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Many studies have shown that the mechanical properties and geometric accuracy of additive manufacturing parts are dependent of many factors such as laser energy density, build orientation, and heat transfer histories. Amongst the factors, heat transfer histories are highly dependent on the geometry of a part, resulting in influencing the mechanical properties and microstructure evolution due to the repeated heating and cooling process. Heat transfer histories are associated with material thermal properties which include thermal conductivity, thermal diffusivity, specific heat capacity, and temperature gradient. The objective of this paper is to understand and observe the microstructure evolution process and microhardness based on variation in geometrical characteristic of the laser-based powder bed fusion (L-PBF). This paper presents the effect of the geometric factors on the mechanical properties and geometric accuracy during the L-PBF process, which benefit future process optimisation and modelling. In this study, samples with varying wall thickness are fabricated in TI6AL4Vand AlSi10Mg alloys by L-PBF. The samples are systematically evaluated by the optical microscope and the Vickers hardness tester. Microstructural characterisation of these samples is further evaluated via scanning electron microscopy. The results show that there is a signification relationship between material thermal properties, microstructure evolution, and mechanical properties with respect to the variation in wall thickness. These results can be used to understand the material thermal behaviour in lattice structures with a thin or small-sized feature and serve as a design guideline to indirectly control the microstructure of a L-PBF part. National Research Foundation (NRF) Accepted version This research was supported by the Singapore Centre for 3D Printing (SC3DP), the National Research Foundation, Prime Minister’s Office, Singapore, under its Medium-Sized Centre funding scheme, Natural Science Foundation of Hebei Province (Project No. F2020208018), and Funding for Introduction of Overseas Researcher of Hebei Province (Project No. C20190331).

The advent of additive manufacturing (AM) has brought about radically new ways of designing and manufacturing of end-use parts and components, by exploiting freedom of design. Due to the unique manufacturing process of AM, both design and process parameters can strongly influence the part properties, thereby enlarging the possible design space. Thus, finding the optimal combination of embodiment design and process parameters can be challenging. A structured and systematic approach is required to effectively search the enlarged design space, to truly exploit the advantages of AM. Due to lowered costs in computing and data collection in the recent years, data-driven strategies have become a viable tool in characterization of process, and researches have starting to exploit data-driven strategies in the design domain. In this paper, a state-of-the-art data-driven design strategy for fused filament fabrication (FFF) is presented. The need for data-driven strategies is explored and discussed from design and process domain, demonstrating the value of such a strategy in designing an FFF part. A comprehensive review of the literature is performed and the research gaps and opportunities are analysed and discussed. The paper concludes with a proposed data-driven framework that addresses the identified research gaps. The proposed framework encompasses knowledge management and concurrent optimization of embodiment design and process parameters to derive optimal FFF part design. Contribution of this paper is twofold: A review of the state-of-the-art is presented, and a framework to achieve optimal FFF part design is proposed. Economic Development Board (EDB) National Research Foundation (NRF) Published version This research was supported by a grant from ST Engineering Aerospace, EDB-IPP, Singapore Centre for 3D Printing (SC3DP), the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

Conformal cooling is an additive manufacturing-based solution and it is a rapidly developing method for reducing the cooling time of the plastic injection process. The present study investigates the thermal and mechanical performances of the 3D-printed conformal cooling channels using computational analyses and multi-objective optimization. For a real injection mold product, two different conformal cooling channel profiles, which are circular and elongated, are analyzed individually. Their cooling time, temperature non-uniformity, and pressure drop are assessed. Compared to the traditional channels, the cooling time of designed CCCs is reduced in the range of 30-60%. The cooling and fatigue life performances of the elongated channel are analyzed for different channel pathways and cross section areas. As for the circular channel, the coolant temperature, volume flow rate, and channel diameter are selected as the parameters within the ranges of 288.0-298.0 K, 1.0-10.0 L/min, and 2.1-2.5 mm, respectively. According to these parameters, the multi-objective optimization study is performed and the best trade-off point is found at the channel diameter of 2.5 mm, coolant temperature of 297 K, and the flow rate of 1 L/min when all the objectives have equal weights in the optimization problem. Accepted version

4D printing provides a means to create dynamic structures for self-actuating devices fabricated by additive manufacturing techniques. Most existing 4D printing processes are only capable of one-way shape morphing using a single mechanism for stimulation. In the limited successful reversible 4D-printing studies, most only demonstrate 2D-to-3D shape morphing. This study introduces a contactless shape setting mechanism to empower reversible 4D printing using a combination of stimuli. The forward shape programming is realised by asymmetric swelling with heated ethanol, and the shape recovery is accomplished by dry heating. The shape morphing can be predicted using a simulation model. Several designs are 4D-printed to achieve different forms of morphing to obtain 3D-to-3D shape morphing. The entire shape morphing cycle could be completed within as short as 3 min. The capability of 3D-to-3D shape morphing and reversibility of our 4D-printing process promise great potential for different applications such as biomimetics and soft robotics. National Research Foundation (NRF) Accepted version This research is supported by the Singapore Centre for 3D Printing, which is funded by the Singapore National Research Foundation.

The process-structure-property (PSP) relationship of continuous carbon fiber reinforced thermoplastics manufactured through fused filament fabrication was investigated for the first time, specifically on mode I Interlaminar Fracture toughness (ILFT). A standard double cantilever beam test is used to evaluate the mode I ILFT. The PSP relationship is established using micro-computed tomography scan and laser optical microscope. This study demonstrates that higher nozzle, bed temperatures, and lower print speed enhances the mode I ILFT. The mode I ILFT is found to be greatly affected by porosity and bond formation between the layers, which are dependent on the process parameters. National Research Foundation (NRF) This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

Additive manufacturing (AM) or 3D printing is growing rapidly in the manufacturing industry and has gained a lot of attention from various fields owing to its ability to fabricate parts with complex features. The reliability of the 3D printed parts has been the focus of the researchers to realize AM as an end-part production tool. Machine learning (ML) has been applied in various aspects of AM to improve the whole design and manufacturing workflow especially in the era of industry 4.0. In this review article, various types of ML techniques are first introduced. It is then followed by the discussion on their use in various aspects of AM such as design for 3D printing, material tuning, process optimization, in situ monitoring, cloud service, and cybersecurity. Potential applications in the biomedical, tissue engineering and building and construction will be highlighted. The challenges faced by ML in AM such as computational cost, standards for qualification and data acquisition techniques will also be discussed. In the authors’ perspective, in situ monitoring of AM processes will significantly benefit from the object detection ability of ML. As a large data set is crucial for ML, data sharing of AM would enable faster adoption of ML in AM. Standards for the shared data are needed to facilitate easy sharing of data. The use of ML in AM will become more mature and widely adopted as better data acquisition techniques and more powerful computer chips for ML are developed. National Research Foundation (NRF) Accepted version This research is supported by the National Research Foundation, Prime Minister’s Ofce, Singapore under its Medium-Sized Centre funding scheme.

