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A brief introduction to the selective laser melting of Ti6Al4V powders by supreme-speed plasma rotating electrode process
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx HOSTED BY
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A brief introduction to the selective laser melting of Ti6Al4V powders by supreme-speed plasma rotating electrode process Yang Liua, Xiao-Hao Zhaoa, Yun-Jin Laia, Qing-Xiang Wanga, Li-Ming Leib, Shu-Jin Lianga,∗ a b
Sino-Euro Materials Technologies of Xi'an Co., Ltd., Xi'an, 710018, China AECC Commercial Aircraft Engine Co., Ltd., Shanghai, 201108, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Particle size distribution Atomization Ti6Al4V Plasma rotating electrode process Selective laser melting Rapid solidification
In the present study, Ti6Al4V spherical powders were prepared by supreme-speed plasma rotating electrode process and the particle size fit log-normal distribution. The average diameter of the powders was successfully determined by a model developed in this work, suggesting that the particle size distribution is mainly affected by the rotating speed. The log-normal distribution factor of the particle size distribution maintains stable as the rotating speed ω varies. The particle size distribution indicates that the main atomization mode of Ti6Al4V under supreme-speed plasma rotating electrode process is of the characteristics of direct drop formation. The mechanical properties of the samples prepared by selective laser melting of Ti6Al4V powders were characterized, indicating that such Ti6Al4V samples with isotropy structure exhibit high yield strength and good ductility.
1. Introduction Additive manufacturing methods are of essential importance to the production of advanced titanium alloy components with relatively low buy-to-fly ratio and near-net-shape [1]. The titanium alloy 3D printing routes such as selective laser melting process and electron beam melting process need a giant amount of spherical powders with good flowability and suitable particle size ranges [2]. Plasma rotating electrode process (PREP) and gas atomization are two usual technologies to prepare highquality titanium spherical powders. Such powders could be employed to produce the titanium alloys meeting the requirements of aerospace and biomedical applications due to the high fracture resistance, corrosion resistance and biocompatibility of the alloys [2–4]. It is reported that PREP is a cost-effective way to prepare powders with desirable particle sizes ranging from 50 to 350 μm. This is because the size distributions of powders by PREP are narrow and easy to modify [5,6]. Previous reports have indicated that the particle size distribution of PREP powders is primarily dependent on the parameters of rotating speed and heat parameters [7,8]. Compared with the powders by gas atomization which could bring impurities, PREP powders contain fewer impurities [9]. The ductility of titanium alloys is very sensitive to impurities such as oxygen. Previous work has pointed out that even a 0.71 wt% oxygen content could cause a sharp decline in the ductility of Ti6Al4V alloy [10,11]. However, by comparison with gas atomization, the current PREP
∗
might be hard to prepare fine powders (< 75 μm) with enough yield. The major limitation of PREP is that available rotating speed applied is not suitable to form the particles with small size [12], suggesting that increasing rotating speed is necessary for the upgrade of PREP. SinoEuro Materials (SMT, Xi'an, China) has developed supreme-speed plasma rotating electrode process (SS-PREP) by applying high-speed bearing systems with air cooling. The novel system could raise the rotating speed into the range of 28000–36000 rpm, which could promote the yield of fine powders (< 75 μm). In this paper, the particle size distribution of the Ti6Al4V powders by SS-PREP was characterized in association with a newly developed log-normal fitting [13]. A basic model to predict the particle size distribution of SS-PREP Ti6Al4V powders was established. The mechanical behaviors of the additive manufacturing components prepared by gas atomization powders and SS-PREP powders were comparatively investigated. 2. Experiments and principles 2.1. The atomization mechanism for PREP titanium alloy powders Earlier research found that particle size distribution could be determined by the atomization model consisting of direct drop formation, ligament disintegration and film distribution [14]. The spray model can be approximately judged by Hine-Milborn number [15].
Corresponding author. #45 Fengcheng 2nd Road, Economic and Technological Development Zone, Xi'an City, Shaanxi Province, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (S.-J. Liang).
https://doi.org/10.1016/j.pnsc.2019.12.004 Received 25 January 2019; Received in revised form 19 December 2019; Accepted 26 December 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Yang Liu, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.12.004
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
Y. Liu, et al.
Hi =
μ0.17 Qρ0.71ω0.60 γ 0.88D 0.68
2.3. The preparations of the components by selective laser melting and properties analysis
(1)
In eq. (1), μ is the viscosity of the molten liquid and γ is the surface tension, ρ stands for the density of the liquid metal, Q is the volume melting rate, ω is the rotating speed and D is the diameter of the electrode. If Hi < 0.07, the main atomization mechanism for centrifugal atomization is direct drop formation. If Hi is 0.07 < Hi < 1.33, the main atomization mechanism is ligament disintegration, and the atomization mechanism is FD if Hi > 1.33. An empirical function based on Hi number has been summarized in previous work [16,17]. For titanium alloy, μ is 3.01 mPa s, ρ is 4.13 g/cm3, γ is 1.65 N/m and Q is usually lower than 1.2 × 10−6 m3/s. Thus, the Hi number of PREP titanium alloy powders is lower than 0.07 with the rotating speeds below 30000 rpm, and the diameter of the rotating bars smaller than 80 mm. This suggests that the most possible spray model for usual PREP titanium alloy is direct drop formation [18].
