In-Situ Reduction of Mo-Based Composite Particles during Laser Powder Bed Fusion

: 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 H 2 reduction. Spherical 1.5 wt.% CNT/Mo composite powders with uniform dispersion were fabricated via FD-POEM. The quantity of MoO 2 decreased signiﬁcantly, and a hexagonally structured Mo 2 C phase was simultaneously formed in the L-PBF build. The Mo 2 C 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.


Introduction
Refractory metals have gained significant attention in the academic and industrial fields [1][2][3]. Molybdenum and its alloys are considered attractive candidates for ultrahightemperature applications, such as in astronautics and aerospace, because of their high melting points, low thermal expansion coefficients, and excellent high-temperature strength [4,5]. Regarding the production of complex geometries and tailored parts, there are several limitations associated with Mo and its alloys, as traditional techniques such as casting and powder metallurgy approaches have limitations for geometric flexibility [6,7]. By contrast, laser powder bed fusion (L-PBF), a novel manufacturing process of additive manufacturing (AM) [8,9], is ideally suited for the production of complex parts with internal or external structures [10].
Several alloys such as high-strength stainless steel, nickel-based superalloys, lightweight aluminum alloys, aerospace-grade titanium, and copper matrix composites manufactured by L-PBF have been investigated extensively [11][12][13][14][15]. L-PBF is capable of fabricating threedimensional products according to computer-aided design models using a high-power density laser beam to selectively fuse and consolidate loose powders in a layered manner [16,17]. L-PBF has competitive advantages in the direct manufacturing of complicated components, feedstock reusability, and high processing flexibility [18]. Compared with conventional alloys, L-PBF for refractory materials has not yet been sufficiently developed. A few studies have focused on fabricating Mo components via L-PBF [19][20][21]. Although some interesting results were obtained in these studies, the final products still suffered from many structural defects such as porosity and cracks attributed to low energy input or high ductile-brittle transition temperatures [21,22]. Thus, while the effects of L-PBF parameters have been investigated for the defect reduction of Mo products during manufacturing [23], the significant influence of feedstock powders on the properties of the L-PBF parts needs to be evaluated. The metallic powders applied for L-PBF should have high sphericity and a small amount of satellite powder [24]. Atomization has been extensively used in the powder bed fusion technology of AM [25,26]. Atomization, such as that of gas, water, and plasma, remains the leading powder manufacturing technologies for AM [27]. During the atomization process, metals are melted, thereby impacting the inert gas jet, water pressure, or high-temperature plasma and atomizing into particles. However, these methods exhibit certain limitations owing to the high melting points of the refractory materials. For instance, a large amount of energy is consumed during melting and atomization, stable crucibles are needed, and satellites are formed on powders. Therefore, new methods for fabricating low-cost Mo and alloy powders for AM are required.
The novel freeze-dry pulsated orifice ejection method (FD-POEM) was proposed to address the demand for refractory materials applied to L-PBF in this study. In the FD-POEM, a slurry is prepared by mixing different raw powders and then ejected from an orifice driven by pulsations of the diaphragm. The droplets were then extruded into liquid nitrogen in the form of spheres due to surface tension. The frozen particles were then transferred to dry in a freeze-drying machine. This method has the advantages of fabrication without melting, a high degree of sphericity, composition flexibility for composite powders, and controllability of particle size distribution.
Because raw materials are mixed in water for the slurry, the oxygen content of the particles increases. Oxides may be formed in the build via the powder and processing atmosphere during the L-PBF process. The oxygen impurities were found to weaken the grain boundaries, thereby inducing the risk of hot cracks and leading to an elevated ductile-to-brittle transition temperature [20]. Leung et al. [28,29] reported that oxides could act as nucleation sites for pore formation owing to the oxidation behavior of powders via Marangoni flow. Therefore, the oxygen should be eliminated or controlled. In general, the conventional process of hydrogen gas serving as a reductant is employed to minimize the formation of oxides in metal powders [30]. Recently, an alternative method of alloying with carbon using pre-alloyed powders by selective laser melting was employed to decrease the oxides in the build because of the alloying elements preventing the segregation of oxygen at the grain boundaries. However, the content of carbon in pre-alloyed powders needs to be precisely controlled, and the process of pre-alloying is complex and energy-consuming [31].
Herein, carbon nanotubes (CNTs) were chosen as ideal reduction agents by adding them to Mo powders processed by FD-POEM to remove oxygen. The reason for this is twofold: first, acid-treated CNTs can exist individually because of their functionalized surfaces [32]. These functionalized CNTs could be uniformly dispersed on the surface of metal nanoparticles because of electrostatic forces. The CNTs could serve as the reduction agent because the reaction MoO 2 + C → Mo + CO 2 is prone to occur according to the Gibbs free energy change during the L-PBF process. Second, the abundance of dangling bonds of carbon atoms on the surface of acid-treated CNTs provides the feasibility of carbide formation at high temperatures. The in situ formation of Mo carbides could play the role of reinforcing the build.
In this study, the feasibility of the in situ removal of oxygen from Mo built by the addition of CNTs into powders processed by FD-POEM during L-PBF was investigated. The effects of oxygen removal in the build were examined via microstructure observation and compared with the traditional method of hydrogen reduction.

