Characterization, preparation, and reuse of metallic powders for laser powder bed fusion: a review

Laser powder bed fusion(L-PBF) has attracted significant attention in both the industry and academic fields since its inception, providing unprecedented advantages to fabricate complex-shaped metallic components. The printing quality and performance of L-PBF alloys are infuenced by numerous variables consisting of feedstock powders, manufacturing process,and post-treatment. As the starting materials, metallic powders play a critical role in infuencing the fabrication cost, printing consistency, and properties. Given their deterministic roles, the present review aims to retrospect the recent progress on metallic powders for L-PBF including characterization, preparation, and reuse. The powder characterization mainly serves for printing consistency while powder preparation and reuse are introduced to reduce the fabrication costs.Various powder characterization and preparation methods are presented in the beginning by analyzing the measurement principles, advantages, and limitations. Subsequently, the effect of powder reuse on the powder characteristics and mechanical performance of L-PBF parts is analyzed, focusing on steels, nickel-based superalloys, titanium and titanium alloys, and aluminum alloys. The evolution trends of powders and L-PBF parts vary depending on specific alloy systems, which makes the proposal of a unified reuse protocol infeasible. Finally,perspectives are presented to cater to the increased applications of L-PBF technologies for future investigations. The present state-of-the-art work can pave the way for the broad industrial applications of L-PBF by enhancing printing consistency and reducing the total costs from the perspective of powders.

Characteristics of Inconel Powders for Powder-Bed Additive Manufacturing

In this study, the flow characteristics and behaviors of virgin and recycled Inconel powder for powder-bed additive manufacturing （AM） were studied using different powder characterization techniques. The results revealed that the particle size distribution （PSD） for the selective laser melting （SLM） process is typically in the range from 15 μm to 63 μm. The flow rate of virgin Inconel powder is around 28 s·（50 g）^-1. In addition, the packing density was found to be 60%. The rheological test results indicate that the virgin powder has reasonably good flowability compared with the recycled powder. The inter-relation between the powder characteristics is discussed herein. A propeller was successfully printed using the powder. The results suggest that Inconel powder is suitable for AM and can be a good reference for researchers who attempt to pro- duce AM powders.

Multienzyme co-immobilization-based bioelectrode:Design of principles and bioelectrochemical applications

Enzyme cascade reactions play significant roles in bioelectrochemical processes because they permit more complex reactions. Co-immobilization of multienzyme on the electrode could help to facilitate substrate/intermediate transfer among different enzymes and electron transfer from enzyme active sites to the electrode with high stability and retrievability. Different co-immobilization strategies to construct multienzyme bioelectrodes have been widely reported, however, up to now, they have barely been reviewed. In this review, we focus on recent state-of-the-art techniques for constructing co-immobilized multienzyme electrodes including random and positional co-immobilization. Particular attention is given to strategies such as multienzyme complex and surface display. Cofactor co-immobilization on the electrode is also crucial for the enhancement of catalytic reaction and electron transfer, yet, few studies have been reported. The up-to-date advances in bioelectrochemical applications of multienzyme bioelectrodes are also presented. Finally, key challenges and future perspectives are discussed.

Crack-free and high-strength AA2024 alloy obtained by additive manufacturing with controlled columnar-equiaxed-transition

Laser powder bed fusion(LPBF)of high-strength Al alloys is challenging due to the formation of both hot and cold cracks.In the present work,highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass(MG)powders.The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1μm and relatively coarser grains.The grain refinement can be associated with the formation of Al3Zr particles,serving as the heterogeneous nucleation sites for theα-Al matrix.Complete routes for columnar-equiaxed-transition(CET)have been revealed by tailoring the concentration of nucleation particles and solidification conditions.CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling.Between these two regions,columnar grains epitaxially grow with orientations determined by the thermal gradient.The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains.The as-fabricated AA2024/5%MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement,surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders.The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.

Protein engineering of oxidoreductases utilizing nicotinamide-based coenzymes,with applications in synthetic biology

Two natural nicotinamide-based coenzymes(NAD and NADP)are indispensably required by the vast majority of oxidoreductases for catabolism and anabolism,respectively.Most NAD(P)-dependent oxidoreductases prefer one coenzyme as an electron acceptor or donor to the other depending on their different metabolic roles.This coenzyme preference associated with coenzyme imbalance presents some challenges for the construction of high-efficiency in vivo and in vitro synthetic biology pathways.Changing the coenzyme preference of NAD(P)-dependent oxidoreductases is an important area of protein engineering,which is closely related to product-oriented synthetic biology projects.This review focuses on the methodology of nicotinamide-based coenzyme engineering,with its application in improving product yields and decreasing production costs.Biomimetic nicotinamide-containing coenzymes have been proposed to replace natural coenzymes because they are more stable and less costly than natural coenzymes.Recent advances in the switching of coenzyme preference from natural to biomimetic coenzymes are also covered in this review.Engineering coenzyme preferences from natural to biomimetic coenzymes has become an important direction for coenzyme engineering,especially for in vitro synthetic pathways and in vivo bioorthogonal redox pathways.

Stoichiometric Conversion of Maltose for Biomanufacturing by In Vitro Synthetic Enzymatic Biosystems

Maltose is a natural α-(1,4)-linked disaccharide with wide applications in food industries and microbial fermentation. However,maltose has scarcely been used for in vitro biosynthesis, possibly because its phosphorylation by maltose phosphorylase (MP)yields β-glucose 1-phosphate (β-G1P) that cannot be utilized by α-phosphoglucomutase (α-PGM) commonly found in in vitrosynthetic enzymatic biosystems previously constructed by our group. Herein, we designed an in vitro synthetic enzymaticreaction module comprised of MP, β-phosphoglucomutase (β-PGM), and polyphosphate glucokinase (PPGK) for thestoichiometric conversion of each maltose molecule to two glucose 6-phosphate (G6P) molecules. Based on this syntheticmodule, we further constructed two in vitro synthetic biosystems to produce bioelectricity and fructose 1,6-diphosphate (FDP),respectively. The 14-enzyme biobattery achieved a Faraday efficiency of 96.4% and a maximal power density of 0.6mW/cm^(2),whereas the 5-enzyme in vitro FDP-producing biosystem yielded 187.0mM FDP from 50 g/L (139mM) maltose by adopting afed-batch substrate feeding strategy. Our study not only suggests new application scenarios for maltose but also provides novelstrategies for the high-efficient production of bioelectricity and value-added biochemicals.