Pulsed mode selective laser melting of porous structures: Structural and thermophysical characterization

In this paper, the potential of selective laser melting (SLM) of stainless steel CL 20ES powder was investigated with a focus on controlled fabrication of porous structures with strongly reduced pore sizes, i.e. feature sizes significantly below conventional minimum SLM feature sizes. By controlling laser scan properties interacting with the powder bed directly, porous structures can be generated by selectively sintering powder particles. A wide range of porous samples was manufactured following this strategy, aiming to increase porosity while keeping pore sizes low. The effect of process parameters, including laser power and focal point positioning, was evaluated for a fibre laser operated in pulsed wave (PW) emission mode. The first part of this study focuses on characterization of key porous structure properties, i.e., porosity, average mass density, average pore sizes and structures at microscopic scales. The second part deals with the influence of porosity and pore sizes on thermal and fluid properties, i.e., the effective thermal conductivity (ETC) and wettability. We have quantified the directional dependence (build direction plane and scan direction plane) off the structural and thermophysical properties of porous structures. For a range of porosities and pore sizes, we have observed that porosity and surface morphology influence the thermal properties and contact angle of droplets on the printed surface. Thermal conductivity was measured and the associated analysis was compared with available models and correlations in literature. The average thermal conductivity of fabricated porous structures was determined between 6−14 W/m K and found to be a function of porosity. Furthermore, the capillary wicking performance of additively manufactured stainless steel porous structures having an average pore radius from 9 to 23 μm was determined.