Selective laser melting (SLM) is an additive manufacturing technology that utilizes a multi-channel overlapping process to form a single layer, followed by powder deposition and formation of multiple layers to accumulate the final part. The morphology of the single-layer multi-channel directly affects the powder deposition process and the quality of interlayer bonding and porosity in the subsequent SLM process. Additionally, owing to the heat transfer boundary conditions and thermal accumulation effects in the layer-by-layer process, there is a certain fluctuation in the width of the molten channel along the scanning direction, as well as differences in the morphology between the first and last formed melt tracks and between the surface and bottom layers. Currently, research on the quality of SLM parts mainly focuses on the final surface roughness and porosity. However, the quality and precision of SLM parts depend largely on the quality of single-channel and single-layer multi-channel formation during the manufacturing process. In this study, we investigate the SLM process and analyze the quality of single-channel and single-layer multi-channel formation. This study aims to provide insights into controlling the surface and internal quality of SLM parts.

This study employed a 316L stainless steel material. First, a powder bed model was established using the discrete element method, considering various factors such as surface tension, evaporation, recoil pressure, Marangoni effect, and gravity. A fluid dynamic model of the powder melting process was then constructed in FLUENT to simulate the process of single-channel formation in SLM. The dynamic behavior of the molten pool during the process was analyzed. Single-channel SLM experiments were designed, and the formed parts were observed and measured microscopically. The simulation and experimental results were combined to identify the optimal process parameters for achieving high-quality single-channel formation. Finally, a geometric reconstruction of the single layer multi-channel model was performed by MATLAB based on the experimental and simulation findings.

The molten channel width and its coefficient of standard deviation of the SLM process were investigated by varying the laser power and scanning speed for 316L stainless steel powder. When the laser power is maintained at a constant 200 W, increasing the scanning speed to 2 m·s^-1 results in a decrease in the molten channel width to 46.65 μm, accompanied by an increase in the coefficient of standard deviation of the molten channel width to 22.65% (Fig. 7). High-quality single-channel formation is difficult to achieve at excessive scanning speeds, as indicated by interruptions in the molten channel at scanning speeds greater than 2 m·s^-1. When the laser scanning speed is maintained at 1.3 m·s^-1, within the laser power range 100?300 W, the molten channel width initially increases to 69.64 μm before decreasing. Similarly, the coefficient of standard deviation initially decreases to 13.11% and then increases (Fig. 11). Notably, different molten channel widths and coefficients of standard deviation are obtained under the same line energy density, and the coefficient of standard deviation varies significantly, fluctuating between 12.26% and 22.65% (Table 4). Based on these findings, a laser power of approximately 200 W and a scanning speed of around 1.0 m·s^-1 are found to be optimal for achieving high-quality single-channel formation (Fig. 15). To further analyze the molten channel morphology, a mathematical representation of the molten channel cross-section was developed, dividing the contour curve into upper and lower parts. The computed contour curve exhibits good agreement with the actual contour curve, indicating that the mathematical model accurately represents the molten channel cross-section shape (Fig. 17). Furthermore, based on this representation, a three-dimensional reconstruction of the multi-channel morphology was performed, providing a basic characterization of the single-layer multi-channel morphology (Fig. 19).

In this study, the SLM process of 316L stainless steel powder was investigated through a multi-field coupled simulation, experimental tests, and microscopic observations of the molten channel morphology. The results reveal that the molten channel width is inversely proportional to the scanning speed, whereas the coefficient of standard deviation is directly proportional to the scanning speed, at a constant laser power. When the scanning speed is constant, an increase in laser power initially leads to an increase in the molten channel width, followed by a decrease, whereas the coefficient of standard deviation initially decreases and then increases. The line energy density has no significant effect on the signal molten channel and its coefficient of standard deviation, it is concluded that a laser power of approximately 200 W and a scanning speed of around 1.0 m·s^-1 are optimal for achieving a coefficient of standard deviation below 15%, that is, high-quality single-channel formation. Furthermore, a mathematical model based on the well-formed single molten channel cross-section and overall contour was developed, which accurately represents the molten channel shape. This model provides a new method for further research on controlling the surface roughness and porosity of SLM-formed parts.