Discrete Element simulations were carried out to model the sandpile formation with micro-sized spherical particles, with aim to gain in-depth understanding of particle spreading processes involved in additive manufacturing. Analysis of the piling process demonstrates that the friction and particle size distribution are the two key factors that control the structural properties of the sandpile. Both sliding friction and rolling friction hinder the particle flowability, leading to a higher angle of repose and a lower packing fraction of the sandpile. When the sliding friction is high, the rolling friction predominantly affects the final sandpile morphology, and vice versa. Interestingly, the effect of the rolling friction vanishes when its coefficient exceeds 0.4 due to multiple rolling restrictions. There is a competing effect for the particle size distribution: on one hand, with different sized particles, both the packing fraction and the angle of repose are reduced; on the other hand, when the size segregation is significant, the small particles sink down to fill the gaps between the big-particles, greatly enhancing the packing fraction and reverting the angle of repose to the value of single size distribution case. The present work not only provides an in-depth understanding of the particle piling mechanism under various working conditions, but also presents a practical way to extract particle properties for optimizing the additive manufacturing processes. (C) 2019 Elsevier B.V. All rights reserved.