- Email: [email protected]
Influence of the vapour channel on processing in laser powder bed fusion
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Procedia Manufacturing 36 (2019) 80–87 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
Influence of the vapour channel on processing in laser powder bed Influence of the vapour channel on processing in laser powder bed fusion Manufacturing Engineering Society International fusionConference 2017, MESIC 2017, 28-30 June 2017, Vigo (Pontevedra), Spain
Jan Frostevarga, *, Jörg Volppa, Cassidy Thompsona, Himani Siva Prasada, Tatiana a a a Jan Frostevarga, *, Jörg Volpp , Cassidy Thompson Himani Siva Prasada, Tatiana Fedina , Frank Brückner, a,b Costing models for capacity a a,b Industry 4.0: Trade-off Fedinaoptimization , Frank Brücknerin a a
between used capacity and operational efficiency
Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå 97187, Sweden b Fraunhofer for Material Technology, Winterbergstraße 28, 01277 Luleå Dresden, Germany Department ofInstitute Engineering Sciencesand andBeam Mathematics, Luleå University of Technology, 97187, Sweden b Fraunhofer Institute for Material and Beam Technology, Winterbergstraße 28, 01277 Dresden, Germany
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
Abstract a University of Minho, 4800-058 Guimarães, Portugal b Abstract Unochapecó, 89809-000 Chapecó, SC, Brazil Additive Manufacturing provides many opportunities to design and manufacture parts that are difficult or not possible to produce with conventional methods. In Selective Laser Melting in powder bed fusion poolordynamics and to stability is Additive Manufacturing provides many opportunities to (SLM) design and manufacture parts (PBF), that aremelt difficult not possible produce dependent on a large numberInofSelective factors, Laser e.g. laser power output, power density, travel speed, reflectivity of powder bed, rapid with conventional methods. Melting (SLM) in powder bed fusion (PBF), melt pool dynamics and stability is Abstract heating andon vaporization. Sinceoftravel speeds often veryoutput, fast andpower the laser interaction is small, the physical events become dependent a large number factors, e.g.are laser power density, travelzone speed, reflectivity of powder bed, rapid difficultand to predict but alsoSince to observe. This work describe thethe formation and geometrical characteristics of the vaporization heating vaporization. travel speeds are aims often to very fast and laser interaction zone is small, the physical events become zone during processing. a combination of theoretical descriptions, resulting material structures and a comprehensive analysis Under the concept ofUsing "Industry 4.0",work production processes will be and pushed to be characteristics increasingly difficult to predict but also to observe. This aims to describe the formation geometrical ofinterconnected, the vaporization of high-speed imageson ofUsing processing zone fortheoretical different descriptions, heatmuch inputs more and travel speeds, the dynamic melt pool zone during processing. atime combination of resulting material structures andfor acapacity comprehensive analysis information based athe real basis and, necessarily, efficient. Inexplanations this context, optimization behaviour are the derived. pressures from processing involved moves particlesfor next it, changing the of high-speed images ofThe the melting processing zone formaximization, different heat inputs and travel speeds, explanations the to dynamic melt pool goes beyond traditional aim of and capacity contributing also for powder organization’s profitability and value. conditionslean for to lack of material. findings can provide a powder basiscapacity forparticles creating more efficient and stable behaviour are neighbouring derived. The tracks melting and pressures from These processing involved moves next to it, changing the Indeed, management andduecontinuous improvement approaches suggest optimization instead of SLM processing, with fewertracks imperfections. conditions for neighbouring due to lack of material. These findings can provide a basis for creating more efficient and stable maximization. The study of capacity optimization and costing models is an important research topic that deserves SLM processing, with fewer imperfections. contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical © 2019 The Author(s). Published by Elsevier B.V. model for capacity management based on different costing models (ABC and TDABC). A generic model has been © 2019 The Authors. Published B.V. Peer-review under responsibility ofbyElsevier the scientific committee of the 17th Nordic Laser Material Processing Conference. © 2019 The Author(s). Publishedby Elsevier B.V. Peer-reviewand under responsibility of the scientific committee of design the 17thstrategies Nordic Laser Material Processing Conference. developed it was used to analyze idle capacity and to towards the maximization of organization’s Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference.
