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Progress in Powder-Based Additive Manufacturing

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A special issue of Journal of Manufacturing and Materials Processing (ISSN 2504-4494).

Deadline for manuscript submissions: closed (31 March 2023) | Viewed by 35588

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Prof. Dr. Dietmar Drummer

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Guest Editor

Institute of Polymer Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Am Weichselgarten 10, 91058 Erlangen, Germany
Interests: polymer processes; polymer AM

Prof. Dr. Michael Schmidt

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Guest Editor

Institute of Photonic Technologies, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Konrad-Zuse-Straße 3/5, 91052 Erlangen, Germany
Interests: photonics; laser applications

Prof. Dr. David Bourell

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Guest Editor

Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA
Interests: additive manufacturing; powder bed fusion; materials selection; microstructure-property relationships; laser sintering

Special Issue Information

Dear Colleagues,

Additive manufacturing (AM) is widely acknowledged to be a revolutionary manufacturing technology. Regarding powder-based AM, beam-based technologies have attracted the most research and industrial attention, and have grown significantly during the last decade. Due to its high potential for forming almost unrestrictedly complex shapes, powder-based AM allows the production of individualized products and fully functional parts for various engineering problems.

To exploit the full potential of AM processes, current research focuses on process-adapted materials to achieve defined part characteristics in processing and process development for reproducible component performance in terms of locally and globally defined part properties. Further, simulation methods to create a virtual lab for predictive part behavior are needed for the additional development of the process, leading to a highly interdisciplinary field of research.

This Special Issue, “Progress in Powder-Based Additive Manufacturing”, will focus on advances in powder manufacturing, material design, process development, and simulation techniques for powder bed fusion. Of particular significance are original papers dealing with metal and polymer powders as well a laser or electron-beam powder bed fusion. Topics of interest include (but are not strictly limited to) the following:

• Powder manufacturing for AM;
• Characterization of powder properties under process conditions;
• New materials for additive manufacturing;
• Polymers and metals;
• Effect of powder properties on the properties of AM components;
• Fundamental investigations on the powder–process–component relationship;
• Advanced additive manufacturing strategies;
• Modeling and simulation applied to powder bed fusion;
• Product design and topology optimization;
• Smart materials and multi-functional components.

Prof. Dr. Dietmar Drummer
Prof. Dr. Michael Schmidt
Prof. Dr. David Bourell
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Journal of Manufacturing and Materials Processing is an international peer-reviewed open access semimonthly journal published by MDPI.

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Published Papers (13 papers)

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Research

20 pages, 5675 KiB

Open AccessEditor’s ChoiceArticle

Innovative Process Strategies in Powder-Based Multi-Material Additive Manufacturing

by Robert Setter, Jan Hafenecker, Richard Rothfelder, Sebastian-Paul Kopp, Stephan Roth, Michael Schmidt, Marion Merklein and Katrin Wudy

J. Manuf. Mater. Process. 2023, 7(4), 133; https://doi.org/10.3390/jmmp7040133 - 24 Jul 2023

Cited by 3 | Viewed by 2992

Abstract

Multi-material additive manufacturing (AM) attempts to utilize the full benefits of complex part production with a comprehensive and complementary material spectrum. In this context, this research article presents new processing strategies in the field of polymer- and metal-based multi-material AM. The investigation highlights the current progress in powder-based multi-material AM based on three successfully utilized technological approaches: additive and formative manufacturing of hybrid metal parts with locally adapted and tailored properties, material-efficient AM of multi-material polymer parts through electrophotography, and the implementation of UV-curable thermosets within the laser-based powder bed fusion of plastics. Owing to the complex requirements for the mechanical testing of multi-material parts with an emphasis on the transition area, this research targets an experimental shear testing set-up as a universal method for both metal- and polymer-based processes. The method was selected based on the common need of all technologies for the sufficient characterization of the bonding behavior between the individual materials. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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8 pages, 2632 KiB

Open AccessCommunication

Investigation of the Shape and Detectability of Pores with X-ray Computed Tomography

by Benjamin Baumgärtner, Juan Hussein and Tino Hausotte

J. Manuf. Mater. Process. 2023, 7(3), 103; https://doi.org/10.3390/jmmp7030103 - 23 May 2023

Cited by 1 | Viewed by 2409

Abstract

Component porosity is a quality attribute in additive manufacturing (AM). One possibility for the non-destructive three-dimensional determination of porosity or pore shape is X-ray computed tomography (CT), which enables an investigation of the influence of AM process parameters on the appearance and characteristics of the pores. Since there is no porosity standard for CT, a traceable determination of the measurement uncertainty is not possible. Using a digital twin of the CT system, an estimation of the CT measurement uncertainty is in principle possible. In this contribution, experimental CT analyses of powder bed fusion samples made of Ti64 and PA12 are compared with CT simulations. The results show a size-dependent influence on the shape and detectability of the pores. Using the CT model, a simulated shape- and material-dependent probability of detection (POD) is calculated. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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16 pages, 5737 KiB

