Influences of particle collision treatments on surface characteristics and fatigue strength of Ti-6Al-4V alloy

In this study, the particle collision treatments (fine particle bombarding (FPB) and conventional shot peening (SP)) were performed for Ti–6Al–4V alloy in order to comprehensively investigate the influences of their treatment conditions on the surface conditions, microstructures, hardness distributions and residual stress. Moreover, we examined the relationship between the surface characteristics of the treated materials and their mechanical properties including fatigue strength in detail. According to the obtained experimental results, the values of the maximum height waviness increased with increasing in the size of collision particles and injection pressure. Among all the treated materials, the differences in hardness distribution were relatively small because the work hardening exponent of Ti–6Al–4V alloy is low and the degree of work hardening is insensitive to the amount of plastic deformation. The absolute values of compressive residual stress generated at the surfaces became higher when finer particles were used as collision particles. The increase in injection pressure increased the surface temperature and released the introduced residual stress. As the particle collision treatments affected only the surface regions, the mechanical properties of the treated materials were almost the same as those of the annealed one. In the range of the conditions examined in this study, the most effective treatment to improve the fatigue strength of Ti–6Al–4V alloy was FPB under a relatively low injection pressure. This was because the value of the maximum height waviness was the smallest and the highest compressive residual stress was introduced at the same time.