Submit to Applied Sciences Review for Applied Sciences Propose a Special Issue

Journal Menu

Journal Browser

Metal Additive Manufacturing and its Applications: From the Material to Components Service Life

Print Special Issue Flyer
Special Issue Editors
Special Issue Information
Keywords
Benefits of Publishing in a Special Issue
Published Papers

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Materials Science and Engineering".

Deadline for manuscript submissions: closed (31 May 2022) | Viewed by 57721

Share This Special Issue

Special Issue Editors

Prof. Dr. Volker Wesling

E-Mail Website
Guest Editor

Institute of Welding and Machining, Clausthal University of Technology, Agricolastraße 2, 38678 Clausthal-Zellerfeld, Germany
Interests: materials; welding; machining; mechanical engineering; composites; materials engineering; powders; manufacturing engineering
Special Issues, Collections and Topics in MDPI journals

Dr. Kai Treutler

E-Mail Website
Guest Editor

Institute of Welding and Machining, Clausthal University of Technology, Agricolastraße 2, 38678 Clausthal-Zellerfeld, Germany
Interests: welding; additive manufacturing; multi-material compunds; waam; high-strength steels; materials engineering
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

These days, additive manufacturing processes have a large representation in current research and in the focus of industrial applications. In addition to process and geometric aspects, material properties are also increasingly in focus, as well as modern materials, such as titanium aluminides, high entropy alloys and nickel base superalloys are more and more qualified for additive manufacturing.

The more the knowledge about additive manufacturing increases, the more these processes are used in industrial manufacturing. Here, they are used in many industries, from aviation to energy technology, and ensure the continuous further development of technical equipment with appropriate post-processing, such as milling or heat treatment.

In order to take this fact into account, a Special Issue, which is open to a wide range of topics, is the adequate approach, will provide a good overview of the current state of research, and is intended to provide an overview of the latest research results and industrial applications, both across applications and materials.

Possible topics are listed below.

Potential topics include, but are not limited to, the following:

Progress in processes for metal additive manufacturing;
- Arc-based ( WAAM)
- Beam-based (powderbed, electron/laser powder/wire)
Generation path planning
Process surveillance and control
- Surveillance of temperature
- Surveillance of geometry
- Indirect surveillance
Microstructural propagation of additively manufactured materials
Multi-material-systems and -design for metal additive manufacturing
Properties of additive manufactured materials;
- Static, fatigue, crash, wear
Additional post-processing
- Machining
- Heat-treatment
- Impact treatment
Modelling for additive manufacturing
- Modelling of weldability
- Modelling of phase transformation
- Modelling of heat-transfer
Industrial applications and service life properties

Please stick to metal applications and research.

Prof. Dr. Volker Wesling
Dr. Kai Treutler
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. Applied Sciences 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 2400 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
Direct energy deposition (DED)
Mechanical properties
Process surveillance
WAAM
Powderbed AM
Electron beam AM
Fatigue
Modelling
Path planning
Wear
Industrial applications
Microstructure
Weldability
Phase transformation
Post weld heat treatment (PWHT)

Benefits of Publishing in a Special Issue

Ease of navigation: Grouping papers by topic helps scholars navigate broad scope journals more efficiently.
Greater discoverability: Special Issues support the reach and impact of scientific research. Articles in Special Issues are more discoverable and cited more frequently.
Expansion of research network: Special Issues facilitate connections among authors, fostering scientific collaborations.
External promotion: Articles in Special Issues are often promoted through the journal's social media, increasing their visibility.
e-Book format: Special Issues with more than 10 articles can be published as dedicated e-books, ensuring wide and rapid dissemination.

Further information on MDPI's Special Issue policies can be found here.

Published Papers (13 papers)

Download All Papers

Order results

Result details

Show export options Show export options

Select all

Export citation of selected articles as:

Editorial

Jump to: Research, Review

3 pages, 188 KiB

Open AccessEditorial

Special Issue on Metal Additive Manufacturing and Its Applications: From the Material to Components Service Life

by Kai Treutler and Volker Wesling

Appl. Sci. 2022, 12(18), 8994; https://doi.org/10.3390/app12188994 - 7 Sep 2022

Viewed by 1259

Abstract

These days, additive manufacturing processes have a large representation in current research and in the field of industrial applications [...] Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