value. The trade-off capacity vsPressure operational efficiency is highlighted and it is shown that capacity Keywords: High speed imaging; SLM;maximization Powder movement; optimization might hide operational inefficiency. Keywords: High speed imaging; SLM; Powder movement; Pressure © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 1. Introduction 2017. 1. Introduction
Additive manufacturing is a new, and rapidly growing technique that uses a digital 3D design to build a product. Additive is awith new,aand rapidly growingdeposition, technique that uses fusion, a digitaland 3Dbinding design to a product. This is donemanufacturing layer upon layer process involving melting, of build the preselected This is done layer upon layer with a process involving deposition, melting, fusion, and binding of the preselected
Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction * Corresponding author. Tel.: +46 920 49 1675 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 49 1675 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.2019.08.012
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material. One main advantage of additive manufacturing is that it enables the production of complex shapes. Products that before required a simplified design due to manufacturing techniques, can now be produced by adding multiple layers of a material. This also significantly decreases the number of parts needed to build or assemble the product. Products that previously consisted of complex assemblies now can be made as one individual part [1]. While there are many advantages to additive manufacturing, there are also some drawbacks. The main challenge faced by additive manufacturing is meeting engineering quality standards [2]. Unlike conventional material processing, using additive manufacturing leads to a limited number of options concerning the modification of the microstructures after manufacturing. Also, because of the different thermal history across the material, the part contains anisotropic properties parallel and perpendicular to the deposited layers [1]. The final product of the manufacturing process can be affected by many parameters. In fact, previous indications determined at least 157 parameters could change the output of AM [3]. There are different methods to use powder for Additive Manufacturing: blown powder (e.g. named Direct Metal Deposition or Laser Metal Deposition) [4] or pre-placed powder a.k.a. Powder Bed Fusion (PBF). PBF can be made by using e.g. a laser as a heat source to locally melt a layer of the powder bed (e.g. named Selective Laser Melting, Laser Metal Fusion). Selective Laser Melting (SLM) is widely used in industry; however, some basic effects during the process are not yet well understood.
Fig. 1. Principle of SLM processing.
A main quality criterion for SLM is the production of defect-free parts [5]. The main imperfections resulting from the processing are the formation of porosity or loss of material due to spattering. Mishra et al. [6] concluded that the parameters used for SLM are usually conservative and lead to an inefficient energy use, mainly to reduce number of cavities and porosity. Volpp et al. [7] could show that the powder availability significantly effects the track geometries and leads to fluctuations of the track size and depths. One destabilizing effect could be varying absorptivity of the laser beam on the powder bed and the dense material [3,8]. Yadroitsev et al. [9] found in single-track experiments that SLM processes show a high amount of spattering, which results in a redistribution of the powder in the powder bed. This effect can lead to changed powder bed surfaces when processing subsequent tracks and layers. Buchbinder et al. [10] confirmed that track dimensions can significantly vary even when using the same processing parameters. Those effects are not fully understood and are usually not considered when using SLM. One reason might be the varying laser energy absorption at the surfaces of the powder bed and the dense material. Another possible reason for this phenomenon can be the vapour plume occurring due to the high laser energy input. The created vapour plume can induce gas flows that lead to powder spattering and denudation effects [5]. Denudation is the apparent clearing of powder around a single track bed [11]. The depletion of metal powders found near the laser scan path is caused by ambient gas pressure and laser parameters. Denudation zones have been found to be caused due to entrainment of powder particles in a shear flow of gas that is driven by a metal vapour jet found at the melt
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track, and metal vapour flux directed away from the laser spot. These two causations vary their dominancy depending on the processing parameters. The high vapour gas flow velocities from the processing zone can lead to both pushing the powder particles away from the processing zone and movement of powder particles towards the melt pool due to the Bernoulli-effect. This flow is induced by the extreme evaporation that occurs within the laser spot and pressure drop inside the associated vapour jet. The entrainment also can also cause the ejection of particles both vertically and rearward in relation to the laser scan direction. These particles are then redistributed in varying positions on the powder bed [11]. In addition, it was found that the vapour inclination can vary depending on the processing speed and laser power. This effect leads on the one hand to highly dynamic behaviour of the processing zone, which lead on the other hand to dynamic powder particle movement and denudation effects. While increasing the power of the laser also increases the width of the denuded zone, it was found that varying the laser scan rate has little effect on the size of denudation. Although another study found that decreasing the laser power or increasing the laser scan speed leads to the reduction of the normalized enthalpy. Hence, the molten pool will have higher stability and lower spatter particles may be created during the SLM process [12]. When reducing the ambient pressure (Ar atmosphere), it is found that the denudation zone increases. However, at lower pressures (less than 10 torr) the trend for increasing/decreasing denudation width varies. It is also found that the denudation zone depends on powder particle shape, where more spherical shapes provide wider denudation zones [13]. A powerful way of getting insight into the process behaviour to observe and explain effects during the processing is high speed imaging (HSI). Therefore, this work aims to identify phenomena related to the impact of the dynamic vapour from the processing zone to conclude about their origin using HSI analysis. 2. Methodology HSI can provide photographic evidence of phenomena that cannot be observed under ordinary circumstances. Examples are melting or boiling of material, plume behaviour etc. However, it can be challenging to interpret the videos [14]. Experiments made is not to produce good tracks, rather to use HSI to observe phenomena occurring during processing on a powder bed. Processing speeds are therefore made at much lower and faster travel speeds than is normally used for SLM. For the HSI, a Photron Fastcam SX series high speed camera was used with 200 mm Nikon telemacro optics with a narrow band pass filer (808 nm) while the surface of processing was illuminated by a Cavitar Cavilux HF diode laser light source (808 nm wavelength, 500 W peak pulse). For the relatively slow laser travel speeds, 1/60 m/s, the laser power was set to 300W and having a top hat 75 µm wide spot at the surface, inclined with 8 degrees from the normal of the surface in the travel direction. A 316L GA powder with PSD -63 microns from Höganäs AB, was preplaced with a layer thickness of 100-150 µm. The process was shielded by feeding Ar gas through a tube (2 sm). For high laser travel speeds, a scanner was used to build tracks using MAR M 247TM, Table 1, with different laser power, travel speeds and grain sizes, as seen in Table 2. For these experiments, the beam had an elliptical shape, major axis was 0.250 µm and that of the minor axis was 8 μm, with the major axis along the beam travel direction, producing very thin tracks. The powder layers were built using a 0.1 mm thick aluminium frame and flat scrapers on a 4 mm thick carbon steel sheet as substrate, in ambient air atmosphere.
