Special Issue "Challenges in Additive Manufacturing: From Coupon Studies to Components"

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Materials Simulation and Design".

Deadline for manuscript submissions: 30 June 2021.

Special Issue Editor

Dr. Jan Haubrich
E-Mail Website
Guest Editor
Institute of Materials Research, German Aerospace Center (DLR), Linder Hoehe, Cologne, 51147, Germany
Interests: additive manufacturing; material characterization; processing and part manufacturing strategies; process monitoring; titanium alloys, surfaces and interfaces; surface chemistry and physics; adhesive bonding; theoretical modelling; density functional theory

Special Issue Information

Dear Colleagues,

Additive manufacturing (AM) of metallic materials still faces serious hurdles that need to be overcome before a widespread utilisation, particularly for highly loaded and safety-relevant components, can be achieved. A variety of technologies, such as laser powder bed fusion, direct energy deposition and metal binder jetting, are available today and new approaches, such as multi-material printing, are under development.

On the coupon level, efforts are being made to address challenges in processing strategies and materials characterisation. In this respect, much potential is recognised in the development of novel alloys tailored to the metallurgical conditions that prevail in AM.

Another crucial step in AM is to transfer the knowledge that is obtained from these fundamental studies to component manufacturing while acknowledging the role that local part geometry plays in the resulting thermal history and material properties. Individual and often complex build, processing and post-treatment strategies must be developed. Simulations of AM processes are underway that can enhance our understanding of AM materials and guide the development of component manufacturing strategies.

This Special Issue shall be dedicated to new developments in metal and multi-material AM, encompassing (yet not limited to):

  • materials properties–process relationships;
  • multi-material AM and new AM approaches;
  • process modelling, simulations and digitalisation;
  • in operando process monitoring;
  • alloy development for AM;
  • transfer of process and materials knowledge from coupons to components;
  • component manufacturing: build and post-processing strategies; and
  • material characterisation in complex AM components.

I invite you to contribute original, high-quality submissions to this Special Issue. Regular and review articles as well as short communications from materials research, theoretical modelling, engineering and process simulation are welcome.

Dr. Jan Haubrich
Guest Editor

Manuscript Submission Information

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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. Materials is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • additive manufacturing (AM)
  • laser-, electron- and sintering-based AM techniques
  • hybrid additive manufacturing
  • manufacturing chains: part-specific build and post-treatment strategies
  • materials characterisation (e.g., defects, microstructure, texture, residual stress, surface properties)
  • special scanning and processing strategies (including high-temperature processing)
  • heat and surface post-treatments
  • in operando monitoring
  • part and materials validation
  • process modeling and simulation
  • quantum-chemical materials design
  • digitalisation

Published Papers (8 papers)

