Surface Integrity in Dry and Cryogenic Machining of AZ31B Mg Alloy with Varying Cutting Edge Radius Tools

How surface-to-volume ratio affects degradation of magnesium: in vitro and in vivo studies

Despite the many studies carried out over the past decade to determine the biodegradation performance of magnesium and its alloys, few studies focused on the effect of altered surface area to volume ratio on in vitro and in vivo degradation rate and osteogenesis. Here, high purity magnesium cylindrical rods with gradient of surface area to volume ratio were processed by excavating different numbers of grooves on the side surface. The immersion test in SBF solution and the rat femoral condylar bone defect model were used to evaluate the degradation of magnesium rods in vitro and in vivo, respectively. We demonstrated that, the increased number of grooves on the HP magnesium surface represented a decrease in the percentage of residual volume over time, not necessarily an increase in absolute degradation volume or a regular change in corrosion rate. Furthermore, there were strong linear correlations between the relative degradation volume and the initial surface-to-volume ratio of HP magnesium rods both in vitro and in vivo. The difference in the slope of this relationship in vitro and in vivo might help to determine the possible range of in vivo degradation rates via in vitro data. In addition, the corrosion rate is more suitable for evaluating bone formation surrounding the different HP magnesium rods. Our findings in this work may facilitate adjusting the in vivo degradation and osteogenesis of different kinds of orthopedic implants made of the same magnesium-based material, and thus, accelerate the clinical popularization and application.

Enhancement of stress corrosion cracking of AZ31 magnesium alloy in simulated body fluid thanks to cryogenic machining

Magnesium and its alloys have recently attracted great attention as potential materials for the manufacture of biodegradable implants. Unfortunately, their inadequate resistance to the simultaneous action of corrosion and mechanical stresses in the human body have hampered their use as implant materials. This work aims at evaluating the Stress Corrosion Cracking (SCC) susceptibility of the AZ31 Mg alloy after being machined under cryogenic cooling. The SCC behaviour was evaluated by means of Slow Strain Rate Tests (SSRTs) in Simulated Body Fluid (SBF) at 37 °C. Prior to testing, a full characterization of the machined surface integrity, including microstructural observations, residual stress, nano-hardness measurements and surface texture analysis was carried out together with the assessment of the corrosion properties through potentiodynamic polarization curves. In addition, the morphology of the fracture surfaces after SSRTs was analysed by means of 3D optical profiler and Scanning Electron Microscopy (SEM). The improved corrosion resistance due to the increased extension of the nano-surface layer and to the compressive residual stresses represents the reason of the reduced SCC susceptibility of cryogenically machined AZ31 samples as compared to dry machined ones.

Biodegradable magnesium alloys as temporary orthopaedic implants: a review

The study of innovative biodegradable implant materials is one of the most interesting research topics at the forefront in the area of biomaterials. Biodegradable implant materials in the human body can be gradually dissolved, absorbed, consumed or excreted, so there is no need for the secondary surgery to remove implants after the surgery regions have healed. However, most of the biodegradable materials, usually polymers, do not have good mechanical properties to be reliable for bearing the load of the body. Magnesium and its alloys due to the excellent biodegradability and biocompatibility as well as the suitable mechanical compatibility with human bone are very promising candidates for the development of temporary, degradable implants in load-bearing applications. However, Mg alloys are corrosion susceptible in a biological environment. Besides, the high corrosion rate and the low bioactivity of magnesium implants are the challenging problems, which need to be resolved before employing them in clinical applications. This paper provides a review of state-of-the-art of magnesium alloy implants for orthopedic and tissue engineering applications and describes recent progress in the design of novel structure design Mg alloys and potential approaches to improve their biodegradation performance.