Magnesium alloys and methods of fabricating articles therefrom

Abstract

Disclosed are methods of forming articles from magnesium alloys using additive manufacturing (e.g., binder jet additive manufacturing) that involve a liquid sintering process. These methods can utilize a shortened liquid sintering process while still producing articles having improved properties including corrosion resistance. Also described are articles formed by these methods.

Description

[0001] Attorney Docket No.103362-018WO1 MAGNESIUM ALLOYS AND METHODS OF FABRICATING ARTICLES THEREFROM CROSS-REFERENCE TO RELATED APPLICATIONS This application claims benefit of priority of U.S. Provisional Application No. 63 / 664,496, filed June 26, 2024, which is incorporated herein by reference. BACKGROUND Magnesium (Mg) alloys have been used as a lightweight material in automotive, aerospace and consumer product industries, and most recently have been considered an attractive metallic candidate for biodegradable implant applications. It should be noted that about 70% of medical implants approved by the United States Food and Drug Administration (FDA) are metal-based. In particular, metallic implants are the dominant material used in clinical orthopedic applications, such as skeletal reconstruction. Compared to conventional metallic materials used in orthopedic implants such as titanium (Ti) alloys, stainless steel, or cobalt-chromium (Co-Cr) alloys, Mg alloys are distinguished because of the similarity of their elastic modulus to human bone. Compared to the other alloys used in orthopedic devices, the lower stiffness of Mg alloys, as well as their eventual resorption, may prevent the commonly observed negative effect of stress-shielding induced bone loss. Also of note, Mg alloys eliminate the need for second surgical operations for implant removal due to their excellent biocompatibility and biodegradability. Therefore, Mg alloys could become a next generation biodegradable metallic material for applications in vascular stents, bone fracture fixation, and bone grafting fixation, as they eventually degrade in biological media. Following extensive research on biodegradable Mg alloys, several candidate alloys have entered various stages of clinical trials and commercialization. Biotronik (Berlin, Germany) launched a Mg coronary stent, Magmaris, and received a CE mark (European conformity) in 2016 to become the first biodegradable metallic vascular stent approved for human use. The Magmaris stent is formed from a Mg-RE based alloy, i.e., WE43 (Mg- 4wt.%Y-3wt.%RE-0.7wt.%Zr), and is expected to become more widely available. Also, two companies have demonstrated clinical success and achieved regulatory approval in Europe and South Korea for Mg alloy skeletal repair devices. In 2013, Syntellix (Hanover, Germany) received a CE mark for the MagnezixR RE-bearing Mg alloy fracture Attorney Docket No.103362-018WO1 compression screw. Indeed, it was the first Mg alloy-based (i.e., MgYREZr) skeletal implant. In 2015, the U&I Corporation (Gyeonggi-do, Korea) received a certification from the Ministry of Food and Drug Safety in South Korea for orthopedic bone screws made from a bioresorbable Mg-Ca alloy. In spite of their potential, Mg alloys have yet to be widely adopted in many biomedical applications. Improved Mg alloys and methods of forming articles from Mg alloys are needed to address many applications in the biomedical space. The compounds and methods disclosed herein address these and other needs. SUMMARY Described herein are methods of forming an article from a magnesium alloy using an additive manufacturing process (e.g., binder jet additive manufacturing (BJAM)). These methods can comprise (a) forming a green body in a powder bed deposition region of an additive manufacturing apparatus from a powder comprising a magnesium alloy; and (b) liquid sintering the green body to produce an article formed from the magnesium alloy. In some embodiments, the liquid sintering of the green body can be performed in a total time of less than 12 hours, such as a total time of from 2 hours to 10 hours, from 2 hours to 8 hours, from 3 hours to 8 hours, from 4 hours to 8 hours, from 2 hours to 6 hours, from 3 hours to 6 hours, or from 4 hours to 6 hours. In certain embodiments, liquid sintering the green body can comprise (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to less than 12 hours (e.g., from 10 minutes to 1 hour); and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 24 hours (e.g., from 2 hours to 6 hours). In certain embodiments, the article formed from the magnesium alloy can have a porosity of less than 10%, such as a porosity of from 8% to 1%. In certain embodiments, the article formed from the magnesium alloy can exhibit improved corrosion resistance as compared to an as cast article formed from the magnesium alloy, as evidenced by decreased hydrogen evolution during the immersion in Hank’s solution. In certain embodiments, the article formed from the magnesium alloy can exhibit a yield strength, an ultimate tensile strength, an elongation, or a combination thereof that is similar or improved relative to an as cast article formed from the magnesium alloy. Attorney Docket No.103362-018WO1 BRIEF DESCRIPTION OF THE FIGURES The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate several aspects described below. Figure 1. WE43 alloy green parts were prepared by BJAM for (panel a) microstructural analysis and micro-tensile test, (panel b) electrochemical analysis, and (panel c) immersion testing. Figure 2. Heat treatment profile for liquid phase sintering. Figure 3. Schematic of the experimental set-up for the hydrogen evolution test. Figure 4. Schematic of (panel a) the location of the micro-tension specimens from the sintered part and (panel b) the micro-tension specimen. Figure 5. Solid fraction of WE43 alloy calculated based on Scheil's model using Pandat software. Figure 6. The representative optical micrographs showing porosity levels for (panel a) 180 min, (panel b) 240 min, and (panel c) 300 min at 600 °C. Figure 7. XRD pattern of sintered WE43 alloy at 600 for 300 min. Figure 8. SEM images at different sintering time and temperature: (panel a) 660 °C, (panel b) 680 °C, (panel c) 680 °C for 10 min, (panel d) 680 °C for 30 min then 600 °C for 180 min, (panel e) 680 °C for 30 min then 600 °C for 240 min, and (panel f) EDS results of sintered WE43 alloy at 680 °C for 30 min then 600 °C for 300 min. Figure 9. BSE image of the cross-sectional as-received WE43 powder. Figure 10. Photograph of binder jetting printed and sintered WE43 alloy. Figure 11. Hydrogen evolution as measured during the first 6 h of immersion in Hank’s solution. Figure 12. SEM images of the corroded surface of sintered WE43 alloy specimens after (panel a) 30 min, (panel b) 1 h, (panel c) 2 h, and (panel d) EDS result of the corroded surface after 2 h in Hank’s solution. Figure 13. Potentiodynamic polarization curves of sintered WE43 alloys after immersion in Hank's solution. Figure 14. (panel a) Nyquist plots and (panel b) equivalent circuit of sintered WE43 alloys immersed in Hank’s solution. Figure 15. Stress-strain curves of sintered WE43 alloys. Attorney Docket No.103362-018WO1 Figure 16. SEM images of the fracture surface found after the micro-tensile tests of: (panel a, panel c) sintered WE43 alloy for 180 min at 600 °C and (panel b, panel d) sintered WE43 alloy for 300 min at 600 °C. Figure 17. Schematic illustration of the additive manufacturing methods described herein. DETAILED DESCRIPTION The materials, compounds, compositions, articles, and methods described herein may be understood more readily by reference to the following detailed description of specific aspects of the disclosed subject matter and the Examples and Figures included therein. Before the present materials, compounds, compositions, and methods are disclosed and described, it is to be understood that the aspects described below are not limited to specific synthetic methods or specific reagents, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Also, throughout this specification, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which the disclosed matter pertains. The references disclosed are also individually and specifically incorporated by reference herein for the material contained in them that is discussed in the sentence in which the reference is relied upon. General Definitions In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings: Throughout the description and claims of this specification the word “comprise” and other forms of the word, such as “comprising” and “comprises,” means including but not limited to, and is not intended to exclude, for example, other additives, components, integers, or steps. As used in the description and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a composition” includes mixtures of two or more such compositions, Attorney Docket No.103362-018WO1 reference to “the compound” includes mixtures of two or more such compounds, reference to “an agent” includes mixture of two or more such agents, and the like. “Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. It is understood that throughout this specification the identifiers “first” and “second” are used solely to aid in distinguishing the various components and steps of the disclosed subject matter. The identifiers “first” and “second” are not intended to imply any particular order, amount, preference, or importance to the components or