While significant progress has been made in laser powder bed fusion (L-PBF) for metal additive manufacturing (AM), there is still limited large scale adoption of this advanced manufacturing technique by the industry. This paper covers the recent developments in L-PBF with discussions from the materials and process perspectives. High entropy alloys and high strength aluminium alloys have been identified as key materials development for L-PBF. Then, scanning strategies and multi-lasers applications for the process are also discussed. Other research trends and topics such as powder recycling, shape memory alloys and magnetic alloys are illustrated. The final part of this paper provides an outlook on the recent advancements while suggesting potential research topics for L-PBF. National Research Foundation (NRF) Accepted version The authors acknowledge the support from the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

PurposeSoft robotics is currently a rapidly growing new field of robotics whereby the robots are fundamentally soft and elastically deformable. Fabrication of soft robots is currently challenging and highly time- and labor-intensive. Recent advancements in three-dimensional (3D) printing of soft materials and multi-materials have become the key to enable direct manufacturing of soft robots with sophisticated designs and functions. Hence, this paper aims to review the current 3D printing processes and materials for soft robotics applications, as well as the potentials of 3D printing technologies on 3D printed soft robotics.Design/methodology/approachThe paper reviews the polymer 3D printing techniques and materials that have been used for the development of soft robotics. Current challenges to adopting 3D printing for soft robotics are also discussed. Next, the potentials of 3D printing technologies and the future outlooks of 3D printed soft robotics are presented.FindingsThis paper reviews five different 3D printing techniques and commonly used materials. The advantages and disadvantages of each technique for the soft robotic application are evaluated. The typical designs and geometries used by each technique are also summarized. There is an increasing trend of printing shape memory polymers, as well as multiple materials simultaneously using direct ink writing and material jetting techniques to produce robotics with varying stiffness values that range from intrinsically soft and highly compliant to rigid polymers. Although the recent work is done is still limited to experimentation and prototyping of 3D printed soft robotics, additive manufacturing could ultimately be used for the end-use and production of soft robotics.Originality/valueThe paper provides the current trend of how 3D printing techniques and materials are used particularly in the soft robotics application. The potentials of 3D printing technology on the soft robotic applications and the future outlooks of 3D printed soft robotics are also presented.

Magnetic digital microfluidics (MDM) manipulates fluids in the form of droplets on an open substrate, and incorporates surface energy traps (SETs) to facilitate the droplet manipulation. Conventional MDM devices are fabricated monolithically, which makes it difficult to modify the device configuration without completely overhauling the original design. In this paper, we present a modular MDM architecture that enables rapid on-demand configuration and re-configuration of MDM platforms for customized bioanalyses. Each modular component contains a SET and a Lego-like antistud that fits onto a base board with Lego-like studs. We illustrate the versatility of the modular MDM architecture in biomarker sensing, pathogen identification, antibiotic resistance determination, and biochemical quantification by demonstrating immunoassays, phenotypical assays and enzymatic assays on various modular MDM platforms configured on demand to accomplish the fluidic operations required by assorted bioanalytical assays. The modular MDM architecture promises great potential for point-of-care diagnostics by offering on-demand customization of testing platforms for various categories of diagnostic assays. It also provides a new avenue for microfluidic assay development with its high configurability which would significantly reduce the time and cost of the development cycle. A Lego-inspired, modular magnetic digital microfluidic architecture enables customizable bioanalysis. Magnetic digital microfluidics controls droplets on a surface via magnetic particles, with the liquid droplets themselves acting as mini bioreactors for analysis. Such devices often include surface energy traps to aide droplet manipulation. However, because they are fabricated as monolithic systems, they cannot be altered for different bioanalysis post-fabrication. Here, a team led by Yi Zhang from Nanyang Technological University, Singapore, reports a modular design for magnetic digital microfluidic devices, in which functional modules are configured and reconfigured via Lego-like studs. This modular design allows users to build testing platform on demand for a wide range of bioanalyses.

Organically modified montmorillonite/polypropylene (OMMT/PP) composite powders for selective laser sintering (SLS) were developed using four powder preparation methods including melt extrusion combined with cryogenic grinding, thermally induced phase separation (TIPS), surface coating, and mechanical mixing. Evaluations were conducted on the shape and size, powder-bed surface roughness, and thermal behavior of the developed composite powders as well as the mechanical properties of their injection-moulded tensile specimens. Melt extrusion proved to be the most favorable as the powders exhibited the highest thermo-oxidative stability (Td,max = 378 °C) and enhanced mechanical strength and ductility. TIPS could produce powders with reduced crystallization temperatures and increased melt enthalpies that were beneficial for SLS, and with powder morphology ideal for SLS without post-processing. It was found that at low loadings, nanofillers with sheet-like structures such as OMMT could limit the increase in crystallization temperatures of the composite powder, thus ensuring their wide sintering windows for SLS. Economic Development Board (EDB) Accepted version The work was supported by the Economic Development Board (EDB), Singapore and Robert Bosch (SEA) Pte Ltd. through the Industrial Postgraduate Program with School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.

3D Concrete Printing (3DCP) has been gaining popularity in the past few years. Due to the nature of line-by-line printing and the slump of the material deposition in each extruded line, 3D printed structures exhibit obvious lines or marks at the layer interface, which affects surface finish quality and potentially affect bonding strength between layers. This makes it necessary to control the extrudate formation in 3DCP. However, it is difficult to directly analyse the extrudate formation process because the extrudate shape depends on many parameters. In this paper, a machine learning technique is applied to correlate the formation of the extrudate to the printing parameters using an Artificial Neural Network model. The training data for the model development was obtained from extrudates printed in 3DCP experiments. The performance of the trained model was experimentally validated and the predicted extrudate geometry resulting from the developed model showed good agreement to the actual extrudate geometry. Subsequently, the developed model was used to find proper nozzle shapes to produce designated extrudate geometries. Significant improvement on the printing quality was demonstrated using nozzle shapes generated from the model on 3D printed objects consisting a vertical wall, an inclined wall and a curved part. National Research Foundation (NRF) Accepted version

Fe-Based bulk metallic glasses (BMGs) with a composition of Fe48Cr15Mo14C15B6Y2 have recently been reported with good application performance due to their excellent mechanical and chemical properties, showing excellent corrosion resistance, remarkable forming and processing ability, ultrahigh yield strength and large elasticity. Here, we report on a new functional application for such Fe-based BMGs, which can exhibit better catalytic performance than the pristine Fe surface. The hydrogen evolution and dissociation processes of one and two H2O molecules on both BMG and pristine Fe surfaces are investigated using first-principles calculations. The energy barriers of the dissociation processes on the BMG surface are lower than those on the pristine Fe surface. Moreover, the structural configurations along the dissociation path during hydrogen evolution show that it is easier for H2O molecules to dissociate into H2 on the surface of the BMG, rendering it a more active catalyst than the pristine Fe surface. Analyses on the electronic structures show further evidence that the BMG surface has a stronger ability to facilitate charge transfer at the interface and is more inclined to accept transferred charges, thereby promoting its catalytic activity. These findings shed light on understanding the functional applications of BMGs and are anticipated to be highly meaningful for further experimental investigations. Nanyang Technological University National Research Foundation (NRF) The authors gratefully acknowledge financial support from the National Research Foundation Medium-Sized Centre, Singapore through the Marine and Offshore Programme and the Nanyang Environment and Water Research Institute (Core Fund), Nanyang Technological University, Singapore.