The powders of 15–45 μm were selected for selective laser melting process (by EOS M290) with the max power of 400 W, laser spot diameter of 100 μm and scanning speed of 15 mm/s the selective laser melting parts were annealed under 850 °C for 2 h in order to reduce thermal stress cracking [19] and improve the mechanical properties of Ti6Al4V alloy [20,21]. 3. Results and discussion Previous researches have pointed out that the main atomization mechanism of titanium alloys by plasma rotating electrode process is direct drop formation [18]. The physical model of direct drop formation is that the centrifugal force on the boundary of the rotating molten pool is almost equal to the surface tension of a newly formed metal droplet, which can be described by eq. (2) and eq. (3):
π 3 D 2 d ρ ω = πdγ 6 2
2.2. The preparations of powders and analysis The as-received Ti6Al4V rotating electrodes were machined to diameter 55 mm and length < 700 mm to satisfy the requirements of SS-PREP. The atomization chamber was filled with Ar gas atmosphere. The powders were manufactured under a 75 kW plasma torch with angular speeds of 9000–23000 rpm. A schematic of the steps is shown in Fig. 1. After atomization, the powders were collected under 99.999% Ar atmosphere to avoid oxidation and pollution. The as-received powders were packed in a glove box, with the vapor < 20 ppm and the oxygen < 1000 ppm. The powders were mixed before the laser particle size distribution test. The morphologies of the prepared powders were observed on a scanning electron microscope (SEM) with energy dispersive spectrometer. The particle size distributions of the powders prepared at different rotating speeds ranging from 9000 rpm to 23000 rpm were manually measured by Malvern MASTERSIZER 2000 laser particle size analyzer.
d=
1 ω
12γ ρD
(2)
(3)
here d is the mean diameter of powders, which is similar to d50 or dpeak. eq. (3) is similar to the result report by Mohanty et al. [22]. The comparison between the data by eq. (3) and the values of d10, d50, d90 by tests (laser particle analyzer) is shown in Fig. 2 and Table 1 with the same rotating speeds ranging from 9000 to 23000 rpm. As shown in Fig. 2, the d50 of the Ti6Al4V powders prepared by SSPREP with the different rotating speeds (ω) strictly match the expected values. All 9 groups of data with different ω are in accordance with the trend by eq. (3). This phenomenon indicates that it is possible to obtain the mean particle sizes of the SS-PREP Ti6Al4V powders before atomization. Fig. 3 shows the comparisons of experimental data and fitting data of the laser particle size (< 200 μm) of the powders prepared with different rotating speeds (15000–23000 rpm). The particle size distribution shown in Fig. 3 suggests that the particle size distribution
Fig. 1. The schematic of the plasma rotating electrode process. 2
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Table 2 The relationship between the shape factor σ and the rotating speed ω of the SSPREP Ti6Al4V PSD system. ω(rpm)
15000
17000
20000
21000
22000
23000
σ
0.3624
0.3601
0.3603
0.3961
0.3011
0.3339
∫0
Table 1 The laser PSDs of the SS-PREP Ti6Al4V powders produced with the different rotating speed ranging from 9000 to 23000 rpm. d10 (μm)
d50 (μm)
d90 (μm)
9000 11000 13000 15000 17000 20000 21000 22000 23000
236.798 183.453 152.902 107.086 96.489 85.255 70.984 88.806 82.632
320.165 250.443 209.98 173.094 155.321 136.787 120.504 131.076 127.269
431.018 342.825 289.309 271.304 243.018 214.785 196.216 192.148 194.487
1n f (ξ )dξ =
1n d − 1n daverage ⎞ 1 erfc ⎛− 2 σ 2 ⎝ ⎠ ⎜
⎟
(5)
here σ is the shape factor of the log-normal distribution. daverage could be regarded as the calculated value from eq. (3). The parameter σ could be numerically solved by eq. (5) from the as-calculated average diameters and the data of d10 to d90 in Table 1. The calculated σ data are shown in Table 2, in which the obvious trend of σ is not presented as the rotating speed varies. The mean value of σ in the present study is about 0.35, indicating that the width of the laser particle size distribution peak for SS-PREP Ti6Al4V powders could be regarded as a constant. As such, a simulation could be performed by eqs. (4) and (5) with the calculated σ listed in Table 2 and the daverage calculated from eq. (3). Fig. 3 shows the calculation and the experimental particle size distribution of the powders (< 200 μm) prepared by SS-PREP with different rotating speeds. As shown in Fig. 3, the particle size distribution of the powders by SS-PREP is in good agreement with the log-normal characteristic fitted by eq. (2) through eq. (5). This result indicates that the functions of the present work could be applied to accurately predict the particle size distribution before atomization. Accordingly, the following equation could be obtained:
Fig. 2. The comparison between the particle sizes by eq. (3) and the particle sizes of the powders by SS-PREP with different rotating speeds.
