2023 Volume 64 Issue 6 Pages 1119-1124
Freeze-dry pulsated orifice ejection method (FD-POEM) is a promising approach of fabricating spherical refractory alloy or composite powders for laser powder bed fusion (L-PBF). However, due to the weak particle strength induced by their intrinsic pore characters, the FD-POEM powders were likely crushed during the powder recoating process. Taking MoSiBTiC alloys as an example, in this work, plasma spheroidization (PS) treatment was utilized to strengthen the FD-POEM powders. The MoSiBTiC powders were completely melted and rapidly solidified during PS, leading to the evolution of mesh-pore to a fully dense structure. Correspondingly, the PS powders displayed an increased sphericity of 0.98, while showing a significantly decreased particle size of 53.1 µm. Moreover, the laser absorptivity of PS powder was reduced because of the decreased multiple reflections via the smooth powder surface. Microstructure evaluations illustrated that the PS MoSiBTiC powders mainly consisted of the elemental (Ti, B, and C)-supersaturated Mo phase, as well as small amounts of Mo5SiB2, TiC, and Mo2B, indicating the occurrence of in-situ alloying during PS. Furthermore, the internal and inter-particle microstructures of PS powders were highly homogeneous, attributing to the uniformly dispersed elemental powders of FD-POEM powders. The results of this work suggest that the combination of FD-POEM and PS is an effective approach of fabricating refractory powders for L-PBF.
Refractory intermetallics possessing superior mechanical properties at elevated temperatures are highly demanded in many industrial fields.1,2) The MoSiBTiC alloys are deemed as one of promising refractory materials in the application of aerospace industries, due to the high melting point and excellent mechanical properties at high-temperatures.3,4) However, traditional methods such as casting, or powder metallurgy have limitations in producing refractory components with complex structures for practical uses. In this regard, additive manufacturing (AM) is considered as a promising alternative.5,6) Laser powder bed fusion (L-PBF) is one of AM techniques.7,8) The L-PBF process uses a laser beam for selectively fusing powders to build up 3-dimensional objects in a layer-by-layer manner.9) Comparing with conventional manufacturing methods, L-PBF has many benefits, involving the direct fabrication of complex structural parts, reduced time-to-market, as well as enhanced processing flexibility.10–13) L-PBF has offered a novel approach of processing intermetallics with complex structures.14,15)
As is known that the quality of L-PBF parts is largely influenced by building parameters and powder characteristics. The effects of building parameters on the porosity, microstructures, and mechanical properties of L-PBF parts have been extensively studied.7–10) However, there lacks sufficient attentions on powder properties. The powder properties can influence the powder bed quality, the melt pool condition and the quality of the final build.5,16–19) The fabrication of suitable L-PBF powders with spherical shape, suitable particle size and sufficient laser absorptivity is thus highly required.16,20,21) The powders applied for L-PBF were primarily produced using gas atomization,22–25) water atomization,22,26) and plasma rotating electrode process.27,28) However, due to the high melting points of refractory intermetallics, their powders are difficult to be made by these traditional methods. Recently, several new techniques have been developed to fabricate MoSiBTiC powders. For instance, Zhou et al.5,29) fabricated the MoSiBTiC powders by high-energy ball milling and subsequent sieving towards to L-PBF. However, certain internal defects were included in L-PBF parts due to the included ZrO2 contamination and poor flowability of ball-milled particles. Through the plasma spheroidization (PS) processing of cast-crashed powders, Higashi et al.30,31) prepared spherical MoSiBTiC powders without any satellites. However, the microstructure varied from particle to particle because of the chemical composition inhomogeneity of cast-crashed powders.
In our group, a novel method named freeze-dry pulsated orifice ejection method (FD-POEM) has been recently put forward to fabricate composite or refractory metallic powders without fusion processes.21,32,33) The FD-POEM particles had a uniform elemental distribution, narrow particle size distribution, and high laser absorptivity. However, due to the mesh-pore structure and weak strength, the FD-POEM powders were fractured easily during the powder recoating process. In this work, therefore the post-treatment of PS technique was utilized to strengthen the FD-POEM powders by considering its ultra-high temperature and rapid heating/cooling rate. The effects of PS treatment on the powder morphology, size distribution, chemical composition or laser absorptivity were investigated. The microstructure evolution of FD-POEM MoSiBTiC powders during PS was also illustrated.
