Effect Of Composite Heat Treatment On The Microstructure And Properties Of AZ31B Magnesium Alloy Laser Welded Joints

Magnesium alloy has the advantages of low density,good electromagnetic frequency closure performance,good damping and vibration reduction,high specific strength and stiffness,and is a promising structural material in lightweight design under the background of the"dual carbon"strategy.It has wide attention in industries such as automobiles,aerospace,and 3C products.Laser welding technology is suitable for the welding of magnesium alloys due to its highly concentrated energy,fast welding speed,and high degree of automation,However,due to the active chemical properties,low melting point,low surface tension,high thermal conductivity and coefficient of thermal expansion of magnesium alloys,it is extremely difficult to obtain welded joints with excellent comprehensive properties in industrial production even if laser welding is used.In this regard,this article uses fiber laser to study the laser welding process of AZ31B magnesium alloy,and attempts to improve the microstructure and properties of laser welded joints through post weld deep cooling treatment and composite heat treatment.It explores suitable post weld deep cooling treatment parameters and composite heat treatment system,providing reference for the industrial application of magnesium alloy laser welding technology.The main research conclusions are as follows:Firstly,the laser power and welding speed affect the welding heat input,and have an impact on the weld seam fusion width,collapse amount,joint structure and performance.When the defocusing amount is 0,the laser power is 2500W,and the welding speed is 30mm/s,the welding heat input is moderate,and beautiful and well formed welds can be obtained.The front and back surfaces are continuous fish scale patterns,and the weld seam collapse is small.The front and back fusion widths are moderate,and the fusion width ratio is close to 1,The tensile strength and elongation after fracture of the joint are 209.8MPa and 5.3%,respectively,reaching 94.9%and 65.4%of the base metal.The fracture mode is quasi cleavage fracture.The microstructure of the weld seam is composed of the substrateα-Composed of Mg phase and second phase,the second phase is eutectic Mg₁₇（Al-Zn）₁₂ phase and eutectic The eutectic structure ofα-Mg phase coexisting and growing together.Secondly,there are certain differences in the sensitivity of different regions of welded joints to cryogenic treatment.The effect of cryogenic treatment time on joint performance is not linear,and cryogenic treatment can cause the second phase to aggregate at the defect and promote Mn precipitation,while the second phase is eutectic freeβ-Mg₁₇Al₁₂ phase and eutectic The eutectic structure ofα-Mg phase coexisting and growing together,also containingη-Al₈Mn₅ phase.Different cryogenic treatment times can lead to lattice distortion,grain refinement,deformation,rotation,and the combined effect of precipitation and fracture of the second phase,resulting in differences.After 4 hours of cryogenic treatment,the welded joint has good mechanical properties,with tensile strength and elongation after fracture of223.0MPa and 9.2%,respectively,reaching 106.3%and 174.0%of the welded joint,and reaching 100.9%and 113.6%of the rolled base metal.After 12 hours of cryogenic treatment,the average microhardness of the weld seam significantly increased,reaching 112.5%of the welded joint,and the joint microhardness is extremely uneven,with the highest microhardness reaching 71HV_0.1 and the lowest value only 51HV_0.1.Finally,the combination of cryogenic treatment,solid solution treatment,and artificial aging treatment improved the uniformity of the joint structure,improved the disadvantages of cryogenic treatment,enhanced the strengthening effect of heat treatment,and promoted the precipitation of Mn elements.The highest Mn element content in the second phase can reach30.43%.Different composite heat treatment systems have different effects on the microstructure and properties of joints,but they can all improve the strength and plasticity of welded joints,with the most significant improvement in joint plasticity.The joint after solid solution treatment,deep cooling treatment,and aging treatment has the best comprehensive performance,with better strength and plasticity than the joint after single heat treatment or deep cooling treatment.At the same time,the overall microhardness uniformity of the joint is significantly improved,with tensile strength and elongation after fracture of 226.6MPa and12.5%,respectively,reaching 108.0%and 234.6%of the welded joint,102.5%and 153.4%of the rolled base metal.