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Laser powder bed fusion (LPBF) has the advantages of high resolution and geometric freedom but can be susceptible to process failures and defects caused by inappropriate process parameters and powder conditions. This study aims to reveal and quantify the moisture effect on the qualities and properties of as-built parts with various process parameters. The results showed that the density was decreased by 7.86% with humid powder (60.0% relative humidity (RH)) compared to dry powder (3.4%RH). Expectedly, the observed low density led to the property degradation in the hardness, yield strength (YS), and ultimate tensile strength (UTS) of the humid powder by 11.7, 15.02, and 21.25%, respectively, compared to that of dry powder (3.4%RH). Interestingly, the elongation at break of the parts fabricated with humid powder (60.0%RH) was increased by 2.82%, while their YS and UTS were decreased significantly. It seems that the water molecules on the powder surface hindered the reaction between the N2 shielding gas and melted powder, which resulted in the reduction in the austenite (γ) phase by up to 4.05 wt.%. This could be mainly responsible for the decrease in both the YS and UTS of the humid powder by approximately 100 and 150 MPa, respectively. This study demonstrates that the moisture of the metal powder used for LPBF should be carefully controlled to ensure desirable as-built qualities and properties. [ABSTRACT FROM AUTHOR]

Although equiatomic CoCrFeNi medium-entropy alloys (MEAs) show excellent ductility, their strength is not high enough for the use in various engineering applications. It is necessary for their strength to be improved, while maintaining their great ductility. In this study, the in-situ TiC x N y nanoparticle reinforced equiatomic CoCrFeNi MEA matrix nanocomposites were fabricated by laser powder bed fusion (LPBF) technique and the addition of sub-micron TiC particles. Interestingly, the LPBF-fabricated CoCrFeNi MEAs have been found to exhibit both poor ultimate tensile strength (UTS) (∼582 MPa) and uniform elongation (δ u) (∼11.7 %), attributed to the formation of hot cracks. However, the in-situ formation of TiC x N y nanoparticles could eliminate the formation of hot cracks. The near defect-free nanocomposites with substantial sub-grains were fabricated, and their UTS and δ u were increased by 74.4 % (∼1015 MPa) and 92.3 % (∼22.5 %), respectively. The microstructure of as-built nanocomposites was mainly composed of FCC phase and TiC x N y nanoparticles, and a few M 23 C 6 nanoparticles were found. The high strength of as-built nanocomposites was mainly attributed to Orowan and dislocation strengthening mechanisms. To the best of our knowledge, this is the first study to introduce high strength and great ductility into CoCrFeNi MEAs via the in-situ formation of TiC x N y nanoparticles with the optimized LPBF process. [Display omitted] • It is the first time that hot cracks were found in LPBF-fabricated CoCrFeNi medium entropy alloys. • The in-situ formation of TiC x N y nanoparticles could eliminate the generation of hot cracks. • Well-bonded interfaces were observed between TiC x N y nanoparticles and FCC matrix. • This is the study to introduce high strength and ductility into CoCrFeNi medium entropy alloys. [ABSTRACT FROM AUTHOR]

The employment of grain refinement approaches is regarded as a favorable strategy to increase mechanical properties of duplex stainless steels (DSSs). In this work, a new grain refinement route was successfully developed via laser powder bed fusion (LPBF) of micro-duplex stainless steel (SS) matrix composites along with a subsequent quenching process. For as-built composites, TiC x N y nanoparticles were first formed in-situ via the introduction of submicron/micron-sized TiC particles with low cost into 2205 DSSs, and fine equiaxed high-temperature ferrite (δ-ferrite) grains were formed due to the ultra-high cooling rate of LPBF and heterogeneous nucleation sites provided by in-situ formed TiC x N y nanoparticles. The formation of TiC x N y nanoparticles inhibited the epitaxial growth of δ-ferrite grains, and no obvious textures were found in as-built composites. Subsequently, fine austenite grains were produced at grain boundaries of fine unrecrystallized δ-ferrite during quenching treatment of as-built composites, which enabled heat-treated composites to achieve an ensemble of outstanding ultimate tensile strength and uniform elongation. The δ-ferrite recrystallization was mainly inhibited by the existence of TiC x N y nanoparticles. This study can provide a groundwork for designing grain refinement methods to produce high-performance DSS parts with complex geometry based on LPBF technique. • A new grain refinement route was successfully developed for duplex stainless steels (DSSs). • Fine δ-ferrite grains were obtained by utilizing ultra-high cooling rate of SLM and TiC x N y nanoparticles. • Fine austenite grains were formed at grain boundaries of parent unrecrystallized δ-ferrite grains. • Micro-duplex SS matrix composites showed an outstanding combination of ultimate tensile strength and uniform elongation. [ABSTRACT FROM AUTHOR]

