Modeling of nanoparticle agglomeration and powder bed formation in microscale selective laser sintering systems

Additive manufacturing (AM) has received a great deal of attention for the ability to produce three dimensional parts via laser heating. One recently proposed method of making microscale AM parts is through microscale selective laser sintering (mu-SLS) where nanoparticles replace the traditional powders used in standard SLS processes. However, there are many challenges to understanding the physics of the process at nanoscale as well as with conducting experiments at that scale; hence, modeling and computational simulations are vital to understand the sintering process physics. At the sub-micron (mu m) level, the interaction between nanoparticles under high power laser heating raises additional near-field thermal issues such as thermal diffusivity, effective absorptivity, and extinction coefficients compared to larger scales. Thus, nanoparticle's distribution behavior and characteristic properties are very important to understanding the thermal analysis of nanoparticles in mu-SLS process. This paper presents a discrete element modeling (DEM) study of how copper nanoparticles of given particle size distribution pack together in a mu-SLS powder bed. Initially, nanoparticles are distributed randomly into the bed domain with a random initial velocity vector and set boundary conditions. The particles are then allowed to move in discrete time steps until they reach a final steady state position, which creates the particle packing within the powder bed. The particles are subject to both gravitational and cohesive forces since cohesive forces become important at the nanoscale. A set of simulations was performed for different cases under both Gaussian and log-normal particle size distributions with different standard deviations. The results show that the cohesive interactions between nanoparticles has a great effect on both the size of the agglomerates and how densely the nanoparticles pack together within the agglomerates. In addition, this paper suggests a potential method to overcome the agglomeration effects in mu-SLS powder beds through the use of colloidal nanoparticle solutions that minimize the cohesive interactions between individual nanoparticles. (C) 2016 Elsevier B.V. All rights reserved.