Design, manufacturing and validation of cellular Zn-1Mg with controlled internal architecture as degradable orthopaedic implant.

Some implants only have a temporary function and are therefore preferably made from a biodegradable material which is removed from the body by dissolution after several months or years. Zn and its alloys are suitable for this application, both from a biological and degradation rate point of view. However, little is known about Zn and Zn alloys in cellular (porous) form. In addition, one of the manufacturing processes which is most promising for the production of (orthopedic) implants with porous volumes, laser powder bed fusion or Direct Metal Printing (DMP), has not been used for Zn and its alloys yet. Therefore, the aim of this thesis was to develop the DMP process for the production of cellular Zn alloys, compare their mechanical properties with those of cellular Zn alloys produced by two spaceholder-based production processes, and study the influence of the meso- and microstructure on the mechanical properties.

NaCl was used as a spaceholder for the production of cellular Zn alloys with Field Assisted Sintering Technology (FAST) and Liquid Metal Infiltration (LMI). This leads to a limited mesostructural control, combined with a sintered microstructure for FAST and a cast microstructure for LMI. The mechanical properties of these materials were tested in compression for a porosity ranging from 75 to 88%. After optimization of the process parameters, the DMP process was used to produce non-cellular and cellular Zn, the latter with a closely controlled mesostructure and DMP microstructure. Non-cellular DMP Zn was studied by X-Ray Diffraction to measure its crystallographic texture and its mechanical properties were measured by tensile testing and impulse excitation. Cellular DMP Zn with five different unit cells – diamond, dodecahedron, octet truss, face centered cubic and Kagome – and a porosity ranging from 49 to 80% was tested in compression.

This research showed that FAST is the least suitable production process for cellular Zn, as the oxide skin, originally at the powder surface, causes a brittle fracture of the sintered struts. Both LMI and DMP resulted in ductile deformation and the influence of the mesostructure on the plateau stress was shown to be more important than the influence of the microstructure. The plateau stress is 6±0.6 MPa for LMI cellular Zn with a porosity of 76% and 8±0.2 MPa for DMP cellular Zn with 74% porosity and a bending-dominated mesostructure. In contrast, the plateau stress for DMP cellular Zn with a more stretching-dominated mesostructure, the Kagome unit cell, is 14±0.4 MPa for 77% porosity. The use of alloying elements can also increase the strength, as the plateau stress for LMI cellular Zn1.5Mg is 10±0.5 MPa for 77% porosity.

The mechanical properties of non-cellular Zn with a density of 98.5% produced by DMP are anisotropic due to the preferential orientation of the crystallographic  direction in the building plane. The yield strength and stiffness in the building direction are 78±0.4 MPa and 110±0.2 GPa, respectively, while in the building plane they are only 55±0.7 MPa and 81±0.4 GPa. Elongation at break is around 10%, independent of the orientation.

Overall, these results show that the plateau stress of cellular Zn and Zn1.5Mg is at the lower end of the strength range for bone (5-200 MPa). The strength of DMP cellular Zn is higher than of LMI cellular Zn and Zn1.5Mg: up to 48±3 MPa for 66% porosity (Kagome unit cell). A further increase in strength for a given density is not trivial for DMP, as manufacturing inaccuracies prevent the experimental strength from reaching its theoretical value. In addition, the DMP process requires a full powder characterization and process parameter optimization in case alloying elements would be added to further increase the strength.