steps modified by these terms. “Additive manufacturing” refers to “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies,” as defined in ASTM F2792-l2a, entitled “Standard Terminology for Additively Manufacturing Technologies.” Non-limiting examples of additive manufacturing processes for producing parts and other articles from feedstocks include, for example, binder jet additive manufacturing (BJAM), direct metal laser sintering (DMLS), selective laser melting (SLM), selective laser sintering (SLS), and electron beam melting (EBM). In certain embodiments described herein, the additive manufacturing methods described herein relate to binder jet additive manufacturing (BJAM). As used herein, “powder” refers to a material comprising a plurality of particles. Powder may be used in a powder bed in an additive manufacturing system or process to produce a tailored alloy product via additive manufacturing. Powder, as used herein, may comprise a single material or a blend of two or more materials. As used herein, the term “depowder” or “depowdering” refers to removing loose powder from the cured polymer powder composite (e.g., a green body). As used herein, a “median particle size” of a powder refers to the diameter at which 50% of the volume of the particles in the powder has a smaller diameter (e.g., D50). As used herein, median particle size is determined in accordance with the methods described in ASTM standard B822-20, entitled “Standard Test Method for Particle Size Distribution of Metal Powders and Related Compounds by Light Scattering,” which is incorporated herein Reference will now be made in detail to specific aspects of the disclosed materials, compounds, compositions, articles, and methods, examples of which are illustrated in the accompanying Examples and Figures. Attorney Docket No.103362-018WO1 Methods Described herein are methods of forming articles from magnesium alloys using an additive manufacturing process (e.g., binder jet additive manufacturing (BJAM)). These methods can comprise (a) forming a green body in a powder bed deposition region of an additive manufacturing apparatus from a powder comprising a magnesium alloy; and (b) liquid sintering the green body to produce an article formed from the magnesium alloy. In some embodiments, the liquid sintering of the green body can be performed in a total time of less than 12 hours, such as a total time of from 2 hours to 10 hours, from 2 hours to 8 hours, from 3 hours to 8 hours, from 4 hours to 8 hours, from 2 hours to 6 hours, from 3 hours to 6 hours, or from 4 hours to 6 hours. In some embodiments, liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to less than 12 hours; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 24 hours. In certain embodiments, liquid sintering of the green body comprises: (1) heating the green body at a temperature at or above the liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 10 hours, from 10 minutes to 8 hours, from 10 minutes to 6 hours, from 10 minutes to 5 hours, from 10 minutes to 4 hours, from 10 minutes to 3 hours, from 10 minutes to 2 hours, from 10 minutes to 1 hour, from 15 minutes to 12 hours, from 15 minutes to 10 hours, from 15 minutes to 8 hours, from 15 minutes to 6 hours, from 15 minutes to 5 hours, from 15 minutes to 4 hours, from 15 minutes to 3 hours, from 15 minutes to 2 hours, or from 15 minutes to 1 hour. In certain embodiments, liquid sintering of the green body comprises: (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 18 hours, from 1 hour to 12 hours, from 1 hour to 8 hours, from 1 hour to 6 hours, from 1 hour to 4 hours, from 1 hour to 2 hours, from 2 hours to 24 hours, from 2 hours to 18 hours, from 2 hours to 12 hours, from 2 hours to 8 hours, from 2 hours to 6 hours, from 2 hours to 4 hours, from 4 hours to 24 hours, from 4 hours to 18 hours, from 4 hours to 12 hours, from 4 hours to 8 hours, or from 4 hours to 6 hours. Attorney Docket No.103362-018WO1 In some embodiments, the liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 1 hour; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 2 hours to 6 hours. In some embodiments, heating the green body at the temperature at or above the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of at least 640 ºC, such as a temperature of from 640 ºC to 750 ºC, a temperature of from 640 ºC to 700 ºC, a temperature of from 650 ºC to 750 ºC, a temperature of from 650 ºC to 700 ºC, a temperature of from 660 ºC to 750 ºC, or a temperature of from 660 ºC to 700 ºC. In some embodiments, heating the green body at the temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of from 540 ºC to 630 ºC, such as a temperature of from 540 ºC to 620 ºC, from 560 ºC to 620 ºC, or from 580 ºC to 620 ºC. In certain embodiments, the liquid sintering the green body comprises: (1) heating the green body at a temperature of from 660 ºC to 700 ºC for a period of time of from 15 minutes to 1 hour; and (2) heating the green body at a temperature of from 560 ºC to 620 ºC for a period of time of from 2 hours to 6 hours. In some embodiments, liquid sintering of the green body is performed in a controlled atmosphere, such as an argon atmosphere. The magnesium alloy can comprise any suitable magnesium alloy, such as a magnesium alloy known for use in biomedical or aerospace applications. Examples of magnesium alloys include AZ31, AZ91, AM50, ZK60, and WE43. In certain embodiments, the magnesium alloy can comprise a rare-earth magnesium alloy. In certain embodiments, the magnesium alloy comprises yttrium, neodymium, zirconium, or a combination thereof. In certain embodiments, the magnesium alloy comprises from 90-95 wt% magnesium, from 3.7-4.3 wt% yttrium, from 2.4-4.4 wt% rare earth elements, and from 0.4-1.0 wt% zirconium. In certain embodiments, the magnesium alloy comprises WE43, WE54, or a combination thereof. In some embodiments, the powder comprising the magnesium alloy can comprise a population of magnesium alloy particles having an average particle size of from 5 microns Attorney Docket No.103362-018WO1 to 100 microns (e.g., from 10 microns to 100 microns, from 25 microns to 100 microns, from 50 microns to 100 microns, from 75 microns to 100 microns, from 5 microns to 75 microns, from 10 microns to 75 microns, from 25 microns to 75 microns, from 50 microns to 75 microns, from 5 microns to 50 microns, from 10 microns to 50 microns, from 25 microns to 50 microns, from 5 microns to 25 microns, from 10 microns to 25 microns, from 25 microns to 50 microns, or from 5 microns to 10 microns). In certain embodiments, the powder comprising the magnesium alloy can comprise a population of magnesium alloy particles having an average particle size of from 25 microns to 50 microns. In some embodiments, the magnesium alloy particles comprise a layer of metal oxide present on a surface of the particles. The metal oxide can comprise, for example, Y2O3, Nd2O3, MgO, or a combination thereof. In certain embodiments, the metal oxide present on a surface of the particles can have a melting temperature, and liquid sintering of the green body can comprise, in part, (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy but below the melting temperature of the metal oxide. In some embodiments, the method can comprise binder jetting additive manufacturing (BJAM). In some embodiments, forming the green body in the powder bed deposition region of the additive manufacturing apparatus comprises selectively depositing a binder onto the powder bed in a pattern corresponding to a cross-sectional layer of the article. The binder can comprise any suitable binder known for use in a BJAM process. In certain embodiments, the binder can comprise an aqueous binder and / or polymeric binder, such as a blend of ethylene glycol monobutyl ether and ethylene glycol. In some embodiments, the method further comprises curing or drying the green body prior to the liquid sintering. In certain embodiments, curing or drying the green body prior to the liquid sintering can comprise heating the green body at a temperature below the solidus temperature of the magnesium alloy. In certain embodiments, curing or drying the green body prior to the liquid sintering can comprise heating the green body at a temperature of from 50 ºC to 300 ºC, such as a temperature of from 100 ºC to 200 ºC. In certain embodiments, curing or drying the green body prior to the liquid sintering can comprise heating the green body for a period of time of at least 1 hour (e.g., at least 2 hours, at least 3 hours, or at least 4 hours). In certain embodiments, curing or drying the green body prior to the liquid sintering can comprise heating the green body for a period of time of from 1 hour to 12 hours (e.g., from 2 hours to 6 hours). Attorney Docket No.103362-018WO1 In some embodiments, the method further comprises depowdering the cured or dried green body prior to the liquid sintering. In some embodiments, the article formed from the magnesium alloy has a porosity of less than 10% (e.g., 9% or less, 8% or less, 7.5% or less, 7% or less, 6% or less, 5% or less, 4% or less, 3% or less, 2.5% or less, 2% or less, or 1% or less). In some embodiments, the article formed from the magnesium alloy has a porosity of at least 0.5% (e.g., at least 1%, at least 2%, at least 2.5%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, at least 7.5%, at least 8%, or at least 9%). The article formed from the magnesium alloy can have a porosity ranging from any of the minimum values described