Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Journal Pre-proof Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Marie Le Dantec, Lucas Güniat, Matthias Leistner, Renato Figi, Davide Bleiner, Marc Leparoux, Patrik Hoffmann PII:

S0032-5910(19)31011-3

DOI:

https://doi.org/10.1016/j.powtec.2019.11.052

Reference:

PTEC 14934

To appear in:

Powder Technology

Received date:

13 July 2019

Revised date:

9 November 2019

Accepted date:

18 November 2019

Please cite this article as: M. Le Dantec, L. Güniat, M. Leistner, et al., Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon, Powder Technology(2019), https://doi.org/10.1016/ j.powtec.2019.11.052

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier.

Journal Pre-proof Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Marie Le Danteca*, Lucas Güniata,1, Matthias Leistnera, Renato Figib, Davide Bleinerb, Marc Leparouxa, Patrik Hoffmanna

*Corresponding author: Marie Le Dantec, +41 76 632 08 81, [email protected]

a

Laboratory for Advanced Materials Processing, Empa (Swiss Federal Laboratories for Materials Science Laboratory for Advanced Analytical Technologies, Empa (Swiss Federal Laboratories for Materials

oo

b

f

and Technology), Feuerwerkerstrasse, 39, CH-3602 Thun, Switzerland Science and Technology), Überlandstrasse, 129, CH-8600 Dübendorf, Switzerland

pr

Abstract

Powder additive manufacturing (AM) of materials is affected by the oxygen present in the raw material,

e-

especially if they possess a native oxide. The latter influences the properties of the laser molten track. We carried out a study on the interactions between silicon fine powder (<3 µm) pellets and a pulsed

Pr

Nd:YAG laser (wavelength 1064 nm), with the aim of preparing AM of such material. The influence of oxygen content in the initial silicon powders on the continuous tracks morphology was investigated. It was found that the initial oxygen content of the processed powders must be lower than 0.1 wt% to

al

produce a smooth silicon melted track. This result is explained as due to the formation of gaseous

rn

silicon monoxide (SiO) by reaction between silicon and its native oxide during the process.

Jo u

Keywords

Laser melting, fine silicon powders, silicon oxide, oxygen, density, track morphology, additive manufacturing

1. Introduction

Additive manufacturing (AM) has been growing attention for the past few years. It is especially popular in the fields of aerospace, medical applications and manufacturing industries, since it provides advantages over competing manufacturing technologies, such as a high degree of customization, efficient material utilization, cost saving, tailored design of complex parts and creation of lightweight pieces[1–5].

Powder-bed AM presents various challenges, such as the understanding of the role of oxygen during the powder melting process. Indeed, oxygen is known to influence the melt pool and the final AM parts through diverse mechanisms explained in the following. Oxides were mostly found to disturb the wetting of the molten metal on the solid part, which leads to the so-called balling effect[6–8]. They can also change the melt pool dynamics[9] and the laser absorptivity[10], as well as trigger the formation

Journal Pre-proof of oxide inclusions, and defects such as porosity in the final parts[11,12]. Louvis et al.[13] demonstrated that oxide formation was the main responsible for material defects after Selective Laser Melting (SLM) of aluminium in their experiments. Indeed, the oxide formed on top of the melt pool was vaporized by the laser, and the oxide at the bottom of the melt pool was most likely broken by means of the forces stirring the melt pool induced by the Marangoni effect. However, the oxide formed on the sides of the melt pool remained, causing the molten metal not to be able to wet the previously solidified material. This phenomenon led to porosity and defects. Oxygen content in the raw powders also has an influence on the final parts properties. For example, incorporation of oxygen from a raw powder material containing more than 0.33 wt% oxygen, the tensile ductility of Ti-6Al-4V parts drops significantly[14,15]. Dubenskaia et al.[16] demonstrated that oxygen led to an increase of the melt pool temperature in SLM of stainless steel, mostly due to exothermal oxidation reaction and increase of

oo

f

absorptivity brought by the presence of oxide.

