Fundamental analysis of the influence of powder characteristics in selective laser melting of molybdenum based on a multi-physical simulation model

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1 19 th Plansee Seminar RM 26/1 Fundamental analysis of the influence of powder characteristics in selective laser melting of molybdenum based on a multi-physical simulation model K.-H. Leitz*, C. Grohs*, P. Singer*, B. Tabernig*, A. Plankensteiner*, H. Kestler*, L. S. Sigl* * Plansee SE, Metallwerk-Plansee-Straße 71, 6600 Reutte, Austria Abstract Selective Laser Melting (SLM) offers great potential for the processing of molybdenum as it allows the production of parts that can hardly be fabricated in a classical way. The workpiece is built up by melting up powder layer by layer. Besides the applied laser parameters, the characteristics of the powder and the deposited powder layer have a crucial influence on the dynamics of the process and the final processing result. In order to get a fundamental understanding of these influences, numerical simulations are a versatile tool as they allow a detailed look on isolated parameters. In this contribution a coupled thermo-fluid dynamical simulation model for SLM is applied for a fundamental analysis of the influence of powder characteristics in SLM of molybdenum. The results allow a deeper insight into the influences of powder particle size and powder particle arrangement on the SLM process and the formation of the molten track. Keywords Selective laser melting, multi-physical simulation, powder, molybdenum, Comsol Multiphysics, Liggghts Selective Laser Melting A Promising Technology for the Processing of Molybdenum In industrial manufacturing significant changes occur currently, caused by the increasing digitalization of production. Those changes due to the connection between production and information technologies are predicted to be as dramatic as those observed when mechanization, mass production and electronics led to revolutions of industrial fabrication in the past. Therefore, it is generally talked about the fourth industrial revolution or Industry 4.0 [1]. In order to make Industry 4.0 work new manufacturing technologies are required that allow a fast, precise, efficient and flexible fabrication of functional products from digital data. At this additive manufacturing by Selective Laser Melting (SLM) is highly promising as it allows the fabrication of fully functional metal machine and tool parts. SLM offers unique design possibilities at arbitrary lot sizes and is highly material efficient [2]. In SLM a workpiece is built up from a powder bed. A thin powder layer as well as a part of the subjacent layer is molten up by a laser

19 th Plansee Seminar RM 26/2 controlled by a scanner system. When the material solidifies again, a melt metallurgic connection between adjacent and subjacent lines is formed.

The principle of SLM is shown in Fig. 1 a. Fig. 1 b shows the SLM process during operation.

2 19 th Plansee Seminar RM 26/2 controlled by a scanner system. When the material solidifies again, a melt metallurgic connection between adjacent and subjacent lines is formed. The building platform is lowered, a new powder layer is delivered and the process starts from the beginning [3]. At this layer-wise 3D printing of massive, metal free form components can be realized. The principle of SLM is shown in Fig. 1 a. Fig. 1 b shows the SLM process during operation. Whereas SLM of materials like aluminum, steel, titanium, nickel and cobalt chromium alloys is already well established and already applied in industrial production, the processing of molybdenum by SLM is still a big technical challenge as the processing window is significantly narrower. Nevertheless, for complex workpiece geometries, SLM is an attractive alternative to classical powder metallurgical fabrication routes and by a proper choice of processing parameters complex geometrical structures can be fabricated. Fig. 1 c and d show examples of thin walled, tapered grid structures as well as massive demonstrator parts that were fabricated at Plansee SE by SLM of molybdenum. For a successful, stable and repeatable processing of molybdenum by SLM a fundamental process understanding is essential. For this aim multi-physical transient process simulations are a powerful tool in order to analyze the process on a mesoscopic level, study defect formation mechanisms and to learn about the influence of processing parameters and powder characteristics on process dynamics and processing result. However, modelling of laser beam-matter interaction is a challenging task, as it requires a coupling between optics, thermo- and fluid-dynamics. Approaches for multi-physical laser beam-matter interaction modelling have been developed for a variety of laser processes [4 10] including SLM [11 15]. Similar models have been developed for the electron beam melting process [16 18]. At Plansee SE a thermo-fluid dynamical simulation model following those approaches was developed and applied for an analysis of SLM of molybdenum [19 22]. Figure 1: Selective Laser Melting (SLM): (a) technology principle, (b) process during operation and (c, d) molybdenum demonstrator parts fabricated by SLM at Plansee SE.

19 th Plansee Seminar RM 26/3 Multi-Physical Simulation Model for Selective Laser Melting The multi-physical simulation model for SLM is based on Comsol Multiphysics.

