Numerical Modelling of Processes Governing Selective Laser Sintering

This study is focused on numerical investigation of heat transfer, melting, solidification and coalescence of metallic particles invoked by laser heating in application to selective laser sintering. The selective laser sintering is a promising additive manufacturing technology, applicable to a wide range of materials and products demanding a high degree of customization. Besides having potential advantages over conventional manufacturing, layer-by-layer synthesis poses challenges concerning mechanical properties, repeatability, limited choice of materials, slow production rate etc. This is due to the lack of understanding of the process variables and underlying physics. The metal powder is subjected to local heating by constant or pulsed irradiation from a moving laser source. The powder is sintered due to liquid-liquid contact growth during partial melting of particles retaining an unmelted solid core. The interparticle contacts are developed by capillary flow of the melt driven by high curvature gradients in the vicinity of the contact. The densification is accompanied by mutual approaching of solid cores, growing size of liquid neck between the particles and finally by particle cooling and melt solidification. The process involves multiple modes of heat transfer, phase change, mass and momentum transport in powder bed and is therefore very complex. Separate thermal models are developed to describe heat transfer at different length scales. The thermal model for radiation absorption and its transport in powder bed is based on non-continuum approach considering the discrete nature of medium. On the other hand, particle scale model describes evolution of the temperature field and phase change in a particle, while assuming surrounding granular powder as homogeneous medium. Model predictions are compared with experiments conducted on a commercial sintering system Eosint M270. Finally a representative case of two particles in contact is considered for coupled thermal and hydrodynamic analysis. The governing equations for heat transfer and flow of liquid melt are solved using boundary element method. The coupled model accounts for heat transfer, phase change during laser heating cycle and resulting melt flow due to surface tension forces simultaneously. Detailed study of the effect of process parameters on particle temperature evolution, contact growth and densification rate is conducted and discussed