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Table 1. Specified composition of powders used in the experiments (wt-%) Element MAR M 247TM (Seco) 316L (Höganäs)
Ni
W
Co
Cr
Al
Si
66.16-68.24
9.3-9.7
9-9.5
8-8.5
5.4-5.7
0.25
10.0-14.0
16.0-18.0
1.0
S
P
Mn
Mo
C
0.1 0.045
0.03
0.06-0.09
2.0
1.2-2.75
0.03
Table 2. Process settings used for high speed processing Case
Laser power output (W)
Grain size (µm)
Laser spot travel speed, vt (m/s)
Line energy output (mW/mm)
1
800
47-80 (coarse)
10
80
2
800
24-25 (fine)
10
80
3
1500
47-80 (coarse)
10
150
4
1500
47-80 (coarse)
100
15
3. Powder bed particle movement during processing By applying HSI during processing of the SLM process, some phenomena can be seen. During SLM processing with the lower speed, Fig. 2, it can be seen that powder particles are pushed away from the front of the process zone, Fig. 2a. The direction of particles are seemingly random, as well as the velocity, within a certain range. Some particles move with smaller angles and thus collide with particles in the powder bed. Particles that have higher angles and do not collide with other particles also seem to have higher speeds. The movement is likely caused by the thermal expansion during the rapid evaporation of metal exposed to the laser irradiation. This causess a denuded area in close proximity to the melt pool front to form, limiting the amount of material being fed into the melt pool and resulting track. However, there is a lot of particles being pulled from the powder bed on the sides of the process zone into rear end of the melt pool. This is very similar to the Bernoulli-effect also identified by other authors [11,15]. It is rather the material being drawn into the melt pool from the powder bed that makes up for much the produced solidified tracks. The powder flying from the process will land elsewhere on the powder bed, possibly producing an uneven powder bed. When having a more uneven powder bed, the solidified tracks also have less even morphology, Fig. 2b. (a)
200 µm
0 ms
3 ms
0 ms
3 ms
0 ms
3 ms
(b)
(c)
Fig. 2. HSI of slow speed SLM processing; (a) on uniform and (b) uneven powder bed with (c) neighboring track 30% overlapping.
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Also, the powder particles will fly even more unevenly since the conditions for impacting particles also vary more, possibly causing decreasing processing stability. When producing a neighbouring track with 30% overlap, the produced track is smaller in size, in both height and width. This is not surprising since the second track has only one side to draw particles from the powder bed (the first track is solid). It is however also found that a few powder particles from the other side of the original first track is pulled over this into the melt pool, Fig. 2c. HSI of the relatively high speed SLM process reveals that much of the movement of particles occurs some still time after the process zone, Fig. 3a. The particles move in the same manner as a bow wave, where the initial particles have much higher movement than the bulk of particles. Movement seems to be directed towards the side of the track, in such a manner that the faster particles also fly higher and possibly lands further away from the clad track. The movement of the laser beam is in this case faster than the movement of the particles, gradually building up inertia on particles (impacting other particles in the powder bed), causing a movement of particles towards the sides where the powder bed impedes and redirects the moving particles. The interaction time of particles to be pulled into the melt pool from the powder bed is in this case very limited, especially since the particles have momentum away from the process. This consequently leads to that a produced track is basically only constituted by particles that the laser melts and are also not ejected as spatters (which much reduced in this case). This particular behaviour can be considered as a different mode of SLM.
Fig. 3. Frames from HSI, showing upwards movement of particles for cases 1-4, (a)-(d) respectively.