Fabrication of CNT/Mo Particles Using the FD-POEM
CNT/Mo composite particles used for L-PBF were prepared using FD-POEM. Raw Mo powder was obtained from A.L.M.T. Corp., Japan. The CNTs were treated with an acidic mixture of HNO 3 /H 2 SO 4 (1:3 in volume) under ultrasonication and mechanical stirring at 323 K for~4-6 h [32]. CNT/Mo composite particles were synthesized via FD-POEM. A schematic of the processing strategy is presented in Figure 1. First, the acid-treated CNTs were dispersed in 10 mL of water via ultrasonication. Raw Mo powder was added to a suitable amount of deionized water. The slurry of 1.5 wt.% CNT/Mo particles mixtures were prepared through a combination of mechanical blending and ultrasonication at 273 K for 1 h, as shown in Figure 1a. Second, the slurry was dropped into liquid nitrogen using a POEM apparatus (Figure 1b). The setup included a POEM body, a diaphragm, an orifice pipe, and liquid nitrogen. A diaphragm was used to form droplets from the orifice pipe. In the POEM process, the slurry was extruded from an orifice, which was attributed to the vibration of a diaphragm at a certain frequency with one pulse followed by a rectangular waveform. The ejected droplets were spherical during falling because of the surface tension. They were instantly frozen in liquid nitrogen to form particles. Finally, spherical CNT/Mo composite particles were obtained via complete freeze-drying for at least 24 h, as illustrated in Figure 1c. In addition, spherical Mo particles were prepared using FD-POEM for comparison.

Reduction of Particles Processed by FD-POEM
Mo or CNT/Mo particles processed through FD-POEM were partially reduced by H 2 at 800 • C for 1 h. They are denoted as H 2 -reduced Mo powders or H 2 -reduced CNT/Mo powders, respectively.

L-PBF Processing of Particles Processed by FD-POEM
All bulk specimens were produced using in-house developed L-PBF equipment with a Yb: YAG fiber laser source (Raycus Fiber Laser Technologies Co., Ltd., Wuhan, China). The laser beam had a wavelength of 1070 nm and a maximum power of 22 W in continuous mode. The process was performed under Ar shielding gas to prevent the oxidation of materials during L-PBF. A set of L-PBF parameters is listed in Table 1. The energy input per unit volume, E, was calculated as: E = P/hvt E was 412 J/mm 3 , higher than that utilized for processing Mo in previous studies [19,22]. Ti substrate was chosen because of its low thermal conductivity. Rectangular specimens of 4 mm × 4 mm × 1.4 mm were fabricated using FD-POEM Mo, H 2 -reduced Mo, CNT/Mo, or H 2 -reduced CNT/Mo particles on Ti substrates, respectively. Due to the weak intrinsic strength of particles processed by FD-POEM, they were fractured during the recoating process; however, they were still applicable for L-PBF. These builds were subjected to microstructural analysis and mechanical characterization.

Characterizations
The zeta potential of the slurries was tested using a nanoparticle analyzer (SZ-100, HORIBA, Japan). The morphology and structure of the particles processed by FD-POEM were characterized using scanning electron microscopy (SEM; JSM-6010 LV, JEOL, Japan). The phase constitution of the builds was confirmed along the cross section through X-ray diffraction (XRD) using a 9 kW diffractometer with Cu Kα radiation (Smartlab, Rigaku, Japan). The microstructure of the L-PBF builds was examined using field-emission SEM (FESEM; JSM-6500F, JEOL, Japan) and annular dark-field scanning transmission electron microscopy (STEM; JEOL JEM-ARM200F, Japan). Specimens for microstructural investigation were cut from builds and a general metallurgical polishing method was executed. The transmission electron microscopy (TEM) specimens were acquired from composite builds using a focused ion beam (JEOL JIB-4600F) system. The hardness test was carried out for the polished specimens. The mechanical properties were investigated using Vickers hardness measurements on a micro-hardness tester (HM-200, Mitutoyo, Japan) with a peak load of 0.3 N and a holding time of 10 s, based on ISO standard 14, 577-1:2015. The hardness test was conducted at least 15 times, and the results were averaged.

