Critical role of electrostatic forces in powder spreading in micro laser powder bed fusion

Micro laser powder bed fusion (micro-LPBF) is a highly precise additive manufacturing technique typically utilizing fine powder particles smaller than 20 $\mu m$. However, the small size of powder particles in micro-LPBF results in serious agglomeration, leading to poor quality of the powder layer, characterized by reduced packing density and worsened surface roughness. This is crucial as the powder layer quality directly impacts the quality of as-printed parts. In this study, we leverage experimental and numerical approaches to investigate the powder spreading mechanisms in micro-LPBF. Both rubber and metal scrapers are used in the experiments, and the rubber scraper achieves a denser powder layer with the relative packing density of ∼ 40 %, outperforming the metal scraper (∼ 30 %). Both simulation and experiments confirm the existence and critical role of the electrostatic force originating from friction, which is the first report to the best of our knowledge. Compared to the metal scraper, the rubber scraper with a larger work function and a higher surface energy density produces stronger electrostatic forces between powder particles. Consequently, the attractive effects of van der Waals forces are counteracted by the repulsive electrostatic forces, alleviating the powder agglomeration and thus resulting in a denser powder layer. On the other hand, it is also found that more powder particles adhere to the bottom of the rubber scraper, leading to a less optimal powder layer, especially when the layer thickness is very small. This study has provided new scientific understanding and can offer practical guidance on selecting scrapers according to the powder materials to achieve high-quality powder layer by adjusting the electrostatic forces between powder particles.