above to any of the maximum values described above. For example, in some embodiments, the article formed from the magnesium alloy can have a porosity of from 0.5% to 10% (e.g., from 1% to 8%, or from 2% to 8%). In some embodiments, the article formed from the magnesium alloy exhibits improved corrosion resistance as compared to an as cast article formed from the magnesium alloy, as evidenced by decreased hydrogen evolution during the immersion in Hank’s solution. In some embodiments, the article formed from the magnesium alloy exhibits a yield strength, an ultimate tensile strength, an elongation, or a combination thereof that is similar or improved relative to an as cast article formed from the magnesium alloy. EXAMPLES To further illustrate the principles of the present disclosure, the following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compositions, articles, and methods claimed herein are made and evaluated. They are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperatures, etc.); however, some errors and deviations should be accounted for. Unless indicated otherwise, temperature is °C or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of process conditions that can be used to optimize product quality and performance. Only reasonable and routine experimentation will be required to optimize such process conditions. Attorney Docket No.103362-018WO1 Example 1. Low Carbohydrate High Fat Diet Curbs Glioblastoma by Shifting Lipids and Limiting Cancer Stem Cell Metabolic Flexibility. Summary This Example investigates full liquid phase sintering as a process of fabrication parts from WE43 (Mg-4wt.% Y-3wt.% RE-0.7wt.% Zr) alloy using binder jetting additive manufacturing (BJAM). This fabrication process is being developed for use in producing structural or biomedical devices. Specifically, this example focuses on achieving a near- dense microstructure with WE43 Mg alloy while substantially reducing the duration of sintering post-processing after BJAM part rendering. The optimal process resulted in microstructure with 2.5% porosity and significantly reduced sintering time. The improved sintering can be explained by the presence of Y2O3and Nd2O3oxide layers, which form spontaneously on the surface of WE43 powder used in BJAM. These layers appear to be crucial in preventing shape distortion of the resulting samples and in enabling the development of sintering necks, particularly under sintering conditions exceeding the liquidus temperature of WE43 alloy. Sintered WE43 specimens rendered by BJAM achieved significant improvement in both corrosion resistance and mechanical properties through reduced porosity levels related to the sintering time. Introduction Additive manufacturing (AM) offers additional potential for metallic implants by providing opportunities to develop complex, personalized geometries that are difficult or impossible to make using conventional manufacturing processes. More specifically, AM makes it possible to personalize both external and internal morphology. Prior to manufacture, one can interactively simulate the effect of personalized device shape, location, material properties, and mechanical properties on post-implantation performance prior to sending the optimized computer-aided design (CAD) files through a layer-by-layer fabrication process. On the other hand, conventional manufacturing methods such as casting, machining, rolling, forging, joining, deformation, and extrusion have been shown to reduce cost in industries where mass production reduces cost through economies of scale. Approximately 98% of all structural applications for Mg alloys have been manufactured by casting processes such as die casting and gravity casting. In contrast, wrought Mg alloys have limited applications, primarily due to insufficient formability and workability at room temperature. While additive-manufacturing of Mg alloy-based medical implants have been the topic of research, none has been translated to the clinic despite the significant Attorney Docket No.103362-018WO1 opportunities for personalized medical devices due, primarily to their biomedically relevant mechanical properties and capacity to resorb. As an AM technique, binder jetting, offers a much faster material deposition rate than most fusion-based AM processes, including metallic laser powder bed fusion (LPBF) that uses a laser or an electron beam powder bed fusion (EBPBF) heat source. Binder jetting AM (BJAM) is a two-step process which 1) prints “green” (binder present, prior to post- processing heat treatments) parts with the required shape, and 2) transforms them into dense (non-porous), functional parts. In the first step, an Mg alloy powder is bound into the desired shape by extruding a polymer into the powder bed, at a near-ambient temperature. Next, the 3D printed green parts are sintered products, usually under a protective atmosphere. Therefore advanced sintering processes are needed for the BJAM of Mg alloys to take full advantage of the expedited rate of production, increased freedom of product design for complex shapes, and the ability to produce residual stress-free printed components. Furthermore, a significant problem encountered in the BJAM of Mg alloy parts is the difficulty in producing sufficiently dense 3D printed parts. Another difficulty is that forming can take a long time due to the oxide layer on the surface of Mg powders. Unlike Fe and Cu and their alloys, the diffusivity of Mg alloys, once oxidized, is several orders of magnitude lower than its self-diffusivity. Thus, the MgO film on an Mg alloy powder’s surface will significantly retard mass transport which is needed for sintering neck formation between powder particles in a conventional solid state sintering process. Attempts to solve this problem have utilized many techniques, such as spark plasma sintering (SPS), microwave (MW) sintering, and liquid phase sintering. Notably, SPS is expected, as an alternative sintering process, to obtain geometrically stable shapes. In MW sintering, the compacted specimen couples with microwaves, volumetrically absorbing the electromagnetic energy per se, and converting it to heat, whereas in conventional sintering, heat is generated by an external heating source and then transferred to the specimen. As a result of completely different heating mechanisms, MW sintering offers many advantages over conventional sintering methods, including enhanced diffusion kinetics, reduced sintering time, improved physical and mechanical properties, substantial energy savings, and eco-friendliness. A detailed comparative analysis of the consolidation of as-printed Mg- 5.06wt.%Zn-0.15wt.%Zr green parts by microwave sintering has been conducted. In this study, employing microwave sintering for densification reduced the overall sintering time by three fold as compared to sintering in a conventional furnace. This time saving is Attorney Docket No.103362-018WO1 attributed to a difference in the sintering mechanisms triggered at a comparatively lower temperature in a microwave furnace. Despite these merits, there are still questions about the sintering mechanisms and the densification behavior related to SPS parameters, and the effects of SPS parameters on microstructure and properties require further in-depth investigation. WE43 Mg alloy, with a nominal composition of Mg-(3.7-4.3) wt.% yttrium (Y)- (0.4-1.0) wt.%) zirconium (Zr)-(2.4-4.4) wt. % rare earth (RE), has been studied for lightweight structural applications and potential bioresorbable applications, due to its superior mechanical strength and good corrosion properties. WE43 alloy has been manufactured using laser powder bed fusion (LPBF) in recent years, and the samples rendered by LPBF have shown mechanical properties superior to those of cast parts. However, the high porosity and formation of intermetallic compounds in LPBF samples leads to accelerated corrosion propagation and enhanced micro-galvanic corrosion effects. Thus, the high corrosion rates and inhomogeneous corrosion behavior of WE43 specimens rendered by LPBF alloy is an issue that needs to be addressed to improve the overall performance of 3D printed WE43 specimens in contexts where a slower corrosion rate is desired. In this study, a liquid phase sintering process has been developed for WE43 green parts rendered BJAM 3D printing. The effect of this sintering process on the microstructure, mechanical properties, and corrosion properties of WE43 alloy was investigated. Additionally, we attempted to reduce the total sintering time significantly for these WE43 alloy specimens compared to what is needed for the post binder jetting sintering of AZ91D alloy specimens. After high densification was achieved while maintaining the original shape of the green part during the liquid sintering process, the sintering mechanism of the liquid phase process was studied using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS). The mechanical properties of the final products were evaluated by micro-tension testing. Moreover, their corrosion properties were studied by an electrochemical tests including potentiodynamic polarization, electrochemical impedance spectroscopy, and an immersion test in Hanks’ solution. Finally, we expect that a successful liquid phase sintering process following BJAM for WE43 would result in opportunities to investigate the additive manufacturing of the WE54 Mg alloy (Mg- 5wt.%Y-4wt.