ω (rpm)
x
F (a, b) =
1n b − 1n daverage ⎞ 1n a − 1n daverage ⎞ 1 1 erfc ⎛− − erfc ⎛− 2 2 σ 2 σ 2 ⎝ ⎝ ⎠ ⎠ ⎜
⎟
⎜
⎟
(6) here, the fraction of a random size range (a,b) could be easily simulated with the parameter σ (about 0.35 for SS-PREP Ti6Al4V) and the calculated daverage by eq. (3). The particle size distribution of powders by PREP and electrode induction melting gas atomization has been reported in previous study [23]. Fig. 4 shows the comparison of the calculated particle size distribution with SS-PREP (speed > 30000 rpm and diameter of 55 mm) from eq. (6) and the particle size distribution by PREP and electrode induction melting gas atomization. The calculated data showed that the SS-PREP Ti6Al4V PSD portion of < 80 μm was close to 50% if under an extreme speed of > 30000 rpm. Furthermore, The SS-PREP had the narrowest distribution, and the fine powder yield (< 80 μm) could be much higher than the current PREP, being similar to EIGA. The SEM morphology of the SLM used (< 50 μm) Ti6Al4V powder
Fig. 3. The calculated and experimental data of the particle size distribution (< 200 μm) of the powders (< 200 μm) prepared by SS-PREP with different rotating speeds.
follows a log-normal distribution model. The probability density function and cumulative density function (known as a non-elementary function) of as-known log-normal distribution could be written as follows.
f (1n d ) =
(1n d − 1n daverage )2 ⎞ 1 exp ⎛⎜− ⎟ 2σ 2 σ 2π ⎝ ⎠
(4)
Fig. 4. The weight percentages comparison of the commercial PREP [23], EIGA [23], and the SS-PREP at > 30000 rpm speed. 3
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Fig. 5. The morphology and microstructure of the SS-PREP Ti6Al4V (< 50 μm); (a) SEM × 100; (b) SEM × 2000; (c) metallurgical structure SEM × 1000; (d) metallurgical structure SEM × 2000; (e) Chemical content analysis (EDS).
annealing could reduce the yield strength and improve the elongation at the same time. This relationship of the AM Ti6Al4V was reported and the varying of the performances was mostly affected by the portion of acicular α’ and lamellar (α+β) structures. These published discussions described a fitting law of the AM Ti6Al4V performance with a group of yield strength and elongation data, shown in Fig. 6 [27]. The yield strength and elongation data points above the “balance line” displayed the higher comprehensive performances but the points under the “balance line” showed poor performances. Fig. 6 concludes that the SSPREP-HT Ti6Al4V points are closer to the high-performance zone of the picture but the PA-HT [27] and GA-HT [19] points are located on the nearby zone of the “balance line”. The result supported the findings that the PREP powders supply fewer defects with better ductility [9]. The morphologies of the tensile fractures (SS-PREP-SLM-HT) and the microstructures of the as-built AM specimens of all directions were shown in Fig. 7 (a) to (c) ( × 1000) and (i) to (j) ( × 5000). The tensile fracture structures were composed of the ductile dimples with torn edges, which indicated the character of the fully developed plastic fracture. The diameters of the dimples of all the directions were about 2–7 μm. The morphologies of all the directions fracture dimples were homogeneous, which could infer that the fracture behaviors of the different direction tensile tests were almost equal, being in harmony with the similar data in Table 3. Thus, the fundamental microstructures should also resemble in all the directions to maintain the tensile performances agreed with the structures. The comparisons that the microstructures of the XY and Z directions of the × 500 (d and e) and × 2000 (g and h) scopes were shown in Fig. 7. It could be deduced that the microstructures of both the XY direction and the Z direction after SLM-HT were indistinguishable, leading to a conclusion that the
prepared by the SS-PREP was shown in Fig. 5 (a) and (b). Fig. 5 shows that the particles formed by the SS-PREP are highly regular in shapes, and the surfaces of the particles are smooth and round without any satellite particles or other irregular particles. The detailed microstructure and EDS results were shown in Fig. 5 (c)–(e). It could be concluded that the microstructures of the SS-PREP Ti6Al4V powders were the typical rapid solidification structures with full martensite and beta grain sizes about 35 μm. In this structure, the cooling speed for the phase transformation was about 105 K/s because the alpha morphology developed was lenticular [24]. The EDS data displayed in Fig. 5 (e) revealed that there was no remarkable micro segregations or pollution, the chemical content varying was also acceptable. Therefore, the experiment confirmed that the morphology, microstructure and chemical content of the SS-PREP were all qualified. The mechanical properties comparisons of the HT-SLM components prepared by the SS-PREP Ti6Al4V powders above and the other atomization routes were shown in Table 3. It was reflected that the comprehensive mechanical properties of the HT-SLM parts of SS-PREP were significantly higher than the contrasting groups of the traditional PREP powder [25], plasma atomization (PA) powders [26] and GA [19] powders, although there were small differences among the HT conditions. However, it was worth noting that the elongation of the SS-PREP sample was markedly higher than the others. The possible reason was that the powders prepared by centrifugal atomization (for example, PREP and SS-PREP) were denser and cleaner, so the original cavities or impurities which injured the ductility were fewer for the components of the SS-PREP powder [9]. The high comprehensive performances for the SLM Ti6Al4V parts required both high yield strength and high elongation. In general,
Table 3 The comparison of the heat-treated SLM properties between the SS-PREP and the current PA and GA. SLM Materials
