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In the present work, an approach of freeze-dry pulsated orifice ejection method (FD-POEM) was utilized to fabricate monodispersed MoSiBTiC-La2O3 composite powders for laser powder bed fusion (L-PBF). The FD-POEM powders were spherically shaped, possessing a narrow size range and uniform element distribution. As revealed by the single-track and single-layer experiments, the porous FD-POEM particles were sufficiently fused under laser irradiation, leading to the generation of continuous laser tracks and low surface roughness layers, which proved a feasible L-PBF processability of MoSiBTiC-La2O3 powders. Careful microstructural observations confirmed that the microstructure of the molten pools was primarily composed of Mo solid solution dendrites reinforced with La2O3 nanoparticles. Consequently, the single MoSiBTiC-La2O3 track had a high Martens hardness of 3955 HM. The result of this work reveals that the combination of FD-POEM and L-PBF has a great potential of developing advanced heat-resistant Mo-based alloys with tailored structures for ultrahigh-temperature applications. [ABSTRACT FROM AUTHOR]

Novel TiC-reinforced titanium matrix composites (TMCs) were in situ synthesized by laser powder bed fusion (L-PBF) of unique carbon nanotube (CNT)-decorated Ti-6Al-4V powders. Acid-treated CNTs were coated on the surface of Ti-6Al-4V particles by electrostatic self-assembly without varying the powder sphericity, leading to improved printability, as proved by laser-absorption and single-track experiments. During L-PBF, the CNTs were completely transformed into monocrystalline TiC dispersed in the α'-Ti matrix via a dissolution/precipitation mechanism. As illustrated by high-resolution transmission electron microscopy, the in situ-synthesized TiC crystals were closely bonded to the matrix, exhibiting typical TiC [001]//Ti [00−1] and TiC (220)//Ti (−100) orientation relationships. Significantly, the morphology of TiC underwent an interesting evolution from nanorods to micro-spheres, and to dendrites with an increase in the CNT content, causing a gradual increase in hardness of TMCs. This study may provide insights into the design of high-performance TMCs with unique microstructures, excellent properties, and tailored architectures. Unlabelled Image • CNTs were coated on the surface of Ti-6Al-4V powders by electrostatic self-assembly. • Effect of CNT addition on the powder characteristics and 3D-printability of Ti-6Al-4V particles was investigated. • Novel TiC nanorods were first synthesized by L-PBF. • Microstructure evolution and formation mechanism of in situ TiC was clarified. [ABSTRACT FROM AUTHOR]

Abstract The MoSiBTiC alloy is one promising candidate for ultrahigh–temperature materials. However, it faces severe challenges in the machining of complex shapes because of its significant brittleness, high melting point, and stiffness. To overcome this challenge, we have successfully fabricated MoSiBTiC alloy parts via laser powder bed fusion (L-PBF). A combination of arc-melting and controllable high-energy ball milling (HEBM) was employed to prepare suitable MoSiBTiC powders for L-PBF. The evolution of powder morphology, constituent phases, and laser absorptivity, as well as particle size and distribution during HEBM, was investigated. Moreover, the effects of L-PBF parameters on the densification, microstructure, and mechanical performance of MoSiBTiC alloy builds were studied. A dense MoSiBTiC alloy, mainly consisting of a Mo solid-solution, Mo 5 SiB 2 , Mo 2 C, and TiC phases, was obtained at an energy density of 156 J mm−3 using the laser power of 70 W. Compared to the as-cast alloy, the L-PBF-processed MoSiBTiC alloy possessed more uniform and finer grain structures, while exhibiting a lower Vickers hardness due to the existence of internal microcracks. It was also proved that the small quantity of ZrO 2 particles from HEBM processing was uniformly imbedded in L-PBF builds. This work may offer significant guidance for designing and producing complexly shaped refractory intermetallics with unique microstructures in ultrahigh–temperature applications. Graphical abstract Image 1 Highlights • Additive manufacturing of MoSiBTiC alloy for ultrahigh–temperature material was firstly attempted. • An effective approach was developed and optimized for large-scale production of starting MoSiBTiC powders. • High-density MoSiBTiC alloy (∼99.8%) was produced via optimizing laser processing conditions. • Additive-manufactured MoSiBTiC alloy had more uniform and finer grain structures than as-cast one. [ABSTRACT FROM AUTHOR]