%RE, mainly Nd) for biomedical and structural applications. Attorney Docket No.103362-018WO1 Materials and Methods Material Preparation. The atomized metal powder of WE43 alloy is made from the MAP43 Mg alloy and is provided by Luxfer MEL Technologies (Flemington, NJ, USA). The WE43 powder we used for BJAM sample production had a manufacturer-guaranteed particle size range of 20 to 63 μm, and its particle size distribution was D106.9 μm; D5039.5 μm; D9055.9 μm. The chemical composition of that powder is shown in Table 1. Table 1. The chemical composition of the atomized WE43 powder. Samples were formed via BJAM using an Innovent+TM3D printer (ExOne, Irwin, PA, USA). An aqueous binder was provided by ExOne with a composition of ethylene glycol monobutyl ether and ethylene glycol. After printing, the green parts were cured at 150 °C for 4 h in a furnace followed by a depowderizing step. The green parts were then prepared for microstructural analysis, tensile testing, electrochemical analysis, and immersion testing. For microstructural analysis and micro-tensile testing, cubic specimens with 15x15x15 mm dimensions were prepared. For electrochemical analysis, specimens in the shape of a cylinder with a diameter of 22 mm and a thickness of 3.5 mm were manufactured. For the immersion test, cylindrical specimens with a diameter of 13 mm and a length of 19 mm were fabricated (Figure 1). To study sintering behavior, each green part was sintered in a furnace (VBF-1200X- H8, MTI Corporation, Richmond, CA, USA) under Ar atmosphere while the furnace chamber was slightly pressurized to ensure that there is no inflow of air to the chamber. The controlled atmosphere in the furnace before the heating temperature was obtained by a purging process. The process of vacuum and purging was repeated three times to dilute the oxygen level as much as possible in the chamber. Then, the heat treatment profile for liquid phase sintering was followed by heating at 10 °C / min from room temperature to 300 °C, holding for 30 min at 300 °C, heating at 10 °C / min to 680 °C, holding for 30 min, cooling at 1 °C / min to 600 °C, holding for 180, 240 and 300 min, respectively. Then it was cooled down to room temperature in the furnace (Figure 2). Additionally, as part of the densification through sintering process, a green part manufactured by a binder jetting Attorney Docket No.103362-018WO1 process undergoes a debinding process. This involves heat treatment for about 30 min to an hour at 200 to 300 °C. The binder is mostly decomposed through pyrolysis and completely removed during this time. Microstructure. In order to determine the sintering conditions, the solidification behavior of WE43 Mg alloy was simulated based on the Scheil’s model by using Pandat software and PanMg2018 database. The microstructure of the sintered parts was characterized using optical microscopy (OM) to analyze the porosity of each sintered part with image analysis software (ImageJ), which was used to measure the pores seen in the middle area of each sample. At least 10 randomly taken images were analyzed for each sample. The phases and oxide layers in the sintered parts were analyzed by X-ray diffraction (XRD, Rigaku, MiniFlex 600) with radiation generated 20 mA and 40 kV in the 2θ range from 20° to 80° at a scanning rate of 1° / min and a step size of 0.02°. And the microstructures of each sintered parts were observed by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS). To observe the cross-section of the as-received WE43 Mg alloy powder, the powder was mounted in a conductive resin (Struers Polyfast, Struers Ltd. Canada) and mechanically ground and polished using progressively finer SiC paper between 320 and 800 grit. Then, the SEM micrograph was obtained in backscattered electron (BSE) mode. All specimens for microstructural observation were polished with a 2000-grit silicon carbide (SiC) paper and then finally polished a near-mirror finish using 0.05 μm alumina powder suspended in ethyl alcohol. Corrosion Properties. To measure the volume of hydrogen evolution during the immersion in Hank’s solution, the testing equipment was set up as shown in Fig.3. The specimens were soaked in a beaker containing Hank’s solution at the ambient temperature (15-20 ºC), respectively. The hydrogen that evolved during immersion was collected into a burette with a scailing of 0.1 ml over the specimen. And to investigate corrosion initiation and propagation of the sintered part, the corroded surface was observed by SEM for the initial 2 h. The potentiodynamic polarization and EIS measurements were conducted in Hank’s solution (Thermo Fisher Scientific, Waltham, MA, USA) using a Gamry Reference 600+ potentiostat (Gamry Instruments, Warminster, PA) at room temperature. The exposed areas of the specimens were used as the working electrodes through with a platinum counter electrode. A saturated calomel reference electrode (SCE) was used as the reference electrode. Potentiodynamic polarization tests were performed at a scan rate of 0.167 mV / s Attorney Docket No.103362-018WO1 according to ASTM G59-23. EIS was conducted in the frequency range of 100 kHz to 10 mHz with a perturbation amplitude of 10 mV in an open circuit for 30 min according to ASTM G106-89. All specimens tested for corrosion properties were polished with SiC paper up to 2000 grit and then underwent potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and an immersion test. Mechanical Properties. Micro-tension specimens were machined from the center of sintered cube samples with a gauge length of 1.5 mm, gauge width of 0.36 mm, and thickness of 0.20 mm by electrical discharge machining (EDM) as shown in Figure 4. The micro-tensile specimens were sectioned and then carefully polished with 800 grit SiC paper. Tensile tests were conducted on at least 5 specimens per sintered cube sample with a strain rate of 0.06 / s on a custom micro-tensile testing system. After the tensile testing was completed, the resulting fracture surfaces were observed. Results and Discussion Microstructure. To perform the full liquid phase sintering, calculations based on the Scheil model were conducted using Pandat software to predict the solid fraction of WE43 alloy as a function of temperature, as shown in Figure 4. As a result, the solidus temperature was calculated to be 530 °C, and the temperature at which the liquid fraction becomes 99% was calculated to be 640 °C. Moreover, when the green part was maintained at 680 °C for 30 min and then at 600 °C for 180, 240, and 300 min before cooling, the porosities measured through OM images were 7.2, 4.3, and 2.5%, respectively (Figure 5), showing that a decrease in porosity was associated with an increase in sintering time. There are different mechanisms of densification at work during the sintering process, including particle rearrangement, pore elimination, and grain growth related to migration and diffusion. Moreover, temperature is a dominant factor in terms of sample densification during sintering. As the sintering temperature increases, mass transport becomes more active due to increased diffusion. If a liquid phase forms, the mass transport can be significantly faster compared to materials that only undergo solid-state diffusion. Therefore, the liquid phase considerably reduces the time needed for densification. However, theoretically, for excessive holding time at high sintering temperatures, the stability of the oxide film on the surface of Mg powder decreases due to excessive the volume of the liquid phase or its fluidity. The uneven filling of pores by the liquid phase not only hinders the densification process but can also cause uneven shrinkage and distortion. Therefore, especially for pure Mg powder, which includes vulnerable MgO film on its surface, Attorney Docket No.103362-018WO1 sintering is usually performed at a temperature below its liquidus temperature or even below the solidus temperature to maintain the original shape of the part and prevent expansion. Recent studies reported significant research in producing dense structures of MgO film on AZ91D alloy powders through a complete liquid phase sintering process at temperatures higher than its liquidus. While examining the high-temperature stability of the spontaneously formed MgO on the powder of AZ91D, in which the liquidus temperature is 600 °C, sintering was performed at temperatures from 590 to 640 °C for 12 h. As a result, the highest density of 93.16% was observed at 620 °C. Additionally, specimens have been produced with a density of 97.8% through a sintering method that involved artificially forming a net-like skeleton of MgO by drying a binder jetting processed AZ91D green part in an air atmosphere at 90 °C for 10 h, maintaining it at 700 °C for 30 min, and then at 600 °C for 6 h. Therefore, the full liquid sintering process maintained at 680 °C for 30 min to form a full liquid phase in WE43 alloy particles and then at 600 °C for 300 min indicates a significant reduction in the overall process time compared to the reported the full liquid sintering processes. Figure 6 is the result of analyzing the XRD pattern of the WE43 alloy cooled after holding at 600 °C for 300 min, where the presence of intermetallic compounds such as Mg, Mg12Nd, and oxides like MgO, Nd2O3, and Y2O3was analyzed. Figure 7 shows the microstructure of WE43 alloy sintered in a liquid phase sintering process as a function of sintering time and temperature, respectively. Figure 7 (panel a) shows the microstructure observed immediately after cooling upon reaching 