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Characterization of nanoparticle mixed 316 L powder for additive manufacturing
Journal Pre-proof Characterization of nanoparticle mixed 316 L powder for additive manufacturing Wengang Zhai, Wei Zhou, Sharon Mui Ling Nai, Jun Wei
PII:
S1005-0302(20)30158-4
DOI:
https://doi.org/10.1016/j.jmst.2020.02.019
Reference:
JMST 2007
To appear in:
Journal of Materials Science & Technology
Please cite this article as: Zhai W, Zhou W, Ling Nai SM, Wei J, Characterization of nanoparticle mixed 316 L powder for additive manufacturing, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2020.02.019
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Research Article
Characterization of nanoparticle mixed 316L powder for additive manufacturing Wengang Zhai 1, Wei Zhou 1,*, Sharon Mui Ling Nai 2,*, Jun Wei 2
1
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
2
Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, 637662, Singapore
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[Received 23 September 2019; Received in revised form 18 November 2019; Accepted 8 December 2019] * Corresponding authors.
E-mail addresses: [email protected] (W. Zhou); [email protected] (M. L. S.
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Nai).
Nanoparticles reinforced steels have many advantaged mechanical properties.
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Additive manufacturing offers a new method for fabricating nanoparticles reinforced high performance metal components. In this work, we report the application of low energy ball milling in mixing nanoparticles and micron 316L powder. With this
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method, 0.3 and 1.0 wt% Y 2O3 nanoparticles can be uniformly distributed on the surface of 316L powder with the parameters of ball-to-powder ratio at 1:1, speed at 90 rpm and 7 h of mixing. The matrix 316L powders remain spherical in shape after the mixing process. In the meantime, the effect of low energy ball milling and the addition of Y2O3 nanoparticles on the powder characteristics (flowability, apparent
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density and tap density) are also studied. Results show that the process of low energy ball milling itself can slightly decrease the flowability and apparent density of the 316L powder. The addition of 0.3 and 1.0 wt% Y 2O3 nanoparticles can also decrease the flowability, the tap density and the apparent density compared with the original 316L powder. All of these changes result from the rough surface of the mixed powder produced by ball milling and the addition of Y 2O3 nanoparticles. The powder’s rough surface can increase the coefficient of friction of powders. The mixture of 316L powder and Y2O3 nanoparticles can be successfully used for selective laser melting 1
(SLM). The relative density of SLM 316L-Y2O3 is measured at 99.5%. However, Y2O3 agglomerations were observed which is due to the poor wettability between 316L and Y2O3.
Keywords: Powder mixing; Powder characterization; Flowability; Apparent density; Tap density; Additive manufacturing
1. Introduction
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Metal matrix composites (MMCs) have many applications in cutting tools, automotive disc brakes, driveshaft, sports equipment. Metal matrix nanocomposites (MMNCs) are those MMCs where the nanosized reinforcement is employed, typically less than 100 nm in size [1-4]. Usually, the mechanical properties and wear resistance of MMCs and MMNCs are improved.
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The commonly used reinforcements are high melting point carbides (SiC, TiC, WC, BC 4), oxides (Al2O3, Y2O3), nitrides (AlN, ZrN) and borides (TiB 2).
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MMCs can be fabricated by casting[5] but more often by powder metallurgy[1, 6-8]. In a powder metallurgy process, the reinforcements are usually blended with the matrix powder by
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high energy ball milling to achieve atomic bonding of different powders [7]. The ball milling is followed by consolidation processes, for example, Hot Rolling (HR), hot pressing (HP), hot
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isostatic pressing (HIP) and spark plasma sintering (SPS)[1, 3, 6-10]. During the high energy ball milling, the powders are repeatedly welded, fractured, and rewelded, which lead to a considerable change of powder shape, as reported in Refs.[11-15]. Although there are some efforts have been done for the preparation of ceramic particles reinforced metal powder for additive manufacturing using high energy ball milling [16-19], efficiency can be a problem that
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should be considered when the high energy ball milling process is applied due to that after the higher energy ball milling, the milled powder must be sieved. After the high energy ball milling process, the shape of 316L powder was changed dramatically. Furthermore, the high energy ball milling process increases not only possibly lead times, but also capital costs. Therefore, high energy ball milling is very difficult to be applied in additive manufacturing, where powder morphology plays an important role. Additive manufacturing, or 3D printing, is a particular technique designed to use high power-density laser or electron beam to melt powders [20-22]. This technology offers a new 2
opportunity to fabricate MMNCs. Gao et al.[23,
24]
fabricated oxide dispersion strengthened
stainless steel by electron beam selective melting. They mixed nano Y 2O3 powder and the metal powder by ball milling with the ball-to-powder ratio at 5:1, speed at 150 rpm, for 6 h in alcohol. The mixing method allowed successful electron beam printing of the nanocomposites; however, it is noted that the relative density achieved is low at 96.5% only [23]. Improvement of the mixing method may help to improve the relative density. MMCs [25] or MMNCs[26-28] are also produced by selective laser melting; however, no work has been carried out to study how powder mixing process or addition of nanoparticles affects the properties of the mixed powder. In the current research, we report a method of low energy ball milling process to mix Y2O3
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nanoparticles and micron 316L powder for additive manufacturing. The research reports the first systematic study of effect of the milling process and addition of the nanoparticles on the powder properties affecting the quality of additive manufacturing, including powder
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morphology, flowability, apparent density and tap density. 2. Materials and methods
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2.1. Materials
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Materials used were 316L stainless steel powder (Hoganas, Belgium) and Y 2O3 nanoparticles (Nanostructured & Amorphous Materials, Inc., USA). The mean size of the 316L powder was measured using an LA-960 laser scattering particle size distribution analyzer and
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was found to be 42.3 µm. The Y 2O3 particles had sizes ranging from 32 nm to 36 nm and purity of 99.9%. Chemical composition of the 316L powder is as follows (wt%): 0.011% C, 2.5% Mo, 12.7% Ni, 1.5% Mn, 16.8% Cr, 0.7% Si, 0.074% O, and Fe balance. Fig. 1 shows the morphology of the 316L powder and Y 2O3 nanoparticles.
