Improvement of Fatigue Strength of Ti-6Al-4V Alloy by Plasma Carburizing and FPB Treatment

Abstract

This study was conducted to improve fatigue strength of Ti-6Al-4V alloy by hybrid surface modification which was composed of plasma carburizing and fine-particle bombarding (FPB) treatment. Plasma carburizing at the first stage was conducted for 14.4ks at 1023K to form a hardened layer with no remarkable growth of microstructure in the substrate. Subsequent FPB treatment was carried out to eliminate the brittle compound layer (TiC) formed on the outermost surface by plasma carburizing and to introduce compressive residual stress. The result showed that the plasma carburizing slightly increased the fatigue strength of the titanium alloy. However, this treatment greatly reduced fatigue life because fatigue cracks rapidly propagated along the compound layer. When FPB treatment was further performed for the plasma-carburized material, the compound layer was perfectly eliminated and a relatively high compressive residual stress was introduced. As a result, initiation of fatigue cracks from the surface was strongly suppressed so that the crack initiation site existed under the hardened layer. At the same time, the fatigue life and the fatigue strength were greatly improved and the maximum percentage of the improvement in the fatigue strength reached to 30%.