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By measuring the travelled distance after the laser-particle interaction zone, rough movement speeds can be determined, Table 3. The speed of particles seems to be only partially related to the power output of the beam, but rather the quantity of particles which also depends on particle size. The speed of the powder particles is much less than the movement of the laser beam. If the laser beam makes the consecutive track close enough in time, the powder particles could be “caught” before they leave the surface. Movement of powders is somewhat irregular. Particles flying high upwards also moves due to the shielding gas movement. Using the fine sized powder, there is much more movement of particles, Fig. 3b. It is however uncertain if the mass of powders remain the same as in the larger sized particles (Case 1 vs case 2). When increasing the power, Fig. 3c and case 3, there is much more powders flying from the process zone and producing a larger denudation zone, but behaving in a similar fashion. When increasing the travel speed even further, Fig. 3d, the process behaves in a similar fashion but with less high speed particles. This might however be correlated to a lower line energy input. Note that for these videos, any sound tracks where not produced. Table 3. Range of particle movement speed after process zone Case
Particle movement range (m/s) Very fast (very few)
Fast (few)
Mid (many)
1
~1.64
~0.55
~0.4
Slow (most) ~0.19
2
~1.40
~0.75
~0.16
~0.060
3
~1.44
~0.76
~0.24
~0.085
4
-
~0.16
~0.10
~0.059
4. High speed powder bed additive manufacturing As derived from the State of the Art, the underlying effects during PBF are not fully understood yet. The results from high-speed-imaging in this work indicate that the vapour plume has a significant impact on the powder movement on the powder bed and thereby on the subsequent tracks and layers. If the travel speed of the laser beam on the surface is fast enough, powder particles in the path of the laser beam will be caught by the laser beam and the produced melt pool to a much larger extent. The produced bow wave of powders, Fig. 4a will produce a denudation zone with increased height of the powder bed in close proximity to the produced track (some particles will still land on the denudation zone), Figs. 4b-c. If the speed is not too high, some particles in the powder bed might still be able to act as feedstock to the melt pool.
Fig. 4. Principles for SLM with fast travel speeds; (a) side view of powder movement during processing, resulting bead (b) side view and (c) top view.
Figure 5a-d illustrates how the profiles of a series of tracks might look like if process settings are adjusted so that proper fusion of the clad track to the base material is achieved. Depending on how much powder is available to feed the process, weld depth will vary. This consequently leads to varying profiles of the bead. For the second layer, Fig. 5e-h, the varying height of the layer beneath and the added powder layer consequently leads to varying required depths required for the produced track in order to not produce cavities. Keeping record on the produced height of the powder bed might help to adjust process settings when scanning over selected regions, leading to not having to overcompensate over the whole build section during processing, potentially reducing power consumption and increasing processing efficiency.
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Fig. 5. Laser powder bed melting side view of (a) one track and (b-d) consecutive partially overlapping tracks. (e-h) second layer tracks.
5. Conclusions Powder particles fly from the front of the processing zone and the melt pool is dominantly fed by powder particles on the side of the melt pool. This consequently leads to: • Flying powder particles generally have increased speeds at higher angles due to less impact with other particles • Uneven height of the powder bed, possibly caused by spatter from neighbouring tracks, leads to increased instability of produced tracks and powder movement • Neighbouring tracks are consequently much smaller in size because they are only fed from the powder bed from one side When processing at high travel speeds, the behaviour of the process has changed dramatically and can be considered a separate mode for SLM processing: • The feedstock to the produced clad track from the powder bed by the Bernoulli-effect is severely reduced • Particles ejected from the front of the process zone are also reduced, being caught by laser beam and produced melt pool • Clad tracks would likely be dominantly fed by particles directly hit and melted by the laser beam • The height of the powder bed close to the denudation zone will be increased, requiring adaptation of process settings for the neighbouring tracks Acknowledgements The authors are grateful for funding granted by EU-ERDF-InterregNord projects C3TS and CINEMA References [1] [2] [3] [4] [5] [6] [7] [8]
H. Fayazfar, M. Salarian, A. Rogalsky, D. Sarker, P. Russo, V. Paserin, E. Toyserkani, A critical review of powder-based additive manufacturing of ferrous alloys: Process parameters, microstructure and mechanical properties, Mater. Des. 144 (2018) 98–128. doi:10.1016/J.MATDES.2018.02.018. 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 Mater. 108 (2016) 36–45. doi:10.1016/J.ACTAMAT.2016.02.014. J. Trapp, A.M. Rubenchik, G. Guss, M.J. Matthews, In situ absorptivity measurements of metallic powders during laser powder-bed fusion additive manufacturing, Appl. Mater. Today. 9 (2017) 341–349. doi:10.1016/J.APMT.2017.08.006. F. Brueckner, M. Riede, F. Marquardt, R. Willner, A. Seidel, S. Thieme, C. Leyens, E. Beyer, Process characteristics in high-precision laser metal deposition using wire and powder, J. Laser Appl. 29 (2017) 022301. doi:10.2351/1.4983237. 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, J. Laser Appl. 29 (2017) 022303. doi:10.2351/1.4983259. P. Mishra, T. Ilar, F. Brueckner, A. Kaplan, Energy efficiency contributions and losses during selective laser melting, J. Laser Appl. 30 (2018) 032304. doi:10.2351/1.5040603. J. Volpp, F. Brueckner, A.F.H. Kaplan, Track geometry variations in Selective Laser Melting processes, in: Proc. ICALEO, 14-18 Oct., LIA, Orlando, FL, U.S.A., 2018. N.K. Tolochko, Y. V. Khlopkov, S.E. Mozzharov, M.B. Ignatiev, T. Laoui, V.I. Titov, Absorptance of powder materials suitable for laser sintering, Rapid Prototyp. J. 6 (2000) 155–161. doi:10.1108/13552540010337029.