660 °C. Interestingly, an oxide film remains a shell, with some particles partially hollow inside and others where liquid has flowed out and solidified. These powders are seen to form sintering necks with each other. Since the liquidus temperature calculated in Figure 3 is around 640 °C, it can be inferred that at this temperature, Mg found inside the film, except for the oxide film composed of MgO, Nd2O3, and Y2O3, is in a liquid state. The liquid breaks through weak points in the film and flows out, forming necking between particles. As shown in Figure 7(panel b), even when heated to 680 °C, the particles maintain their shape while forming thicker necks and a larger number of necks. It is evident in Figure 7(panel c), after maintaining at 680 °C for 20 min and then cooling, that the liquid flowing out from the area around the particles into the pores and the liquid from different particles are combining to form necking. Generally, when pores exist between solids, capillary force, which aids in quickly filling these pores, can accelerate liquid transfer over the solid particles. Moreover, if the solid phase, i.e., the Attorney Docket No.103362-018WO1 oxide film, is soluble in the liquid, the solid framework can soften, distorting the sintering shape, or the change in the interface energy between the solid and liquid can reduce the effect of capillary force. Therefore, these results demonstrate that the presence of Nd2O3and Y2O3ensures a more stable oxide film that covers the particles, maintains the sample’s shape at high temperatures, and allows liquid Mg to flow out and fill the voids while simultaneously forming sintering necks between particles. Figure 7(panel d)-(panel f) show the microstructures after cooling to 600 °C where it is held for 180, 240, and 300 min, respectively, before cooling to room temperature. It can be seen that the amount of pores between particles decreases significantly as the holding time increases. The EDS mapping results in Figure 7(panel f) suggest that the alloy elements Nd, Y, Zr are mostly present in the oxide layer surrounding the outer of the powders, confirming the positions of the Nd2O3and Y2O3layers detected in the XRD pattern analysis (Figure 6). The alloying elements Nd, Y, Zr inside the particles are analyzed to be very uniformly distributed in minute quantities. In contrast, Figure 8 shows the backscattered electron image of the cross-sectional, as- received WE43 powder and it indicates that there is a segregation of alloying elements between interdendritic regions. Therefore, it can be suggested that most of the alloying elements, such as Nd, Y, and Zr, move towards the outer surface of WE43 particles during the solidification process of the full liquid phase sintering. These alloying elements could be considered to have enhanced the stability of the oxide layer on WE43 particles during the full liquid sintering process, preventing ununiform shrinkage and distortion. Finally, Figure 9 and Table 2 show photos and volume changes of the green part samples labeled (A) and the final sintered WE43 specimens labeled (B), confirming that the overall volume has shrunk uniformly. Table 2. Comparison of the measured sizes between the green part and the sintered part after full liquid phase sintering process. Attorney Docket No.103362-018WO1 Corrosion Properties. Hydrogen evolution is closely associated with the dissolution of the Mg alloy in two distinct ways. First, an electrochemical reaction, as depicted in Eq.1, serves to balance the Mg dissolution process described in Eq.2. Additionally, hydrogen is directly evolved during the interaction of Mg ions with water or aqueous solutions. The overall reaction yields one molecule of hydrogen gas per each Mg atom that dissolves. Meanwhile, experimental analysis has demonstrated that during the corrosion of Mg or Mg alloys, univalent Mg+ intermediates exist with an apparent valency between 1 and 2. This research is based on a univalent Mg model and the negative difference effect (NDE) theory which describes hydrogen evolution behavior in Mg alloys during their dissolution. Experiments have proved that the presence of univalent Mg+ ions ultimately leads to an increase in hydrogen production. This phenomenon is a particularly distinctive corrosion reaction found in Mg alloys, explaining why the amount of hydrogen gas produced is greater than the amount predicted by Faraday's law. Despite this, ultimately the amount of hydrogen produced during immersion is directly proportional to the corrosion rate of Mg. Figure 11 presents the volume of evolved hydrogen immersed in Hank’s solution during the first 6 hours. The surface area was calculated by assuming the samples were a perfectly dense cylinder, and the evolved hydrogen volume per the surface area was calculated from the initial surface area of those cylindrical specimens. For all specimens, the evolved hydrogen volume increased with increasing time. Furthermore, the specimens used were subjected to the same composition and the same corrosion environment and the distinct microstructural difference is the decrease in porosity level related to sintering time. Through this observation, it is evident that the decrease in porosity led to a reduction in hydrogen gas generation. Additionally, when the evolved hydrogen was quantitatively calculated over a period of 6 hours, in the WE43 alloy containing 7.2, 4.3, 2.5% porosity for microstructures sintered for 180, 240, and 360 min, respectively, the associated gas evolution rate was measured as 0.31, 0.15, and 0.11 ml cm-2hr-1(Table 3), respectively. Attorney Docket No.103362-018WO1 Table 3. Hydrogen evolution as measured during the first 6 hours of immersion in Hank’s solution. Figure 12 shows SEM images of the WE43 alloy samples, sintered at 600 °C for 300 min, revealing the corrosion surface over 2 h for the initial corrosion behavior. Figure 12(panel a) shows that cracks, indicated by white arrows, have formed on the surface of this sample as part of the corrosion process after 30 min of immersion. These cracks were likely caused by partial cracking of the corrosion product film due to volume differences between the Mg substrate and the corrosion product film and dehydration that occurs after the initial post-immersion specimen drying. Additionally, such cracks in the corrosion product film are reported to be caused by hydrogen generation and release from the Mg matrix during the corrosion process. It was difficult to determine the dominant corrosion behavior during the short 30-minute immersion due to the formation of corrosion products on the entire surface. However, as shown in Figure 12(panel b), localized corrosion was observed along the boundary of the powder surface's oxide film and the internal Mg alloy matrix. Figure 12(panel c) shows that this corrosion attack occurs only in the Mg region inside the particle. Fig.12(panel d) shows the EDS mapping analysis of the corroded surface after 2 h of immersion, focusing on areas where corrosion has progressed to the interior of the grain. As observed in the microstructural analysis, the region that was the surface of the sintered WE43 particles contained the formation of layers containing Y, Zr, and Nd and the Mg12Nd phase. The corrosion attack has penetrated in the Mg region within the particle. This phenomenon is obviously caused by the micro-galvanic corrosion effect due to the potential difference between Mg and the alloying element layer and between Mg and the precipitate. Therefore, the dominant cause of corrosion initiation in the binder-jetted WE43 alloy after the full liquid-phase sintering is likely due to the micro-galvanic corrosion effect. It is inferred that the Mg matrix within the specimen decomposes first, leaving the surrounding oxidation layer intact prior to a period where corrosion is more significantly advanced. This phenomenon may make the corrosion behavior of Mg alloys appear similar due to the micro-galvanic effect, but there are clear and distinct differences. When noble intermetallics are formed due to alloying, they generate micro-galvanic couplings with Mg matrix due to potential differences between the two phases, causing the relatively active Mg to corrode Attorney Docket No.103362-018WO1 rapidly. However, forming these noble metal intermetallics along the grain boundaries as a continuous network can act as a corrosion barrier. In typical commercial Mg alloys like as- cast WE43 (Mg-Y-Zr-Nd), AZ91D (Mg-Al-Zn), and AM50 (Mg-Al-Mn), it seems that continuous networks of precipitates or intermetallics are not formed without artificial treatment. Therefore, the oxidation films observed in the sintered structure can act as electrical insulators, wrapping the Mg matrix and delaying corrosion propagation after Mg decomposition. Specifically, the presence of Y2O3and Nd2O3in MgO films formed in the WE43 alloy in corrosive environments significantly reduces the corrosion rate of the WE43 alloy. Moreover, it is intriguing that in this example, we observe that oxidation films were formed before exposure to Hank’s solution. Nevertheless, the initiation of corrosion between the Mg matrix interface and the oxide layer is presumed to be due to the presence of Mg12Nd phase or non-oxide Y, Zr, Nd, which cause corrosion initiation as a result of potential differences with Mg. Figure 13 shows that the potentiodynamic polarization measurements of these specimens were performed after OCP reached a steady state for 30 