采用光纤激光-熔化极活性气体保护焊(MAG)复合焊接进行X80管线钢6 mm厚根部的打底焊接，以实现单面焊双面成形。对工艺规范参数优化后的焊接接头进行宏观成形分析和显微硬度、拉伸、冲击、弯曲等力学性能研究。在此基础上，重点研究了激光-MAG复合焊热循环对打底焊焊缝接头微观组织的影响。结果表明，在优化的工艺参数范围内，可获得成形良好、无缺陷的焊缝；焊缝截面呈高脚杯状，可分为电弧作用区和激光作用区；电弧作用区的硬度高于激光作用区；拉伸试样断裂于母材；电弧作用区焊缝组织主要为针状铁素体和少量的先共析铁素体，激光作用区焊缝组织主要为针状铁素体和贝氏体类型组织(上贝氏体+粒状贝氏体)；激光作用区和电弧作用区的热影响区组织差异不大，但电弧作用区的过热区明显宽于激光作用区。

Fiber laser-metal active-gas welding (MAG) hybrid welding on 6-mm-thick root of X80 pipeline steel is carried out to attain one side welding both sides formation. Mechanical properties of welded joint in optimal parameters such as microhardness, tensile strength, impact toughness, bending strength are studied. Based on this, the effect of thermal cycle caused by hybrid welding on welded joint is investigated. The results indicate that good appearance, defect free joint can be achieved in the optimized parameters, and the cross-section of welded joint exhibits feature of “goblet”, which can be divided into two parts: arc zone and laser zone. The microhardness in arc zone is higher than that in laser zone. Fracture of tensile test occurs in the base metal. The microstructures of weld in arc zone mainly consist of acicular ferrite and a small amount of proeutectoid ferrite. Whereas, The microstructures of weld in laser zone mainly compose of acicular ferrite and bainite-like (upper bainite and granular bainite). The difference in microstructure of heat affected zone (HAZ) between arc zone and laser zone is small, but the coarse zone in arc zone is obviously wider than that in laser zone.