The silicon and its oxide differ with the aforementioned metals as silicon oxide in contact with the

pr

elemental silicon decomposes[17,18] at a temperature of about 1000°C, well below the melting temperature of silicon (1414°C). Therefore its impact on the melt pool might be different in our case.

e-

Silicon powder native oxide is well-known for hindering silicon melting properties. Seo et al.[19] have attempted melting Si powders recycled from kerf loss in a graphite crucible. They observed the

Pr

formation of a slag composed of silicon, oxygen and carbon, with porosity. Success was reached when they attempted the melting in a reducing atmosphere (H 2 + Ar) and opted for directional solidification to push the gas bubble upwards. Eyer et al.[20] tried to melt a Si powder layer by Zone Melting

al

Recrystallization (ZMR) and noticed that uniform melting was prevented by Silicon monoxide (SiO)

rn

evaporation.

Jo u

However, limited literature is available concerning additive manufacturing of materials for which the decomposition and reaction of their native oxide produces a gaseous compound. Our own successful laser direct metal deposition of silicon pillars from blown sub 10 micrometer diameter silicon powder is a recent breakthrough that could be one solution to the difficulties presented in this paper[21,22]. In this work, we studied the laser-silicon powder interactions in a powder-bed fashion. Pellets of silicon powders of various oxygen content were realized and molten tracks were produced with a millisecond pulse duration pulsed near infrared fiber Laser under N2 and O2 free controlled atmosphere. A very fine powder (< 3 μm) was used to increase the surface-to-volume ratio and anticipate the use of this powder in AM. We show that oxidation has to be avoided in order to obtain smooth silicon tracks. We also propose an explanation for the mechanisms as indicated by the obtained experimental results.

2. Materials and Methods 2.1. Material The samples used in this study were prepared from commercial silicon powders (Dalian King Choice Non-Ferrous Metals Products Co., Ltd) with 99.99% purity. The supplier gave the following

Journal Pre-proof particle size distribution: D10=1.535 um, D50=2.966 um, D90=4.817 um. A laser diffraction method CILAS 1090 (Cilas, France) was used to measure the particle size distribution after dispersion in water. Figure 1 shows the particle size measurement performed on the silicon powders used in this study. It shows a broad particle size distribution, centered around 3 um, and a negative skew. The collected data by means of laser particle size analysis suggested the following distribution: D10=0.53 μm, D50=3.09 μm and D90=10.58 μm. The particle morphology was observed by scanning electron microscopy (S-4800 Hitachi, Japan) (see Figure 2, a). We observed irregular silicon chunks up to sizes of 5 μm, surrounded by micron to sub-micron sized particles. The broad particle size distribution facilitated having pellets of high density because the small particles occupied the voids

2.1.2.

Pellet preparation

oo

Sample preparation

pr

2.1.1.

f

between the larger ones.

The silicon powder was pressed into pellets (Figure 2, b) of 10 mm diameter and about 920 μm height

e-

with 140 bar held for 45 s by hydraulic pressing in a stainless steel die. This process led to pellets of an estimated (mass/volume) pellet density of about 60%, compared to the density of monocrystalline

Pr

silicon (2.33 g/cm3). The powders were pressed into pellets in order to avoid the raw powders to fly away during the argon flushing procedure, which required several phases of vacuuming (see

Deoxidation

rn

2.1.3.

al

paragraph 2.2).

The deoxidation process was carried out according to the following procedure. A particle suspension

Jo u

was formed using 5g of as-received powder and about 600 mL of water. This suspension was ultrasonicated for 30 min to deagglomerate. 20 mL of concentrated hydrofluoric acid (49%HF) was then added to the suspension. The solution was again ultrasonicated for 30 min. The suspension was then vacuum filtered through a 0.3 µm paper filter to recover the deoxidized powder as wet filter cake. The wet filter cake was dried at 50°C by continuous pumping vacuum over night at a pressure of 15 mbar. After this deoxidation and drying process, the powders were thermosealed in aluminium polyethylene bags under argon, and placed into a desiccator to avoid moisture uptake.

