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Powder particle movement during Powder Bed Fusion
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Procedia Manufacturing 36 (2019) 26–32 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
17th Nordic Laser Material Processing Conference (NOLAMP17), 27 – 29 August 2019 17th Nordic Laser Material Processing Conference (NOLAMP17), 27 – 29 August 2019
Powder particle movement during Powder Bed Fusion Powder particle movement during Powder Bed Fusion
Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June Joerg Volpp* 2017, Vigo (Pontevedra), Joerg Volpp* Spain Department of Applied Physics and Mechanical Engineering, Luleå University of Technology, 971 87 Luleå, Sweden Department of Applied Physics and Mechanical Engineering, Luleå University of Technology, 971 87 Luleå, Sweden
Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency
Abstract Abstract Powder Bed Fusion is a widely used technique to produce complex parts with different materials. In principle, a pre-placed powder layer is locally melted a laserused and thereby to thea,* previous tracks and This technique offers high flexibility at Powder Bed Fusion is abywidely technique complex with different materials. Inbprinciple, a pre-placed powder a to produce b layers. A.beam Santana , P. fused Afonso , parts A. Zanin , R. Wernke fast High-speed laserand scanning theprevious one hand, the fast buttechnique induces heat, pressure layerprocessing is locally speeds. melted by a laser beam therebyenables, fused toonthe tracks andprocessing layers. This offersforces high and flexibility at aother hand. in and around the processing zone on thescanning The behavior of the single particles on the powder bed around the processing fast processing speeds. High-speed laser enables, on the one hand, the fast processing but induces heat, forces and pressure University of Minho, 4800-058 Guimarães, Portugal b zone hard tothe observe and therefore in this work track the the processing movement in andisaround processing zone onnot thesufficiently other hand.investigated. The89809-000 behaviorHigh-speed-imaging of the single onused the powder bed to around Unochapecó, Chapecó, SC, particles Brazil was of theispowder of the powdernotbed during Powder Bed Fusion in order to observe their behaviour. could be zone hard to particles observe and therefore sufficiently investigated. High-speed-imaging wasand usedexplain in this work to track the It movement observed that powder move towards the melt poolBed affecting large area pool,their which changes the powder of the powder particlesparticles of the powder bed during Powder Fusiona in order to around observethe andmelt explain behaviour. It could be distribution ofpowder the powder bed. move This indicates a strong flow isaconstantly which is thought to be observed that particles towards that the melt poolgas affecting large area present around during the meltprocessing, pool, which changes the powder Abstract due to the metal vapour induced by laser evaporation but can alsoisinduced when no vapour is present due to the temperature distribution of the powder bed. This indicates that a strong gasbeflow constantly present during processing, which is thought toand be pressure aroundinduced the processing due to theincrease metal vapour by laser zone. evaporation but can be also induced when no vapour is present due to the temperature and Under concept ofthe "Industry pressurethe increase around processing4.0", zone.production processes will be pushed to be increasingly interconnected, © 2019 The Author(s). by Elsevier B.V. information based onPublished a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2019 2019 The The Authors. Published by Elsevier B.V. Peer-review under responsibility ofby the scientific committee of the 17th Nordic Laserfor Material Processing Conference. © Author(s). Published Elsevier B.V. goes beyond the traditional aim of capacity maximization, contributing organization’s profitability Peer-review under responsibility of the scientific committee of the 17th Nordicalso Laser Material Processing Conference.and value. Peer-review under responsibilityand of the scientific committee of the 17th Nordic Laser Material Processing Conference.instead of Indeed, lean management continuous improvement approaches suggest capacity optimization Keywords: Selective laser melting; particle tracking; denudation; particle movement; vapor flow maximization. The study of capacity optimization and costing models is an important research topic that deserves Keywords: Selective laser melting; particle tracking; denudation; particle movement; vapor flow contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Additive Manufacturing using pre-placed powder became a significant manufacturing method during the last years, optimization might hide operational inefficiency. Additive Manufacturing using pre-placed powder became amedical significant manufacturing method during the the last whole years, which is used in most industrial sectors such as aerospace, or automotive technologies through © 2017 The Authors. Published by Elsevier B.V. which ischain used [1]. in most industrial sectorscalled such Selective as aerospace, medical or or automotive technologies through theproduce whole process This process, often Laser Melting Powder Bed Fusion (PBF), can Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference process Thisquality. process, often called Selective Bed Fusion (PBF), can produce complexchain parts [1]. in high However, the basic effectsLaser of theMelting processorarePowder not completely understood, which leads 2017. complex partsunnecessary in high quality. However, theinefficient basic effects of the process are not completely understood, which leads to partly high energy input and re-melting of already processed structures as Mishra et al. [2] to partly high unnecessary energy input and inefficient re-melting of already processed structures as Mishra et al. [2] could show or unstable processing when using non-standard materials. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency could show or unstable processing when using non-standard materials.