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2.2. Methods for powder mixing
Powder mixing machine used was Inversina Tumbler Mixer (Bioengineering, Inc.). During
the powder mixing, 316L stainless steel balls with a diameter of 12 mm were added to the powders. The ball-to-powder ratio (weight) was fixed at 1:1. The mixing speed was 90 rpm. The powders were mixed for 7 h. 0.3 wt% and 1.0 wt% of Y 2O3 nanoparticles were mixed with 316L powder.
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Table 1 presents the comparison of parameters used in high energy ball milling and our method. In our method, the ball-to-powder ration is only 1:1, mixing time is only 7 h and the mixing speed is only 90 rpm. Compared with the parameters used for high energy ball milling process, the energy is quite low. Hence, here we refer the method as Low Energy Ball Milling. 2.3. Powder characterization The powders were observed under the scanning electron microscope (SEM, JEOL JSM-7600F and JSM-5600), and their various properties were characterized according to ASTM standards.
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The flowability was measured according to ASTM B213 standard test method for flow rate of metal powders using the hall flowmeter funnel. 50 g powder were weighed and a 2.54 mm hall flowmeter funnel was used. The 50 g powder was timed as it flowed through the 2.54 mm orifice of the hall flowmeter funnel. The flow rate in unit of second per 50 g was used to
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describe the powder flowability.
The apparent density was measured according to ASTM B212 standard test method for
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apparent density of free-flowing metal powders using the hall flowmeter funnel. The powders were carefully placed in a hall flowmeter and then let the powder flow into a 25 cm 3 density
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cup. When the cup was fully filled, the powder was weighed, and the apparent density was calculated as the ratio of the weight to the cup volume.
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The tap density was measured according to ASTM B527 standard test method for tap density of metal powders and compounds. 100 g powder was weighed and placed in a cylinder with a capacity of 25 cm 3. The powders were tapped for 3000 times, and then their volume was measured to calculate the powder’s tap density.
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2.4. SLM using different powders
Selective laser melting (SLM) of the original 316L powder and ball milled powders were
performed using the ProX DMP 300 (3D Systems, South Carolina, US) equipped with a 500 Watt continuous fiber laser. The laser spot size is 75 μm. The SLM processing was performed under the vacuum condition and the oxygen content was below 300 ppm. Ar gas was purged directly to the laser scanned area to protect the melt pool. The printing parameters were fixed as follows: laser power 250 W, scan speed 1200 mm/ s,
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layer thickness 40 μm, and hatching space of 55 μm. The scanning angle between adjacent layers is 90°. 2.5. Density measurement After the printing is finished, the cubes are cut by electrical discharge machining from the substrate. The density of the printed cubes is measured by Archimedes Principle with the equation:
sample (Mair Mwater) = water Mair
(1)
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where sample is the density of sample, water is the density of DI water (1.0 g/cm 3), Mair is the mass of sample in air, Mwater is the mass of sample in water. Therefore, with this method, only the mass of sample in air and water are needed. Before the density measurement, the surfaces
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of printed 316L cubes are grinded by SiC paper. Because during the mass measurement in water, air bubbles would attach on the surface of the printed sample due to the rough original surface which can affect the result of density. Here, the relative density of 316L is measured
3. Results and discussion
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with respect to the density of bulk material at 8.0 g/cm 3.
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3.1. Distribution of Y 2O3 in 316L powder mixture
Fig. 2(a) shows the typical image of the powders after mixing for 1 h. Y 2O3 particles appeared as white dots due to their non-conducting property. Fig. 2(b) shows the EDS result of the area pointed by the arrow. It indicates the agglomeration of Y2O3. As agglomeration could still be observed, 1 h is insufficient for the Y 2O3 particles to be uniformly distributed on the
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surface of the 316L powder.
