Laser Powder Bed Fusion (LiM 2021)

Adjusting the surface roughness of WE43 components manufactured by laser-based powder bed fusion
Tjorben Griemsmann, Niclas Söhnholz, Christian Hoff, Jörg Hermsdorf, Stefan Kaierle

The outstanding characteristics of magnesium alloys make them promising materials for biomedical or lightweight construction applications, especially in combination with the advantages of laser-based powder bed fusion. While most research in this field focusses porosity and microstructural properties, the surface quality is left out. Because the surface is an important factor for corrosion and notch effects, this work addresses the adjustment of the surface roughness from parts made out of a WE43 alloy. Using design of experiments contour scan trials are carried out for vertical and down skin surfaces. As a result, the roughness of vertical surfaces is reduced from approximately 27.1 μm (Ra) and 172.2 μm (Rz) without contour scans to 13.4 μm and 109.8 μm with contour scans. The applicability of the contour parameters is approved by cross sections to investigate the porosity of the contour volume interface.

Keywords: Laser-based powder bed fusion; Magnesium; Surface roughness; Design of experiments


Influence of laser focus shift on porosity and surface quality of additively manufactured Ti-6Al-4V
Nicole Emminghaus, Christian Hoff, Jörg Hermsdorf, Stefan Kaierle

In laser-based powder bed fusion of metals (PBF-LB/M) an increase of the laser spot size by shifting the focus position (defocusing) offers the opportunity of reducing the overall scanning time as well as achieving a more stable melt pool behavior. However, the influence on porosity and surface roughness of bulk samples has received little attention so far. In this work, the influence of laser defocusing (Yb-fiber laser, minimum beam diameter of 35 μm) on part porosity as well as top and side surface roughness is investigated for additively manufactured Ti-6Al-4V. Therefore, the focusing lens position relative to its standard setting is investigated in a range between 1.2 mm and -8.7 mm. Additionally, the main processing parameters are varied and their influences and interaction effects are statistically evaluated according to the design of experiments approach. Optimum settings for low porosity and surface roughness are presented.

Keywords: Laser powder bed fusion; additive manufacturing; Ti-6Al-4V; defocusing; porosity; surface quality


Machine-comprehensive study of comparability and reproducibility for laser powder bed fusion of corrosion resistant steel 316L/1.4404
Florian Bittner, Bernhard Müller, Aitor Echaniz, Sebastian Matthes, Burghardt Klöden, Christian Kolbe

Additive Manufacturing of metallic components by means of laser-based powder bed techniques earns increasing importance for industrial applications due to the increased sustainability resulting from the high resource efficiency and an effective value chain. However, for further industrial penetration different challenges have to be overcome. The most urging challenge is the warranty and control of a constant high quality of the components. This includes the requirement of a reliable good machine-comprehensive comparability of components goodness. Important factors are the respective machine concept, which differs remarkably in inert gas conduction or powder supply, the quality of the powder with its morphological properties, age and storage conditions, as well as the respective system parameters. The results of a standard VDI 3405-2 based round robin test for the steel 316L (1.4404) are discussed, at which five partners with different machines participated. The implementation is not based on ideal conditions, but addresses the respective best practice of the participants, which covers industrial reality at a high degree. Thereby, the differences between included machine concepts and scattering within a manufacturing order is discussed. With this, the existing gap of standardization of properties for laser powder bed fusion of the well-established material 316L/1.4404 shall be closed analogue to a series of other materials within the VDI-standard family 3405. Based on the results obtained, manufacturing processes can be designed to be more resource-efficient and thus contribute to the sustainability of the used additive manufacturing processes.

