Preparation of Spherical Ta10W Alloy Powder and Its Selective Laser Melting (SLM) Printability Study
Release time:
2026-02-09
Ta10W alloy is an ultra-high-temperature structural material with high density, high melting point and high strength. Its tensile strength still exceeds 120 MPa at 2000 ℃, and it exhibits good ductility and weldability, making it suitable for service environments with high temperature and high pressure [1-2]. Components manufactured from this alloy have been critically applied in the aerospace field under high-temperature and high-pressure conditions, such as combustion chambers of spacecraft, nose cones of missile engines, nozzles, exhaust pipes and other key components [3-4]. However, these components are characterized by small size and complex geometry, and technical problems including difficult machining, long forming time and extremely high cost still exist in practical production [5].
Selective Laser Melting (SLM) is a metal 3D printing technology developed on the basis of the original Selective Laser Sintering (SLS) technique. It features high dimensional accuracy, high material utilization rate and low surface roughness, and has overwhelming advantages in forming complex geometric components. It has been widely used in the fabrication of key components in aviation, aerospace and medical device fields [6-7]. SLM can realize internal complex structures (e.g., internal cooling channels, porous structures) that are unachievable by conventional processing methods, which is of great significance for improving the performance and lightweight design of aerospace components [8]. Nevertheless, SLM imposes strict requirements on powder composition, particle size and morphology; powders for SLM must possess high sphericity and excellent flowability [9].
Due to the ultra-high melting point of refractory metals, traditional atomization and rotating electrode methods are difficult to produce high-quality spherical powders. In contrast, radio-frequency (RF) plasma processing technology uses plasma as a heat source, with advantages including ultra-high temperature (≈10000 K), fast heating/cooling rates, high sphericity, good flowability and high purity of the prepared powders, which is applicable for spherical powder processing of high-melting-point materials [10-11]. At present, this technology has become the primary method for preparing spherical powders of high-melting-point metals and ceramics, including metals such as W, Mo, Ta, Nb, Ti and ceramics such as Al₂O₃ and SiO₂ [12-15].
At present, relatively few studies focus on spherical Ta and its alloy powders suitable for SLM forming. Qin et al. [16] treated irregular Ta powder via RF plasma spheroidization, achieving a powder spheroidization rate of over 90%. Mao Xinhua et al. [17] used irregular Ta powder produced by sodium reduction as raw material, investigated the effects of RF plasma process parameters on the characteristics of spherical powder, and studied the microstructure of SLM-formed parts. The formability of 3D printing is highly sensitive to impurity elements; in particular, excessive oxygen (O) content easily induces cracking during printing. Ta is extremely prone to oxygen absorption, so impurity content must be strictly controlled during powder preparation [18]. To reduce the impurity content of spherical powder, it is necessary to adopt high-purity powder as the raw material for plasma spheroidization.
In this work, pure Ta ingot prepared by electron beam melting (EBM) was used as raw material and treated via plasma spheroidization, successfully obtaining spherical Ta powder applicable for SLM forming. However, there are no reports on Ta10W alloy powder suitable for SLM up to now. Inspired by previous work, EBM-fabricated Ta10W alloy ingot was subjected to hydrogenation, crushing and sieving to obtain irregular non-spherical powder, which was further processed by plasma treatment, followed by dehydrogenation and deoxidation. High-quality spherical powder was successfully prepared, and its performance was evaluated via SLM; the mechanical properties of 3D-printed bulk Ta10W alloy were tested. The powder preparation strategy in this work is expected to be extended to the entire Ta alloy system, and can provide a reference for solving the cracking issue of 3D-printed complex geometric components.
1 Experimental Materials and Methods
1.1 Experimental Materials
Spherical powder was prepared using EBM-fabricated ingot as raw material. The ingot was produced from high-purity Ta and W metal powders (purity > 99.95 wt%) mixed at a mass ratio of 10:1. The mixture was compacted into a green billet of 25 mm×25 mm×400 mm via isostatic pressing, and then subjected to three rounds of EBM to ensure chemical homogeneity, yielding a Ta10W alloy ingot of φ150 mm × 400 mm.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was employed to measure the chemical composition of the ingot. Samples were taken from three positions (top, middle and bottom) of the Ta10W alloy ingot for composition analysis. The results show that the alloy ingot has uniform composition with negligible gravity segregation; main impurities include O, N, Fe and Nb, all of which meet the specifications of ASTM B708 standard, as listed in Table 1.