Fig. 3 presents the distribution of Y 2O3 nanoparticles on the surface of 316L powder with
7 h of mixing using low energy ball milling process. After the ball milling process, the 316L powder remains in spherical shape. Fig. 3(a) shows the mixing result of 316L-0.3% Y2O3. High magnification SEM image given in Fig. 3(b) shows that the Y2O3 nanoparticles are uniformly distributed on the surface of 316L powder. Fig. 3(c) shows the mixing result of 316L-1.0% Y2O3. High magnification SEM image given in Fig. 3(d) shows that the Y2O3 nanoparticles are
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uniformly distributed on the surface of 316L powder. It can be concluded that the diluted low energy ball milling is an effective method for mixing micron and nano powders. Nanosized particles have the nature to agglomerate and attach on other surface. Van der Walls’ force and Coulomb's force are the driven force for this phenomenon. During the mixing, steel balls with the diameter of 12 mm were added to break the Y 2O3 clusters. After 7 h mixing, the nanosized Y2O3 particles were attached on the surface of 316L powder. 3.2. Properties of powder The microstructure and mechanical property of additive manufactured components can be
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affected by the properties of powder [29-31], for instance, morphology, size distribution, flowability, apparent density, tap density, etc. In this work, two factors have been fou nd to affect the powder characteristics. One factor is the addition of Y 2O3. Another factor is the ball milling process. The measurement of flowability, apparent density and tap density were
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conducted according to ASTM B213, B212, B527 standards. Figure 4 indicates the powder characterization results of flowability, apparent density and tap density of original 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders. Detailed values are shown in
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Table 2.
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Fig. 4(a) shows the flow rate of the powders. The flow rate of the virgin 316L powder is 15.92 s/50 g, which means that 50 g of 316L powder flowed over in 15.92 s through a hall flowmeter funnel. The flow rate of ball milled 316L is 16.79 s/50 g. With the addition of 0.3
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wt% Y2O3 nanoparticles, the flow rate is increased to 17.07 s/50 g. With 1.0 wt% Y2O3 nanoparticles addition, the flow rate climbed to 20.44 s/50 g sharply. Fig. 4(b) presents the apparent density of the powders. The apparent density of the virgin 316L powder is 4.386 g/cm3. The apparent density of the ball milled 316L powder decreases to
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4.168 g/cm3. With the addition of Y 2O3 nanoparticles, the apparent densities of ball milled 316L-0.3Y2O3 and 316L-1.0Y2O3 are 4.202 and 4.206 g/cm 3, respectively. Fig. 4(c) depicts the tap density of the powders. The tap density of 316L powder is
decreased by the low energy ball milling process and the addition of Y2O3 nanoparticles. The tap density of the virgin 316L powder is 5.025 g/cm3. The tap density of the ball milled 316L powder is 4.926 g/cm3. With the addition of Y 2O3 nanoparticles, the tap densities of the ball milled 316L-0.3Y2O3 and 316L-1.0Y2O3 are 4.854 and 4.831 g/cm3, respectively.
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To understand the reason of the changes of powder properties by the low energy ball milling process, the ball milled 316L powder was observed using the SEM. After low energy ball milling, the surface of 316L powder is observed to be rougher than that of the original 316L powder, as shown in Fig. 5(a) and (b). The deformation of 316L powders caused by the compression of 316L milling balls used in the low energy ball milling process results in the roughed powder surface. Because of this phenomenon, the flowability is compromised. The apparent densities of the 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders are increased slightly due to the filling of gaps among powders by the nanoparticles. However, the tap densities of 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders are observed to decrease.
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During the tapping, the powders are more difficult to move around. This results from the increase of the coefficient of friction between particles due to the ball milling process and the addition of Y2O3 particles.
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Fig. 6 illustrates the mechanism of powder mixing using low energy ball milling (Fig. 6(a)) and high energy ball milling (Fig. 6(b)). After 7 h low energy ball milling, Y 2O3 nanoparticles are uniformly distributed on the surface of the 316L powder. The shape of the
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ball milling process to the 316L powder.
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316L powder remains spherical. However, a rougher surface is introduced by the low energy
The powder mixing method used in powder metallurgy is called high energy ball milling due to the (i) high ball-to-powder ratio (ranging from 5:1 to 30:1), (ii) high mixing speed
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(ranging from 300 to 600 rpm) and (iii) long mixing time (ranging from 20 to 150 h). During the high energy ball milling, powders are repeatedly welded, fractured, and rewelded. As a result, the powders are alloyed mechanically and the shapes are changed significantly. Usually, the resultant powder shape is irregular. The mechanism of high energy ball milling is depicted in Fig. 6(b). The high energy ball milled powder does not flow through the hall flowmeter
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funnel. Therefore, the flowability of high energy ball milled powder cannot be measured. As the powder preparation of apparent density uses the same flowability measurement setup, the apparent density of high energy ball milled powder also cannot be measured. Additive manufacturing offers a new opportunity to fabricate MMCs. It is easy to achieve the atomic bonding of the additions and metal matrix. To achieve a uniform dispersion of nanoparticles reinforced MMNCs, the mixing of additions and metal powders before the additive manufacturing process is crucial. Nanoparticles can be added into the alloy ingot
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before the powder atomization
[32]
. However, agglomeration of nanoparticles would occur due
to the nature of nanosized particle. There are two reasons to explain the agglomeration of nanoparticle: (i) the surface energy of nanosized particle is very high. To decrease the high surface energy, the nanosized particle would agglomerate spontaneously; (ii) Van der Walls’ force and Coulomb's force are the driven force for agglomeration. In addition, the density difference of nanoparticles and metal matrix also poses as processing challenge. Some researchers attempted to print PM 2000 ODS steel and MA 956 ODS steel using high energy ball milled powder in irregular shape by selective laser melting. However, agglomerations still can be observed on the top surface and the site of lack of fusion in the printed samples [33-36]. One of the reasons is the poor powder quality. Therefore, for additive manufacturing, the
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additions must be uniformly distributed on the surface of metal powder, and the spherical shape of the powders cannot be significantly altered after the mixing process.
