applied sciences Review Applications of Magnesium and Its Alloys: A Review Jovan Tan and Seeram Ramakrishna * Center for Nanotechnology and Sustainability, Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore; [email protected] * Correspondence: [email protected] Abstract: Magnesium is a promising material. It has a remarkable mix of mechanical and biomedical properties that has made it suitable for a vast range of applications. Moreover, with alloying, many of these inherent properties can be further improved. Today, it is primarily used in the automotive, aerospace, and medical industries. However, magnesium has its own set of drawbacks that the industry and research communities are actively addressing. Magnesium’s rapid corrosion is its most significant drawback, and it dramatically impeded magnesium’s growth and expansion into other applications. This article reviews both the engineering and biomedical aspects and applications for magnesium and its alloys. It will also elaborate on the challenges that the material faces and how they can be overcome and discuss its outlook. Keywords: materials; engineering materials; biomaterials; magnesium; magnesium alloys; properties; applications 1. Introduction Citation: Tan, J.; Ramakrishna, S. As an alkaline earth metal, magnesium is shiny and silvery-white in appearance. It Applications of Magnesium and Its is also highly reactive and never found free in nature [1,2] with terrestrial and cosmic Alloys: A Review. Appl. Sci. 2021, 11, abundance [3]. It exists as a chemical compound until Joseph Black (in 1754), with prior 6861. https://doi.org/10.3390/ contributions from Friedrich Hoffman (in 1729), recognized it as an element [4]. Even app11156861 though Humphry Davy discovered the first magnesium metal in 1808 [1,4–8], the first production of pure magnesium was made by Antoine-Alexander Bussy in 1828 [4,6]. Academic Editors: Ana M. Camacho, Since its discovery, magnesium has played an influential role in society. In its early Frank Walther and Filippo Berto days, military applications and wars fueled its growth [8]. For example, magnesium was weaponized to construct incendiary bombs, flares, and ammunitions that were subse- Received: 13 May 2021 quently deployed in World War II, and it caused massive conflagrations and widespread Accepted: 23 July 2021 devastations [9]. Post-War, magnesium’s availability and unique blend of properties were Published: 26 July 2021 explored and were found to be highly attractive for an extensive range of applications. Today, magnesium is used for engineering applications in automotive, aerospace, and con- Publisher’s Note: MDPI stays neutral sumer electronics. In addition, it has a role in organic chemistry and pharmaceuticals [10] with regard to jurisdictional claims in and is used to construct several general-purpose applications, such as sporting goods, published maps and institutional affil- household products, and office equipment [8]. iations. Furthermore, with its superior biological properties, especially its ability to biodegrade in vivo, magnesium has received accelerated interest as a promising biomaterial by both the research and industry communities [11]. As a result, the global magnesium market is forecasted to grow at a compound annual growth rate of 4.9% and reach 1.6 million metric Copyright: © 2021 by the authors. tons (Mt) by 2027, increasing from 1.1 Mt in 2020 [12]. China alone produced 0.9 Mt in the Licensee MDPI, Basel, Switzerland. same year, accounting for more than 80% of global production [13]. This article is an open access article Henceforth, given its continued interest and developments, both a comprehensive distributed under the terms and state of knowledge and a primer for readers interested in the properties and applications conditions of the Creative Commons of magnesium and its alloys are imperative. Therefore, this article aims to synthesis and Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ present the recent progress and developments of this domain. Mainly, this article focuses 4.0/). on its engineering and biomedical applications. First, the article succinctly introduces Appl. Sci. 2021, 11, 6861. https://doi.org/10.3390/app11156861 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 17 Appl. Sci. 2021, 11, 6861 2 of 16 es on its engineering and biomedical applications. First, the article succinctly introduces the main methods in producing magnesium. It also delves into the recent progress and the main methods in producing magnesium. It also delves into the recent progress and developments in its production methods. Then, it presents the key considerations ena- developments in its production methods. Then, it presents the key considerations enabling blingmagnesium magnesium and and its alloys its alloys to be to considered be considered for engineeringfor engineering and and biomedical biomedical purposes. pur- poses.Finally, Finally, an extensive an extensive discussion discussion on the recenton the progress recent andprogress developments and developments surrounding sur- its roundingapplications its applications follows. follows. 