焊后热处理对镁合金搅拌摩擦焊接接头的影响

Influences of Post-weld Heat Treatments on Friction Stir-welded AZ31B Magnesium Alloy Joints

对AZ31B镁合金进行搅拌摩擦焊接,研究热处理前后焊接接头组织及性能的变化。结果表明:焊后热处理可以显著改善焊接接头的力学性能;当搅拌头的旋转速度为600r.min-1,焊接速度为47.5mm.min-1,接头的抗拉强度为168MPa,延伸率为3.6%。热处理后,接头的抗拉强度和延伸率最高可达205MPa和8.6%,比热处理前分别提高22.02%和138.89%;接头的最佳热处理工艺为:退火温度250℃,保温1h;随着退火温度的升高,焊核区晶粒的长大速度大于热影响区/热机影响区混合区域(HAZ/TMAZ)。当退火温度高于250℃时,焊接接头的拉伸断口主要为扁条状韧窝,呈现出微孔聚合韧性断裂特征。

Friction stir welding was conducted on AZ31B magnesium alloy.Variation of microstructures and properties of the welding joints before and after heat treatments was studied.The results show that post-weld heat treatments can markedly improve the mechanical properties of the friction stir-welded AZ31B magnesium alloy joints;when the rotating speed of stirring pin is 600 r·min-1 and the welding speed is 47.5 mm-min-1,the ultimate tensile strength can reach 168 MPa and the elongation is 3.6%.After heat treatments,the highest ultimate tensile strength can reach 205 MPa,which is 22.02% higher than that before heat treatments and the elongation is 8.6%,which is 138.89% higher than before heat treatments.The ideal heat treatment technique of the joint is keeping the annealing temperature at 250℃ for 1 h.As the annealing temperature increases,the growing speed of welding nugget zone grain is higher than the heat affected zone/thermo-mechanically affected zone.When the annealing temperature is higher than 250 ℃,the morphologies of tensile fracture of welding joints mainly exhibit oblate dimples with a typical ductile dimple-fracture.