1. Introduction * Corresponding author. Tel.: +46 920 493969. The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address:author. [email protected] * Corresponding Tel.: +46 920 493969. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured E-mail address: [email protected] in several©ways: tons of production, hours of manufacturing, etc. The management of the idle capacity 2351-9789 2019 The Author(s). Published byavailable Elsevier B.V. * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 Peer-review©under the scientific committee of741 the 17th Nordic Laser Material Processing Conference. 2351-9789 2019responsibility The Author(s).ofPublished by Elsevier B.V. E-mail address: [email protected] Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference. 2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference. 10.1016/j.promfg.2019.08.005
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Usually, the PBF machines use a pre-defined parameter set for processing for all tracks. However, it was found that the boundary conditions for building single tracks can significantly vary. In particular, the denudation effect leads to different amounts of powder that is available for building the following track [3]. Yadroitsev & Smurov [4] found that the second track is lower than the first one. At different hatch distances between the tracks, Yadroitsev et al. [5] described that the denudation effect leads to a decrease in track height. The widths of the denudation zone were calculated to about two particle widths (~60 µm). However, no ambient gas flow was considered, and the calculated denudation zone was smaller than the ones seen in high-speed-videos. This effect changes the absorptivity of the laser beam due to the different reflectivity of powder and flat melt pool or solid surfaces on the one hand [4]. On the other hand, the heat conduction can change since heat conduction through powder is lower compared to the one through compact material, which both leads to different shapes of the track. Ly et al. [6] observed in high-speed-video recordings of the process zone that particles are pushed away from the melt pool to the side and rear directions. Other works, e.g. Gunenthiram et al. [7] observed that powder particles move towards the melt pool at an average velocity of 0.4 m/s. The possible reasons for the denudation effect was discussed in different works, whereas three main impacts were identified: 1. Melt pool movement and capturing of surrounding particles According to calculations of Ly et al. [6], 60% of the observed spatters are ejections of hot material, 25% from cold ejections, and 15% from recoil pressure induced ejections. They observed that the accelerated particles from the powder bed either incorporate into the melt pool, miss the laser beam and form cold ejections or are heated by the laser beam and form the hot ejections. 2. Partial vaporization of single particles and acceleration of particles away from the melt pool Gladush et al. [8] observed that the melt pool surface during PBF can reach vaporization temperature, which induces a recoil pressure on the melt pool surface. This recoil pressure can induce a melt flow outwards and supports thereby the surface tension driven Marangoni flow as Khairallah et al. [9] observed. According to Ly et al. [6], the melt movement is directly related to the energy input. The expanded melt pool showing dynamic behavior due to the varying recoil pressure can incorporate adjacent particles into the melt pool leading to the denudation effect. 3. Vapor induced gas flow pushing particles in the direction to the melt pool Matthews et al. [10] found that it is possible that particles can partially be heated to vaporization temperature within 5 µs, which leads to an acceleration of the particles away from the processing zone with speeds around 20 m/s. Ho et al. [11] calculated the velocity of the vapor plume from boiling surfaces to 700 m/s vapor when assuming a vapor stream size similar to the laser spot size. This leads to a Reynolds number of Re=3500, which indicates a turbulent flow of the vapor stream. Therefore, it is likely that the vapor jet pulls ambient gas towards the processing zone. Khairallah et al. [12] modeled the PBF process using Finite-Element-Method (FEM) and confirmed that, besides an increased pore formation, a keyhole occurs, which is the vapor channel known from laser deep penetration welding processes (e.g. [13]). Matthews et al. [10] concluded that the gas flow is the dominating effect for the observed spattering compared to the recoil pressure acceleration, which can be explained by the Bernoulli effect. Matthews et al. [10] used an FEMmodel to show that the vapor is ejected in normal and rearward direction, which is in good agreement with the observed powder movement directions. Gunenthiram et al. [7] observed that the vapor plume in high-speed-videos shows varied ejection directions, which can, in addition, lead to fluctuations of particle attraction and thereby of the denudation zone widths. In this work, the denudation effect was observed using high-speed-imaging to observe the impact of the denudation effect on the particle flow in the powder bed and thereby increase the understanding of the PBF-process in general using a laboratory setup.
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2. Process setup and methods 2.1 Process setup Figure 1 shows a sketch of the used setup, which is a substitute experiment in order to observe and explain physical aspects of the process. One powder layer of 100 µm thickness was applied on the base material and the laser beam was lead in a linear movement over the powder bed. A laser (300 W IPG-Yttrium fiber laser) with a laser power of 250 W and a beam velocity of 1 m/min was used at a laser spot size on the powder layer surface of 75 µm. Argon shielding gas was applied through a nozzle to shield the processing zone from the surrounding atmosphere.
