It is our pleasure to announce the five winners of the Metals 2021 Highly Cited Paper Award, which recognizes the most cited papers published in Metals (ISSN: 2075-4701) from 1 January 2019 to 31 December 2020. The papers selected for this award were assessed by the Metals Award Committee, led by the Editor-in-Chief, Prof. Dr. Hugo F. Lopez. The recipients of the Metals 2021 Highly Cited Paper Award are as follows:

First Prize (CHF 800 and a certificate)

“A Review of the Serrated-Flow Phenomenon and Its Role in the Deformation Behavior of High-Entropy Alloys”
by Jamieson Brechtl, Shuying Chen, Chanho Lee, Yunzhu Shi, Rui Feng, Xie Xie, David Hamblin, Anne M. Coleman, Bradley Straka, Hugh Shortt et al.
Metals 2020, 10(8), 1101; https://doi.org/10.3390/met10081101
Available online: https://www.mdpi.com/2075-4701/10/8/1101

Winning Article Introduction

High-entropy alloys (HEAs) are a novel class of alloys that have many desirable properties. The serrated flow that occurs in high-entropy alloys during mechanical deformation is an important phenomenon since it can lead to significant changes in the microstructure of the alloy. In this article, we review the recent findings on the serration behavior in a variety of high-entropy alloys. Relationships among the serrated flow behavior, composition, microstructure, and testing condition are explored. Importantly, the mechanical-testing type (compression/tension), testing temperature, applied strain rate, and serration type for certain high-entropy alloys are summarized. The literature reveals that the serrated flow can be affected by experimental conditions such as the strain rate and test temperature. Furthermore, this type of phenomenon has been successfully modeled and analyzed, using several different types of analytical methods, including the mean-field theory formalism and the complexity-analysis technique. Importantly, the results of the analyses show that the serrated flow in HEAs consists of complex dynamical behavior. It is anticipated that this review will provide some useful and clarifying information regarding the serrated-flow mechanisms in this material system. Finally, suggestions for future research directions in this field are proposed, such as the effects of irradiation, additives (such as C and Al), the presence of nanoparticles, and twinning on the serrated flow behavior in HEAs.

Acceptance Speech from the Authors:

We are extremely grateful to MDPI for having been selected to receive the “Highly Cited Paper Award” as it is an honor for our work to be recognized in this way. We would also like to thank our team members since winning this award would not have been possible without their exceptional efforts. Our team hopes that this work will be both an inspiration as well as a source of knowledge for years to come. Lastly, we will also continue to push the boundaries of the field of serrated plastic flow.

Second Prize (CHF 500 and a certificate)

1. “Recent Development in Beta Titanium Alloys for Biomedical Applications”
by Liang-Yu Chen, Yu-Wei Cui and Lai-Chang Zhang
Metals 2020, 10(9), 1139; https://doi.org/10.3390/met10091139
Available online: https://www.mdpi.com/2075-4701/10/9/1139

Winning Article Introduction

Beta-type titanium (Ti) alloys, which primarily consist of beta phase with a body-centered cubic structure, have attracted a lot of attention as novel biomedical materials in the past decades owing to their low elastic moduli and good biocompatibility compared with the other Ti alloys. This article provides a broad and extensive review of beta-type Ti alloys in terms of alloy design, preparation methods, mechanical properties, corrosion behavior, and biocompatibility. As reviewed, beta-type Ti alloys have lower elastic moduli, better corrosion behavior, and higher biocompatibility compared with other types of Ti alloys. Additive manufacturing technologies give the opportunity for further controlling the elastic moduli of beta-type Ti alloys by producing lattice structures. Hence, beta-type Ti alloys are suitable for use as implant materials. However, due to their inert characteristics, the bioactivity of beta-type Ti alloys needs to be improved.

Acceptance Speech from the Authors:

We are very grateful to have been selected to receive the “Highly Cited Paper Award” from MDPI. Thank you all so much for being here to share this occasion. We are so honored to have our work recognized by MDPI. It means so much to us that the work we are so passionate about resonates with others. We would also like to thank our group members and our family for their support throughout the writing of this article. Lastly, thanks to Jiangsu University of Science and Technology and Edith Cowan University for recognizing us. We hope that this recognition of our work can serve as an inspiration to others in the field. We will continue our efforts in the field of biomedical titanium alloys and look forward to bringing positive changes to biomedical titanium alloys for many years to come. We are humbled and appreciative.