Highly active catalysts from the earth-abundant metals are essential to materialize the low-cost production of hydrogen through water splitting. Herein, nickel porous networks codoped with Cu and Fe prepared by thermal reduction of presynthesized Cu, Fe-codoped Ni(OH)2 nanowires are reported. The sample consists of nanoparticles of ∼80 nm, which form highly porous network clusters of ∼1 μm with a pore size of 10–100 nm. Among the various doped compositions, the NiCu0.05Fe0.025 porous network exhibits the best catalytic activity with a low overpotential of 60 mV for a hydrogen evolution reaction (HER) in 1 M KOH solution and a specific activity of 0.1 mA cm–2 at 117 mV overpotential calculated based on the electrochemical active surface area (ECSA). The density functional theory calculations reveal that codoping of Fe and Cu into the Ni lattice results in a shift of d-bands of nickel to lower energy levels and thus in the reduced hydrogen adsorption energy (ΔGH = −0.131 eV), which is close to ΔGH for Pt (−0.09 eV). When NiCu0.05Fe0.025(OH)2 nanowires is used as an oxygen evolution reaction (OER) catalyst and is coupled with NiCu0.05Fe0.025 porous networks for overall water splitting, the NiCu0.05Fe0.025∥NiCu0.05Fe0.025(OH)2 catalyst couple achieves a current density of 10 mA cm–2 at 1.491 V, similar to that of the Pt/C∥RuO2 couple and offers a negligible loss in the performance when operated at 20 mA cm–2 for 30 h. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Three-dimensional (3D) printed Sc-modified Al alloy by powder bed fusion (PBF) provides significant strength and ductility without hot tearing during the process. This kind of 3D-printable high specific strength materials exhibits great potential in lightweight applications. Due to the lesser design limitation through the 3D printing process, the degree of lightweight is greatly affected by the specific strength of the materials. Hence, to further improve the mechanical properties of the material through process optimisation or post-treatment is of great importance. Microstructure feature variations due to different processing parameters are well known for traditional processes and materials. This study explores the parameter–microstructure–performance relationship of 3D printed Sc-modified Al alloys from the perspective of melt pool interactions. According to the stress concentration effect and Hall–Petch effect, the mechanical properties of the 3D printed materials vary greatly depending on the difference in defect size, shape and grain size. Accepted version

3D printing of cementitious materials is an innovative and promising approach in the construction sector, attracting much attention over the past few years. Use of waste cementitious materials in the production of 3D printable components increases the sustainability and cost-effectiveness of this process. This work proposes an environmentally friendly 3D printable cementitious material involving the use of magnesium potassium phosphate cement (MKPC) with various ratios of fly ash replacement ranging from 0 to 60 wt% to increase the working time of the binder. Silica fume was used at up to 10 wt% to adjust rheological and mechanical properties. The performance of the developed MKPC binders with different formulations in the context of 3D printing was assessed via a detailed investigation of the workability, extrudability, buildability, compressive strength, porosity and microstructural analysis. Amongst the mixtures studied, the optimum MKPC formulation involving 60 wt% fly ash and 10 wt% silica fume with a borax-to-magnesia ratio of 1:4 was selected for a small-scale printing demonstration in line with its rheological and mechanical properties. Finally, a 20-layer component with a height of 180 mm was printed in 5 min to demonstrate the feasibility of the adopted mixture in 3D printing. Accepted version

In-situ monitoring systems for additive manufacturing processes drive the possibility for identification of defects during printing, rendering both time and cost savings as the process can be stopped if a failure is detected. However, the relationship between defects identified during printing to the actual mechanical performance of the printed part has yet to be established. In this paper, we report the design and development of an optical based image processing in-situ monitoring system implemented on the selective laser melting process. The system consisted of an optical camera, a mirror and a set of light emitting diode lights. 316 L stainless steel specimens were fabricated with varying energy densities. Features captured in the optical images during printing were identified and quantified by image processing techniques. Results from density tests based on Archimedes’ principle and mechanical properties from tensile tests have demonstrated a correlation with the processed optical images of the specimens, validating the potential of inferring mechanical performance of printed parts based on features identified through the optical based image processing in-situ monitoring system. Accepted version

Next-generation hybrid optics will provide superior performances over traditional optics by combining the advantages of refractive, reflective, and diffractive optics and metasurfaces. Hybrid optics have been realized by integrating diffractive optical structures to the top surface of traditional bulk refractive or reflective optics. However, high-resolution manufacturing requirement of diffractive patterns on top of free-form refractive or reflective optical surfaces have hindered the wide-spread dissemination of hybrid optics. In this paper, we demonstrate a transferable ultra-thin micro-optics having multi-level transmittance and phase profiles which are arbitrarily patterned by tunable photoreduction and photoablation of graphene oxides (GO) using femtosecond (fs) direct laser writing. A 5 × 5 array of multi-level ultra-thin micro diffractive lens having a focal length of 15 mm was exemplarily patterned with real-time laser power control; the resulting spot size was smaller than 14 μm with the suppression of diffractive side peaks by 14.9% at the first order and 10.8% at the second order ones. This laser-patterned diffractive lens array was successfully transferred to the surface of a refractive cylindrical lens via polydimethylsiloxane (PDMS) as the flexible/stretchable substrate; the resulting optical performance agrees well with the theoretical simulation result. This new fabrication method will pave a way to novel hybrid optical systems. Accepted version

This paper considers the motion planning problem for multiple tethered planar mobile robots. Each robot is attached to a fixed base by a flexible cable. Since the robots share a common workspace, the interactions amongst the robots, cables, and obstacles pose significant difficulties for planning. Previous works have studied the problem of detecting whether a target cable configuration is intersecting (or entangled). Here, we are interested in the motion planning problem: how to plan and coordinate the robot motions to realize a given non-intersecting target cable configuration. We identify four possible modes of motion, depending on whether (i) the robots move in straight lines or following their cable lines; (ii) the robots move sequentially or concurrently. We present an in-depth analysis of Straight & Concurrent, which is the most practically-interesting mode of motion. In particular, we propose algorithms that (a) detect whether a given target cable configuration is realizable by a Straight & Concurrent motion, and (b) return a valid coordinated motion plan. The algorithms are analyzed in detail and validated in simulations and in a hardware experiment., 17 pages, 20 figures, 1 table

Rather than utilizing a sophisticated robot which is trained—and tuned—for a scenario in a specific environment perfectly, most people are interested in seeing robots operating in various conditions where they have never been trained before. In accordance with the goal of utilizing aerial robots for daily operations in real application scenarios, an aerial robot must learn from its own experience and its interactions with the environment. This paper presents an instantaneous learning-based control approach for the precise trajectory tracking of a 3D-printed aerial robot which can adapt itself to the changing working conditions. Considering the fact that model-based controllers suffer from lack of modeling, parameter variations and disturbances in their working environment, we observe that the presented learning-based control method has a compelling ability to significantly reduce the tracking error under aforementioned uncertainties throughout the operation. Three case scenarios are considered: payload mass variations on an aerial robot for a package delivery problem, ground effect when the aerial robot is hovering/flying close to the ground, and wind-gust disturbances encountered in the outdoor environment. In each case study, parameter variations are learned using nonlinear moving horizon estimation (NMHE) method, and the estimated parameters are fed to the nonlinear model predictive controller (NMPC). Thanks to learning capability of the presented framework, the aerial robot can learn from its own experience, and react promptly—unlike iterative learning control which allows the system to improve tracking accuracy from repetition to repetition—to reduce the tracking error. Additionally, the fast C++ execution of NMPC and NMHE codes facilitates a complete onboard implementation of the proposed framework on a low-cost embedded processor. NRF (Natl Research Foundation, S’pore) Accepted version

Re-melting strategies with same and opposite directions to the first scanning routine were performed in AlSi10Mg parts by selective laser melting (SLM) technology. Surface roughness and porosity were investigated with confocal microscopy, micro-computed tomography (CT) and optical microscopy (OM). Re-melting facilitates the top surface finish with Ra value decreasing from 20.67 μm to 11.67 μm (same direction) and 10.87 μm (opposite direction), almost at the same level. On side surface there is a contradictory trend. Pores at SLM parts include spherical pores due to entrapped gases, irregular pores for lack of fusion, and keyhole pores because of laser movement. The former two kinds form in the central areas while the latter one is located at edges of melting tracks and exhibits different distribution at both sides. Re-melting allows more chances for pores (spherical and keyhole pores) to escape from the melting pools. Irregular pores decrease because smoother surface allows powders to be fully melted. Porosity decrease of both re-melting strategies in central areas of the SLM parts is almost on the same level while same directional re-melting exhibits superior ability to release porosity at edges because of the porosity distribution difference at the head and wake of the melting tracks. National Research Foundation (NRF) Accepted version This research is supported by National Natural Science Foundation of China (Project No. 51371109) and National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme. Wenhui Yu appreciates the sponsorship from China Scholarship Council (Sponsorship No.201706220205).

