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Improved method for High Strength Magnesium Alloys in the as-cast Condition

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Development of low cost casting methodology (or significant improvement of existing casting methods) for the production of magnesium alloys with significantly improved microstructural and mechanical properties/performance 

DESCRIPTION: Due to their low density and high specific properties, magnesium (Mg) alloys are often considered for applications in which weight savings are an important selection factor. Typically, the alloys used in these situations are in the wrought condition, as they display significantly higher strengths relative to cast alloys. However, there are numerous components on US Army platforms that would benefit from improved performance and weight savings of higher strength cast Mg alloys (transmission casings, structural members, non load bearing part, etc.). Despite these potential opportunities, the use of as-cast magnesium alloys is still hampered by their lower strengths in the as processed condition despite the advent of alloys that contain an appreciable amount of precipitates (such as the LPSO containing Mg-Y-Zn and Mg-Al-Gd alloys and/or rare-earth containing alloys, e.g., Mg-Gd-X-Y). Oftentimes, the precipitates in these alloys suffer from a lack of homogeneous and uniform distribution in the matrix. In addition, in some cases, the precipitates can actually reduce strength properties as they serve to nucleate cracks during loading (e.g., Mn-rich precipitates). Thus, in order for Mg castings to become more widely accepted/used in the as-cast state, it is imperative that the strength and ductility of these materials be significantly improved over current high strength cast alloys (e.g., WE43, Mg-Gd(Y)-Zn, etc.). (An approximate 20% or more improvement in tensile performance over average values for high strength Mg alloy castings of ~375 MPa and 5-7% elongation is desired.) In an attempt to overcome the above limitation, the US Army is interested in the development of a low cost, highly robust (e.g., consistent) casting method that will produce magnesium alloys with significant property improvements over current methods. In addition to defense related applications, the development of higher strength Mg alloys would readily find applications in automotive, aerospace, and other industries currently faced with increasing demand for higher strength, lighter weight materials solutions. It is desired that this method will use readily available elemental additions (e.g., four 9s purity or better) – and the method may or may not utilize the application of electromagnetic fields. The alloy should contain a significantly refined grain structure with uniform distribution of precipitates which could be coherent, semi-coherent, or incoherent with the matrix. The precipitates may either form through in-situ reactions during the casting process or may be added (for example, nano-oxides) during the melting/stirring/casting process. Furthermore, it is desired that the precipitates should be present over a relatively broad size range (from nanosized oxide particles to 10-20 micron precipitates) in order to maximize strengthening effects. Modifications of existing Mg alloy compositions (such as well known AZ or ZK series) as well as novel ones developed specifically for this topic may be used. For further insight on potential reinforcements, the reader is referred to the series of papers by JF Nie (Monash University) on the desired combination of precipitate size, shape and distribution in Mg alloys. Those interested in the use of oxide nanoparticles are referred to the work by M Gupta (National University of Singapore). 

PHASE I: Select a standard (baseline) commercially available alloy chemistry and develop an improved fabrication methodology for successful ingot melting and casting. In addition to delivering sample materials which demonstrate the fidelity of the methodology, quantify the alloy material according the following requirements: - Deliver one (1) ingot casting with dimensions of 7 inches x 7 inches x 4 inch. - Provide a detailed composition evaluation: Full chemical assay and analysis for both all metallic and non-metallic constituents Impurities, especially, the low atomic number interstitials - Microstructural characterization: Identify macro- and micro-scale morphology Phase identification, precipitate chemistry Size, and distribution, and texture of the alloy matrix Verification to be performed using optical, scanning, and transmission electron microscopy, electron backscatter diffraction, and X-ray diffraction analyses to verify the grain morphology, constituent phases, precipitate types, and texture present. - Demonstrate compositional homogeneity and uniformity within the delivered ingot material, subject to the constraints: No more than 1.5 atomic percent variation Tested at four (4) random locations - Mechanical tensile properties at quasi-static strain rates (in three orthogonal directions) of the as-cast ingot: Desired: UTS – 450 MPa, Elongation – 15%; Minimum acceptable: UTS – 325 MPa, Elongation – 15% Degree of anisotropy: primary UTS value should drop no more than 10% -Downselect a second Mg-based alloy composition, provide the reasoning for its selection (e.g., ease of fabrication, cost of raw materials, strengthening mechanisms, etc.) and identify the relevant processing protocols for the successful fabrication. 