Research

Jump to: Editorial, Review

12 pages, 4574 KiB

Open AccessFeature PaperArticle

Investigation on the Acoustical Transmission Path of Additively Printed Metals

by Raphael Rehmet, Swenja Lorenz, Vincent Hahn and Armin Lohrengel

Appl. Sci. 2023, 13(1), 180; https://doi.org/10.3390/app13010180 - 23 Dec 2022

Viewed by 1377

Abstract

When additive manufacturing is used, the freedom of design plays a decisive role. It allows designers of mechanically stressed parts to generate special functional surfaces, integrate very efficient conformal cooling channels, or adapt the parts to the specific load in order to increase the stress ratio. In combination with the functional principle of particle dampers, the usage of metal additive manufacturing could be useful to implement damping directly in the transmission path of machine parts and elements. To achieve this goal, it is necessary to qualify the influence on the damping capability of the particles, outer geometry, frequency behaviour and the shape of the inner geometry. In this approach, different inner geometries are used to analyse the distinctions of damping behaviour caused by unmelted metal powder. The focus lies on the geometrical shape of the inner geometry. It is recognisable that the shape has a non-negligible impact on the dynamic behaviour of the structure. The results show that structures with a high number of sharp edges in the inner shape oriented orthogonally to the excitation or under a certain angle provide a better energy decay than structures without an inner shape or a force-parallel oriented shape. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

12 pages, 4779 KiB

Open AccessArticle

Micromagnetic Properties of Powder Metallurgically Produced Al Composites as a Fundamental Study for Additive Manufacturing

by Maraike Gräbner, Henning Wiche, Kai Treutler and Volker Wesling

Appl. Sci. 2022, 12(13), 6695; https://doi.org/10.3390/app12136695 - 1 Jul 2022

Cited by 7 | Viewed by 1690

Abstract

Resource-efficient manufacturing with a high degree of freedom in terms of component shape can be realised through additive manufacturing. The focus can lie not only on the manufacturing process in terms of geometrical correctness, stability, etc., but also on the targeted development of specific material properties. This study shows the development of hybrid material systems made of aluminium and the ferromagnetic particles iron, cobalt, and nickel. The aim is to use the ferromagnetic properties as sensor properties to enable the easy sensing of material properties such as the microstructure, fatigue, or occurring stresses. To easily adopt different compositions, hot isostatic pressing was selected for the characterisation of the material composites Al-Fe, Al-Ni, and Al-Co with regard to their magnetic properties. Subsequently, transfer to the additive manufacturing process of wire and arc additive manufacturing gas metal arc welding was carried out by mixing the powder separately into the weld pool. The study shows that it is possible to prevent a complete transformation of Ni and Co into intermetallic phases with Al by adjusting the influencing variables in the HIP process. Magnetic properties could be detected in the composites of Al-Co and Al-Fe. This work serves as a preliminary work to realise additive components made of hybrid material systems of Al-Fe, Al-Co, and Al-Ni with the GMA welding process. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

12 pages, 2701 KiB

Open AccessArticle

Development of an Alternative Alloying Concept for Additive Manufacturing Using PVD Coating

by Marcel Leicher, Kai Treutler and Volker Wesling

Appl. Sci. 2022, 12(13), 6619; https://doi.org/10.3390/app12136619 - 30 Jun 2022

Cited by 4 | Viewed by 1794

Abstract

New alloys are needed to adapt the material properties and to improve the weldability of arc-based additive manufacturing processes. The classic development of welding filler materials is time-consuming and cost-intensive. For this reason, an alternative alloy concept is investigated and qualified here. This is based on the thin-film coating of welding filler materials by means of PVD coating. An HSLA steel DIN EN ISO 14341-A G 50 7 M21 is used as the base material. This is alloyed with the elements Al, Cr, Nb, Ni and Ti by means of PVD thin-film coating. This procedure represents an alternative alloy concept. In the scope of the qualification, the influence of the process and material properties is investigated, and the alternative alloying concept is compared with the classical alloying concept of secondary metallurgy. The investigations have shown that the thin film coating on the surface of the welding filler metal affects the process properties in the form of a changed arc length. Furthermore, the mechanical properties and the effect on the microstructure morphology were investigated. These were compared in the same chemical composition with a Mn4Ni2CrMo produced by secondary metallurgy. The results are in agreement with regard to the mechanical properties and the effect on the microstructure morphology. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