Poor understanding of composite powder properties is a bottleneck in the design of high-performance metallic components via laser powder bed fusion (L-PBF). Traditional particle- or fiber-like fillers are known to result in reduced powder flowability and the uncontrollable morphology of composite powders. In this work, we demonstrate a first example of improving the powder properties and L-PBF manufacturability of metallic powders decorated by two-dimensional (2D) nanofillers. 0.5 wt% graphene oxide (GO) sheets were uniformly adhered onto the surface of Ti–6Al–4V (Ti64) powders via an electrostatic self-assembly, with their spherical morphology and particle size uncompromised. The notably enhanced quality of the GO-decorated Ti64 (GO@Ti64) powder bed, indicated by the results of a novel centrifugal separation testing and a recoating experiment, was attributed to the significant reduction in powder/powder and powder/substrate adhesive forces. Moreover, while the thermal conductivity of the GO@Ti64 powders was found to decrease, the laser absorptivity increased as a result of multiple local absorptions of wrinkled GO. The optimal process zone of GO@Ti64 powders was therefore shown to have been extended and to have become wider, enhancing L-PBF printability. High-resolution transmission electron microscopy inspections revealed that the GO/Ti64 build fully consisted of ultrafine α' martensite structures with a lath width of ∼91 nm. This novel structure was attributed to the high-temperature, nonequilibrium densification of L-PBF; the complete dissolution of the 2D GO nanosheets into the Ti64 matrix generated a novel carbon-supersaturated, precipitation-free structure. The grain refinement and solid-solution strengthening behaviors led to an increase in the yield strength of the GO/Ti64 build from 1375 MPa to 1793 MPa, and also an increase in its compressive strength, from 1796 MPa to 2032 MPa. The results of this investigation contribute to a better understanding of composite powder properties and can be seen as a significant step toward the preparation of high-performance metallic components via L-PBF. [ABSTRACT FROM AUTHOR]

Raw powders are processed in water during the freeze-dry pulsated orifice ejection method (FD-POEM), leading to the inclusion of oxygen impurities. This study proposes a strategy for removing the oxygen content and enhancing the mechanical performance of laser powder bed fusion (L-PBF) builds from powders using carbon nanotubes (CNTs) and H2 reduction. Spherical 1.5 wt.% CNT/Mo composite powders with uniform dispersion were fabricated via FD-POEM. The quantity of MoO2 decreased significantly, and a hexagonally structured Mo2C phase was simultaneously formed in the L-PBF build. The Mo2C with network structure was distributed along the boundaries of equiaxed Mo grains, leading to an increased Vickers hardness of the matrix. This study demonstrates the feasibility of fabricating oxygen-free and high-strength refractory parts during L-PBF for ultrahigh-temperature applications. [ABSTRACT FROM AUTHOR]

The low laser absorptivity and high thermal conductivity of Al are the foremost concerns when developing Al matrix composites (AMCs) through laser powder bed fusion (L-PBF) process. In this study, we demonstrated an example of improving the 3D-printability of AMCs by means of powder surface modification. Flexible graphene oxide (GO) sheets were carefully coated onto the surface of AlSi10Mg powders under electrostatic self-assembly via a hetero-agglomeration process. In addition to maintaining a shape and particle size similar to the initial metallic powders, the GO-coated AlSi10Mg powders exhibited enhanced laser absorptivity and decreased thermal conductivity, beneficial to their fusion. Under high-energy irradiation, the GO sheets were partially transformed to Al 4 C 3 nanorods individually distributed in the matrix, while the un-reacted parts floated within molten pools under buoyancy, forming an in-situ carbon layer tightly deposited on the surface of the composite build. This work may provide significant guidance for the design and production of high-performance AMCs with advanced architectures for practical applications. • Graphene oxide (GO) sheets were uniformly coated onto the surface of AlSi10Mg powders via electrostatic self-assembly. • The mixed powders exhibited enhanced laser absorptivity and lower thermal conductivity, beneficial to the 3D-printability. • The microstructure evolution of GO was investigated systematically during laser additive manufacturing. • A carbon layer was in-situ deposited on the surface of composite builds. [ABSTRACT FROM AUTHOR]

