Investigating natural ageing and static recrystallisation behaviour of additively manufactured pure Zn and Zn1Mg alloy for biodegradable applications

Sammanfattning

Biodegradable implants provide temporary support and promote healing in damaged body tissues, disintegrating after fulfilling their function and thereby decreasing the risks associated with implant removal. Additive manufacturing (AM), especially Powder Bed Fusion – Laser Beam (PBF-LB), offers advantages for producing these implants, including design flexibility, lower cost and reduced lead time. Zinc (Zn) is a biodegradable metal with good biocompatibility and a favourable corrosion rate, but must be alloyed with elements such as magnesium (Mg) to achieve suitable mechanical properties for implant applications. Additionally, due to its low melting point, Zn alloys can undergo natural ageing (NA) and static recrystallisation (SRX) even at low temperatures, raising concerns regarding their microstructural stability at body temperature during service life. This study investigates the potential effects of NA and SRX at 37°C and room temperature (RT) in pure Zn and Zn-1Mg (wt.%) powder-mixed alloy samples produced by PBF-LB over 30, 60 and 90 days. Characterisation was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness, and compression testing. Results revealed the unexpected presence of the hard and brittle MgZn2 intermetallic phase, likely related to a non-uniform Mg distribution associated with the powder mixing technique. XRD analysis showed a decrease in the MgZn2 phase over time, consistent with a decrease in microhardness in the same period, possibly indicating NA. SEM images suggested a reduction in dendrite structures after 90 days at 37°C, potentially reflecting SRX. These findings provide groundwork for future studies to further explore these phenomena over longer periods and with additional characterisation techniques. Furthermore, using pre-alloyed powders instead of powder mixtures may help eliminate the formation of MgZn2 phase and its associated effects.