振荡扫描激光束-电弧复合焊接的过程控制与冶金行为研究

Based on the previous finding that high arc current must be used to advoid the pore formation within the weld during laser-arc hybrid welding of thick aluminum alloys, which is very adverse to improve the joint mechanical properties including strength and toughness due to the high heat input, we put forward to a new idea named as oscillating laser beam-arc hybrid welding based on high frequency laser beam oscillator to overcome this quesiton. During this new hybrid welding, the regularly oscillating laser beam would stir the molten pool to refine the grains and improve the uninformity of the weld microstructure as the laser-arc interaction reduces the weld porosity. By the syniergic effect of the stirring impact induced by osicillating laser beam and the laser-arc interaction of hybrid welding, not only the weld porosity can be well avoided but also the joint mechanical propertes can be improved. This study aims to investigate the effects of the high frequency osicillating of laser beam on the arc characteristics, dorplet transfer and laser-arc interaction during hybrid welding, explore the effects of the strong stirring and regular melt flow indcued by laser beam oscillating on the compicalted metallurgical behaviors in molten pool including heat and mass transfer, burning-loss and migration of alloying elements, crystallization of weld metal, the formation of defects, etc., and subsquently discuss the relationships betweent the metallurgical behaviors and the joint mcirostrucutre and mechanical properties. According to the research results, the controlling strategies for process stability and joint quality would be developed, and the database of welding parameters for oscillating laser beam-arc hybrid welding of aluminum alloys would be estabilished. The results obtained in this study shall be a good reference for high quality welding of aluminum alloys, and also useful to enrich the fundamental theory of laser-arc hybrid welding.

研究发现，激光-MIG电弧复合焊接中厚铝合金必须采用大电弧电流来抑制焊缝气孔，这导致热输入较大，接头强韧性不够理想。为此，本项目提出基于振镜的振荡扫描激光束-电弧复合焊接铝合金新工艺、新方法：利用激光束的高频扫描强制搅拌熔池，该搅拌效应和激光-电弧相互作用能够优势互补，实现既消除焊缝气孔又改善焊缝组织、提高接头强韧性的目的。本项目将围绕其中的关键科学问题进行研究，揭示激光束高频振荡扫描的动态行为对电弧特性和激光-电弧相互作用的影响规律；探索由高频振荡扫描激光束驱动的熔池搅拌效应和熔体有序流动对熔池传热传质、合金元素烧损迁移、缺陷形成、焊缝结晶等焊接冶金行为的影响规律，及其和接头组织性能的内在关联。在此基础上，提出铝合金振荡扫描激光束-MIG复合焊接过程控制和接头组织性能的调控方法，建立工艺参数数据库系统，为铝合金的高质量焊接提供理论依据和技术支撑，并丰富激光-电弧复合焊接基础理论。

围绕振荡扫描激光束-电弧复合焊接新方法的关键科学问题开展了系统研究，实现了降低消除气孔电弧电流阈值、提高接头强韧性的预期目标，掌握了其工艺特性，明晰了激光束振荡扫描效应对激光-电弧相互作用的影响机理，及其对熔池行为和焊缝组织性能的影响机制。研究结果不但为铝合金高质量焊接提供了一种新工艺，而且对加深和丰富激光-电弧复合焊接工艺的理解有重要意义。主要研究结果如下：. （1）激光束振荡扫描行为能够增强激光光致等离子体和电弧热等离子之间带电粒子的输运行为，从而提高激光-电弧相互作用程度，增强工艺稳定性。与无扫描复合焊接相比，振荡扫描复合焊接激光辐射区光谱强度（Mg II 279.76nm谱线）最大可提高近6.5倍，电子温度和电子密度分别提升17%和45%。. （2）激光振荡扫描对复合焊接熔池有强烈的搅拌效应。随着激光束扫描振幅和频率的增加，由振荡激光束驱动的搅拌效应在熔池中逐渐占据主导地位，引导上层液态金属向下层流动，促进合金元素向焊缝根部过渡，从而减少焊缝溶质偏析。振荡扫描复合焊缝激光区合金元素含量最大可增加至电弧区的4/5，相对于常规复合焊接提高近3倍。. （3）圆形扫描路径是振荡扫描复合焊接的最优扫描模式，具有最有效的熔池搅拌效应和最好的工艺稳定性。该发现为后续研究指明了发展方向。. （4）突破了常规复合焊接必须采用250A以上大电流才能消除气孔的工艺瓶颈，激光束振荡扫描可将获得I级焊缝的阈值电流降低至150A，证明了项目预期设想。. （5）构建了熔池流场有限元数值分析模型阐述了激光束振荡扫描行为捕获气泡，促进其逸出并降低焊缝气孔率的物理机制。. （6）掌握了激光束振荡扫描行为对复合焊缝组织性能的影响规律，为其工业应用提供了有效的数据支撑。在优化的工艺参数下，振荡扫描复合焊缝抗拉强度比常规焊缝提高20MPa左右，延伸率提升43%。焊缝强韧性增强的主要原因为激光束搅拌效应打乱了树枝晶生长方向并细化了晶粒，同时使强化相析出更均匀。