Purpose The capability of microparticle/objects patterning in the three-dimensional (3D) printing structure could improve its performance and functionalities. This paper aims to propose and evaluate a novel acoustic manipulation approach. Design/methodology/approach A novel method to accumulate the microparticles in the cylindrical tube during the 3D printing process is proposed by acoustically exciting the structural vibration of the cylindrical tube at a specific frequency, and subsequently, focusing the 50-μm polystyrene microparticles at the produced pressure node toward the center of the tube by the acoustic radiation force. To realize this solution, a piezoceramic plate was glued to the outside wall of a cylindrical glass tube with a tapered nozzle. The accumulation of microparticles in the tube and printing structure was monitored microscopically and the accumulation time and width were quantitatively evaluated. Furthermore, the application of such technology was also evaluated in the L929 and PC-12 cells suspended in the sodium alginate and gelatin methacryloyl. Findings The measured location of pressure and the excitation frequency of the cylindrical glass tube (172 kHz) agreed quite well with our numerical simulation (168 kHz). Acoustic excitation could effectively and consistently accumulate the microparticles. It is found that the accumulation time and width of microparticles in the tube increase with the concentration of sodium alginate and microparticles in the ink. As a result, the microparticles are concentrated mostly in the central part of the printing structure. In comparison to the conventional printing strategy, acoustic excitation could significantly reduce the width of accumulated microparticles in the printing structure (p Originality/value This paper proves that the proposed acoustic approach is able to increase the accuracy of printing capability at a low cost, easy configuration and low power output.

Spattering is an unavoidable phenomenon during Selective Laser Melting (SLM). The distinctively large and dark solidified molten particles have always been observed on the powder bed, leading to the potential deterioration of the final printed parts. In commercial SLM machines, inert gas flow is pumped into the chamber over the powder bed to remove spatter and other unwanted by-products such as metal vapour and plasma plumes. However, traces of spatter still remain on the powder bed and the regions close to the chamber outlet. In this work, the trajectories of spatter particles were tracked using the Discrete Phase Model (DPM). The continuous fluid domain was solved using three-dimensional Computational Fluid Dynamics (CFD) simulations. The entrainment effects on the hot ejections as well as the recoil pressure driven ejections were considered. In other words, it was assumed that the latter were also entrained by the metal vapour and laser plume. The final distribution characteristics downstream of the cross-flow (-x direction) were validated by an earlier experimental study. The results show that a gradually decreasing trend in terms of the spatter size and mass distributions were obtained from the simulations which showed good agreement with the earlier experiments. It was also shown that the trajectories of the spatter particles were governed mainly by their initial momentum when the inert gas flow velocity was increased. While complete removal of the spatter particles from the powder bed was not achieved, the presence of the inert gas flow aided in the transport of a majority of the particles further downstream giving a more even mass distribution. Also, in spite of not accounting for the complete multi-physics of the SLM process and the associated recoil pressure driven ejections, the proposed spatter transport simulations can be applied in future gas cross-flow optimisation designs for the effective removal of spatter over the powder bed in commercial SLM machines. Accepted version

In this study, symmetrical and dichotomous fractal flow channel designs with various configurations in terms of the number of channels being split into smaller channels, were investigated experimentally for their flow boiling heat transfer performance with FC-72 as the working fluid. A multi-channel parallel channel design depicted as Parallel was used as a benchmark. The channels were fabricated by Selective Laser Melting (SLM). Three mass fluxes were studied, viz., 600, 900 and 1200 kg/m2⋅s. The peak heat transfer coefficient and pressure drop were investigated. High-speed visualization studies were also employed to examine the two-phase behaviors in the channels. The fractal flow channels were found to perform comparably with the Parallel channel at low mass fluxes but achieved higher heat transfer coefficients at higher mass fluxes, with the design n = 2, i.e., channel splitting into smaller channels twice, being the highest. The pressure drops of the fractal flow channel designs are higher than the Parallel channel due to their longer channel lengths, and the flow impacting multiple bifurcations at an angle, with n = 4, i.e., channel splitting into smaller channels four times, experiencing the highest pressure drop for all mass fluxes. Visualization studies show that fractal flow channels cause an early annular flow, enhancing heat transfer. Flow reversals in the Parallel channel were observed especially at higher mass fluxes, which suggest no enhancement in the heat transfer coefficient. The fractal flow channels, however, do not experience any flow reversal which could be attributed to the acceleration of the two-phase flow after each bifurcation, which in turn increases the inertia force to overcome the force from vapor expansion. The visualization studies also suggest that increasing the complexity does not result in better flow boiling heat transfer performance. This is due to the multiple flow separations which resulted in no two-phase heat transfer interaction and thus, under-utilization of the entire channel surface area for heat transfer. A comparison with a general correlation for flow boiling gives mean absolute errors of 31.6% and 35.3% for the fractal flow channels and Parallel, respectively. National Research Foundation (NRF) Accepted version The SLM 250 HL equipment used in this research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

3D printing is a novel construction method, which utilizes sequential deposition of printable material to build structures. It contributes to the automation in civil engineering and offers advantages of design, greenness and efficiency. Similarities between conventional spray technology and 3D printing indicate the feasibility of spray-based 3D printing, which could enhance the automation in vertical and overhead construction. However, low dimensional accuracy of sprayed profiles with conventional materials greatly affects the quality of spray-based 3D printing. This study contributes to the development of cementitious material to improve the dimensional accuracy of spray-based 3D printing. In this study, fly ash cenosphere and air entraining agent were introduced to obtain the optimal mixture design considering the delivery and deposition requirements. Subsequent spray tests show that the optimal mixture has the smallest splash width and most uniform material distribution among all the designed mixtures. Analysis of deposition process reveals that the distribution of sprayed material is closely related with its rheological properties, which could guide the future research work on spray-based printing of cementitious materials. Accepted version

Additive manufacturing (AM) or 3D printing is a manufacturing technique where successive layers of material are layered to produce parts. The design freedom afforded by AM is ideal for the space industry, where part production is low volume and highly customized. The objective of this paper is to review research in the area of additive manufacturing for space (AMFS) in all areas, from propulsion to electronics to printing of habitats, and to identify the gaps and directions in the research. In this paper, we investigate the AMFS research by splitting it into two domains: space and ground-based. Space-based AMFS has been performed on the International Space Station using polymers, and we also discuss the future of in space AM, a subject closely related to more general in space manufacturing. The ground-based research is split into three categories based on the printing material: metal, polymer, and others. The last category includes regolith, cement, and ceramic. This paper explores AMFS by bringing together as much research information as possible using a combination of papers, presentations, and news articles. We expect that the paper will allow the reader to gain an understanding of the current status of AMFS research and will contribute to the field as a reference and research guidelines. NRF (Natl Research Foundation, S’pore) Accepted version

