激光选区熔化微尺度熔池特性与凝固微观组织

发布时间：2018-01-17 23:28

本文关键词：激光选区熔化微尺度熔池特性与凝固微观组织　出处：《清华大学》2016年博士论文　论文类型：学位论文

【摘要】：激光选区熔化是金属增材制造的主要技术途径之一,常用于带内部复杂结构的异形金属零部件成形,目前已得到广泛重视和大量研究。但由于对其微结构和微缺陷形成机制的认识不足,其潜力尚未得到充分发挥,也制约了其应用范围。本文从微尺度熔池特性出发,对激光选区熔化微结构调控和跨尺度微观组织形成等问题开展研究。以高熔点、高热导金属钨为对象,开展了激光选区熔化表面球化问题研究,观察分析了典型的表面球化凝固形貌,建立了将材料物性、工艺参数与球化现象相关联的“铺展/凝固竞争模型”,揭示了球化现象的物理机制,并提出了抑制球化、提高致密度的方法。开展成形缺陷问题研究,采用同步辐射显微CT对激光选区熔化Co-Cr-Mo合金缺陷进行了三维形貌重构。通过缺陷形貌分析和薄液膜流动稳定性分析,认为表面粗糙、熔池振荡、脉冲激光反冲力、保护气流剪切力、随机分布的粉末粒子等各类扰动源是导致成形缺陷的主要因素。研究发现基于扫描策略的激光选区熔化“缺陷自愈合现象”,逐层旋转激光扫描方向有助于减少扰动源影响,抑制缺陷。不同的激光扫描策略形成不同的温度场和熔池散热方向,是调控、抑制缺陷的重要手段,同时也具有调控晶粒生长的能力。研究发现长条柱状晶逐层外延生长现象,阐明了温度场、流场耦合作用下的激光选区熔化晶粒择优生长规律。研究发现多种合金存在亚微米尺度的六角胞状、伸长胞状和条状亚结构,认为微熔池非平衡快速凝固与非线性热毛细对流具有相互耦合作用,亚微米尺度结构的产生与熔体热毛细对流失稳、贝纳德对流胞和非平衡快速凝固的相互耦合作用有关,受控于温度梯度、表面张力梯度以及熔体粘度、热扩散系数等本征物理量。激光选区熔化的熔池尺寸较小(~100μm),重力影响可以忽略,表面张力主导。微尺度熔池内存在极高的温度梯度和表面张力梯度,可产生强对流失稳和贝纳德自组织结构;由于凝固速度极快,对流结构可保留在凝固组织之中,进而形成独特的亚微米尺度结构。这些亚微米尺度结构,与表面凝固形貌和快速凝固晶粒组织共同组成了激光选区熔化独特的跨尺度微结构。
[Abstract]:Laser selective melting is one of the main technical approaches for metal augmentation, which is often used in the forming of special-shaped metal parts with complex internal structures. At present, extensive attention has been paid and a lot of research has been done. However, due to the lack of understanding of its microstructure and mechanism of microdefect formation, its potential has not been fully exploited. Based on the characteristics of the micro-scale melting pool, the control of laser selective melting microstructure and the formation of cross-scale microstructure are studied in this paper. The high melting point and high thermal conductivity tungsten metal are taken as the object. The study on selective melting surface spheroidization of laser was carried out, and the typical surface spheroidization morphology was observed and analyzed. A "spreading / solidification competition model" was established which could relate material properties, process parameters and spheroidization phenomena. The physical mechanism of spheroidization was revealed, and the methods of restraining spheroidization and increasing density were put forward. Synchrotron radiation microCT was used to reconstruct the 3D morphology of laser fused Co-Cr-Mo alloy defects. The surface was considered to be rough by analyzing the defect morphology and the flow stability of thin liquid film. Molten pool oscillation, pulsed laser backstroke, protection of airflow shear force. Random distribution of powder particles and other disturbance sources are the main factors leading to forming defects. It is found that laser selective melting based on scanning strategy is "defect self-healing phenomenon". Layer by layer rotation of laser scanning direction is helpful to reduce disturbance source influence and suppress defects. Different laser scanning strategies form different temperature field and melting pool heat dissipation direction, which is an important means to control and suppress defects. At the same time, it has the ability to regulate grain growth. The phenomenon of epitaxial growth of long columnar crystals is found, and the temperature field is clarified. The preferential growth rule of laser selective melting grain under the interaction of flow field. It is found that there are sub-micron scale hexagonal, elongated and striped substructures in many alloys. It is considered that the nonequilibrium rapid solidification and nonlinear thermal capillary convection of the micromelting pool are coupled with each other, and the generation of submicron scale structure and the instability of the melt thermal capillary convection are also discussed. The coupling of Bernard convection cell and nonequilibrium rapid solidification is controlled by temperature gradient, surface tension gradient and melt viscosity. Thermal diffusion coefficient and other intrinsic physical quantities. The size of melting pool of laser selective melting is smaller than 100 渭 m, and the influence of gravity can be neglected. High temperature gradient and surface tension gradient exist in the micro-scale molten pool, which can produce strong convection instability and Bernard self-organization structure. Because of the high solidification rate, the convective structure can be retained in the solidified structure, and then form a unique sub-micron scale structure. Combined with surface solidification morphology and rapidly solidified grain structure, laser selective melting has a unique cross scale microstructure.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG665

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