3 19 th Plansee Seminar RM 26/3 Multi-Physical Simulation Model for Selective Laser Melting The multi-physical simulation model for SLM is based on Comsol Multiphysics. It includes the absorption of laser radiation on the metal surface, conductive and convective heat transfer in metal and atmosphere as well as melting, solidification, evaporation and condensation processes. The thermo-fluid dynamical multi-phase description is based on the phase field approach [23]. Depending on the temperature the model distinguishes between solid, liquid and vapor metal. The solid metal is treated as a high viscous fluid and the surface tension is restricted to the liquid phase. Evaporation modelling takes into account the density change between solid and vapor as well as the resulting mass flux by following an approach described in [24]. Fig. 2 shows a result obtained with the simulation model for SLM of steel at typical processing parameters for this material and demonstrates the included physics. The model geometry consists of a powder layer on a massive base plate. The laser radiation is absorbed on the top surface of the metal powder. The material heats up, a melt pool is formed and evaporation occurs. The laser moves along the powder bed and when the material solidifies again, the molten track is formed. The simulation model uses temperature dependent material data taken from [25 29] (see Table I). A detailed description of the simulation model, the contained physics and its implementation in Comsol Multiphysics can be found in [22]. In the past the model was applied in order to analyze the influence of process parameters in line and layer buildup from steel and molybdenum [19 21]. Fig. 3 shows simulation results for SLM of molybdenum at standard parameters for volume buildup. It was shown that the model is able to describe material specific differences in SLM. Whereas for steel at typical processing parameters there is a long melt pool and evaporation occurs (compare Fig. 2), for molybdenum the melt pool is significantly shorter and no evaporation can be observed in the simulation at the regarded parameters (compare Fig. 3). In accordance to experimental data the simulation results showed that for molybdenum the dependency of line width and surface roughness on the power P [W] is severely pronounced, whereas for steel the energy input ρ E [J m] is the dominant process parameter. This is due to the high thermal conductivity of molybdenum [19 21]. Figure 2: Multi-physical simulation model for SLM (SLM of 40 µm steel powder at standard parameters for volume buildup (standard power P 0,steel and standard energy input ρ E0,steel )).

4 19 th Plansee Seminar RM 26/4 Table I: Temperature dependent material data for molybdenum and air atmosphere [25 29]. Figure 3: Simulation results for SLM of 40 µm molybdenum powder at standard parameters for volume buildup (standard power P 0,Mo and standard energy input ρ E0,Mo ): (a) line buildup and (b) layer buildup. Simulation Results and Discussion Beside material and processing parameters, in SLM the process dynamics and the processing result are also significantly determined by the characteristics of the deposited powder layer. Therefore, the multiphysical simulation model for SLM described in the previous section shall be applied as follows in order to analyze the influence of powder size and arrangement in SLM of molybdenum. Beside regular arrangements of powder particles in the powder bed (i.e. periodic simple cubic packaging), a more realistic powder distribution with irregular powder packaging obtained from a discrete element simulation model shall be considered. Idealized Powder Arrangement In a first step four idealized molybdenum powder arrangements on a massive base plate were considered in order to analyze the influence of particle size, layer thickness and packing density. The following powder configurations were compared to each other: one and two layers of simple cubic arranged 20 µm powder, one simple cubic layer of 40 µm powder and one layer of simple cubic 20 µm powder with one line of particles missing. Emanating from standard energy input ρ E0,Mo and standard