3D printing, or additive manufacturing, is a technology which adopts layer-by-layer additive deposition process to build three-dimensional objects. Over the past decade, 3D printing has been attracting more and more attention in the building and construction industry. Compared with conventional concrete casting techniques, 3D printing contributes to higher efficiency with freeform construction, greatly reduced labor and much less construction waste. However, 3D printable cementitious materials are different from conventional concrete in terms of rheology, printability, and mechanical performances. This paper aims to systematically bridge the gap between the requirement and research and development of 3D printable cementitious materials to date. Guided by 3D printing process and multi-level design of cementitious materials, the requirements for 3D printable cementitious material at different material development levels are discussed. This paper provides insights for the future development of 3D printable cementitious materials for building and construction by controlling the basic inputs of materials to obtain desired structural performance. Accepted version

Variations in the physical and mechanical properties of parts made by laser power bed fusion (L-PBF) could be affected by the choice of processing or post-processing strategies. This work examined the influence of build orientation and post-processing treatments (annealing or hot isostatic pressing) on the fatigue and fracture behaviours of L-PBF stainless steel 316L in the high cycle fatigue region, i.e. 104 – 106 cycles. Experimental results show that both factors introduce significant changes in the plastic deformation properties, which affect fatigue strength via the mechanism of fatigue-ratcheting interaction. Cyclic plasticity is characterised by hardening, which promotes mean stress insensitivity and improved fatigue resistance. Fatigue activities, involving the initiation of crack at defects and microstructural heterogeneities, are of greater relevance to the longer life region where the global deformation mode is elastic. As the simultaneous actions of ratcheting and fatigue generate complex nonlinear interactions between the alternating stress amplitude and mean stress, the fatigue properties could not be effectively predicted using traditional stress-based models. A modification to the Goodman relation was proposed to account for the added effects of cyclic plasticity and was demonstrated to produce good agreement with experimental results for both cyclic hardening and softening materials. EDB (Economic Devt. Board, S’pore) Accepted version

The disposal of post-consumer glass has been a major issue due to its inert properties that may cause environmental effects, while recycling of these glasses is only feasible if the waste glass recovered is sorted into its different colour to prevent chemical incompatibility in the manufacturing process. 3D printing in building and construction has gain increasing attention in the past decade and provides a potential to sustainably utilize the recycled unsorted glasses. This paper examines the use of recycled glass as the fine aggregates for 3D concrete printing applications. Despite the several studies done on the use of recycled glass in concrete, there is a lack of focus on the rheology of the material which is essential to the performance of the material in 3D concrete printing. Although results have shown that the mechanical strength for the recycled glass concrete is lower than the sand aggregates concrete, yet the flow properties of the former is better than the latter. Nonetheless, a balance between the mechanical strength and flowability of the mix design should be studied. The future work revolves around the optimization of the mix design using a combination of sand and recycled glass, adjusting the gradation of recycled glass particles and addition of accelerators to improve its buildability and mechanical strengths. Accepted version

Digital construction has recently become the subject of very rapidly growing research activities all over the world. In this regard, our current research demonstrates the possibility of using high volume fly ash mortar, modified with micro silica for 3D concrete printing application. To assess the effect of the micro silica on rheological performance, a rotational rheometer was used to measure yield stress, viscosity and reversible stiffening behavior of ternary blends of fly ash–cement–micro silica. Since, the disposal of fly ash from coal power plants has increased rapidly due to the feed-in tariff established by the government, this research will pave a way to utilize high volume of fly ash for digital construction application. National Research Foundation (NRF) The authors would like to acknowledge Sembcorp Architects & Engineers Pte Ltd and National Research Foundation (NRF), Singapore for their funding and support in this research project.

This article provides a database of the mechanical properties of additively manufactured polymeric materials fabricated using material extrusion (e.g., fused filament fabrication (FFF)). Mechanical properties available in the literatures are consolidated in table form for different polymeric materials for FFF. Mechanical properties such as tensile, compressive, flexural, interlayer, fatigue, and creep properties are discussed in detail. The effects of printing parameters such as raster angle, infill, and specimen orientation on properties are also provided, together with a discussion of the possible causes (e.g., texture, microstructure changes, and defects) of anisotropy in properties. In addition to that, research gaps are identified which warrant further investigation. NRF (Natl Research Foundation, S’pore) Accepted version

Knowledge of thermodynamics of multimetallic nanoparticles is of great importance in prediction and advancing the understanding of synthesis, characterization, and applications of metal nanoparticles. In this work, molecular dynamics simulations were performed to investigate the melting characteristics and behaviors of a ternary Fe-Ni-Cr nanoparticle (19.17 wt.% Cr, 11.72 wt.% Ni, and the rest Fe). It was found that the melting of the nanoparticles starts from the surface and proceeds gradually inwards to the core, indicating a liquid nucleation and growth melting mode. During heating, severe Cr segregation with increasing temperature were observed, and the nano Cr clusters prefer to aggregate mostly at the surface due to lower surface energy and stronger cohesive interactions of Cr atoms than Fe and Ni. Moreover, the melting temperature of the nanoparticles decreases as the particle radius decreases, and there exists a linear relationship between the melting point and the inverse of the radius. This signifies the feasibility of the linear depression effect for the size-dependent melting of Fe-Ni-Cr nanoparticles accompanying surface segregation and aggregation. The findings in this work are believed to provide the atomic scale understanding of mechanisms of melting and surface segregation of ternary Fe-Ni-Cr nanoparticles. National Research Foundation (NRF) Accepted version This work is supported by the National Natural Science Foundation of China (51603160), the Excellent Dissertation Cultivation Funds of Wuhan University of Technology (2017-YS-011), and Fundamental Research Funds for the Central Universities (WUT:172401008). C. H. Wong, Y. M. Zhang and C. J. W. Lim are supported by the National Research Foundation, Prime Minister’s Office, Singapore under the National Additive Manufacturing Innovation Cluster (NAMIC) programme (RCA-17/357) and under the Medium-Sized Centre funding scheme.

In this paper, a novel method for printing path design in the three-dimensional (3D) concrete printing process (3DCP) has been developed and applied to a large-scale construction process. Despite the large number of research works in digital concrete printing over the last decade, few studies have explored the method of automatic printing path design, specifically designing the printing path by directly extracting building information modeling (BIM) information from the BIM platform in 3DCP. To obtain BIM information and automatically design the printing path, a script package was developed using a single scripting environment called Dynamo, a Revit plug-in. The proposed method was evaluated using the results of simulation and real-time printing. Compared with the conventional method, the simulation results show that the proposed approach can reduce data loss from a 3D BIM model to printing path generation. The real-time printing test implies that the proposed approach is perfectly suitable for integrating BIM with 3DCP. National Research Foundation (NRF) Accepted version

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided. Ministry of Education (MOE) Nanyang Technological University The first author would like to acknowledge the financial support for his research appointment at the University of Illinois at Urbana Champaign, USA under the College of Engineering (CoE) International Postdoctoral Fellowship Scholarship (IPS) provided jointly by the Ministry of Education, Singapore and Nanyang Technological University, Singapore.