14 pages, 94811 KiB

Open AccessArticle

Wire and Arc Additive Manufacturing of a CoCrFeMoNiV Complex Concentrated Alloy Using Metal-Cored Wire—Process, Properties, and Wear Resistance

by Kai Treutler, Swenja Lorenz, Jens Hamje and Volker Wesling

Appl. Sci. 2022, 12(13), 6308; https://doi.org/10.3390/app12136308 - 21 Jun 2022

Cited by 13 | Viewed by 2333

Abstract

The field of complex concentrated alloys offers a very large number of variations in alloy composition. The achievable range of properties varies greatly within these variants. The experimental determination of the properties is in many cases laborious. In this work, the possibility of using metal-cored wires to produce sufficient large samples for the determination of the properties using arc-based additive manufacturing or in detail wire and arc additive manufacturing (WAAM) is to be demonstrated by giving an example. In the example, a cored wire is used for the production of a CoCrFeNiMo alloy. In addition to the process parameters used for the additive manufacturing, the mechanical properties of the alloy produced in this way are presented and related to the properties of a cast sample with a similar chemical composition. The characterization of the resulting microstructure and wear resistance will complete this work. It will be shown that it is possible to create additively manufactured structures for a microstructure and a property determination by using metal-cored filler wires in arc-based additive manufacturing. In this case, the additively manufactured structure shows an FCC two-phased microstructure, a yield strength of 534 MPa, and a decent wear resistance. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

20 pages, 5505 KiB

Open AccessArticle

Smart Web Service of Ti-Based Alloy’s Quality Evaluation for Medical Implants Manufacturing

by Ivan Izonin, Roman Tkachenko, Zoia Duriagina, Nataliya Shakhovska, Viacheslav Kovtun and Natalia Lotoshynska

Appl. Sci. 2022, 12(10), 5238; https://doi.org/10.3390/app12105238 - 22 May 2022

Cited by 15 | Viewed by 2429

Abstract

The production of biocompatible medical implants is accompanied by technological and time costs. As a result, to be used in the human body, such a product must be of the highest quality. Assessing the quality of biomedical implants made of titanium alloys is relevant given their impact on the health and life of their wearer. In the case of the production of such implants by additive technologies, an important task is to evaluate the properties of the alloys from which it is made. The modern development of Artificial Intelligence allows replacing traditional assessment methods with machine learning methods for such assessment. Existing machine learning methods demonstrate very low classification accuracy, and existing hybrid systems, although increasing classification accuracy, are not sufficient to apply such schemes in practice. The authors improved the hybrid PNN-SVM system to solve this problem in this paper. It is based on the combining use of PNN, Ito Decomposition, and SVM. The PNN’s summation layer outputs were used as additional attributes to an initial dataset. Ito decomposition was used to nonlinearly model relationships between features of an extended dataset. Further classification is carried out using SVM with a linear kernel. The proposed approach’s modeling is performed based on a real-world dataset using the smart web service designed by the authors. Experimentally found an increase in the classification accuracy by 6% of the proposed system compared to existing ones. It makes it possible to use it in practice. Designed smart web service, in which the authors implemented both improved and existing hybrid classification schemes allows to quickly, easily, and without high qualification of the user to implement and explore in more detail chosen classification scheme when classification tasks in various fields of industry. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

17 pages, 5389 KiB

Open AccessArticle

Bead Geometry Prediction in Laser-Wire Additive Manufacturing Process Using Machine Learning: Case of Study

by Natago Guilé Mbodj, Mohammad Abuabiah, Peter Plapper, Maxime El Kandaoui and Slah Yaacoubi

Appl. Sci. 2021, 11(24), 11949; https://doi.org/10.3390/app112411949 - 15 Dec 2021

Cited by 25 | Viewed by 5382

Abstract

In Laser Wire Additive Manufacturing (LWAM), the final geometry is produced using the layer-by-layer deposition (beads principle). To achieve good geometrical accuracy in the final product, proper implementation of the bead geometry is essential. For this reason, the paper focuses on this process and proposes a layer geometry (width and height) prediction model to improve deposition accuracy. More specifically, a machine learning regression algorithm is applied on several experimental data to predict the bead geometry across layers. Furthermore, a neural network-based approach was used to study the influence of different deposition parameters, namely laser power, wire-feed rate and travel speed on bead geometry. To validate the effectiveness of the proposed approach, a test split validation strategy was applied to train and validate the machine learning models. The results show a particular evolutionary trend and confirm that the process parameters have a direct influence on the bead geometry, and so, too, on the final part. Several deposition parameters have been found to obtain an accurate prediction model with low errors and good layer deposition. Finally, this study indicates that the machine learning approach can efficiently be used to predict the bead geometry and could help later in designing a proper controller in the LWAM process. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

24 pages, 35944 KiB

Open AccessFeature PaperArticle

Full-Field Strain Determination for Additively Manufactured Parts Using Radial Basis Functions

by Stefan Hartmann, Lutz Müller-Lohse and Jendrik-Alexander Tröger

Appl. Sci. 2021, 11(23), 11434; https://doi.org/10.3390/app112311434 - 2 Dec 2021