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Research

Open AccessArticle
Microstructures and Mechanical Properties of Hybrid, Additively Manufactured Ti6Al4V after Thermomechanical Processing
Materials 2021, 14(4), 1039; https://doi.org/10.3390/ma14041039 - 22 Feb 2021
Viewed by 505
Abstract
In the present study, we propose a hybrid manufacturing route to produce high-quality Ti6Al4V parts, combining additive powder laser directed energy deposition (L-DED) for manufacturing of preforms, with subsequent hot forging as a thermomechanical processing (TMP) step. After L-DED, the material was hot [...] Read more.
In the present study, we propose a hybrid manufacturing route to produce high-quality Ti6Al4V parts, combining additive powder laser directed energy deposition (L-DED) for manufacturing of preforms, with subsequent hot forging as a thermomechanical processing (TMP) step. After L-DED, the material was hot formed at two different temperatures (930 °C and 1070 °C) and subsequently heat-treated for stress relief annealing. Tensile tests were performed on small sub-samples, taking into account different sample orientations with respect to the L-DED build direction and resulting in very good tensile strengths and ductility properties, similar or superior to the forged material. The resulting microstructure consists of very fine grained, partially globularized alpha grains, with a mean diameter ~0.8–2.3 µm, within a beta phase matrix, constituting between 2 and 9% of the sample. After forging in the sub-beta transus temperature range, the typical L-DED microstructure was no longer discernible and the anisotropy in tensile properties, common in additive manufacturing (AM), was significantly reduced. However, forging in the super-beta transus temperature range resulted in remaining anisotropies in the mechanical properties as well as an inferior tensile strength and ductility of the material. It was shown, that by combining L-DED with thermomechanical processing in the sub-beta transus temperature range of Ti6Al4V, a suitable microstructure and desirable mechanical properties for many applications can be obtained, with the advantage of reducing the material waste. Full article
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Open AccessCommunication
Epitaxial Growth of Silicon on Silicon Wafers by Direct Laser Melting
Materials 2020, 13(21), 4728; https://doi.org/10.3390/ma13214728 - 23 Oct 2020
Viewed by 446
Abstract
Additive manufacturing (AM) of brittle materials remains challenging, as they are prone to cracking due to the steep thermal gradients present during melting and cooling after laser exposition. Silicon is an ideal brittle material for study since most of the physical properties of [...] Read more.
Additive manufacturing (AM) of brittle materials remains challenging, as they are prone to cracking due to the steep thermal gradients present during melting and cooling after laser exposition. Silicon is an ideal brittle material for study since most of the physical properties of single-element materials can be found in the literature and high-purity silicon powders are readily available. Direct laser melting (DLM) of silicon powder was performed to establish the conditions under which cracks occur and to understand how the solidification front impacts the final microstructure. Through careful control of process conditions, paying special attention to thermal gradients and the growth velocity, epitaxial pillars free of cracks could be grown to a length of several millimeters. Full article
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Open AccessArticle
Pyrometric-Based Melt Pool Monitoring Study of CuCr1Zr Processed Using L-PBF
Materials 2020, 13(20), 4626; https://doi.org/10.3390/ma13204626 - 16 Oct 2020
Viewed by 436
Abstract
The potential of in situ melt pool monitoring (MPM) for parameter development and furthering the process understanding in Laser Powder Bed Fusion (LPBF) of CuCr1Zr was investigated. Commercial MPM systems are currently being developed as a quality monitoring tool with the aim of [...] Read more.
The potential of in situ melt pool monitoring (MPM) for parameter development and furthering the process understanding in Laser Powder Bed Fusion (LPBF) of CuCr1Zr was investigated. Commercial MPM systems are currently being developed as a quality monitoring tool with the aim of detecting faulty parts already in the build process and, thus, reducing costs in LPBF. A detailed analysis of coupon specimens allowed two processing windows to be established for a suitably dense material at layer thicknesses of 30 µm and 50 µm, which were subsequently evaluated with two complex thermomechanical-fatigue (TMF) panels. Variations due to the location on the build platform were taken into account for the parameter development. Importantly, integrally averaged MPM intensities showed no direct correlation with total porosities, while the robustness of the melting process, impacted strongly by balling, affected the scattering of the MPM response and can thus be assessed. However, the MPM results, similar to material properties such as porosity, cannot be directly transferred from coupon specimens to components due to the influence of the local part geometry and heat transport on the build platform. Different MPM intensity ranges are obtained on cuboids and TMF panels despite similar LPBF parameters. Nonetheless, besides identifying LPBF parameter windows with a stable process, MPM allowed the successful detection of individual defects on the surface and in the bulk of the large demonstrators and appears to be a suitable tool for quality monitoring during fabrication and non-destructive evaluation of the LPBF process. Full article
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Open AccessArticle
Direct Energy Deposition of TiAl for Hybrid Manufacturing and Repair of Turbine Blades
Materials 2020, 13(19), 4392; https://doi.org/10.3390/ma13194392 - 01 Oct 2020
Cited by 2 | Viewed by 701
Abstract
While repair is mainly used to restore the original part geometry and properties, hybrid manufacturing aims to exploit the benefits of each respective manufacturing process regarding either processing itself or resulting part characteristics. Especially with the current implementation of additive manufacturing in the [...] Read more.
While repair is mainly used to restore the original part geometry and properties, hybrid manufacturing aims to exploit the benefits of each respective manufacturing process regarding either processing itself or resulting part characteristics. Especially with the current implementation of additive manufacturing in the production of TiAl, turbine blades for both hybrid manufacturing and repair new opportunities are enabled. One main issue is the compatibility of the two or more material types involved, which either differ regarding composition or microstructure or both. In this study, a TNMTM-alloy (Ti-Nb-Mo) was manufactured by different processes (casting, forging, laser additive manufacturing) and identically heat-treated at 1290 °C. Chemical compositions, especially aluminum and oxygen contents, were measured, and the resulting microstructures were analyzed with Scanning Electron Microscopy (SEM) and High-energy X-ray diffraction (HEXRD). The properties were determined by hardness measurements and high-temperature compression tests. The comparison led to an overall assessment of the theoretical compatibility. Experiments to combine several processes were performed to evaluate the practical feasibility. Despite obvious differences in the final phase distribution caused by deviations in the chemical composition, the measured properties of the samples did not differ significantly. The feasibility of combining direct energy deposition (DED) with either casting or laser powder bed fusion (LPBF) was demonstrated by the successful build of the dense, crack-free hybrid material. Full article
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Open AccessFeature PaperArticle
Laser Powder-Bed Fusion as an Alloy Development Tool: Parameter Selection for In-Situ Alloying Using Elemental Powders
Materials 2020, 13(18), 3922; https://doi.org/10.3390/ma13183922 - 04 Sep 2020
Cited by 4 | Viewed by 1319
Abstract
The design of advanced alloys specifically tailored to additive manufacturing processes is a research field that is attracting ever-increasing attention. Laser powder-bed fusion (LPBF) commonly uses pre-alloyed, fine powders (diameter usually 15–45 µm) to produce fully dense metallic parts. The availability of such [...] Read more.
The design of advanced alloys specifically tailored to additive manufacturing processes is a research field that is attracting ever-increasing attention. Laser powder-bed fusion (LPBF) commonly uses pre-alloyed, fine powders (diameter usually 15–45 µm) to produce fully dense metallic parts. The availability of such fine, pre-alloyed powders reduces the iteration speed of alloy development for LPBF and renders it quite costly. Here, we overcome these drawbacks by performing in-situ alloying in LPBF starting with pure elemental powder mixtures avoiding the use of costly pre-alloyed powders. Pure iron, chromium, and nickel powder mixtures were used to perform in-situ alloying to manufacture 304 L stainless steel cube-shaped samples. Process parameters including scanning speed, laser power, beam diameter, and layer thickness were varied aiming at obtaining a chemically homogeneous alloy. The scientific questions focused on in this work are: which process parameters are required for producing such samples (in part already known in the state of the art), and why are these parameters conducive to homogeneity? Analytical modelling of the melt pool geometry and temperature field suggests that the residence time in the liquid state is the most important parameter controlling the chemical homogeneity of the parts. Results show that in-situ alloying can be successfully employed to enable faster and cost-efficient rapid alloy development. Full article
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Open AccessArticle
Laser Powder Bed Fusion of Metal Coated Copper Powders
Materials 2020, 13(16), 3493; https://doi.org/10.3390/ma13163493 - 07 Aug 2020
Cited by 4 | Viewed by 915
Abstract
Laser powder bed fusion (L-PBF) of copper alloys with high copper content is difficult due to the high infrared reflectivity and thermal conductivity of these alloys. In this study a simple and scalable method for coating copper powder with tin and nickel is [...] Read more.
Laser powder bed fusion (L-PBF) of copper alloys with high copper content is difficult due to the high infrared reflectivity and thermal conductivity of these alloys. In this study a simple and scalable method for coating copper powder with tin and nickel is presented, and suggested as an alloying strategy for such alloys. The coated powders were processed in a commercial L-PBF-machine at various scanning speeds. The samples made from coated powders show a lower amount of porosity compared to samples made from in-situ alloyed powders of similar composition. Full article
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Open AccessArticle
Pandora’s Box–Influence of Contour Parameters on Roughness and Subsurface Residual Stresses in Laser Powder Bed Fusion of Ti-6Al-4V
Materials 2020, 13(15), 3348; https://doi.org/10.3390/ma13153348 - 28 Jul 2020
Viewed by 661
Abstract
The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. [...] Read more.
The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 µm to 13 µm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 µm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF. Full article
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Open AccessArticle
In Situ and Ex Situ Characterization of the Microstructure Formation in Ni-Cr-Si Alloys during Rapid Solidification—Toward Alloy Design for Laser Additive Manufacturing
Materials 2020, 13(9), 2192; https://doi.org/10.3390/ma13092192 - 10 May 2020
Viewed by 964
Abstract
Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this [...] Read more.
Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this work, the phase formation of four Ni-Cr-Si alloys was studied as a function of cooling rate and chemical composition using a liquid droplet rapid solidification technique. Post mortem x-ray diffraction, scanning electron microscopy, and in situ synchrotron microbeam X-ray diffraction shows the present and evolution of the rapidly solidified microstructures. Furthermore, the obtained results were compared to standard laser deposition tests. In situ microbeam diffraction revealed that due to rapid cooling and an increasing amount of Cr and Si, metastable high-temperature silicides remain in the final microstructure. Due to more sluggish interface kinetics of intermetallic compounds than that of disorder solid solution, an anomalous eutectic structure becomes dominant over the regular lamellar microstructure at high cooling rates. The rapid solidification experiments produced a microstructure similar to the one generated in laser coating thus confirming that this rapid solidification test allows a rapid pre-screening of alloys suitable for laser beam-based processing techniques. Full article
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

