Investigations on processing copper-titanium powder blends via PBF-LB/M
Christoph Hecht, Daniel Schüller, Daniel Utsch, Thomas Stoll, Jörg Franke

Copper-titanium containing alloys are widely used in active metal brazing of metal-ceramic-joints due to the ability of titanium to wet ceramic components. An important application for such active metal brazed substrates is the production of power electronic circuit carriers, because ceramics, compared to other commonly used insulation materials like epoxy, provide superior characteristics with regard to thermal management and reliability. In this work, a parametric study on processing copper-titanium powder blends with a titanium share of 5 wt.-% and 10 wt.-% via laser-based powder bed fusion of metals is presented. The way of alloying the materials in-situ with the laser is a cost-effective approach to avoid expensive pre-alloyed powders. The study shows suiting process windows to generate samples with a residual porosity of 1 % and less. Additionally, the alloying behavior of the samples is assessed via energy-dispersive X-ray spectroscopy before and after subsequent thermal treatments of the samples.

Keywords: PBF-LB/M; In-Situ Alloying; Copper; Titanium; Active Metal Brazing

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Process-structure-property relationships of additively manufactured lattice structures based on triply periodic minimal surfaces (TPMS)
F. Günther, M. Wagner, S. Pilzc, A. Gebert, M. Zimmermann

Lattices based on triply periodic minimal surfaces (TPMS) are attracting increasing interest due to their excellent structure-property relationships. However, the potential can only be exploited if their structural integrity is ensured. This requires a fundamental understanding of the impact of imperfections that arise during additive manufacturing. Therefore, in the present study, the structure-property relationships of TPMS lattices, including their imperfect morphologies, are investigated experimentally and numerically. Specifically, the focus is on biomimetic TPMS lattices fabricated by laser powder bed fusion (LPBF) from the biocompatible alloy Ti-42Nb. Based on computed tomography analyses, typical LPBF-imperfections are identified before a modeling procedure is developed for reconstruction of the as-built morphology. Finally, compression tests are performed and compared with the accompanying finite element studies. This work highlights the central importance of process-related imperfections for the structure-property relationships of LPBF-processed TPMS lattices and provides a numerical tool to capture their effects. Given high simulation accuracy and flexibility, this approach might become a key factor in the future design process of additively manufactured structures.

Keywords: lattice structures; triply periodic minimal surfaces; additive manufacturing; imperfect lattices; numerical reconstruction

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Influences of optical errors in the SLM process with high separation angle beam splitting
Florian Joachim Oskar Spieth, Thorsten Heeling, Hans-Christian Moehring

Higher productivity, especially higher build rates are necessary for a widespread use of additive manufacturing in series production. In selective laser melting (SLM) these improvements are currently focusing on strategies for beam shaping and splitting to increase the power input in the melt pool. Nevertheless, the power to be absorbed in the melt pool is limited by its size which leads to the trade-off between accuracy and build rate. To avoid this limit, the use of a beam splitter with resulting spot distances of over 25 mm is investigated. This could be used to build up three parts in parallel. While this offers great potential for productivity increases, optical errors like distance, focus and rotation errors are inherent to the setup and need to be understood for future compensation strategies. The influences of these errors are analyzed and discussed for stainless steel 316L specimens.

Keywords: beam splitting; selective laser melting; stainless steel 316L

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Development of a tree-support software module for PBF-LB/M
Jan Hünting, José Manuel Crego Lozares, Maria Isabelle Maiwald, Jochen Michael, Claus Emmelmann, Tim Röver

By combining simulation and generative design, a software tool was developed that generates individual support structures in the form of trees. The objective is that even users with little experience can manufacture components with efficient support structures using PBF-LB/M. The key element of the method is the use of tree structures whose shape is generated by algorithmic, biological growth using botanical methods. Topology optimization, which has already proven itself in the design of optimal support structures, was used to optimize the support structures of three benchmark parts. The generated trees were then compared to commercially available tree-like supports and block supports using Finite element analysis to simulate the PBF-LB/M process. The simulation also included a stress relief heat treatment and the removal of the part from build plate and supports. Based on the knowledge gained from the numerical assessment of the generated tree supports, the created tool allows the user to generate highly material-usage efficient supports with block support-comparable capabilities without requiring any previous experience of the subject.