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3.3. SLM using original 316L powder and Y2O3 nanoparticles mixed powder
Fig. 7 provides the unetched structure of the SLM printed samples using the different powders. It can be seen that with the increasing addition of Y 2O3 nanoparticles, porosity and
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lack of fusion can be observed in the printed samples. It is noticed that the black dots in Fig . 7(c) is the resultant of agglomeration of Y 2O3 nanoparticles as illustrated in Fig. 7(d). EDS
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mapping of the agglomeration of Y 2O3 is presented in Fig. 8. The main reason of the formation of the Y2O3 agglomeration is because of the poor wettability between 316L and Y 2O3
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nanoparticles. Another reason could be attributed to the ultra-high temperature of the melt pool during SLM. Simulation studies show the highest temperature of the melt pool is higher than 3000 °C and the temperature near the keyhole can even reach 5000 °C[37, 38], which is much higher than the melting point of Y 2O3 (the melting point of Y2O3 is only 2425 °C). Exposure in such high temperature, the Y 2O3 nanoparticles can be sintered or melted in such high
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temperature. Therefore, Y 2O3 agglomeration is formed in the SLM samples. It can be seen from Fig. 7(d) that the Y2O3 agglomeration presents a polycrystal morphology which further indicates the agglomeration is a result of the sintering or melting of Y 2O3 nanoparticles.
The density of SLM printed sample using the original 316L powder is 7.961 g/cm 3 and the relative density is 99.5%. The relative density was measured with the respect to 316L bulk density of 8.0 g/cm3. The density of SLM printed sample using ball milled 316L-0.3Y2O3 powder is 7.955 g/cm 3, which is 99.6% of the theoretical density of 316L-0.3Y2O3. Likewise,
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the density of 316L-1.0Y2O3 is 7.888 g/cm3, which is 99.2% of the theoretical density of 316L-1.0Y2O3. The densities of the SLM printed samples were higher than 99% which is acceptable. However, further work needs to be done to better disperse the Y 2O3 nanoparticles in the 316L matrix.
4. Conclusions (1) Low energy ball milling of 0.3 wt% Y 2O3 nanoparticles and 316L powder for 7 h with ball-to-powder ratio of 1:1 resulted in uniform distribution of the nanoparticles on the surface
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of 316L powder. (2) However, the process of low energy ball milling itself can result in a rough surface of the 316L powder. Therefore, the flowability, apparent density and tap density are decreased slightly.
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(3) The addition of Y2O3 nanoparticles further decreases the flowability, apparent density and
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tap density of the mixed powder.
(4) The mixture of 316L powder and Y 2O3 nanoparticles can be successfully used for SLM. The
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relative density of SLM 316L-Y2O3 is measured at 99.5%. However, Y 2O3 agglomerations were
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observed which is due to the poor wettability between 316L and Y2O3.
Acknowledgements
The work was supported by A*STAR Industrial Additive Manufacturing Program: The A*STAR Additive Manufacturing Centre (AMC) Initiative: Work Package 1 (High
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Temperature Materials Development for 3D Additive Manufacturing, Grant No. 1426800088). W. Zhai and W. Zhou would like to acknowledge the financial support from Nanyang Technological University.
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Fig. 1. SEM images of (a) 316L powder and (b) Y2O3 nanoparticles.
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Fig. 2. (a) Distribution of Y 2O3 on the surface of 316L powder after mixing for 1 h and (b) EDS
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result of the area pointed by the arrow in (a) showing Y 2O3 agglomeration.
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Fig. 3. SEM images of (a) 0.3% Y2O3 after mixing for 7 h, (b) higher magnification image
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showing the distribution of 0.3% Y 2O3 on the surface of 316L, (c) 1.0% Y2O3 after mixing for 7 h and (d) higher magnification image showing the distribution of 1.0% Y 2O3 on the surface of
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316L.
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Fig. 4. Powder properties of original 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0% Y2O3: (a) flowability; (b) apparent density; (c) tap density.
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Fig. 5. Surface morphologies of (a) original 316L powder and (b) ball milled 316L powder.
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Fig. 6. Mechanism of powder mixing: (a) low energy ball milling (our method); (b) high
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energy ball milling.
Fig. 7. SLM printed samples using (a) original 316L powder, (b) ball milled 316L-0.3% Y2O3 16
powder. The image insert shows the presence of Y2O3 particles; (c) ball milled 316L-1.0% Y2O3 powder, (d) SEM image of Y 2O3 agglomeration in SLM 316L-1.0% Y2O3 sample.
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Fig. 8. EDS mapping of the Y2O3 agglomeration in SLM 316L-1.0% Y2O3.