2. “3D Printing of Highly Pure Copper”
by Thang Q. Tran, Amutha Chinnappan, Jeremy Kong Yoong Lee, Nguyen Huu Loc, Long T. Tran, Gengjie Wang, Vishnu Vijay Kumar, W. A. D. M. Jayathilaka, Dongxiao Ji, Mrityunjay Doddamani et al.
Metals 2019, 9(7), 756; https://doi.org/10.3390/met9070756
Available online: https://www.mdpi.com/2075-4701/9/7/756

Winning Article Introduction

Copper has been widely used in many applications due to its outstanding properties such as malleability, high corrosion resistance, and excellent electrical and thermal conductivities. While 3D printing can offer many advantages from layer-by-layer fabrication, the 3D printing of highly pure copper is still challenging due to the thermal issues caused by copper’s high conductivity. This paper presents a comprehensive review of recent work on 3D printing technology of highly pure copper over the past few years, including Selective Laser Melting (SLM), Electron Beam Melting (EBM), Binder Jetting (BJ) and Ultrasonic Additive Manufacturing (UAM). Additionally, the advantages, challenges, and performance of the copper parts fabricated by each method have been identified and compared.

In particular, direct SLM requires higher energy power than EBM to heat the copper powder to its melting point for part fabrication owing to the issue of low energy absorption of pure copper with conventional lasers. Due to rapid melting/solidification of copper powder, the copper parts fabricated by both direct SLM and EBM have many issues in the full melting process, such as thermal residual stresses and thermally induced deformation. These issues are not severe for the indirect SLM, BJ and UAM processes as their printing temperatures are much lower (below 50% of copper’s melting temperature). Both indirect SLM and BJ need quite low power to fabricate green parts, but post processing such as green part curing, debinding, and sintering are required to achieve the final metal parts. Additionally, their final sintered parts usually have low density and high porosity, which require further post processing to obtain fully dense structures. The UAM can produce fully dense parts without post processing but high power is required to process pure copper due to material hardening and oxidation issues. Regarding printed parts’ properties, EBM can fabricate copper parts with the best value of relative density (99.95%), electrical conductivity (102% IACS), tensile strength (177 MPa), and thermal conductivity (411.89 W/mK). In contrast, copper parts from indirect SLM have the poorest performance with a relative density of 84.8% and strength of only 8 MPa.

The potential applications of the 3D printed copper parts in thermal management systems, heat exchange devices, RF cathodes, and induction heat coils have been demonstrated in the paper. In the near future, the 3D printing of pure copper will most likely grow further to address all the current issues of the printing methods while exploring more potential applications of the printed copper parts. New 3D printers equipped with a green laser source have been developed to overcome the low absorption of pure copper to conventional laser sources and component damages. The technical issues and the optimized process of 3D printing highly conductive copper components can be addressed more effectively to open up new industrial fields of application such as in mobility, electronics, robotics, hydraulics, medicine and aerospace.

Acceptance Speech from the Authors:

We are honored to accept the Metals “Highly Cited Paper Award 2021”. This award is fantastic recognition and encourages our researchers and scientists to publish more impactful research work on metal-related research areas. In recent years, 3D printing has been growing rapidly and has become one of the key manufacturing technologies in the 4^th industrial revolution. It is also considered more sustainable than conventional manufacturing processes and is therefore suitable for lowering the carbon footprint of manufactured products. As one of the highly cited papers last year, we believe that our review paper on “3D Printing of Highly Pure Copper” has contributed remarkably to scientific research and technology development in additive manufacturing fields.

We would like to express our special thanks to the Editorial Office Board of Metals for selecting our paper. In addition, we are extremely grateful to those who nominated and supported us. We are also thankful to the Lloyd’s Register Foundation, our collaborators at the University of Cambridge and Texas A&M University, as well as the research team who have made incredible contributions to this paper. The teams from different universities, institutes, and countries have been working very hard to deliver such impactful work. This is a fantastic award that strengthens our team’s passion for science during this challenging period.

