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As an emerging powder-bed fusion additive manufacturing technique, selective electron beam melting (SEBM) attracts numerous attentions due to its great potential applications in orthopaedic implants and aerospace components. The microstructure and mechanical properties of SEBM-built Co-Cr-Mo alloy have been investigated. Microstructural characterisation was carried out using scanning electron microscopy, and X-ray diffraction techniques... NRF (Natl Research Foundation, S’pore) Published version

Selective laser melting (SLM) is a disruptive additive manufacturing technology that makes metal parts directly from 3D models in an automate layer-wise manner. Numerous studies have been carried out to examine the effects of various factors, such as laser power, scanning parameter, powder feedstock shape, substrate temperatures etc, on the microstructure and mechanical properties of SLM-built parts. The present work focused on the influence of chamber oxygen concentration towards the SLM-built stainless steel 316L (SS316L) parts. Chamber oxygeninduced amorphous silicon-enriched nano-particles have been found to be ubiquitous in SLM-built SS316L parts. However, the contribution of these nano-particles towards the built part’s mechanical properties is still unclear. Three batches of SS316L samples with varying chamber oxygen concentrations of 0.08 mol%, 0.16 mol% and 0.24 mol% were fabricated by SLM. Tensile and Vickers hardness tests were conducted. Backscatter Electron Microscopy was employed to elucidate the mechanisms of these amorphous nano-particles on the overall mechanical performance. Published version

Good mechanical properties are essential for steel parts additively manufactured (AM), especially, for load bearing components. For example, a proper impact toughness is necessary for structural steels used in marine and offshore (M & O) industry to prevent low temperature brittleness of the steels while a high fatigue strength warrant a long service span of the steels. Although the works on charpy and fatigue characteristics of AM parts are lacking, some related studies on their fracture behavior have been reported, which cover a range of steels including 316L stainless steel, maraging steel, 17-4 PH stainless steel, H13 tool steel, etc. This paper critically summaries the reported works and discusses a future direction for the study of the impact and fatigue characteristics of AM steels. NRF (Natl Research Foundation, S’pore) Published version

Single crystal (SX) nickel-based superalloy blade forms a key element of hightemperature gas turbines that are vital to aviation and power industries, owing to its excellent creep properties at elevated temperatures. Conventional manufacturing of SX Ni-based superalloy components is a tedious and expensive process due to stringent tolerance on part geometry and SX quality. Additive manufacturing (AM) provides unique features such as economic sustainability, automated manufacturing process and capability of printing identical parts. Moreover, the future of AM technologies shows significant promise towards achieving complete layer-wise control, in terms of varying process parameters. This study employs selective electron beam melting (SEBM), a powder-bed metal AM technique, to additively manufacture a first-generation Nickel-based SX superalloy. High vacuum environment and nearly unidirectional thermal gradient, inherent with SEBM, make it the most promising AM technique for SX superalloy manufacturing. Detailed microstructural characterization using optical microscopy, scanning electron microscopy and X-ray diffraction reveal that dominant columnar grains aligned with the build direction were formed into a strongly textured superalloy sample. However, there are still some cracks occurred along the columnar grain boundaries. The correlation between SEBM processing and microstructure is discussed. Published version

Metal additive manufacturing (AM) technologies have attracted a lot of interests from many industrial sectors due to its unlimited capability of producing complex geometries. Metal parts with complex shapes at large scales are highly demanded in marine and offshore (M&O) industries. The aim of this paper is to ascertain the feasibility of Additive Manufacturing (to be more specific Laser Engineered Net Shaping) of ASTM A131 EH 36 steel. After doing mechanical testing for LENS processed ASTM A131 EH 36 samples, the LENS processed samples with horizontal build direction have much better mechanical properties comparing with conventionally made parts, however samples with vertical build direction or 45 degree vertical build directions have properties comparable or lower than parts made of conventional methods due to build defects. NRF (Natl Research Foundation, S’pore) Published version

An experimental study of process parameter optimization for stainless steel 316L (SS316L) parts additively manufactured by selective electron beam melting (SEBM) was carried out. The process parameters for different stages, particularly in the in-fill hatch melting stage, were optimized in this study. Near-fully dense (>99%) SS316L parts have been successfully fabricated with well-melted surfaces. Microstructural characterization was performed on the as-SEBM-built SS316L parts with the optimal process parameters. It revealed that near-equiaxed grains were formed, which is distinctive from the counterparts additively manufactured by other SEBM processes. The mechanism for the formation of near-equiaxed grains was discussed in detail. This paper provides an insight into fabricating SS316L parts with high density and desirable microstructure via SEBM process. NRF (Natl Research Foundation, S’pore) Published version

In an additive manufactured metallic part, distinct and different microstructure and mechanical properties may exist on different areas due to differences in shape and location. Two parts, one with straight-finned structure and one with curvefinned structure, were fabricated by selective electron beam melting method using prealloyed Ti-6Al-4V powder. Microstructural characterization of these two parts that have varying fin thickness and shape were carried out in order to investigate the synthetically influence of build geometry and in-fill hatching strategy on selective electron beam melting. It was found that the β-spacing is larger in the curve-finned structure, leading to a lower micro-hardness as compared to the straight-finned structure. It suggests a slower cooling rate in the curve-finned structure due to the differences in build geometry and infill hatching strategy. Published version