The Effect of Powder Size and Morphology on the Sinterability of Bioresorbable Mg-Sr/Ca Alloys

Possessing outstanding biocompatibility and bioresorbability, magnesium (Mg) alloys with strontium (Sr) and calcium (Ca) additions have shown potential to be used as temporary implants in orthopaedic applications. Having a low elastic modulus (45 GPa) close to the human bone lowers the stress shielding effects. Low temperature Additive Manufacturing (AM) techniques (e.g., Fused Deposition Modelling) have potential to be used for the fabrication of complex Mg components while avoiding safety concerns associated with high temperature AM. However, low sinterability of common Mg-alloys is the main limiting factor. The objective of this work is to investigate the effect of powder particle size / morphology on the sinterability of Mg-Ca/Sr based alloys produced via powder metallurgy. Laser Diffraction and Scanning Electron Microscopy (SEM) were used to characterize particle size and morphology. The study also focused on assessing the role of liquid phase sintering (LPS) mechanism by thermodynamic calculations and microstructural characterisation (SEM). Porosity measurements using density analysis and image processing were employed to determine the effects of powder size and morphology on sinterability of the alloys. It was found that the non-homogeneous particle size distribution with more spherical powder particles, facilitated the compaction and accordingly higher densification was obtained. This was achieved for powders milled at higher speeds (900 rpm), resulting in significantly lower porosity levels (~ 6-8 %) compared to the dry-milled state (~40-60 %).