Effect of multi-pass friction stir processing on mechanical, microstructural, bioactivity, and corrosion properties of novel magnesium-hopeite composite for biomedical applications

Abstract

The mechanical properties of magnesium and its alloys are remarkably close to those of natural bone, and they are biodegradable, thus they have been studied for use in consumable bio-implant applications. However, their applicability is constrained by the unpredictable corrosion rate and the moderate bioactivity of Mg-based materials in a biological environment. Considering this, a Hopeite-reinforced magnesium composite was fabricated using friction stir processing (FSP). Along with the many other advantages of FSP, it significantly reduces grain size. The impact of reduced grain size and the presence of Hopeite particles on the control of magnesium degradation was studied. After FSP, there was a significant reduction in grain size owing to severe plastic deformation and dynamic recyclization. The effect of the number of passes was studied using samples with one pass, two passes, and three passes of Mg-Hopeite composite. There was a correlation between increasing the number of FSP passes and improvements in the material's mechanical characteristics, corrosion resistance, and bioactivity behavior. The samples were immersed in simulated body fluid for in vitro bioactivity studies, where it was determined that the 3 passes FSP Mg-Hopeite composite had the greatest Ca/P ratio and hence the best bioactivity, outperforming the 2 passes composite, the 1 pass composite, and pure Mg. Apatite layer formation in FSP Mg-Hopeite composite samples has also conformed to their bioactive behavior. Electrochemical and immersion studies designed to evaluate the corrosion behavior of the material in simulated bodily fluid revealed that grain refinement and an increase in biomineralization led to an improvement in the material's corrosion resistance. Considering the findings, which indicate that 3 passes of FSP Mg-Hopeite composite possess superior corrosion resistance, and bioactive and mechanical properties in comparison to either 1 pass and 2 passes of FSP Mg-Hopeite composite or pure Mg, this material is ideally suited for use as temporary implants for biomedical applications.