Figure 1. Sketch of processing setup showing the two used high-speed-camera positions
High-speed-imaging was used to observe the process zone from a 0° and 45° angle position (Figure 1) in order to observe the movement of the powder particles around the processing zone. Frame rates of 4350 fps and 6250 fps were used. In addition, an illumination laser (Cavitar, wavelength 870 nm) was installed to illuminate the processing zone, while the camera has a band-pass filter in front of the lens to only record the light from the illumination laser and avoid impacts of the processing laser light. 2.2 Materials Stainless steel 316L was used as base material and as powder material. The powder was of the size 45 µm to 90 µm (Figure 2a) and was pre-placed on the base material in one layer of 100 µm (Figure 2b).
(a)
(b) Figure 2. (a) Magnification of the used 316L powder particles; (b) powder layer on the base material surface
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3. Results High-speed-videos were recorded during processing at different angles. Figure 3 shows sequences of high-speedimages at a 0° and 45° angle position of the camera. The particles move towards the melt pool and can be incorporated into the melt pool.
(a)
(b)
Figure 3. Sequences of high-speed-images observing single particle movement towards the melt pool (a) recorded from a 45° and (b) from 0° angle (processing the second track at a hatch distance of 200 µm) with marked (red arrows) particles moving towards the melt pool (every fourth frame of the video is shown)
In the high-speed-videos, it is possible to detect the particle movement. The zone, in which particles show movement in the videos, was detected and marked in Figure 4. This zone shows a width, which is 3.7 times the width of the melt pool.
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Figure 4. Visual detection of the denudation zone (camera angle 45°)
4. Discussion The experiment simulating the PBF process shows physical effects that happen during PBF. Although a simplified setup of the PBF process (e.g. slow process velocity and related larger melt pool, adapted larger powder layer thickness) had to be used in order to provide access with the recording equipment, basic effects were visualized and can be explained in the current study. The surprisingly large area, where visible powder movement occurs can be further explored and explained in order to increase the understanding of the physical effects and help to predict the powder availability for the subsequent tracks. Yadroitsev et al. [14] measured the denudation zone to be 30 µm on each side at a laser spot size of 70 µm, which is a 1.9 times increase. However, when comparing this measured denudation zones, the particle movement zone observed in the actual work is much larger (3.7 times the melt pool size). Therefore, it seems that the observed particle movement zone does not necessarily indicate that the whole powder material is removed from the surface. Apparently, some powder particles can still be present, when just the top layer of particles was removed. The detected denudation zone by Yadroitsev et al. [14] may have been the zone of complete removal of powder particles in experiments. As Matthews et al. [10] summarize: “Denudation is a complex interplay between melt pool geometry, metal vapor flow and ambient gas pressure.” The Bernoulli-effect is efficient when a high-speed gas stream, as induced by vaporization, is present. However, in the recorded high-speed-videos of the actual work, a melt pool was created with the used parameter set that did not show a keyhole. Only a flattening of the melt pool was observed, which indicates a recoil pressure on the melt pool surface that is likely to happen since the intensity 𝐼𝐼𝐼𝐼 =
𝑃𝑃𝑃𝑃𝐿𝐿𝐿𝐿
(1)
𝐴𝐴𝐴𝐴𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
with 𝑃𝑃𝑃𝑃𝐿𝐿𝐿𝐿 being the laser power and 𝐴𝐴𝐴𝐴𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 being the laser beam area on the material surface, with the used parameter set is 5.66 ∙ 106 𝑊𝑊𝑊𝑊/𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐2 . Since in the actual work, no vapor ejection was visible that can lead to the Bernoulli-effect, another mechanism might cause the particle movement on the powder bed. The fact of the absence of vapor ejection offers the possibility to learn more about the denudation effect since the effect apparently happens also without vapor ejection unlike it happens during standard PBF. The used shielding gas pressure was set to low values that the powder movement was not affected. The pressure due to temperature differences might support the powder particle movement. A rough calculation of the pressure that can be built up around the processing zone due to the temperature increase close to vaporization temperature was conducted. Assuming an isochoric heating of the ambient gas around the melt pool, the pressure 𝑝𝑝𝑝𝑝2 increases to 𝑝𝑝𝑝𝑝2 = 𝑝𝑝𝑝𝑝1 ∙
𝑇𝑇𝑇𝑇2 𝑇𝑇𝑇𝑇1
(2)
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from the pressure before heating 𝑝𝑝𝑝𝑝1 based on the temperature before heating 𝑇𝑇𝑇𝑇1 to the vaporization temperature of the material 𝑇𝑇𝑇𝑇2 . A temperature increase from 300 K to 3300 K at 1 bar leads to a pressure of 11 bar around the melt pool. The heated gas under high pressure will expand mainly straight upwards due to the relatively flat surface shape of the melt pool surface. This effect seems to lead to a suction of ambient gas above the powder bed in the direction towards the upward streaming gas. Due to the expected high pressures, the gas stream can be a cause for the observed particle movement (Figure 5).