2.2. Laser, chamber and in-situ measurements A CF flanges gas tight vacuum chamber dedicated to the study of traces of reactive gases (water vapor, oxygen) and pressure influences on laser-material interaction processes such as pulsed laser welding, equipped with different laser sources including a millisecond pulsed near infrared fiber laser (IPG Photonics, model YLR-150/1500-QCW-MM-AC) was used to perform the experiments (Figure 3). The laser beam profile was top hat and the beam diameter, 600 μm. The laser head was fixed on a vertical position above the window port of the chamber at 90° angle from the pellet surface. The sample was mounted on an xyz moving stage inside the chamber. The chamber was filled with 1 bar Ar after

Journal Pre-proof evacuation and several purging cycles to reach an atmosphere containing less than 100 ppm of water measured by a humidity sensor (Pura Sensor 0-100 ppm, Michell Instruments, UK). All melted tracks presented in this article were performed at a pulse energy of 0.3 J, a pulse duration of 1 ms, and a scanning speed of 1 mm/s. With the repetition rate of 30 Hz this resulted in an overlap of consecutive pulses of 98 %. In-situ observations of the plume created during laser processing of silicon powder pellets were carried out with a high speed camera Videal MotionPro at 20000 frames/s and 0.1 μs exposure. The camera was placed at 90° angle with respect to the laser beam in order to observe the plume from the side (Figure 4).

2.3. Sample analysis Oxygen measurements

f

2.3.1.

oo

The oxygen content in the silicon powder measurements were performed by combustion and infrared analysis with a LECO TC-500 after developing a reproducible and quantitative reliable oxygen content measurement process. The latter consisted in placing dried powder in a tin capsule with nickel chips,

pr

and melting in a graphite crucible in an electrode furnace at about 3000°C. The oxygen was detected

Estimation of the oxygen weight percent within the material

Pr

2.3.2.

e-

as CO2 after reaction with the graphite crucible by infrared quantitative analysis.

We calculated the oxygen weight percent within the material by calculating the mass of perfect silicon spheres with Radius R and a 2 nm thick dense SiO2 layer around. The latter thickness is an upper value,

al

because the natural oxide layer is often thinner and the thickness measurements depend on the measuring method. For reference, the surface oxide on 3 um diameter monodisperse Si powder

2.3.3.

Jo u

rn

particles with the 2nm native oxide layer corresponds to 0.24 wt% oxygen (see Figure 5).

Sample observation and EDX measurements

Sample track morphology was observed by optical microscopy (ZEISS Axioplan, Germany) with a magnification of 30x. The EDX analysis was performed using an EDX detector (EDAX, USA) mounted in a high resolution scanning electron microscope S-4800 (Hitachi, Japan). The pellets were attached to the sample holder with a carbon tape. The measurements were made with an acceleration voltage of 15 kV and a beam current of 10 μA.

1. Results

1.1. Track morphology Silicon powder pellets were produced with the as-received and two different deoxidized silicon powders. Measurements by combustion and infrared analysis showed that an oxygen content of 2.6±0.1, 0.4±0.1 and less than 0.1 wt% were respectively contained in these 3 different powders. The morphology of the laser melted silicon tracks was sensitively dependent on the raw powder oxygen

Journal Pre-proof content, as shown on the optical microscope pictures, in Figure 6. Laser processing of pellets made from powders containing 2.6 wt% oxygen resulted in balling, spattering, loss of material and traces of oxide deposition (Figure 6, a). Energy-dispersive X-ray Spectroscopy (EDX) analysis of the oxide trace revealed the presence of a high amount of Oxygen besides the Silicon and also some Carbon (Figure 7). The presented weight % and atomic % values are only correcting the X-ray creation probability, not the real composition due to higher electron penetration depth than the redeposit layer thickness. The real oxygen content of the redeposit layer might be underestimated. After deoxidation of the powder down to 0.4 wt% of oxygen, no more oxide redeposit traces were observed, and only few spattering occurred (Figure 6, b). Finally with powder deoxidized to less than 0.1 wt%, a smooth shiny track was obtained (Figure 6, c). The irregularities in the tracks were due to cracks already present in the pellet after pressing, because it was challenging to take the pellets out of the die after compacting without

oo

f

breaking.

pr

1.2. Plume analysis

In-situ observations with a high speed camera on a single laser shot of an energy per pulse of 0.3 J are

e-

reported on Figure 8 for silicon powder pellets of two different oxygen contents. The plume formed during laser processing of oxidized silicon powder pellets (2.6 wt% O in Figure 8, a) expands as the material evaporates. Numerous droplets of liquid are spattered out of the melt pool as seen as white

Pr

500 μm

round regions in the video picture sequence. Furthermore the light emission of the hot gases and the turbulent vortex like movements are clearly seen. On the contrary, in the case of powder deoxidized to duration.

