Influence of Plasma Immersion Ion Implantation on Ti-6Al-4V Alloy Tensile–Tensile Fatigue Behavior: The Introduction of Shot Peening as a Pretreatment

The purpose of this study was to investigate the influence of combined plasma immersion ion implantation (PIII) and shot peening (SP) treatments on the axial fatigue strength of the Ti-6Al-4V alloy. The specimens were superficially treated with SP and PIII separately and in a combined way. The tensile–tensile fatigue behavior was investigated based on microstructural analysis, residual stress measurements, profilometry, Vickers microhardness, scanning electron microscopy, and transmission electron microscopy. Axial fatigue tests at room temperature were performed for the Ti-6Al-4V base material, PIII at 500 and 800 °C, SP process and the combination of both surface treatments. The residual stress measurements indicated that PIII treatment at 500 °C induced tensile residual stresses of 189 MPa on the material surface. The PIII treatment at 500 °C maintained the fatigue resistance of the Ti-6Al-4V alloy in the low-cycle fatigue region, while the PIII treatment at 800 °C reduced the fatigue life at all stress levels, from 32 to 96% reduction in lifetime. The combination of SP pretreatment and PIII treatment at 500 °C decreased the tensile residual stress value at the surface from 189 to 23 MPa, resulting in an improvement of the fatigue strength compared to only PIII treatment, for N = 106, the fatigue strength doubled in value. The analysis and tests presented in this work showed the potential of combining PIII and SP treatments to promote the surface protection without the substantial fatigue strength loss frequently associated to surface treatments.