Figure 5. Sketch of pressure zones around the melt pool during PBF leading to powder particle movement
4. Summary and conclusion Denudation is an effect that captures powder particles during powder bed fusion in a large area, which is much larger than the laser spot area and therefore affects the subsequent processing. Besides the incorporation of powder particles around the track, in this work, powder movement in a much larger area around the processing zone was observed. This effect indicates that even around the usually identified denudation zone with no particles, particle movement happens. The observations lead to the conclusion that besides the vapor induced gas flow also the temperature increase including the pressure increase around the processing zone can induce the particle movement towards the melt pool. These basic physical effects can also influence the standard powder bed fusion processes. Acknowledgements The author kindly acknowledges the support of David Santín Lopez during his student thesis “Investigation of mechanisms involved in SLM: Spatter and denudation” and the funding of • C3TS – arctic platform to Create, 3D-print, Test and Sell (Interreg Nord), • CINEMA - Towards circular economy via ecodesign and sustainable remanufacturing (Interreg Nord), • SPAcEMAN - Sustainable Powders for Additive Manufacturing (KIC raw materials) and • SAMOA - Sustainable Aluminium additive Manufacturing fOr high performance Applications (KIC raw materials). References [1] F. Geyer, From preform to polishing - lasers in metal based additive manufacturing, Proc. of the 36th International Congress on Applications of Lasers and Electro-Optics (ICALEO), paper 204 (2013). [2] P. Mishra, T. Ilar, F. Brueckner, A. F. H. Kaplan, Energy efficiency contributions and losses during SLM, Journal of Laser Applications (in press) (2018). [3] J. Volpp, F. Brueckner, A. F. H. Kaplan, Track geometry variations in Selective Laser Melting processes, J. Laser Appl. 31, 022310 (2019); doi: 10.2351/1.5096107 [4] I. Yadroitsev, I. Smurov, Surface morphology in selective laser melting of metal powders, Physics Procedia, 12. Jg., 264-270 (2011).
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[5] I. Yadroitsev, P. Bertrand, G. Antonenkova, S. Grigoriev, I. Smurov, Use of track/layer morphology to develop functional parts by selective laser melting, Journal of Laser Applications, 25(5), 052003 (2013). [6] S. Ly, A. M. Rubenchik, S. A. Khairallah, G. Guss, M. J. Matthews, Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing, Scientific reports, 7(1), 4085 (2017). [7] V. Gunenthiram, P. Peyre, M. Schneider, M. Dal, F. Coste, R. Fabbro, Analysis of laser–melt pool–powder bed interaction during the selective laser melting of a stainless steel, Journal of Laser Applications, 29(2), 022303 (2017). [8] G. G. Gladush, I. Smurov, Physics of laser materials processing: theory and experiment (Vol. 146), Springer Science & Business Media (2011). [9] S. A. Khairallah, A. T. Anderson, A. Rubenchik, W.E. King, Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones, Acta Materialia, 108, 36-45 (2016). [10] M. J. Matthews, G. Guss, S. A. Khairallah, A. M. Rubenchik, P. J. Depond, W. E. King, Denudation of metal powder layers in laser powder bed fusion processes, Acta Materialia, 114, 33-42 (2016). [11] J. R. Ho, C.P. Grigoropoulos, J.A.C. Humphrey, Computational study of heat transfer and gas dynamics in the pulsed laser evaporation of metals, Journal of Applied Physics, 78(7), 4696-4709 (1995). [12] S. A. Khairallah, A. Anderson, Mesoscopic simulation model of selective laser melting of stainless steel powder, Journal of Materials Processing Technology, 214(11), 2627-2636 (2014). [13] J. Volpp, D. Freimann, Indirect measurement of keyhole pressure oscillations during laser deep penetration welding, Proc. of the 32nd International Congress on Applications of Lasers and Electro-Optics (ICALEO), paper 1301, 334-340 (2013) [14] I. Yadroitsev, L. Thivillon, P. Bertrand, I. Smurov, Strategy of manufacturing components with designed internal structure by selective laser melting of metallic powder, Applied Surface Science, 254(4), 980-983 (2007).
