Objective During the coaxial powder feeding laser deposition process, the mass transfer, heat transfer, and fluid flow in the molten pool are closely related to the surface and internal quality of the formed part. Numerical simulation technology can provide an effective means for studying this series of coupled complex physical phenomena. In recent years, numerical modeling of powder feeding laser deposition has made great progress. However, the deposition layer morphologies in such model are mostly pre-set rather than being computed. In fact, the establishment and development of the deposition layer morphology is a combined result of powder feeding, melting, and solidification. During the development of the technology towards precision manufacturing, it is of great significance for understanding the deposition mechanism and improving the internal and surface quality of the parts to build a coupled mathematical model that can accurately describe the deposition formation and the physical processes in the molten pool. In this paper, a mathematical model of the coaxial powder feeding laser deposition is established, and the numerical computation of the single pass and overlap forming processes is completed. The shape of the deposition layer, the characteristics, and the cause of the temperature field of the molten pool are analyzed. We hope that this research can provide help for understanding the deposition mechanism dominating the forming process, which promotes laser deposition manufacturing towards high dimensional accuracy and high internal quality.
Methods In this study, a three-dimensional mathematical model of the coaxial powder feeding laser deposition is established by combining the volume of fluid (VOF) method with the powder feeding equation. The shape of the deposition layer and the morphology of the molten pool of single pass and overlap forming processes for IN718 alloy are simulated and verified by experiments with the same parameters. Based on the good agreement between the simulation results and the experimental results, the mutual influence of the temperature between the first pass deposition and the overlap pass deposition for the overlap forming is evaluated. The heat accumulation effect due to the first pass deposition on the temperature of the overlap and its molten pool is analyzed by comparing their maximum temperatures. At the same time, the effect of the current high temperature of the overlap pass deposition on the temperature gradient and cooling rate of the first pass deposition is analyzed by comparing temperature re-rising. The potential effect of the first pass deposition on the solid phase transition is also pointed out.
Results and Discussions The shape and size of the deposition layer of single pass forming are achieved by using the three-dimensional mathematical model presented in this paper, which are further compared with the experimental results (Fig.4 and Fig.5); the temperature field and the development of the three-dimensional molten pool morphology under different moments are obtained (Fig.7 and Fig.8); the molten pool depth calculated by the simulation is basically consistent with the experimental results (Fig.9). On the basis of single pass forming simulation, the deposition layer shape, molten pool morphology, and temperature field are obtained for overlap forming process with 30% overlapping rate (Fig.10, Fig.11 and Fig.12). Because of the thermal accumulation of the first pass deposition, the molten pool of the overlap pass deposition is 4.19% larger than that of single pass process and absorbs more latent heat of fusion, which leads to that the maximum surface temperature of the overlap pass deposition is slightly lower than that of the single pass forming (Fig.13 and Fig.14). At the same time, affected by the high temperature state of the overlap pass deposition, the temperature of the first pass deposition re-rises in the range of 1000--1600 K, and the amplitude is 100--300 K (Fig.16).
Conclusions In this paper, a three-dimensional mathematical model is put forward by combining the VOF method with the powder feeding equation, and the simulations of single pass and overlap laser deposition forming processes with coaxial powder feeding are realized. When the scanning speed is increased by 75%, that is, from 8 mm/s to 14 mm/s, the height and width of the deposition layer are reduced by 57.1% and 21.6%, respectively. The calculated height, width, and depth of the single pass deposition are in good agreement with the experimental results. In the overlap forming process with 30% overlapping rate for one-way parallel scanning, influenced by the heat accumulation of the first pass deposition, the molten pool formed by the overlap pass deposition is 4.19% larger than that formed by the single pass forming, and more latent heat of fusion is absorbed, which causes that the highest surface temperature of 2038 K during the overlap forming is slightly lower than that of 2068 K during single pass forming. At the same time, the first pass deposition is also affected by the high temperature state of the overlap, which presents the temperature re-rise to some extent at different positions. Because the temperature re-rise occurs within 1000--1600 K with an amplitude of 100--300 K, the solid phase transition of the first pass deposition will be affected for IN718 alloy. The results of this study are of great significance for process optimization and quality improvement in laser deposition technology.
