Improvement of Fatigue Strength of Ti–6Al–4V Alloy by Hybrid Surface Treatment Composed of Plasma Nitriding and Fine-Particle Bombarding

Abstract

This study was conducted to improve the fatigue strength of Ti­6Al­4V alloy by hybrid surface treatment composed of plasma nitriding and fine-particle bombarding (FPB). Plasma nitriding to form a hardened layer was performed at relatively low temperature to suppress the growth of the microstructure of the substrate as far as possible. Subsequent FPB eliminated the outermost brittle compound layer leading to a reduction in fatigue strength, and also introduced compressive residual stress at the same time. Hybrid surface treatment formed a hardened layer that can improve wear resistance without marked deterioration of the mechanical properties. Since the formed layer possessed high compressive residual stress and relatively high hardness, initiation of fatigue cracks from the surface was strongly suppressed and the crack initiation site was located in the substrate under the formed layer. As a result, the fatigue strength improved by 47% and the fatigue ratio (fatigue strength/tensile strength) reached a very high level of 91%. [doi:10.2320/matertrans.M2013228]