Open AccessArticle

Thin-Walled Part Properties in Powder Bed Fusion of Polymers—A Comparative Study on Temperature Development and Part Performance Depending on Part Thickness and Orientation

by Andreas Jaksch, Simon Cholewa and Dietmar Drummer

J. Manuf. Mater. Process. 2023, 7(3), 96; https://doi.org/10.3390/jmmp7030096 - 11 May 2023

Cited by 7 | Viewed by 2406

Abstract

To develop new areas of application for laser-based powder bed fusion of polymers (PBF-LB/P), a deeper process understanding of the resulting mechanical properties, particularly for thin-walled and complex structures, is needed. This work addresses the influence of part thickness and orientation in detail. For a general understanding, two PBF systems were used. For comparison, the normalized energy density was determined for specimens of various thicknesses and orientations. It could be seen that the normalized energy density exhibited opposing trends for the two systems for progressively thinner samples. During the process, the exposure temperature development was observed using an infrared camera for a greater understanding of the developing part properties. To further investigate the fracture behavior, an infrared camera was used during tensile testing, which revealed various patterns depending on the PBF-System used. The results showed a machine-dependent difference in the exposure temperatures and elongation at break for z-oriented parts. While the surface roughness was independent of the thickness, the density, porosity, and the mechanical properties were affected significantly by the part thickness. The parts showed a brittle breaking behavior with a crack initiation from the short side of the tensile bar. These results improved process expertise, and in particular the mechanical performance of thin-walled structures caused by temperature variations in PBF-LB/P. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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17 pages, 7196 KiB

Open AccessEditor’s ChoiceArticle

Evaluation of Additively-Manufactured Internal Geometrical Features Using X-ray-Computed Tomography

by Benjamin Baumgärtner, Richard Rothfelder, Sandra Greiner, Christoph Breuning, Jakob Renner, Michael Schmidt, Dietmar Drummer, Carolin Körner, Matthias Markl and Tino Hausotte

J. Manuf. Mater. Process. 2023, 7(3), 95; https://doi.org/10.3390/jmmp7030095 - 10 May 2023

Viewed by 2390

Abstract

X-ray-computed tomography (CT) is today’s gold standard for the non-destructive evaluation of internal component defects such as cracks and porosity. Using automated standardized evaluation algorithms, an analysis can be performed without knowledge of the shape, location, or size of the defects. Both the measurement and the evaluation are based on the fact that the component has no internal structures or cavities. However, additive manufacturing (AM) and hybrid subtractive procedures offer the possibility of integrating internal structures directly during the building process. The examination of powder bed fusion (PBF) samples made of Ti64 and PA12 showed that the standardized evaluation methods were not able to identify internal structures correctly. Different evaluation methods for the CT-measured values were analyzed and recommendations on a procedure for measuring internal structures are given. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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16 pages, 3605 KiB

Open AccessArticle

Influence of Novel Beam Shapes on Laser-Based Processing of High-Strength Aluminium Alloys on the Basis of EN AW-5083 Single Weld Tracks

by Florian Nahr, Dominic Bartels, Richard Rothfelder and Michael Schmidt

J. Manuf. Mater. Process. 2023, 7(3), 93; https://doi.org/10.3390/jmmp7030093 - 9 May 2023

Cited by 13 | Viewed by 3027

Abstract

The commonly used Gaussian intensity distribution during the laser-based processing of metals can negatively affect melt pool stability, which might lead to defects such as porosity, hot cracking, or poor surface quality. Hot cracking is a major factor in limiting production rates of high-strength aluminium alloys in laser-based processes such as welding or the powder bed fusion of metals (PBF-LB/M). Going away from a Gaussian intensity distribution to ring-shaped profiles allows for a more even heat distribution during processing, resulting in more stable melt pools and reduced defect formations. Therefore, the aim of this study is to investigate the influence of different laser beam profiles on the processing of high-strength aluminium alloys by using a multicore fiber laser, allowing for in-house beam shaping. Single weld tracks on the aluminium alloy EN AW-5083 are produced with varying laser powers and weld speeds, as well as different beam profiles, ranging from Gaussian intensity distribution to point/ring profiles. The molten cross sections are analyzed regarding their geometry and defects, and the surface roughness of the weld tracks is measured. By using point/ring beam profiles, the processing window can be significantly increased. Hot cracking is considerably reduced for weld speeds of up to 1000 mm/s compared to the Gaussian beam profile. Furthermore, the melt pool width and depth are more stable, with varying parameters for the point/ring profiles, while the Gaussian beam tends to keyhole formation at higher beam powers. Finally, a strong decrease in surface roughness for the point/ring profiles, accompanied by a significantly reduced humping effect, starting even at lower beam powers of 200 W, can be observed. Therefore, these results show the potential of beam shaping for further applications in laser-based processing of high-strength aluminium alloys. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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21 pages, 10759 KiB