Results and Discussion
The features of the raw Mo powders are shown in Figure 2a. The Mo nano powders were irregularly shaped with a size of D 50 , 655 nm. The zeta potential of Mo in water was −54.9 mV. The negative charge makes the Mo powder hydrophilic and mutually repels in water. It is noteworthy that the dispersion state of the slurry is an important factor in obtaining a stable slurry for FD-POEM because a heterogeneous deposition phenomenon does not occur during slurry fabrication. The typical morphology of acid-treated CNTs is shown in Figure 2b. It was obvious that certain circumference-type nano defects were introduced on the outer layers of the CNTs, as observed by TEM, indicating a defective character. The individual CNTs were hydrophilic and negatively charged, with a value of 48 mV in ethanol. The addition of CNTs into the Mo slurry improved the dispersion of the slurry owing to the repulsive forces between the two species.      Pores were observed, while unmelted metallic particles were not found in the build, as indicated by Figure 5a. Moreover, two morphologies of black-contrasted precipitates with banded or spherical shapes were observed. Further analyses were performed using the STEM. The precipitates mainly consisted of O and Mo from the STEM-EDS mappings (Figure 5b,c). Figure 5d shows the microstructure of the precipitate within the cross section. The selected-area electron diffraction (SAED) patterns in the inset suggest that the structure of the Mo matrix is the body centered cubic phase, and the banded precipitate is MoO 2 with a monoclinic structure. This is consistent with the XRD results ( Figure 4). As shown in Figure 5e, the interface between the banded precipitates and the matrix was clearly observed without microcracks. An interfacial orientation relationship of Mo [11] //MoO 2 [1] or Mo 011 //MoO 2 110 was detected, indicating the presence of a lowenergy Mo-MoO 2 interface.  Figure 6 shows the microstructure of the build using H 2 -reduced Mo powder by L-PBF. Columnar grains were also observed. Not only the quantity, but also the size of precipitates decreased remarkably in comparison with the microstructure of the pure Mo build in Figure 5, indicating a decreased content of the MoO 2 phase.  Figure 7 presents the microstructures of the build using the CNT/Mo particles processed by FD-POEM. In contrast to the pure Mo build, pores were also observed in the FESEM image, while the distributed precipitates decreased significantly. There were many segregations among the network shapes generated in the cross-section. The highmagnification image in Figure 7b reveals that the segregation consists of carbides and oxides in the build. They were demonstrated by EDS analysis, as shown in Figure 7c,d, corresponding to blue and orange crosses, respectively. The result agrees well with those of the XRD in Figure 4, which implied that the diffraction peaks of MoO 2 decreased because of its small quantity and Mo 2 C with hexagonal phase appeared. A cell-like microstructure of L-PBF build using H 2 -reduced 1.5 wt.% CNT/Mo powders is shown in Figure 8. Equiaxed grains were also observed. The grain diameter was in the range of 2 µm. The grains are surrounded by a network of segregated phases, as shown in Figure 8a. Further TEM and diffraction patterns confirmed that the grain and segregated phases were Mo and Mo 2 C, respectively. Figure 8c shows the interface between the Mo matrix and the carbides. No microcracks were observed at the interface. The in situ Mo 2 C could serve as the nucleation sites for the formation of uniform equiaxed grains and cause grain refinement during L-PBF, subsequently impeding crack initiation and propagation along the grain boundaries [31,33].
Furthermore, no oxides were identified in the build. The method in which powders were treated by a combination of CNT addition and H 2 reduction proved to be the most effective in removing oxides of the build. The oxygen content in the particles processed by FD-POEM was partially reversed by H 2 . Subsequently, residual oxygen was consumed by the in situ reaction with the CNTs during L-PBF. The residual CNTs contributed to the formation of Mo 2 C in the build. The Vickers hardness values of these builds are shown in Figure 9. The Vickers hardness of the Mo build was found to be 170 ± 2.6 HV. The hardness of the H 2 -reduced Mo or CNT/Mo build increased slightly (~215 HV). Notably, the build using H 2 -reduced CNT/Mo particles exhibited the highest Vickers hardness (417 ± 26 HV), which is larger than that reported in reference [31], possibly because of two main reasons: (i) the strengthening effect is induced by grain refinement. The mean grain size of H 2 -reduced 1.5 wt.% CNT/Mo build was about 2 µm, smaller than that of pure Mo build (~4 µm). (ii) The formation of strong Mo 2 C was verified along the grain boundary by TEM investigations, leading to an increased hardness of the Mo build. This result indicates that CNTs played the dual role of removing oxygen from the powders and strengthening the Mo matrix during the L-PBF process.

Conclusions
A strategy of acid-treated CNTs as a carbon source was proposed to address the removal of oxides in L-PBF builds from FD-POEM particles. Spherical pure Mo particles and 1.5 wt.% CNT/Mo composite particles were fabricated via FD-POEM. The oxygen was partially removed, and the Mo 2 C phase was formed in the build by means of the addition of CNTs. Combining the addition of CNTs with the pretreatment of H 2 reduction, the STEM observation revealed that the oxides were completely removed and the network shape of Mo 2 C was distributed around the matrix, resulting in the highest Vickers hardness of the build. This research demonstrated that CNTs can be exploited as an effective reduction agent to remove oxygen from FD-POEM powders and can be used as reinforcement of refractory material by L-PBF.