异种合金激光-MIG复合熔钎焊界面组织不均匀性与接头力学性能调控机理研究

Considerable work has been performed on laser welding-brazing of steel to aluminum alloys in thin sheets at home and abroad. The results show that the relatively high tensile strength of the joints has been achieved. However, it has been found that the microstructural inhomogeneity of brittle intermetallic compounds is formed at the welding-brazing interface. With increasing the thickness of the welded joint, the problem of the microstructural inhomogeneity formed at the welding-brazing interface will become more serious. The inhomogeneity of intermetallic compounds formed at the welding-brazing interface has a significant detrimental effect on the mechanical properties of the welded joints. However, very few data have been published on the microstructural inhomogeneity of the intermetallic compounds at the welding-brazing interface for the welded joints in the middle-thickness plates. In addition, the understanding of the formation mechanism of the microstructural inhomogeneity occurred at the welding-brazing interface and the correlation between the microstructural inhomogeneity and the mechanical properties of the joints is inadequate. Therefore, in this study, a novel laser-MIG hybrid welding-brazing process will be proposeded to joining of aluminum to steel plates in the middle thickness, in a butt configuration. The welding temperature field for the laser-MIG hybrid welding-brazing interface by means of the numerical simulation will be calculated to obtain the promising temperature field distribution at the welding-brazing interface. The effects of the different welding parameters on the weld geometry and mechanical properties of the joints will be studied. Based on the the above results, a further investigation on the microstructural evolution of the welding-brazing interface will be carried out by the thermodynamic analysis of the formation of intermetallic compounds and growth kinetics experiment of the intermetallic compounds at the welding-brazing interface. The formation mechanism of the microstructural inhomogeneity of the intermetallic compounds at the laser-MIG hybrid welding-brazing interface will be elucidated. The effects of the microstructural inhomogeneity of the intermetallic compounds at the welding-brazing interface on the tensile fracture properties will be discussed in details. The quantitive relationships between the microstructural inhomogeneity of the intermetallic compounds at the welding-brazing interface and the tensile strength of the welded joints will be established. The results and findings of this study will contribute to an improved understanding of the laser-MIG hybrid welding-brazing process and provide a theoretical basis for laser-MIG hybrid welding-brazing joining of aluminum to steel plates in the middle thickness for future practical engineering applications.

目前国内外对薄板异种合金激光熔钎焊已进行了大量研究，可获得较高力学性能的接头，但是激光熔钎焊存在界面组织不均匀性的问题，随着板厚的增加，界面组织不均匀性问题将更加严重，对接头力学性能影响更为重要。目前对中厚板激光熔钎焊界面组织不均匀性的研究鲜见报道，对界面组织不均匀性的形成机理及其与接头力学性能相关性还缺乏认识。本项目以中厚板铝/钢异种合金对接接头为对象，提出一种聚焦的光纤激光-MIG复合热源熔钎焊新方法，通过接头温度场分布的模拟计算，研究不同工艺参数对界面组织不均匀性及接头力学性能的影响；通过热物理模拟实验、热力学分析计算和生长动力学实验，对界面组织不均匀性进行深入研究，揭示界面金属间化合物不均匀性的形成机理，探讨界面金属间化合物不均匀性对接头拉伸断裂行为的影响，建立界面金属间化合物不均匀性与接头抗拉强度的定量关系，为中厚板异种合金激光-MIG复合熔钎焊的工程应用提供理论基础。

针对单束激光焊接钢/铝异种接头界面金属间化合物(IMC)不均匀性问题，进行了双光束激光深熔焊接铝/钢接头新方法研究，光束能量比、相对位置、排布方式等参数对接头成形、界面组织和接头力学性能有重要影响。双光束激光深熔焊接在较宽工艺参数范围获得良好焊缝成形，界面IMC层厚度达10 µm以下，接头机械抗力最大为115.6 N/mm。.针对单束激光焊接钢/铝异种接头焊缝凹陷及微裂纹问题，开展了激光-MIG复合深熔焊接铝/钢接头新方法研究，改善了焊缝的凹陷缺陷，减少了界面IMC处微裂纹数量。界面IMC层厚度在10 μm以下，填充Al-Si焊丝接头界面形成了延性的Fe8Al2Si相，降低IMC层硬度至620.6 HV，接头拉伸抗力最高为13.0 kN。.针对铝/钢接头界面IMC脆性问题，探索了填充粉末和中间涂层对接头焊缝成形、界面IMC组织、显微硬度及力学性能的影响。填充Zn、Ni、Sn粉末改善了焊缝成形以及微裂纹、气孔等缺陷，明显减小了界面IMC厚度，显著降低了IMC层显微硬度值。添加Ni、Cu和AlSi12中间层，有效减少了界面IMC中Fe4Al13相的数量和厚度。.采用热物理模拟试验研究了加热温度及合金元素对界面IMC形成机制的影响。发现加热温度高于1000℃有利于控制界面Fe2Al5层厚度，700℃~900℃加热温度有利于控制粗大针状Fe4Al13相的数量和尺寸。加入Ni元素改善了界面IMC形态，降低了IMC厚度，降低了界面显微硬度，提高接头机械抗力约20%。.钢/铝异种接头界面IMC厚度及形态对接头拉伸断裂有重要影响。激光-MIG电弧复合焊接头为沿晶脆性断裂，但热源作用在钢侧接头断裂路径在厚度方向上呈现了一种混合断裂特征，提高了接头的抗拉强度。EBSD分析证实拉伸断裂发生在Fe2Al5与Fe4Al13相的相界处，或在Fe2Al5层的内部。.钢/铝接头界面IMC层主要由连续片状Fe2Al5层和靠近Fe2Al5层呈针状或锯齿状、分布于铝合金基体的Fe4Al13相组成。此外，EBSD相分布图证实，钢焊缝区晶粒的晶界上形成了细小的Fe4Al13和Fe2Al5相。.研究为异种合金激光焊接技术的应用提供理论依据，在航空航天、船舶、汽车工业领域有广阔的应用前景。