Testing Direction
YS(MPa)
UTS (MPa)
EL (%)
RA (%)
SS-PREP-HT (< 45 μm) (850 °C 2 h)
XY plate Z-axis 45° – – XY plate Z-axis –
983.0 ± 2.6 1003.6 ± 5.1 1007 ± 3.1 1097 825 974 ± 7 937 ± 9 1000
1061 1082 1086 957 895 1065 1052 1090
15.0 ± 1.0 16.3 ± 0.3 15.8 ± 1.0 3.4 6-10 7 ± 0.5 9.6 ± 0.9 8.3
48.0 ± 2.0 44.7 ± 2.1 50.6 ± 1.5 – – – – –
PREP-LENS-HT (45–150 μm) (1050 °C 2 h) [25] ASTM F2924 PA-HT (730 °C 2 h) [26] GA-HT (800 °C 4 h) [19]
4
± 1.5 ± 1.5 ± 2.6
± 21 ± 11
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homogeneity of the SS-PREP SLM-HT component was excellent in this work. It was clear that the microstructures of the SLM Ti6Al4V should consist of α′ formed by the rapid solidification upon 1050 °C [21,28,29]. When heated, the decomposition α’→ α+β started at approximately 760 °C and finished at 1050 °C. When heat treated at 850 °C, the α’ is converted to α+β and the α fraction was reported about 73% [20,21]. After that, the samples were furnace cooled and contained the most β and finest lamellar α, contributing to a well comprehensive performance of good ductility and acceptable high tensile strength. The homogeneous structures made the mechanical properties equal in all directions. This advantage could significantly decrease the difficulty of the setting of the AM printing strategies because the mechanical performance in all the different directions was similar. 4. Conclusions The laser PSDs of the spherical Ti6Al4V powders prepared by the SS-PREP matched the log-normal law of 2 factors of σ and u. The σ for the SS-PREP Ti6Al4V was about 0.35, and u could be regarded as the average diameter, exactly calculated from the known metal surface tension, density, rotating speed, and the diameter of the cylinder.
Fig. 6. The relationship [27] of yield strength and elongation of Ti6Al4V AM parts and the comparison of the differently prepared powders (including PA [26], GA [19], PREP [25] and SS-PREP) performances.
Fig. 7. The morphologies of the tensile fractures and microstructures of all directions (SS-PREP-HT): (a) the SEM morphology of the fracture surface of the 45° direction sample ( × 1000); (b) the SEM morphology of the fracture surface of the XY direction sample ( × 1000); (c) the SEM morphology of the fracture surface of the Z direction sample ( × 1000); (d) the microstructure of the XY direction ( × 500); (e) the microstructure of the Z direction ( × 500); (f) the direction instruction; (g) the SEM microstructure of the XY direction ( × 2000); (h) the SEM microstructure of the Z direction ( × 2000); (i) the SEM morphology of the fracture surface of the 45° direction sample ( × 5000); (j) the SEM morphology of the fracture surface of the XY direction sample ( × 5000); (k) the SEM morphology of the fracture surface of the Z direction sample ( × 5000).
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The portion of a random size range (a,b) could be pre-known by the deduced formulas in this work with precise data. The SS-PREP Ti6Al4V powders explored the high performances after the SLM-HT process for the reason that both the yield strength and elongation could reach outstanding values, being much higher than the statistical level of the current AM Ti6Al4V. The microstructures of the different directions groups were similar to the SS-PREP SLM-HT samples. Thus, as a new supplement of the AM raw powder materials, using SS-PREP Ti6Al4V could easily reach the higher isotropism.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by the National Key R&D Program of China (2018YFB1106400) (China) and Key R&D Program of Shaanxi (2017KJXX-19) (China). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.12.004. References [1] P. Sun, Z.Z. Fang, Y. Xia, Y. Zhang, C. Zhou, Powder Technol. 301 (2016) 331–335, https://doi.org/10.1016/j.powtec.2016.06.022. [2] H. Gong, K. Rafi, H. Gu, T. Starr, B. Stucker, Addit. Manuf. 1 (2014) 87–98, https:// doi.org/10.1016/j.addma.2014.08.002. [3] H. Attar, M. Calin, L.C. Zhang, S. Scudino, J. Eckert, Mater. Sci. Eng. A 593 (2014) 170–177, https://doi.org/10.1016/j.msea.2013.11.038. [4] W. Xu, M. Brandt, S. Sun, J. Elambasseril, Q. Liu, K. Latham, K. Xia, M. Qian, Acta Mater. 85 (2015) 74–84, https://doi.org/10.1016/j.actamat.2014.11.028. [5] R. Kumagae, M. Yoshitake, O. Iwatsu, M. Tokizane, J. Jpn. Soc. Powder Powder
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A brief introduction to the selective laser melting of Ti6Al4V powders by supreme-speed plasma rotating electrode process Yang Liua, Xiao-Hao Zhaoa, Yun-Jin Laia, Qing-Xiang Wanga, Li-Ming Leib, Shu-Jin Lianga,∗ a b
Sino-Euro Materials Technologies of Xi'an Co., Ltd., Xi'an, 710018, China AECC Commercial Aircraft Engine Co., Ltd., Shanghai, 201108, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Particle size distribution Atomization Ti6Al4V Plasma rotating electrode process Selective laser melting Rapid solidification
In the present study, Ti6Al4V spherical powders were prepared by supreme-speed plasma rotating electrode process and the particle size fit log-normal distribution. The average diameter of the powders was successfully determined by a model developed in this work, suggesting that the particle size distribution is mainly affected by the rotating speed. The log-normal distribution factor of the particle size distribution maintains stable as the rotating speed ω varies. The particle size distribution indicates that the main atomization mode of Ti6Al4V under supreme-speed plasma rotating electrode process is of the characteristics of direct drop formation. The mechanical properties of the samples prepared by selective laser melting of Ti6Al4V powders were characterized, indicating that such Ti6Al4V samples with isotropy structure exhibit high yield strength and good ductility.
