Understanding the dwell-fatigue-damage mechanism of powder metallurgy Ti-6Al-4V alloys fabricated by hot isostatic pressing

Abstract

Dwell fatigue of titanium alloys used in aeroengine industry has threatened the reliability of aircraft for decades. Powder metallurgy (PM) through hot isostatic pressing (HIP) can fabricate titanium-alloy components with complex structures, and the mechanical properties are close to wrought alloys. Unfortunately, the dwell-fatigue behavior of the as-HIPed titanium-alloy powder components has not been reported in previous studies. The present work firstly investigated the effects of the peak stress (σp) and stress ratio (R) on the dwell-fatigue behavior and damage mechanism of the as-HIPed Ti-6Al-4V powder compact. With increasing σp, the dwell fatigue lives (NLCDF) decrease, but the dwell debit increases. The lowest NLCDF has been obtained at R = 0.1. With increasing R, the fatigue behavior of dwell fatigue is approaching to creep fatigue. When R is negative, the introduction of a lower reverse stress reduces the cumulated max plastic strain, resulting in the increment of NLCDF. Moreover, a crack-initiation and propagation model based on soft-hard grain pairs has been proposed to describe the dwell-fatigue mechanism. In addition, the dwell debit of PM Ti-6Al-4V alloys is similar to or even lower than that of wrought alloys. Improving the microstructure homogeneity of the material may be the key to further reducing the dwell effect.