The influence of material and process parameters on powder spreading in additive manufacturing

Additive manufacturing (AM) or 3D printing is beginning to mature from a rapid prototyping to an industrial production technology. However, there are still a lot of fundamental questions that must be addressed in order to make this leap forward. There are many different AM technologies; here, we focus on laser powder bed fusion (LPBF). A key step in LPBF is the initial spreading of the powder layer before it is melted in a solid object, via interaction with a laser. Ideally the powder should be spread as a dense, uniform layer. However, developing a spreading process that can produce a consistent layer, across the wide range of powders used, is a challenge for LPBF manufactures. Therefore, we investigate the influence of materials and process parameters on layer quality. To perform this study we perform computing simulations using the discrete particle method (DPM), a.k.a. discrete element method. This allows us to define metrics to evaluate the powder layer quality, allowing direct comparisons of different tools and parameters. We emulated the effect of the complex particle shape and surface roughness via rolling resistance and interparticle sliding friction. Additionally we investigated the effect of particle cohesion and type of spreading tool. We found that all these factors have a major, albeit sometimes surprising influence on the powder layer quality. In particular, more irregular shaped particles, rougher particle surfaces and/or higher interfacial cohesion usually, but not always, lead to worse spreadability. In general, there is a trade-off between material and process parameters. For example, increasing the spreading speed decreases layer quality for non- and weakly cohesive powders, but improves it for strongly cohesive ones. On the other hand, using a counter-clockwise rotating roller as a spreading tool improves the powder layer quality compared to spreading with a blade. For both tools, a unique correlation between the quality criteria uniformity and mass fraction is reported allowing an easily measured experimental value to be related to the layer quality. Finally, we showed that size-segregation occurs during spreading and this effect is able to explain some of our results.