Third Prize (CHF 300 and a certificate)

1. “Wire and Arc Additive Manufacturing of Aluminum Components”
by Markus Köhler, Sierk Fiebig, Jonas Hensel and Klaus Dilger
Metals 2019, 9(5), 608; https://doi.org/10.3390/met9050608
Available online: https://www.mdpi.com/2075-4701/9/5/608

Winning Article Introduction

Since their introduction, additive manufacturing (AM) processes gained growing interest for customizable fabrication of metal components in both research and industrial applications. Thereby, the wire and arc additive manufacturing provides an emerging technology for the near net-shape additive manufacturing of large structures with complex geometry as well as the customization of semi-finished components (incremental manufacturing), using cost efficient production resources such as arc welding technology and wire materials. In this context, the presented experimental study aimed to describe the basic effects of the welding process on the buildup accuracy and material properties for the wire arc additive manufacturing of aluminum components.

Experiments were carried out comparing two different aluminum alloys, Al-4047 and Al-5356. Due to the layer-by-layer material deposition, process conditions such as energy input, arc characteristics, and material composition result in a different processability during the manufacturing process and were considered within the scope of this study. Therefore, linear wall samples were manufactured under variation of the filler wire composition and analyzed in terms of surface finishing, hardness, and residual stress. Furthermore, the mechanical properties were determined in different building directions using tensile testing.

In conclusion, the results from the experimental studies have shown that the accuracy and deposition properties during WAAM depend on the material composition. Thereby, a wide solidification range of the aluminum alloy is more suitable for uniform deposition. In addition, it was found that the arc length and pulse energy result in higher dynamic forces during droplet transition, thus affecting the deposition accuracy and surface finish appearance. The mechanical properties showed dependencies on the direction of loading with regard to the deposition direction. Samples taken vertically to the deposition direction resulted in a significant loss of elongation. Furthermore, the material properties were found to be evenly distributed over the thin-walled component.

Acceptance Speech from the Authors:

We are very pleased that various researchers, contributing to the field of additive manufacturing with metals, have cited our work and that we could contribute to improving the significance of Metals.

2. “Mechanical and Microstructural Characterization of TIG Welded Dissimilar Joints between 304L Austenitic Stainless Steel and Incoloy 800HT Nickel Alloy”
by Grzegorz Rogalski, Aleksandra Świerczyńska, Michał Landowski and Dariusz Fydrych
Metals 2020, 10(5), 559; https://doi.org/10.3390/met10050559
Available online: https://www.mdpi.com/2075-4701/10/5/559

Winning Article Introduction

This article presents the results of experimental tests on the quality of dissimilar welded joints between 304L austenitic stainless steel and 800HT nickel alloy. TIG butt welded joints of 7.47 mm tubes were subjected to non-destructive testing (visual, penetrant and radiographic), destructive testing (static tensile test, bending test, and microhardness measurements) and structure observations (macro- and microscopic examinations, SEM, element distribution characteristics). Non-destructive tests and metallographic examinations showed that the welded joints meet the acceptance criteria for B level in accordance with the EN ISO 13919–1 standard. Significant differences were found in the morphologies of the zones between the weld and the base materials: type A fusion boundary from the side of the Incoloy 800HT alloy and type B fusion boundary in the zone: weld–304L steel. These results were confirmed by image analyses obtained on a confocal microscope. During microstructural analysis no precipitates were found that could reduce the corrosion resistance of the joints. The high quality of the joints was also confirmed by the results of the destructive tests. The tensile strength of the welded joints was higher than the joined materials (Incoloy 800HT) and a 180° bending angle was obtained confirming the high plasticity of the joints. As a consequence of structural changes caused by the influence of the welding thermal cycle microhardness reached the highest values in the weld, the lowest in HAZ from the 304L austenitic stainless-steel side. The presented procedure can be used for TIG welding of 304L–800HT nickel alloy dissimilar welded joints with the use of S Ni 6082 nickel filler metal.

Acceptance Speech from the Authors:

The authors would like to thank the employees of the Secespol Sp. z o.o. from Nowy Dwór Gdański for their help in preparing materials for welding; the technical staff from the Faculty of Mechanical Engineering of Gdańsk University of Technology for technical assistance; and Dariusz Karubin, Ryszard Buza, and Dr. Michał Dobrzyński for consultations during the analysis of the observation results at confocal microscopy. In addition, we would like to thank the editors of the Metals journal for their professionalism and for carrying out the publishing process in short time.