Keywords: Support structure; Additive manufacturing; Waste reduction; Generative design; Tree support; PBF-LB/M

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Characterization of the bistable melting regime for processing of copper with an NIR laser under LPBF conditions
Marvin Kippels, Daniel Heußen, Norbert Pirch, Constatin Leon Häfner

LPBF of pure copper using an infrared laser is considered challenging because only limited parameter ranges are usable to achieve highest densities (>99.5%). The absorption coefficient of copper for the infrared wavelength is often used as a central influencing parameter but does not reflect the characteristic transition behavior. An abrupt transition to keyhole welding when the energy input is increased limits the usable parameter range. In the present work, the transition behavior is characterized on basis of single weld tracks generated under LPBF conditions. In addition to the process parameters laser power, beam diameter and scanning speed, different surface qualities are included in the consideration of the transition behavior. In this context, a process parameter range defined by a melt pool alternating between heat conduction and keyhole welding is observed. This bistable range can be characterized, among other things, with the quotient of laser power and beam diameter.

Keywords: Keyhole; copper; LPBF; infrared; melt pool stability

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High nitrogen steels produced by PBF-LB/M – Influence of particle size distribution and process parameters on melting behavior of additivated steel powders
Felix Radtke, Louis Becker, Simone Herzog, Jonathan Lentz, Sebastian Weber, Christoph Broeckmann

Additivated powders allow an extension of suitable materials for the powder bed fusion laser beam process (PBF-LB/M). High nitrogen steel (HNS) powders and components are limited by the nitrogen solubility of the melt and thus possible interstitial solid solution strengthening. In this study, Si3N4 powder is added to the austenitic steel powder AISI 304L. The PBF-LB/M process is adapted to shape components in which the Si3N4 particles are only partially dissolved in the melt. In a subsequent hot isostatic pressing (HIP) process, nitrogen diffuses into the component, taking advantage of the high nitrogen solubility of the austenite. The particle size distribution of the powders and the process parameters of the PBF-LB/M process were varied in the context of this work. Undissolved Si3N4 particles are identified and the total nitrogen contents are quantified to evaluate the process parameters and melting behavior of Si3N4 additivated austenitic steel powders. The final material is evaluated regarding density and microstructure.

Keywords: PBF-LB/M; high nitrogen steels; 304L Si3N4; additivation; melting behavior

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Laser powder bed fusion of high-density glass
Brian Seyfarth, Tobias Ullsperger, Hagen Peter Kohl, Burak Yürekli, Lisa Matthäus, Sven Padutsch, Stefan Nolte

Glass as a material poses a challenge for laser powder bed fusion (L-PBF) due to the comparably high viscosity even at high temperatures and low absorption for laser wavelengths in the visible and near infrared spectrum. As its superb optical properties, low thermal and electrical conductivity and chemical properties makes it desirably for various applications, several alternative approaches were developed for making the additive manufacturing of glass feasible. Here we demonstrate the printing of glass parts obtaining densities above 99% directly from the powder bed without the need for thermal post processing by using suitable process parameters and heating of the powder bed. Although not completely free of bubbles, with the advantages of the L-PBF process, the realization of almost any desired geometry impossible to create with common molding or subtractive processes is within reach.

Keywords: Additive manufacturing; L-PBF; Glass; Density; Porosity; Soda Lime; CO2-Laser

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Bimetallic structure formation from powder mixture by laser powder bed fusion
Ada Steponavičiūtė, Karolis Stravinskas, Aušra Selskienė, Genrik Mordas

