PVC-LOT-008-J-038

Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by ~16% and ~40% respectively, with the engineered textured microstructure compared to the common textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility.NRF (Natl Research Foundation, S’pore)Published versio

Microstructure of a spark-plasma-sintered Fe2VAl-type Heusler alloy for thermoelectric application

The influence of microstructure on thermoelectricity is increasingly recognized. Approaches for microstructural engineering can hence be exploited to enhance thermoelectric performance, particularly through manipulating crystalline defects, their structure, and composition. Here, we focus on a full-Heusler Fe2VAl-based compound that is one of the most promising thermoelectric materials containing only Earth-abundant, non-toxic elements. A Fe2VTa0.05Al0.95 cast alloy was atomized under a nitrogen-rich atmosphere to induce nitride precipitation. Nanometer- to micrometer-scale microstructural investigations by advanced scanning electron microscopy and atom probe tomography (APT) are performed on the powder first and then on the material consolidated by spark-plasma sintering for an increasing time. APT reveals an unexpected pick-up of additional impurities from atomization, namely W and Mo. The microstructure is then correlated with local and global measurements of the thermoelectric properties. At grain boundaries, segregation and precipitation locally reduce the electrical resistivity, as evidenced by in-situ four-point probe measurements. The final microstructure contains a hierarchy of structural defects, including individual point defects, dislocations, grain boundaries, and precipitates, that allow for a strong decrease in thermal conductivity. In combination, these effects provide an appreciable increase in thermoelectric performance

P2X7 nucleotide receptors mediate caspase-8/9/3-dependent apoptosis in rat primary cortical neurons

Apoptosis is a major cause of cell death in the nervous system. It plays a role in embryonic and early postnatal brain development and contributes to the pathology of neurodegenerative diseases. Here, we report that activation of the P2X7 nucleotide receptor (P2X7R) in rat primary cortical neurons (rPCNs) causes biochemical (i.e., caspase activation) and morphological (i.e., nuclear condensation and DNA fragmentation) changes characteristic of apoptotic cell death. Caspase-3 activation and DNA fragmentation in rPCNs induced by the P2X7R agonist BzATP were inhibited by the P2X7R antagonist oxidized ATP (oATP) or by pre-treatment of cells with P2X7R antisense oligonucleotide indicating a direct involvement of the P2X7R in nucleotide-induced neuronal cell death. Moreover, Z-DEVD-FMK, a specific and irreversible cell permeable inhibitor of caspase-3, prevented BzATP-induced apoptosis in rPCNs. In addition, a specific caspase-8 inhibitor, Ac-IETD-CHO, significantly attenuated BzATP-induced caspase-9 and caspase-3 activation, suggesting that P2X7R-mediated apoptosis in rPCNs occurs primarily through an intrinsic caspase-8/9/3 activation pathway. BzATP also induced the activation of C-jun N-terminal kinase 1 (JNK1) and extracellular signal-regulated kinases (ERK1/2) in rPCNs, and pharmacological inhibition of either JNK1 or ERK1/2 significantly reduced caspase activation by BzATP. Taken together, these data indicate that extracellular nucleotides mediate neuronal apoptosis through activation of P2X7Rs and their downstream signaling pathways involving JNK1, ERK and caspases 8/9/3

Selective laser melting of multi material

With the increased demand for customised products with unique geometries and features, current manufacturing methods are challenged to produce complex shapes. Additive manufacturing is gradually replacing the traditional manufacturing methods to come up with complex parts required in small quantities. Additive manufacturing for multi materials in polymers has been researched more comprehensively compared to multi materials for metals. This is due to the lower working requirement for the polymers and plastics. However, metallic multi materials possess huge potential for application in our lives. One example would be making use of the strength of stainless steel for the outer support structure and the high heat conductivity of copper inside to control the direction of heat flow. This project aims to use selective laser melting to fabricate multi materials (316L stainless steel and Hovadur K220 copper) in two directions. Previous research work has shown the capability of fabricating multi materials in one direction via selective laser melting. This project successfully manufactured multi material parts in two directions with the help of a multi compartment hopper design. Scanning Electron Microscopic and Energy-disperse X-ray spectroscopy tests have shown successful bond formed between the two materials side by side to each other. This project serves as one of the first steps for metallic multi material fabrication using additive manufacturing methods. There are still many areas needed to be developed for metallic multi materials to be successfully fabricated into any shape and geometry.Bachelor of Engineering (Aerospace Engineering

Additive manufacturing of austenitic alloys with high strength and high ductility by selective laser melting