Laser metal deposition (LMD) additive manufacturing was utilized to fabricate a Cu‒9Al‒5Fe‒5Ni alloy with a hierarchical microstructure and superior mechanical strength. An optimized processing window of LMD was established for printing the alloy with a relative density greater than 99% using laser power of 1000–1500 W, scanning speed of 0.5–1.5 m/min and hatch space of 1.5–2 mm. The LMD-printed alloy exhibited a microstructure consisting of a martensite β* phase, a Widmanstätten α phase, Fe Al and NiAl nanoprecipitates, and nanotwins. The hierarchical microstructure comprising microscale cellular structures, sub-microscale grains, and nanoscale precipitates and twins was achieved. The cellular structures were formed by the martensite β* and α phases. The nanotwins were formed at the interface of the plate-like β* phase, which was induced by the low stacking fault energy of the alloy and high cooling rate of LMD. The Fe Al precipitates were formed within the β* and α phases, while the NiAl precipitates were distributed in the β* phase. The yield strength, ultimate strength, and elongation of the LMD-printed alloy were 593–713 MPa, 769–949 MPa, and 10–12%, respectively. The yield strength of the LMD-printed alloy was 160% and 76% higher than that of the counterparts fabricated by casting and wire arc additive manufacturing, respectively, which was attributed to the synergistic effects of the underlying mechanisms including the Hall-Petch type strengthening, dislocation strengthening, precipitation strengthening, and solid solution strengthening. These findings validated the applicability of LMD for printing the Cu‒9Al‒5Fe‒5Ni alloy and facilitated the potential applications in marine and offshore industries. National Research Foundation (NRF) Accepted version

The influences of microstructure and solidification parameters on the hot cracking behaviour of an additively manufactured Ni-based superalloy via selective electron beam melting (SEBM) have been investigated. Severe intergranular hot cracking is found to occur at grain boundaries (GBs) in the SEBM-built superalloy. A generalised cracking criterion that considers various process parameters along with the GB inclination is developed on the basis of a prevailing hot cracking criterion for columnar-grained microstructure. A parametric evaluation of influencing factors is quantitatively conducted to predict the optimal conditions that could prevent hot cracking while maintaining the desirable fine microstructure. With the aid of numerical simulations, it is determined that while micro-segregation occurs at all types of GBs, its occurrence at the divergent ones tend to promote cracking. Using the results of this work, hot cracking in additively manufactured Ni-based superalloy single crystals can be potentially mitigated. National Research Foundation (NRF) Accepted version

Observing micro-organisms with depth-resolving capability is important in optical microscopy for biomedical sciences and industries. We demonstrate the fabrication and use of a two-photon polymerized micro-axicon lens that generates a self-reconstructing pencil-like Bessel beam for light-sheet fluorescence microscopy (LSFM), providing 3D internal structures of micro-organisms. The fabricated SU-8 micro-axicon of 100 µm diameter transforms the input Gaussian beam from a single-mode fiber into a non-diffractive Bessel-Gauss beam. The focused spot size of the Bessel-Gauss beam is 2.3 ± 0.25 µm with a long propagation distance over 160 µm, which is well-suited for LSFM. The self-reconstruction capability of the generated Bessel-Gauss beam is investigated thoroughly through both simulations and experiments. Since this micro-axicon can be directly 3D-printed on single-mode fibers’ end facets or small mobile substrates, this can replace the bulky objective lens from conventional light-sheet microscopes. This will facilitate the wide-spread use of 3D tomographic imaging of micro-organisms, especially in compact micro-fluidic devices and lab-on-a-chip architectures. National Research Foundation (NRF) Published version National Research Foundation of the Republic of Korea (NRF2012R1A3A1050386); Research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA-15/027); Korea Forest Service (Korea Forestry Promotion Institute) through the R&D Program for Forest Science Technology (2020229C10-2022-AC01); Basic Research Program (NK224C) funded by the Korea Institute of Machinery and Materials.

This study focused on the development of 3D printed building components incorporating MgO–SiO2 binders. The effects of parameters such as MgO/SiO2 and water/binder ratios and superplasticizer dosage were evaluated to produce 3D printable MgO–SiO2 binders. The assessment of rheological and mechanical properties, which led to an optimum mix that satisfied the printability criteria, was supported by microstructural characterization. The best-performing mix in terms of printability and mechanical performance was characterized via x-ray diffraction (XRD), thermogravimetric analysis/derivative thermogravimetry (TG/DTG) and field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS). The selected mix exhibited thixotropic behaviour with an ability to recover ~80% of its original viscosity within 60 s of extrusion and a shape retention factor of ~1. The extrudability, shape retention and buildability aspects were successfully demonstrated by printing a 5-layered 3D structure without any deformations. The final product contained brucite and M-S-H as hydration products, which were the main contributor to mechanical performance and microstructural densification. National Research Foundation (NRF) Accepted version The authors would like to acknowledge Sembcorp Architects & Engineers Pte. Ltd. and National Research Foundation (NRF) Singapore for their funding and support in this research project.

In this paper, a direct laser deposition (DLD) process was used to print ASTM A131 EH36 high strength low alloy steel samples in four different orientations, namely, horizontal 0° (XY_0°) & 45° (XY_45°) and vertical 90° (XZ_90°) & 45° (XZ_45°), with an EH36 steel powder. The microstructures, mechanical properties and fracture behavior of the printed materials, were evaluated in detail. Tensile, charpy impact and fatigue tests were conducted to measure the mechanical properties of the materials. The results showed that all the EH36 steel samples printed in the four orientations satisfied the ASTM standards for the tensile and charpy impact properties of EH36 steel. The microstructures of the EH36 steel samples and the thermal cycles encountered in the different build orientations could be tuned by controlling the printing process parameters and scanning method. The microstructure was closely correlated with the mechanical properties of the built samples. The fatigue fracture mechanism of the printed EH36 steel samples was also proposed. National Research Foundation (NRF) This work was supported by the Singapore Centre for 3D Printing, Nanyang Technological University, Singapore and funded by the National Research Foundation, Prime Minister's Office, Singapore under the Medium-Sized Centre funding scheme and the Research Collaboration Agreement between NTU and Keppel Marine & Deepwater Technology, Singapore.

A powder-scale multi-physics model is developed to investigate the keyhole dynamics and porosity in products fabricated by selective laser melting (SLM) under the sub-atmospheric pressure. The effects of the sub-atmospheric pressure on the plume attenuation of the laser beam, vaporization temperature and vapor recoil pressure are taken into account. The thermo-fluid behaviour in the melt pool under the atmospheric and sub-atmospheric ambient pressure is analysed in the SLM process. The simulation results suggest that the reduction of the sub-atmospheric pressure can increase the melt pool depth, whereas the effects are insignificant under the low sub-atmospheric pressure. As compared with the atmospheric pressure, the sub-atmospheric pressure leads to the decrease of the average temperature and the increase of the average recoil pressure on the keyhole surface, which is in favour of increasing keyhole stability and maintaining keyholes open. Consequently, the sub-atmospheric pressure can help to improve the keyhole stability and thus minimize pore defects in SLM-built products. National Research Foundation (NRF) Accepted version The authors would like to acknowledge the financial support from the National Research Foundation Medium Sized Center, Singapore through the Marine and Offshore Program.