1. Introduction Additive manufacturing methods are of essential importance to the production of advanced titanium alloy components with relatively low buy-to-fly ratio and near-net-shape [1]. The titanium alloy 3D printing routes such as selective laser melting process and electron beam melting process need a giant amount of spherical powders with good flowability and suitable particle size ranges [2]. Plasma rotating electrode process (PREP) and gas atomization are two usual technologies to prepare highquality titanium spherical powders. Such powders could be employed to produce the titanium alloys meeting the requirements of aerospace and biomedical applications due to the high fracture resistance, corrosion resistance and biocompatibility of the alloys [2–4]. It is reported that PREP is a cost-effective way to prepare powders with desirable particle sizes ranging from 50 to 350 μm. This is because the size distributions of powders by PREP are narrow and easy to modify [5,6]. Previous reports have indicated that the particle size distribution of PREP powders is primarily dependent on the parameters of rotating speed and heat parameters [7,8]. Compared with the powders by gas atomization which could bring impurities, PREP powders contain fewer impurities [9]. The ductility of titanium alloys is very sensitive to impurities such as oxygen. Previous work has pointed out that even a 0.71 wt% oxygen content could cause a sharp decline in the ductility of Ti6Al4V alloy [10,11]. However, by comparison with gas atomization, the current PREP
∗
might be hard to prepare fine powders (< 75 μm) with enough yield. The major limitation of PREP is that available rotating speed applied is not suitable to form the particles with small size [12], suggesting that increasing rotating speed is necessary for the upgrade of PREP. SinoEuro Materials (SMT, Xi'an, China) has developed supreme-speed plasma rotating electrode process (SS-PREP) by applying high-speed bearing systems with air cooling. The novel system could raise the rotating speed into the range of 28000–36000 rpm, which could promote the yield of fine powders (< 75 μm). In this paper, the particle size distribution of the Ti6Al4V powders by SS-PREP was characterized in association with a newly developed log-normal fitting [13]. A basic model to predict the particle size distribution of SS-PREP Ti6Al4V powders was established. The mechanical behaviors of the additive manufacturing components prepared by gas atomization powders and SS-PREP powders were comparatively investigated. 2. Experiments and principles 2.1. The atomization mechanism for PREP titanium alloy powders Earlier research found that particle size distribution could be determined by the atomization model consisting of direct drop formation, ligament disintegration and film distribution [14]. The spray model can be approximately judged by Hine-Milborn number [15].
Corresponding author. #45 Fengcheng 2nd Road, Economic and Technological Development Zone, Xi'an City, Shaanxi Province, China. E-mail addresses: [email protected] (Y. Liu), [email protected] (S.-J. Liang).
https://doi.org/10.1016/j.pnsc.2019.12.004 Received 25 January 2019; Received in revised form 19 December 2019; Accepted 26 December 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Yang Liu, et al., Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.12.004
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Hi =
μ0.17 Qρ0.71ω0.60 γ 0.88D 0.68
2.3. The preparations of the components by selective laser melting and properties analysis
(1)
In eq. (1), μ is the viscosity of the molten liquid and γ is the surface tension, ρ stands for the density of the liquid metal, Q is the volume melting rate, ω is the rotating speed and D is the diameter of the electrode. If Hi < 0.07, the main atomization mechanism for centrifugal atomization is direct drop formation. If Hi is 0.07 < Hi < 1.33, the main atomization mechanism is ligament disintegration, and the atomization mechanism is FD if Hi > 1.33. An empirical function based on Hi number has been summarized in previous work [16,17]. For titanium alloy, μ is 3.01 mPa s, ρ is 4.13 g/cm3, γ is 1.65 N/m and Q is usually lower than 1.2 × 10−6 m3/s. Thus, the Hi number of PREP titanium alloy powders is lower than 0.07 with the rotating speeds below 30000 rpm, and the diameter of the rotating bars smaller than 80 mm. This suggests that the most possible spray model for usual PREP titanium alloy is direct drop formation [18].