5 19 th Plansee Seminar RM 26/5 power P 0,Mo for volume buildup from molybdenum, the influences of a reduced relative energy input ρ E ρ E0,Mo and a reduced relative power P P 0,Mo were analyzed. Relative energy input ρ E ρ E0,Mo was varied from 100 % to 44 %, the relative power P P 0,Mo from 100 % to 50 %. The simulated molten tracks and cross sections along the processing axis at the borders of the regarded parameter window are shown in Figs In Fig. 8 width and depth of the molten tracks are plotted for the regarded powder configurations and parameter sets. Width was defined as the difference of maximum and minimum y coordinate of the simulated molten track. Melting depth was measured with respect to the original substrate surface at z = 0 m. Width and depth of the molten tracks of the simple cubic layer of 20 µm powder show no significant dependence on the energy input, but depend strongly on the power (compare Fig. 8). This result is in accordance to earlier results on the influence of processing parameters in SLM of molybdenum [18, 19]. Whereas at a reduced relative energy input of ρ E ρ E0,Mo = 44 % the width of the molten track is comparable to that obtained at standard processing parameters along with still good connection between molten track and substrate, a reduced relative power of P P 0,Mo = 50 % is no longer sufficient to completely melt up the particles in the simulation and no connection to the substrate is obtained (see Fig. 4). For two layers of simple cubic 20 µm powder the widths of the molten tracks are slightly higher as the energy removal in a thicker powder layer is worse. Therefore, in contrast to one powder layer, even at a reduced relative power of P P 0,Mo = 50 % still a significant amount of powder is molten up, even if no connection to the substrate is obtained (compare Fig. 5). Compared to the previous regarded powder configuration the absolute value of melting depth is smaller as more powder materials needs to be molten up. Furthermore, a slight dependence of width and depth of the molten track on the energy input is obtained, whereas power dependency is less pronounced (compare Fig. 8). This can be explained by the effectively lower thermal conductivity of the powder bed, leading to a more pronounced energy input dependency [19, 20]. Figure 4: Simulation results for molten tracks generated by SLM of one simple cubic layer of 20 µm molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 50 %.

19 th Plansee Seminar RM 26/6 Figure 5: Simulation results for molten tracks generated by SLM of two simple cubic layers of 20 µm molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E

6 19 th Plansee Seminar RM 26/6 Figure 5: Simulation results for molten tracks generated by SLM of two simple cubic layers of 20 µm molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 50 %. Figure 6: Simulation results for molten tracks generated by SLM of one simple cubic layer of 40 µm molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 50 %. The melt pool widths for one simple cubic layer of 40 µm powder are slightly higher than for one layer of finer powder particles. At a relative power of P P 0,Mo = 100 % connection between molten track and substrate is obtained, whereas at a lower relative power of P P 0,Mo = 50 % the powder particles are not molten up completely. Beside this the molten tracks show higher width variations caused by wetting of neighboring powder particles (compare Fig 6). It becomes obvious that bigger particles in the powder layer are critical as incomplete melting can lead to insufficient connection of powder layers. Furthermore, line roughness can be higher due to wetting effects and partially molten adhering particles. The energy input dependency of the 40 µm powder is on a similar scale as for the two layers of 20 µm powder, whereas the power dependency is comparable to that of one layer of 20 µm powder (compare Fig 8). Formation of track width and depth of the fourth considered powder configuration with a line defect in a simple cubic layer of 20 µm powder are on the same scale as those of the first one (compare Fig 8). However, at the position of the line defect the molten track shows a necking as there is less material to be melted and the energy removal through the substrate is more efficient (compare Fig 7). These results demonstrate that a loose or irregular powder arrangement in SLM incorporates the risk of line width variations or, even worse, an interruption of the molten track.

7 19 th Plansee Seminar RM 26/7 Figure 7: Simulation results for molten tracks generated by SLM of one simple cubic layer of 20 µm molybdenum powder with a line defect: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 50 %. Figure 8: Width and depth of lines fabricated by SLM obtained from the simulation model for different powder configurations (sc20: 1 simple cubic layer of 20 µm powder, 2sc20: 2 simple cubic layers of 20 µm powder, sc40: 1 simple cubic layer of 40 µm powder, sc20defect: 1 simple cubic layer of 20 µm powder with a line defect): (a) energy input dependency and (b) power dependency. The simulation results obtained for the four idealized powder configurations clearly demonstrate that size and arrangement of the powder have a significant influence on the SLM process and the processing result. A multi-physical simulation based analysis of SLM must not only look at processing parameters but also needs to take into account the morphological characteristics of the powder layer. Therefore, in the following a realistic powder distribution with irregular particle arrangement shall be considered. Realistic Powder Arrangement Getting information about the 3D powder arrangement in an SLM powder layer by experimental methods is not trivial. Other authors have used random packing algorithms [15 18] or discrete element modelling [14, 20] to generate a realistic powder distribution. In this work also a discrete element based particle modelling approach was chosen. For this purpose the open source discrete element method particle

19 th Plansee Seminar RM 26/8 simulation software LIGGGHTS (LAMMPS Improved for General Granular and Granular Heat Transfer Simulations) was applied.