As reported elsewhere, the FD-POEM method was applied for the fabrication of MoSiBTiC powders.31) Mo, Si, MoB, and TiC powders with d50 of 1.0, 0.57, 4.3, and 0.67 µm, respectively, were used as starting powders. Their quantities were determined using the nominal molar composition of 65Mo–5Si–10B–10Ti–10C (mol%). The starting elemental powders were put into deionized water with a concentration of 10 vol%, followed by the sufficiently ultrasonic dispersion at 273 K. The slurry mixture was then ejected from the pulsating orifice which was froze instantly after falling into the liquid N2. Eventually, the spherical MoSiBTiC particles were received through a complete freeze-drying process.
2.2 PS treatment of FD-POEM powdersThe PS treatment was performed using mixed Ar/H2 (8:1 in volume) as carrier gas. The flow rate and plasma input power were 45 L/min and 35 kW, respectively. To reduce the risk of oxidation, the final PS FD-POEM powder was ultrasonically cleaned in xylene before using.
2.3 CharacterizationsThe powder morphology was observed using an optical microscope with an image manipulation software (WinROOF, MITANI Ltd., Japan). Scanning electron microscopy (SEM; JSM-6010 LV, JEOL, Japan), transmission electron microscopy (TEM; HF-2000EDX, Hitachi, Japan), and electron probe micro-analyzer (EPMA; JEOL JXA-8530F, Japan) were used for microstructural analysis. Laser absorptivity of powders were determined by a UV–VIS–NIR spectrophotometer (V-670, JASCO, Japan). The phase constitution was obtained from X-ray diffraction (XRD, Rigaku D/Max-2550 PC, Japan).
Figure 1(a) displays the morphology of spherical MoSiBTiC particles produced by FD-POEM. The powder had a high sphericity of ∼0.94. As displayed in Fig. 1(b), the powder size distribution of FD-POEM powders was within a narrow range, meeting atypical Gaussian fitting. The particle dimensions d10, d50, and d90 values were measured as 82.2, 115.7, and 154.9 µm, respectively.
(a) SEM image and (b) particle size distribution of the MoSiBTiC powders produced by FD-POEM.
As illustrated by the high-magnification surface and cross-sectional SEM image in Fig. 2 and Fig. 3, the FD-POEM powder presents a mesh-pore structure. The mesh-pore feature was further verified by X-ray CT image as shown in Fig. S1. The porosity was determined to 90.51% according to the spherical volume of the FD-POEM particle and the actual volume of its constituent raw powders.32) Such mesh-pore structure was induced by the generation and sublimation of ice crystals during the FD-POEM process.32) The porous powders were easily fractured during the morphology observations, owing to their intrinsically weak strength. In this case, tiny particle debris (white arrows) were observed in Fig. 1(a). As shown in Fig. 2(b), the Mo, Si, and Ti elements were found to be uniformly distributed on the surface of FD-POEM powders. That is due to the good dispersion of starting elemental powders in the prepared slurry mixture.
(a) SEM image of a FD-POEM powder; (b) the enlarged view and corresponding EDS mappings taken from the red box in (a).
(a) Low- and (b) high-magnification cross-sectional SEM images of the FD-POEM powders.
As shown in Fig. 4(a), the PS powder displayed a higher-sphericity of 0.98 than the FD-POEM powder. The satellite particles, which are typically induced from gas atomization, were not detected. Moreover, the powder surface became smoother after PS. Few PS powders show a crumpled surface. Figure 4(b) shows the particle size distribution of PS powders, retaining the narrow Gaussian distribution. The d10, d50, and d90 were identified as 38.2, 53.1 and 76.4 µm, respectively, showing significantly decreased particle sizes after PS.
(a) SEM image and (b) particle size distribution of the PS MoSiBTiC powders.
The volume of PS powder was significantly decreased by 90.33% through the geometry calculation, indicating the occurrence of densification of individual FD-POEM particles during PS. Since the porosity of FD-POEM powders was determined by the working parameters, it is expected that the size of PS powders can be effectively controlled by altering the ejection method or slurry concentration.
As displayed by the SEM image (Fig. 5(a)) and the corresponding EDS mappings (Fig. 5(b)–(d)), the elemental Mo, Ti, and Si were uniformly distributed on the powder surface, benefiting from the uniform characters of FD-POEM powders (Fig. 2).
(a) SEM image of a PS MoSiBTiC powder, and corresponding EDS mappings of elemental (b) Mo, (c) Ti, and (d) Si.