Bimetallic structures are an excellent solution for a lot of engineering applications which require varying properties at different locations of the same object. Implementation of such structures into engineering fields can lead to easier maintenance, economical and space savings and can also open wider application possibilities. In recent years, production of bimetallic structures has been made possible with help of additive manufacturing (AM) technologies. Using laser powder-bed fusion AM, bimetallic structures can be created by depositing different materials in a layer-by layer fashion.
In this work, two materials in powder form were used for bimetallic structure formation – CoCrMo and 17-4 PH stainless-steel. Two different bimetallic structures – one sandwich-like and one made from the two-material powder mixture, were successfully produced by using the L-PBF technology. In-depth analysis of the 17-4 PH and CoCrMo materials and microstructural properties of the produced bimetallic samples were investigated. A gradual change in chemical element distribution is observed at the two-material fusion zone of the sandwich-like specimen while the mixed powder specimen showed even elemental distribution throughout the alloy. The thickness of the fusion zone in the sandwich-like specimen is around 600-630 μm. The hardness values of the fusion zone (46±1 HRC) and of the mixed powder alloy (44±1) are higher than the hardness of 17-4 PH (43±1 HRC) but lower than CoCrMo (50±1 HRC). The experimentally evaluated density of the sandwich-like and mixed powder bimetallic specimens are 7.96 g/cm3 and 8.06 g/cm3 respectively. The difference in values proves that the bimetallic alloy possesses unique characteristics that are not specific to either of the materials.

Keywords: additive manufacturing; laser powder bed fusion; bimetallic structure

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Manufacturing of transparent quartz glass via laser powder bed fusion
T. Schmidt, C. Scholz, S. Kasch, M. Kahle

Recent developments for the additive manufacturing of glass components, the process parameters, the employed system technology and methods of material qualification are presented. The process of laser based powder bed fusion (PBF-LB) – often called selective laser melting as well – for the manufacturing of transparent components from quartz glass powder uses a defocused laser beam for layer-by-layer melting of the glass powder and a focused laser beam for a parallel contour processing during the process. The focused, pulsed laser beam has a higher pulse peak power, and adhering particles in the edge area of the melted component are removed. After the particle edge is removed, the produced samples exhibit relatively low roughness, high transparency and usual density values of fused quartz.
The process will be explained in detail, as well as perspectives and challenges. The PBF-LB process with glass materials will be compared to metals, and properties (roughness, density, stress, accuracy) of produced quartz-glass pieces will be shown.

Keywords: PBF-LB; quartz glass powder; CO2 laser; laser beam melting; 3D quartz glass components; transparency

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Additive manufacturing of highly resolved pure copper parts
Hagen Peter Kohl, Tobias Ullsperger, Brian Seyfarth, Burak Yürekli, Stefan Nolte

The general feasibility of pure copper using the laser assisted powder bed fusion (L-PBF) process has been repeatedly demonstrated over the last few years. The high thermal and electrical conductivity combined with the geometric freedom of the L-PBF process offers a wide range of applications. However, these studies also reveal that it is difficult to achieve a high density combined with a high surface quality and resolution. This shows that the thermal management, which in general is crucial in L-PBF processes, is even more important for high thermally conductive materials. We demonstrate the fabrication of pure copper parts offering a homogeneous and high density above 99 %, superior surface quality with an average roughness around 3 μm combined with a high resolution with features below 250 μm.

Keywords: laser assisted powder bed fusion; copper; high resolution; thermal management

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Fabrication of an athermal mirror from a hyper-eutectic AlSi alloy via LPBF
Christoph Wilsnack, Juliane Moritz, Arnd Reutlinger, Sebastian Eberle, Lukas Stepien, Elena Lopez, Frank Brückner, Christoph Leyens

This study explores the use of a combination of the hypereutectic aluminum-silicon alloy AlSi40 and electroless nickel (NiP) for optical mirrors in space-borne instruments. The combination of AlSi40 and NiP offers a solution to the trade-offs between optical performance, structural integrity, manufacturing time and price that traditional materials used for optical space applications have. AlSi40 is processable by additive technologies like Laser Powder Bed Fusion (LPBF), which allows the design and fabrication of optical components with optimized internal structures (e.g. lattice structures) resulting in increased stiffness-to-mass ratio of the component, which is crucial for space applications. The process chain, including parameter development and material characterization, was conducted and a demonstrator mirror was printed and tested under representative operational conditions. The study also includes the transfer process of the parameters and the experimental conditions between several AM machines, to ensure the scalability and reproducibility of the process.

