Selective Laser Melting, SLM) is a widely used metal additive manufacturing technology, which melts metal powder layer by layer by high-energy laser beam, and finally forms metal parts with high precision and complex structure. In SLM process, metal powder is used as raw material, and its performance directly affects the forming quality and mechanical properties of finished parts. Among them, the choice of powder particle size is particularly critical, which is one of the important factors affecting forming accuracy, surface roughness, compactness and printing efficiency.

First, the influence of particle size on the quality of powder spreading

SLM equipment spreads metal powder evenly on the substrate by scraper or roller for layer-by-layer printing. If the particle size of powder is too large, it will easily lead to uneven powder distribution, cavity and spheroidization; However, although the particle size is too small, it is helpful to the uniformity of powder spreading, but it may lead to the decline of powder fluidity and lead to the difficulty of powder spreading. Generally, the recommended particle size range is 15 ~ 53 microns. The powder in this range can not only ensure good fluidity and powder laying quality, but also reduce the risk of splashing and oxidation.

Second, the relationship between particle size and forming accuracy

Fine powder has higher specific surface area and is easier to melt under the action of laser energy, which is beneficial to improve forming accuracy and surface quality. However, too fine powder is easy to produce "smoke effect" in the forming process, which affects the laser penetration and forming stability. Therefore, in the pursuit of high-precision parts, it is necessary to strike a balance between particle size and process stability.

Third, the effect of particle size on density

Density is one of the key indexes to measure the quality of SLM products. The powder with moderate particle size is easier to fully wet and fuse in the melting process, reducing internal defects, thus improving the density. The research shows that the powder with a particle size of 15 ~ 45 microns can obtain a high density (usually greater than 99%), which is suitable for applications requiring high mechanical properties, such as aerospace and medical implants.

Fourth, the difference of particle size selection of different materials

Different kinds of metal materials have different sensitivity to particle size. For example, because of its low density and high surface oxidation tendency, aluminum alloy powder usually needs finer particle size (such as 10 ~ 30 microns) to improve melting efficiency; Titanium alloy and stainless steel powder can be used in a wide range (15 ~ 53 microns) to give consideration to forming quality and production efficiency.

V. Importance of particle size distribution

In addition to the average particle size, particle size distribution (PSD) is also a factor that cannot be ignored. The ideal particle size distribution should be narrow distribution or bimodal distribution: narrow distribution is beneficial to powder fluidity and powder spreading uniformity, while bimodal distribution can fill the gap between coarse powder with fine powder, improve packing density and improve forming quality.

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To sum up, the selection of SLM metal powder particle size is a multi-factor weighing process, which needs to be comprehensively evaluated in combination with specific materials, equipment parameters and forming requirements. Reasonable selection of powder particle size can not only improve the forming efficiency and parts performance, but also reduce the defect rate, and promote the in-depth application of SLM technology in high-end manufacturing field. In the future, with the development of material science and additive manufacturing technology, the control and optimization of metal powder particle size will be more refined and intelligent.