Keywords: laser powder bed fusion; steel; reproducibility, round robin test


Laser powder bed fusion of pure copper using a 1000 W green laser
G. Nordet, C. Gorny, P. Lapouge, A. Effernelli, E. Blanchet, F. Coste and P. Peyre

Additive manufacturing of copper is still considered as complex using the usual IR laser wavelength. For the L-PBF process, the density of copper parts stays at an average value of less than 99 %. These low densities of parts could be explained by the high reflectance at IR wavelength and the high conductivity of pure copper, which necessitates using more power than other materials. In this context, changing the laser wavelength from near IR to green, with the recently developed industrial sources, allows to improve the laser-matter interaction and reduce the power needed to satisfactorily densify copper. In the current study, we demonstrate the possibility to implement a 1000 W green laser on a dedicated L-PBF test bench using a 15-45 μm pure copper powder. A parametric study is then carried out to create dense parts. Such an innovative work allows creating up to 99.9 % dense parts of pure copper which is much better than all the recent investigations using IR laser sources.


Manufacturing knowledge: model instead of experience, a big step towards reproducibility and first-time-right in the production of complex component geometries using PBF-LB/M
Hannes Korn, Stefan Holtzhausen, Felix Gebhardt, Claudia Ortmann, Ralph Stelzer, Welf-Guntram Drossel

The cost structure and geometry freedom of laser powder bed fusion (PBF-LB/M) holds great potential for lightweight-capabilities, customization and on-demand manufacturing of metal parts. Obstacles currently exist in first time right manufacturing and reliable reproducibility under changing process conditions. Reasons are the many setting variables (laser parameters, process parameters, scan strategy) and disturbance variables (powder batch, operator, ambient conditions), which have a difficult to quantify influence on the quality characteristics of the component (warpage, surface roughness, porosity).
Compared to the so far widespread experience based parameterization of the process, statistical modeling has great potential for describing and understanding quantitatively the effects of the setting- and disturbance variables on the quality characteristics. The influence of scan strategy and laser parameters on the warpage and surfaces of PBF-LB/M-components is evaluated on cantilever-like bridge specimens according to an optimized experimental plan. The relation between setting variables and quality characteristics is quantified in a linear model approach and its predictive power is evaluated.

Keywords: scan strategy; parameter development; statistical modeling; warpage; complex geometry


Oxide dispersion strengthened steel manufactured by laser powder bed fusion and directed energy deposition
C.Doñate-Buendia, P.Kürnsteiner, M.B.Wilms, B.Gault, B.Gökce

Additive manufacturing technologies are ideally suited for the generation of custom geometries and parts. In the context of specific applications such as high-temperature industrial processes like gas turbines or furnaces, the development of parts with enhanced high-temperature strength and oxidation resistance is highly desired. Oxide dispersion strengthened (ODS) steels are considered as suitable materials for such high temperature applications. To assess the effect of the processing technique on the manufacturing of ODS steels and its properties, an Fe-Cr based steel powder coated with 0.08 wt% of laser generated Y2O3 nanoparticles is processed by laser powder bed fusion (LPBF) and directed energy deposition (DED). We show that the produced specimens show superior mechanical properties at 600o C compared to the reference part built without nanoparticle-addition. The enhanced mechanical properties are explained by the microstructure and nanoparticle dispersion in the generated ODS steels.

Keywords: Laser powder bed fusion; Laser ablation in liquids; Selective laser melting; Directed energy deposition; Laser metal deposition


Investigation of Kovar in PBF-LB/M
Arvid Abel, Jakob Pufal, Vitaly Rymanov, Christian Hoff, Jörg Hermsdorf, Sumer Maklouf, Jörg Lackmann, Andreas Stöhr, Stefan Kaierle

The iron-nickel-cobalt alloy Kovar is highly desirable in glass-to-metal hybrid components, e.g., hermetic seals, or as packaging material in high-frequency microsystems due to its thermal expansion coefficient similar to borosilicate glass. Hitherto, the processability of Kovar in additive manufacturing has only been insufficiently investigated, leaving the potential of this material for functional integrated components unused. This paper describes the processing in PBF-LB/M and the understanding of the process parameters to achieve a relative density over 99.9 % in test specimens, large volumes, and complex structures. The investigated factors were laser power, scanning speed, and hatch distance. The initial experiments were done as full factorial designs. Subsequent investigations were done within the design of experiments to develop an empirical process model for the fabrication of Kovar in the PBF-LB/M. The best results were fabricated with volumetric energy densities between 200 to 350 to achieve a maximum density of 99.96 %.

