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

摘要:

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600 r·min-1，焊接速度为47.5 mm·min-1，接头的抗拉强度为168 MPa，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205 MPa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1 h；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。

Abstract:

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 oC 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 oC, the morphologies of tensile fracture of welding joints mainly exhibit oblate dimples with a typical ductile dimple-fracture.