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Article

Selective Laser Melting of 60NiTi Alloy with Superior Wear Resistance

by
Fangmin Guo
1,2,†,
Hui Shen
2,†,
Zhiwei Xiong
2,†,
Ying Yang
3,
Xin Tong
4,
Yanbao Guo
1,* and
Shijie Hao
2,*
1
College of Mechanical and Transportation Engineering, China University of Petroleum, Beijing 102249, China
2
College of New Energy and Materials, China University of Petroleum, Beijing 102249, China
3
College of Chemical Engineering and Environment, China University of Petroleum, Beijing 102249, China
4
Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou 510632, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Metals 2022, 12(4), 620; https://doi.org/10.3390/met12040620
Submission received: 20 February 2022 / Revised: 24 March 2022 / Accepted: 1 April 2022 / Published: 3 April 2022
(This article belongs to the Special Issue Shape Memory Alloys 2022)
Browse Figures
Versions Notes

Abstract

:
In this work, the selective laser melting (SLM) 60NiTi alloy was successfully fabricated. Through designing an orthogonal experiment of parameters optimization including laser power (P) and scanning speed (v), the optimal parameters window with both high forming quality and appropriate composition proportion was established. The SLM 60NiTi can exhibit high relative density (>98%) and low Ni loss (<0.2 at.%) at the parameter window of P = 80–90 W, v = 300–350 mm/s, and energy density of 145–155 J/mm3. The optimally-selected SLM 60NiTi exhibits a high compression strength of 2.2 GPa and large reversible strain of 7% due to the reversible stress-induced martensitic transformation of the NiTi phase and the large elastic strain of the Ni4Ti3 phase. It also exhibits superior wear resistance to conventional casting solution treated 60NiTi because the NiTi phase formed in an SLM repeated thermal cycle possesses a lower solution Ni atom and thus lower critical stress for martensitic transformation, and is more prone to undergo martensitic transformation upon friction and wear.

1. Introduction

The 60NiTi alloy is a well-known Ni-rich NiTi shape memory alloy (SMA) with 60 wt% Ni and 40 wt% Ti. It usually consists of a soft NiTi matrix phase and a hard Ni4Ti3 phase. The volume fraction of the Ni4Ti3 phase with nanometer-scale accounts for ~70%, which contributes to the high strength, high hardness, and high wear resistance [1,2,3]. In addition, the NiTi phase that is capable of reversible stress-induced martensitic transformation contributes to the superelasticity and toughness [2,4]. These advantages make 60NiTi alloy applications able to be widely used as triboelements, bearings, gears, tools, etc. in aerospace, marine, automotive, and medical industries [5,6,7]. However, due to the high hardness and superelasticity, 60NiTi displays poor machinability and can only be fabricated into parts with simple shapes and structures by conventional methods [3,8,9].
As an additive manufacturing technique, selective laser melting (SLM) has been successfully used to fabricate Ti-based, Al-based, and Fe-based metallic parts with complex three-dimensional structures [10,11,12]. Recently, NiTi shape memory alloys are also widely attracted to be fabricated by SLM, expecting better exhibition of their functional properties by forming complex structures. There has been a lot of research work. Wang et al. studied the effect of each individual parameter on the phase transformation and tensile mechanical behavior of SLM-NiTi [13]. Yang et al. improved the tensile recovery strain (2.25–3.73%) stability of SLM-NiTi by nano Ni4Ti3 precipitation [14]. Our recent study demonstrated the damage-tolerant ability and shape recoverability of the SLM-NiTi honeycomb structure based on the unique deformation features and shape memory function of SMA [15].
However, almost all existing SLM-NiTi studies focus on near-equiatomic NiTi shape memory alloys. Up to now, there is no literature about the SLM fabrication of 60NiTi alloy. Based on the unique forming mode of line-by-line and layer-by-layer of the SLM, a high-energy laser beam melted and bonded powder particles on a powder bed [16,17]. During this process, it is considered that there could be the following problems when fabricating 60NiTi parts by SLM. One is poor formability. Due to the local rapid cooling rate and large discrepancy of thermal expansivity between NiTi and Ni4Ti3 phases, the SLM 60NiTi can easily exert high internal stress and evolve to crack under the large thermal gradient [18,19]. The other is the difficulty in accurately controlling the Ni/Ti atom ratio of composition [20,21,22]. It has been demonstrated that the appropriate composition proportion of 60 wt% Ni is essential for the excellent performance of 60NiTi [23]. However, in the SLM process, Ni loss is unavoidable because the Ni element has higher vapor pressure than the Ti element [13,24]. Hence, it is crucial to explore the optimal process parameter window of SLM 60NiTi alloy that could exhibit both the high forming quality and appropriate composition.
In this work, via designing an orthogonal experiment with variable process parameters of laser power and scanning speed, we constructed a series of parameter maps of forming quality to establish the optimal parameter window of SLM 60NiTi. Simultaneously, we studied the effect of parameters on Ni content to establish the parameter window with appropriate composition. After that, we explored the superelasticity and wear property of the optimally-selected SLM 60NiTi sample and revealed the micro-mechanism of the superior properties through a transmission electron microscope (TEM).

