异种金属激光焊接技术凭借激光热源能量密度集中、加工效率高效、易实现自动化等优势，从各种异种金属连接技术中脱颖而出，在化工、航天航空、车辆工程等行业得到大量使用。相比于同种金属的激光焊接，异种金属热物性之间的差异给焊件的可焊性提出了更高的挑战。同时，焊接过程中合金成分的混合与重分布使得影响焊缝质量的机制更为复杂。比如，熔点的差异导致两种金属的熔化先后不一，从而对焊缝的力学性能造成损害。金属成分在混合过程中，合金元素可能因达到一定浓度而导致金属间化合物（intermetallic compounds，IMCs）的形成。在异种金属的激光焊接过程涉及诸多物理效应，影响最终焊缝形成的工艺参数与实验条件繁杂，要想对焊缝的缺陷与成分进行调控，并且获得热应力、应变小、微观结构致密细小的目标焊缝，具有极大的难度。这些目的的实现均与熔池中的热量输运和合金元素浓度的动态变化有着密切关联，将介观尺度上熔池中的热质输运行为与焊缝的宏观力学性能、微观组织结构等建立联系也是目前研究的主要内容之一。因此，本文结合工程实际应用，针对异种金属激光焊接涉及到的跨尺度多物理场中的科学问题，以工业纯镍（Ni）与304不锈钢（304SS）的熔焊为例，利用自编程开发的针对激光焊接中三维传热传质问题的求解器（基于有限体积法与SIMPLE算法），分别对传导模式、匙孔模式下的异种金属焊接熔池中的热质输运行为特性进行了研究。本文的主要内容与研究成果如下：

1. 基于Navier-Stokes方程，耦合温度场、熔池流场、浓度场建立了三维数值模型，模型考虑了基体金属的相变传热、固液相混合区对动量的耗散、熔池Marangoni对流对热输运与质量输运的耦合效应等多种复杂物理机制。

2. 针对304SS与Ni的激光传导焊接进行了数值模拟，将计算的熔池尺寸与合金元素分布值与实验结果进行了对比，验证了模型的有效性。首先，通过量纲分析了传导模式激光焊接的熔池演化中传热传质的主导机制。然后将数值模拟结合正交参数设计与极差分析，系统地研究了激光功率、光斑偏移量、扫描速度对Ni与304SS激光传导焊接中接头的合金元素重分布的影响。以流入Ni侧的Fe元素平均含量表征熔池中元素分布情况，通过极差分析研究了工艺参数的相对重要性：扫描速度各水平的极差为9.45%，光斑偏移量为9.17%，功率为1.11%。流入Ni侧的Fe元素平均浓度与扫描速度呈负相关，与偏移量呈正相关。为了更好地调控焊缝中溶质元素的分布，进而调控微观组织，得到优化的焊缝性能，进一步探讨了探讨了工艺参数影响元素分布背后的物理机制，适当地降低扫描速度，向304SS侧偏移光斑有利于Fe元素稀释更充分，分布更均匀。

3. 保持离焦量与扫描速度的不变，通过变化激光功率进行了304SS与Ni薄板深熔焊接实验。研究了功率密度对焊缝几何尺寸的影响，发现随功率升高，焊缝的深宽比逐渐增加。针对薄板深熔焊接的特点，建立了304SS与Ni的激光深熔焊接的三维数值模型。通过模拟结果可以看出，不同参数下模拟的焊缝轮廓结果均与实验一致。通过量纲分析，发现热传导为304SS与Ni薄板深熔焊接熔池中的主要传热机制，而传质机制由熔池对流主导。然后，研究了准稳态下的熔池不同位置的热输运特征、熔池的流动与溶质输运行为的关联。

With the advantages of concentrated energy density of the laser heat source, high processing efficiency and easy automation, laser welding technology for dissimilar metals stands out from various dissimilar metal fusion technologies and is used in large numbers in industries such as chemical, aerospace and vehicle engineering. The differences between the thermal properties of dissimilar base metals can lead to more challenging weldability than laser welding with the same metal, and the mixing and redistribution of alloy components during the welding process makes the mechanisms affecting the quality of the weld more complex. For example, differences in melting points lead to different melting sequences between the two metals, which can cause damage to the mechanical properties of the weld, and the formation of intermetallic compounds (IMCs), which can occur when the alloying elements reach a certain concentration during the mixing of the metal components. However, the process of laser welding of dissimilar metals involves many physical effects, and the parameters and process conditions that influence the final weld are so complex that it is extremely challenging to regulate the defects and composition of the weld and to achieve a target depth of melt with low thermal stress, and strain and a dense and fine microstructure. These objectives are closely related to the dynamic changes in heat and alloying element concentration in the melt pool, and it is one of the main trends in research to link the mesoscopic transport behaviour of the melt pool with the macroscopic mechanical properties and microstructure of the weld. Therefore, this paper combines the practical application of engineering for the important needs of laser welding of dissimilar metals, based on the status of relevant research, for dissimilar metal laser welding multi-physical field phenomenon, refining the scientific aspects of the impact of thermal mass transport in the melt pool during the welding process. This paper investigates the behavioural characteristics of heat and mass transport in the melt pool of dissimilar metals welding in conduction mode and keyhole mode, respectively, using a solver (based on the finite volume method and SIMPLE algorithm) for three-dimensional heat and mass transfer problems in laser welding as an example. The main contents and findings of this paper are as follows.

1. Based on the Navier-Stokes equation, coupled temperature field, fluid flow, concentration field to establish a three-dimensional numerical model, The model takes into account the complex physical mechanisms of phase change heat transfer in the base metal, the dissipation of momentum in the mushy zone, and the coupling effect of Marangoni convection on heat transport and mass transport.

2. For numerical simulation of laser conduction welding of 304SS and Ni, the calculated melt pool size and alloy element distribution values and experimental results were compared to verify the validity of the model. Firstly, the dominant mechanism of heat and mass transfer in the evolution of the melt pool of conduction mode laser welding was analyzed by means of dimensional analysis. The numerical simulations were then combined with orthogonal parameter design and extreme difference analysis to systematically investigate the effects of laser power, spot offset, and scan speed on the redistribution of alloying elements in the joint of nickel and 304SS. The average elemental content of Fe flowing into the Ni side characteristics the elemental distribution in the melt pool and the relative importance of the process parameters is investigated by means of a polar difference analysis: 9.45% for the scanning speed levels, 9.17% for the spot offset and 1.11% for the power. The mean elemental concentration of Fe flowing into the Ni side was negatively correlated with the scanning speed and positively correlated with the offset. The physical mechanism behind the influence of process parameters on elemental distribution was further explored. Appropriately lowering the scanning speed and shifting the spot towards the 304SS side facilitated a more adequate dilution and uniform distribution of Fe elements.

3. Keeping the amount of defocusing and scanning speed constant, deep penetration welding experiments were carried out by varying the laser power between 304SS and industrial pure nickel thin plate. The impact of power density on the geometry of the weld seam was investigated, found that with the increase in power, the depth to width ratio of the weld seam gradually increased. For the characteristics of deep fusion welding of thin plates, a three-dimensional numerical model was established for laser deep fusion welding of 304SS with industrial pure nickel. After calculation, the results of the simulated weld profile under different parameters are consistent with the experiment. Through dimensional analysis, conduction was found to be the main mechanism in the melt pool of 304SS and industrial pure nickel sheet deep fusion welding, while the mass transfer mechanism was dominated by convection in the melt pool. The quasi-steady state of the melt pool was then studied in relation to the heat transport characteristics of different locations, the flow of the melt pool and the solute transport behaviour.