Keywords: Powder Bed Fusion by Laser Beam; Kovar; Additive Manufacturing; Design of Experiments; Additive Processing


Comparison of different density measurement techniques for laser assisted powder bed fusion
Lisa Schade, Gabor Matthäus, Hagen Kohl, Roland Ramm, Burak Yürekli, Tobias Ullsperger, Brian Seyfarth, Stefan Nolte

One of the major quality control criteria for additively manufactured parts is the density achieved. Besides fundamental properties like microstructure, residual strain or impurities, the density fundamentally defines how the final product matches the intended material properties. In general, mostly surface inspections of randomly prepared cross sections are undertaken. On the one hand side, this approach delivers important information regarding the morphology and distribution of pores, however, on the other hand side, this characterization only considers a small fraction of the entire sample volume and therefore cannot reflect the true density without a significant level of uncertainty. In this work, we investigate four different measurement techniques, sizing and weighing, surface inspections, x-ray tomography and Archimedes´ principle with a focus on their advantages and disadvantages. The results show significant differences of the obtained density values with deviations in the range of several percent depending on the underlying material and sample size.

Keywords: Powder bed fusion; Selective laser melting; Additive manufacturing; 3D printing; Density measurement


Influence of process-relevant parameters and heat treatments on the microstructure and resulting mechanical behavior of additively manufactured AlSi10Mg via Laser Powder Bed Fusion
Andreas Kempf, Leonardo Agudo Jácome, Kai Hilgenberg

Within the group of additive manufacturing (AM) technologies for metals, laser powder bed fusion (L-PBF) has a leading position. Nevertheless, reproducibility of part properties has not reached sufficient maturity hindering the use for industrial applications especially for safety-relevant components. This article presents the results of various experimental tests performed with the aluminium alloy AlSi10Mg identifying reasons for the high deviations in mechanical properties. Herein, it is discussed how microstructure is influenced by different process parameters (laser power, scanning speed, energy density, building height) and how it can be adjusted by suitable post process heat treatments. The impact of resulting changes in microstructure is shown by monotonic tensile and cyclic fatigue tests considering specimens manufactured with different L-PBF machines.

Keywords: Additve manufacturing; Laser powder bed fusion; AlSi10Mg; Microstructure; Mechanical behavior


Development of SLM 3D printing system using Galvano scanner for pure copper additive manufacturing by 200 W blue diode laser
Keisuke Takenaka, Yuji Sato, Koji Tojo, Masahiro Tsukamoto

Selective laser melting (SLM) is one of laser additive manufacturing technologies. Because absorptance of blue light on pure copper materials is higher than that of conventional near-infrared light, a blue diode laser is expected to effective in shaping pure copper parts. In our previous study, we developed a high power and high intensity blue diode laser with the wavelength of 450 nm. Output power and fiber core diameter was 200 W and 100 μm, respectively. In this study, we have developed a SLM machine using Galvano scanner and the 200 W blue diode laser. The number of stacked layers were changed to form a pure copper parts in the SLM method, and the influence of them on the cross-sectional area of the parts was investigated.