1. Title: “DED of TiAl for hybrid manufacturing and repair of turbine blades”
Authors: Silja-Katharina Rittinghaus 1, Janett Schmelzer 2, Marcus Rackel 3, Ulrike Hecht 4 and A. Weisheit 1
Affiliations:
1 Fraunhofer Institute for Laser Technology (FhG-ILT), Aachen, Germany; [email protected];
2 Otto-von-Guericke-Universit?t Magdeburg, Magdeburg, Germany; [email protected]
3 Helmholtz Zentrum Geesthacht, Geesthacht, Germany; [email protected]
4 Access e.V., Aachen, Germany; [email protected]

2. Title: “On the influence of nitrogen and argon as process gas atmosphere on the mechanical and corrosion properties of ASS 304L produced by L-BPF“
Authors: E. A. J?gle   1,2,3, P. Kürnsteiner  1, D. Kim  1, E. Tada 2 and N. Nakada  2
Affiliations:
1 Department Microstructure Physics and Alloy design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany; [email protected]; [email protected]; [email protected]
2 World Reserch Hub Initiative, School of Materials and Chemical Technology, Tokyo Institute of Technology, Tokyo, Japan; [email protected]; [email protected]
3 Institute of Materials Science, Department of Aerospace Engineering, Bundeswehr University Munich, Germany

3. Title: “Measurement and Prediction of Heat Transfer Within Metal Powder Beds”
Authors:  W. Sparling 1, C. W. Sinclair 2
Affiliation:
1 The University of British Columbia, Department of Materials Engineering, 309-6350 Stores Road, Vancouver, BC Canada V6T 1Z4; [email protected]; [email protected]


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