The Effect of Annealing, Overheating, and a Sintered Porous Coating Thermal Treatments on the Fatigue Properties of Ti-6.0wt.%Al-4.0wt.%V ELI Alloy Plate Specimens

Abstract

The primary objective of this investigation was to determine the effect on the mechanical and fatigue properties induced by sintering powder on Ti-6wt.%Al-4wt.%V ELI (Extra Low Interstitial) alloy substrates as compared to annealing, and overheating thermal treatments. Fatigue tests were conducted using a variable speed plate fatigue machine on various specimens in the different thermally-treated conditions so that a comparison study of the fatigue characteristics could be performed. The heat treatments, which included as-annealed, as-overheated, and sintered porous-coated, were performed on the plate fatigue specimens prior to testing in the variable speed fatigue plate machine. A comparison study of the fatigue results/properties of the samples subjected to the three thermal treatments revealed that the annealed specimens exhibited superior fatigue resistance as compared to the as-overheated and porous-coated samples. However, a substantial decrease in the fatigue resistance of the porous coated sintered to the as-overheated thermal treatment was not evidenced. It was determined that the decrease in the fatigue resistance of the as-overheated, as compared to the annealed heat treatment, was a result of an increase in the macroscopic grain size. Furthermore, for the porous-coated specimens, it was determined that the presence of stress concentrations at the surface irregularities of the sintered bond sites resulted in the decrease fatigue resistance. It was found, from optical metallography on the sintered porous-coated samples, that microscopic fatigue cracks originated at the necked regions of both the particle-substrate interface and the particle-particle interface. From these findings it was determined that the fatigue cracks at the particle-substrate interface contributed to the ultimate fatigue failure of the sintered porous-coated plate fatigue specimens.