Keywords: SLM; blue laser; copper


Development of a machine concept for the processing of Ti-6Al-4V in the PBF-LB/M process under silandized argon atmosphere
Marijan Tegtmeier, Nicole Emminghaus, Jannes August, Marius Lammers, Christian Hoff, Jörg Hermsdorf, Ludger Overmeyer, Stefan Kaierle

The presence of oxygen in the PBF-LB/M process leads to embrittlement in the workpiece in materials with high affinity to oxygen. Especially the metal powder Ti-6Al-4V requires a special protective atmosphere during processing. By doping the argon 1.5 % with monosilane, the residual oxygen of a usual argon atmosphere is bound and reduced to a value typical for XHV (Extreme High Vacuum).
Basically, the development of an PBF-LB/M system according to VDI 2221 is presented. The admixture of silane requires an innovative machine concept in order to ensure the compatibility of the materials used and to prevent the process gases from becoming hazardous. The phases of development are accompanied by comprehensive reaction studies and flow simulations. The resulting concept relies on a compact machining area (Ø100x100mm) and breaks new ground in the processing of special materials, not only through the process gases used, but also in powder and workpiece management.

Keywords: Laser powder bed fusion; PBF-LB/M, Design; CFD; flow simulation; Ti-6Al-4V; oxygen free


Laser-based powder bed fusion with 16 kW
Artur Leis, Stefan Bechler, Rudolf Weber, Thomas Graf

Laser-based Powder Bed Fusion (LPBF) is typically performed at laser powers between 500 - 1000 W, and diameters of the laser beam between 50 μm - 500 μm. As the build rate is directly connected with the applied laser power, a reduction of the process time requires an increase of the applied laser power. In order to build large parts in a sufficient time, the implementation of high laser powers in LBPF is of high interest.
To increase the build rate, the laser power was set to be 16 kW and the diameter of the laser beam was determined to generate continuous melt beads. Additively manufactured samples of AlSi10Mg were used for the high-power experiments. The melting process was recorded with a high-speed camera. The generated beads were analysed metallographically to determine the extent and shape of the molten region and the porosity.
Diameters of the laser beam between 2.5 - 3.8 mm, feed rates within a range of 0.5 - 1.5 m/s lead at the laser power of 16 kW to continuous melt beads but show also strong hydrogen-induced porosity.

Keywords: laser-based powder bed fusion; high power; additive manufacturung; hydrogen; porosity


Effect of microstructure for additively manufactured Ti64 plate on modulated pulses by vacuum SLM
Yuta Mizuguchi, Tuneyoshi Arimura, Masahiro Ihama, Yuji Sato, Norio Yoshida, Minoru Yoshida, Masahiro Tsukamoto

Selective Laser Melting (SLM), one of additive manufacturing technologies, can fabricate complex shapes such as lattice structures, porous structures, and biological structures. In addition, the ability to create the final shape allows for cost reduction and weight reduction of parts. Since the laser is irradiated layer by layer, the lower layers are affected by heat. As heat accumulates, the temperature of the entire object increases, and the aging effect promotes the growth of crystal grains in the lower layer, resulting in anisotropy in the object. In this study, we attempted to control the grain size by precisely controlling the amount of heat input for each layer in the layer-by-layer fabrication of Ti64 plates. The amount of heat input was controlled using a modulated pulsed laser. One layer was formed by changing the frequency of the modulation pulse, and the effects on the surface system and crystal grains of the formed object were investigated. As a result, the surface roughness increased and the grain size decreased as the frequency increased. A 10-layer Ti64 sample was fabricated and compared with samples fabricated using a modulated pulse laser and a continuous wave (CW) laser. The results showed that the grain uniformity of the samples fabricated using the modulated pulse laser was higher than that of the samples fabricated using the CW laser. This suggests that it is possible to control the material structure by modulated pulse laser.