Jo u

2. Discussion

rn

al

less than 0.1 wt% oxygen, the plume is much smaller and disappears quickly after the end of the pulse

2.1. Oxide deposit and spattering effect The 300 micrometer large side deposits on both sides of the laser track shown in figure 6(a) are probably silicon monoxide (SiO) deposits. Higher oxidation to SiO2 , especially on the surface posterior exposed to ambient, is highly probable, but due to the processing under pure Argon and the relatively fast condensation time the deposit is most likely SiO. The spatter droplets observed in figure 6 and strong spattering observed by high speed imaging (see figure 8a) during laser processing of the “oxidized” powder suggest that a gas is formed during the process and its formation and expansion by heating up ejects molten particles from the melt pool. Both observations can be attributed to the formation of SiO gas produced from the surface oxide on the silicon particles. Silicon Monoxide (SiO) is probably produced, evaporated and heated during laser irradiation. It partially condenses onto the cold substrate on the sides of the tracks transported downwards by the plume induced vortex like flow seen in figure 8a in the time snapshots from 711ns, 1011ns and 1711ns after the beginning of the pulse. As the temperature reached during laser processing is at least equal to the melting temperature of silicon (1414°C), and that SiO is formed at very high rates at temperatures above 1150°C, as stated

Journal Pre-proof by Moore et al.[17], where 200nm thick SiO2 films on Silicon where transformed into SiO within seconds, SiO is certainly produced and released much faster in our case. It would be released from all the oxidized surfaces of the particles in powder layers ahead and also below the surface of the melt pool and would eject molten droplets as spatter due to overpressure during heating. Each powder particle is probably formed of a pure Si core and a SiO2 shell. The formation of SiO gas from liquid silicon and its oxide at high temperatures has been well-established, according to the following equation[23]: Si(l) + SiO2(s) -> 2 SiO(g) This reaction start to release significant amounts of products from about 1’000°C[24,25], and its evaporation rate increases with temperature[26]. Investigations of liquid silicon drops on solid SiO2 for wetting and contact angle measurements revealed Si droplet vibration from the surface due to SiO gas

oo

f

formation and trapping[23,27].

pr

It is very unlikely that adsoption of water on the oxidized silicon particles is the cause of gas release and spattering during laser processing. Indeed, additional experiments were made with silicon powder

e-

containing 2.6 wt% oxygen, where the powder was dried in a furnace for several hours before being pressed. The morphology of the track did not show any improvement. The decrease in surface tension

Pr

due to increase in local oxygen partial pressure[28] could also enhance the spattering effect

2.2. Balling effect

al

(Marangoni effect).

rn

After processing, oxidation traces were also observed at the bottom of the track, on and in between the solidified droplets, as shown in figure 6a. Indeed, the important loss of material as spattering

Jo u

(Figure 6) prevents the rest of the material to percolate. Therefore, it tends to ball under the effect of surface tension. Moreover, as the wetting angle of molten Si on SiO2 at 1693 K has been reported to be between 85 and 90°[29–32], liquid silicon is expected not to wet its oxide very well, leading to balling. The bottom of the track also has a rough surface as the pellet is made out of powder onto which oxide might condense, which enhances the phenomenon. In this study, the oxygen partial pressure in the chamber was kept constant at a low level of less than 42 ppm, but the local oxygen pressure can be higher due to SiO gas formation. It has been reported in the literature for diverse materials that an increasing oxygen partial pressure in the atmosphere led to balling. Li et al.[33] showed that an increase in the oxygen partial pressure from 0.1% to 1% in the atmosphere in the chamber led to balling for stainless steel and nickel powders. They attributed this effect to oxidation of the molten pool and consequently a dewetting of the molten material on the oxidized substrate.

