中国科学院力学研究所机构知识库

送粉式激光熔覆技术以其加工过程柔性化程度高、适用粉末材料体系广泛及制造过程绿色无污染的优势，广泛应用于航空航天、大型工程机械和船舶等领域构件的直接成形制造，同时可以实现具有特定力学、材料学和化学性能的功能涂层与基体构件的冶金结合。因此，送粉式激光熔覆技术得到了国内外研究者和工程技术人员的高度关注和广泛的研究。在送粉式激光熔覆工艺过程中，粉末流-激光、粉末流-气流的耦合作用决定粉末流空间传输行为、空间气流场和衰减的激光能量分布。粉末的空间传输特性、衰减后的激光能量分布和气流的动力学行为直接作用于熔池的气液界面，进而决定熔池特征及其演化规律。同时，熔覆过程高温度梯度和高冷却速率的特点使得熔池内存在复杂的热-力耦合作用，导致精确控制熔池形貌特征，实现无缺陷熔覆工艺存在很大的困难和挑战。因此，深入分析送粉式激光熔覆的工艺过程并建立合理的分析方法，总结熔池形貌特征、缺陷特征的形成规律并探究其形成机理是深入研究送粉式激光熔覆技术、提高其工程价值的根本途径。针对这一情况，本文采用数值模拟和工艺实验相结合的方式，基于激光和粉末流的相互作用研究熔池特征，分析送粉式激光熔覆粉末的两相传输行为、激光-粉末相互作用、熔池形貌特征成形规律及工艺过程导致的缺陷形貌和演化行为。本文主要内容及研究成果如下：

1. 采用数值模拟结合在线光学诊断的方法研究了同轴送粉过程中粉末的传输机理，开发了一种试错匹配数值模拟和实验结果来量化描述颗粒与喷嘴壁之间非弹性碰撞恢复系数的方法。实验统计和数值计算的粒子出口速度和空间浓度分布的一致性表明，恢复系数取值0.9可用于描述颗粒与喷嘴壁之间的非弹性碰撞行为。基于确定的恢复系数，提出了一种基于光学在线观测半定量和数值模拟定量分析粉末空间传输行为的方法，研究粉末从单层到多层射流的传输特性。研究发现：外保护气流对粉末在多层射流流场的传输行为有很大影响，当外环保护气流量为20L/min时最有利于粉末聚焦。由同轴送粉头内部结构确定的载粉气流的速度分布和颗粒与壁之间的非弹性碰撞是决定颗粒出口速度的两个相互耦合的因素。频繁的非弹性碰撞和载粉气流速度的降低导致粒子速度的发散和传输轨迹的波动。当颗粒的入口速度为1.33m/s时，出口速度范围为0.4m/s至0.9m/s。本部分研究提供一种确定粒子与同轴送粉头壁面非弹性碰撞恢复系数的通用方法，全面探究了粉末颗粒在同轴送粉头内部和自然射流区的输送机理。

2. 从理论和实验两个角度研究了激光熔覆在大工艺范围的激光能量输入和粉末质量添加条件下单道熔覆工艺及熔池几何特征的成形规律。建立了9个熔池形貌的几何特征与激光功率和送粉速率组合工艺参数(记为P^αF^β)之间的线性回归模型，以定量分析熔池几何特征与工艺参数之间的关系，大的回归系数和残差的均匀分布证明了所建立模型的正确性。可以得出结论：随着能量输入的增加，熔池内具有向外流动特征的Marangoni对流增强。因此，熔池经历了从无稀释、浅稀释、平坦稀释到波动稀释的演变。熔池宽度W与激光功率P之间几乎是线性关系，表明熔池表面激光能量积累在熔池宽度W的演变中起主要作用。随着送粉速率的增加，熔池高度h_c的增加率在640%至360%之间，说明熔池高度h_c的演变由送粉速率主导。熔池横截面的总面积A、熔池在基体之上的面积A_c、基体上熔池的面积A_m、熔池总高度H、熔池在基体之上的高度h_c、熔池在基体内的深度h_m、熔池宽度W、稀释率D、润湿角θ由能量输入和质量累积的复杂耦合关系确定，分别与组合工艺参数P^0.5F^0.2、P^0.2F^0.5、P^0.5/F^0.2、P^0.3F、P^0.5/F^0.2、P^0.2/F^0.2和P^0.2/F^0-2构成线性回归模型。这项工作旨在探究激光能量输入和粉末质量添加对熔池几何特征的影响及其影响机制，为促进送粉式激光熔覆技术的实际工程应用提供重要的参考价值。

