Improved fretting fatigue mechanism of surface-strengthened Ti-6Al-4V alloy induced by ultrasonic surface rolling process

Surface plastic deformation methods are prevalently applied to improve the fretting fatigue (FF) property of titanium alloys. Understanding the intrinsic relation between surface plastic deformation severity and the FF property is of great significance to the application of these methods. Here, the influence of surface plastic deformation severity on the FF behavior of Ti-6Al-4V alloy is investigated. Two different levels of surface plastic deformation severity are prepared at the surface of Ti-6Al-4V alloy by employing ultrasonic surface rolling process. Uniaxial FF tests illustrate that both of the deformed samples have improved FF properties and although their fatigue life is comparable, the influences of their refined microstructures and compressive residual stresses are different. Pre- and post-mortem microstructural analyses reveals that the grains are coarsened or crystallized from amorphous material at the topmost surface under the multiaxial stresses to accommodate the plastic strain for the high surface deformation severity, while the grains are refined at the subsurface for the low surface deformation during FF loading.