Additive manufacturing and mechanical properties of titanium lattices for biomedical applications

Abstract

While stochastic porous structures are still largely used in the design of metallic biomaterials, their optimization is limited by the poor control over the morphological characteristics of the pore network. Periodic cellular materials, commonly known as lattices, are able to fulfil their function in a more efficient way: they can provide superior strength for a reduced weight when compared to stochastic architectures. The recent advances in metal additive manufacturing (AM) technologies have enlarged the design possibilities for biomedical applications. As a result, research has focused on novel lattice geometries such as the triply periodic minimal surfaces (TPMS), which are deemed to have superior biological and mechanical performances over conventional strut-based unit-cells.Although promising outcomes have been reported with additively manufactured titanium TPMS, their manufacturing and mechanical properties have not been explored comprehensively. The complex physics behind the SLM process can lead to surface or internal defects as well as morphological discrepancies. It is thus essential to evaluate the processability and mechanical properties of such materials in the context of load-bearing implant applications. In addition to the topological aspects, novel non-toxic alloys should also be explored as alternatives to the industry standard Ti-6Al-4V.In the present thesis, the limitations of stochastic porous titanium are introduced through a preliminary study. Samples with varying levels of porosity were produced using powder sintering with a novel space holder material. The mechanical properties of the materials were investigated thoroughly with both experimental and numerical methods. Although the space holder method is considered as a cost-efficient manufacturing process, the findings notably highlighted the need for a better control over the pore network, as reduced strength and lack of pore connectivity were reported.In order to address this lack of control, the processability and mechanical properties of TPMS are approached in different ways through this work using selective laser melting (SLM). Firstly, Ti-6Al-4V lattices with varying levels of porosity were produced using a Schwartz primitive unit-cell. Their suitability for bone implants applications was assessed through microstructural and macrostructural observations, as well as compression mechanical testing. The elastic mechanical properties of the SLM-produced lattices were found to be suitable for hard tissue engineering and yield strengths were reported higher than stochastic titanium. The discrepancies in surface state and morphological features observed between the CAD models and the reconstructed microcomputed tomography scans have led to further investigations in this regard.The surface irregularities brought about by the SLM process are known to be detrimental to the high cycle fatigue behaviour. Given the importance of fatigue life in the development of biomaterials, tension-tension high cycle fatigue tests were conducted on three TPMS geometries, namely the gyroid, diamond and Schwartz primitive. Based on the first results with the Schwartz primitive lattices, a high level of porosity of 70% was chosen for each of the sample. The fatigue life of the Ti-6Al-4V TPMS were found to be higher than conventional strut-based designs, although lower than bulk SLM-produced titanium. Compared to bulk material, lattices are known to be more sensitive to fatigue crack initiations due to their larger surface area. The fractography analysis confirmed that the fatigue crack initiation occurs at the surface of the struts, where micronotches are present. Based on the results presented in this study and the general consensus from the literature, additional work was carried out towards the surface improvement of SLM-produced TPMS lattices.Although deemed as crucial in the context of biomedical applications, the surface cleaning of TPMS lattices has not been addressed. Using hydrofluoric and nitric acids, an incremental polishing was conducted in this work. The evolution of the topology and of the surface states were reported with the increase in polishing time. The polishing was successful in removing the unmelted powder particles that adhere to the surface of the struts and in conserving the topology. This study also pointed out the importance of conducting topology-specific parametric studies for the polishing of titanium lattices, as the homogeneity of the polishing was found to be dependent to the base unit-cell of the lattices.In addition to the work conducted with the Ti-6Al-4V lattices, an alternative Ti-25Ta was also explored as a non-toxic metallic biomaterial. This work represents the first attempt at selective laser melting lattices with a titanium-tantalum powder blend. Using identical CAD models has allowed for direct comparison between the two alloys. The titanium-tantalum alloy presented higher elastic admissible strains than the industry standard Ti-6Al-4V and an increased ductility brought by the tantalum. Although the quasi-static mechanical properties were found promising for a use in biomedical applications, the unmelted tantalum particles observed with the Ti-Ta matrix have raised concerns regarding their fatigue life and further investigation should be conducted in this direction.