Keywords: Laser Powder Bed Fusion; Additive Manufacturing; Quality Assurance; Space; AlSi40; hyper-eutectic alloy

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Enhancement of additive manufactured soft magnetic components by precise air gaps combining laser powder bed fusion and ultrafast laser ablation
David Kolb, Markus Hofele, Manuel Henn, Matthias Buser, Volkher Onuseit, Thomas Graf, Harald Riegel

Laser Powder Bed Fusion (PBF-LB) additive manufacturing offers great potential for the production of efficiency- and performance-enhancing soft magnetic materials and components for electrical machines. One way to reduce remagnetization losses in soft magnets is to inhibit the propagation of eddy currents through geometric design. This results in the need for very fine eddy current inhibiting insulating air gaps. However, PBF-LB is limited in terms of achievable accuracy and minimum size of electrically separated structures >100 μm due to the inherent micro-melting process. In this work, the PBF-LB process is combined with layer-wise ultrafast laser ablation to create precise air gaps in the range of 25-45 μm in soft magnets made of pure iron. Magnetic characterization revealed a reduction of iron losses in the alternating magnetic field with frequencies of 25-200 Hz of up to 44% in the as-built condition and up to 39% in the heat-treated condition.

Keywords: Selective laser melting; Ultrashort pulsed laser; Magnets; Laser ablation; Internal structures; Combined machining

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Enhancing Laser Powder Bed Fusion processes thanks to beam shaping with Multi-Plane Light Conversion
Adeline Orieux, Avinash Kumar, Aymeric Lucas, Simone Barani, Martina Vincetti, Francesca Lazzarini, Adrien Douard, Gwenn Pallier, Guillaume Labroille

This paper focuses on processing new materials, such as difficult-to-process Ni-based superalloys, using Laser Powder Bed Fusion (L-PBF) in the additive manufacturing industry. L-PBF involves scanning a laser over a bed of powder to melt it where a 3D part needs to be constructed. The two main challenges in L-PBF are improving process yield and processing new materials, which both may be solved with the shaping of the laser beam.
Multi-Plane Light Conversion (MPLC) technology enables industrial robust beam-shaping solutions compatible with L-PBF machines. In this paper we will describe the process improvements using different beam shapes. Especially, these beam-shapers allow for better management of the thermal gradient during the process, resulting in a finer and equiaxed microstructure, reduced residual stress, and decreased risk of solidification cracking. The paper will describe the beam-shapers’ performance and process results on different materials, including their mechanical performance analysis.

Keywords: Laser; Additive Manufacturing; powder; Nickel; 3D-Printing; beam shaping; MPLC; cracking; porosity

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Advanced laser irradiation strategies for tailoring temperature profiles for laser-based powder bed fusion (PBF-LB/P) of highly absorbing polymers
Vadim Medvedev, Sebastian-Paul Kopp, Stephan Roth

Additively manufactured parts consisting of polymer powders blended with carbon black have increased protection against ultraviolet radiation and thus are beneficial for applications involving intensive exposure to sunlight. However, this powder blends are challenging to be processed in laser-based powder bed fusion of polymers (PBF-LB/P) due to their high absorptivity of infrared radiation. Thus, precisely tailoring the energy input provided by a CO2-laser is crucial for enhancing process stability for processing highly absorptive polymers. Adapting the laser energy input by applying advanced irradiation strategies offers the possibility to tailor the temperature profiles during PBF-LB/P process. In this study, the effect of advanced laser irradiation strategies with pixel-encoded energy densities on the adaption of the temperature profile of polymer powders blended with carbon black is investigated. Finally, irradiation patterns are defined to achieve spatially tailored temperature profiles across the powder layer for demonstration the capabilities of the developed scanning strategy.