3. 气孔、裂纹和欠熔合是送粉式熔覆缺陷的主要形式。系统分析了不同激光能量输入和粉末质量输入导致的缺陷大小、形状及其分布规律。建立了熔覆过程的热-力耦合模型，解释了裂纹的动态演化机制。分析了涂层的微观形貌特征、裂纹萌生行为、扩展规律及其扩展机理。结果表明：熔池内气孔形貌是激光能量输入和粉末质量输入相互耦合作用的结果，其分布规律反映熔池内部的物理过程机制。单道裂纹主要来源于熔池表面，并沿熔池深度方向扩展至基体。激光功率和扫描速度是影响裂纹萌生和扩展行为的主要因素。当扫描速度相对较低时，可以诱发基体产生裂纹，并且裂纹在平行于激光扫描速度的方向上扩展，表现为基体被撕裂。在多道搭接熔覆的工艺过程，裂纹主要表现为网状裂纹的形成和横向裂纹的持续扩展。合理工艺参数可以实现无裂纹熔覆工艺。未熔合缺陷是熔池能量输入不足的表现，并且影响裂纹缺陷的动态演化行为。本工作旨在深入理解熔池内的缺陷形貌及其动态演化行为，并为无缺陷熔覆工艺提供理论指导。

The powder fed laser cladding technology has been widely used in the direct manufacturing of components in many fields such as aerospace, large engineering machinery, shipbuilding with its advantages of high flexibility process, wide range of application of powder material systems and pollution-free manufacturing process. Furthermore, by employing powder fed laser cladding technology, a metallurgical combination of functional coatings with specific mechanical, material and chemical properties with the substrate can be achieved. Therefore, the powder fed laser cladding technology has been attracted highly concerned and widely studied by researchers and engineers at home and abroad. In the process of powder fed laser cladding, the coupling behavior of powder flow, laser and gas flow determines the spatial transmission behaviors of powder flow, spatial gas flow field and attenuation of the laser energy distribution. The spatial transmission characteristics of the powder, the distribution of the attenuated laser energy, and the dynamic behavior of the gas flow directly act on the gas-liquid interface of the molten pool, thereby determining the characteristics and evolution behavior of the molten pool. At the same time, the characteristics of high temperature gradient and high cooling rate in the cladding process lead to complex thermal-mechanical coupling behaviors in the molten pool, resulting in significant difficulties and challenges in accurately controlling the geometric characteristics of the molten pool and forming defect-free morphology. Therefore, in-depth analysis of the process of powder fed laser cladding and establishment of reasonable analysis methods, summarizing the formation rules of the morphology and defect characteristics of the molten pool, and exploring their formation mechanisms are the fundamental ways to improve the engineering value of powder fed laser cladding technology. In response to this situation, this paper uses a combination method of numerical simulation and process experiments to study the characteristics of the molten pool based on the interaction between laser and powder flow. The two-phase transport behavior of powder and gas flow, between laser and powder, the formation rules of the morphological characteristics of the molten pool, as well as the morphology and evolution behavior of the defects caused by the process are analyzed. The main content and research results are as follows:

1. The transmission mechanism of powder in the coaxial feeding nozzle is studied by the combined methods of numerical simulation and online optical diagnosis. A trial-and-matching method is developed to quantify the restitution coefficient which is used to describe the inelastic collision between particles and the nozzle wall. The consistency of the outlet velocity and the spatial concentration distribution of particles between the experimental statistic and the numerical calculation shows that the restitution coefficient of 0.9 can be used to measure the inelastic collision behavior between particles and the nozzle wall. Employing the determined restitution coefficient, a semi-quantitative method based on optical diagnostic and quantitative analysis derived from numerical calculation is proposed to study the powder-gas flow transport characteristics from single-layer to multi-layer jet flow. It is found that the outer shielding gas flow has a great influence on the multi-layer jet flow field, and it is most conducive to powder focusing at the outer shielding gas flow of 20L/min. The velocity distribution of carrying gas flow determined by the inner structure of the coaxial feeding nozzle and the inelastic collision between particles and the wall are the two mutually coupled factors that determine the outlet velocity of particles. Frequent inelastic collisions and the decrease of the carrying gas flow velocity lead to velocity dispersion and trajectory fluctuation of particles. When the inlet velocity of particles is 1.33m/s, the outlet velocity ranges from 0.4m/s to 0.9 m/s. This section aims to provide a general method to determine the restitution coefficient and offer a comprehensive understanding of the transportation mechanism of power particles both inside the coaxial powder feeding nozzle and the multi-layer jet zone between the substrate and the nozzle outlet.

2. The formation rule of single-track geometrical characteristics under a wide range of laser energy and powder mass input is studied from both theoretical and experimental points of view. The linear regression models between nine geometrical characteristics and the combined process parameters of laser power and powder feeding rate written as P^αF^β are established to quantitatively analyze the geometrical characteristics of the molten pool, which are confirmed by large correlation coefficients and the analysis of residuals. It can be concluded that more laser energy input leads to the enhanced outward direction of Marangoni convection and thus the molten pool undergoes the evolution from no dilution, shallow dilution, flat dilution to fluctuating dilution. The almost linear relationship between the cladding width W and the laser power has been found, indicating that laser energy accumulation plays a major role in the evolution of cladding width W. The increased ratio of cladding height h_c ranges from 640% to 360% with the increase of powder feeding rate, implying that the evolution of cladding height h_c is dominated by the powder feeding rate. The total area of the cross-section A, the area of the clad A_c, the area of the molten substrate A_m, the total height of the cross-section H, the penetration depth h_m, the dilution ratio D, and the cladding angle θ are determined by complex coupling of energy input and mass accumulation and they are proportional to P^0.5F^0.2, P^0.2F^0.5, P^0.5/F^0.2, P^0.3F, P^0.5/F^0.2, P^0.2/F^0.2, and P^0.2/F^0.2, respectively. This work aims to provide a general knowledge of the influence of laser energy input and powder mass addition on the geometrical characteristics of the molten pool and its related influence mechanism, which is very helpful in providing reference and laying a basis for promoting the practical application of laser cladding technology.

3. Porosity, cracks, and lack of fusion are the main characteristics morphology of defects in the molten pool. The effects of different laser energy input and powder mass input on the size, shape, distribution of defects and their dynamic evolution during the cladding process are systematically analyzed. A thermal-mechanical coupling model for the cladding process is established to explain the dynamic evolution mechanism of cracks. The microstructure morphology characteristics of the coating, the crack initiation behavior, the law of crack propagation and its propagation mechanism are analyzed. The results show that the pore morphology in the molten pool is the result of the interaction between laser energy input and powder mass input, and its distribution reflects the physical process mechanism inside the molten pool. The cracks mainly originate from the surface of the clad and propagate along the depth direction of the molten pool to the substrate. The laser power and scanning speed are the main factors that affect crack initiation and propagation behavior. When the cladding speed is relatively low, the crack in the substrate can be induced, and the crack propagates in the direction parallel to the laser scanning speed, which is manifested as the substrate being torn. The continuously expanding transverse cracks and the network-shaped cracks can be formed in the multi-track overlapping cladding process. The crack-free cladding process can be achieved by a reasonable selection of processing parameters. Lack-of-fusion defect is a manifestation of insufficient energy input in the molten pool and affects the dynamic evolution behavior of crack defects. The purpose of this work is to deeply understand the defect morphology and its dynamic evolution behavior in the molten pool, and provide theoretical guidance for the defect-free cladding process.