1.2 Preparation and Characterization of Spherical Ta10W Alloy Powder
The process flow for preparing spherical Ta10W alloy powder is shown in Fig. 1. Firstly, the Ta10W alloy ingot was hydrogenated: Ta absorbs hydrogen to form tantalum hydride at a certain temperature, resulting in embrittlement. The chemical reaction equation [19] is:
where x = 0.2~0.8 (i.e., H/Ta = 0.2~0.8). Studies have shown that the solubility of hydrogen in Ta depends on hydrogenation temperature and hydrogen pressure; the solubility is relatively high at low temperatures and decreases with increasing temperature. The reaction rate between Ta and hydrogen is proportional to the square root of hydrogen pressure [17]. The hydrogenation process in this experiment was set as 900 ℃, holding time 800 min, hydrogen pressure 200 Pa.
Subsequently, the hydrogenated alloy ingot was crushed to obtain irregular particles, which were sieved to a particle size range of 15~65 μm. Further fluidized bed treatment was conducted for dehydrogenation and deoxidation, followed by acid pickling to obtain pre-spheroidized particles.
A TEKNA-40 kW (SY166) RF plasma powder preparation system was used to spheroidize the pre-spheroidized Ta10W alloy powder. Argon (inert gas with low ionization energy) was adopted as the central gas to stabilize the plasma arc. To improve the thermal conductivity of plasma, a mixture of argon and helium (flow ratio 5:3) was used as sheath gas; detailed spheroidization parameters are listed in Table 2.
ICP-MS was used to analyze the chemical composition before and after spheroidization (referring to GB/T 15076—2008), and a LECO oxygen-nitrogen-hydrogen analyzer was employed to measure the contents of H, O and N in the powder. A Malvern particle size analyzer was used to characterize the particle size distribution (referring to GB/T 19077—2016). The flowability of spherical powder was tested by a Hall flowmeter (following YS/T 1297—2019), and the apparent density was measured by a JL-A3 powder property tester. The morphology of spherical powder was observed via a JSF-7500F Scanning Electron Microscope (SEM).
1.3 Evaluation of Printability of Spherical Ta10W Alloy Powder
To evaluate the printability of the alloy powder, a Farsoon FS271M 3D printing system was used to fabricate bulk Ta10W alloy specimens (10 mm×10 mm×10 mm) and cylindrical rods (φ10 mm×60 mm). The optimal printing power and scanning speed range were determined experimentally, with printing parameters listed in Table 3.
The Archimedes drainage method was used to measure the density of bulk specimens under different printing parameters, and a metallographic microscope was adopted to observe the specimen morphology and evaluate internal defects of printed parts. Cylindrical rods were machined into tensile specimens with a gauge diameter of 5 mm and gauge length of 25 mm. Uniaxial tensile properties were tested using an Instron 4505 mechanical testing machine (referring to GB/T 228.1—2021). Finally, SLM forming of complex geometric components was carried out to further verify the printability of the spherical powder.
2 Results and Discussion
2.1 Physical Properties of Spherical Ta10W Alloy Powder
Fig. 2 compares the SEM images of Ta10W alloy powder before and after plasma spheroidization. As shown in Fig. 2(a), the pre-spheroidized particles exhibit irregular morphology. Malvern particle size analysis shows that D₅₀ and D₉₀ are 25.5 μm and 37.7 μm, respectively. Fig. 2(b) presents the SEM morphology after plasma spheroidization: the Ta10W alloy particles are perfectly spherical with clean surfaces, no satellite powders or agglomerates are observed, and the particle size distribution is relatively uniform.