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Table list: Table 1 Parameters used in high energy ball milling and our method. Low energy ball
Parameter
High energy ball milling
Ball-to-powder ratio
5:1 - 30:1
1:1
Mixing speed (rpm)
200 - 600
90
Mixing time (h)
20 - 150
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Table 2 Powder properties of 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0 %Y2O3. 316L
Ball milled 316L 316L-0.3%Y2O3 316L-1.0%Y2O3
Flow rate (s/50 g)
15.92
16.79
17.07
20.44
Apparent density (g/cm 3)
4.386
4.168
4.202
4.206
Tap density (g/cm3)
5.025
4.926
4.854
4.831
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Powder
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PII:
S1005-0302(20)30158-4
DOI:
https://doi.org/10.1016/j.jmst.2020.02.019
Reference:
JMST 2007
To appear in:
Journal of Materials Science & Technology
Please cite this article as: Zhai W, Zhou W, Ling Nai SM, Wei J, Characterization of nanoparticle mixed 316 L powder for additive manufacturing, Journal of Materials Science and amp; Technology (2020), doi: https://doi.org/10.1016/j.jmst.2020.02.019
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Research Article
Characterization of nanoparticle mixed 316L powder for additive manufacturing Wengang Zhai 1, Wei Zhou 1,*, Sharon Mui Ling Nai 2,*, Jun Wei 2
1
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
2
Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, 637662, Singapore
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[Received 23 September 2019; Received in revised form 18 November 2019; Accepted 8 December 2019] * Corresponding authors.
E-mail addresses: [email protected] (W. Zhou); [email protected] (M. L. S.
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Nai).
Nanoparticles reinforced steels have many advantaged mechanical properties.
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Additive manufacturing offers a new method for fabricating nanoparticles reinforced high performance metal components. In this work, we report the application of low energy ball milling in mixing nanoparticles and micron 316L powder. With this
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method, 0.3 and 1.0 wt% Y 2O3 nanoparticles can be uniformly distributed on the surface of 316L powder with the parameters of ball-to-powder ratio at 1:1, speed at 90 rpm and 7 h of mixing. The matrix 316L powders remain spherical in shape after the mixing process. In the meantime, the effect of low energy ball milling and the addition of Y2O3 nanoparticles on the powder characteristics (flowability, apparent
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density and tap density) are also studied. Results show that the process of low energy ball milling itself can slightly decrease the flowability and apparent density of the 316L powder. The addition of 0.3 and 1.0 wt% Y 2O3 nanoparticles can also decrease the flowability, the tap density and the apparent density compared with the original 316L powder. All of these changes result from the rough surface of the mixed powder produced by ball milling and the addition of Y 2O3 nanoparticles. The powder’s rough surface can increase the coefficient of friction of powders. The mixture of 316L powder and Y2O3 nanoparticles can be successfully used for selective laser melting 1
(SLM). The relative density of SLM 316L-Y2O3 is measured at 99.5%. However, Y2O3 agglomerations were observed which is due to the poor wettability between 316L and Y2O3.
Keywords: Powder mixing; Powder characterization; Flowability; Apparent density; Tap density; Additive manufacturing
1. Introduction
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Metal matrix composites (MMCs) have many applications in cutting tools, automotive disc brakes, driveshaft, sports equipment. Metal matrix nanocomposites (MMNCs) are those MMCs where the nanosized reinforcement is employed, typically less than 100 nm in size [1-4]. Usually, the mechanical properties and wear resistance of MMCs and MMNCs are improved.
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The commonly used reinforcements are high melting point carbides (SiC, TiC, WC, BC 4), oxides (Al2O3, Y2O3), nitrides (AlN, ZrN) and borides (TiB 2).
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MMCs can be fabricated by casting[5] but more often by powder metallurgy[1, 6-8]. In a powder metallurgy process, the reinforcements are usually blended with the matrix powder by
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high energy ball milling to achieve atomic bonding of different powders [7]. The ball milling is followed by consolidation processes, for example, Hot Rolling (HR), hot pressing (HP), hot
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isostatic pressing (HIP) and spark plasma sintering (SPS)[1, 3, 6-10]. During the high energy ball milling, the powders are repeatedly welded, fractured, and rewelded, which lead to a considerable change of powder shape, as reported in Refs.[11-15]. Although there are some efforts have been done for the preparation of ceramic particles reinforced metal powder for additive manufacturing using high energy ball milling [16-19], efficiency can be a problem that
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should be considered when the high energy ball milling process is applied due to that after the higher energy ball milling, the milled powder must be sieved. After the high energy ball milling process, the shape of 316L powder was changed dramatically. Furthermore, the high energy ball milling process increases not only possibly lead times, but also capital costs. Therefore, high energy ball milling is very difficult to be applied in additive manufacturing, where powder morphology plays an important role. Additive manufacturing, or 3D printing, is a particular technique designed to use high power-density laser or electron beam to melt powders [20-22]. This technology offers a new 2
opportunity to fabricate MMNCs. Gao et al.[23,
24]
fabricated oxide dispersion strengthened
stainless steel by electron beam selective melting. They mixed nano Y 2O3 powder and the metal powder by ball milling with the ball-to-powder ratio at 5:1, speed at 150 rpm, for 6 h in alcohol. The mixing method allowed successful electron beam printing of the nanocomposites; however, it is noted that the relative density achieved is low at 96.5% only [23]. Improvement of the mixing method may help to improve the relative density. MMCs [25] or MMNCs[26-28] are also produced by selective laser melting; however, no work has been carried out to study how powder mixing process or addition of nanoparticles affects the properties of the mixed powder. In the current research, we report a method of low energy ball milling process to mix Y2O3
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nanoparticles and micron 316L powder for additive manufacturing. The research reports the first systematic study of effect of the milling process and addition of the nanoparticles on the powder properties affecting the quality of additive manufacturing, including powder
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morphology, flowability, apparent density and tap density. 2. Materials and methods
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2.1. Materials
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Materials used were 316L stainless steel powder (Hoganas, Belgium) and Y 2O3 nanoparticles (Nanostructured & Amorphous Materials, Inc., USA). The mean size of the 316L powder was measured using an LA-960 laser scattering particle size distribution analyzer and
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was found to be 42.3 µm. The Y 2O3 particles had sizes ranging from 32 nm to 36 nm and purity of 99.9%. Chemical composition of the 316L powder is as follows (wt%): 0.011% C, 2.5% Mo, 12.7% Ni, 1.5% Mn, 16.8% Cr, 0.7% Si, 0.074% O, and Fe balance. Fig. 1 shows the morphology of the 316L powder and Y 2O3 nanoparticles.
