A study of fatigue notch sensibility on titanium alloy TiAl6V4 parts manufactured by selective laser melting

Abstract

Titanium Ti6Al4V alloy is a light alloy characterized by having excellent mechanical properties and corrosion resistance combined with low specific weight, commonly used in biomedical applications, automotive and aerospace components. Current work analyses the fatigue behavior of titanium alloy TiAl6V4 parts, manufactured by selective laser melting (SLM), intending to characterize fatigue strength from low to high life range, under constant amplitude strain control. Fatigue tests were carried out at room temperature, using round dog bone specimens where laser powder deposition occurred in layers perpendicular to the sample axle. All specimens were subjected to stress release treatment. A second batch of specimens was tested in order to investigate the notch sensibility of the material. All tests were performed under displacement control. The material was characterized in terms of the tensile mechanical properties, cycle curve, Basquin and Coffin equations. The analysis of the results showed a strain-softening behavior that increased with applied strain, and non-linear response in and plastic regime. In addition, this alloy exhibited a low transition life, about 250 reversals, which can be attributed to the combination of high strength and relatively low ductility. The material revealed a notch sensibility factor, that was quantified for the round notch with a stress concentration factor K t =1.7 (with respect to the effective cross section), increasing with fatigue life, from one for low cycle fatigue tending to 1.42 for high cycle fatigue (N f of about one million cycles). SEM analysis showed that fatigue crack initiated from the surface and propagated through the cross section, occurring in many cases multi-nucleation.