MoSiB-based alloys are extremely promising ultrahigh–temperature materials, owing to their high melting points, good thermal stability, and excellent elevated-temperature mechanical properties. However, their fabrication is limited to conventional manufacturing techniques. In this work, a MoSiBTiC multiphase alloy, mainly consisting of Mo solid solution (Mo ss), Mo 5 SiB 2 (T 2), and TiC phases, was fabricated by laser powder bed fusion (L-PBF) using ball-milled alloy powders; subsequently, the effect of hot isostatic pressing (HIP) on its microstructure and fracture toughness was investigated. Due to the non-equilibrium metallurgical characteristics of L-PBF, the as-built alloy included uniform and fine-grained structures, as well as internal microcracks. The length of microcracks was effectively reduced with the assistance of the TiC bridging effect during HIP. As revealed by a high-resolution transmission electron microscope, the TiC was uniformly dispersed and was in close contact with the Mo ss or T 2 phase. Consequently, the MoSiBTiC multiphase alloy exhibited a high fracture toughness of 9.0 MPa(m)1/2 after HIP at 1973 K. This work could be an important step towards the fabrication of high-performance refractory components with advanced properties in ultrahigh–temperature use. Unlabelled Image • A high-performance MoSiBTiC alloy was designed by laser powder bed fusion (L-PBF) and hot isostatic pressing (HIP). • L-PBF process was optimized to fabricate a MoSiBTiC alloy using ball-milled powders. • Microstructural evolution during HIP was observed via high-resolution TEM. • TiC phase acted as a healing agent for repairing internal microcracks. [ABSTRACT FROM AUTHOR]

A novel Mo-based composite with simultaneously improved mechanical and oxidation properties was fabricated by laser powder bed fusion (L-PBF), using uniform Al 2 O 3 -nanoparticle-decorated MoTiAl powders bridged by functionalized carbon nanotubes. A tight ceramic coating ∼2.57 μm in thickness consisting of an α-Al 2 O 3 matrix with dispersed TiC particles was formed on the surface of the Al 2 O 3 -CNT/MoTiAl composite, which has been proved to effectively increase resistance to oxidation at 1173 K. Meanwhile, the nanoparticles were homogenously dispersed and tightly contacted with the matrix, giving rise to an enhanced Vickers hardness. This work shed light on designing and producing high-performance Mo-based composites for application to ultrahigh-temperature materials. • A novel Mo-based composite with simultaneously improved mechanical and oxidation properties was fabricated by laser powder bed fusion. • A tight ceramic coating consisting of an α-Al 2 O 3 matrix with dispersed TiC particles was formed on the composite surface. • The ceramic coating could effectively increase resistance to oxidation at 1173 K. • The nanoparticles were homogenously dispersed and tightly contacted with the matrix, resulting in an enhanced Vickers hardness. [ABSTRACT FROM AUTHOR]

A novel MoSiBTiC alloy was fabricated via laser powder bed fusion (L-PBF) combined with the freeze-dry pulsated orifice ejection method (FD-POEM). Spherical MoSiBTiC powders with uniform size distribution and good dispersion of component elemental particles were prepared by FD-POEM, without a melting process. Due to their distinctive mesh-like structure, the laser absorptivity of the FD-POEM particles was as high as 87.3%, allowing the elemental powders to be sufficiently melted under laser irradiation. The L-PBF build was primarily composed of a unique supersaturated solid solution Mo (Mo sss) dendrites enriched with Si, Ti and B elements, Mo + Mo 5 SiB 2 nanostructures, and TiC nanoparticles. The Mo sss phase exhibited much higher hardness than Mo phase of the as-cast alloy attributing to the grain refinement and the solid strengthening effect. These results of this study provided an approach of fabricating high-performance Mo-based materials in ultrahigh-temperature applications. [Display omitted] • FD-POEM enables elemental fabrication of spherical MoSiBTiC powders for 3D-printing. • A novel MoSiBTiC alloy with enhanced mechanical performance was in situ fabricated via L-PBF of FD-POEM powders. • A unique supersaturated solid solution Mo phase enriched with Si, Ti or B elements generated due to rapid solidification. [ABSTRACT FROM AUTHOR]