激光熔覆是材料科学及智能制造学科交叉发展的一项新兴表面改性技术。以其巨大的灵活性与优越的性能，广泛应用于大型装备关键零部件表面修复与强化，为军工以及民用重大装备的维护、升级乃至革新提供了新的方式。激光熔覆工艺具有小空间、瞬时性、高能量、剧烈变化等特点，伴有强光、高热、粉尘等，因此熔池内小尺度的材料成形机理一直是理论研究的热点与难点。本文针对42CrMo基材表面T15粉末同轴送粉激光熔覆工艺，对高温条件下能量与物质输运、凝固晶粒组织演化等物理现象进行数值建模，分析流动、传热及晶粒生长行为，并与试验进行对比验证，为激光熔覆工艺机理分析提供基础的理论指导与量化数据。首先，本文建立了熔池微流动的多组分多相流模型。模型耦合传热、传质、流动等多物理过程，通过能量源项与质量源项描述了激光照射与粉末添加，采用CLSVOF捕捉自由表面，并考虑了表面张力引起的Marangoni效应。采用单、多道次激光熔覆试验，对比熔覆层高度、宽度、深度、稀释率等，验证多相流模型的准确性。粉末中添加稀土Ce元素后，由于表面张力系数变化，熔池内流动方向由内向转为外向。多道次搭接的模拟结果显示，熔池温度场、流场具有不对称性，搭接区存在重熔现象，导致熔覆层呈现粗晶区与细晶区的交替出现。其次，本文建立了熔池凝固微观尺度的晶粒生长相场法模型。模型耦合了固液相变、界面推移、成分分布等因素，通过序参量?和组分C对体系自由能最小化进行求解。模拟结果中，计算域下方率先出现柱状晶，上方出现等轴晶。过冷度是晶粒生长的重要条件，晶间竞争与成分偏析是晶粒生长的主要现象。本文模拟了T ?=-4×10^4~-6×10^4 K/s三种不同边界冷却条件，冷却速率较大时，柱状晶高度低，晶粒生长快。模拟结果与熔覆试验金相表征结果进行了比较验证，两者基本一致。最后，本文建立了微流动条件下相场法耦合玻尔兹曼格子法的晶粒生长模型。引入熔池多相流模型的速度边界，进行了流动过程单、双、多晶粒生长模拟。模拟结果显示了液相在晶粒周边的绕流，且晶粒具有沿流动方向优先生长的趋势。进一步模拟了晶粒在流场内随机生成并生长过程。微观尺度的精细建模有助于揭示熔池内剧烈变化的物理细节，是激光熔覆工艺设计的理论基础。
Laser cladding is an emerging surface modification technology developed at the intersection of material science and intelligent manufacturing disciplines. With its great flexibility and superior performance, it is widely used in the surface repair and strengthening of key parts in large-scale equipment, providing a new way for the maintenance, upgrading and even innovation of major military and civilian equipment. The laser cladding process has the characteristics of small space, instantaneity, high energy, and drastic changes, accompanied by strong light, high heat, dust, etc. Therefore, the small-scale material forming mechanism in the molten pool has always been a hot and difficult point of theoretical research. In this paper, based on the coaxial powder feeding laser cladding process of T15 powder on the surface of 42CrMo substrate, numerical modeling is carried out on physical phenomena such as energy and material transport, solidified grain structure evolution under high temperature conditions, and the flow, heat transfer and grain growth behavior are analyzed. Comparison between computation and experiments is made, so as to provide basic theoretical guidance and quantitative data for the mechanism analysis of laser cladding process.Firstly, this paper establishes a multi-component and multi-phase flow model for the molten pool. The model couples multi-physical processes such as heat transfer, mass transfer, and flow. The energy source and mass source terms describing laser irradiation and powder addition are customized. CLSVOF is used to capture the free surface and the Marangoni effect caused by surface tension is considered. Single and multi-pass laser cladding tests are used to compare the height, width, depth, dilution rate of the cladding layer to verify the accuracy of the multiphase flow model. After the addition of rare earth Ce element, due to the change of the surface tension coefficient, the flow direction in the molten pool changes from inward to outward. The simulation results of multi-pass overlap show that the temperature field and flow field of the molten pool are asymmetry. There is a re-melting phenomenon in the overlap area, which causes the cladding layer shows alternate coarse and fine grain regions.Secondly, this paper establishes the phase field method model of the micro-scale grain growth of the molten pool solidification. The model couples factors such as solid-liquid phase transition, interface transition, composition distribution, and solves the minimization of system energy expressed by order parameter ? and component C. In the simulation results, columnar grains appear first in the lower half of the computational domain, while equiaxed grains grow in the upper half of the computational domain. Undercooling is critical for the grain growth, while intergranular competition and composition segregation are common phenomena. Three different boundary cooling rates, T ?=-4×10^4~- 6×10^4 K/s, are simulated. The faster the cooling, the lower the height of columnar grains and the faster the grain growth. The simulation results and the metallographic characterization results of the cladding test are compared and verified, and similarity is shown.Lastly, to simulate the grain growth process under the condition of micro flow, a phase field coupled Boltzmann lattice model is established. The velocity boundary of the multiphase flow model of the molten pool is introduced, and the single, double, and multi-grain growth simulations in the flow process are carried out. The simulation results show that the liquid phase flows around the grains, and the grains have a tendency to preferentially grow along the flow direction. The process of randomly generating and growing grains in the flow field is further simulated. The fine modeling of the micro-scale helps to reveal the physical details of the drastic changes in the molten pool, which is the theoretical basis of the laser cladding process design.
