increasing welding speed and welding power and reducing welding heat input can improve the weld forming quality and obtain approximately straight weld section morphology.The electron beam welding joint of Mg-10Gd magnesium alloy is mainly composed of fusion zone,partially melted zone,heat affected zone and base metal.The grains in the weld area are obviously refined and consist of fine equiaxed grains,and the size of the grains in the upper part of the weld is larger.The weld boundary is mainly composed of equiaxed grains and a small number of columnar grains.Compared with the base metal zone,the microstructure of the heat-affected zone has little change,but the??(Mg₇Gd)phase precipitated in the zone will change or dissolve in the welding process,which is the weak area of the joint.The strengthening phase in the base metal is??phase,and the main strengthening phase in the weld is Mg₅Gd phase.The transition and dissolution of??phase in the heat-affected zone can be reduced by reducing the welding heat input,which is beneficial to improving the joint strength.The weld zone is composed of fine equiaxed grains whose hardness is obviously higher than that of the base metal zone.The maximum strength of the joint is 227.5MPa,which can reach 97.0%of the base metal.The fracture type is quasi cleavage fracture.At high temperature tensile,the joint strength is equivalent to the base material.The precipitation strengthening is the main strengthening mechanism in the base metal zone,the grain boundary strengthening and the second phase strengthening are the main strengthening mechanisms in the weld zone,and the heat affected zone is the solid solution strengthening.In this paper,the formation mechanism and control measures of the blowhole defects in EBW joint of Mg-10Gd magnesium alloy were investigated.The hydrogen pores in the weld are mainly caused by the great difference in the solubility of hydrogen in solid magnesium and liquid magnesium,but the base metal contains little hydrogen,so the number of hydrogen pores formed is small.However,most of the process characteristic pores were caused by the instability of key holes.Fluent finite element simulation software was used to simulate the flow behavior of weld pool under low speed and high speed welding.The simulation results show that the use of high-speed welding can reduce the welding heat input,improve the stability of the keyhole,and obtain a relatively straight keyhole wall,and the molten pool metal mainly flows to the upper and lower surface of the molten pool,which is conducive to the escape of bubbles,and can effectively control the problem of blowhole defect in the weld.