Cited by 7 | Viewed by 1914

Abstract

Additively manufactured components, especially those produced in deposition welding processes, have a rough curvilinear surface. Strain and surface deformation analysis of such components is increasingly performed using digital image correlation (DIC) methods, which raises questions regarding interpretability of the results. Furthermore, in triangulation or local tangential plane based DIC strain analysis, the principal strain directions are difficult to be calculated at any point, which is due to the non-continuity of the approach. Thus, both questions will be addressed in this article. Apart from classical local strain analysis based on triangulation or local linearization concepts, the application of globally formulated radial basis functions (RBF) is investigated for the first time, with the advantage that it is possible to evaluate all interesting quantities at arbitrary points. This is performed for both interpolation and regression. Both approaches are studied at three-dimensional, curvilinear verification examples and real additively manufactured cylindrical specimens. It is found out that, if real applications are investigated, the RBF-approach based on interpolation and regression has to be considered carefully due to so-called boundary effects. This can be circumvented by only considering the region that has a certain distance to the edges of the evaluation domain. Independent of the evaluation scheme, the error of the maximum principal strains increases with increasing surface roughness, which has to be kept in mind for such applications when interpreting or evaluating the results of manufactured parts. However, the entire scheme offers interesting properties for the treatment of DIC-data. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

16 pages, 4665 KiB

Open AccessArticle

A Novel Laser-Aided Machining and Polishing Process for Additive Manufacturing Materials with Multiple Endmill Emulating Scan Patterns

by Mohammad Masud Parvez, Sahil Patel, Sriram Praneeth Isanaka and Frank Liou

Appl. Sci. 2021, 11(20), 9428; https://doi.org/10.3390/app11209428 - 11 Oct 2021

Cited by 8 | Viewed by 2798

Abstract

In additive manufacturing (AM), the surface roughness of the deposited parts remains significantly higher than the admissible range for most applications. Additionally, the surface topography of AM parts exhibits waviness profiles between tracks and layers. Therefore, post-processing is indispensable to improve surface quality. Laser-aided machining and polishing can be effective surface improvement processes that can be used due to their availability as the primary energy sources in many metal AM processes. While the initial roughness and waviness of the surface of most AM parts are very high, to achieve dimensional accuracy and minimize roughness, a high input energy density is required during machining and polishing processes although such high energy density may induce process defects and escalate the phenomenon of wavelength asperities. In this paper, we propose a systematic approach to eliminate waviness and reduce surface roughness with the combination of laser-aided machining, macro-polishing, and micro-polishing processes. While machining reduces the initial waviness, low energy density during polishing can minimize this further. The average roughness (

R_{a} = 1.11 μ

m) achieved in this study with optimized process parameters for both machining and polishing demonstrates a greater than 97% reduction in roughness when compared to the as-built part. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

8 pages, 2095 KiB

Open AccessArticle

Correlations between Process and Geometric Parameters in Additive Manufacturing of Austenitic Stainless Steel Components Using 3DPMD

by Kevin Hoefer

Appl. Sci. 2021, 11(12), 5610; https://doi.org/10.3390/app11125610 - 17 Jun 2021

Cited by 4 | Viewed by 2264

Abstract

The additive manufacturing of components is characterized by a layered build-up. The stability of the build-up process with regard to the component geometry and the layer thickness is essential for the success of the entire system. A prerequisite for this is the exact knowledge of the interrelationships between the process, construction parameters and the resulting component geometry, respectively. These correlations are determined within the study using the 3D Plasma Metal Deposition Process (3DPMD). For this purpose, the process is first subjected to a system analysis. Possible influencing variables were identified with regard to the question “Which system parameters influence the component geometry?” and then prioritized. Then, the influence of control factors (welding current intensity, welding speed, and powder mass flow) was investigated according to the specifications of the Design of Experiments (DOE) method by means of a full-factorial experimental design and evaluated on the basis of metallographic cross-sections. As a result, it was determined that the system parameter powder mass flow only influences the layer thickness and not the wall thickness and is, therefore, available as a process control variable. In sum, comprehensive knowledge of complex relationships between the control parameters and the component geometry in additive manufacturing using 3DPMD was achieved and forms the basis for further scientific work. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

15 pages, 4386 KiB

Open AccessEditor’s ChoiceArticle

Geometry and Distortion Prediction of Multiple Layers for Wire Arc Additive Manufacturing with Artificial Neural Networks

by Christian Wacker, Markus Köhler, Martin David, Franziska Aschersleben, Felix Gabriel, Jonas Hensel, Klaus Dilger and Klaus Dröder

