Effects of thermal processing variations on microstructure and high cycle fatigue of beta-STOA Ti-6Al-4V

Abstract

Titanium alloys are often used in fatigue-limited structural applications within the aerospace industry. Because of the primary processing control and reactivity of titanium, the fatigue life of the material is predominantly dependent upon the α-phase microstructure, rather than internal defects such as voids or inclusions. For titanium alloy Ti – 6 wt % aluminum - 4 wt % vanadium (Ti-6Al-4V), the influence of cooling rate from above the ß-transus is known to affect the resulting microstructure and influence high cycle fatigue life. The transfer time from the furnace to the water-quenching bath significantly influences the cooling rate, and thus controls the microstructural development and fatigue properties. A hydraulic actuator produced from a Ti-6Al-4V forging failed prematurely during fatigue testing, and provided the industrial motivation for this work. A quench dilatometer was used, along with subsequent scanning electron microscopy, to explore the microstructural variations produced as a function of thermal history. Rotating bending fatigue testing in the high cycle fatigue regime highlighted a two orders of magnitude reduction in fatigue life due to an increased quenching transfer time. The increased quench transfer time was shown to create packets of co-oriented α laths that facilitated crack initiation. Fatigue crack growth rate measurements were also used to quantify post-initiation crack growth rates in microstructures produced by different quench delay times. Long crack growth rates were found to be similar for short and long quench delay times. Post-mortem fractographic analysis and electron back scattered diffraction aided in determining the microstructural influence on fatigue crack initiation and propagation, indicating that long, planar basal slip lengths contribute to crack nucleation. Based upon these findings, it is found that even minor variations in quench transfer time can significantly influence the high cycle fatigue life of titanium alloys.