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Available Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Procedia Manufacturing 00 (2019) 000–000 Procedia Manufacturing 00 (2019) 000–000
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www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Manufacturing 36 (2019) 26–32 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
17th Nordic Laser Material Processing Conference (NOLAMP17), 27 – 29 August 2019 17th Nordic Laser Material Processing Conference (NOLAMP17), 27 – 29 August 2019
Powder particle movement during Powder Bed Fusion Powder particle movement during Powder Bed Fusion
Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June Joerg Volpp* 2017, Vigo (Pontevedra), Joerg Volpp* Spain Department of Applied Physics and Mechanical Engineering, Luleå University of Technology, 971 87 Luleå, Sweden Department of Applied Physics and Mechanical Engineering, Luleå University of Technology, 971 87 Luleå, Sweden
Costing models for capacity optimization in Industry 4.0: Trade-off between used capacity and operational efficiency
Abstract Abstract Powder Bed Fusion is a widely used technique to produce complex parts with different materials. In principle, a pre-placed powder layer is locally melted a laserused and thereby to thea,* previous tracks and This technique offers high flexibility at Powder Bed Fusion is abywidely technique complex with different materials. Inbprinciple, a pre-placed powder a to produce b layers. A.beam Santana , P. fused Afonso , parts A. Zanin , R. Wernke fast High-speed laserand scanning theprevious one hand, the fast buttechnique induces heat, pressure layerprocessing is locally speeds. melted by a laser beam therebyenables, fused toonthe tracks andprocessing layers. This offersforces high and flexibility at aother hand. in and around the processing zone on thescanning The behavior of the single particles on the powder bed around the processing fast processing speeds. High-speed laser enables, on the one hand, the fast processing but induces heat, forces and pressure University of Minho, 4800-058 Guimarães, Portugal b zone hard tothe observe and therefore in this work track the the processing movement in andisaround processing zone onnot thesufficiently other hand.investigated. The89809-000 behaviorHigh-speed-imaging of the single onused the powder bed to around Unochapecó, Chapecó, SC, particles Brazil was of theispowder of the powdernotbed during Powder Bed Fusion in order to observe their behaviour. could be zone hard to particles observe and therefore sufficiently investigated. High-speed-imaging wasand usedexplain in this work to track the It movement observed that powder move towards the melt poolBed affecting large area pool,their which changes the powder of the powder particlesparticles of the powder bed during Powder Fusiona in order to around observethe andmelt explain behaviour. It could be distribution ofpowder the powder bed. move This indicates a strong flow isaconstantly which is thought to be observed that particles towards that the melt poolgas affecting large area present around during the meltprocessing, pool, which changes the powder Abstract due to the metal vapour induced by laser evaporation but can alsoisinduced when no vapour is present due to the temperature distribution of the powder bed. This indicates that a strong gasbeflow constantly present during processing, which is thought toand be pressure aroundinduced the processing due to theincrease metal vapour by laser zone. evaporation but can be also induced when no vapour is present due to the temperature and Under concept ofthe "Industry pressurethe increase around processing4.0", zone.production processes will be pushed to be increasingly interconnected, © 2019 The Author(s). by Elsevier B.V. information based onPublished a real time basis and, necessarily, much more efficient. In this context, capacity optimization © 2019 2019 The The Authors. Published by Elsevier B.V. Peer-review under responsibility ofby the scientific committee of the 17th Nordic Laserfor Material Processing Conference. © Author(s). Published Elsevier B.V. goes beyond the traditional aim of capacity maximization, contributing organization’s profitability Peer-review under responsibility of the scientific committee of the 17th Nordicalso Laser Material Processing Conference.and value. Peer-review under responsibilityand of the scientific committee of the 17th Nordic Laser Material Processing Conference.instead of Indeed, lean management continuous improvement approaches suggest capacity optimization Keywords: Selective laser melting; particle tracking; denudation; particle movement; vapor flow maximization. The study of capacity optimization and costing models is an important research topic that deserves Keywords: Selective laser melting; particle tracking; denudation; particle movement; vapor flow contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical model for capacity management based on different costing models (ABC and TDABC). A generic model has been 1. Introduction developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s 1. Introduction value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Additive Manufacturing using pre-placed powder became a significant manufacturing method during the last years, optimization might hide operational inefficiency. Additive Manufacturing using pre-placed powder became amedical significant manufacturing method during the the last whole years, which is used in most industrial sectors such as aerospace, or automotive technologies through © 2017 The Authors. Published by Elsevier B.V. which ischain used [1]. in most industrial sectorscalled such Selective as aerospace, medical or or automotive technologies through theproduce whole process This process, often Laser Melting Powder Bed Fusion (PBF), can Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference process Thisquality. process, often called Selective Bed Fusion (PBF), can produce complexchain parts [1]. in high However, the basic effectsLaser of theMelting processorarePowder not completely understood, which leads 2017. complex partsunnecessary in high quality. However, theinefficient basic effects of the process are not completely understood, which leads to partly high energy input and re-melting of already processed structures as Mishra et al. [2] to partly high unnecessary energy input and inefficient re-melting of already processed structures as Mishra et al. [2] could show or unstable processing when using non-standard materials. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency could show or unstable processing when using non-standard materials.