min. The corrosion potentials and corrosion current densities obtained via Tafel extrapolations are shown in Table 4. The corrosion potential was observed to move in a noble direction as the sintering time increased. Similarly, the corrosion current density was observed to move in a decreasing direction as sintering time increased. This result also corresponds with the corrosion rate trend measured by hydrogen evolution. Since there were no significant microstructural differences, except for porosity, it can be inferred that the increase in porosity affects the values of corrosion potential and current density. Table 4. Fitting results of polarization curves of sintered WE43 alloys. Figure 14 illustrates a Nyquist plot after 3h of exposure to Hank’s solution, as well as the respective equivalent circuits under those conditions. The Nyquist diagrams display capacitive loops at high and low frequencies. Typically, the capacitive loop at the high frequencies was attributed to the features of the double electric layer at the interfaces Attorney Docket No.103362-018WO1 between the alloy surfaces and the electrolyte solution, while the small medium frequency capacitive loops represent the capacity of the corrosion products. The low-frequency inductive loops indicate corrosive pitting and destruction of the layers of corrosion products. In Figure 13, the absence or very small size of the capacitive loops at medium and low frequencies in the Nyquist plots of all alloys suggests that there are minimal corrosion products or that they are easily destroyed in all specimens. This behavior can be attributed to the fact that, as seen in microstructural analyses, the solute elements move to the oxide layer of the WE43 particles after the entire liquid sintering process. Consequently, there are hardly any alloy elements left inside the particles, which in turn might contribute minimally or hardly to the formation of stable corrosion products during corrosion behavior. In detail, capacitive loops are usually attributed to charge transfer, film effects, and mass transport. Also, the area under the arc is useful in evaluating the corrosion resistance of the alloy and indicates corrosion resistance. It can be seen that corrosion resistance increases with increasing sintering time. The equivalent circuit is also useful for analyzing charge transfer, oxide film and mass transfer factors. One constant phase element (CPE) factor, Cf, represents the capacitance of the corrosion product layer, and Rfis the resistance of that layer. Cdlrepresents the double layer capacitance at the interface between the specimen and the Hank’s solution. Rtis the charge transfer resistance associated with the electrochemical reaction in the same region. Rsis the resistance between the reference and working electrodes and is therefore not usually considered. RLand L occur in the induced loop of the EIS spectrum. The fitting EIS data by using the same equivalent circuit shows the corrosion resistance increases with the increase in sintering time when comparing the Rfand the Rtvalue of each alloy (Table 4). By considering the EIS impedance curves, which are similar to those of each alloy in shape, it can be understood that the same corrosion mechanism occurs, and the resistance regarding the corrosion film is the main reason for the increase in corrosion resistance. Furthermore, considering Cfand Rf, which are factors related to corrosion products, it is judged that research is necessary on the correlation between the oxide film that existed before exposure to the corrosion environment, such as Hank's solution, and the corrosion products. Mechanical Properties. Figure 15 shows the stress-strain curves for each sintered WE43 alloy sample relative to sintering time. The results did not show a difference in the elastic modulus of the sintered samples due to the difference in sintering time. However, the elongation and tensile strength appeared to increase as sintering time increased. As the Attorney Docket No.103362-018WO1 sintering time increased from 180 to 240 to 300 min, all at 600 °C, yield strength (YS) was measured at 87, 104, and 115 MPa, ultimate tensile strength (UTS) at 145, 191, and 230 MPa, and elongation length (EL) at 2.1, 2.2, and 2.6%, respectively. This is due to the decrease in porosity and the increase in sinter bonding with increased sintering time. Although commercial WE43 alloys are mainly used after age hardening heat treatment has improved a part’s mechanical properties, the YS, UTS, and EL of as-cast WE43 alloys were found to be 132 - 150 MPa, 179 - 206 MPa, and 5.3 - 6.5%, respectively. Therefore, the mechanical properties seen in this study showed higher UTS but slightly lower YS and EL compared to as-cast WE43 alloy specimens. Figure 16 shows the fracture surfaces and the surface of the gauge length direction of the tensile specimens sintered for 180 and 300 min. By comparing Figure 16 (panel a) and (panel b), it can be observed that Figure 16(panel b) had a relatively smoother fracture propagation in the direction of the ideal direction of shear stress by unidirectional load compared to Figure 16 (panel a). In addition, observing the fracture surfaces of Figure 16(panel c) and (panel d), it can be seen that Figure 16(panel c) had a fracture propagation from the location presumed to be where porosity was present (white arrow) and from the interface between the powder and the oxide film (black arrow), whereas Figure 16(panel d) showed a very smooth fracture surface rather than being induced or propagated from the interface between the powder and the oxide film. This fracture behavior can be distinguished from cases where failure occurs at the particle boundary due to a weak bonding force between the powders in sintered Mg alloys. Therefore, these observations confirmed that the mechanical properties were improved due to the reduction of porosity and enhancement of sinter bonding with increased sintering time. Conclusions This study has successfully developed a full liquid phase sintering process, based on CALPHAD modeling, for binder jetting printed WE43 magnesium alloy specimens. The samples tested show minimal porosity. Increased sintering time correlated directly with enhanced mechanical and corrosion properties, which are crucial for structural and biomedical applications. Overall, these findings highlight the potential of using liquid phase sintering processes in the production of high-quality magnesium alloys manufactured via powder-based processes. The presence of Y2O3and Nd2O3in the MgO layer on the surface of WE43 alloy powder plays a role in maintaining structural integrity at high sintering temperatures, Attorney Docket No.103362-018WO1 thereby contributing to the stability and densification of the alloy. The Y2O3and Nd2O3layers are stably present at 680°C, exceeding the liquidus temperature of 640°C. As a result, even when the fully melted Mg liquid inside the oxide layer escaped, it was determined that this contributed to maintaining the shape, forming sintering necks, and filling the pores during the sintering process due to the capillary effect. Consequently, when the porosity of the coarse microstructure was analyzed after heating at 680°C for 30 min, followed by full liquid phase sintering at 600°C for 180, 240, and 300 minutes, porosity was reduced to 7.2%, 4.3%, and 2.5%, respectively, achieving a dense microstructure. A significant decrease in hydrogen gas evolution was observed with reduced porosity levels, indicating an improvement in the corrosion resistance of the sintered WE43 alloy. Additionally, as analyzed through potentiodynamic polarization measurements and EIS analysis, it was observed that with the increase in sintering time, i.e., the reduction in porosity levels, the corrosion potential increased, and the corrosion current decreased, indicating an overall increase in corrosion resistance. Also, this result appears to be related to the role of the outer oxide layer on the powder in preventing the propagation of corrosion, which initiates from within the powder, as observed in the corrosion behavior. The mechanical properties, influenced by the reduction in porosity levels due to increased sintering times, were enhanced as evidenced by the micro-tension testing reported here. Additionally, observations of the fracture surface indicate that the influence of porosity, along with the sintering temperature, play a crucial role in improving mechanical properties by enhancing sintered bond strength. Finally, the sintered WE43 alloy exhibited its best mechanical properties with a yield strength of 115 MPa, ultimate tensile strength of 230 MPa, and elongation of 2.6%, which is comparable to that of as-cast alloy specimen properties. Ultimately, the research aimed at reducing defects in the final products of the binder jetting process and post-processing can be considered crucial for improving mechanical and corrosion properties. Therefore, it is judged necessary to simultaneously conduct studies on binder saturation, powder layer thickness, and the average size and distribution of powder in the binder jetting process. References [1] S. N. Mathaudhu, A. A. Luo, N. R. Neelameggham, E. A. Nyberg, and W. H. Sillekens, ESSENTIAL READINGS IN MAGNESIUM TECHNOLOGY. 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[0050] Y. Li, Z. Shi, X. Chen, and A. Atrens, “Anodic hydrogen evolution on Mg,” Journal of Magnesium and Alloys, vol.9, no.6, pp.2049–2062, Nov.2021.

