Additively manufactured beta–ti alloy for biomedical applications
Metallic biomaterials have an essential portion of uses in biomedical applications. Their properties can be tuned by many factors resulting in their process tuneability. Among metallic biomaterials for biomedical and specifically orthopedic applications, titanium and its alloys exhibit the most suitable characteristics as compared to stainless steels and Co-Cr alloys because of their high biocompatibility, specific strength (strength to density ratio), and corrosion resistance. According to their phase constitution, Ti-alloys are classified into three main groups, namely alpha, beta, and alpha+beta alloys. Depending on the degree of alloying and thermomechanical processing path, it is possible to tune the balance of α and β phases, which permits to tailor properties like strength, toughness, and fatigue resistance. (alpha+beta) Ti alloys, especially Ti-6Al-4V, are widely used alloys in biomedical applications; however, they have some drawbacks like the presence of toxic elements i.e., V and relatively high elastic modulus to that of bones. In view of the lower elastic modulus of body center cubic beta phase (50GPa<100GPa) compared to the alpha+beta, as well as due to their good mechanical properties, excellent corrosion resistance, and biocompatibility, beta-Ti alloys have been recently proposed as a valid alternative to alpha+beta ones.
The growing interest in additive manufacturing (AM) techniques opens new and very interesting perspectives to the production of biomedical prosthetic implants. AM will prospectively allow implant customization to the patient and produce it on demand, with large savings on times and costs. Moreover, AM is gaining increasing interest due to the possibility of producing orthopedic implants with functionally graded open-cell porous metals. The main advantages of porous materials are the reduction of the elastic modulus mismatch between bone and implant alloy reducing the stress shielding effect and improving implant morphology providing biological anchorage for tissue in-growth. In this scenario, the first goal of the present PhD thesis work was to identify a high-performance β-Ti alloy formulation suitable to Laser-Powder Bed Fusion (L-PBF) additive manufacturing. In particular, it explores the potential use of a β-metastable Ti alloy, namely Ti-15Mo-2.7Nb-3Al-0.2Si (Beta Ti21S, 21 wt.% of alloying additions, including Silicon) for biomedical applications.
Through microstructural, mechanical, and cytotoxicity analyses, it could be shown that this alloy grade exhibits i) an unprecedented ultra-low elastic modulus, ii) improved cytocompatibility due to the lack of Vanadium, and iii) no martensitic transformation responsible for hard and brittle solidification structures. A second goal was to assess the manufacturability of metamaterials made of β-Ti21S via L-PBF. For this purpose, cubic cellular lattice structures of different unit cell sizes (and therefore different strut thickness) have been fabricated and characterized through microstructural analysis using different techniques, and computed tomography combined with linear elastic finite element simulations to identify the minimum cell size that can be printed with adequate dimensional and geometrical accuracy.
Samples of the selected unit cell size were also tested to determine their static and fatigue properties. The main results show that i) the suitable manufacturing quality is obtained for strut thickness above 0.5 mm, ii) the mechanical tests place the present cellular structures among the best stretching dominated cellular lattice materials investigated to date in the literature, and iii) the fatigue tests showed a normalized fatigue strength at 107 cycles of close to 0.8, similar to cubic lattices made of Ti-6Al-4V, and higher than most cellular structures in the literature. In the last part of the thesis, a more complex octet truss structure was fabricated in the manufacturable cell size, and its mechanical properties were investigated. The octet truss topology can be beneficial both in terms of mechanical properties and biocompatibility by providing the different types of porosity suitable for bone in-growth.