Additive manufacturing of biodegradable Zn-xMg alloys: Effect of Mg content on manufacturability, microstructure and mechanical properties

Additive Manufacturing (AM) exhibits tremendous advantages in producing customized medical implants and has achieved successful clinical applications for non-degradable metals such as titanium, stainless steel and cobalt alloys, particularly in the form of porous scaffolds for bone implants. Biodegradable materials allow the metal to degrade while natural tissue grows thereby reducing implant lifetime within the body to a minimum. The design freedom of AM and especially Laser Powder Bed Fusion (LPBF) overcomes technical difficulties of conventional technologies to manufacture complex structures. Within this work test geometries and porous scaffolds were manufactured via LPBF with pre-alloyed Zinc-Magnesium (Zn-xMg) powder, where x = 0, 1, 2 and 5 wt% to investigate the effect of Mg on manufacturability, microstructure and mechanical properties. Relative material densities of cubic specimens above 99.5% were achieved with all pre-alloyed powders. The addition of Mg reduces the processing window compared to pure Zn. A decreased grain size and an increase in intermetallic precipitates is observed with the addition of Mg. Zn-1Mg samples exhibited 3.2x the maximum ultimate tensile strength (UTS) of pure Zn (i.e., 119 MPa). However, the tensile elongation is only a fifth compared to pure Zn. With increasing Mg content, the amount of Zn+Mg2Zn11 rises and MgZn2 forms. These brittle phases characteristically increase hardness but reduce ductility. Under the chosen processing conditions, all Zn-xMg scaffolds demonstrated good processability. The geometric porosity was designed to be 80.5% with the achieved value of 70.8 % for compression testing. Zn-1Mg scaffold showed the highest compressive strength and Young's modulus (19.1 MPa and 0.65 GPa respectively). This paper summarizes the technical challenges of manufacturing Zn-xMg alloys via LPBF for tissue engineering applications.