Selective laser melting (SLM) is a popular powder bed fusion technique used for manufacturing of metal parts. Due to its high-energy laser heat source, SLM process creates a high thermal gradient and fast cooling rate. As a result, it tends to generate a fine microstructure with high dislocation densities and small grain size. Since it’s a layer-by-layer manufacturing process, there is a potential for microstructural control within a single build. This study thus aims to understand the mechanisms of microstructural control for SLM process. Specifically, it aims to achieve a desirable microstructure with improved the mechanical properties. Two types of alloys with austenitic crystallographic structures were fabricated in the current work, namely stainless steel 316L (SS316L) and high-entropy alloy AlxCoCrFeNi. It is found that with proper parameter optimization, the microstructure of SS316L could be successfully manipulated to enhance both strength and ductility concurrently as compared to the base material. As for AlxCoCrFeNi, hot cracking was found to occur in CoCrFeNi, proper composition adjustment is shown to successfully minimize the hot crack density and improve its mechanical performance.Doctor of Philosoph

Effects of chamber oxygen concentration on microstructure and mechanical properties of stainless steel 316L parts by selective laser melting

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 versio

High density lipoprotein promoting proliferation and migration of type II alveolar epithelial cells during inflammation state

Abstract Background To investigate the effect and mechanism of high density lipoprotein (HDL) on type II alveolar epithelial cells during inflammation state. Methods The original generation of type II alveolar epithelial cells were separated in rats and treated with PBS/LPS/HDL/HDL + LPS. To observe the proliferation and migration of type II alveolar epithelial cells with bromodeoxyuridine(BrdU) assay, transwell assay and wound healing experiments. In addition, western blot detected the expression of TP-binding cassette transporter A1 (ABCA1), cystic fibrosis transmembrane conductance regulator (CFTR) and the phosphorylation of AKT/extracellular signal-regulated kinase(ERK)/mitogen-activated protein kinase(MAPK). Enzyme-linked immunosorbent assay (ELISA) tested the secretion of tumor necrosis factor a(TNF-a)/interleukin 1a(IL-1a)/IL-6. Results HDL promoted the proliferation (↑17%, p < 0.001 HDL+ LPS vs. LPS) and migration (wounding healing: ↑93%, p < 0.001 HDL+ LPS vs. LPS; transwell migration: ↑154%, p < 0.001 HDL+ LPS vs. LPS) of type II alveolar epithelial cells. Furthermore, HDL increased the phosphorylation of MAPK, but not AKT/ERK. And HDL decreased the secretion of TNF-a (↓46%, p < 0.01 HDL+ LPS vs. LPS) and IL-1a (↓45%, p < 0.001 HDL+ LPS vs. LPS), but not IL-6. In addition, HDL up-regulated the expression of ABCAI (↑99%, p < 0.001 HDL vs. CON) and down-regulated the expression of CFTR (↓25%, p < 0.05 HDL vs. CON) in type II alveolar epithelial cells. Conclusions HDL increases the phosphorylation of MAPK, which promotes the proliferation and migration of type II alveolar epithelial cells. And it decreased the secretion of TNF-a/IL-1a and the expression of CFTR. All these suggest that HDL plays an important role in anti-inflammatory effect in inflammation state of lung

Selective laser melting of stainless steel 316L with low porosity and high build rates

The present study employs fast scanning speeds to fabricate high-density stainless steel 316L (SS316L) parts via selective laser melting (SLM). It aims to improve the production rate while maintaining a low porosity for the SLM-built parts. Density values of > 99% were recorded for all the fabricated samples in this study. The scanning speed of the laser could be much improved due to the use of 380 W power laser. The overall build rate in this study is supposed to be enhanced by ~ 72% as compared to commonly used processing parameters. Detailed microstructural characterization was carried out in order to obtain an in-depth understanding of the microstructure of SLM-built SS316L. The microhardness of built parts is between 213 and 220 HV, which is much higher than that of the standard annealed counterpart of ~ 155 HV. This study provides an insight on how to improve SLM build rates without any loss of parts' density and mechanical properties.Accepted versio

Comparative Study on Tribological Behavior of Ti-6Al-4V and Co-Cr-Mo Samples Additively Manufactured with Electron Beam Melting

Ti-6Al-4V (Ti64) and Co-Cr-Mo (CoCr) samples of two different thicknesses were manufactured via powder bed electron beam melting (EBM). The tribological behaviour of the samples was evaluated using a ball-on-disc microtribometer and the corresponding coefficients of friction (COFs) of the samples were obtained. The wear rate of the worn samples were estimated through measurement of the widths and depths of their wear tracks using a surface profilometer. The microstruture and Vickers microhardness of the samples were also analyzed, which were related to the respective wear rate.Published versio