Effects of particle elongation on the binary coalescence dynamics of powder grains for Laser Sintering applications

Abstract

In this paper, the effects of particle shape and viscoelasticity on the binary coalescence rates of a pair of PEK HP3 particles in the Laser Sintering (LS) process are investigated in detail. PEK HP3 powder was characterised and the sintering rates were determined using Hot Stage Microscopy (HSM). On average, the neck growth rate shows a slower dynamic compared to the theory. Furthermore, in some of the trials an initial delay in growth and/or a contraction in the neck size were observed. To understand this behaviour, the dynamics of individual non-spherical particles were studied under similar conditions. The overall perimeter of the particles shrank and they gradually attained a final spherical state under the effect of surface tension forces, which indicates that the shape of individual coalescing units influences the sintering rates. In addition, the effects of viscoelasticity on the sintering rate were investigated using the available theoretical approaches and results show that viscoelasticity can change the rate of coalescence but the neck growth rate shows a strictly increasing trend even for very large Deborah numbers. To investigate the shape effects, volume-of-fluid (VoF) simulations of elongated ellipsoidal particles in head-to-head contact were performed and the neck growth rates were determined. These analyses show that the contraction of the neck size can occur purely due to geometrical effects and local surface curvatures of the coalescing particles. In addition, a critical aspect ratio exists beyond which the coalescence does not complete and the contraction continues until the pair of coalescing particles eventually separate forming two independent spheres. To explain this phenomenon, the local curvatures along the surface of coalescing particles were calculated and the results show that local minima in the curvature at the neck's contact points with the particles' main body form which is believed to drive the separation.