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I. Yadroitsev, A. Gusarov, I. Yadroitsava, I. Smurov, Single track formation in selective laser melting of metal powders, J. Mater. Process. Technol. 210 (2010) 1624–1631. doi:10.1016/J.JMATPROTEC.2010.05.010. D. Buchbinder, H. Schleifenbaum, S. Heidrich, W. Meiners, J. Bültmann, High Power Selective Laser Melting (HP SLM) of Aluminum Parts, Phys. Procedia. 12 (2011) 271–278. doi:10.1016/J.PHPRO.2011.03.035. P. Bidare, I. Bitharas, R.M. Ward, M.M. Attallah, A.J. Moore, Fluid and particle dynamics in laser powder bed fusion, Acta Mater. 142 (2018) 107–120. doi:10.1016/J.ACTAMAT.2017.09.051. M. Taheri Andani, R. Dehghani, M.R. Karamooz-Ravari, R. Mirzaeifar, J. Ni, A study on the effect of energy input on spatter particles creation during selective laser melting process, Addit. Manuf. 20 (2018) 33–43. doi:10.1016/J.ADDMA.2017.12.009. U. Scipioni Bertoli, G. Guss, S. Wu, M.J. Matthews, J.M. Schoenung, In-situ characterization of laser-powder interaction and cooling rates through high-speed imaging of powder bed fusion additive manufacturing, Mater. Des. 135 (2017) 385–396. doi:10.1016/J.MATDES.2017.09.044. I. Eriksson, High Speed Imaging Analysis of Laser Welding, Luleå University of Technology, 2013. 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 Mater. 114 (2016) 33–42. doi:10.1016/J.ACTAMAT.2016.05.017.
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Procedia Manufacturing 36 (2019) 80–87 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
Influence of the vapour channel on processing in laser powder bed Influence of the vapour channel on processing in laser powder bed fusion Manufacturing Engineering Society International fusionConference 2017, MESIC 2017, 28-30 June 2017, Vigo (Pontevedra), Spain
Jan Frostevarga, *, Jörg Volppa, Cassidy Thompsona, Himani Siva Prasada, Tatiana a a a Jan Frostevarga, *, Jörg Volpp , Cassidy Thompson Himani Siva Prasada, Tatiana Fedina , Frank Brückner, a,b Costing models for capacity a a,b Industry 4.0: Trade-off Fedinaoptimization , Frank Brücknerin a a
between used capacity and operational efficiency
Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå 97187, Sweden b Fraunhofer for Material Technology, Winterbergstraße 28, 01277 Luleå Dresden, Germany Department ofInstitute Engineering Sciencesand andBeam Mathematics, Luleå University of Technology, 97187, Sweden b Fraunhofer Institute for Material and Beam Technology, Winterbergstraße 28, 01277 Dresden, Germany
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
Abstract a University of Minho, 4800-058 Guimarães, Portugal b Abstract Unochapecó, 89809-000 Chapecó, SC, Brazil Additive Manufacturing provides many opportunities to design and manufacture parts that are difficult or not possible to produce with conventional methods. In Selective Laser Melting in powder bed fusion poolordynamics and to stability is Additive Manufacturing provides many opportunities to (SLM) design and manufacture parts (PBF), that aremelt difficult not possible produce dependent on a large numberInofSelective factors, Laser e.g. laser power output, power density, travel speed, reflectivity of powder bed, rapid with conventional methods. Melting (SLM) in powder bed fusion (PBF), melt pool dynamics and stability is Abstract heating andon vaporization. Sinceoftravel speeds often veryoutput, fast andpower the laser interaction is small, the physical events become dependent a large number factors, e.g.are laser power density, travelzone speed, reflectivity of powder bed, rapid difficultand to predict but alsoSince to observe. This work describe thethe formation and geometrical characteristics of the vaporization heating vaporization. travel speeds are aims often to very fast and laser interaction zone is small, the physical events become zone during processing. a combination of theoretical descriptions, resulting material structures and a comprehensive analysis Under the concept ofUsing "Industry 4.0",work production processes will be and pushed to be characteristics increasingly difficult to predict but also to observe. This aims to describe the formation geometrical ofinterconnected, the vaporization of high-speed imageson ofUsing processing zone fortheoretical different descriptions, heatmuch inputs more and travel speeds, the dynamic melt pool zone during processing. atime combination of resulting material structures andfor acapacity comprehensive analysis information based athe real basis and, necessarily, efficient. Inexplanations this context, optimization behaviour are the derived. pressures from processing involved moves particlesfor next it, changing the of high-speed images ofThe the melting processing zone formaximization, different heat inputs and travel speeds, explanations the to dynamic melt pool goes beyond traditional aim of and capacity contributing also for powder organization’s profitability and value. conditionslean for to lack of material. findings can provide a powder basiscapacity forparticles creating more efficient and stable behaviour are neighbouring derived. The tracks melting and pressures from These processing involved moves next to it, changing the Indeed, management andduecontinuous improvement approaches suggest optimization instead of SLM processing, with fewertracks imperfections. conditions for neighbouring due to lack of material. These findings can provide a basis for creating more efficient and stable maximization. The study of capacity optimization and costing models is an important research topic that deserves SLM processing, with fewer imperfections. contributions from both the practical and theoretical perspectives. This paper presents and discusses a mathematical © 2019 The Author(s). Published by Elsevier B.V. model for capacity management based on different costing models (ABC and TDABC). A generic model has been © 2019 The Authors. Published B.V. Peer-review under responsibility ofbyElsevier the scientific committee of the 17th Nordic Laser Material Processing Conference. © 2019 The Author(s). Publishedby Elsevier B.V. Peer-reviewand under responsibility of the scientific committee of design the 17thstrategies Nordic Laser Material Processing Conference. developed it was used to analyze idle capacity and to towards the maximization of organization’s Peer-review under responsibility of the scientific committee of the 17th Nordic Laser Material Processing Conference.