Appl. Sci. 2021, 11(10), 4694; https://doi.org/10.3390/app11104694 - 20 May 2021

Cited by 38 | Viewed by 4936

Abstract

Wire arc additive manufacturing (WAAM) is a direct energy deposition (DED) process with high deposition rates, but deformation and distortion can occur due to the high energy input and resulting strains. Despite great efforts, the prediction of distortion and resulting geometry in additive manufacturing processes using WAAM remains challenging. In this work, an artificial neural network (ANN) is established to predict welding distortion and geometric accuracy for multilayer WAAM structures. For demonstration purposes, the ANN creation process is presented on a smaller scale for multilayer beads on plate welds on a thin substrate sheet. Multiple concepts for the creation of ANNs and the handling of outliers are developed, implemented, and compared. Good results have been achieved by applying an enhanced ANN using deformation and geometry from the previously deposited layer. With further adaptions to this method, a prediction of additive welded structures, geometries, and shapes in defined segments is conceivable, which would enable a multitude of applications for ANNs in the WAAM-Process, especially for applications closer to industrial use cases. It would be feasible to use them as preparatory measures for multi-segmented structures as well as an application during the welding process to continuously adapt parameters for a higher resulting component quality. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

15 pages, 47280 KiB

Open AccessArticle

Hysteresis Measurements and Numerical Losses Segregation of Additively Manufactured Silicon Steel for 3D Printing Electrical Machines

by Hans Tiismus, Ants Kallaste, Anouar Belahcen, Toomas Vaimann, Anton Rassõlkin and Dmitry Lukichev

Appl. Sci. 2020, 10(18), 6515; https://doi.org/10.3390/app10186515 - 18 Sep 2020

Cited by 39 | Viewed by 5067

Abstract

Samples from FeSi4 powder were fabricated with a low power selective laser melting (SLM) system using a laser re-melting strategy. The sample material was characterized through magnetic measurements. The study showed excellent DC magnetic properties, comparable to commercial and other 3D printed soft ferromagnetic materials from the literature at low (1 T) magnetization. Empirical total core losses were segregated into hysteresis, eddy and excessive losses via the subtraction of finite element method (FEM) simulated eddy current losses and hysteresis losses measured at quasi-static conditions. Hysteresis losses were found to decrease from 3.65 to 0.95 W/kg (1 T, 50 Hz) after the annealing. Both empirical and FEM results confirm considerable eddy currents generated in the printed bulk toroidal sample, which increase dramatically at high material saturation after annealing. These losses could potentially be reduced by using partitioned material internal structure realized by printed airgaps. Similarly, with regard to the samples characterized in this study, the substantially increased core losses induced by material oversaturation due to reduced filling factor may present a challenge in realizing 3D printed electrical machines with comparable performance to established 2D laminated designs. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Figure 1

Review

Jump to: Editorial, Research

54 pages, 14354 KiB

Open AccessFeature PaperReview

The Current State of Research of Wire Arc Additive Manufacturing (WAAM): A Review

by Kai Treutler and Volker Wesling

Appl. Sci. 2021, 11(18), 8619; https://doi.org/10.3390/app11188619 - 16 Sep 2021

Cited by 151 | Viewed by 23280

Abstract

Wire arc additive manufacturing is currently rising as the main focus of research groups around the world. This is directly visible in the huge number of new papers published in recent years concerning a lot of different topics. This review is intended to give a proper summary of the international state of research in the area of wire arc additive manufacturing. The addressed topics in this review include but are not limited to materials (e.g., steels, aluminum, copper and titanium), the processes and methods of WAAM, process surveillance and the path planning and modeling of WAAM. The consolidation of the findings of various authors into a unified picture is a core aspect of this review. Furthermore, it intends to identify areas in which work is missing and how different topics can be synergetically combined. A critical evaluation of the presented research with a focus on commonly known mechanisms in welding research and without a focus on additive manufacturing will complete the review. Full article

(This article belongs to the Special Issue Metal Additive Manufacturing and its Applications: From the Material to Components Service Life)

► Show Figures

Journal Menu

Journal Browser

Metal Additive Manufacturing and its Applications: From the Material to Components Service Life

Share This Special Issue

Special Issue Editors

Special Issue Information

Keywords

Benefits of Publishing in a Special Issue

Published Papers (13 papers)

Editorial

Research

Review

Further Information

Guidelines

MDPI Initiatives

Follow MDPI