Selective laser melting of high strength-to-modulus ratio tntz and tntz-o alloys tailored for load-bearing biomedical applications

Weihuan, Kong ORCID: 0000-0002-1723-1732 (2022). Selective laser melting of high strength-to-modulus ratio tntz and tntz-o alloys tailored for load-bearing biomedical applications. University of Birmingham. Ph.D.

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Abstract

Titanium alloys, combined with human-friendly β stabilizers such as niobium, molybdenum, and tantalum have been investigated for low-modulus biomedical applications. This project characterised the relationships between microstructure, mechanical properties, and biocompatibility of selective laser melting (SLM) manufactured β titanium alloys. In the study of TNT and TNTZ alloy design and manufacturing, the authors investigated Ti-Nb-Ta based β alloys with different zirconium additions (0, 5, 9 wt. %) manufactured by SLM. BF-TEM images combining SAD patterns of TNT(Z) alloys show single β phase obtained inside the beta matrix; a low level of as-fabricated defects is obtained and zirconium functions as a neutral element in these high β-stabilized Ti-Nb-Ta based alloys. Corrosion ions of TNT(Z) alloys released from immersion testing at each time intervals show extremely small concentrations (<10 μg/L).

The TNTZ post-processing treatment work shows the existence of single beta grain matrix and alpha precipitates along grain boundary in SLM+HIP manufactured TNT5Zr alloy, and ellipsoidal nano-sized intragranular α'' precipitates were introduced after aging treatment. Including the inferior notch-like surface of the test-pieces, slip-band cracking occurs in this ductile SLM+HIP+aging manufactured TNT5Zr alloy are regarded as the main factors to determinate its fatigue strength (170 MPa). In vitro short-term biocompatibility evaluation reveals that almost no significant difference of biocompatibility results between TNT5Zr alloy and the reference biomaterial (Ti-6Al-4V).

In the study of Ti-34Nb-13Ta-5Zr-0.2O (TNT5Zr-0.2O, wt. %) post-processing treatment, advanced HIP subjected to high and intermediate cooling rate (HCR & ICR) were exploited to close keyholes and tune the microstructure of SLMed TNT5Zr-0.2O alloys. High-angle annular dark-field (HAADF) micrographs show discrete large Ti-rich α grain boundary precipitates in TNT5Zr-0.2O-ICR alloy. Tensile properties show that TNT5Zr-0.2O-AF alloy possessed high UTS of 975 ± 12 MPa, and elongation of 4.9% ± 0.3%; the TNT5Zr-0.2O-ICR alloy obtained slightly higher UTS (1036 ± 26 MPa) and lower elongation (3.0% ± 0.3%). Advanced HIP subjected to intermediate cooling rate functions well to close SLM-processed keyholes but the resistance to fatigue is not markedly enhanced.

In the study of TNT5Zr-0.2O surface treatment, the authors investigated the feasibility of thermal oxidation (TO) for improving the wear and fatigue properties of TNT5Zr-0.2O alloys manufactured by SLM. A mixture of rutile, Nb2O5, Ta2O5, and ZrO phases were formed as an oxide layer after TO. Plain fatigue strength of CE treated alloy (150 MPa) was 1.5 times higher than the value of CE+TO treated alloy (60 MPa), as a result of multiple premature fatigue cracks possibly developing in the compounds region after TO. In vitro biocompatibility results showed no significant differences in metabolic activity of pre-osteoblasts seeded on the treated surfaces. Overall, though the oxide layer is corrosion-resistant in the aggressive environment (3M HCl solution), showing potential application of TO in additively manufactured titanium medical devices.

3D porous structures have been receiving more attention for orthopedic implant development mainly due to their lower elastic moduli to prevent aseptic loosening, however, their low yield strengths may increase failure risk in load-bearing implants. In the study of TNT5Zr-0.2O lattice design and manufacturing, scaffolds infilled with sheet-based triply periodic minimal surface (TPMS) unit cells were manufactured with different design porosity via SLM. Quasi-static compression tests showed that low elastic moduli (10~22 GPa) and high yield strengths (358~1045 MPa) were obtained in the varying TPMS cylindrical specimens. A good balance of high strength and low modulus is obtained in low-porosity TNT5Zr-0.2O TPMS scaffold implants, which potentially work well in human body and provides long service time.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):