value. The trade-off capacity vsPressure operational efficiency is highlighted and it is shown that capacity Keywords: High speed imaging; SLM;maximization Powder movement; optimization might hide operational inefficiency. Keywords: High speed imaging; SLM; Powder movement; Pressure © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 1. Introduction 2017. 1. Introduction
Additive manufacturing is a new, and rapidly growing technique that uses a digital 3D design to build a product. Additive is awith new,aand rapidly growingdeposition, technique that uses fusion, a digitaland 3Dbinding design to a product. This is donemanufacturing layer upon layer process involving melting, of build the preselected This is done layer upon layer with a process involving deposition, melting, fusion, and binding of the preselected
Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction * Corresponding author. Tel.: +46 920 49 1675 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 49 1675 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.2019.08.012
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material. One main advantage of additive manufacturing is that it enables the production of complex shapes. Products that before required a simplified design due to manufacturing techniques, can now be produced by adding multiple layers of a material. This also significantly decreases the number of parts needed to build or assemble the product. Products that previously consisted of complex assemblies now can be made as one individual part [1]. While there are many advantages to additive manufacturing, there are also some drawbacks. The main challenge faced by additive manufacturing is meeting engineering quality standards [2]. Unlike conventional material processing, using additive manufacturing leads to a limited number of options concerning the modification of the microstructures after manufacturing. Also, because of the different thermal history across the material, the part contains anisotropic properties parallel and perpendicular to the deposited layers [1]. The final product of the manufacturing process can be affected by many parameters. In fact, previous indications determined at least 157 parameters could change the output of AM [3]. There are different methods to use powder for Additive Manufacturing: blown powder (e.g. named Direct Metal Deposition or Laser Metal Deposition) [4] or pre-placed powder a.k.a. Powder Bed Fusion (PBF). PBF can be made by using e.g. a laser as a heat source to locally melt a layer of the powder bed (e.g. named Selective Laser Melting, Laser Metal Fusion). Selective Laser Melting (SLM) is widely used in industry; however, some basic effects during the process are not yet well understood.
Fig. 1. Principle of SLM processing.
A main quality criterion for SLM is the production of defect-free parts [5]. The main imperfections resulting from the processing are the formation of porosity or loss of material due to spattering. Mishra et al. [6] concluded that the parameters used for SLM are usually conservative and lead to an inefficient energy use, mainly to reduce number of cavities and porosity. Volpp et al. [7] could show that the powder availability significantly effects the track geometries and leads to fluctuations of the track size and depths. One destabilizing effect could be varying absorptivity of the laser beam on the powder bed and the dense material [3,8]. Yadroitsev et al. [9] found in single-track experiments that SLM processes show a high amount of spattering, which results in a redistribution of the powder in the powder bed. This effect can lead to changed powder bed surfaces when processing subsequent tracks and layers. Buchbinder et al. [10] confirmed that track dimensions can significantly vary even when using the same processing parameters. Those effects are not fully understood and are usually not considered when using SLM. One reason might be the varying laser energy absorption at the surfaces of the powder bed and the dense material. Another possible reason for this phenomenon can be the vapour plume occurring due to the high laser energy input. The created vapour plume can induce gas flows that lead to powder spattering and denudation effects [5]. Denudation is the apparent clearing of powder around a single track bed [11]. The depletion of metal powders found near the laser scan path is caused by ambient gas pressure and laser parameters. Denudation zones have been found to be caused due to entrainment of powder particles in a shear flow of gas that is driven by a metal vapour jet found at the melt