3. Conclusions The morphology of laser melted Si tracks made on powder pellets of three different oxygen contents were compared. The results showed that the raw powder must contain less than 0.1 wt% of oxygen to

Journal Pre-proof produce smooth tracks. In-situ observations of the plume and EDX analysis of the deposit around tracks made from oxidized powder allowed concluding that the presence of oxide led to a reaction between Si and SiOx giving SiO gas that was the main responsible for material depletion and oxidation traces due to gas release and condensation. Reducing the particle size to less than 5 µm also increases the surface to volume ratio, increasing the proportion of oxygen in the raw powders. This fact will constitute an additional challenge to the transportation problems that already arise with the use of sub 10 micrometer diameter particle sized powders. The mechanism behind the plume formation by evaporation of matter and circulation can be compared to the plume formation observed for pulsed laser welding but needs further investigations for further conclusions. This study is of impact to any research on laser powder bed fusion of powders regarding materials that are easily prone to oxidation. The obtained results are of further guidance to Selective Laser Melting

oo

f

(SLM) of a silicon track on a wafer.

pr

4. Acknowledgments

The authors would like to thank Peter Ramseier, Hans-Rudolf Sieber and Oliver Nagel for their help

e-

and participation to the project.

Pr

This research did not receive any specific grant from funding agencies in the public, commercial, or

rn

5. Short Biographies

al

not-for-profit sectors.

Marie Le Dantec

Jo u

In 2013, Marie Le Dantec obtained her engineering diploma in micro and nanotechnologies for integrated systems from Phelma-Grenoble INP, France. This diploma is a joint degree with Politecnico di Torino, Italy and EPFL, Lausanne. She completed her PhD on additive fabrication of silicon pillars on silicon wafers by a direct energy deposition additive manufacturing technology at the laboratory of advanced materials at EMPA Thun, Switzerland in 2018 (enrolled at EPFL, Lausanne, Switzerland). Her PhD focuses on crack formation and epitaxial growth of those silicon pillars from the monocrystalline wafer during the process.

Lucas Güniat Lucas Güniat obtained his Master from EPFL in Materials Science and Engineering in 2016. He is now a PhD student at EPFL working in the laboratory of semiconductor materials (LMSC - Prof. Anna Fontcuberta I Morral) on the kinetics of growth of III/V Semiconductor nanowires.

Matthias Leistner

Journal Pre-proof Matthias Leistner received the Diploma degree in chemistry from Technical University Dresden, Dresden, Germany, in 2007. In 2007, he became a Staff Member of the Process Monitoring Group, Fraunhofer Institute for Material and Beam Technology, Dresden. In 2014, he joined Empa as post-doctoral fellow to work on laser welding.

Marc Leparoux Marc Leparoux is head of the group Nanoparticles & Nanocomposites at the Laboratory for Advanced Materials Processing at Empa. He received his Magistère in Materials Science and a DEA in Solid Chemistry in 1992 at the University of Rennes and his Ph.D. in Physical Chemistry in 1995 from the University of Orléans, France. He then worked on high temperature process monitoring at the Fraunhofer

oo

f

Institute for Material and Beam Technology in Dresden (Germany). He joined Empa in 2001 where he develops the activities on thermal plasma synthesis of nanoparticles. Now he is focusing on powder

pr

processing including laser direct deposition and on metal matrix composites.

e-

Renato Figi

Renato Figi is an analytical chemist at the laboratory of advanced analytical technologies at EMPA,

Pr

Dübendorf, Switzerland. He is a team leader in the field of analytical chemistry since 1987. He special-

al

izes in wet chemistry methods, sample preparation, ICP-OES, ICP-MS, XRF, IC, ISE, Photometry.

rn

Davide Bleiner

PD Dr. habil. Davide Bleiner got is PhD on Laser Ablation Chemistry at the ETH Zurich in 2002. From

Jo u

2002 till 2005 he was postdoc at the Empa, while 2005-2007 was postdoc at the University of Antwerp in Belgium. In 2007 joined the ETH Zurich in the Dept. of Mechanical Engineering, on laser plasma for Extreme UV lithography. In 2011 was appointed SNF professor at the University of Bern, working on plasma X-ray lasers. Since 2014 he is head of the Advanced Analytical Technologies at the Empa Dübendorf. He teaches at the University of Zurich classes for Laser Ablation Chemistry.