The powders of 15–45 μm were selected for selective laser melting process (by EOS M290) with the max power of 400 W, laser spot diameter of 100 μm and scanning speed of 15 mm/s the selective laser melting parts were annealed under 850 °C for 2 h in order to reduce thermal stress cracking [19] and improve the mechanical properties of Ti6Al4V alloy [20,21]. 3. Results and discussion Previous researches have pointed out that the main atomization mechanism of titanium alloys by plasma rotating electrode process is direct drop formation [18]. The physical model of direct drop formation is that the centrifugal force on the boundary of the rotating molten pool is almost equal to the surface tension of a newly formed metal droplet, which can be described by eq. (2) and eq. (3):
π 3 D 2 d ρ ω = πdγ 6 2
2.2. The preparations of powders and analysis The as-received Ti6Al4V rotating electrodes were machined to diameter 55 mm and length < 700 mm to satisfy the requirements of SS-PREP. The atomization chamber was filled with Ar gas atmosphere. The powders were manufactured under a 75 kW plasma torch with angular speeds of 9000–23000 rpm. A schematic of the steps is shown in Fig. 1. After atomization, the powders were collected under 99.999% Ar atmosphere to avoid oxidation and pollution. The as-received powders were packed in a glove box, with the vapor < 20 ppm and the oxygen < 1000 ppm. The powders were mixed before the laser particle size distribution test. The morphologies of the prepared powders were observed on a scanning electron microscope (SEM) with energy dispersive spectrometer. The particle size distributions of the powders prepared at different rotating speeds ranging from 9000 rpm to 23000 rpm were manually measured by Malvern MASTERSIZER 2000 laser particle size analyzer.
d=
1 ω
12γ ρD
(2)
(3)
here d is the mean diameter of powders, which is similar to d50 or dpeak. eq. (3) is similar to the result report by Mohanty et al. [22]. The comparison between the data by eq. (3) and the values of d10, d50, d90 by tests (laser particle analyzer) is shown in Fig. 2 and Table 1 with the same rotating speeds ranging from 9000 to 23000 rpm. As shown in Fig. 2, the d50 of the Ti6Al4V powders prepared by SSPREP with the different rotating speeds (ω) strictly match the expected values. All 9 groups of data with different ω are in accordance with the trend by eq. (3). This phenomenon indicates that it is possible to obtain the mean particle sizes of the SS-PREP Ti6Al4V powders before atomization. Fig. 3 shows the comparisons of experimental data and fitting data of the laser particle size (< 200 μm) of the powders prepared with different rotating speeds (15000–23000 rpm). The particle size distribution shown in Fig. 3 suggests that the particle size distribution
Fig. 1. The schematic of the plasma rotating electrode process. 2
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Table 2 The relationship between the shape factor σ and the rotating speed ω of the SSPREP Ti6Al4V PSD system. ω(rpm)
15000
17000
20000
21000
22000
23000
σ
0.3624
0.3601
0.3603
0.3961
0.3011
0.3339
∫0
Table 1 The laser PSDs of the SS-PREP Ti6Al4V powders produced with the different rotating speed ranging from 9000 to 23000 rpm. d10 (μm)
d50 (μm)
d90 (μm)
9000 11000 13000 15000 17000 20000 21000 22000 23000
236.798 183.453 152.902 107.086 96.489 85.255 70.984 88.806 82.632
320.165 250.443 209.98 173.094 155.321 136.787 120.504 131.076 127.269
431.018 342.825 289.309 271.304 243.018 214.785 196.216 192.148 194.487
1n f (ξ )dξ =
1n d − 1n daverage ⎞ 1 erfc ⎛− 2 σ 2 ⎝ ⎠ ⎜
⎟
(5)
here σ is the shape factor of the log-normal distribution. daverage could be regarded as the calculated value from eq. (3). The parameter σ could be numerically solved by eq. (5) from the as-calculated average diameters and the data of d10 to d90 in Table 1. The calculated σ data are shown in Table 2, in which the obvious trend of σ is not presented as the rotating speed varies. The mean value of σ in the present study is about 0.35, indicating that the width of the laser particle size distribution peak for SS-PREP Ti6Al4V powders could be regarded as a constant. As such, a simulation could be performed by eqs. (4) and (5) with the calculated σ listed in Table 2 and the daverage calculated from eq. (3). Fig. 3 shows the calculation and the experimental particle size distribution of the powders (< 200 μm) prepared by SS-PREP with different rotating speeds. As shown in Fig. 3, the particle size distribution of the powders by SS-PREP is in good agreement with the log-normal characteristic fitted by eq. (2) through eq. (5). This result indicates that the functions of the present work could be applied to accurately predict the particle size distribution before atomization. Accordingly, the following equation could be obtained:
Fig. 2. The comparison between the particle sizes by eq. (3) and the particle sizes of the powders by SS-PREP with different rotating speeds.
