Mechanical and Corrosion Response of Friction Stir Extruded Magnesium Alloys

Abstract

Magnesium (Mg) and its alloys are attractive candidates for lightweight applications owing to their high specific mechanical properties. These properties make magnesium comparable to many polymers in terms of weight while offering a higher strength. Also, Mg is a biocompatible metal that does not trigger toxicological tissue reactions since it exists naturally in the human body. Such capabilities of Mg make it an excellent candidate to be used in biomedical applications. However, Mg has low ductility at ambient temperature as a result of its hexagonal closed packed (HCP) crystal structure, which necessitates heat input to activate additional slip systems to promote stable flow and accommodate high strains. Therefore, Mg requires multi-stage high-temperature processing to achieve the desired form. In this context, this research utilizes Friction Stir Extrusion (FSE), a new solid-state process based on the principles of friction stir welding that uses frictional heat and severe plastic deformation to manufacture the metal into a rod in single-stage processing. WE43, an important Mg alloy, was selected as a model material in this study. WE43 is already being used in the transportation sector, such as in helicopter transmissions and aero-engines. Also, due to its proven biocompatibility, it has the potential to be utilized in biomedical devices. Yet, the precipitates nature in WE43 alloy creates a complex mechanical and corrosion response which makes it crucial to investigate and necessitates engineering its microstructure to achieve the desired behavior. The study aimed to understand the microstructural features that evolve due to the FSE and their corresponding effect on mechanical and corrosion behavior. FSE process produced a composite microstructure with refined grains in the outer region, while coarse grains were observed in the rod center. It is believed that this composite microstructure results due to the strain, strain rate, and temperature variations during the process. The composite microstructure significantly impacted the rod’s mechanical properties where the microhardness was higher towards the edge of the rod, while the core had lower hardness owing to larger grains. The mechanical properties were later studied by nano-, micro, and macro-scale mechanical tests. A relative comparison of the corrosion behavior showed that the corrosion rate of the rod increased as a result of the FSE process due to the variation in the grain size and the distribution of the precipitates in the different regions on the extruded rod. However, the corrosion resistance improved by removing the refined grains region from the rod and keeping only the rod’s core. This research provides a preliminary assessment of using FSE to produce Mg alloy rods for potential applications in biocompatible medical devices such as stents and dental implants.