PHASE II: Demonstrate feasibility of scaling the fabrication methodology, identified and developed in Phase I for both the baseline and second alloy compositions. Furthermore, demonstrate repeatability of the process and construct an up-scaled pilot-scale facility that is suitable for batch, semi-continuous or continuous production of alloy material. In addition to delivering sample materials which demonstrate the fidelity of the methodology, quantify larger-scale alloy materials according the following requirements: - Construct a pilot-scale melting and casting system, capable of producing batch-mode and semi-continuous castings, and develop a manufacturing operations and commercialization plan - Three (3) ingot castings with dimensions of (minimum) 15 inches x 15 inches x 6 inch - As was performed in Phase I, demonstrate compositional homogeneity and uniformity within each of the castings. - Mechanical tensile properties at quasi-static strain rates (in three orthogonal directions) of the as-cast ingots: Desired: UTS – 450 MPa, Elongation – 15%; Minimum acceptable: UTS – 325 MPa, Elongation – 15% Degree of anisotropy: primary UTS value should drop no more than 10% - Evaluate high strain rate properties (at a strain rate of 10^3 /sec or higher): Tested in three orthogonal directions, High strain rate properties to be consistent with quasi-static strain rate properties and the strain rate sensitivity of Mg alloys - Develop a commercialization strategy and identify potential partnering and transition opportunities in the automotive or other relevant industrial sector. Provide cost benefit analysis of the use of as-cast Mg based alloy versus currently used material. 

PHASE III: Establish up-scaled fabrication facility based on key factors identified during Phases I and II. Within a manufacturing environment, demonstrate viability of the process that can be operated in continuous production mode. Since higher strength as-cast Mg components could most certainly find numerous insertion points in automotive and/or aerospace components in both commercial and military vehicles (e.g., engine blocks/housings, transmission housings in helicopters, framing, etc.), identify a tangible and practical application for the demonstration of the new or improved technology. With the commercial partner, develop the required implementation plan to transition the technology and show the benefits of the higher fidelity Mg-based alloy in the specific application selected. 

REFERENCES: 

1: P. Fu, L. Peng, H. Jiang, W. Ding, and C. Zhai, "Tensile properties of high strength cast Mg alloys at room temperature: A review," China Foundry 11 (2014) 277-286.

2:  M. Gupta and W.L.E. Wong, "Magnesium based nanocomposites: Lightweight materials of the future," Materials Characterization 105 (2015) 30-46.

3:  V. Hammond, "Magnesium Nanocomposites: Current status and prospects for Army applications," US Army Laboratory Tech Report, ARL-TR-5728, September 2011.

4:  A. Khandelwal, K. Mani, N. Srivastava, R. Gupta, and G.P. Chaudhari, "Mechanical behavior of AZ31/Al2O3 magnesium alloy nanocomposites prepared using ultrasound assisted stir casting," Composites B123 (2017) 64-73.

5:  A. Luo, "Magnesium casting technology for structural applications," Journal of Magnesium and Alloys 1 (2013) 2-22.

6:  A. Luo, "Recent magnesium alloy development for elevated temperature applications," International Materials Review 49 (2004) 13-30.

7:  J.F. Nie, "Effects of precipitate strength and orientation on dispersion strengthening in magnesium alloys," Scripta Materialia 48 (2003) 1009-1015.

8:  J.F. Nie, "Precipitation and hardening in magnesium alloys," Metallurgical and Materials Transactions A43 (2012) 3891-39

KEYWORDS: Magnesium, Casting, Texture, Microstructure, Mechanical Properties, High Strain Rates 

CONTACT(S): 

Vincent Hammond 

(410) 278-2752 

vincent.h.hammond.civ@mail.mil 

Dr. Laszlo Kecskes 

(410) 306-0811 

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