Previous studies [4] have shown that plasma spheroidization parameters significantly affect the quality of spherical powder. This work focused on the effects of reaction chamber pressure and powder feeding rate on spheroidization rate. Fig. 3(a) illustrates the influence of powder feeding rate on the spheroidization rate of Ta10W alloy powder at a constant reaction chamber pressure. The spheroidization rate gradually decreases with the increase of powder feeding rate; when the feeding rate is lower than 2.7 kg/h, the spheroidization rate is close to 100%.
In general, the energy required for complete spheroidization of a single particle can be expressed by Eq. (2) [19-20]:(Note: The original text omitted the formula body; the formula description is retained as in the source paper)
where d is particle diameter, ρ is theoretical density of the particle, Cₚ is specific heat capacity, Tₘ is melting point, T₀ is room temperature, and Hₘ is latent heat of fusion. The total heat required to melt all particles in the powder is Q (sum of heat for melting the i-th particle, n = total number of particles). Complete powder spheroidization is achieved when the effective plasma heat Qₑ ≥ Q.
Under constant plasma energy, Ta10W alloy has a higher melting point and latent heat of fusion than pure Ta powder, requiring more heat for melting. Similarly, a higher powder feeding rate increases the total heat demand for melting. Therefore, with constant plasma energy, the spheroidization rate decreases as the powder feeding rate increases (Fig. 3(a)).
Samples of more than 1 kg were randomly taken from three groups of powder with spheroidization rate close to 100%, mixed and re-sampled for particle size testing. The particle size distribution (Fig. 3(b)) shows that spherical Ta10W alloy powder is mainly distributed in 10~100 μm, following a normal distribution. D₁₀, D₅₀ and D₉₀ are 16.7 μm, 27.3 μm and 53.8 μm, respectively. SEM observation of the cross-sectional morphology of spherical alloy particles reveals almost no hollow spheres (Fig. 4).
The apparent density and flowability of the spherical powder were tested to verify its adaptability for SLM. The results show an apparent density of 10.3 g/cm³ and a Hall flow rate of 4 s/(50 g), both superior to those of spherical Ta powder reported in the literature [21]. 3D printing practice has proven that printability is closely related to chemical composition; oxygen content is extremely detrimental to printability and mechanical properties, and Ta has a strong affinity for oxygen [22-23].
The chemical compositions of powder before and after spheroidization are compared in Table 4. Compared with pre-spheroidized powder, the content of low-melting-point Mg in spherical alloy powder decreases significantly, W content remains nearly unchanged, O content surges to 950×10⁻⁶, while N and H increase slightly (45×10⁻⁶ and 15×10⁻⁶, respectively). The high temperature during plasma spheroidization may induce reactions between powder surfaces and ambient oxygen; elevated oxygen content can form oxide inclusions inside the alloy, which act as stress concentration points and reduce toughness and fatigue properties.
To improve the SLM printability, spherical powder was further treated by fluidized bed. Finally, the O content of spherical alloy powder was reduced to 180×10⁻⁶, with N and H contents of 15×10⁻⁶ and 13×10⁻⁶, respectively. Compared with spherical pure Ta powder and other Ta alloy powders reported in relevant literature, the comprehensive physical properties of the as-prepared Ta10W spherical powder in this experiment are more excellent, indicating superior SLM formability [16].
2.2 Printability of Spherical Ta10W Alloy Powder
SLM process parameters for Ta10W alloy were optimized experimentally. The results show that the highest density of printed specimens is achieved at a laser power of 400 W and scanning speed of 600 mm/s, with an average density of 16.71 g/cm³, reaching the theoretical density of Ta10W alloy (16.7~16.9 g/cm³). This confirms that dense Ta10W alloy parts can be fabricated via SLM with the as-prepared spherical powder. Excessively high scanning speed (>600 mm/s) leads to a significant increase in pores and cracks in specimens.
Fig. 5(a) shows SLM-printed bulk and cylindrical specimens with smooth surfaces, demonstrating excellent printability of the powder. Fig. 5(b) presents SLM-printed complex geometric parts, which are free of cracking and meet the dimensional accuracy requirements. Thus, the spherical Ta10W alloy powder prepared via EBM ingot-hydrogenation-crushing-plasma spheroidization is suitable for 3D printing of products with complex geometries.
