激光熔覆熔池动态演化及缺陷工艺调控研究进展

激光熔覆是一种高能束增材修复技术,具有热影响区小、组织性能可控性强、材料选择范围广等系列优势,目前已广泛应用于能源动力等领域关键金属构件的增材制造成形与受损零部件的修复再制造中。激光熔覆是以“激光”为热源的能量沉积技术,包括高能激光束冲击、表面熔池熔化快凝及熔覆表面层形成等多种物理、化学过程,其中熔池内金属热流体动力演化行为与熔覆层缺陷及表层组织性能调控密切相关。金属熔池具有“急热骤冷”的凝固特征,其内部对流、传热和传质等行为决定了熔覆层中温度及应力分布状态,是诱导熔覆层内气孔、裂纹等组织内部缺陷形成的关键因素。从激光熔覆过程中熔池内部对流、传热与传质的动态物理特性出发,论述了激光热源的理论模型设计、动态熔池中“流场+温度场+应力场”的多物理场数值模拟等方面的相关研究。在此基础上,分析了激光熔覆层典型缺陷-裂纹和气孔的形成机理及特征,总结了“材料-工艺-熔凝行为-涂层缺陷”的内在关联机制。同时,针对单一工艺方式调控熔池内熔凝过程的局限性,概述了多种复合能量场调控技术对熔覆层内部缺陷的作用机制与调控效果。最后,总结了当前激光熔覆层缺陷动态形成过程中存在的问题,并对其发展趋势进行了展望。

Review of Dynamic Evolution of Laser Cladding Molten Pool and Defect Process Regulation

Laser cladding is a high-energy beam additive repair technology, which has a series of advantages such as small heat-affected zone, strong controllable microstructure properties and wide range of material selection, which can effectively improve and enhance the mechanical, metallurgical, physical and other properties of the substrate surface. At present, laser cladding technology has been widely used in the additive manufacturing and molding of key components such as fuel nozzles and eddies in the aerospace field, as well as the repair and re-manufacturing of vulnerable parts such as molds and rails in the field of energy and power. Laser cladding is an energy deposition technology with "laser" as the heat source. The whole process involves powder transportation, material melting and coating solidification and other links, including convection, heat transfer, mass transfer and crystallization and other physical and chemical phenomena, affected by laser process parameters, powder and matrix materials and processing environment and other factors. During the whole cladding process, the surface temperature changes drastically, the solidification rate of the molten pool is large, and the metallurgical defects of the cladding layer such as pores and cracks are not in place due to the lack of matching and regulation of various factors such as heat source characteristics, process parameters and material properties, which in turn adversely affect the mechanical and microstructure properties of the processed or repaired structure. The metal molten pool has the solidification characteristics of "rapid heat and quenching", and its internal convection, heat transfer and mass transfer behaviors determine the temperature and stress distribution state in the cladding layer, which is the key factor inducing the formation of internal defects such as pores and cracks in the cladding layer. From the perspective of analyzing the dynamic physical characteristics of convection, heat transfer and mass transfer in the molten pool during the laser cladding process, this paper discussed the formation mechanism of the liquid molten pool of laser cladding, the reasonable selection of laser heat source model, and the multiphysics numerical simulation of "flow field + temperature field + stress field" in the dynamic molten pool. On this basis, the formation mechanism and characteristics of cracks and pores of typical defects of laser cladding coatings were analyzed, and the intrinsic correlation mechanism and regulation mode of "material-process-melting behavior-coating defect" were summarized from the perspective of single process regulation. At the same time, considering that only through material design, process parameter optimization and other single methods had limited effect on the control of hydrodynamic behavior in the molten pool, it was difficult to get rid of the dilemma that traditional process control methods relied too much on a large amount of data and were greatly affected by test errors. The three energy fields of ultrasonic energy field, electromagnetic energy field and induction heat energy field were summarized in the laser cladding process, and the influence mechanism on the molten pool and the regulation effect of defects was analyzed. Finally, the problems existing in the dynamic formation of laser cladding layer defects were summarized, and its development trend was prospected.