2. Materials and Methods

Pre-alloyed 60NiTi powders fabricated by a gas-atomizing method were used. Figure 1a,b show the SEM image of power morphology and size distribution of 60NiTi powders, respectively. The powders consist of well-dispersed and smooth-surfaced sphere particles of diameters ranging from 18–46 μm (D50 = 33 μm).
The SLM fabrication was conducted using an Eplus M100-T machine (e-Plus 3D Tech. Co. Ltd., Beijing, China) equipped with a maximum 200 W Yb fiber laser of 50 μm in diameter. The fabrication was carried out under a high purity Ar atmosphere with an oxygen level below 500 ppm. An orthogonal experiment of laser powers (P, 40/50/60/70/80/90/100/110/120 W) and scanning speeds (v, 150/200/250/300/350/400/450/500 mm/s) was conducted. Each scanning speed and each power were combined in pairs under fixed strategy, layer thickness, and hatch spacing to fabricate a sample. The hatch spacing (h) is 50 μm, and the powder layer thickness (t) is 30 μm. The energy density (E) is calculated by E = P/(v × h × t). The stripe width is 4 mm, and the rotation angle is 67° (Figure 1c). This rotation angle follows a principle that is not divisible by 360°, which aims at avoiding the overlap of laser tracks among multilayers and guaranteeing the blanket coverage melting of powders [13,25]. Series of block samples with a size of 6 × 6 × 3 mm3 were fabricated by SLM (Figure 1d).
The samples were polished for the observation of FEI Quanta 200F scanning electron microscope (SEM) (FEI Company, Hillsboro, Oregon, USA) and OLYMPUS DSX510optical microscopy (OM) (OLYMPUS Co., Tokyo, Japan). The relative density was calculated from the average porosity value of three optical micrographs at the same scale. The Ni contents were detected from the energy dispersive spectrometry (EDS) equipped in SEM. Each sample was calculated three times at different locations and took the average for final Ni content. The phase analysis was performed by a transmission wide-angle X-ray diffraction NANOPIX-WE system (XRD) (Rigaku Co., Tokyo, Japan). The NANOPIX-WE system is configured with a Mo rotating anode target X-ray source (corresponding to a wavelength of λ, 0.7093 Å). The microstructures were observed by FEI Tecnai G2 F20 transmission electron microscope (TEM) (FEI Company, Hillsboro, Oregon, USA). The sample disk for TEM was ground and then electropolished using a RL-I twinning-jet thinning electropolishing device (Beijing Ruiling innovation Tech. Co. Ltd., Beijing, China) and an electrolyte consisting of 33% HNO3 in methanol at 243 K and 30 V.
The compression testing sample was a cylinder with a height of 3 mm and diameter of 1.5 mm, which was cut from the as-built 60NiTi. The compression loading direction was along the building direction. Compression tests were conducted at room temperature (20 °C) using a MDT series universal compression machine (KQL Test Instrument Co. Ltd., Shenzhen, China). The strain rate was 10−3 s−1. The reciprocating sliding wear tests were performed using reciprocating friction and wear testing machine (HSR-2M, ZKKH Tech. Co. Ltd., Lanzhou, China). Before the test, the samples were ground with abrasive papers and polished by diamond paste. These tests were conducted at 5 N loads under the unlubricated condition at room temperature with a constant wear track length of 3 mm. Zirconia (ZrO2) balls with a diameter of 3 mm were used as the counter friction pair. The worn surface was observed by a Keyence VK-X100 laser scanning confocal microscope (LSCM) (Keyence Co., Osaka, Japan) and analyzed by Keyence multifile analyzer software (VK-H1XMC, Keyence Co., Osaka, Japan).