Figure 6(a) and (b) illustrate the internal morphology of PS MoSiBTiC particles. It can be noted that the pore FD-POEM powders were fully densified during PS. When the FD-POEM powder was passing through the high-temperature (3000–10,000 K) plasma region during PS, it was instantly fused and shrunk into a spherical shape because of the surface tension effect, resulting in a dense structure during rapid cooling (∼106 K/s).34) It is expected that the powder strength will be largely enhanced by forming such dense powders, which can be uniformly deposited during the L-PBF recoating process.
(a) Low- and (b) high-magnification cross-sectional SEM images of the PS MoSiBTiC powders.
It should be noted that visible microcracks were not detected inside most of the PS powders even during the rapid, ultrahigh-temperature PS process. Only individual PS powders were found to contain pores inside. Moreover, the internal and inter-particle microstructures of PS powders were highly homogeneous, which was different from that in the powders produced by mechanical crashing and PS.30,31) This unique phenomenon was attributed to more uniform compositions of the FD-POEM powders compared to the crashed powder.
3.3 Laser absorptivity of PS MoSiBTiC powdersFigure 7(a) compares the absorptivity of the FD-POEM and PS MoSiBTiC powders at a wide range of wavelengths. The laser absorptivity of the PS powder was ∼72.2% at λ = 1070 nm. It was smaller than that of the FD-POEM particle (∼87.1%). This was caused by the changed surface states. As schematically illustrated in Fig. 7(b), the multiple reflections of laser occurred at the mesh structure of FD-POEM powders, leading to higher laser absorptivity. In contrast, the laser rays were easily reflected and escaped from the smooth surface of the PS powders. Importantly, the laser absorptivity of 72.2% was sufficient for additively manufactured metallic parts by L-PBF.
(a) Laser absorptivity of the FD-POEM and PS MoSiBTiC powders; (b) the schematic of different laser absorptivity between FD-POEM and PS powders.
Figure 8 shows the XRD pattern of the FD-POEM and PS MoSiBTiC powders. The PS powder was dominantly composed of Mo (JCPDS#42-1120), small amount of Mo5SiB2 (T2) (JCPDS#65-8686), Mo2B (JCPDS#25-0561), and TiC (JCPDS#65-5408). The XRD peaks of starting elemental MoB and Si particles in PS powders were not observed. This result reveals that the in-situ alloying process occurred, since the extremely high temperature plasma was applied during PS.
XRD pattern of the FD-POEM and PS MoSiBTiC powder.
Figure 9(a) shows the high-magnification SEM-backscattered electron (BSE) image of a PS MoSiBTiC powder. The powder mainly consisted of white coarse region (red arrow), dark nanoparticle (yellow arrow) and fine eutectic structure (green arrow) in Fig. 9(a), which were elemental Mo-, Ti- and Si-rich, respectively, as confirmed by the EPMA analysis in Fig. 9(b). Further microstructural analysis was carried out using TEM observations in Fig. 10. According to the selected area electron diffraction (SAED) pattern, the Si-, Mo-, and Ti-rich phases were corresponding to the Mo, T2, and TiC, respectively.
(a) High-magnification BSE image and (b) corresponding elemental EPMA mapping of the cross-section of the PS MoSiBTiC powder.
TEM image of the PS MoSiBTiC powders. Inserts show the SAED patterns of Mo, T2 and TiC phases taken from the red, green, and yellow spots, respectively.
It is noted that the contents of T2 and TiC phases were much lower than that in the cast MoSiBTiC alloy. This was due to the formation of non-equilibrium, (Ti, B, C)-supersaturated solid-solution structures during the ultrafast solidification of PS.30,31) In addition, the size of TiC phase was measured as ∼100 nm, which was smaller than that of the starting TiC powders (∼670 nm). The results indicates that refractory TiC particles suffers a dissolution/precipitation process, resulting in the formation of nano-TiC dispersions in the PS powder.29,33)
The effects of PS on the powder characteristics and microstructures of MoSiBTiC powders produced by FD-POEM were investigated. The following is a summary of the main conclusions.
It is concluded that the combination of FD-POEM and PS is an effective approach of fabricating MoSiBTiC powders.
The present study was partially supported by JST-MIRAI Program with Grant Number of JPMJMI17E7 and the MEXT Program: Data Creation and Utilization Type Material Research and Development Project Grant Number JPMXP1122684766. The authors would like to thank Dr. Kosei Kobayashi and Dr. Takamichi Miyazaki at Tohoku University for providing technical assistance during the EPMA and TEM analyses.