1. Introduction * Corresponding author. Tel.: +46 920 493969. The cost of idle capacity is a fundamental information for companies and their management of extreme importance E-mail address:author. [email protected] * Corresponding Tel.: +46 920 493969. in modern production systems. In general, it is defined as unused capacity or production potential and can be measured E-mail address: [email protected] in several©ways: tons of production, hours of manufacturing, etc. The management of the idle capacity 2351-9789 2019 The Author(s). Published byavailable Elsevier B.V. * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 Peer-review©under the scientific committee of741 the 17th Nordic Laser Material Processing Conference. 2351-9789 2019responsibility The Author(s).ofPublished by Elsevier B.V. E-mail address: [email protected] Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference. 2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference. 10.1016/j.promfg.2019.08.005
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Usually, the PBF machines use a pre-defined parameter set for processing for all tracks. However, it was found that the boundary conditions for building single tracks can significantly vary. In particular, the denudation effect leads to different amounts of powder that is available for building the following track [3]. Yadroitsev & Smurov [4] found that the second track is lower than the first one. At different hatch distances between the tracks, Yadroitsev et al. [5] described that the denudation effect leads to a decrease in track height. The widths of the denudation zone were calculated to about two particle widths (~60 µm). However, no ambient gas flow was considered, and the calculated denudation zone was smaller than the ones seen in high-speed-videos. This effect changes the absorptivity of the laser beam due to the different reflectivity of powder and flat melt pool or solid surfaces on the one hand [4]. On the other hand, the heat conduction can change since heat conduction through powder is lower compared to the one through compact material, which both leads to different shapes of the track. Ly et al. [6] observed in high-speed-video recordings of the process zone that particles are pushed away from the melt pool to the side and rear directions. Other works, e.g. Gunenthiram et al. [7] observed that powder particles move towards the melt pool at an average velocity of 0.4 m/s. The possible reasons for the denudation effect was discussed in different works, whereas three main impacts were identified: 1. Melt pool movement and capturing of surrounding particles According to calculations of Ly et al. [6], 60% of the observed spatters are ejections of hot material, 25% from cold ejections, and 15% from recoil pressure induced ejections. They observed that the accelerated particles from the powder bed either incorporate into the melt pool, miss the laser beam and form cold ejections or are heated by the laser beam and form the hot ejections. 2. Partial vaporization of single particles and acceleration of particles away from the melt pool Gladush et al. [8] observed that the melt pool surface during PBF can reach vaporization temperature, which induces a recoil pressure on the melt pool surface. This recoil pressure can induce a melt flow outwards and supports thereby the surface tension driven Marangoni flow as Khairallah et al. [9] observed. According to Ly et al. [6], the melt movement is directly related to the energy input. The expanded melt pool showing dynamic behavior due to the varying recoil pressure can incorporate adjacent particles into the melt pool leading to the denudation effect. 3. Vapor induced gas flow pushing particles in the direction to the melt pool Matthews et al. [10] found that it is possible that particles can partially be heated to vaporization temperature within 5 µs, which leads to an acceleration of the particles away from the processing zone with speeds around 20 m/s. Ho et al. [11] calculated the velocity of the vapor plume from boiling surfaces to 700 m/s vapor when assuming a vapor stream size similar to the laser spot size. This leads to a Reynolds number of Re=3500, which indicates a turbulent flow of the vapor stream. Therefore, it is likely that the vapor jet pulls ambient gas towards the processing zone. Khairallah et al. [12] modeled the PBF process using Finite-Element-Method (FEM) and confirmed that, besides an increased pore formation, a keyhole occurs, which is the vapor channel known from laser deep penetration welding processes (e.g. [13]). Matthews et al. [10] concluded that the gas flow is the dominating effect for the observed spattering compared to the recoil pressure acceleration, which can be explained by the Bernoulli effect. Matthews et al. [10] used an FEMmodel to show that the vapor is ejected in normal and rearward direction, which is in good agreement with the observed powder movement directions. Gunenthiram et al. [7] observed that the vapor plume in high-speed-videos shows varied ejection directions, which can, in addition, lead to fluctuations of particle attraction and thereby of the denudation zone widths. In this work, the denudation effect was observed using high-speed-imaging to observe the impact of the denudation effect on the particle flow in the powder bed and thereby increase the understanding of the PBF-process in general using a laboratory setup.