3. Results and Discussion

Figure 2 shows the process parameter map (Figure 2a) of the orthogonal experiment with variable laser power and scanning speed and some SEM images (Figure 2b–f) of the polished SLM bulk 60NiTi. The parameter map shows the tendency of forming quality with scanning speed and laser power. The SEM images show the typical defects under different forming conditions. It is seen that the good forming quality with fewer defects (such as #1 sample, P = 80 W, v = 300 mm/s, Figure 2b) is obtained at a suitable laser power of 80–90 W but a low scanning speed of 150–200 mm/s. In such a parameter window, sufficient energy input can be achieved and the alloy powders can get sufficient fusion. As the scanning speed increases, it is preferred to form cracks. When the laser power is low, the energy input becomes insufficient and laminar cracking appears (#2 sample, P = 60 W, v = 350 mm/s, Figure 2c), causing poor formability. This laminar cracking is usually formed because of the insufficient powder fusion between layers. As the laser power increases, the high scanning speed still causes the formation of micro-cracking (#3 sample, P = 80 W, v = 400 mm/s, Figure 2d). This micro-cracking is mainly formed after solidification due to the different thermal expansivity between the NiTi phase and Ni4Ti3 phase. With the laser power further increasing, spherical pores of 20–50 μm (#4 sample, P = 100 W, v = 400 mm/s, Figure 2e) and larger keyholing of 100–130 μm (#5 sample, P = 100 W, v = 200 mm/s, Figure 2f) gradually appear. This is because too high energy input easily generates the boiling melt of powders and causes amounts of stuck gases [17,26].
Figure 3 shows the evolutions of relative density with parameters including laser power (P), scanning speed (v), and energy density (E). It is important to note that the groups of P = 60 W and 70 W, and some samples of high scanning speeds (for example v = 600 mm/s, P = 80–120 W) were not counted because they did not build successfully for lack of fusion or severe laminar cracking [27,28]. Figure 3a shows the change of relative density with laser power. It is seen that the relative density exhibits a decreasing tendency with the increase of laser power. This is mostly because of the large number of sphere holes appearing at higher energy input, as sample #5 shown in Figure 2f. Figure 3b shows the change of relative density with scanning speed, which does not exhibit a definite evolution tendency in all groups. It is found that at the laser powers of 80 W and 90 W, and all the successfully fabricated samples exhibit a high relative density of >97%, mostly >99%. For the group samples with P = 100 W, the relative density shows a remarkable increase with the scanning speed increasing. Except for the group samples of P = 120 W, other group samples at higher P all exhibit small fluctuation first but rapid increase when v > 450 mm/s. This is because, at high laser power, the increase of scanning speed is capable of achieving a suitable energy input, thus avoiding the boiling melt of powders and decreasing spherical pores [13,29] (compare samples #4 and #5 in Figure 2).
Figure 3c shows the change of relative density with energy density. It is noted that, at P = 80 W, v = 150–350 mm/s and P = 90 W, v = 150–200 mm/s, the relative densities are always high. However, for other groups, the highest relative density in every group only appears at the lowest energy density while most concentrated at 130–170 J/mm3. This is because higher energy input easily generates spherical holes or keyholing holes, thus decreasing relative density [30,31]. In addition, it is noted that the relative density and defect condition (morphology and quantity) are different at different P/v combinations, although at similar energy densities. As shown in Figure 3d, with the increase of both laser power and scanning speed, the relative density decreases sharply from 99.7% to 88.7%, and the spherical pores become more and larger. This indicates that an optimal parameter window consists of more than one suitable E but also a suitable P/v combination.
Figure 4 shows the effect of processing parameters on the Ni content of SLM 60NiTi. Note that some samples that did not build successfully were not counted here. Figure 4a,b shows the effect of laser power and scanning speed on Ni content, respectively. It is seen that the Ni content in most samples exhibits a decreasing tendency with the increasing laser power and decreasing scanning speed, respectively. However, the effect of one parameter can be influenced or regulated by another parameter. For example, when scanning speed is 350 mm/s, the Ni content decreases by 1.4 at.% (from 54.9 at.% to 53.5 at.%) with the laser power increasing from 80 W to 140 W, whereas the Ni content decreases by 3.2 at.% (from 54.5 at.% to 51.3 at.%) when the v = 200 mm/s.
Figure 4c plots the Ni contents under different energy densities. It shows that there is a negative correlation between Ni content and energy density. Figure 4d plots the Ni loss (the gap of as-built Ni content from standard 55 at.% Ni composition) as a function with energy density. It is found that the Ni loss increases as energy density increases. It is considered mainly because of the higher energy input leading to more severe Ni element volatilization [28]. The minimum Ni loss is ~0.1 at.% when the energy density is 152 J/mm3, which is the closest to the standard value of 55 at.%. The maximal Ni loss is up to 3.6 at.% when the energy density increases to 600 J/mm3. The calculated values show that the average Ni loss of 60NiTi is about 0.83 at.%/(100 J/mm3) during the SLM process, which is more than three times that of near-equiatomic Ni50.6Ti49.4 alloys (0.24 at.%/(100 J/mm3)) during the SLM process [13]. The main cause of this high Ni loss of SLM 60NiTi is thought to be the 60NiTi powders possessing more Ni elements and thus requiring higher energy input for favorable fusion.
