Giulio Cavaliere: Sustainability in additive manufacturing of biodegradable magnesium alloys

Abstract

Powder bed fusion with laser beam (PBF-LB) of magnesium (Mg) alloys shows potential for biodegradable, patient-specific implants. However, challenges attributed to material performance continue to constrain clinical translation, while sustainability and powder safety represent key areas for improvement. This thesis addresses these limitations through two aims.

The first aim was to improve material sustainability and operator safety in PBF-LB processing of WE43 (Mg–4Y–3RE–Zr) alloy. Powder reuse was investigated using a closed-loop cycle, evaluating changes in size distribution, sphericity, chemical composition, and rheological behavior. The impact on printed components was assessed through density measurements, microstructural analysis, and hardness testing. While powder reuse led to the consumption of both fine (<20 µm) and coarse (>75 µm) particles and reduced sphericity, the impact on bulk sample density and hardness was minimal. However, elevated oxide content in printed components may affect corrosion resistance and warrants further investigation. To evaluate the effect of powder handling risks, cytotoxicity of virgin and processed powders was assessed in fibroblasts (L929), macrophages (RAW 264.7), and keratinocytes (KERTr). A clear dose-dependent cytotoxic response was observed, with keratinocytes being the most sensitive cell type. Laser-altered particles exhibited increased cytotoxicity, attributed to morphological and surface compositional changes, indicating the need for appropriate safety measures during handling.

The second aim was to develop a rare-earth (RE)-free, amorphous Mg alloy with improved corrosion resistance, leveraging the high cooling rates in PBF-LB. This was driven mainly by performance limitations of existing alloys, but also sustainability concerns related to RE extraction and supply as well as toxicity concerns.  A Mg–Zn–Ca glass-forming composition was selected to exploit the favorable properties of bulk metallic glasses (BMGs). Printability was demonstrated, revealing a trade-off between density and crystallinity, along with promising corrosion rate (0.36 mm·y-1) and good biocompatibility. However, cracking limited processability, particularly in larger components. Scanning strategy optimization and substrate preheating were explored as mitigations strategies. Shorter scan vectors and laser rescanning reduced cracking, while substrate preheating had the most significant effect, improving printability without substantially altering the microstructure.

Overall, this work advances both process sustainability and material design for PBF-LB of Mg-based implants.