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track, and metal vapour flux directed away from the laser spot. These two causations vary their dominancy depending on the processing parameters. The high vapour gas flow velocities from the processing zone can lead to both pushing the powder particles away from the processing zone and movement of powder particles towards the melt pool due to the Bernoulli-effect. This flow is induced by the extreme evaporation that occurs within the laser spot and pressure drop inside the associated vapour jet. The entrainment also can also cause the ejection of particles both vertically and rearward in relation to the laser scan direction. These particles are then redistributed in varying positions on the powder bed [11]. In addition, it was found that the vapour inclination can vary depending on the processing speed and laser power. This effect leads on the one hand to highly dynamic behaviour of the processing zone, which lead on the other hand to dynamic powder particle movement and denudation effects. While increasing the power of the laser also increases the width of the denuded zone, it was found that varying the laser scan rate has little effect on the size of denudation. Although another study found that decreasing the laser power or increasing the laser scan speed leads to the reduction of the normalized enthalpy. Hence, the molten pool will have higher stability and lower spatter particles may be created during the SLM process [12]. When reducing the ambient pressure (Ar atmosphere), it is found that the denudation zone increases. However, at lower pressures (less than 10 torr) the trend for increasing/decreasing denudation width varies. It is also found that the denudation zone depends on powder particle shape, where more spherical shapes provide wider denudation zones [13]. A powerful way of getting insight into the process behaviour to observe and explain effects during the processing is high speed imaging (HSI). Therefore, this work aims to identify phenomena related to the impact of the dynamic vapour from the processing zone to conclude about their origin using HSI analysis. 2. Methodology HSI can provide photographic evidence of phenomena that cannot be observed under ordinary circumstances. Examples are melting or boiling of material, plume behaviour etc. However, it can be challenging to interpret the videos [14]. Experiments made is not to produce good tracks, rather to use HSI to observe phenomena occurring during processing on a powder bed. Processing speeds are therefore made at much lower and faster travel speeds than is normally used for SLM. For the HSI, a Photron Fastcam SX series high speed camera was used with 200 mm Nikon telemacro optics with a narrow band pass filer (808 nm) while the surface of processing was illuminated by a Cavitar Cavilux HF diode laser light source (808 nm wavelength, 500 W peak pulse). For the relatively slow laser travel speeds, 1/60 m/s, the laser power was set to 300W and having a top hat 75 µm wide spot at the surface, inclined with 8 degrees from the normal of the surface in the travel direction. A 316L GA powder with PSD -63 microns from Höganäs AB, was preplaced with a layer thickness of 100-150 µm. The process was shielded by feeding Ar gas through a tube (2 sm). For high laser travel speeds, a scanner was used to build tracks using MAR M 247TM, Table 1, with different laser power, travel speeds and grain sizes, as seen in Table 2. For these experiments, the beam had an elliptical shape, major axis was 0.250 µm and that of the minor axis was 8 μm, with the major axis along the beam travel direction, producing very thin tracks. The powder layers were built using a 0.1 mm thick aluminium frame and flat scrapers on a 4 mm thick carbon steel sheet as substrate, in ambient air atmosphere.
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Table 1. Specified composition of powders used in the experiments (wt-%) Element MAR M 247TM (Seco) 316L (Höganäs)
Ni
W
Co
Cr
Al
Si
66.16-68.24
9.3-9.7
9-9.5
8-8.5
5.4-5.7
0.25
10.0-14.0
16.0-18.0
1.0
S
P
Mn
Mo
C
0.1 0.045
0.03
0.06-0.09
2.0
1.2-2.75
0.03
Table 2. Process settings used for high speed processing Case
Laser power output (W)
Grain size (µm)
Laser spot travel speed, vt (m/s)
Line energy output (mW/mm)
1
800
47-80 (coarse)
10
80
2
800
24-25 (fine)
10
80
3
1500
47-80 (coarse)
10
150
4
1500
47-80 (coarse)
100
15
3. Powder bed particle movement during processing By applying HSI during processing of the SLM process, some phenomena can be seen. During SLM processing with the lower speed, Fig. 2, it can be seen that powder particles are pushed away from the front of the process zone, Fig. 2a. The direction of particles are seemingly random, as well as the velocity, within a certain range. Some particles move with smaller angles and thus collide with particles in the powder bed. Particles that have higher angles and do not collide with other particles also seem to have higher speeds. The movement is likely caused by the thermal expansion during the rapid evaporation of metal exposed to the laser irradiation. This causess a denuded area in close proximity to the melt pool front to form, limiting the amount of material being fed into the melt pool and resulting track. However, there is a lot of particles being pulled from the powder bed on the sides of the process zone into rear end of the melt pool. This is very similar to the Bernoulli-effect also identified by other authors [11,15]. It is rather the material being drawn into the melt pool from the powder bed that makes up for much the produced solidified tracks. The powder flying from the process will land elsewhere on the powder bed, possibly producing an uneven powder bed. When having a more uneven powder bed, the solidified tracks also have less even morphology, Fig. 2b. (a)
200 µm
0 ms
3 ms
0 ms
3 ms
0 ms
3 ms
(b)
(c)
Fig. 2. HSI of slow speed SLM processing; (a) on uniform and (b) uneven powder bed with (c) neighboring track 30% overlapping.