Based on the above results, we established the optimal parameter window of P = 80–90 W, v = 300–350 mm/s, and E = 145–155 J/mm3, where the fabricated samples possess both high forming quality (relative density >98%) and near standard composition (Ni loss <0.2 at.%). Particularly, the sample fabricated with P = 80 W, v = 350 mm/s, and E = 152 J/mm3 exhibits the best forming qualities of both high relative density of 99.9% and suitable composition of 54.9 at.% Ni.
Figure 5 shows the XRD pattern and TEM images of the above optimally-selected sample fabricated by P = 80 W and v = 350 mm/s. Figure 5a shows that the sample consists of the rhombohedral Ni4Ti3 phase and cubic NiTi B2 phase. Figure 5b shows the TEM bright-field image and Figure 5c,d show the TEM dark-field images for the Ni4Ti3 phase and NiTi phase, respectively. It is noted that the NiTi phase exhibits a network distribution, and the Ni4Ti3 phase was surrounded and divided by a slender NiTi phase (Figure 5c). The unit of the network consists of the inner Ni4Ti3 phase and boundary NiTi phase. The size of the Ni4Ti3 phase is 30–40 nm, and the width of the NiTi phase is 10–20 nm. The Ni4Ti3 phase is slightly larger than that of conventional Casting-S&Q 60NiTi (~20 nm) [4,32]. It is mainly because of the remelting/reheating effect in SLM’s repeated thermal cycle process that could promote the growth of precipitates.
Figure 6 shows the compression stress–strain curves with incremental strains until fracture. It is seen that the SLM 60NiTi exhibits a fracture strength of 2.2 GPa and fracture strain of 8.5%. Inset is the amplifying loading and unloading stress–strain curve at 8% strain, which demonstrates an ultra-large reversible strain of 7%. It is speculated that such an ultra-large reversible strain may come from the coupling effect of two phases [6]. Upon loading, the NiTi phase would undergo a stress-induced martensitic transformation; meanwhile, the nanoscale Ni4Ti3 phase would display large elastic strain, which has ever been proved in Ni-rich NiTi alloys with similar Ni4Ti3 nano-precipitate [33,34]. Upon unloading, the NiTi phase could undergo inverse martensitic transformation to austenite and the Ni4Ti3 phase also undergoes elastic recovery simultaneously. The reversible martensitic transformation and large elasticity contribute together to the favorable superelasticity of SLM 60NiTi.
Figure 7 shows the results of reciprocating sliding wear tests and the microstructures of the optimally-selected sample fabricated by optimal parameters of P = 80 W and v = 350 mm/s. In comparison, the conventionally used casting 60NiTi treated by solid-solution and water-quenching (Casting-S&Q 60NiTi) was also tested with the same condition. Figure 7a shows the friction coefficient curves of the two samples. It shows that the curve of SLM 60NiTi exhibits a lower plateau value than that of Casting-S&Q 60NiTi. Insets are the optical microscopic pictures of the wear surface. It was observed that the Casting-S&Q 60NiTi displays a bigger wear area and poor wear condition than the SLM 60NiTi. Figure 7b shows that the SLM 60NiTi exhibits both a lower friction coefficient and less wear volume than Casting-S&Q 60NiTi, indicating that the former exhibits better wear properties.
Although the SLM 60NiTi exhibits a larger Ni4Ti3 size compared to conventional Casting-S&Q 60NiTi (Figure 5c), it still presents better wear properties than the latter. As shown in Figure 7c, for the 60NiTi alloy, it is known that the stress-induced reversible martensitic transformation that occurred in the NiTi phase is the major contributor to the excellent wear properties [6,35,36]. Upon external sliding friction, the existence of the NiTi phase can effectively relieve stress concentration by phase transformation and recover to its original shape and morphology by the inverse phase transformation upon unloading [37,38]. For the 60NiTi treated by conventional solution and quenching, the solution Ni atom has a high fraction in the NiTi phase for the rapid cooling rate during quenching. However, for the SLM 60NiTi, the repeated thermal cycle will bring multiple remelting/reheating effects, promoting the Ni precipitation in the NiTi phase. Hence, the solution Ni atom in the NiTi phase of SLM 60NiTi is lower than that of Casting-S&Q 60NiTi. For the NiTi shape memory alloy, the lower Ni content will make the NiTi exhibit higher martensitic transformation temperature (MTT) [39,40]. It is known that the MTT of 60NiTi is far lower than room temperature (~20 °C). The higher MTT means the SLM 60NiTi requires lower critical stress (σS in Figure 7c) than the Casting-S&Q 60NiTi (σC in Figure 7d) and is thus more prone to undergo a martensitic transformation when loaded under the same temperature [41,42]. In addition, the high critical stress makes the Casting-S&Q 60NiTi easily experience dislocation glide compared to SLM 60NiTi, causing irrecoverable deformation and damaging the wear resistance. As a result, the SLM 60NiTi exhibits better wear resistance than conventional Casting-S&Q 60NiTi.