8 19 th Plansee Seminar RM 26/8 simulation software LIGGGHTS (LAMMPS Improved for General Granular and Granular Heat Transfer Simulations) was applied. In order to generate a covering thin powder layer, three wipes of a blade with a gap width of 40, 80 and 120 µm were simulated in the particle simulation. Before each wipe molybdenum powder was inserted with a medium particle size of 20 µm. Fig. 9 shows intermediate particle configurations at four different time instants of the third wipe of the discrete element particle simulation. From this calculated particle distribution a volume of 500 µm x 220 µm x 130 µm was cut out and taken as an initial geometry for the multi-physical simulation model for SLM. Fig. 10 shows different time steps during line fabrication from this more realistic powder distribution with irregular particle arrangement at standard parameters for volume buildup. It can be seen that powder particle arrangement significantly influences the process dynamics in SLM. Larger particles need more energy to melt up and wetting of adjacent particles influences the fluid dynamics in the melt pool. The molten tracks obtained from the simulation at the regarded parameter sets are shown in Fig. 11. Compared to the idealized simple cubic powder distributions, the molten tracks obtained from the realistic powder arrangements show much more irregularities, both in shape and topology of the molten track. Melting depth variations are higher compared to an idealized simple cubic powder arrangement. Furthermore, line width variations occur due to wetting of big neighboring particles. Figure 9: Discrete element simulation results for different time instants (time t1 < t2 < t3 < t4) of the powder deposition process.

19 th Plansee Seminar RM 26/9 Figure 10: Simulation results for different stages (time t1 <

powder arrangement at standard parameters for volume buildup (standard power P 0,Mo and

Figure 11: Simulation results for molten tracks generated by SLM of molybdenum powder with

9 19 th Plansee Seminar RM 26/9 Figure 10: Simulation results for different stages (time t1 < t2 < t3 < t4) of line buildup by SLM of molybdenum powder (d = 20 µm) with a realistic powder arrangement at standard parameters for volume buildup (standard power P 0,Mo and standard energy input ρ E0,Mo ). Figure 11: Simulation results for molten tracks generated by SLM of molybdenum powder with a realistic powder arrangement: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, P P 0,Mo = 50 %.

19 th Plansee Seminar RM 26/10 Figure 12: Scanning electron microscope images and microsections of lines fabricated by SLM of molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo P P 0,Mo = 50 %.

10 19 th Plansee Seminar RM 26/10 Figure 12: Scanning electron microscope images and microsections of lines fabricated by SLM of molybdenum powder: (a) ρ E ρ E0,Mo = 100 %, P P 0,Mo P P 0,Mo = 50 %. = 100 %, (b) ρ E ρ E0,Mo = 44 %, P P 0,Mo = 100 % and (c) ρ E ρ E0,Mo = 100 %, Figure 13: Width and depth of lines fabricated by SLM obtained from simulation and experiment: (a) energy input dependency and (b) power dependency. For a validation of the simulation results the latter were compared to experimental data. Fig. 12 shows scanning electron microscope (SEM) images as well as microsections of lines fabricated at corresponding parameters by SLM of a monolayer of molybdenum powder. Width was determined from SEM images taken at view angles of 0. From the microsections both width and depth could be measured. Track widths independently determined by those different methods show good correlation. For each parameter set three samples were analyzed. Width and depth of the molten tracks obtained from simulation and experiment are plotted in Fig. 13 and show good correlation. The simulated values show the right size scale and correct trend. The energy input dependency appears more pronounced in the experimental data. The simulated widths and

