对6.3 mm厚AZ91D变形镁合金进行CO2激光对接焊试验，激光功率为3500 W，焊接速度为2.5 m/min，离焦量为+2 mm，氩气流量为10 L/min，并采用光学显微镜（OM）、扫描电镜（SEM）和电子万能拉伸试验机考察接头的微观组织、拉伸性能和接头的断口形貌。结果表明：在此工艺参数下，接头焊缝成形良好；焊缝区由细小的α-Mg等轴晶基体组织和沿晶界分布的β-Mg17Al12组成，但存在微观气孔和微裂纹等缺陷；部分熔化区宽度很窄，为50～80 μm，靠近焊缝区一侧晶粒局部熔化，靠近母材一侧晶粒未熔化；接头抗拉强度(UTS)为232 MPa，延伸率约为6%，分别为母材的90%和25%；接头在焊缝区断裂，断口形貌表明为韧脆混合断裂。

AZ91D wrought magnesium alloy plates, with thickness of 6.3 mm, are welded by a CO2 laser beam with a laser power of 3500 W, welding speed of 2.5 m/min, defocusing of +2 mm and a constant argon gas flow of 10 L/min. The microstructure, tensile properties and fracture morphology of the joint are investigated. The results show that a favorable weld can be obtained. The fusion zone consists of finer α-Mg equiaxed matrix and β-Mg17Al12 precipitates distributed along the grain boundaries with pores and microcracks frequently found. The width of the partly melted zone is very narrow, about 50～80 μm, and the grains near the fusion zone are locally melted while those near the base metal are not. Tensile results indicate that the ultimate tensile strength (UTS) and elongation of the joint are respectively 232 MPa and 6%, about 90% and 25% of that of the base metal. The final fracture of the joint occurs at the center of the fusion zone and the fracture morphology shows a combination of brittle and ductile features.