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2. Process setup and methods 2.1 Process setup Figure 1 shows a sketch of the used setup, which is a substitute experiment in order to observe and explain physical aspects of the process. One powder layer of 100 µm thickness was applied on the base material and the laser beam was lead in a linear movement over the powder bed. A laser (300 W IPG-Yttrium fiber laser) with a laser power of 250 W and a beam velocity of 1 m/min was used at a laser spot size on the powder layer surface of 75 µm. Argon shielding gas was applied through a nozzle to shield the processing zone from the surrounding atmosphere.
Figure 1. Sketch of processing setup showing the two used high-speed-camera positions
High-speed-imaging was used to observe the process zone from a 0° and 45° angle position (Figure 1) in order to observe the movement of the powder particles around the processing zone. Frame rates of 4350 fps and 6250 fps were used. In addition, an illumination laser (Cavitar, wavelength 870 nm) was installed to illuminate the processing zone, while the camera has a band-pass filter in front of the lens to only record the light from the illumination laser and avoid impacts of the processing laser light. 2.2 Materials Stainless steel 316L was used as base material and as powder material. The powder was of the size 45 µm to 90 µm (Figure 2a) and was pre-placed on the base material in one layer of 100 µm (Figure 2b).
(a)
(b) Figure 2. (a) Magnification of the used 316L powder particles; (b) powder layer on the base material surface
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3. Results High-speed-videos were recorded during processing at different angles. Figure 3 shows sequences of high-speedimages at a 0° and 45° angle position of the camera. The particles move towards the melt pool and can be incorporated into the melt pool.
(a)
(b)
Figure 3. Sequences of high-speed-images observing single particle movement towards the melt pool (a) recorded from a 45° and (b) from 0° angle (processing the second track at a hatch distance of 200 µm) with marked (red arrows) particles moving towards the melt pool (every fourth frame of the video is shown)
In the high-speed-videos, it is possible to detect the particle movement. The zone, in which particles show movement in the videos, was detected and marked in Figure 4. This zone shows a width, which is 3.7 times the width of the melt pool.
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Figure 4. Visual detection of the denudation zone (camera angle 45°)
4. Discussion The experiment simulating the PBF process shows physical effects that happen during PBF. Although a simplified setup of the PBF process (e.g. slow process velocity and related larger melt pool, adapted larger powder layer thickness) had to be used in order to provide access with the recording equipment, basic effects were visualized and can be explained in the current study. The surprisingly large area, where visible powder movement occurs can be further explored and explained in order to increase the understanding of the physical effects and help to predict the powder availability for the subsequent tracks. Yadroitsev et al. [14] measured the denudation zone to be 30 µm on each side at a laser spot size of 70 µm, which is a 1.9 times increase. However, when comparing this measured denudation zones, the particle movement zone observed in the actual work is much larger (3.7 times the melt pool size). Therefore, it seems that the observed particle movement zone does not necessarily indicate that the whole powder material is removed from the surface. Apparently, some powder particles can still be present, when just the top layer of particles was removed. The detected denudation zone by Yadroitsev et al. [14] may have been the zone of complete removal of powder particles in experiments. As Matthews et al. [10] summarize: “Denudation is a complex interplay between melt pool geometry, metal vapor flow and ambient gas pressure.” The Bernoulli-effect is efficient when a high-speed gas stream, as induced by vaporization, is present. However, in the recorded high-speed-videos of the actual work, a melt pool was created with the used parameter set that did not show a keyhole. Only a flattening of the melt pool was observed, which indicates a recoil pressure on the melt pool surface that is likely to happen since the intensity 𝐼𝐼𝐼𝐼 =
𝑃𝑃𝑃𝑃𝐿𝐿𝐿𝐿
(1)
𝐴𝐴𝐴𝐴𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏
with 𝑃𝑃𝑃𝑃𝐿𝐿𝐿𝐿 being the laser power and 𝐴𝐴𝐴𝐴𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 being the laser beam area on the material surface, with the used parameter set is 5.66 ∙ 106 𝑊𝑊𝑊𝑊/𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐2 . Since in the actual work, no vapor ejection was visible that can lead to the Bernoulli-effect, another mechanism might cause the particle movement on the powder bed. The fact of the absence of vapor ejection offers the possibility to learn more about the denudation effect since the effect apparently happens also without vapor ejection unlike it happens during standard PBF. The used shielding gas pressure was set to low values that the powder movement was not affected. The pressure due to temperature differences might support the powder particle movement. A rough calculation of the pressure that can be built up around the processing zone due to the temperature increase close to vaporization temperature was conducted. Assuming an isochoric heating of the ambient gas around the melt pool, the pressure 𝑝𝑝𝑝𝑝2 increases to 𝑝𝑝𝑝𝑝2 = 𝑝𝑝𝑝𝑝1 ∙
𝑇𝑇𝑇𝑇2 𝑇𝑇𝑇𝑇1
(2)
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from the pressure before heating 𝑝𝑝𝑝𝑝1 based on the temperature before heating 𝑇𝑇𝑇𝑇1 to the vaporization temperature of the material 𝑇𝑇𝑇𝑇2 . A temperature increase from 300 K to 3300 K at 1 bar leads to a pressure of 11 bar around the melt pool. The heated gas under high pressure will expand mainly straight upwards due to the relatively flat surface shape of the melt pool surface. This effect seems to lead to a suction of ambient gas above the powder bed in the direction towards the upward streaming gas. Due to the expected high pressures, the gas stream can be a cause for the observed particle movement (Figure 5).