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Also, the powder particles will fly even more unevenly since the conditions for impacting particles also vary more, possibly causing decreasing processing stability. When producing a neighbouring track with 30% overlap, the produced track is smaller in size, in both height and width. This is not surprising since the second track has only one side to draw particles from the powder bed (the first track is solid). It is however also found that a few powder particles from the other side of the original first track is pulled over this into the melt pool, Fig. 2c. HSI of the relatively high speed SLM process reveals that much of the movement of particles occurs some still time after the process zone, Fig. 3a. The particles move in the same manner as a bow wave, where the initial particles have much higher movement than the bulk of particles. Movement seems to be directed towards the side of the track, in such a manner that the faster particles also fly higher and possibly lands further away from the clad track. The movement of the laser beam is in this case faster than the movement of the particles, gradually building up inertia on particles (impacting other particles in the powder bed), causing a movement of particles towards the sides where the powder bed impedes and redirects the moving particles. The interaction time of particles to be pulled into the melt pool from the powder bed is in this case very limited, especially since the particles have momentum away from the process. This consequently leads to that a produced track is basically only constituted by particles that the laser melts and are also not ejected as spatters (which much reduced in this case). This particular behaviour can be considered as a different mode of SLM.
Fig. 3. Frames from HSI, showing upwards movement of particles for cases 1-4, (a)-(d) respectively.
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By measuring the travelled distance after the laser-particle interaction zone, rough movement speeds can be determined, Table 3. The speed of particles seems to be only partially related to the power output of the beam, but rather the quantity of particles which also depends on particle size. The speed of the powder particles is much less than the movement of the laser beam. If the laser beam makes the consecutive track close enough in time, the powder particles could be “caught” before they leave the surface. Movement of powders is somewhat irregular. Particles flying high upwards also moves due to the shielding gas movement. Using the fine sized powder, there is much more movement of particles, Fig. 3b. It is however uncertain if the mass of powders remain the same as in the larger sized particles (Case 1 vs case 2). When increasing the power, Fig. 3c and case 3, there is much more powders flying from the process zone and producing a larger denudation zone, but behaving in a similar fashion. When increasing the travel speed even further, Fig. 3d, the process behaves in a similar fashion but with less high speed particles. This might however be correlated to a lower line energy input. Note that for these videos, any sound tracks where not produced. Table 3. Range of particle movement speed after process zone Case
Particle movement range (m/s) Very fast (very few)
Fast (few)
Mid (many)
1
~1.64
~0.55
~0.4
Slow (most) ~0.19
2
~1.40
~0.75
~0.16
~0.060
3
~1.44
~0.76
~0.24
~0.085
4
-
~0.16
~0.10
~0.059
4. High speed powder bed additive manufacturing As derived from the State of the Art, the underlying effects during PBF are not fully understood yet. The results from high-speed-imaging in this work indicate that the vapour plume has a significant impact on the powder movement on the powder bed and thereby on the subsequent tracks and layers. If the travel speed of the laser beam on the surface is fast enough, powder particles in the path of the laser beam will be caught by the laser beam and the produced melt pool to a much larger extent. The produced bow wave of powders, Fig. 4a will produce a denudation zone with increased height of the powder bed in close proximity to the produced track (some particles will still land on the denudation zone), Figs. 4b-c. If the speed is not too high, some particles in the powder bed might still be able to act as feedstock to the melt pool.
Fig. 4. Principles for SLM with fast travel speeds; (a) side view of powder movement during processing, resulting bead (b) side view and (c) top view.
Figure 5a-d illustrates how the profiles of a series of tracks might look like if process settings are adjusted so that proper fusion of the clad track to the base material is achieved. Depending on how much powder is available to feed the process, weld depth will vary. This consequently leads to varying profiles of the bead. For the second layer, Fig. 5e-h, the varying height of the layer beneath and the added powder layer consequently leads to varying required depths required for the produced track in order to not produce cavities. Keeping record on the produced height of the powder bed might help to adjust process settings when scanning over selected regions, leading to not having to overcompensate over the whole build section during processing, potentially reducing power consumption and increasing processing efficiency.
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Fig. 5. Laser powder bed melting side view of (a) one track and (b-d) consecutive partially overlapping tracks. (e-h) second layer tracks.
5. Conclusions Powder particles fly from the front of the processing zone and the melt pool is dominantly fed by powder particles on the side of the melt pool. This consequently leads to: • Flying powder particles generally have increased speeds at higher angles due to less impact with other particles • Uneven height of the powder bed, possibly caused by spatter from neighbouring tracks, leads to increased instability of produced tracks and powder movement • Neighbouring tracks are consequently much smaller in size because they are only fed from the powder bed from one side When processing at high travel speeds, the behaviour of the process has changed dramatically and can be considered a separate mode for SLM processing: • The feedstock to the produced clad track from the powder bed by the Bernoulli-effect is severely reduced • Particles ejected from the front of the process zone are also reduced, being caught by laser beam and produced melt pool • Clad tracks would likely be dominantly fed by particles directly hit and melted by the laser beam • The height of the powder bed close to the denudation zone will be increased, requiring adaptation of process settings for the neighbouring tracks Acknowledgements The authors are grateful for funding granted by EU-ERDF-InterregNord projects C3TS and CINEMA References [1] [2] [3] [4] [5] [6] [7] [8]
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