Additive manufacturing (AM) is a rapid and flexible technique for the production of metal parts and prototypes from metal powders. The quality of parts manufactured using AM is a strong function of powder physical and mechanical properties. Previous work has shown that powders with a large fraction of fine particles produce better parts in terms of smooth finished surface and high mass density. However, an excessive fine fraction in the source powder causes serious flowability issues, leading to unexpected voids or discontinuities in the finished product. This effect of the fine fraction on the flowability of metal powder is widely encountered but poorly understood in the AM industry. This study presents a three-dimensional (3D) discrete element method (DEM) model to simulate the microscale mechanisms of powder flow considering the effects of van der Waals force. This microscale force has a negligible influence on the flowability of coarse grains, but the effect becomes the dominant factor governing the behavior of fine fractions (micrometer scale). The results show that the numerical model presented in this paper is capable of reproducing the experimental dependency of the powder flowability on the fine fraction. Moreover, it also successfully captures the characteristics of particle flow under the influence of microscale van der Waals force. (C) 2019 Elsevier B.V. All rights reserved.