Figure 5. Sketch of pressure zones around the melt pool during PBF leading to powder particle movement
4. Summary and conclusion Denudation is an effect that captures powder particles during powder bed fusion in a large area, which is much larger than the laser spot area and therefore affects the subsequent processing. Besides the incorporation of powder particles around the track, in this work, powder movement in a much larger area around the processing zone was observed. This effect indicates that even around the usually identified denudation zone with no particles, particle movement happens. The observations lead to the conclusion that besides the vapor induced gas flow also the temperature increase including the pressure increase around the processing zone can induce the particle movement towards the melt pool. These basic physical effects can also influence the standard powder bed fusion processes. Acknowledgements The author kindly acknowledges the support of David Santín Lopez during his student thesis “Investigation of mechanisms involved in SLM: Spatter and denudation” and the funding of • C3TS – arctic platform to Create, 3D-print, Test and Sell (Interreg Nord), • CINEMA - Towards circular economy via ecodesign and sustainable remanufacturing (Interreg Nord), • SPAcEMAN - Sustainable Powders for Additive Manufacturing (KIC raw materials) and • SAMOA - Sustainable Aluminium additive Manufacturing fOr high performance Applications (KIC raw materials). References [1] F. Geyer, From preform to polishing - lasers in metal based additive manufacturing, Proc. of the 36th International Congress on Applications of Lasers and Electro-Optics (ICALEO), paper 204 (2013). [2] P. Mishra, T. Ilar, F. Brueckner, A. F. H. Kaplan, Energy efficiency contributions and losses during SLM, Journal of Laser Applications (in press) (2018). [3] J. Volpp, F. Brueckner, A. F. H. Kaplan, Track geometry variations in Selective Laser Melting processes, J. Laser Appl. 31, 022310 (2019); doi: 10.2351/1.5096107 [4] I. Yadroitsev, I. Smurov, Surface morphology in selective laser melting of metal powders, Physics Procedia, 12. Jg., 264-270 (2011).
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[5] I. Yadroitsev, P. Bertrand, G. Antonenkova, S. Grigoriev, I. Smurov, Use of track/layer morphology to develop functional parts by selective laser melting, Journal of Laser Applications, 25(5), 052003 (2013). [6] S. Ly, A. M. Rubenchik, S. A. Khairallah, G. Guss, M. J. Matthews, Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing, Scientific reports, 7(1), 4085 (2017). [7] V. Gunenthiram, P. Peyre, M. Schneider, M. Dal, F. Coste, R. Fabbro, Analysis of laser–melt pool–powder bed interaction during the selective laser melting of a stainless steel, Journal of Laser Applications, 29(2), 022303 (2017). [8] G. G. Gladush, I. Smurov, Physics of laser materials processing: theory and experiment (Vol. 146), Springer Science & Business Media (2011). [9] S. A. Khairallah, A. T. Anderson, A. Rubenchik, W.E. King, Laser powder-bed fusion additive manufacturing: Physics of complex melt flow and formation mechanisms of pores, spatter, and denudation zones, Acta Materialia, 108, 36-45 (2016). [10] M. J. Matthews, G. Guss, S. A. Khairallah, A. M. Rubenchik, P. J. Depond, W. E. King, Denudation of metal powder layers in laser powder bed fusion processes, Acta Materialia, 114, 33-42 (2016). [11] J. R. Ho, C.P. Grigoropoulos, J.A.C. Humphrey, Computational study of heat transfer and gas dynamics in the pulsed laser evaporation of metals, Journal of Applied Physics, 78(7), 4696-4709 (1995). [12] S. A. Khairallah, A. Anderson, Mesoscopic simulation model of selective laser melting of stainless steel powder, Journal of Materials Processing Technology, 214(11), 2627-2636 (2014). [13] J. Volpp, D. Freimann, Indirect measurement of keyhole pressure oscillations during laser deep penetration welding, Proc. of the 32nd International Congress on Applications of Lasers and Electro-Optics (ICALEO), paper 1301, 334-340 (2013) [14] I. Yadroitsev, L. Thivillon, P. Bertrand, I. Smurov, Strategy of manufacturing components with designed internal structure by selective laser melting of metallic powder, Applied Surface Science, 254(4), 980-983 (2007).