[0051] G. Song, A. Atrens, and D. StJohn, “An hydrogen evolution method for the estimation of the corrosion rate of magnesium alloys,” in Essential readings in magnesium technology, 2016, pp.565–572.

[0052] C. Cai, R. Song, L. Wang, and J. Li, “Surface corrosion behavior and reaction product film deposition mechanism of Mg-Zn-Zr-Nd alloys during degradation process in Hank’s solution,” Surf Coat Technol, vol.342, pp.57–68, May 2018.

[0053] K. Zhang et al., “Effects of solute redistribution during heat treatment on micro- galvanic corrosion behavior of dilute Mg-Al-Ca-Mn alloy,” Corros Sci, vol.213, Apr.2023.

[0054] M. C. Zhao, M. Liu, G. Song, and A. Atrens, “Influence of the β-phase morphology on the corrosion of the Mg alloy AZ91,” Corros Sci, vol.50, no.7, pp.1939–1953, Jul.2008.

[0055] L. M. Peng, J. W. Chang, X. W. Guo, A. Atrens, W. J. Ding, and Y. H. Peng, “Influence of heat treatment and microstructure on the corrosion of magnesium alloy Mg- 10Gd-3Y-0.4Zr,” J Appl Electrochem, vol.39, no.6, pp.913–920, Jun.2009.

[0056] Y. Zhang et al., “Effects of Intermetallic Microstructure on Degradation of Mg-5Nd Alloy,” Metall Mater Trans A Phys Metall Mater Sci, vol.51, no.10, pp.5498–5515, Oct. 2020.

[0057] H. Y. Choi and W. J. Kim, “The improvement of corrosion resistance of AZ91 magnesium alloy through development of dense and tight network structure of Al-rich α phase by addition of a trace amount of Ti,” J Alloys Compd, vol.696, pp.736–745, 2017.

[0058] D. Zander and C. Schnatterer, “The influence of manufacturing processes on the microstructure and corrosion of the AZ91D magnesium alloy evaluated using a computational image analysis,” Corros Sci, vol.98, pp.291–303, Sep.2015.

[0059] S. Leleu, B. Rives, J. Bour, N. Causse, and N. Pebere, “On the stability of the oxides film formed on a magnesium alloy containing rare-earth elements,” Electrochim Acta, vol. 290, pp.586–594, Nov.2018. Attorney Docket No.103362-018WO1

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[0064] H. S. Jiang et al., “Microstructure and mechanical properties of WE43 magnesium alloy fabricated by direct-chill casting,” Materials Science and Engineering: A, vol.684, pp.158–164, Jan.2017.

[0065] F. Chen, X. Cai, B. Dong, and S. Lin, “Effect of process modes on microstructure and mechanical properties of CMT wire arc additive manufactured WE43 magnesium alloy,” Journal of Materials Research and Technology, vol.27, pp.2089–2101, Nov.2023.

Claims

Attorney Docket No.103362-018WO1 CLAIMS What is claimed is:

1. A method comprising: (a) forming a green body in a powder bed deposition region of an additive manufacturing apparatus from a powder comprising a magnesium alloy; and (b) liquid sintering the green body to produce an article formed from the magnesium alloy.

2. The method of claim 1, wherein liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to less than 12 hours; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 24 hours.

3. The method of claim 2, wherein the liquid sintering of the green body comprises: (1) heating the green body at a temperature at or above the liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 10 hours, from 10 minutes to 8 hours, from 10 minutes to 6 hours, from 10 minutes to 5 hours, from 10 minutes to 4 hours, from 10 minutes to 3 hours, from 10 minutes to 2 hours, from 10 minutes to 1 hour, from 15 minutes to 12 hours, from 15 minutes to 10 hours, from 15 minutes to 8 hours, from 15 minutes to 6 hours, from 15 minutes to 5 hours, from 15 minutes to 4 hours, from 15 minutes to 3 hours, from 15 minutes to 2 hours, or from 15 minutes to 1 hour.

4. The method of any one of claims 2-3, wherein the liquid sintering of the green body comprises: (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 18 hours, from 1 hour to 12 hours, from 1 hour to 8 hours, from 1 hour to 6 hours, from 1 hour to 4 hours, from 1 hour to 2 hours, from 2 hours to 24 hours, from 2 hours to 18 hours, from 2 hours to 12 hours, from 2 hours to 8 hours, from 2 hours to 6 hours, from 2 hours to 4 hours, from 4 hours to 24 hours, from 4 hours to 18 hours, from 4 hours to 12 hours, from 4 hours to 8 hours, or from 4 hours to 6 hours.Attorney Docket No.103362-018WO1 5. The method of any one of claims 2-4, wherein the liquid sintering of the green body is performed in a total time of less than 12 hours, such as a total time of from 2 hours to 10 hours, from 2 hours to 8 hours, from 3 hours to 8 hours, from 4 hours to 8 hours, from 2 hours to 6 hours, from 3 hours to 6 hours, or from 4 hours to 6 hours.

6. The method of any one of claims 2-5, wherein the liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 1 hour; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 2 hours to 6 hours.

7. The method of any one of claims 2-6, wherein heating the green body at the temperature at or above the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of at least 640 ºC, such as a temperature of from 640 ºC to 750 ºC, a temperature of from 640 ºC to 700 ºC, a temperature of from 650 ºC to 750 ºC, a temperature of from 650 ºC to 700 ºC, a temperature of from 660 ºC to 750 ºC, or a temperature of from 660 ºC to 700 ºC.

8. The method of any one of claims 2-7, wherein heating the green body at the temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of from 540 ºC to 630 ºC, such as a temperature of from 540 ºC to 620 ºC, from 560 ºC to 620 ºC, or from 580 ºC to 620 ºC.

9. The method of any one of claims 2-8, wherein the liquid sintering the green body comprises: (1) heating the green body at a temperature of from 660 ºC to 700 ºC for a period of time of from 15 minutes to 1 hour; and (2) heating the green body at a temperature of from 560 ºC to 620 ºC for a period of time of from 2 hours to 6 hours.Attorney Docket No.103362-018WO1 10. The method of any one of claims 1-9, wherein the liquid sintering of the green body is performed in a controlled atmosphere, such as an argon atmosphere.

11. The method of any one of claims 1-10, wherein the magnesium alloy comprises a rare-earth magnesium alloy.

12. The method of any one of claims 1-11, wherein the magnesium alloy comprises yttrium, neodymium, zirconium, or a combination thereof.