4. Conclusions

In this study, we determined the optimal parameters window of 60NiTi fabricated by SLM. 60NiTi with controllable forming quality and composition with excellent wear resistance was fabricated for the first time. The experimental results and the discussion present above allow the following conclusions to be reached:
(1)
The evolutions of the forming quality with scanning speed and laser power were revealed in the parameters window, respectively. The two parameters have a different impact on the formation mechanism of pore defects. The Ni content of the samples is tightly controlled by the energy density, which determines the amount of Ni loss.
(2)
We established the optimal parameter window of P = 80–90 W, v = 300–350 mm/s, and E = 145–155 J/mm3, where the fabricated samples possess both high forming quality (relative density >98%) and near standard composition (Ni loss <0.2 at.%).
(3)
The phase constitution of 60NiTi is composed of nano-sized Ni4Ti3 (30–40 nm) precipitates and network distributed NiTi, which is completely different from the classic nearly equiatomic SLM-NiTi.
(4)
The optimally-selected SLM 60NiTi exhibits an ultra-large reversible strain of 7% and 2.2 GPa strength, which also cannot occur in the nearly equiatomic SLM-NiTi. The excellent property is attributed to the strong coupling effect between the nano Ni4Ti3 precipitates and the transformable NiTi phase. It also makes SLM 60NiTi have more excellent wear resistance than the conventional casting counterpart.

