RESUMO
Lattice structures produced by additive manufacturing have been increasingly studied in recent years due to their potential to tailor prescribed mechanical properties. Their mechanical performances are influenced by several factors such as unit cell topology, parent material and relative density. In this study, static and dynamic behaviors of Ti6Al4V lattice structures were analyzed focusing on the criteria used to define the failure of lattices. A modified face-centered cubic (FCCm) lattice structure was designed to avoid the manufacturing problems that arise in the production of horizontal struts by laser powder bed fusion. The Gibson-Ashby curves of the FCCm lattice were obtained and it was found that relative density not only affects stiffness and strength of the structures, but also has important implications on the assumption of macroscopic yield criterion. Regarding fatigue properties, a stiffness based criterion was analyzed to improve the assessment of lattice structure failure in load bearing applications, and the influence of relative density on the stiffness evolution was studied. Apart from common normalization of S-N curves, a more accurate fatigue failure surface was developed, which is also compatible with stiffness based failure criteria. Finally, the effect of hot isostatic pressing in FCCm structures was also studied.
RESUMO
Laser powder bed fusion (L-PBF) techniques have been increasingly adopted for the production of highly personalized and customized lightweight structures and bio-medical implants. L-PBF can be used with a multiplicity of materials including several grades of titanium. Due to its biocompatibility, corrosion resistance and low density-to-strength ratio, Ti-6Al-4V is one of the most widely used titanium alloys to be processed via L-PBF for the production of orthopedic implants and lightweight structures. Mechanical properties of L-PBF Ti-6Al-4V lattice structures have mostly been studied in uniaxial compression and lately, also in tension. However, in real-life applications, orthopedic implants or lightweight structures in general are subjected to more complex stress conditions and the load directions can be different from the principal axes of the unit cell. In this research, the mechanical behavior of Ti-6Al-4V diamond based lattice structures produced by L-PBF is investigated exploring the energy absorption and failure modes of these metamaterials when the loading directions are different from the principal axis of the unit cell. Moreover, the impact of a heat treatment (i.e. hot isostatic pressing) on the mechanical properties of the aforementioned lattice structures has been evaluated. Results indicate that the mechanical response of the lattice structures is significantly influenced by the direction of the applied load with respect to the unit cell reference system revealing the anisotropic behavior of the diamond unit cell.
Assuntos
Diamante , Titânio , Corrosão , Lasers , Teste de Materiais , PósRESUMO
Mechanical performance of additively manufactured (AM) Ti6Al4V scaffolds has mostly been studied in uniaxial compression. However, in real-life applications, more complex load conditions occur. To address this, a novel sample geometry was designed, tested and analyzed in this work. The new scaffold geometry, with porosity gradient between the solid ends and scaffold middle, was successfully used for quasi-static tension, tension-tension (R = 0.1), tension-compression (R = -1) and compression-compression (R = 10) fatigue tests. Results show that global loading in tension-tension leads to a decreased fatigue performance compared to global loading in compression-compression. This difference in fatigue life can be understood fairly well by approximating the local tensile stress amplitudes in the struts near the nodes. Local stress based Haigh diagrams were constructed to provide more insight in the fatigue behavior. When fatigue life is interpreted in terms of local stresses, the behavior of single struts is shown to be qualitatively the same as bulk Ti6Al4V. Compression-compression and tension-tension fatigue regimes lead to a shorter fatigue life than fully reversed loading due to the presence of a mean local tensile stress. Fractographic analysis showed that most fracture sites were located close to the nodes, where the highest tensile stresses are located.