Patrik Hoffmann Patrik Hoffmann a chemist from University of Karlsruhe carried out his PhD thesis at EPFL, in Lausanne in Switzerland. After a post doctoral fellowship at IBM ARC, San Jose (USA), he returned to EPFL as project leader before joining Gramm Technik (Germany) an electrochemical company as dental section manager. From 1997 until 2009 he carried out research on Laser Micro-Processing and Focused Electron Beam Induced Processing at EPFL. Since 1997 he is teaching in micro-engineering and materials sciences at EPFL. In April 2009 he became head of the Laboratory of Advanced Materials Processing at Empa where he develops activities on additive manufacturing. Declaration of interests

Journal Pre-proof

☒ 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.

6. References

L. Chougrani, J.P. Pernot, P. Véron, S. Abed, Lattice structure lightweight triangulation for additive manufacturing, CAD Comput. Aided Des. 90 (2017) 95–104. doi:10.1016/j.cad.2017.05.016.

[2]

L. Yang, O. Harrysson, D. Cormier, H. West, H. Gong, B. Stucker, Additive Manufacturing of Metal Cellular Structures: Design and Fabrication, Jom. 67 (2015) 608–615. doi:10.1007/s11837015-1322-y.

[3]

C. Klahn, B. Leutenecker, M. Meboldt, Design strategies for the process of additive manufacturing, Procedia CIRP. 36 (2015) 230–235. doi:10.1016/j.procir.2015.01.082.

[4]

C. Emmelmann, M. Petersen, J. Kranz, E. Wycisk, Bionic lightweight design by laser additive manufacturing (LAM) for aircraft industry, SPIE Eco-Photonics 2011 Sustain. Des. Manuf. Eng. Work. Educ. a Green Futur. 8065 (2011) 80650L. doi:10.1117/12.898525.

[5]

J. Parthasarathy, B. Starly, S. Raman, A design for the additive manufacture of functionally graded porous structures with tailored mechanical properties for biomedical applications, J. Manuf. Process. 13 (2011) 160–170. doi:10.1016/j.jmapro.2011.01.004.

[6]

S. Das, Physical Aspects of Process Control in Selective Laser Sintering of Metals, Adv. Eng. Mater. 5 (2003) 701–712. doi:10.1002/adem.200310099.

[7]

S. Zhang, Q.S. Wei, G.K. Lin, X. Zhao, Y.S. Shi, Effects of Powder Characteristics on Selective Laser Melting of 316L Stainless Steel Powder, Adv. Mater. Res. 189–193 (2011) 3664–3667. doi:10.4028/www.scientific.net/AMR.189-193.3664.

[8]

J. Liu, D. dong Gu, H. yu Chen, D. hua Dai, H. Zhang, Influence of substrate surface morphology on wetting behavior of tracks during selective laser melting of aluminum-based alloys, J. Zhejiang Univ. Sci. A. 19 (2018) 111–121. doi:10.1631/jzus.A1700599.

[9]

S. Ozawa, K. Morohoshi, T. Hibiya, Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation, 54 (2014) 2097–2103. doi: 10.2355/isijinternational.54.2097

[10]

L. He, H. Zhao, W. Niu, Understanding the effect of oxygen on weld pool and keyhole in laser beam welding, J. Laser Appl. 30 (2018) 012003. doi: 10.2351/1.5017703

[11]

L. Thijs, J. Van Humbeeck, K. Kempen, E. Yasa, J.P. Kruth, Investigation on the inclusions in maraging steel produced by SLM, in book: Innovative Developments in Virtual and Physical Prototyping, (2011) 297-304. doi: 10.1201/b11341-48

[12]

X. Lou, P.L. Andresen, R.B. Rebak, Oxide inclusions in laser additive manufactured stainless steel and their effects on impact toughness and stress corrosion cracking behavior, J. Nucl. Mater. 499 (2018) 182–190. doi:10.1016/j.jnucmat.2017.11.036.

