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

Revealing the microstructural evolution of electron beam powder bed fusion and hot isostatic pressing Ti-6Al-4V in-situ shelling samples using X-ray computed tomography

Electron beam powder bed fusion/hot isostatic pressing (E-PBF/HIP), also known as in-situ shelling, is an emerging technology that produces components by only forming their shells whilst retaining sintered powder at the core, and then using HIP to consolidate the entire structure. E-PBF/HIP can boost additive manufacturing productivity, however, the fundamental understanding of the process-microstructure-property correlations remains unclear. Here, we systematically investigate the microstructural evolution of E-PBF/HIP Ti-6Al-4V parts as a function of hatch melting parameters. All HIPped samples achieve full densification, however, their microstructures are significantly different from one another. Using X-ray computed tomography (XCT) and optical microscopy, our results show that the HIPped Ti-6Al-4V microstructure can be controlled by varying the porosity, P (%), pore surface areas and morphology in the as-built parts with a single set of HIP parameters. The HIPped microstructures still exhibit the as-built columnar grains when the as-built porosity, P 5 % with a highly dense pore network. This work suggests two main drivers for the grain morphology transitions during HIP: (1) a dramatic increase in pore volume increases the localised applied pressure up to 4 times at the core region of the sample and (2) minimise lack-of-fusion pores with high surface energies, promoting dynamic recrystallisation. This study provides a fundamental insight into the E-PBF/HIP technology, showing the feasibility to tailor microstructural properties of E-PBF built parts whilst boosting additive manufacturing productivity

Atom probe tomographic study of L10 martensite in a Pt-modified NiCoCrAlYTa bond coating

The L10 martensite formed in a Pt-modified NiCoCrAlYTa bond coating has been investigated by atom probe tomography. It was found that obvious segregation of Co and Cr occurred in the micro-twins zone inside the martensite lath. Based upon the compositional analysis, it is known that Pt destabilizes the β phase and Co and Cr act as β stabilizers with respect to the β→L10 martensitic transformation. In addition, some α-Cr particles precipitated inside the martensite lath

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

Experimental study on random temperature field of ultra-high performance concrete filled steel tube columns under elevated temperature

Due to the change of microstructure and increased heterogeneity of ultra-high-performance concrete (UHPC) in comparison with normal concrete, the thermal properties of UHPC are much less uniform within the material. Consequently, the fire performance of UHPC filled steel tubes (UHPCFST) columns are affected by the random distribution of temperature within the concrete. In order to study the distribution of the random temperature field within UFPCFST columns under elevated temperature, high temperature tests on 16 circular UHPCFST columns subjected to the ISO-834 standard fire and a linear heating mode are carried out. The average temperature, temperature difference, temperature probability density distribution and the temporal and spatial correlation of the random temperature field are investigated. It is found that the columns with more steel fiber and coarse aggregate contents have higher temperature, small temperature gradient and higher temperature difference. It is also found that the correlation of the zero-centered temperature in the circumferential direction between different time is strong when the temperature continues to rise or fall. The aims of this paper is to provide the reference for the analysis of temperature field of UHPCFST considering the heterogeneity of UHPC core

Process parameter optimization and mechanical properties for additively manufactured stainless steel 316L parts by selective electron beam melting

This work presents an experimental study of process optimization of the pair of critical parameters (speed function (SF) and focus offset (FO)) for stainless steel 316 L (SS316L) parts additively manufactured by selective electron beam melting (SEBM). Here, there are two sets of optimized SF-FO parameters that could build SS316L parts with high relative densities (>99%) and well-melted top build surfaces. Tensile test results show that most of the SEBM-built SS316L samples exhibit higher tensile strengths than the conventional cast and wrought counterparts, whereas their ductility is lower. In addition, strong anisotropic tensile properties are observed for the SEBM-built SS316L samples, e.g. they generally have better tensile properties when loaded parallel to the build direction as compared to the horizontal direction. However, a large number of σ phase was found to precipitate at grain boundaries in the SS316L samples fabricated under lower SF and larger FO with a higher build temperature, which evidently deteriorates their tensile properties particularly for the horizontal direction. It is suggested that SEBM process parameters for SS316L must be optimized to avoid σ phase precipitation at elevated temperatures apart from a well-melted top build surface and a high relative density.NRF (Natl Research Foundation, S’pore

Process parameter optimization for additively manufactured stainless steel 316L parts by selective electron beam melting

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 versio

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

Additive manufacturing of multiple materials by selective laser melting : Ti-alloy to stainless steel via a Cu-alloy interlayer

The ability to combine multiple materials (MM) into a single component to expand its range of functional properties is of tremendous value to the ceaseless optimization of engineering systems. Although fusion and solid-state joining techniques have been typically used to join dissimilar metals, additive manufacturing (AM) has the potential to produce MM parts with a complex spatial distribution of materials and properties that is otherwise unachievable. In this work, the selective laser melting (SLM) process was used to manufacture MM parts which feature steep material transitions from 316L stainless steel (SS) to Ti-6Al-4V (TiA) through an interlayer of HOVADUR® K220 copper–alloy (CuA). The microstructure in both the CuA/SS and TiA/CuA interfaces were examined in detail and the latter was found to be the critical interface as it contained three detrimental phases (i.e. L21 ordered phase, amorphous phase, and Ti2Cu) which limit the mechanical strength of the overall MM part. By making use of the non-homogeneity within the melt pool and limiting the laser energy input, the relatively tougher interfacial α′-Ti phase can be increased at the expense of other brittle phases, forming what is essentially a composite structure at the TiA/CuA interface. During tensile testing, the interfacial α′-Ti phase is capable of deflecting cracks from the relatively brittle TiA/CuA interface towards the ductile CuA interlayer and an overall tensile strength in excess of 500 MPa can be obtained. This method of introducing an interfacial composite structure to improve MM bonding is envisioned to be applicable for the SLM of other metallic combinations as well.NRF (Natl Research Foundation, S’pore

Mechanical properties and fracture analysis of additively manufactured EH 36 steel parts by laser engineered net shaping process

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 versio