13. The method of any one of claims 1-12, wherein the magnesium alloy comprises from 90-95 wt% magnesium, from 3.7-4.3 wt% yttrium, from 2.4-4.4 wt% rare earth elements, and from 0.4-1.0 wt% zirconium.

14. The method of any one of claims 1-13, wherein the magnesium alloy comprises WE43, WE54, or a combination thereof.

15. The method of any one of claims 1-14, wherein the powder comprising the magnesium alloy comprises a population of magnesium alloy particles having an average particle size of from 5 microns to 100 microns.

16. The method of any one of claims 1-15, wherein the magnesium alloy particles comprise a layer of metal oxide present on a surface of the particles.

17. The method of claim 16, wherein the metal oxide comprises Y2O3, Nd2O3, MgO, or a combination thereof.

18. The method of any one of claims 16-17, wherein the metal oxide has a melting temperature, and wherein the liquid sintering the green body comprises (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy but below the melting temperature of the metal oxide.

19. The method of any one of claims 1-18, wherein the method comprises binder jetting additive manufacturing (BJAM).Attorney Docket No.103362-018WO1 20. The method of any one of claims 1-19, wherein forming the green body in the powder bed deposition region of the additive manufacturing apparatus comprises selectively depositing a binder onto the powder bed in a pattern corresponding to a cross-sectional layer of the article.

21. The method of any one of claims 1-20, wherein the method further comprises curing or drying the green body prior to the liquid sintering.

22. The method of any one of claims 1-21, wherein the method further comprises depowdering the cured or dried green body prior to the liquid sintering.

23. The method of any one of claims 1-22, wherein the article formed from the magnesium alloy has a porosity of less than 10%, such as a porosity of from 8% to 1%.

24. The method of any one of claims 1-23, wherein the article formed from the magnesium alloy exhibits improved corrosion resistance as compared to an as cast article formed from the magnesium alloy, as evidenced by decreased hydrogen evolution during the immersion in Hank’s solution.

25. The method of any one of claims 1-24, wherein the article formed from the magnesium alloy exhibits a yield strength, an ultimate tensile strength, an elongation, or a combination thereof that is similar or improved relative to an as cast article formed from the magnesium alloy.

26. A method of forming an article from a magnesium alloy using binder jetting additive manufacturing (BJAM), the method comprising: (a) providing a powder bed comprising a powder comprising the magnesium alloy; (b) selectively depositing a binder onto the powder bed in a pattern corresponding to a cross-sectional layer of the article using a binder jetting print head; (c) repeating the steps of providing the powder bed and selectively depositing the binder to form a green body corresponding to the three-dimensional geometry of the article; (d) curing or drying the green body to enhance its mechanical integrity for subsequent handling; (e) depowdering the cured green body; andAttorney Docket No.103362-018WO1 (f) liquid sintering the green body in a controlled atmosphere to produce a densified article formed from the magnesium alloy.

27. The method of claim 26, wherein liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to less than 12 hours; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 24 hours.

28. The method of claim 27, wherein the liquid sintering of the green body comprises: (1) heating the green body at a temperature at or above the liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 10 hours, from 10 minutes to 8 hours, from 10 minutes to 6 hours, from 10 minutes to 5 hours, from 10 minutes to 4 hours, from 10 minutes to 3 hours, from 10 minutes to 2 hours, from 10 minutes to 1 hour, from 15 minutes to 12 hours, from 15 minutes to 10 hours, from 15 minutes to 8 hours, from 15 minutes to 6 hours, from 15 minutes to 5 hours, from 15 minutes to 4 hours, from 15 minutes to 3 hours, from 15 minutes to 2 hours, or from 15 minutes to 1 hour.

29. The method of any one of claims 27-28, wherein the liquid sintering of the green body comprises: (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 1 hour to 18 hours, from 1 hour to 12 hours, from 1 hour to 8 hours, from 1 hour to 6 hours, from 1 hour to 4 hours, from 1 hour to 2 hours, from 2 hours to 24 hours, from 2 hours to 18 hours, from 2 hours to 12 hours, from 2 hours to 8 hours, from 2 hours to 6 hours, from 2 hours to 4 hours, from 4 hours to 24 hours, from 4 hours to 18 hours, from 4 hours to 12 hours, from 4 hours to 8 hours, or from 4 hours to 6 hours.

30. The method of any one of claims 27-29, wherein the liquid sintering of the green body is performed in a total time of less than 12 hours, such as a total time of from 2 hours to 10 hours, from 2 hours to 8 hours, from 3 hours to 8 hours, from 4 hours to 8 hours, from 2 hours to 6 hours, from 3 hours to 6 hours, or from 4 hours to 6 hours.Attorney Docket No.103362-018WO1 31. The method of any one of claims 27-30, wherein the liquid sintering the green body comprises: (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy for a period of time of from 10 minutes to 1 hour; and (2) heating the green body at a temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy for a period of time of from 2 hours to 6 hours.

32. The method of any one of claims 27-31, wherein heating the green body at the temperature at or above the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of at least 640 ºC, such as a temperature of from 640 ºC to 750 ºC, a temperature of from 640 ºC to 700 ºC, a temperature of from 650 ºC to 750 ºC, a temperature of from 650 ºC to 700 ºC, a temperature of from 660 ºC to 750 ºC, or a temperature of from 660 ºC to 700 ºC.

33. The method of any one of claims 27-32, wherein heating the green body at the temperature at or above a solidus temperature of the magnesium alloy but below the liquidus temperature of the magnesium alloy comprises heating the green body at a temperature of from 540 ºC to 630 ºC, such as a temperature of from 540 ºC to 620 ºC, from 560 ºC to 620 ºC, or from 580 ºC to 620 ºC.

34. The method of any one of claims 27-33, wherein the liquid sintering the green body comprises: (1) heating the green body at a temperature of from 660 ºC to 700 ºC for a period of time of from 15 minutes to 1 hour; and (2) heating the green body at a temperature of from 560 ºC to 620 ºC for a period of time of from 2 hours to 6 hours.

35. The method of any one of claims 26-34, wherein the magnesium alloy comprises a rare-earth magnesium alloy.

36. The method of any one of claims 26-35, wherein the magnesium alloy comprises yttrium, neodymium, zirconium, or a combination thereof.Attorney Docket No.103362-018WO1 37. The method of any one of claims 26-36, wherein the magnesium alloy comprises from 90-95 wt% magnesium, from 3.7-4.3 wt% yttrium, from 2.4-4.4 wt% rare earth elements, and from 0.4-1.0 wt% zirconium.

38. The method of any one of claims 26-37, wherein the magnesium alloy comprises WE43, WE54, or a combination thereof.

39. The method of any one of claims 26-38, wherein the powder comprises a population of magnesium alloy particles having an average particle size of from 5 microns to 100 microns.

40. The method of claim 39, wherein the magnesium alloy particles comprise a layer of metal oxide present on a surface of the particles.

41. The method of claim 40, wherein the metal oxide comprises Y2O3, Nd2O3, MgO, or a combination thereof.

42. The method of any one of claims 40-41, wherein the metal oxide has a melting temperature, and wherein the liquid sintering the green body comprises (1) heating the green body at a temperature at or above a liquidus temperature of the magnesium alloy but below the melting temperature of the metal oxide.

43. The method of any one of claims 26-42, wherein the article formed from the magnesium alloy has a porosity of less than 10%, such as a porosity of from 8% to 1%.

44. The method of any one of claims 26-43, wherein the article formed from the magnesium alloy exhibits improved corrosion resistance as compared to an as cast article formed from the magnesium alloy, as evidenced by decreased hydrogen evolution during the immersion in Hank’s solution.

45. The method of any one of claims 26-44, wherein the article formed from the magnesium alloy exhibits a yield strength, an ultimate tensile strength, an elongation, or a combination thereof that is similar or improved relative to an as cast article formed from the magnesium alloy.

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