【作者】 赵旭东；

【导师】 肖荣诗；

【作者基本信息】 北京工业大学 ， 光学工程， 2012， 博士

【摘要】 本文针对铝/钢异种金属接头的焊接问题，提出了一种宽带激光填充粉末熔钎焊方法。本方法中宽带激光光斑有助于扩大熔池、增加接头尺寸，同步送进的金属粉末不仅改善了焊缝组织成分，还提高了激光束与铝合金母材的能量耦合效率和焊接效率。主要研究内容为以下三个方面：1.系统研究了焊接工艺参数对接头成形、焊缝组织和接头力学性能的影响，重点分析了接头中金属间化合物组织与接头力学性能的关系。2.通过实验观察了激光束、粉末束和母材相互作用的物理过程，揭示了激光填充粉末熔钎焊焊接熔池形成的机制，揭示了增强激光能量耦合效率的根本原因。3.通过建立焊接过程中熔池温度场的有限元模型，分析了温度场对接头钎接界面宽度和金属间化合物生长的影响，以及合金元素扩散的距离，并提出了预测激光填充粉末熔钎焊接头性能的方法。采用本文提出的方法实现了无钎剂AA6061/镀锌钢接头熔钎焊连接。采用适当的焊接工艺参数获得了成形饱满，无裂纹、气孔等缺陷的焊缝。接头机械抗力达到152N/mm。焊接试验表明焊接工艺参数对接头成形有很大的影响。离焦量较小时，激光功率和焊接速度的参数范围较宽。接头界面宽度随激光功率和离焦量的增大而增加，随焊接速度增加而减小。离焦量相同时，激光功率和焊接速度对钎接界面宽度的影响相当；激光功率和焊接速度相同时，增大离焦量有助于增加钎接界面的宽度。熔钎焊接头中钎接界面上的金属间化合物由层状和枝晶状结构组成，层状组织以Fe2Al5相为主，Fe4Al13相仅分布在上层靠近铝合金组织区域。枝晶状组织以Fe-Al和Al-Fe-Si化合物为主，主要组成相为τ1、Fe4Al13和τ5。在焊缝/镀锌钢界面上生长有少量Al5Fe2Zn0.4。显微硬度测试表明金属间化合物组织的硬度明显高于母材硬度，其中层状组织的硬度最高。静载拉伸试验中，接头均断裂在焊缝/镀锌钢界面，呈脆性断裂特征。接头强度与的比值呈正比，层状Fe2Al5相是影响接头性能的主要因素。通过选择高功率高焊速的焊接条件能够获得力学性能较高的熔钎焊接头。激光束与粉末束相互作用后，约10-15%的激光能量被衰减。粉末束吸收激光能量后温度升高，但粉末束最高温度仅约100℃。粉末颗粒与母材表面发生非弹性碰撞，粉末束反弹角受保护气流态影响。实验发现粉末颗粒在母材表面熔化形成微熔池对提高能量耦合效率具有重要作用，是填充粉末能够提高激光能量偶和效率的根本原因。采用有限元法模拟了宽带激光填充粉末熔钎焊过程中的温度场。结果表明在钎料/镀锌钢界面会存在一定宽度的温度线。这种温度分布形式有助于增加钎接界面的宽度。金属间化合物层厚度则受热输入量和界面最高温度共同影响。通过温度场模拟计算了熔池凝固的时间，并根据扩散动力学分析结果计算了Fe元素的扩散距离。综合分析结果归纳出预测接头机械抗力的数值表达式，计算结果与实验结果较为吻合。更多还原

【Abstract】 In this dissertation, contraposed to the problems of welding Aluminiumalloy/steel joint, a laser fusion welding-brazing method using rectangular spot withfiller powder is proposed. In this method, the technique of filling powder is adopted.In this method, broadband laser spot could enlarge the molten and the size of joint.The powder filled could improve the composition of weld organization, as well aslaser energy coupling efficiency and welding efficiency.The present work is concerned with the foundational research into the techniqueof laser fusion welding-brazing aluminium alloy to steel with following objectives:(1)The joints formation, weld microstructure, as well as the mechanical properties arestudied, especially the influence of intermetallic compounds on the strength of thejoint.(2) The interactions between the laser beam, the powder steam and base metalare studied by experiments. The weld pool formation mechanism of the fusionwelding-brazing with filler powder is summed up, as well as the basic reason of highutilization ratio between laser power and base metal.(3) A finite element model of thethermal cycle in welding process is established for studies in influences of thermalcycle on the width of weld interface and intermetallic compounds growing. Base onthe results of experiments and analysis, the method for predicting the mechanicalproperties of the joint is announced.A fusion welding-brazing weld of AA6061and galvanized steel without flux canbe achieved using the welding with filler powder. By optimizing the weldingparameters, the favorable weld without crack and porosity can be achieved, and themaximum intensity of152.5N/mm is obtained. The results of welding tests indicatethat the width of the weld interface increased with the laser power and defocusdistance, and decreased with the increasing of welding velocity. When the weldingparameters are same, increasing the defocus distance could wider the weld interface.The Intermetallic compounds on the interaction surface compose by layeredand dendritic structure. The layered intermetallic compound is mainly Fe2Al5phase.Fe4Al13phase was also observed in the joint close to the aluminium alloy. Thedendritic intermetallic compounds are composed of Fe4Al13, τ1and τ5phases.Al5Fe2Zn0.4phase was found on the brazing filler/galvanized steel interface.Microhardness tests indicate that the hardness of the intermetallic compounds layer ishigher than the two base metals, especially the layered intermetallic compound.Tensile tests indicate that samples fractured brittlely on the interface of brazingfiller/galvanized steel. The joint strength increase with the ratio of. Thethickness of intermetallic compounds, especially the thickness of the layered Fe2Al5phase structure, determines mechanical resistance of the joint. Welding technicalparameters, such as high laser power and high welding speed, is the crucial factor formechanical properties in the fusion welding-brazing joint.With the interaction of the laser beam and powder beam, about10-15%of the laser energy is attenuated. The temperature of powder steam raise with theincreasing of laser power output, and enlarged the high-temperature area. Themaximum temperature of the powder is only about100°C. Inelastic collisionhappened when the powder particles impact surface of base metal. The rebound angleof powder steam is influenced by airflow of the shielding gas. There are slightdifferences at different amount of filler powder and shielding gas. After the collision,the height of powder steam is proportional to amount of filler powder and shieldinggas. The formation basis of weld pool with filler powder is the melting of powderparticles on the base metal surface, which is the basic reason why the filler powdercould improve the utilization ratio of the laser power.A finite element model is created for simulating the thermal cycle of the fusionwelding-brazing process with filler powder. A certain width of the temperate zoneappeared on the filler metal/galvanized steel interface, which is benefit to enlarge thewidth of the weld interface. The thickness of intermetallic compounds layer isaffected by laser energy input and maximum temperature of braze metal/steelinterface. Combined with the influence of welding parameters on the weld formationand intermetallic compounds, this model can be used to predict the welding results.更多还原