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

摘要： 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 was observed. To understand this deviation, the dynamics of individual non-spherical particles were studied at 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 shape of the individual coalescing units can influence the sintering rates. In addition, the effects of viscoelasticity 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 the 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 and form two independent spheres. To explain this phenomenon, the local curvatures along the surface of coalescing particles were calculated and the results show that a local minimum in the curvature at the neck’s contact points with the particle’s main body forms which is believed to drives the separation.