Process parameter experimentation for the holistic optimization of surface quality in direct metal laser sintering

Direct metal laser sintering (DMLS) is an established technology in metal additive manufacturing, where metal AM is rapidly rising in industrial use, research and development, and academic research. Continued research is needed to better understand the process and print properties to control and improve build parameters and as-built part quality. Characteristic defects in as-built parts of porosity, residual stress, and surface roughness, namely for overhang geometries with downward-facing surfaces, can lead to part failures and reduced mechanical or related performance. Improving as-built roughness through informed process parameter selection and optimization, without compromising density, can reduce post-processing time and support material while improving part quality and performance. This thesis presents the experimental development of process parameters for the holistic minimization of as-built surface roughness of 316L stainless steel DMLS prints and subsequent verification through application to a developed complex design. An initial benchmarking study of DMLS printers using a novel test artifact is included, followed by surface roughness-focused experiments of N2 and Ar shielding gas, two powder sizes, and optimization of main laser exposure parameters using design of experiments tools. Characterization includes profilometry, 3D scanning, mechanical property measurement, optical microscopy, and residual stress deflection. The resulting optimized down-facing surface ('downskin') arithmetical average height roughness (Ra) was measured and found to be reduced by 28%. An upward-facing surface ('upskin') Ra below 5 μm is achieved for limited surfaces. The DMLS print parameters were used to improve the downskin roughness and pressure drop characteristics of a novel graded cell-size gyroid heat exchanger design.