Insufficient laser energy input typically leads to the well-known balling effect and results in potential porosities. The basis of selective laser melting (SLM) process is the interaction between the laser and feedstock powder which determines the thermo-fluid dynamics of melt pool and the final quality of SLMed parts. An in-depth insight of laser energy deposition during the SLM process is critical for the process’ optimization and defect elimination. However, due to the difficulty of direct observation of laser reflections, the current understanding of laser irradiation mechanisms is still vague and unclear. Numerical modeling is an effective way to simulate the heat and mass transfer during the SLM process at mesoscopic scale. However, the laser heat source models used in existing literature, such as volumetric heat source and vertical ray heat source, have rarely considered the authenticity of laser-material interaction and have neglected the behavior of multiple reflections and absorptions of a laser in SLM. Therefore, a high-fidelity mesoscopic CFD model coupled with the ray-tracing method has been established in this work using which the correlation between melt pool dynamics and laser irradiation behaviors is well visualized and studied.

The numerical simulation is based on the VOF two-phase flow model that fully considers several physical phenomena such as solidification/melting, surface tension, Marangoni effect, recoil pressure, evaporation heat loss. A ray-tracing algorithm (Fig. 1) is developed to describe the laser-material interaction using user defined function in the commercial software FLUENT. A rain drop method is utilized to generate a layer of randomly packed powder particles based on the commercial software EDEM. The Cu-Cr-Zr single track is fabricated by a commercial SLM system (XDM 250, XDM Co., Ltd, China). The surface track morphology in top view and cross-sectional view of the metallographic diagram is observed with a digital microscope to compare with the simulation results. In-situ measurements of the effective laser absorptivity (Figs. 4 and 5) are carried out based on the calorimetric method to validate the modeled laser absorptivity.

Under the laser power of 430 W and scanning speed of 0.6 m/s, the simulated track width is in the range of 105.4-133.2 μm with an average of 122.4 μm. The experimental result on the other hand is in the range of 91.1-140.9 μm with the average of 110.1 μm. Additionally, the simulated and experimental outcomes also match in terms of the track depth (Fig. 6). A continuous melt track is formed with 430 W laser power and 0.6 m/s scanning speed, while a distorted and broken track is observed with 330 W laser power and 1.2 m/s scanning speed (Fig. 7). This can be attributed to the fact that the insufficient energy input leads to the generation of a smaller volume of melted liquid, and the melted liquid tends to aggregate with surface tension at play. By visualizing the laser ray trajectories, different laser reflection behaviors are observed in both cases. Multiple laser reflections are observed in the depression region for the continuous case, while fewer reflections are observed as the balling effect occurs which implies a reduction in global laser absorptivity (Fig. 8). The effective laser absorptivity measurement has good agreement with the simulated global absorptivity using the ray-tracing method with a low relative error of 7.1% (Table 4). In addition, the vertical reflections on the emerging exposed substrate greatly contribute to the reduction of global absorptivity and lead to an intense oscillation (Figs. 8 and 10).

In the present study, a novel mesoscopic CFD model is established to simulate the melt pool dynamics and laser reflection behaviors during SLM. The implementation of the ray-tracing method can well reproduce the laser reflection and absorption behaviors with the evolution of melt pool thermodynamics which provides an in-depth insight into the energy coupling mechanism during the SLM process. The depression region induced by recoil pressure has a light trapping effect which promotes multiple laser reflection and absorption. The exposed substrate surface due to the occurrence of balling effect greatly weakens the light trapping effect and reduces the laser absorption of the powder bed. The global absorptivity under the balling effect has characteristics of violent fluctuation. The unstable melt pool has an adverse effect on energy coupling between the laser and powder bed. The dynamic balance between the melt pool state and laser absorption is the key to forming a single track of good quality.