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Electron beam melting (EBM), as an excellent Additive Manufacturing (AM) technology, enables the printing of Ti-6Al-4 V alloy for a wide range of applications such as aerospace and biomechanical industries. It improves functionality and integrity of components and negates complexities in assembly processes. However, due to the poor surface and sub-surface integrity represented by the rough surface finish and low dimensional accuracy, achieving a favorable surface condition is quite challenging. Therefore, post processing becomes essential for these electron beam melted (EBM-ed) Ti-6Al-4 V alloys. Being the most common technique to improve such parts, milling of Ti-6Al-4 V alloy is very challenging and resulting tool wear issues, due to its unique material properties. Thus, this paper presents a comprehensive study on the surface integrity of EBM-ed Ti-6Al-4 V parts processed by precision grinding and electropolishing, aiming to qualitatively and quantitatively clarify the interrelation between process parameters and processed surface quality. The surface and subsurface characteristics such as profile accuracy, surface roughness, microstructure, defective layer and residual stress before and after post processing were compared and evaluated. The results show that by precision grinding, the profile accuracy was improved from over 300 µm PV to 7 µm PV, while surface roughness (Ra) was reduced from 30 µm to about 2 µm. The layer with partially melt particles was removed, but introduced a deformed subsurface layer with more residual stress. Then by applying electropolishing, the residual stress was released and the deformed layer was removed. In addition, Ra was further reduced to 0.65 µm. The research can serve as a reference for the integration of post machining processes with AM.

There has been limited studies on corrosion behaviour of post-processed Electron Beam Melted (EBM) Ti6Al4V, given that the factors affecting corrosion resistance of AM Ti6Al4V remain unclear. This paper proposes using heat treatment method to improve the pitting corrosion resistance of EBM Ti6Al4V. Different treatment profiles alter the microstructure of EBM Ti6Al4V. A clear trend is observed between microhardness and α lath width. As-printed EBM Ti6Al4V exhibits an inferior pitting potential, while heat treatment provided a significant improvement in the corrosion resistance. This study finds that the β phase fraction is a better indicator than the α lath width for pitting corrosion resistance. Solution air-cooled & ageing heat treated EBM Ti6Al4V exhibits good mechanical and corrosion properties, and even performs better than commercial cast Ti6Al4V.

Additive manufacturing (AM) technology enables a new way for fabricating components with complex internal surfaces. Selective laser melting (SLM), being one of the most common AM techniques, is able to fabricate complex geometries with superior material properties. However, due to the poor surface quality, the fabricated internal surfaces cannot meet the specifications for some real applications. To achieve the required internal surface condition, post-polishing process is essential. As one of the most prominent processes for finishing inaccessible surfaces with a wide range of materials, abrasive flow machining (AFM) shows great potential to polish AM internal surfaces. Hence, this paper presents an analytical and experimental study on the internal surface quality improvement of SLM Inconel 718 by AFM, aiming to verify the feasibility of AFM on internal surface quality improvement. The surface evolution process was modeled, and the effects of process parameters on surface and subsurface quality were evaluated. The results show that good surface roughness was obtained at the medium conditions of high viscosity, large particle size, low extrusion pressure, and low temperature. The surface morphology was greatly affected by the medium particle size which showed consistency with the surface evolution model that small abrasive particles are unable to overcome the width and depth of the valleys, resulting in the formation of craters. The partially melt layer was effectively removed, and no subsurface damage was induced.

RSA 905, a rapidly solidified aluminum alloy, has been widely utilized in optical, automotive, and aerospace industries owing to its superior mechanical properties such as hardness and strength compared to conventional aluminum alloys. However, the surface finishing of RSA 905 to achieve submicron surface roughness is quite challenging and was rarely addressed. This paper presents an experimental and analytical study on magnetic field-assisted finishing (MFAF) of RSA 905 through a systematic investigation on surface integrity in relation to the MFAF process parameters. The effect of abrasive and polishing speed conditions on material removal and surface roughness was quantitatively investigated. The surface and subsurface quality were evaluated by optical microscope and scanning electron microscope (SEM) observations, residual stress measurement, surface microhardness and tribology test. The results show that relatively high material removal and low surface roughness were obtained under conditions using the SiC abrasive with a grit size of 12 µm at polishing speed of 400 rpm or using the Al₂O₃ abrasive with a grit size of 5 µm at polishing speed of 800 rpm. Heat melt layer caused by wire electrical discharge machining (EDM) during the sample preparation was removed by MFAF without inducing new subsurface damage. The MFAF process also helps release the surface residual stress and improve the tribological performance although the surface microhardness was slightly reduced.

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth-abundant electrocatalysts to potentially replace precious platinum-based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low-density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high-yield, large-scale production of water-dispersed, ultrasmall-sized, high-percentage 1T-phase, single-layer TMD nanodots with high-density active edge sites and clean surface, including MoS2 , WS2 , MoSe2 , Mo0.5 W0.5 S2 , and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of -140 mV at current density of 10 mA cm-2 , a Tafel slope of 40 mV dec-1 , and excellent long-term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high-density active edge sites, high-percentage metallic 1T phase, alloying effect and basal-plane Se-vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging. MOE (Min. of Education, S’pore)