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2.2. Methods for powder mixing
Powder mixing machine used was Inversina Tumbler Mixer (Bioengineering, Inc.). During
the powder mixing, 316L stainless steel balls with a diameter of 12 mm were added to the powders. The ball-to-powder ratio (weight) was fixed at 1:1. The mixing speed was 90 rpm. The powders were mixed for 7 h. 0.3 wt% and 1.0 wt% of Y 2O3 nanoparticles were mixed with 316L powder.
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Table 1 presents the comparison of parameters used in high energy ball milling and our method. In our method, the ball-to-powder ration is only 1:1, mixing time is only 7 h and the mixing speed is only 90 rpm. Compared with the parameters used for high energy ball milling process, the energy is quite low. Hence, here we refer the method as Low Energy Ball Milling. 2.3. Powder characterization The powders were observed under the scanning electron microscope (SEM, JEOL JSM-7600F and JSM-5600), and their various properties were characterized according to ASTM standards.
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The flowability was measured according to ASTM B213 standard test method for flow rate of metal powders using the hall flowmeter funnel. 50 g powder were weighed and a 2.54 mm hall flowmeter funnel was used. The 50 g powder was timed as it flowed through the 2.54 mm orifice of the hall flowmeter funnel. The flow rate in unit of second per 50 g was used to
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describe the powder flowability.
The apparent density was measured according to ASTM B212 standard test method for
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apparent density of free-flowing metal powders using the hall flowmeter funnel. The powders were carefully placed in a hall flowmeter and then let the powder flow into a 25 cm 3 density
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cup. When the cup was fully filled, the powder was weighed, and the apparent density was calculated as the ratio of the weight to the cup volume.
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The tap density was measured according to ASTM B527 standard test method for tap density of metal powders and compounds. 100 g powder was weighed and placed in a cylinder with a capacity of 25 cm 3. The powders were tapped for 3000 times, and then their volume was measured to calculate the powder’s tap density.
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2.4. SLM using different powders
Selective laser melting (SLM) of the original 316L powder and ball milled powders were
performed using the ProX DMP 300 (3D Systems, South Carolina, US) equipped with a 500 Watt continuous fiber laser. The laser spot size is 75 μm. The SLM processing was performed under the vacuum condition and the oxygen content was below 300 ppm. Ar gas was purged directly to the laser scanned area to protect the melt pool. The printing parameters were fixed as follows: laser power 250 W, scan speed 1200 mm/ s,
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layer thickness 40 μm, and hatching space of 55 μm. The scanning angle between adjacent layers is 90°. 2.5. Density measurement After the printing is finished, the cubes are cut by electrical discharge machining from the substrate. The density of the printed cubes is measured by Archimedes Principle with the equation:
sample (Mair Mwater) = water Mair
(1)
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where sample is the density of sample, water is the density of DI water (1.0 g/cm 3), Mair is the mass of sample in air, Mwater is the mass of sample in water. Therefore, with this method, only the mass of sample in air and water are needed. Before the density measurement, the surfaces
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of printed 316L cubes are grinded by SiC paper. Because during the mass measurement in water, air bubbles would attach on the surface of the printed sample due to the rough original surface which can affect the result of density. Here, the relative density of 316L is measured
3. Results and discussion
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with respect to the density of bulk material at 8.0 g/cm 3.
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3.1. Distribution of Y 2O3 in 316L powder mixture
Fig. 2(a) shows the typical image of the powders after mixing for 1 h. Y 2O3 particles appeared as white dots due to their non-conducting property. Fig. 2(b) shows the EDS result of the area pointed by the arrow. It indicates the agglomeration of Y2O3. As agglomeration could still be observed, 1 h is insufficient for the Y 2O3 particles to be uniformly distributed on the
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surface of the 316L powder.
Fig. 3 presents the distribution of Y 2O3 nanoparticles on the surface of 316L powder with
7 h of mixing using low energy ball milling process. After the ball milling process, the 316L powder remains in spherical shape. Fig. 3(a) shows the mixing result of 316L-0.3% Y2O3. High magnification SEM image given in Fig. 3(b) shows that the Y2O3 nanoparticles are uniformly distributed on the surface of 316L powder. Fig. 3(c) shows the mixing result of 316L-1.0% Y2O3. High magnification SEM image given in Fig. 3(d) shows that the Y2O3 nanoparticles are
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uniformly distributed on the surface of 316L powder. It can be concluded that the diluted low energy ball milling is an effective method for mixing micron and nano powders. Nanosized particles have the nature to agglomerate and attach on other surface. Van der Walls’ force and Coulomb's force are the driven force for this phenomenon. During the mixing, steel balls with the diameter of 12 mm were added to break the Y 2O3 clusters. After 7 h mixing, the nanosized Y2O3 particles were attached on the surface of 316L powder. 3.2. Properties of powder The microstructure and mechanical property of additive manufactured components can be
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affected by the properties of powder [29-31], for instance, morphology, size distribution, flowability, apparent density, tap density, etc. In this work, two factors have been fou nd to affect the powder characteristics. One factor is the addition of Y 2O3. Another factor is the ball milling process. The measurement of flowability, apparent density and tap density were
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conducted according to ASTM B213, B212, B527 standards. Figure 4 indicates the powder characterization results of flowability, apparent density and tap density of original 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders. Detailed values are shown in
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Table 2.
