Characterization of biomedical alloy components produced by direct metal laser sintering.

Fiori F., Mengucci P., Gatto A., Bassoli E., Girardin E., Rutkowski B., Czyrska-Filemonowicz A., Barucca G.
  Martedì 12/09   09:00 - 13:00   Aula A107   II - Fisica della materia
Direct metal laser sintering (DMLS), based on a layer-by-layer production process, was used to produce specimens in $Co$ - $Cr$ - $Mo$ - $W$ and $Ti$ - $6Al$ - $4V$ alloys, which are utilized in biomedical applications. The mechanical response and microstructure were investigated in the as-sintered state and after post-production thermal treatments for the $Co$ - $Cr$ - $Mo$ - $W$ samples, and after two post-production treatments for the $Ti$ - $6Al$ - $4V$ ones. Roughness and hardness measurements, as well as tensile and flexural tests, were performed to study the mechanical response, while X-ray diffraction (XRD), electron microscopy (SEM, TEM, STEM) and microanalysis (EDX) were used to investigate the microstructure in different conditions. The anisotropy of the $Ti$ - $6Al$ - $4V$ specimens was also investigated. Results on the $Co$ - $Cr$ - $Mo$ - $W$ samples showed an intricate network of $\varepsilon$ - $Co$ (hcp) lamellae in the $\gamma$ - $Co$ (fcc) matrix, responsible of the high UTS and hardness in the as-sintered state. Thermal treatments increase volume fraction of the $\varepsilon$ - $Co$ (hcp) martensite but slightly modify the average size of the lamellar structure. Nevertheless, thermal treatments are capable of producing a sensible increase in UTS and hardness and a strong reduction in ductility. These latter effects were mainly attributed to the massive precipitation of an hcp $Co3(Mo,W)2Si$ phase and the contemporary formation of $Si$-rich inclusions. $Ti$ - $6Al$ - $4V$ specimens reveal extremely low porosity, high mechanical properties, in particular an elongation higher than the literature data. The results do not evidence any anisotropy between the different orientations. The observed microstructure is very fine. A martensitic $\alpha^{\prime}$ - $Ti$ phase is detected after the first stress relieving treatment, while the firing cycle induces a phase transformation to a stable $\alpha + \beta$-Ti phase with the $\beta$ phase growing at the $\alpha$ grains boundaries. These results suggest possible innovative applications of the DMLS technique to the production of mechanical parts in the medical and dental fields.