More recently, three dimensional printing (3D Printing), also known as an additive manufacturing (AM), has been highlighted since it shows a great promise to realize almost any three dimensional parts or structures with computer aided design (CAD). Several different processes are available for 3D printing, which includes fused deposition modeling, selective laser sintering, stereolithography, photopolymerization, and etc. In particular, considerable attention is paid to the 3D printing technique with photopolymerization due to their high resolutions. Unfortunately, the 3D printed products with photopolymerization however possess poor mechanical properties. Understanding of this should be necessary for the advantages of the 3D printing to be fully realized. Here, this study experimentally investigates the mechanical properties of the 3D printed photopolymer through thermomechanical analysis and tensile testing. In this study, it is found that the printed specimens are not fully cured after the 3D printing with photopolymerization. DiBenedetto equation is employed to better understand the relationship between the curing status and tensile properties. In addition to the poor mechanical properties, anisotropic and size dependent tensile properties of the 3D printed photopolymers are also observed. Electron beam treatment is used to ensure the cure of the 3D printed photopolymer and the corresponding tensile properties are characterized and investigated. [ABSTRACT FROM AUTHOR]

In contrast to conventional preforming techniques, additive manufacturing features direct and layer-by-layer fabrication, which provides viable new capabilities for the fabrication of reinforced composites. In this article, we explore the microstructural design as well as additive manufacturing and characterization of 3D orthogonal, short carbon fiber/acrylonitrile-butadiene-styrene (ABS) preforms and composite. First, an array of 3D orthogonal preforms is designed based on topological consideration and validated by fused filament fabrication of pure ABS wire; high fidelity between models and preforms is accomplished. Then, short carbon fibers are introduced into the designed 3D orthogonal preforms as reinforcement, using a short carbon fiber/ABS wire. Lastly, the compressive behavior of a 3D orthogonal, short carbon fiber/ABS preform and that of its silicone infused composite are characterized. The preform design methodology developed in this research as well as the preliminary effort made in composite fabrication and characterization demonstrates the feasibility of additive manufacturing of 3D orthogonal preform based fiber composites. [ABSTRACT FROM AUTHOR]

Austenitic stainless steels (SSs) with excellent mechanical properties can be fabricated by selective laser melting (SLM). To enrich the variety of SLM-fabricated austenitic SSs and develop some SSs which possess desired properties and cost, an austenite-rich composite of stainless steels was fabricated by SLM of a powder mixture composed of 310 S and 430 SSs. The effect of laser energy density on the defects, microstructure and mechanical properties of materials was investigated. At an appropriate energy density (89.3 J/mm3), samples exhibited the best ultimate tensile strength (∼701 MPa) and elongation to failure (∼46.6%). However, the occurrence of strong keyholing and spattering phenomena severely weakened the tensile properties of samples at the highest energy density (113.6 J/mm3). For all samples, their microstructure was mainly composed of lots of austenite phase and some delta (δ) ferrite phase. The existence of ultra-fine sub-grains surrounded by dislocation networks played an important role in the tensile properties of samples. Particularly, the formation of fine δ-ferrite at grain boundaries of austenite could hamper the growth of austenite grains, and lots of dislocations are formed at the interface between austenite and δ-ferrite. The existence of δ-ferrite with a circular flow feature could inhibit the epitaxial growth of austenite grains. • An austenite-rich composite of stainless steels (SSs) is obtained by SLM of a powder mixture of 310 S and 430 SSs. • The SLM-fabricated austenite-rich composite exhibits a favorable combination of strength and ductility. • The distribution of fine δ-ferrite at grain boundaries of austenite inhibits the growth of austenite grains. • The presence of ferrite with a circular flow can hamper the epitaxial growth of austenite grains. [ABSTRACT FROM AUTHOR]