Keywords: Laser powder bed fusion; laser illumination strategy; carbon black; temperature profile; selective laser sintering

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On the challenges of hybrid repair of gas turbine blades using laser powder bed fusion
Benjamin Merz, Konstantin Poka, Ricardo Nilsson, Gunther Mohr, Kai Hilgenberg

Additive manufacturing (AM) processes such as laser powder bed fusion (PBF-LB/M) are rapidly gaining popularity in repair applications. Gas turbine components benefit from the hybrid repair process as only damaged areas are removed using conventional machining and rebuilt using an AM process. However, hybrid repair is associated with several challenges such as component fixation and precise geometry detection. This article introduces a novel fixturing system, including a sealing concept to prevent powder sag during the repair process. Furthermore, a high-resolution camera within an industrial PBF-LB/M machine is installed and used for object detection and laser recognition. Herein, process related inaccuracies such as PBF-LB/M laser drift is considered by detection of reference objects. This development is demonstrated by the repair of a representative gas turbine blade. The final offset between AM build-up and component is analysed. An approximate accuracy of 160 μm is achieved with the current setup.

Keywords: laser powder bed fusion, additive manufacturing, hybrid repair, position detection, high-resolution camera

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Femtosecond laser Additive Manufacturing with self-produced Stainless Steel powder at low Scanning Speed and low Energy
Iñigo Ramon-Conde, Ainara Rodriguez, Santiago Miguel Olaizola, Mikel Gomez-Aranzadi

The use of USP lasers for AM entails a more complex laser-matter interaction and greater difficulty in producing heat accumulation compared to the laser sources present in industrial systems, due to the usage of ultra-short intense pulses. Usually, the strategy followed with USP lasers in order to trigger the melting of particles is to use a high frequency regime (around 10 MHz) to accumulate heat. In this work, a novel low frequency (<1 MHz) and low power (<1 W) strategy is presented. With it, a minimum wall thickness of 100 μm has been achieved thanks to our own-produced stainless steel powder particles, which is in line with other works. This new strategy enables a more precise control of the spatial distribution of heat, where the minimum size will depend not on the type and power level of the laser source, but on the size and quality of the powder grains.

Keywords: Femtosecond laser; Laser Powder Bed Fusion; Ultrashort pulse; Stainless Steel Powder; High precision manufacturing

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In-situ defect detection via active laser thermographic testing for PBF-LB/M
Philipp Peter Breese, Tina Becker, Simon Oster, Christian Metz, Simon J. Altenburg

Great complexity characterizes Additive Manufacturing (AM) of metallic components via laser powder bed fusion (PBF-LB/M). Due to this, defects in the printed components (like cracks and pores) are still common. Monitoring methods are commercially used, but the relationship between process data and defect formation is not well understood yet. Furthermore, defects and deformations might develop with a temporal delay to the laser energy input. The component’s actual quality is consequently only determinable after the finished process.
To overcome this drawback, thermographic in-situ testing is introduced. The defocused process laser is utilized for nondestructive testing performed layer by layer throughout the build process. The results of the defect detection via infrared cameras are shown for a research PBF-LB/M machine.
This creates the basis for a shift from in-situ monitoring towards in-situ testing during the AM process. Defects are detected immediately inside the process chamber, and the actual component quality is determined.

Keywords: Additive Manufacturing; Laser Powder Bed Fusion; Nondestructive Testing; Thermography; Defect Detection

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Investigation on the correlations between heat accumulations and actual part defects during laser powder bed fusion using heat maps
Sven Müller, Ronald Pordzik, Thorsten Mattulat

During laser powder bed fusion processes, heat accumulations can lead to e.g. annealing colors, increased porosity or protruding edges and are one of the reasons for the necessity of support structures. Using constant manufacturing process parameters, heat accumulations often cannot be avoided due to varying heat dissipation cross sections and gradients. In order to selectively avoid heat accumulations, it is necessary to identify in which part sections heat accumulations occur and at what extent heat accumulations become critical for the part quality. In this study, coaxial two-channel pyrometric measurements are used to generate high-resolution heat maps of the individual layers during laser powder bed fusion. Suitable methods for benchmarking heat accumulations with inner and outer part quality (e. g. porosity) are presented for different geometry characteristics like varying overhang angles, part volumes and shapes. The suitability of such heat maps to identify and visualize especially heat accumulations critical for part quality is analyzed.

Keywords: Additive Manufacturing; Laser Powder Bed Fusion; Heat Map; Temperature Measurement; Defect Detection