Open AccessEditor’s ChoiceArticle

Alternating Additive Manufacturing and Forming—An Innovative Manufacturing Approach

by Thomas Papke, Jan Hafenecker, David Römisch, Raphaela März, Oliver Hentschel, Dominic Bartels, Michael Schmidt and Marion Merklein

J. Manuf. Mater. Process. 2023, 7(3), 90; https://doi.org/10.3390/jmmp7030090 - 6 May 2023

Cited by 1 | Viewed by 2878

Abstract

In this work, an innovative manufacturing approach that includes a fully linked and integrated manufacturing system consisting of a laser-based directed energy deposition (DED-LB/M) module and a forming press is presented. The alternating additive manufacturing (AM) process is based on a combination of a DED-LB/M process using a laser power of 600 W and a feed rate of 400 mm/min and a subsequent forming process, in which the structure is upset with a hydraulic press using a constant forming force of 500 kN in order to smooth the surface and influence the accuracy of the components. For the generation of a fundamental process understanding, a cuboid, basic shape was chosen as geometry for the investigations. The aim is to improve part properties by applying the process steps to generate part properties, which are superior to solely additive manufactured material. It is shown that the geometry of additive manufactured structures can be adapted, and the top surface can be smoothed due to the forming operation. The mean roughness value Rz decreases up to 50% after the forming operation. The hardness can be increased by work hardening. Of special interest is that the higher hardness can be kept up even though a further DED-LB/M process step and forming operation are applied to the additively manufactured and formed structure again. Finally, an analysis of the new manufacturing approach regarding its potential is given. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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14 pages, 3231 KiB

Open AccessArticle

Understanding Inhomogeneous Mechanical Properties in PBF-LB/M Manufactured Parts Due to Inhomogeneous Macro Temperature Profiles Based on Process-Inherent Preheating

by Jonas Zielinski, Jan Theunissen, Henrik Kruse, Silja-Katharina Rittinghaus, Johannes Henrich Schleifenbaum, Dongjian Zhu and Mustafa Megahed

J. Manuf. Mater. Process. 2023, 7(3), 88; https://doi.org/10.3390/jmmp7030088 - 5 May 2023

Cited by 5 | Viewed by 2731

Abstract

The mechanical properties in laser-based powder bed fusion (PBF-LB/M) manufactured parts are anisotropic in nature due to the layer-wise build-up but also change due to different solidification conditions in dependence on the process strategy and the geometry. In this work, the latter effect is examined by means of simulating the thermal history on a part scale (macro temperature) and correlating the critical temperature and holding time with the local hardness and microstructure. A macro temperature model is introduced and validated with vector-based thermal simulations and thermo couple measurements from the build-up process. Two cone-shaped geometries are investigated, namely, an upright and an inverted cone. The examinations are performed and validated with Inconel 718. An outlook to further investigations and more complex, real-life applicable geometries is given. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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25 pages, 20408 KiB

Open AccessEditor’s ChoiceArticle

A Ray Tracing Model for Electron Optical Imaging in Electron Beam Powder Bed Fusion

by Jakob Renner, Julian Grund, Matthias Markl and Carolin Körner

J. Manuf. Mater. Process. 2023, 7(3), 87; https://doi.org/10.3390/jmmp7030087 - 26 Apr 2023

Cited by 4 | Viewed by 2387

Abstract

The recent success of the process monitoring method Electron Optical Imaging, applied in the additive manufacturing process Electron Beam Powder Bed Fusion, necessitates a clear understanding of the underlying image formation process. Newly developed multi-detector systems enable the reconstruction of the build surface topography in-situ but add complexity to the method. This work presents a physically based raytracing model, which rationalises the effect of detector positioning on image contrast development and masking. The model correctly describes the effect of multiple scattering events on vacuum chamber walls or heat shields and represents, therefore, a predictive tool for designing future detector systems. Most importantly, this work provides a validated method to compute build surface height gradients directly from experimentally recorded electron-optical images of a multi-detector system without any calibration steps. The computed surface height gradients can be used subsequently as input of normal integration algorithms aiming at the in-situ reconstruction of the build surface topography. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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19 pages, 3261 KiB