Keywords: SLM; Heat input; Modulated pulses; Pulse energy; Grain size


Energy coupling in laser powder bed fusion of copper using different laser wavelengths
Klaus Behler, Daniel Heussen, Marvin Kupper, Norbert Pirch, Tim Lantzsch, J.H. Schleifenbaum

Highly conductive pure copper is crucial for high current applications in electrical and mechanical engineering. Additive manufacturing of components from pure copper using laser powder bed fusion (LPBF) with conventional machine technology and infrared laser radiation at λ = 1070 nm is challenging due to high reflectivity of copper for infrared laser light. Fraunhofer ILT has been investigating possibilities to use lasers within the visible spectral range (green @ λ = 515 nm and blue @ λ = 450 nm) in the LPBF process. It has been shown that there is potential to improve energy coupling and process stability applying such lasers in the LPBF process. In this paper calorimetric absorptivity measurements are presented, showing the influence of wavelength and process parameters as well as material conditions on the effective energy input for the melting of copper substrate material as well as of powder material.

Keywords: Energy coupling, copper, absorptivity, green laser, additive manufacturing, powder bed, laser powder bed fusion


3D printing of Al-Li with increased Li content using laser assisted powder bed fusion
Burak Yürekli, Dongmei Liu, Tobias Ullsperger , Hagen Kohl, Lisa Schade, Gabor Matthäus, Markus Rettenmayr, Stefan Nolte

Based on the extraordinary low atomic mass of Li, Al-Li alloys hold high potential for future lightweight construction materials. In particular, the elastic modulus of Al-Li alloys increases significantly with rising Li content, offering the potential of extremely high stiffness as compared to conventional Al alloys. However, due to the formation of brittle δ-AlLi during conventional casting processes, the maximum Li content in commercial Al-Li alloys is generally limited to about 2 wt. %. Here we present laser assisted 3D printing using Al-Li alloy powders with an increased Li content of about 4 wt. %. The process is based on custom-made Al-Li powders, which is characterized in terms of powder particle size, density, absorption, and thermal conductivity. In contrast to common approaches, ultrashort laser pulses are used for the melting process, delivering 3D printed parts with a drastically reduced fraction of δ-AlLi phase due to the increased solidification rates in the melt pool.

Keywords: Additive manufacturing; Selective laser melting; Powder bed fusion; 3D printing; Aluminum–lithium alloy


3D printing of high-density copper parts using common NIR CW laser systems at moderate powers
Hagen Kohl, Lisa Schade, Gabor Matthäus, Tobias Ullsperger, Burak Yürekli, Brian Seyfarth, Bernd Braun, Stefan Nolte

Additive manufacturing (AM) of pure copper using laser assisted powder bed fusion (LPBF) at a wavelength of 1070 nm is demonstrated. In comparison to established LPBF materials, pure copper exhibits an extremely high reflectivity for wavelengths around 1 μm and the highest thermal conductivity among other AM materials. Although, pure copper is one of the most interesting materials for AM, the interplay of these characteristics still prevents copper to be applied using common laser-based AM machines. In this work, we demonstrate a processing window for 3D-printing of high-density copper parts based on a fiber laser as widely used in common AM machines. These achievements were obtained with the help of a self-developed numerical model that guided our experimental studies during the LPBF process. After process optimization, relative densities over 99 % could be demonstrated without the help of intense preheating or post processing like hot isostatic pressing.

Keywords: Additive manufacturing; Powder bed fusion; 3D printing; Pure copper; Numerical simulation;


Additive manufacturing of conductive copper traces on 3D geometries by laser-sintering
Ejvind Olsen, Ludger Overmeyer

These days, additive manufacturing processes cover an extensive range of materials. A new trend is a growing interest in the implementation of additional functions like electrical circuits. Combining full-surface primer and copper ink coating from printed electronics with laser processing enables integrating conductive traces directly on the surface of 3D-printed components. Priming reduces the roughness of the 3D printed circuit carrier below 100 nm. Afterward, the metal-containing ink is dip-coated, dried, and sintered locally by laser processing. The used laser system includes a focused and pulsed 1064 nm laser beam controlled by a scanner with three optical axes (x, y and z-direction). This research presents a detailed investigation on the influence of 3D geometrical factors like radii and sidewall angles on the resulting conductive trace resistance.

Keywords: Laser sintering; Printed electronics; 3D printing; Copper ink; Epoxy priming