The safety of structure printed by extrusion-based 3D concrete printing (3DCP) is significantly influenced by interlayer bonding, which is governed by fresh material properties and processes, including mixing, printing, and post-processing phases. Much literature focused on improving interlayer bond strength via material tailoring/bonding agent addition but gave insufficient attention to other phases' impacts. This study investigates the effects of parameters on interlayer bond strength from different phases in 3DCP, including superplasticizer dosage, printing speed, and curing condition. In the mixing phase, the superplasticizer dosage was increased to reduce I and increase surface moisture content (SMC), consequently improving interface microstructure and interlayer bond strength. In the printing phase, similar results were observed by increasing pumping speed to increase material shear rate. In the post-processing phase, proper curing also improved interlayer bond strength. While SMC has a critical impact on interlayer bond strength, material thixotropic index (I ) probably has a stronger influence. National Research Foundation (NRF) Accepted version

In this paper, a theoretical model of filmwise condensation of steam on three-dimensional pin fins fabricated by selective laser melting (SLM), an additive manufacturing (AM) technique, is developed. The model considers the effects of surface tension and gravity on the liquid film flow over the pin fin surface. The three-dimensional nature of the liquid film flow over the fin flank and the unique features of the fin structures as a result of the laser melting process are modeled. Visualization studies are performed to verify the assumptions made in the model. From the modeling results, the local heat transfer coefficient and length-averaged heat transfer coefficient are obtained. The liquid film thickness at various locations of the pin fin is analyzed. The effects of fin tip dimensions, fin stem radius and fin pitch on the liquid film characteristics and heat transfer coefficient are systematically investigated. Our results showed that a thin film region exists in the flat and circular segments of the fin tip which cover approximately 25 – 30% of the fin surface. For a fixed fin diameter, it is found that the length-averaged heat transfer coefficient can be optimized by varying the dimensions of the flat and circular segments. A locally thin film region resulting from the suction effect is observed near the fin base for small fin spacings. However, the suction effect reduces with increasing fin spacing. Due to the three-dimensional nature of the pin fins which induces surface tension in the circumferential direction, the liquid film distribution is uniform. The differences in the length-averaged heat transfer coefficient at different circumferential locations are smaller than 5%. Finally, a comparison with existing experimental results demonstrates that a relatively accurate prediction of the average heat transfer coefficient can be achieved by our model with a maximum deviation of 8.3%. Ministry of Education (MOE) Nanyang Technological University J.Y.H. would like to acknowledge the financial support for his research appointment at the University of Illinois at Urbana-Champaign, USA under the College of Engineering (CoE) International Postdoctoral Fellowship Scholarship (IPS) provided jointly by the Ministry of Education, Singapore and Nanyang Technological University, Singapore. N.M. gratefully acknowledges funding support from the Air Conditioning and Refrigeration Center, and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology.

Over the past decade, 3D printing or additive manufacturing (AM) technology has seen great advancement in many aspects such as printing resolution, speed and cost. Membranes for water treatment experienced significant breakthroughs owing to the unique benefits of additive manufacturing. In particular, 3D printing's high degree of freedom in various aspects such as material and prototype design has helped to fabricate innovative spacers and membranes. However, there were conflicting reports on the feasibility of 3D printing, especially for membranes. Some research groups stated that technology limitations today made it impossible to 3D print membranes, but others showed that it was possible by successfully fabricating prototypes. This paper will provide a critical and comprehensive discussion on 3D printing specifically for spacers and membranes. Various 3D printing techniques will be introduced, and their suitability for membrane and spacer fabrication will be discussed. It will be followed by a review of past studies associated with 3D-printed spacers and membranes. A new category of additive manufacturing in the membrane water industry will be introduced here, known as hybrid additive manufacturing, to address the controversies of 3D printing for membrane. As AM technology continues to advance, its possibilities in the water treatment is limitless. Some insightful future trends will be provided at the end of the paper. Economic Development Board (EDB) National Research Foundation (NRF) The Singapore Membrane Technology Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University is supported by the Economic Development Board of Singapore. Singapore Centre for 3D Printing (SC3DP) is funded by the Singapore National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme.

The presented work outlined the development of a new biocompatible hydrogel material that has potential applications in soft tissue engineering. As a proof of concept, human hepatocytes were used to demonstrate the suitability of this material in providing conducive environment for cellular growth and functional development. Herein, a detailed synthesis of novel gelatin derivatives - photo-crosslinkable glycidyl methacrylate (GMA) functionalized gelatins (Gelatin-GMA), and preparation of three-dimensional (3D) hydrogel scaffolds for the encapsulated Huh-7.5 cells is reported. The Gelatin-GMA biopolymers were synthesized at two different pH values of 3.5 (acidic) and 10.5 (basic) where two different photo-crosslinkable polymers were formed utilizing -COOH & -OH groups in acidic pH, and -NH2 & -OH groups in basic pH. The hydrogels were prepared using an initiator (Irgacure I2959) in the presence of UV light. The Gelatin-GMA biopolymers were characterized using spectroscopic studies which confirmed the successful preparation of the polymer derivatives. Rheological measurement was carried out to characterize the mechanical properties and derive the mesh sizes of the 3D hydrogels. Subsequently, detailed in vitro hepatocyte compatibility and functionality studies were performed in the 3D cell seeded hydrogel platform. The 3D hydrogel design with larger mesh sizes utilizes the advantage of the excellent diffusion properties of porous platform, and enhanced cell-growth was observed, which in turn elicited favorable Huh-7.5 response. The hydrogels led to improved cellular functions such as differentiation, viability and proliferation. Overall, it showed that the Gelatin-GMA based hydrogels presented better results compared to control sample (GelMA) because of the higher mesh sizes in Gelatin-GMA based hydrogels. Additionally, the functional group studies of the two Gelatin-GMA samples revealed that the cell functionalities are almost unaffected even after the tripeptide - Arg-Gly-Asp (RGD) in Gelatin-GMA synthesized at pH 3.5 is no longer completely available. Ministry of Education (MOE) The authors acknowledge the NTU-Northwestern Institute for Nanomedicine (NNIN) and MOE Tier 1 (RG 46/18) for the financial support.

Multi Jet Fusion (MJF) is a recently developed polymeric powder bed fusion (PBF) additive manufacturing technique that has received considerable attention in the industrial and scientific community due to its ability to fabricate functional and complex polymeric parts efficiently. In this work, a systematic characterization of the physicochemical properties of MJF-certified polyamide 11 (PA11) and thermoplastic polyurethane (TPU) powder was conducted. The mechanical performance and print quality of the specimens printed using both powders were then evaluated. Both PA11 and TPU powders showed irregular morphology with sharp features and had broad particle size distribution, but such features did not impair their printability significantly. According to the DSC scans, the PA11 specimen exhibited two endothermic peaks, while the TPU specimen exhibited a broad endothermic peak (116–150 °C). The PA11 specimens possessed the highest tensile strength in the Z orientation, as opposed to the TPU specimens which possessed the lowest tensile strength along the same orientation. The flexural properties of the PA11 and TPU specimens displayed a similar anisotropy where the flexural strength was highest in the Z orientation and lowest in the X orientation. The porosity values of both the PA11 and the TPU specimens were observed to be the lowest in the Z orientation and highest in the X orientation, which was the opposite of the trend observed for the flexural strength of the specimens. The PA11 specimen possessed a low coefficient of friction (COF) of 0.13 and wear rate of 8.68 × 10−5 mm3/Nm as compared to the TPU specimen, which had a COF of 0.55 and wear rate of 0.012 mm3/Nm. The PA11 specimens generally had lower roughness values on their surfaces (Ra &lt, 25 μm), while the TPU specimens had much rougher surfaces (Ra &gt, 40 μm). This investigation aims to uncover and explain phenomena that are unique to the MJF process of PA11 and TPU while also serving as a benchmark against similar polymeric parts printed using other PBF processes.