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Fig. 4(a) shows the flow rate of the powders. The flow rate of the virgin 316L powder is 15.92 s/50 g, which means that 50 g of 316L powder flowed over in 15.92 s through a hall flowmeter funnel. The flow rate of ball milled 316L is 16.79 s/50 g. With the addition of 0.3
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wt% Y2O3 nanoparticles, the flow rate is increased to 17.07 s/50 g. With 1.0 wt% Y2O3 nanoparticles addition, the flow rate climbed to 20.44 s/50 g sharply. Fig. 4(b) presents the apparent density of the powders. The apparent density of the virgin 316L powder is 4.386 g/cm3. The apparent density of the ball milled 316L powder decreases to
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4.168 g/cm3. With the addition of Y 2O3 nanoparticles, the apparent densities of ball milled 316L-0.3Y2O3 and 316L-1.0Y2O3 are 4.202 and 4.206 g/cm 3, respectively. Fig. 4(c) depicts the tap density of the powders. The tap density of 316L powder is
decreased by the low energy ball milling process and the addition of Y2O3 nanoparticles. The tap density of the virgin 316L powder is 5.025 g/cm3. The tap density of the ball milled 316L powder is 4.926 g/cm3. With the addition of Y 2O3 nanoparticles, the tap densities of the ball milled 316L-0.3Y2O3 and 316L-1.0Y2O3 are 4.854 and 4.831 g/cm3, respectively.
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To understand the reason of the changes of powder properties by the low energy ball milling process, the ball milled 316L powder was observed using the SEM. After low energy ball milling, the surface of 316L powder is observed to be rougher than that of the original 316L powder, as shown in Fig. 5(a) and (b). The deformation of 316L powders caused by the compression of 316L milling balls used in the low energy ball milling process results in the roughed powder surface. Because of this phenomenon, the flowability is compromised. The apparent densities of the 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders are increased slightly due to the filling of gaps among powders by the nanoparticles. However, the tap densities of 316L-0.3% Y2O3 and 316L-1.0% Y2O3 powders are observed to decrease.
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During the tapping, the powders are more difficult to move around. This results from the increase of the coefficient of friction between particles due to the ball milling process and the addition of Y2O3 particles.
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Fig. 6 illustrates the mechanism of powder mixing using low energy ball milling (Fig. 6(a)) and high energy ball milling (Fig. 6(b)). After 7 h low energy ball milling, Y 2O3 nanoparticles are uniformly distributed on the surface of the 316L powder. The shape of the
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ball milling process to the 316L powder.
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316L powder remains spherical. However, a rougher surface is introduced by the low energy
The powder mixing method used in powder metallurgy is called high energy ball milling due to the (i) high ball-to-powder ratio (ranging from 5:1 to 30:1), (ii) high mixing speed
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(ranging from 300 to 600 rpm) and (iii) long mixing time (ranging from 20 to 150 h). During the high energy ball milling, powders are repeatedly welded, fractured, and rewelded. As a result, the powders are alloyed mechanically and the shapes are changed significantly. Usually, the resultant powder shape is irregular. The mechanism of high energy ball milling is depicted in Fig. 6(b). The high energy ball milled powder does not flow through the hall flowmeter
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funnel. Therefore, the flowability of high energy ball milled powder cannot be measured. As the powder preparation of apparent density uses the same flowability measurement setup, the apparent density of high energy ball milled powder also cannot be measured. Additive manufacturing offers a new opportunity to fabricate MMCs. It is easy to achieve the atomic bonding of the additions and metal matrix. To achieve a uniform dispersion of nanoparticles reinforced MMNCs, the mixing of additions and metal powders before the additive manufacturing process is crucial. Nanoparticles can be added into the alloy ingot
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before the powder atomization
[32]
. However, agglomeration of nanoparticles would occur due
to the nature of nanosized particle. There are two reasons to explain the agglomeration of nanoparticle: (i) the surface energy of nanosized particle is very high. To decrease the high surface energy, the nanosized particle would agglomerate spontaneously; (ii) Van der Walls’ force and Coulomb's force are the driven force for agglomeration. In addition, the density difference of nanoparticles and metal matrix also poses as processing challenge. Some researchers attempted to print PM 2000 ODS steel and MA 956 ODS steel using high energy ball milled powder in irregular shape by selective laser melting. However, agglomerations still can be observed on the top surface and the site of lack of fusion in the printed samples [33-36]. One of the reasons is the poor powder quality. Therefore, for additive manufacturing, the
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additions must be uniformly distributed on the surface of metal powder, and the spherical shape of the powders cannot be significantly altered after the mixing process.