11 19 th Plansee Seminar RM 26/11 absolute melting depths are slightly smaller than measured in experiment. This can be explained by discretization errors in the simulation leading to an overestimation of heat conduction losses. Conclusions Multi-physical modelling is a powerful tool to get a fundamental understanding of SLM. The presented coupled thermo-fluid dynamical simulation model is able to describe the influence of material, processing parameters and powder characteristics on the process dynamics and the processing result in SLM. The results demonstrate that the characteristics of the powder layer have a significant influence on the processing result in SLM of molybdenum. Processing parameters need to be adapted to the characteristics of the powder layer in order to assure a connection between adjacent and superimposed molten tracks. Variations in powder particle size as well as imperfections in the powder layer can lead to variations in the width of the molten track or insufficient connection between the layers. Due to the smaller melt pool in SLM of molybdenum the process is more sensitive to the morphological characteristics of the powder bed compared to other materials. Therefore, in SLM of molybdenum a high quality, customized powder and a proper choice of processing parameters are essential to obtain the high quality and repeatable processing result required in industrial fabrication. References 1. H. Kagemann, W. Wahlster and J. Heblig, Final report of the Industrie 4.0 Working Group, (2013) 2. R. Schiffler, VDI Nachrichten 15, pp , (2015) 3. R. Poprawe, Lasertechnik für die Fertigung, Springer Verlag, (2015) 4. M. Geiger, K.-H. Leitz, H. Koch and A. Otto, Production Engineering 3, pp , (2009) 5. A. Otto, H. Koch, K.-H. Leitz and M. Schmidt, Physics Procedia 12, pp , (2011) 6. K.-H. Leitz, H. Koch, A. Otto and M. Schmidt, Applied Physics A 106, pp , (2012) 7. K.-H. Leitz, Mikro- und Nanostrukturierung mit kurz und ultrakurz gepulster Laserstrahlung, Meisenbach Verlag, (2013) 8. S. Kohl, K.-H. Leitz, M. Schmidt, Proceedings of the 37th MATADOR Conference, Manchester, UK, (2012) 9. M. Dobler, K.-H. Leitz, A. Otto, M. Schmidt, Proceedings of the ICALEO 2013, Miami, USA, (2013) 10. M. Courtois, M. Carin, P. Le Masson, S. Gaied and M. Balabane, Journal of Laser applications 26, , (2014) 11. F.-J. Gürtler, M. Karg, K.-H. Leitz and M. Schmidt, Physics Procedia 41, pp , (2013) 12. F.-J. Gürtler, M. Karg, M. Dobler, S. Kohl, I. Tzivilsky and M. Schmidt, Solid freeform fabrication Symposium, Austin, USA (2014) 13. S. A. Khairallah, A.T. Anderson, A. Rubenchik and W.E. King, Acta Materialia 108, pp , (2016) 14. J. Zielinski, H.-W. Mindt, M. Megahed and S. Vervoort, Proceedings of the MAMC 2016, November 2016, Linz, Austria, (2016)

12 19 th Plansee Seminar RM 26/ C. Panwisawas, C. Qiu, M. J. Anderson, Y. Sovani, R. P. Turner, M. M. Attallah, J. W. Brooks, H. C. Basoalto, Computational Materials Science 126, pp , (2017) 16. E. Attar, Simulation of Selective Electron Beam Melting Process, Dissertation Universität Erlangen- Nürnberg, (2011) 17. C. Körner, A. Bauereiß and A. Attar, Modelling and Simulation in Materials Science and Engineering 21, , (2013) 18. A. Bauereiß, T. Scharowsky and C. Körner, Journal of Materials Processing Technology 214, pp , (2014) 19. K.-H. Leitz, P. Singer, A. Plankensteiner, B. Tabernig, H. Kestler and L.S. Sigl, Proceedings of Euro PM2015, 4 7 October 2015, Reims, France, (2015) 20. K.-H. Leitz, P. Singer, A. Plankensteiner, B. Tabernig, H. Kestler and L.S. Sigl, Metal Powder Report, (2016) - in press 21. K.-H. Leitz, P. Singer, A. Plankensteiner, B. Tabernig, H. Kestler and L.S. Sigl, Proceedings of the MAMC 2016, November 2016, Linz, Austria, (2016) 22. K.-H. Leitz, P. Singer, A. Plankensteiner, B. Tabernig, H. Kestler and L.S. Sigl, Proceedings of the 2016 Comsol Conference, October 2016, Munich, Germany, (2016) 23. P. Yue, J. J. Fengn, C. Liu and J. Shen, J. Fluid Mech. 515, pp , (2004) 24. Y. Sun and C. Beckermann, Physica D 198, pp , (2004) 25. Comsol Multiphysics Material Library Version 5.2, COMSOL AB 26. C.J. Smithells, Metals Reference Book, Butterworth, (1967) 27. H. Stöcker, Taschenbuch der Physik, Harri Deutsch Verlag, (2000) 28. P.-F. Paradis, T. Ishikawa and N. Koike, International Journal of Refractory Metals & Hard Materials 25, pp , (2007) 29. W.D. Wood, H.W. Deem and C.F. Lucks, Handbook of High-Temperature Materials Thermal Radiative Properties, Plenum Press, (1964) 30. E. J. R. Parteli and T. Pöschel, Powder Technology, pp , (2016)

Thermo-Fluiddynamical Modelling of Laser Beam-Matter Interaction in Selective Laser Melting

Thermo-Fluiddynamical Modelling of Laser Beam-Matter Interaction in Selective Laser Melting K.-H. Leitz 1*, P. Singer 1, A. Plankensteiner 1, B. Tabernig 1, H. Kestler 1, L.S. Sigl 1 1 Plansee SE, Metallwerk-Plansee-Straße