ω (rpm)
x
F (a, b) =
1n b − 1n daverage ⎞ 1n a − 1n daverage ⎞ 1 1 erfc ⎛− − erfc ⎛− 2 2 σ 2 σ 2 ⎝ ⎝ ⎠ ⎠ ⎜
⎟
⎜
⎟
(6) here, the fraction of a random size range (a,b) could be easily simulated with the parameter σ (about 0.35 for SS-PREP Ti6Al4V) and the calculated daverage by eq. (3). The particle size distribution of powders by PREP and electrode induction melting gas atomization has been reported in previous study [23]. Fig. 4 shows the comparison of the calculated particle size distribution with SS-PREP (speed > 30000 rpm and diameter of 55 mm) from eq. (6) and the particle size distribution by PREP and electrode induction melting gas atomization. The calculated data showed that the SS-PREP Ti6Al4V PSD portion of < 80 μm was close to 50% if under an extreme speed of > 30000 rpm. Furthermore, The SS-PREP had the narrowest distribution, and the fine powder yield (< 80 μm) could be much higher than the current PREP, being similar to EIGA. The SEM morphology of the SLM used (< 50 μm) Ti6Al4V powder
Fig. 3. The calculated and experimental data of the particle size distribution (< 200 μm) of the powders (< 200 μm) prepared by SS-PREP with different rotating speeds.
follows a log-normal distribution model. The probability density function and cumulative density function (known as a non-elementary function) of as-known log-normal distribution could be written as follows.
f (1n d ) =
(1n d − 1n daverage )2 ⎞ 1 exp ⎛⎜− ⎟ 2σ 2 σ 2π ⎝ ⎠
(4)
Fig. 4. The weight percentages comparison of the commercial PREP [23], EIGA [23], and the SS-PREP at > 30000 rpm speed. 3
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Fig. 5. The morphology and microstructure of the SS-PREP Ti6Al4V (< 50 μm); (a) SEM × 100; (b) SEM × 2000; (c) metallurgical structure SEM × 1000; (d) metallurgical structure SEM × 2000; (e) Chemical content analysis (EDS).
annealing could reduce the yield strength and improve the elongation at the same time. This relationship of the AM Ti6Al4V was reported and the varying of the performances was mostly affected by the portion of acicular α’ and lamellar (α+β) structures. These published discussions described a fitting law of the AM Ti6Al4V performance with a group of yield strength and elongation data, shown in Fig. 6 [27]. The yield strength and elongation data points above the “balance line” displayed the higher comprehensive performances but the points under the “balance line” showed poor performances. Fig. 6 concludes that the SSPREP-HT Ti6Al4V points are closer to the high-performance zone of the picture but the PA-HT [27] and GA-HT [19] points are located on the nearby zone of the “balance line”. The result supported the findings that the PREP powders supply fewer defects with better ductility [9]. The morphologies of the tensile fractures (SS-PREP-SLM-HT) and the microstructures of the as-built AM specimens of all directions were shown in Fig. 7 (a) to (c) ( × 1000) and (i) to (j) ( × 5000). The tensile fracture structures were composed of the ductile dimples with torn edges, which indicated the character of the fully developed plastic fracture. The diameters of the dimples of all the directions were about 2–7 μm. The morphologies of all the directions fracture dimples were homogeneous, which could infer that the fracture behaviors of the different direction tensile tests were almost equal, being in harmony with the similar data in Table 3. Thus, the fundamental microstructures should also resemble in all the directions to maintain the tensile performances agreed with the structures. The comparisons that the microstructures of the XY and Z directions of the × 500 (d and e) and × 2000 (g and h) scopes were shown in Fig. 7. It could be deduced that the microstructures of both the XY direction and the Z direction after SLM-HT were indistinguishable, leading to a conclusion that the
prepared by the SS-PREP was shown in Fig. 5 (a) and (b). Fig. 5 shows that the particles formed by the SS-PREP are highly regular in shapes, and the surfaces of the particles are smooth and round without any satellite particles or other irregular particles. The detailed microstructure and EDS results were shown in Fig. 5 (c)–(e). It could be concluded that the microstructures of the SS-PREP Ti6Al4V powders were the typical rapid solidification structures with full martensite and beta grain sizes about 35 μm. In this structure, the cooling speed for the phase transformation was about 105 K/s because the alpha morphology developed was lenticular [24]. The EDS data displayed in Fig. 5 (e) revealed that there was no remarkable micro segregations or pollution, the chemical content varying was also acceptable. Therefore, the experiment confirmed that the morphology, microstructure and chemical content of the SS-PREP were all qualified. The mechanical properties comparisons of the HT-SLM components prepared by the SS-PREP Ti6Al4V powders above and the other atomization routes were shown in Table 3. It was reflected that the comprehensive mechanical properties of the HT-SLM parts of SS-PREP were significantly higher than the contrasting groups of the traditional PREP powder [25], plasma atomization (PA) powders [26] and GA [19] powders, although there were small differences among the HT conditions. However, it was worth noting that the elongation of the SS-PREP sample was markedly higher than the others. The possible reason was that the powders prepared by centrifugal atomization (for example, PREP and SS-PREP) were denser and cleaner, so the original cavities or impurities which injured the ductility were fewer for the components of the SS-PREP powder [9]. The high comprehensive performances for the SLM Ti6Al4V parts required both high yield strength and high elongation. In general,
