Infoscience

Thesis

Study of physical phenomena of selective laser melting towards increased productivity

This thesis presents an experimentally focused study of three of the key physical phenomena of the Selective Laser Melting (SLM) process. The SLM process increasingly gains momentum in industrial applications. As awareness of the process and its capabilities for the manufacturing of high performance objects increases, some of the current limitations of the process become more apparent. The SLM process is not a fully mature manufacturing process and there are still significant gains to be made in terms of material understanding, process optimization, final part prediction and quality control. The experimental study of the physical aspects contributes to the material understanding on a fundamental level but also provides the tools for facilitating future research. A link to practical applications is presented in a case study on evaluating the influence of beam movement patterns and on the identification of process instabilities for quality control. A broad literature survey covers the state of the art on improving the SLM process. The three physical phenomena which are the subject of this study are: the interaction of laser beam light with the material to be processed, the transport of heat within the material and the behavior of the molten material. These three phenomena take place under conditions which are unlike conventional production processes, in terms of energy intensity, thermal gradients and liquid (metal) dynamics. The laser-material interaction is characterized by the absorptance. An integrating sphere setup is integrated in an SLM machine to measure the absorptance of the material in the pristine and final state. Also the absorptance evolution is characterized. Results are available for Maraging 300 steel powder, silver powder and AlSi10Mg powder. The influence of surface oxidation and surface roughness is discussed. The heat transport within the powder bed is characterized by diffusion. The thermal diffusivity and thermal conductivity are measured using the flash method with a laser as the energy source. The implementation uses a special laser beam intensity profile and data processing by correlation with numerical simulations. Thermal diffusivity and conductivity values are included for Maraging 300 steel, silver and titanium. The results confirm the large difference between bulk and powder conductivity and the limited influence of the material type. The melt pool behavior is studied using an integrated coaxial vision system. This system uses a combination of sensors to monitor the radiation which is emitted by the melt pool and its surroundings. The implementation presents optimized timing and data processing protocols. The relation between the melt pool properties and the process stability is explained. A procedure is also developed for obtaining temperature field profiles of the melt pool.

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