Author Contributions

Conceptualization, F.G. and H.S.; methodology, H.S.; validation, H.S. and F.G.; formal analysis and investigation, F.G. and Y.Y.; resources, X.T.; writing—original draft preparation, F.G. and Z.X.; writing—review and editing, Z.X., S.H., and Y.Y.; project administration, Y.G.; funding acquisition, Y.G., X.T., and S.H. Supervision, S.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Joint Fund of the National Natural Science Foundation Committee and Chinese Academy of Engineering Physics (NSAF) (No. U2130201), the Natural Science Foundation of China (No. 51971244 and No. 51731010), the Advanced Structural Technology Foundation of China (No. 2020-JCJQ-JJ-024), and the Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515011666).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Powder characteristics include (a) Scanning electron microscopy (SEM) image and (b) statistic of size distribution; (c) schematic of the scanning strategy; (d) pictures of the SLM fabricated bulk 60NiTi.
Figure 1. Powder characteristics include (a) Scanning electron microscopy (SEM) image and (b) statistic of size distribution; (c) schematic of the scanning strategy; (d) pictures of the SLM fabricated bulk 60NiTi.
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Figure 2. (a) Process parameter map of the orthogonal experiment with variable laser power and scanning speed; (b–f) SEM images of the polished SLM bulk 60NiTi blocks. Samples #1–#5 correspond to the marked points in (a).
Figure 2. (a) Process parameter map of the orthogonal experiment with variable laser power and scanning speed; (b–f) SEM images of the polished SLM bulk 60NiTi blocks. Samples #1–#5 correspond to the marked points in (a).
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Figure 3. The relative density is a function with (a) laser power; (b) scanning speed; and (c) energy density. (d) Optical micrographs of the samples were fabricated under similar energy density but different P and v combinations.
Figure 3. The relative density is a function with (a) laser power; (b) scanning speed; and (c) energy density. (d) Optical micrographs of the samples were fabricated under similar energy density but different P and v combinations.
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Figure 4. Ni content of SLM 60NiTi as a function with processing parameters: (a) laser power; (b) scanning speed; (c) energy density; (d) Ni loss as a function with energy density.
Figure 4. Ni content of SLM 60NiTi as a function with processing parameters: (a) laser power; (b) scanning speed; (c) energy density; (d) Ni loss as a function with energy density.
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Figure 5. Microstructural analysis of the well-established SLM 60NiTi fabricated by P = 80 W and v = 350 mm/s. (a) XRD pattern and phase identification; (b) bright-field image and corresponding selected area electron diffraction (SAED) pattern; (c) dark-field image of the Ni4Ti3 phase; (d) dark-field image of the B2-NiTi phase.
Figure 5. Microstructural analysis of the well-established SLM 60NiTi fabricated by P = 80 W and v = 350 mm/s. (a) XRD pattern and phase identification; (b) bright-field image and corresponding selected area electron diffraction (SAED) pattern; (c) dark-field image of the Ni4Ti3 phase; (d) dark-field image of the B2-NiTi phase.
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Figure 6. Compressive loading-unloading curves of the well-established SLM 60NiTi fabricated by P = 80 W and v = 350 mm/s.
Figure 6. Compressive loading-unloading curves of the well-established SLM 60NiTi fabricated by P = 80 W and v = 350 mm/s.
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Figure 7. Wearing properties of the well-established SLM 60NiTi and conventional Casting-S&Q 60NiTi. (a) Friction coefficient curves; (b) summary results of friction coefficient and wear volume at three sampling positions for wearing test; (c) schematics reveal the mechanism of wear properties.
Figure 7. Wearing properties of the well-established SLM 60NiTi and conventional Casting-S&Q 60NiTi. (a) Friction coefficient curves; (b) summary results of friction coefficient and wear volume at three sampling positions for wearing test; (c) schematics reveal the mechanism of wear properties.
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MDPI and ACS Style

Guo, F.; Shen, H.; Xiong, Z.; Yang, Y.; Tong, X.; Guo, Y.; Hao, S. Selective Laser Melting of 60NiTi Alloy with Superior Wear Resistance. Metals 2022, 12, 620. https://doi.org/10.3390/met12040620

AMA Style

Guo F, Shen H, Xiong Z, Yang Y, Tong X, Guo Y, Hao S. Selective Laser Melting of 60NiTi Alloy with Superior Wear Resistance. Metals. 2022; 12(4):620. https://doi.org/10.3390/met12040620

Chicago/Turabian Style

Guo, Fangmin, Hui Shen, Zhiwei Xiong, Ying Yang, Xin Tong, Yanbao Guo, and Shijie Hao. 2022. "Selective Laser Melting of 60NiTi Alloy with Superior Wear Resistance" Metals 12, no. 4: 620. https://doi.org/10.3390/met12040620

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