Elastic and fatigue properties of additively manufactured and milled Ti-6Al-4V removable partial denture clasps

Statement of problem. The use of additive manufacturing Ti-6Al-4V (AM Ti64) in removable partial denture (RPD) frameworks has become increasingly prevalent in clinical practice. However, the elastic and fatigue properties of AM Ti64 clasps are not well understood. Purpose. The purpose of this in vitro study was to investigate the elastic and fatigue properties of AM Ti64 clasps by comparing them with milled Ti64 (Milled TC4) clasps. Material and methods. Dumbbell specimens of AM Ti64 and Milled TC4 were prepared for tensile tests following the International Organization for Standardization (ISO) 22674 standard (n=6). Raw (R) and mechanically polished (P) clasp-shaped specimens were prepared for elastic and fatigue tests and divided into 4 groups: AM Ti64R, AM Ti64P, Milled TC4R, and Milled TC4P. The clasp intaglio surface roughness was measured before testing (n=10). The elastic deformation limit (EDL) of the clasp was determined through incremental displacement loading (n=10). Fatigue tests were conducted on 160 clasps (40 specimens per group) at displacements from 0.30 mm to 1.00 mm for 10(5) cycles (n=5 per displacement). The fracture surfaces of failed clasps were observed using a scanning electron microscope (SEM), and the microstructures were analyzed using optical microscopy. Statistical analyses were conducted using t tests and Kruskal-Wallis tests (alpha=.05). Results. AM Ti64 exhibited higher 0.2% offset yield strength and ultimate tensile strength than Milled TC4 (P<.001), with no significant difference in percentage elongation (P=.298) or elastic modulus (P=.151). No significant differences in intaglio surface roughness were found between AM Ti64 and Milled TC4 clasps in both the raw (P=.306) and polished (P>.999) groups. AM Ti64 clasps demonstrated greater EDL values than Milled TC4 clasps in both the raw and polished groups (P<.001). The fatigue life of AM Ti64 clasps was significantly shorter than that of Milled TC4 clasps (P<.001), with no significant difference between the raw and polished groups in both AM Ti64 (P=.611) and Milled TC4 clasps (P>.999). SEM observation revealed fatigue cracks originating from surface defects in AM Ti64 clasps. Microstructural analysis showed lamellar alpha phase dominance in AM Ti64, while Milled TC4 exhibited primarily equiaxed alpha phase. Conclusions. The AM Ti64 clasp showed better elasticity but inferior fatigue performance compared with Milled TC4 clasps. Mechanical polishing did not significantly affect the elasticity or fatigue properties of the titanium clasp.