Isotactic polypropylene homopolymer (iPP) and copolymer (CoPP) were comparatively investigated for selective laser sintering (SLS). The processability of the polymer powders was evaluated in terms of powder morphology, powder flowability, melting behavior, and crystallization kinetics. An isothermal differential scanning calorimetry (DSC) testing protocol was employed as an effective and efficient method to facilitate the determination of suitable powder bed temperature. The results showed that the sintering window of iPP (26.3 ℃) was similar to that of polyamide 12 (PA12) (25.8 ℃), while CoPP had a narrower sintering window (2.2 ℃), making it much less tolerant to temperature deviations that occur during printing, and thus more prone to warping. The crystallization activation energy values of iPP and CoPP were ∼60 % that of PA12, suggesting that both of the PP materials were less sensitive to temperature fluctuations close to their crystallization temperatures. At optimal processing parameters, the ultimate tensile strength and elongation at break values of the printed iPP and CoPP specimens were found to be 14.9 MPa and 1 %, and 19.5 MPa and 207 %, respectively. Importantly, it was established that for PP with high regio- and stereo-irregularities, such as CoPP, SLS serves as a viable alternative manufacturing technique for producing PP parts comprising γ-phase crystals that are difficult to manufacture by conventional moulding processes. Economic Development Board (EDB) Accepted version The work was supported by the Economic Development Board (EDB), Singapore and Robert Bosch (SEA) Pte Ltd through the Industrial Postgraduate Program with School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.

While significant progress has been made in understanding laser powder bed fusion (L-PBF) as well as the fabrication of various materials using this technology, there is still limited adoption in the industry. One of the key obstacles identified is the lack of materials that can truly manufacture functional parts directly with L-PBF. This paper covers the emerging research on in-situ alloying and multi-metal processing. A comprehensive overview of the underlying scientific topics behind them is presented. The current state of research and progress from different perspectives (the materials and L-PBF processing parameters) are reviewed in order to provide a basis for follow-up research and development of these approaches. Defects, especially those associated with these two material processing routes, are also elucidated by discussing the mechanisms of their formation, including the main influencing factors, and the tendency for them to occur. Future research trends and potential topics are illustrated. The final part of this paper summarizes findings from this review and outlines the possibility of in-situ alloying and multi-metal processing using L-PBF. National Research Foundation (NRF) Accepted version

The influence of surface porosity on the fatigue life of additively manufactured (AMed) EH36 steel samples via selective laser melting (SLM) was studied in-depth by using a numerical method, which the pores were simulative AM 3D pores. The surface pores in both simplified semi-ellipsoid/spheroid and more realistic triangular shapes were evaluated in the first place. Stress concentration factor Kt analyzed by ABAQUS and fatigue life assessed by FE-SAFE were closely related to the AM 3D pore size, shape, position, orientation as well as their interactions, which confirmed their effects on the crack initiation stage during fatigue testing. The effectiveness of adding a surface finish factor Kt to a smooth surface fatigue model was subjected to internal pore geometry in terms of Kt values ranging from 1 to 5.5. Different algorithms were attempted with the stress-life method giving a higher estimation than a strain-life method while becoming invalid when the stress concentration situation was rather serious (around Kt = 2.5). There was no much difference between the uniaxial and multi-axial fatigue algorithms because the uniaxial tensile cyclic loading was applied to the model. Theoretically calculated total fatigue life (crack propagation) using linear elastic fracture mechanics (LEFM) fell in the same order as the experimental results for all the samples (within a factor of 2). AMed material parameters were used for the fatigue life prediction where the microstructure element was counted. AMed pores could deteriorate the mechanical properties of the AMed materials a lot, such as strength, Young’s modulus and fatigue lifetime, etc. The statistical analysis showed the scatter band of the SN curves at a factor of 8 with respect to different laser scanning speeds. Accepted version

In recent years, there is an rise in adoption of additive manufactured parts as end use parts. The increase is especially apparent in an aviation industry, where benefits such as shortened supply chain and parts on demand enabled significant cost savings. One of the most established AM methods, Fused Deposition Modelling™ (FDM), is widely adopted in the aviation industry to manufacture the end use parts. However, to ensure that FDM parts are able to meet functional requirements, extensive characterisation are required to be performed to understand the impact of manufacturing process parameters on the part performance. Due to Process-Structure-Property (PSP) linkage, toolpath can have a significant influence on FDM part performance. Existing research has focused extensively on the impact of process parameters on the FDM part performance. However, it is believed that toolpath planning, which is significantly less extensively studied, can have influence on the part performance. To investigate the effect, this research utilises 5 different tensile specimens and 2 different fabrication methods, namely specimens printed to net shape and specimens machined from a printed plate. It is concluded that the net shape specimens are substantially strengthened and stabilised due to presence of toolpath features. Due to the PSP linkage, the features translate into favourable improvement in mechanical performance which strengthened the net shaped specimens by up to 51%. This paper characterises influence of the toolpath on the mechanical properties and discusses how the toolpath features such as travel distance, thermal history and contours, impact FDM parts which can aid in prediction of performance of FDM structures. Economic Development Board (EDB) National Research Foundation (NRF) Published version This research was supported by a grant from ST Engineering Aerospace, EDB-IPP, Singapore Centre for 3D Printing (SC3DP), the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

The nanometer-scale (nano-scale) microstructural evolution in an additively manufactured Re-free Ni-based superalloy, with single crystal compositions, is investigated through field emission scanning electron microscopy, transmission electron microscopy, and atom probe tomography. We find that nano-scale primary γ′ precipitation occurs in the fine as-built microstructure, leading to an exceptional microhardness of 480.0 ± 6.7 HV at room temperature. Presence of ultra-fine γ′ precipitates, ~ 20 nm in size, is observed in the bottom few layers of the as-built samples, which is hitherto undocumented and contrary to the widespread consensus regarding hierarchical γ′ phase evolution in additively manufactured Ni-based superalloys. Moreover, considerable precipitation of tantalum-rich C14 Laves phase at the grain boundaries and interdendritic regions in the as-built samples emphasizes the need for additive manufacturing specific alloy design. National Research Foundation (NRF) Accepted version

We demonstrate a high repetition-rate upconversion green pulsed micro-laser, which is prepared by the fast thermal quenching of lanthanide-doped upconversion nanoparticles (UCNPs) via femtosecond-laser direct writing. The outer rim of the prepared upconversion hemi-ellipsoidal microstructure works as a whispering-gallery-mode (WGM) optical resonator for the coherent photon build-up of third-harmonic ultra-short seed pulses. When near-infrared (NIR) femtosecond laser pulses of wavelength 1545 nm are focused onto the upconversion WGM resonator, the optical third-harmonic is generated at 515 nm together with the upconversion luminescence. The weak third-harmonic (TH) seed pulses are coherently amplified in the hemi-ellipsoidal upconversion resonator as a result of the resonant interaction between the incident femtosecond laser field, the TH, the upconversion luminescence and the WGM. This upconversion lasing preserves the original repetition rate of the NIR pump laser and the output polarization state is also coherently aligned to the pump laser polarization. Because of the isotropic nature of the upconversion micro-ellipsoids, the upconversion lasing shows maximum intensity with a linearly polarized pump beam and minimum intensity with a circularly polarized pump beam. Our scheme devised for realizing high-repetition-rate lasing at higher photon energies in a compact micro platform will open up new ways for on-chip optical information processing, high-throughput microfluidic sensing, and localized micro light sources for optical memories. Accepted version The authors acknowledge funding support received from Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA-15/027), National Research Foundation of the Republic of Korea (NRF-2012R1A3A1050386, NRF-2018R1A4A1025623), Korea Forest Service (Korea Forestry Promotion Institute) through the R&D Program for Forest Science Technology (2020229C10-2022AC01), and Basic Research Program (NK224C) funded by the Korea Institute of Machinery and Materials.