[13]

E. Louvis, P. Fox, C.J. Sutcliffe, Selective laser melting of aluminium components, J. Mater. Process. Technol. 211 (2011) 275–284. doi:10.1016/j.jmatprotec.2010.09.019.

[14]

M. Yan, W. Xu, M.S. Dargusch, H.P. Tang, M. Brandt, M. Qian, Review of effect of oxygen on room temperature ductility of titanium and titanium alloys, Powder Metall. 57 (2014) 251–257. doi:10.1179/1743290114Y.0000000108.

[15]

H.P. Tang, M. Qian, N. Liu, X.Z. Zhang, G.Y. Yang, J. Wang, Effect of Powder Reuse Times on Additive Manufacturing of Ti-6Al-4V by Selective Electron Beam Melting, Jom. 67 (2015) 555– 563. doi:10.1007/s11837-015-1300-4.

Jo u

rn

al

Pr

e-

pr

oo

f

[1]

Journal Pre-proof M. Doubenskaia, D. Kotoban, I. Zhirnov, Study of oxygen effect on the melting pool temperature during selective laser melting, Mech. Ind. 17 (2016) 1–11. doi:10.1051/meca/2016067.

[17]

J.C. Moore, J.L. Skrobiszewski, A.A. Baski, Sublimation behavior of SiO2 from low- and highindex silicon surfaces, J. Vac. Sci. Technol. A. 25 (2007) 812. doi:10.1116/1.2748798.

[18]

Y. Enta, T. Nagai, T. Yoshida, N. Ujiie, H. Nakazawa, Decomposition kinetics of silicon oxide layers on silicon substrates during annealing in vacuum, J. Appl. Phys. 114 (2013) 114104. doi:10.1063/1.4821882.

[19]

K.H. Seo, B.H. Kang, K.Y. Kim, Fast Melting and Refining of Recycled Silicon Powders from the Wafer Back Grinding Process for Solar Cell Feedstock, Mater. Sci. Forum. 706–712 (2012) 859– 864. doi:10.4028/www.scientific.net/MSF.706-709.859.

[20]

A. Eyer, N. Schillinger, S. Schelb, A. Räuber, J. G. Grabmaier, Silicon sheets grown from powder layers by a zone melting process, J. Cryst. Growth. 82 (1987) 151–154. doi:10.1016/00220248(87)90179-5.

[21]

M. Le Dantec, M. Abdulstaar, P. Hoffmann, M. Leistner, M. Leparoux, Additive Manufacturing of Semiconductor Silicon on Silicon Using Direct Laser Melting, in: Ind. Addit. Manuf. - Proc. Addit. Manuf. Prod. Appl. - AMPA2017, 2017: pp. 104–116. doi:10.1007/978-3-319-66866-6_10.

[22]

M. Le Dantec, Additive Fabrication of Silicon Pillars on Monocrystalline Silicon by Direct Laser Melting, 2018. doi:10.5075/epfl-thesis-8827.

[23]

H. Fujii, M. Yamamoto, S. Hara, K. Nogi, Effect of gas evolution at solid-liquid interface on contact angle between liquid Si and SiO 2, J. Mater. Sci. 34 (1999) 3165–3168. doi:10.1023/A:1004673605025.

[24]

G. Hass, Properties, oxidation, decomposition, and applications of silicon monoxide, (1950).

[25]

S. V. Komarov, D. V. Kuznetsov, V. V. Levina, M. Hirasawa, Formation of SiO and related Si-based materials through carbothermic reduction Of silica-containing slag, Mater. Trans. 46 (2005) 827–834. doi:10.2320/matertrans.46.827.

[26]

S. Wetzel, A. Pucci, H.-P. Gail, Vapor Pressure and Evaporation Coefficient Measurements at Elevated Temperatures with a Knudsen Cell and a Quartz Crystal Microbalance: New Data for SiO, J. Chem. Eng. 57 (2012) 1594–1601. doi:10.1021/je300199a.

