Magnesium alloys, particularly Mg-Y-Nd-Zr systems such as WE43, have attracted significant interest for biodegradable orthopedic implants due to their biocompatibility, biodegradability, and mechanical properties closely matching bone. Their ability to promote osteogenic differentiation and bone ingrowth positions them as promising alternatives to auto- and allografts, which face limitations in availability and risks of donor site morbidity or host rejection. The advent of powder bed fusion–laser beam (PBF-LB) additive manufacturing offers the potential to produce complex, patient-specific geometries optimized for bone healing. However, high corrosion rates remain a critical barrier to clinical adoption.
In a recent study, gas-atomized Mg-Y3.9-Nd3.0-Zr0.49 powder was processed via PBF-LB under argon atmosphere using an EOS M290 system, with optimized parameters including 200 W laser power, 1111 mm/s scanning speed, 0.1 mm hatch distance, and 0.03 mm layer thickness. Rectangular specimens were built with a 67° rotation between layers to reduce track overlap. Post-processing by hot isostatic pressing (HIP) at 520°C and 105 MPa for 3 hours aimed to eliminate porosity. An extruded counterpart served as a reference, representing the current commercial implant material.
Porosity measurements showed densities of 99.4% for as-built (AB) and 99.9% for HIP samples, with microscopy confirming pore elimination after HIP. Microstructural analysis revealed melt pool boundaries and laser scanning lines in AB samples, with secondary phases concentrated at track edges. These included Zr-rich particles, Y- and O-rich oxide flakes from powder atomization, and Mg-RE intermetallics such as Mg3Nd. HIP coarsened secondary phases, replacing Mg3Nd with stable Mg41Nd5 and Mg24Y5, and erased melt pool boundaries in SEM images. Extruded material displayed secondary phases mainly along grain boundaries, fewer oxides, and more homogeneous α-Mg grains.
Electron backscatter diffraction (EBSD) showed AB samples with mixed grain morphologies: large grains with strong basal texture aligned to the build direction and smaller equiaxed grains with random orientation. Grain size averaged 19 μm in build direction and 35 μm transverse. HIP increased grain sizes to 33 μm and 41 μm respectively, with stronger texture and loss of equiaxed grains. Extruded material had finer, more uniform grains (~15 μm) and weaker texture.
Potentiodynamic polarization (PDP) tests indicated high corrosion rates for printed samples, with AB showing higher corrosion current densities than HIP, and both exceeding extruded material. The extruded alloy exhibited slight passivation behavior absent in printed counterparts. Short-term PDP results contrasted with long-term immersion data: over 28 days in Dulbecco’s Phosphate Buffered Saline at 37°C, HIP samples had the highest hydrogen evolution and mass loss, followed by AB, with extruded showing minimal degradation. Visual inspection confirmed severe corrosion in HIP samples, moderate in AB, and minimal in extruded, which retained a gold-like surface color.
SEM after 30 minutes immersion revealed localized “rose” corrosion formations around intermetallic particles in all materials, most severe in HIP. The higher intermetallic content in printed samples reduced alloying element dissolution in the Mg matrix, promoting microgalvanic corrosion. Oxide flakes were less active corrosion sites but could deplete Y from the matrix, reducing resistance.
X-ray diffraction after immersion showed Mg3(PO4)2 peaks dominating extruded surfaces, associated with stable, protective layers. AB surfaces had both Mg3(PO4)2 and Mg(OH)2, while HIP surfaces were rich in Mg(OH)2, indicative of less protective, more soluble corrosion products. Energy-dispersive spectroscopy confirmed compositional differences, with extruded samples forming dense, crack-free layers, AB showing cracked double layers, and HIP having thick, uneven hydroxide layers.
The study concluded that HIP did not improve corrosion resistance; coarsening of secondary phases outweighed benefits of densification, increasing microgalvanic activity and hindering protective layer formation. Extruded material’s superior performance stemmed from fewer intermetallics, more dissolved alloying elements, and homogeneous grain structure, enabling stable phosphate-based surface layers. Texture differences between build and transverse directions did not significantly affect long-term corrosion, underscoring the dominant role of secondary phase size and distribution.
Future work should focus on optimizing PBF-LB parameters to control secondary phase formation, grain size distribution, and alloy composition, potentially through inert-atmosphere powder handling to limit oxide formation. Investigating lower HIP temperatures and faster cooling, along with revisiting texture effects once corrosion rates are reduced, could further advance Mg-Y-Nd-Zr alloys toward viable biodegradable implant applications.