As-printed specimens possess high residual stress, so annealing is required to optimize the comprehensive mechanical properties. Fig. 6 shows the metallographic microstructures of Ta10W alloy specimens fabricated by conventional and SLM processes after annealing at different temperatures for 1 h.
With increasing annealing temperature, the microstructure of conventionally processed Ta10W alloy changes significantly: after annealing at 1550 ℃/1 h, the grain morphology transforms to fully recrystallized equiaxed grains, and grain size increases markedly after annealing at 1650 ℃/1 h. Affected by the molten pool, grain boundaries of SLM-fabricated Ta10W alloy are difficult to distinguish in metallographic images. However, grain morphology confirms no obvious change when annealing temperature increases from 1450 ℃ to 1750 ℃, indicating that SLM endows Ta10W alloy with better thermal stability. Its complete recrystallization temperature may exceed 1800 ℃, implying superior high-temperature mechanical properties.
After annealing below the recrystallization temperature, the mechanical properties (especially ductility) of SLM-fabricated Ta10W alloy are significantly improved. Impurity O segregates at grain boundaries and easily forms TaₓOᵧ particles, increasing strength but reducing ductility. Annealing not only eliminates residual stress to improve ductility, but also drives O diffusion from grain boundaries to grain interiors, purifying grain boundaries and enhancing grain boundary bonding strength, thus moderately improving alloy ductility.
Excessively high annealing temperature triggers recrystallization and grain growth, which impairs ductility. Therefore, the comprehensive mechanical properties of Ta10W alloy are optimal after annealing at 1450 ℃ for 1 h, as listed in Table 5.
Fig. 7 shows the uniaxial tensile curves of SLM-formed specimens and conventionally forged Ta10W alloy. The strength and plasticity of 3D-printed specimens are both superior to those of conventionally processed counterparts. The SLM-printed alloy exhibits a maximum yield strength of 600 MPa and ultimate tensile strength of 740 MPa (Table 5), while the conventionally processed specimen has a yield strength of 518 MPa and ultimate tensile strength of 630 MPa (Fig. 7). In addition, the elongation of SLM-formed alloy exceeds 30%, slightly higher than that of conventional processing.
The grain sizes of the two processes are similar. The enhanced strength of the as-SLM-fabricated alloy is attributed to a large number of nano-scale dislocation cells inside grains, which effectively hinder dislocation motion and achieve grain strengthening. Moreover, the higher O concentration in SLM specimens contributes to solid-solution strengthening. According to fracture mechanics theory [24], dislocation cells not only impede dislocation movement but also alleviate stress concentration at grain boundaries, which is beneficial to plasticity. Nevertheless, the microstructure and corresponding deformation mechanism of SLM-fabricated Ta10W alloy require further in-depth investigation.
3 Conclusions
Spherical powder was prepared from fully EBM-fabricated Ta10W alloy ingot via hydrogenation-crushing-plasma spheroidization, and SLM forming tests were conducted. The main conclusions are as follows:
(1) The as-prepared spherical Ta10W alloy powder exhibits excellent physical properties: spheroidization rate close to 100%, D₉₀ = 53.8 μm, almost no hollow particles, apparent density of 10.3 g/cm³ and Hall flow rate of 4 s/(50 g). The novel process can strictly control the contents of impurity elements (N, H, O).
(2) The spherical powder prepared by the novel process has outstanding printability. Under optimized printing parameters, printed specimens reach the theoretical density of the alloy with uniform microstructure, no obvious printing defects or cracks. After annealing at 1450 ℃ for 1 h, the alloy exhibits a yield strength of 600 MPa, ultimate tensile strength of 740 MPa and elongation over 30%, with comprehensive properties superior to those of conventional pressure processing.
(3) Complex component printing tests were carried out using the as-prepared Ta10W spherical powder. The results show that complex components are free of cracking and meet the dimensional accuracy requirements.
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