Open AccessArticle

Geometrical Influence on Material Properties for Ti6Al4V Parts in Powder Bed Fusion

by Florian Nahr, Michael Rasch, Christian Burkhardt, Jakob Renner, Benjamin Baumgärtner, Tino Hausotte, Carolin Körner, Paul Steinmann, Julia Mergheim, Michael Schmidt and Matthias Markl

J. Manuf. Mater. Process. 2023, 7(3), 82; https://doi.org/10.3390/jmmp7030082 - 25 Apr 2023

Cited by 6 | Viewed by 2776

Abstract

One major advantage of additive manufacturing is the high freedom of design, which supports the fabrication of complex structures. However, geometrical features such as combined massive volumes and cellular structures in such parts can lead to an uneven heat distribution during processing, resulting in different material properties throughout the part. In this study, we demonstrate these effects, using a complex structure consisting of three conic shapes with narrow cylinders in between hindering heat flux. We manufacture the parts via powder bed fusion of Ti6Al4V by applying a laser beam (PBF-LB/M) as well as an electron beam (PBF-EB). We investigate the impact of the different thermal regimes on the part density, microstructure and mechanical properties aided by finite element simulations as well as by thermography and X-ray computed tomography measurements. Both simulations and thermography show an increase in inter-layer temperature with increasing part radius, subsequently leading to heat accumulation along the build direction. While the geometry and thermal history have a minor influence on the relative density of the parts, the microstructure is greatly affected by the thermal history in PBF-LB/M. The acicular martensitic structure in the narrow parts is decomposed into a mix of tempered lath-like martensite and an ultrafine

α

β

microstructure with increasing part radius. The EBM part exhibits a lamellar

α

β

microstructure for both the cylindric and conic structures. The different microstructures directly influence the hardness of the parts. For the PBF-LB part, the hardness ranges between 400 HV

_{0.5}

in the narrow sections and a maximum hardness of 450 HV

_{0.5}

in the broader sections, while the PBF-EB part exhibits hardness values between 280 and 380 HV

_{0.5}

. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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15 pages, 4915 KiB

Open AccessArticle

Thermal Intra-Layer Interaction of Discretized Fractal Exposure Strategies in Non-Isothermal Powder Bed Fusion of Polypropylene

by Samuel Schlicht and Dietmar Drummer

J. Manuf. Mater. Process. 2023, 7(2), 63; https://doi.org/10.3390/jmmp7020063 - 10 Mar 2023

Cited by 3 | Viewed by 2291

Abstract

Additive manufacturing of material systems sensitive to heat degradation represents an essential prerequisite for the integration of novel functionalized material systems in medical applications, such as the hybrid processing of high-performance thermoplastics and gelling polymers. For enabling an inherent process stability under non-isothermal conditions at reduced ambient temperatures in laser-based additive manufacturing, maintaining a homogeneous layer formation is of vital significance. To minimize crystallization-induced deflections of formed layers while avoiding support structures, the temporal and spatial discretization of the melting process is combined with the subsequent quenching of the polymer melt due to thermal conduction. Based on implementing superposed, phase-shifted fractal curves as the underlying exposure structure, the locally limited temporal and spatial discretization of the exposure process promotes a mesoscale compensation of crystallization shrinkage and thermal distortion, enabling the essential homogeneous layer formation. For improving the understanding of local parameter-dependent thermal intra-layer interactions under non-isothermal processing conditions, geometric boundary conditions of distinct exposure vectors and the underlying laser power are varied. Applying polypropylene as a model material, a significant influence of the spatial distance of fractal exposure structures on the thermal superposition of distinct exposure vectors can be derived, implicitly influencing temporal and temperature-dependent characteristics of the material crystallization and the emerging thermal material exposure. Furthermore, the formation of sub-focus structures can be observed, contributing to the spatial discretization of the layer formation, representing a decisive factor that influences the structure formation and mesoscopic part properties in non-isothermal powder bed fusion of polymers. Consequently, the presented approach represents a foundation for the support-free, accelerated non-isothermal additive manufacturing of both polymers and metals, demonstrating a novel methodology for the mesoscale compensation of thermal shrinkage. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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14 pages, 4782 KiB