2.2. Production Production Techniques Techniques ThereThere are are two two basic basic methods methods of of producing producing magnesium—(a) magnesium—(a) the the elec electrolysistrolysis of of fused fused anhydrousanhydrous magnesium magnesium chloride chloride and (b)(b) metallothermicmetallothermicreduction reduction of of magnesium magnesium oxide oxide by byferrosilicon ferrosilicon [14 ].[14]. A typicalA typical production production process process using using the electrolytic the electrolytic and thermal and thermal methods methodsis illustrated is illustrated in Figure in1. Figure 1. FigureFigure 1. 1. ProcessProcess flowchart flowchart of of primary magnesium productionproduction viavia a a thermal thermal and and electrolytic electrolytic process. pro- cess.Reproduced Reproduced with with permission permission from from Cherubini, Cherubini, F., et al.,F., et LCA al.,of LCA magnesium of magnesium production; production; published pub- by lishedElsevier, by Elsevier, 2008 [15 ].2008 [15]. ElectrolysisElectrolysis is is a atwo-step two-step approach. approach. First, First, it itinvolves involves the the hydrometallurgical hydrometallurgical prepa- prepa- rationration of of the the feedstock feedstock (dehydrated (dehydrated magnesium magnesium chloride), chloride), followed followed by by feeding feeding it it directly directly throughthrough electrolytic electrolytic cells cells [5]. [5]. Michael Michael Faraday Faraday first first discovered discovered the the core core principles principles of of pro- pro- ducingducing magnesium magnesium metal metal with with this this method method in in 1833 1833 [6]. [6]. InIn 1852, 1852, Robert Robert Bunsen Bunsen improved improved on on Fara Faraday’sday’s process to achieveachieve permanentpermanent separa-sepa- rationtion of of chlorine chlorine and and magnesium, magnesium, and and that that tweak tweak in in the the process process kickstarted kickstarted the the commer- com- mercializationcialization of magnesiumof magnesium [8]. [8]. Preventing Preventing the the recombination recombination of chlorineof chlorine and and magnesium magne- siumwas criticalwas critical to Robert to Robert Bunsen’s Bunsen’s success success as anhydrous as anhydrous magnesium magnesium chloride chloride is hygroscopic, is hy- groscopic,which can which lead tocan the lead formation to the formation of undesirable of undesirable oxides and oxides oxychlorides and oxychlorides during directdur- ingdehydration direct dehydration [14]. Presently, [14]. Presently, it is still it a is technological still a technological challenge challenge to produce to produce anhydrous an- hydrousmagnesium magnesium chloride chloride with minimal with minimal or, ideally, or, no ideally, oxychlorides no oxychlorides [16]. Bunsen’s [16]. electrolytic Bunsen’s electrolyticprocess is illustratedprocess is illustrated in Figure1 .in Figure 1. The second method of producing magnesium metal is with heat. Unlike electrolysis, The second method of producing magnesium metal is with heat. Unlike electroly- intense heat in the thermal reduction process eliminates the need for an elaborate feedstock sis, intense heat in the thermal reduction process eliminates the need for an elaborate preparation [8]. The Pidgeon and Magnetherm processes are the main thermal routes. They are also batched processes [14] that use the same basic chemistry [5]. In the Pidgeon Appl. Sci. 2021, 11, 6861 3 of 16 process, dolomite and ferrosilicon are formed into briquettes with an externally heated retort to attain magnesium vapors. However, the Magnetherm process calcines a mixture of dolomite, ferrosilicon, and alumina to achieve the same by-product. The magnesium vapors obtained from both routes are then cooled and condensed separately before being extracted [5]. A typical thermal route is also illustrated in Figure1. When the thermal reduction process was first introduced in the 1920s [16], it was touted to be the eventual replacement of electrolysis production. Today, the thermal routes, especially the Pidgeon process, are the most widely used due to their ability to produce high purity magnesium and the plethora of its raw material, dolomite [17]. 2.1. Carbothermic Reduction as a New Production Technique However, the current state-of-the-art Pidgeon process is known for its complex man- agement and