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3.3. SLM using original 316L powder and Y2O3 nanoparticles mixed powder
Fig. 7 provides the unetched structure of the SLM printed samples using the different powders. It can be seen that with the increasing addition of Y 2O3 nanoparticles, porosity and
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lack of fusion can be observed in the printed samples. It is noticed that the black dots in Fig . 7(c) is the resultant of agglomeration of Y 2O3 nanoparticles as illustrated in Fig. 7(d). EDS
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mapping of the agglomeration of Y 2O3 is presented in Fig. 8. The main reason of the formation of the Y2O3 agglomeration is because of the poor wettability between 316L and Y 2O3
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nanoparticles. Another reason could be attributed to the ultra-high temperature of the melt pool during SLM. Simulation studies show the highest temperature of the melt pool is higher than 3000 °C and the temperature near the keyhole can even reach 5000 °C[37, 38], which is much higher than the melting point of Y 2O3 (the melting point of Y2O3 is only 2425 °C). Exposure in such high temperature, the Y 2O3 nanoparticles can be sintered or melted in such high
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temperature. Therefore, Y 2O3 agglomeration is formed in the SLM samples. It can be seen from Fig. 7(d) that the Y2O3 agglomeration presents a polycrystal morphology which further indicates the agglomeration is a result of the sintering or melting of Y 2O3 nanoparticles.
The density of SLM printed sample using the original 316L powder is 7.961 g/cm 3 and the relative density is 99.5%. The relative density was measured with the respect to 316L bulk density of 8.0 g/cm3. The density of SLM printed sample using ball milled 316L-0.3Y2O3 powder is 7.955 g/cm 3, which is 99.6% of the theoretical density of 316L-0.3Y2O3. Likewise,
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the density of 316L-1.0Y2O3 is 7.888 g/cm3, which is 99.2% of the theoretical density of 316L-1.0Y2O3. The densities of the SLM printed samples were higher than 99% which is acceptable. However, further work needs to be done to better disperse the Y 2O3 nanoparticles in the 316L matrix.
4. Conclusions (1) Low energy ball milling of 0.3 wt% Y 2O3 nanoparticles and 316L powder for 7 h with ball-to-powder ratio of 1:1 resulted in uniform distribution of the nanoparticles on the surface
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of 316L powder. (2) However, the process of low energy ball milling itself can result in a rough surface of the 316L powder. Therefore, the flowability, apparent density and tap density are decreased slightly.
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(3) The addition of Y2O3 nanoparticles further decreases the flowability, apparent density and
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tap density of the mixed powder.
(4) The mixture of 316L powder and Y 2O3 nanoparticles can be successfully used for SLM. The
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relative density of SLM 316L-Y2O3 is measured at 99.5%. However, Y 2O3 agglomerations were
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observed which is due to the poor wettability between 316L and Y2O3.
Acknowledgements
The work was supported by A*STAR Industrial Additive Manufacturing Program: The A*STAR Additive Manufacturing Centre (AMC) Initiative: Work Package 1 (High
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Temperature Materials Development for 3D Additive Manufacturing, Grant No. 1426800088). W. Zhai and W. Zhou would like to acknowledge the financial support from Nanyang Technological University.
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Fig. 1. SEM images of (a) 316L powder and (b) Y2O3 nanoparticles.
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Fig. 2. (a) Distribution of Y 2O3 on the surface of 316L powder after mixing for 1 h and (b) EDS
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result of the area pointed by the arrow in (a) showing Y 2O3 agglomeration.
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Fig. 3. SEM images of (a) 0.3% Y2O3 after mixing for 7 h, (b) higher magnification image
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showing the distribution of 0.3% Y 2O3 on the surface of 316L, (c) 1.0% Y2O3 after mixing for 7 h and (d) higher magnification image showing the distribution of 1.0% Y 2O3 on the surface of
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316L.
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Fig. 4. Powder properties of original 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0% Y2O3: (a) flowability; (b) apparent density; (c) tap density.
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Fig. 5. Surface morphologies of (a) original 316L powder and (b) ball milled 316L powder.
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Fig. 6. Mechanism of powder mixing: (a) low energy ball milling (our method); (b) high
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energy ball milling.
Fig. 7. SLM printed samples using (a) original 316L powder, (b) ball milled 316L-0.3% Y2O3 16
powder. The image insert shows the presence of Y2O3 particles; (c) ball milled 316L-1.0% Y2O3 powder, (d) SEM image of Y 2O3 agglomeration in SLM 316L-1.0% Y2O3 sample.
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Fig. 8. EDS mapping of the Y2O3 agglomeration in SLM 316L-1.0% Y2O3.
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Table list: Table 1 Parameters used in high energy ball milling and our method. Low energy ball
Parameter
High energy ball milling
Ball-to-powder ratio
5:1 - 30:1
1:1
Mixing speed (rpm)
200 - 600
90
Mixing time (h)
20 - 150
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Table 2 Powder properties of 316L, ball milled 316L, 316L-0.3% Y2O3 and 316L-1.0 %Y2O3. 316L
Ball milled 316L 316L-0.3%Y2O3 316L-1.0%Y2O3
Flow rate (s/50 g)
15.92
16.79
17.07
20.44
Apparent density (g/cm 3)
4.386
4.168
4.202
4.206
Tap density (g/cm3)
5.025
4.926
4.854
4.831
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Powder
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