This study investigates the porosity and microhardness of 316L stainless steel samples fabricated by selective laser melting (SLM). The porosity content was measured using the Archimedes method and the advanced X-ray computed tomography (XCT) scan. High densification level (≥99%) with a low average porosity content (~0.82%) were obtained from the Archimedes method. The highest porosity content in the XCT-scanned sample was~0.61. However, the pores in the SLM samples for both cases (optical

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... scopy and XCT) were not uniformly distributed. The higher average microhardness values in the SLM samples compared to the wrought manufactured counterpart are attributed to the fine microstructures from the localised melting and rapid solidification rate of the SLM process. Metals 2017, 7, 64 2 of 12 316L SS, in particular, is highly attractive for biomedical and marine applications due to its excellent corrosion resistance and relatively superior ductility compared to other materials [14] [15] [16] . Current research on AM of 316L SS is not only limited to single-material processing, but also extends to composites. For example, Al Mangour et al. [17] studied the SLM of TiC-reinforced 316L SS matrix nanocomposites and found that the addition of fine TiC particles remarkably improved the microhardness and wear performance of the fabricated parts. This is because of the increase in the densification level and the homogeneous microstructure distribution as a result of enhanced reinforcement/matrix wettability. In addition, studies on the SLM of TiB 2 /316L SS nanocomposites were also carried out with varying results. For example, superior compression yield strength and ductility were obtained when processing this nanocomposite without a hot isostatic pressing (HIP) post-processing due to the formation of homogenously dispersed TiB2 particles forming nanoscaled structures [18] . However, HIP treatment was found to reduce the hardness and wear resistance due to the high-temperature annealing effect [19] . Nevertheless, the flexibility of AM processes to fabricate such composites provides a promising future, especially for parts requiring complex geometries. Although SLM is able to manufacture almost fully dense parts (~98%-99%), the presence of residual porosity in SLM-fabricated parts hinders high-strength and fatigue resistance applications [20] . Similar to conventionally manufactured parts, the mechanical properties of components built by SLM are influenced by the resulting microstructure and porosity profiles (size and morphology) [1, 21] . Hence, it is important to understand the microstructure and porosity formation and how their behaviour influences the mechanical properties of the completed parts. Thus, this study aims to investigate the microstructure, porosity distribution and microhardness of 316L SS parts fabricated by SLM, in particular by using the advanced X-ray computed tomography (XCT) technique.