Effect Of Post-weld Heat Treatments On Microstructure And Property Of AZ31B Magnesium Alloy Laser Welded Joints

Magnesium alloy is the lightest structural metallic material in industrial production,it is also one of the most promising industrial materials in China.Laser welding is one of the most common connection method for magnesium alloys.Post-weld heat treatment can effectively eliminate the residual stress of the joint and improve the uniformity of the weld.However,there is little research on the post-weld heat treatment of magnesium alloy laser welding at home and abroad.It is significant to study the effect of post-weld heat treatment on the structure and properties of laser welded joints of magnesium alloys for the application of laser welding of magnesium alloys in industrial production.In this paper,the high-power fiber laser is used to carry out the laser welding process and post-weld heat treatment of AZ31B magnesium alloy.Main research results are as follow:When the laser power P=2.5kw,welding speed v=30mm/s,defocusΔf=+2mm,the positive and negative surfaces are well formed,having the features of continuous and uniform fine fish scales,no obvious defects,besides the melt ratio is moderate.The tensile fracture position is the base material,the tensile strength is 198 MPa,reaching to 98%of base metal.And the elongation after fracture is 6%,reaching to 94%of base metal.There is no Mg₁₇Al₁₂in weld zone,and the coarse spherical particles areα-Mg segregation rich in Al and Mn.Post-weld annealing treatment causes segregation of the weld zone to dissolve in the matrix and grain recrystallization,which improves the uniformity of the weld seam.After annealing at 300°C for 1h,most of the segregation in the weld zone dissolve in the matrix,and recrystallization complete.The crystallites are fine and uniform,the hardness reaching to65HV,and the tensile strength reaches 228MPa.After annealing at 400°C,the segregation in the matrix dissolve substantially,and the grain size is significantly coarse.The tensile strength and hardness reduce,the plasticity improves greatly,and the elongation after fracture is 9.2%,which reaching to 153%of the welded joint.After solution treatment,the Al-segregation in the weld zone and theβ-Mg₁₇Al₁₂ in the base metal and heat-affected zone dissolve basically,the grain size in each region increases obviously,and the Al content in theα-Mg matrix increase.The hardness of each part of the joint decrease significantly and the distribution is uniform.The tensile strength and yield strength decrease slightly,the plasticity increases obviously,and the elongation after fracture reaching to 10.6%.After artificial aging treatment,a large amount ofβ-Mg₁₇Al₁₂ phase is precipitated continuously in the grain boundary and the crystal in each region of the welded joint.The hardness of each area improvs significantly,the average value reaching to 68HV,the tensile strength improves greatly,reaching to 240MPa,the plasticity is obviously deteriorated,and the elongation is only 2.9%.After solution treatment and artificial aging treatment,theβ-Mg₁₇Al₁₂ phase in each part of the welded joint are de-dissolved and re-dissolved from the matrix.The shape is fine and distributed,and the microhardness of the joint was compared with that of the joint after only solution treatment.Both the yield strength and the tensile strength improve significantly,and the elongation decreases slightly.Unheated welded joints have poor corrosion resistance compared to base metals.The relationship among electrochemical corrosion performance after several heat treatment methods is:solution treatment>solid solution+artificial aging>400°C annealing treatment>200°C annealing treatment>no heat treatment>300°C annealing treatment>artificial aging.The larger the grain size,the smaller the size of segregation and second phase,the smaller the number,the better the corrosion resistance.