奥氏体不锈钢材料以其优良的耐腐蚀性、塑性和可焊性，被广泛应用于地铁不锈钢车体的制造中。本文以下一代地铁用8mm厚SUS301L-MT不锈钢为研究对象，开展了激光-MAG复合焊和窄间隙激光填丝焊工艺特性及接头性能研究，通过对焊丝熔入行为、电弧熔滴特性、焊接工艺特性、接头微观组织和综合力学性能的分析、比较，结合EBSD技术探索焊接接头微观成形机制，为进一步指导推广激光-MAG复合焊和窄间隙激光填丝焊技术的工程应用提供了实验基础和理论依据。
本文利用高速摄像技术研究了激光填丝焊的焊丝熔入行为以及激光热源对熔池的影响规律，分析了激光电弧的耦合机制，结果表明：光丝间距、焊接速度、送丝速度、激光功率和离焦量几个主要工艺参数，对熔滴过渡以及焊缝成形有着重要的影响，只有这些参数在一定范围内合理匹配才能获得良好成形的焊接接头。与激光填丝焊相比，激光-MAG复合焊的熔滴过渡稳定性以及焊缝成形较差，但是工艺参数可控范围更宽。
分析比较了301L不锈钢激光-MAG复合焊和窄间隙激光填丝焊的接头显微组织以及焊接热循环过程，结果表明：复合焊与填丝焊接头的微观组织类似，焊缝区均为奥氏体柱状晶与树枝状铁素体，热影响区和母材为奥氏体和形变马氏体；填丝焊的焊接热循环峰值温度低，冷却速度快，因此过冷度大，实际结晶温度低，晶粒更小。
比较了301L不锈钢激光-MAG复合焊和窄间隙激光填丝焊接头的综合力学性能，并绘制了纵向残余应力云图，结果表明：填丝焊、复合焊拉伸试样均断于焊缝，断裂机制为韧窝断裂；填丝焊接头的屈服强度为576MPa，抗拉强度为824MPa，断后延伸率为27.9%，均高于复合焊；填丝焊接头的硬度大于复合焊，接头软化区范围窄；两种焊接方法的纵向残余应力分布类似，均在焊缝及熔合线附近出现“应力凹陷”，复合焊的纵向残余应力峰值为600MPa，高于填丝焊的304MPa。
基于EBSD技术，通过分析焊接接头的熔池凝固结晶取向特征、晶粒形貌以及晶粒尺寸，从材料微观结构角度解释了激光-MAG复合焊与窄间隙激光填丝焊接头性能的差异，结果表明：焊缝晶粒是以<100>//<100>，{100}//{100}为主要取向关系择优生长，并形成{100}<100>织构；窄间隙激光填丝焊接头晶粒尺寸小于激光-MAG复合焊，在单向拉伸载荷条件下，填丝焊接头晶粒在{111}<110>滑移模式下的施密特因子普遍高于复合焊，位错滑移机制更易启动，因此窄间隙激光填丝焊接头的强度和塑性均优于激光-MAG复合焊。

Based on the excellent performance of corrosion resistance, plasticity and weldability, austenitic stainless steel is widely applied in the manufacture of metro’s stainless steel car-body. This thesis takes 8mm thickness SUS301L-MT as research objective to study process acteristics and joint performance of Laser-MAG hybrid welding and narrow gap laser welding through the analysis of wire melting behavior, arc droplet’s acter, welding process acter, microstructure of welded joint and integrated mechanical performance. EBSD was used to explore welded joint’s microcosmic forming mechanism. This thesis provides the experimental foundation and theoretical basis for the further engineering application of Laser-MAG hybrid welding and narrow gap laser welding technology.
This thesis applies high speed video technology to study the fusion of welding wire of laser welding with filler wire and the law that exert the influences on the weld pool by laser thermal source; analyzes the laser arc’s coupling mechanism. The result indicates: wire distance, welding speed, wire feed speed, laser power and defocusing amount exert significant influences on droplet transition and formation of weld. Only when previous parameters get a reasonable matching within the certain range, can the good forming welded joint appears. Compared with laser weld with filler wire, laser-MAG hybrid welding owns worse droplet transition stability and formation of weld but the wider controllable range of process parameters.
After analyzing the weld joint’s microstructure and welding thermal cycle of laser-MAG hybrid welding and narrow gap laser welding with filler wire with 301L stainless steel respectively, the study gets a result: weld joint’s microstructures in hybrid weld and filler wire weld are similar, their weld zone are Austenitic columnar crystal and dendritic ferrite; the heat affected zone and base metal are Austenite and strain-induced Martensite; The filler wire weld’s thermal cycle owns  low peak temperature but the high cooling speed so that the condenser depression is strong, and the crystallization temperature is low, the grain is small.
The thesis compares the integrated mechanical performance of laser-MAG hybrid welding and narrow gap laser welding with filler wire with 301L stainless steel respectively and draws the stress cloud of longitudinal residential stress, the result is shown as follows: the tensile tests of filling wire welding and composite welding are broken in the weld, the fracture mechanisms are dimple fracture; the yield strength of weld joint in filler wire welding is 576MPa; the tensile strength is 824MPa, elongation is 27.9%. The three process parameters are higher than in hybrid weld. The hardness of filler wire weld’s joint is greater than hybrid weld’s, the joint soften zone is small. The distributions of residential stress in two weld process are similar that the “stress dip” appears around the weld and fusion line. The hybrid weld’s peak of longitudinal residential stress is 600MPa, which is higher than filler wire weld’s 304MPa.
Based on the EBSD technology, this study analyzes the weld joints molten pool solidification oriented feature, grain morphology and grain size. From the perspective of material microstructure, the study tries to explain the differences of property between the joints of laser-MAG hybrid welding and narrow gap laser welding with filler wire. The result is shown as follows: weld grain tends to the growth with planes of <100>//<100>,{100}//{100} and form the{100}<100> texture; narrow gap laser welding with filler wire joints’s grain size is smaller than laser-MAG hybrid’s, in the condition of uniaxial tensile load, the Sch factor of filler wire weld’s grain in {111}<110> sliding pattern is higher than hybrid weld’s and the dislocation slide mechanism is easier to be activated so that narrow gap laser welding with filler wire joints strength and plasticity are better than laser-MAG hybrid joints.

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