Ultrafine Cellular Eutectic Biodegradable Zn–Ge Alloys Fabricated by Laser Powder Bed Fusion: Process Manipulation and Performance Improvement

Abstract

Zn has been recognized as a promising biodegradable metal for its good biocompatibility and favorable biodegradability. However, it has always struggled with poor mechanical properties, which restrict its application prospects as a bone implant. In the present study, the laser powder bed fusion (LPBF) process was applied to fabricate biodegradable Zn–Ge eutectic alloys for mechanical reinforcement. The effects of volumetric energy density on the relative density and surface roughness of LPBF-fabricated (LPBF-ed) Zn–Ge alloys were investigated by orthogonal tests, which contributed to an optimal relative density >99.5%. Because of the differences in growth rates of constituent phases under nonequilibrium solidification, the LPBF-ed Zn–Ge alloys exhibited a distinctive ultrafine cellular structure (∼1.2 μm), where the α-Zn matrix was surrounded by a Zn–Ge network eutectic via semicoherent interface. Meanwhile, the size of the Zn–Ge eutectic phase was also significantly refined owing to the ultrafast cooling and high temperature gradient of the LPBF process. Moreover, the Ge-doping was found to enable heterogeneous nucleation and significantly refine the grain size within the LPBF-ed Zn–Ge alloys, especially in the Zn-3Ge alloy. Consequently, the LPBF-ed Zn-3Ge alloy presented substantially improved yield strength (156.81 ± 7.47 MPa) and ultimate tensile strength (178.61 ± 6.41 MPa) owing to the numerous semicoherent interfaces induced by refined eutectic structure. Furthermore, the LPBF-ed Zn–Ge alloys also demonstrated good cytocompatibility, as well as appropriate degradation rates (0.265 ± 0.010 mm/y for LPBF-ed Zn-3Ge alloy) owing to the microbattery effect and the barrier effect. Accordingly, these findings indicated the promising application of the LPBF-ed ultrafine cellular eutectic structure in the mechanical reinforcement of biodegradable Zn alloys.