Z. Pu

Microstructural Changes of AZ31 Magnesium Alloys Induced by Cryogenic Machining and Its Influence on Corrosion Resistance in Simulated Body Fluid for Biomedical Applications

Poor corrosion resistance is one of the major disadvantages of magnesium alloys that inhibits their wide application. It was reported frequently that the alloys’ microstructure has a significant influence on their corrosion resistance. In this study, cryogenic machining is used as a severe plastic deformation tool to modify the surface and subsurface microstructures of an AZ31 Mg alloy. Liquid nitrogen is applied to suppress grain growth caused by large heat generation during machining. “White layers”, where grain boundaries were invisible, were shown to form on the surface and subsurface after machining. The hardness of this layer was about 60% larger than the bulk material. The tool edge radius and the cutting speed have profound influence on the microstructures. Preliminary results from immersion tests in simulated body fluid showed that the corrosion resistance of the AZ31 Mg alloy was enhanced due to the formation of white layer.

Cryogenic Processing of Biomaterials for Improved Surface Integrity and Product Sustainability

Improved functional performance and longer service life of biomedical products offer great sustainability benefits. Surface integrity, which can be modified by severe plastic deformation (SPD) processes, affects the functional performance of materials. Two SPD processes – burnishing and machining – were studied under cryogenic conditions. Cryogenic burnishing of a Co-Cr-Mo biomedical alloy using a novel burnishing tool led to significant grain refinement and 80% greater surface microhardness relative to the bulk. Cryogenic burnishing of AZ31 Mg alloy led to a more than 2 mm thick SPD surface layer with remarkably refined microstructure (grains <300 nm), where hardness was increased 95%. The SPD layer formed on AZ31 Mg alloy after cryogenic machining was about 60% harder than the bulk material. Furthermore, this layer enhanced corrosion resistance during incubation in simulated body fluid. The present results demonstrate significant surface and subsurface property improvement due to cryogenic processes of both alloys, thus providing improved sustainability.

Cryogenic Burnishing of AZ31B Mg Alloy for Enhanced Corrosion Resistance

Poor corrosion resistance is limiting applications of Mg alloys. However, the corrosion performance of an Mg alloy can be enhanced through modification of its microstructure. It has been reported in the literature that the microstructure, especially grain size of AZ31 Mg alloy, has a significant influence on its corrosion resistance. In this study, AZ31B discs were subjected to a novel mechanical processing method-cryogenic burnishing; the surface of AZ31B work piece was burnished with a custom tool under a liquid nitrogen spraying condition. The processing led to a more than 3 mm thick surface layer with remarkably changed microstructures formed on the disc surface. Significant grain refinement occurred within this surface layer due to dynamic recrystallization induced by severe plastic deformation and effective cooling by liquid nitrogen. Both electrochemical method and hydrogen evolution method indicate that the corrosion resistance of the burnished surface was significantly improved.