Study of the inter-particle necks in selective laser sintering Jamasp Jhabvala, Eric Boillat and Re ´my Glardon Laboratoire de Gestion et Proce ´de ´s de Production, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland Abstract Purpose – Since pulsed lasers are mainly used in selective laser sintering (SLS) – contrarily to selective laser melting (SLM) – only the exterior of the powder particles is molten while their core stays solid. The purpose of this paper is to investigate the binding mechanism between two particles of titanium powder. Design/methodology/approach – A dedicated experimental setup is used to isolate the particles. They are then irradiated by the laser. SEM micrographs are taken at each step and image analysis is performed. The obtained results are compared with the predictions of a thermal model allowing for the incorporation of the latent heat of fusion and for a realistic surrounding. The absorbed laser intensity is modeled by means of the Mie theory. Findings – The growing of the interparticular necks and the volume of liquid formed for different repetition rates are measured and compared with numerical simulations. A good agreement is found. A new method to easily find the absorption coefficient of the laser into the grain and the heat exchange coefficient with the exterior is developed. Originality/value – This paper leads to a better understanding of the necking phenomena involved in the SLS consolidation process. An experimental set-up has been developed to observe and quantify the final state of a small amount of laser sintered grains. This process has been shown to be replicable and trustful. The thermal model leads to good predictions of the particle final sintering state. Keywords Titanium, Sintering, Lasers, Selective laser sintering, Simulation, Inter-particle necks Paper type Research paper 1. Introduction The selective laser sintering (SLS) process is limited by two antagonist processes. If too much energy is given, all the material melts and part precision is lost. On the other hand, if the particles do not receive enough energy, the bonding between the particles is weak and the parts have poor mechanical properties. The understanding of the molten phase evolution and of its influence on the inter-particle necks evolution is essential to play between these two limitations. In the SLS process, pulsed lasers are mostly used. Since only the surface of the powder particles is molten, consolidation can be achieved at much lower average power. Liquid bridges appear between the powder particles and are responsible for the bonding. The mechanical properties of the final part depend on the shape of these bonds. The goal of this study is to understand how the evolution of the liquid bridges (also called necks) is influenced by the laser parameters. The idea is to collect enough experimental information concerning the sintered state of a few particles under different laser conditions. The comparison of these results with the liquid phase evolution (predicted by the thermal model) leads to a better understanding of the binding mechanisms involved in SLS. This work is divided into two parts. The first one is an experimental study. The geometrical features of a few particles are analyzed after different types of laser treatment corresponding to different values of the power, repetition rate and pulse duration. These results are obtained thanks to the development of an adequate system for the separation and the transport of the two particles. A P-shaper is also mounted on the laser, so that the Gaussian shape of the beam becomes a top-hat profile. The evolution of the inter-particle neck is then obtained by image processing on electron micrographs. The experimental results are used to clarify the exact relationships between the melting kinetics and the bounding mechanisms. They should be a keystone in developing a comprehensive model of the SLS process taking the thermal phenomena and the flow of molten material into account. In the second part of this work a thermal model of the interaction between the laser beam and a single (metallic) grain is presented. This model demonstrates the deep influence of the laser parameters, like power, pulse duration and repetition rate, onto the melting kinetics and the liquid phase behavior. The experimental results are compared to the thermal model output. The current issue and full text archive of this journal is available at www.emeraldinsight.com/1355-2546.htm Rapid Prototyping Journal 19/2 (2013) 111–117 q Emerald Group Publishing Limited [ISSN 1355-2546] [DOI 10.1108/13552541311302969] The authors would like to thank Me ´lanie Dafflon and Benoit Lorent at LSRO2 (EPFL) for their important involvement in the Delta3 robot experiments. They are also thankful to the Interdisciplinary Centre for Electron Microscopy (EPFL) for providing access to electron microscopes. Received: 7 July 2011 Revised: 23 December 2011 Accepted: 21 March 2012 111