[27]

H. Kanai, S. Sugihara, H. Yamaguchi, T. Uchimaru, N. Obata, T. Kikuchi, F. Kimura, M. Ichinokura, Wetting and reaction between Si droplet and SiO 2 substrate, J. Mater. Sci. 42 (2007) 9529– 9535. doi:10.1007/s10853-007-2090-z.

[28]

Z.F. Yuan, K. Mukai, W.L. Huang, Surface Tension and Its Temperature Coefficient of Molten Silicon at Different Oxygen Potentials, Langmuir. 18 (2002) 2054–2062. doi:10.1021/la0112920.

[29]

Z. Yuan, W.L. Huang, K. Mukai, Wettability and reactivity of molten silicon with various substrates, Appl. Phys. A Mater. Sci. Process. 78 (2004) 617–622. doi:10.1007/s00339-002-20018.

[30]

K. Mukai, Z. Yuan, Wettability of ceramics with molten silicon at temperatures ranging from 1693 to 1773 K, Mater. Trans. JIM. 41 (2000) 338–345. doi:10.2320/matertrans1989.41.338.

[31]

J. Li, H. Hausner, Wetting and adhesion in liquid silicon/ceramic systems, Mater. Lett. 14 (1992) 329–332. doi:10.1016/0167-577X(92)90047-N.

[32]

B. Drevet, N. Eustathopoulos, Wetting of ceramics by molten silicon and silicon alloys: A review, J. Mater. Sci. 47 (2012) 8247–8260. doi:10.1007/s10853-012-6663-0.

[33]

R. Li, J. Liu, Y. Shi, L. Wang, W. Jiang, Balling behavior of stainless steel and nickel powder during selective laser melting process, Int. J. Adv. Manuf. Technol. 59 (2012) 1025–1035. doi:10.1007/s00170-011-3566-1.

Jo u

rn

al

Pr

e-

pr

oo

f

[16]

Jo u

rn

al

Pr

e-

pr

oo

f

Journal Pre-proof

Journal Pre-proof Figure captions

Figure 1 Particle size measurement in volume of the silicon powder used in the process.

Figure 2 (a) SEM picture of Si-powders as-received from the manufacturer, (b) photo of a silicon powder pellet of diameter 10 mm, thickness 950 μm and density of about 60% obtained by hydraulic pressing of the initial powders, and (c) schematic of the process under investigation: a laser beam is moved on a

oo

f

straight line across the pellet to form a Si molten track.

Figure 3

pr

Dedicated setup for laser-material interaction real time observation.

e-

Figure 4

Schematic of the in-situ observation of the plume with a high speed camera. The plume is observed on

Pr

the side at a 90° angle with respect to the laser beam.

al

Figure 5

Theoretical estimation of total Oxygen content in monodispersed naturally oxidized silicon powders in

Figure 6

Jo u

surrounding the Si particles.

rn

function of the particle radius R. It is assumed that 2 nm of dense SiO2 layer is homogeneously

Track morphology for different oxygen contents in initial silicon powders after laser processing with an Laser energy of 0.3 J, pulse duration 1 ms and scanning speed 1 mm/s (a) 2.6 wt% (b) 0.4 wt% (c) less than 0.1 wt%. Higher oxygen concentration in the initial powder lead to balling, spattering, material loss and oxide redeposit.

Figure 7 (a) SEM picture of the track made from powder containing 2.6 wt% oxygen. (b) EDX spectrum of the oxide deposit with Weight % and Atom % values. The analysis is carried out in the region pointed by the arrow, besides the Laser melted track.

Figure 8

Journal Pre-proof Picture sequences of in-situ high speed camera observations of single laser shots on silicon powder pellets of different oxygen contents: (a) 2.6 wt% and (b) less than 0.1 wt%, otherwise identical

Jo u

rn

al

Pr

e-

pr

oo

f

conditions, pulse energy, pulse duration, spot size, inert gas pressure and composition etc.

Journal Pre-proof The laser processing of fine powders strongly depends on its initial oxygen content



Smooth melted tracks are obtained from Si powders with less than 0.1 wt% oxygen



The formation of silicon monoxide during the process led to lower quality tracks

Jo u

rn

al

Pr

e-

pr

oo

f



Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8