Table 3 The comparison of the heat-treated SLM properties between the SS-PREP and the current PA and GA. SLM Materials
Testing Direction
YS(MPa)
UTS (MPa)
EL (%)
RA (%)
SS-PREP-HT (< 45 μm) (850 °C 2 h)
XY plate Z-axis 45° – – XY plate Z-axis –
983.0 ± 2.6 1003.6 ± 5.1 1007 ± 3.1 1097 825 974 ± 7 937 ± 9 1000
1061 1082 1086 957 895 1065 1052 1090
15.0 ± 1.0 16.3 ± 0.3 15.8 ± 1.0 3.4 6-10 7 ± 0.5 9.6 ± 0.9 8.3
48.0 ± 2.0 44.7 ± 2.1 50.6 ± 1.5 – – – – –
PREP-LENS-HT (45–150 μm) (1050 °C 2 h) [25] ASTM F2924 PA-HT (730 °C 2 h) [26] GA-HT (800 °C 4 h) [19]
4
± 1.5 ± 1.5 ± 2.6
± 21 ± 11
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homogeneity of the SS-PREP SLM-HT component was excellent in this work. It was clear that the microstructures of the SLM Ti6Al4V should consist of α′ formed by the rapid solidification upon 1050 °C [21,28,29]. When heated, the decomposition α’→ α+β started at approximately 760 °C and finished at 1050 °C. When heat treated at 850 °C, the α’ is converted to α+β and the α fraction was reported about 73% [20,21]. After that, the samples were furnace cooled and contained the most β and finest lamellar α, contributing to a well comprehensive performance of good ductility and acceptable high tensile strength. The homogeneous structures made the mechanical properties equal in all directions. This advantage could significantly decrease the difficulty of the setting of the AM printing strategies because the mechanical performance in all the different directions was similar. 4. Conclusions The laser PSDs of the spherical Ti6Al4V powders prepared by the SS-PREP matched the log-normal law of 2 factors of σ and u. The σ for the SS-PREP Ti6Al4V was about 0.35, and u could be regarded as the average diameter, exactly calculated from the known metal surface tension, density, rotating speed, and the diameter of the cylinder.
Fig. 6. The relationship [27] of yield strength and elongation of Ti6Al4V AM parts and the comparison of the differently prepared powders (including PA [26], GA [19], PREP [25] and SS-PREP) performances.
Fig. 7. The morphologies of the tensile fractures and microstructures of all directions (SS-PREP-HT): (a) the SEM morphology of the fracture surface of the 45° direction sample ( × 1000); (b) the SEM morphology of the fracture surface of the XY direction sample ( × 1000); (c) the SEM morphology of the fracture surface of the Z direction sample ( × 1000); (d) the microstructure of the XY direction ( × 500); (e) the microstructure of the Z direction ( × 500); (f) the direction instruction; (g) the SEM microstructure of the XY direction ( × 2000); (h) the SEM microstructure of the Z direction ( × 2000); (i) the SEM morphology of the fracture surface of the 45° direction sample ( × 5000); (j) the SEM morphology of the fracture surface of the XY direction sample ( × 5000); (k) the SEM morphology of the fracture surface of the Z direction sample ( × 5000).
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The portion of a random size range (a,b) could be pre-known by the deduced formulas in this work with precise data. The SS-PREP Ti6Al4V powders explored the high performances after the SLM-HT process for the reason that both the yield strength and elongation could reach outstanding values, being much higher than the statistical level of the current AM Ti6Al4V. The microstructures of the different directions groups were similar to the SS-PREP SLM-HT samples. Thus, as a new supplement of the AM raw powder materials, using SS-PREP Ti6Al4V could easily reach the higher isotropism.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was financially supported by the National Key R&D Program of China (2018YFB1106400) (China) and Key R&D Program of Shaanxi (2017KJXX-19) (China). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pnsc.2019.12.004. References [1] P. Sun, Z.Z. Fang, Y. Xia, Y. Zhang, C. Zhou, Powder Technol. 301 (2016) 331–335, https://doi.org/10.1016/j.powtec.2016.06.022. [2] H. Gong, K. Rafi, H. Gu, T. Starr, B. Stucker, Addit. Manuf. 1 (2014) 87–98, https:// doi.org/10.1016/j.addma.2014.08.002. [3] H. Attar, M. Calin, L.C. Zhang, S. Scudino, J. Eckert, Mater. Sci. Eng. A 593 (2014) 170–177, https://doi.org/10.1016/j.msea.2013.11.038. [4] W. Xu, M. Brandt, S. Sun, J. Elambasseril, Q. Liu, K. Latham, K. Xia, M. Qian, Acta Mater. 85 (2015) 74–84, https://doi.org/10.1016/j.actamat.2014.11.028. [5] R. Kumagae, M. Yoshitake, O. Iwatsu, M. Tokizane, J. Jpn. Soc. Powder Powder
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