Open AccessEditor’s ChoiceArticle

Effect of the Sintering Conditions on the Neck Growth during the Powder Bed Fusion with Electron Beam (PBF-EB) Process

by Giovanni Rizza, Manuela Galati, Paolo Antonioni and Luca Iuliano

J. Manuf. Mater. Process. 2023, 7(2), 55; https://doi.org/10.3390/jmmp7020055 - 1 Mar 2023

Cited by 2 | Viewed by 2184

Abstract

A distinctive characteristic of the powder bed fusion with electron beam (PBF-EB) process is the sintering of the powder particles. For certain metallic materials, this is crucial for the success of the subsequent step, the melting, and, generally, the whole process. Despite the sintering mechanisms that occur during the PBF-EB process being similar to well-known powder metallurgy, the neck growth rates are significantly different. Therefore, specific analyses are needed to understand the influence of the PBF-EB process conditions on neck growth and neck growth rate. Additionally, some aspects, such as the rigid body motion of the particles during the sintering process, are still challenging to analyze. This work systematically investigated the effects of different particle diameters and particle diameter ratios. Additionally, the impact of the rigid body motion of the particles in the sintering was analyzed. This work demonstrated that the sintering results significantly depended on the EB-PBF process conditions. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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20 pages, 6330 KiB

Open AccessEditor’s ChoiceArticle

Process-Structure-Property Interdependencies in Non-Isothermal Powder Bed Fusion of Polyamide 12

by Samuel Schlicht, Simon Cholewa and Dietmar Drummer

J. Manuf. Mater. Process. 2023, 7(1), 33; https://doi.org/10.3390/jmmp7010033 - 30 Jan 2023

Cited by 6 | Viewed by 2550

Abstract

Non-isothermal laser-based powder bed fusion (LPBF) of polymers suggests the potential for significantly extending the range of materials applicable for powder-based additive manufacturing of polymers, relying on the absence of a material-specific processing window. To allow for the support-free manufacturing of polymers at a build chamber temperature of 25 °C, applied processing strategies comprise the combination of fractal exposure strategies and locally quasi-simultaneous exposure of distinct segments of a particular cross section for minimizing crystallization-induced part deflection. Based on the parameter-dependent control of emerging cooling rates, formed part morphologies and resulting mechanical properties can be modified. Thermographic in situ measurements allow for correlating thermal processing conditions and crystallization kinetics with component-specific mechanical, morphological, and microstructural properties, assessed ex situ. Part morphologies formed at crystallization temperatures below 70 °C, induced by reduced laser exposure times, are characterized by a nano-spherulitic structure, exhibiting an enhanced elongation at break. An ambient temperature of 25 °C is associated with the predominant formation of a combined (α + γ)-phase, induced by the rapid cooling and subsequent laser-induced tempering of distinct layers, yielding a periodic microstructural evolution. The presented results demonstrate a novel approach for obtaining nano-spherulitic morphologies, enabling the exposure-based targeted adaption of morphological properties. Furthermore, the thermographic inline assessment of crystallization kinetics allows for the enhanced understanding of process-morphology interdependencies in laser-based manufacturing processes of semi-crystalline polymers. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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27 pages, 7438 KiB

Open AccessArticle

Al-Cu-Mg Alloy Powder Reinforced with Graphene Nanoplatelets: Morphology, Flowability and Discrete Element Simulation

by Mulla Ahmet Pekok, Rossitza Setchi, Michael Ryan, Heng Gu, Quanquan Han and Dongdong Gu

J. Manuf. Mater. Process. 2022, 6(6), 148; https://doi.org/10.3390/jmmp6060148 - 21 Nov 2022

Cited by 4 | Viewed by 3057

Abstract

Research in metal matrix composites (MMCs) indicates that superior mechanical properties may be achieved by embedding reinforcement materials. However, the development of new composite powder for additive manufacturing requires an in-depth understanding of its key characteristics prior to its use in the fabrication process. This paper focuses on the low-energy ball milling (LEBM) of aluminium 2024 alloy (AA2024) reinforced with graphene nanoplatelets (GNPs). The main aim is to investigate the effect of the milling time (from 0.5 to 16 h) on the morphology and flowability of the powder. The study shows that, while short milling times (under 2 h) could not break the Van der WaRals forces between nanoparticles, GNPs were well separated and sufficiently covered the powder surface after 4 h of milling, thanks to the continuously applied impact energy. Longer milling time provides increasingly similar flowability results, confirmed by both the experimental work and discrete element model (DEM) simulations. Moreover, the ball milling process decreases the crystallite size of the milled powder by 24%, leading to a 3% higher microhardness. Lastly, the surface energy of the powder was determined as 1.4 mJ/m² by DEM, using the angle of repose of the as-received powder from experimental work. Full article

(This article belongs to the Special Issue Progress in Powder-Based Additive Manufacturing)

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