Influence of process parameters of ultrasonic shot peening on surface nanocrystallization and hardness of pure titanium

Ultrasonic shot peening (USP) is a mechanical surface treatment to rapidly realize surface nanocrystallization of metals. In this paper, the influence rules and mechanism of process parameters of USP on surface nanocrystallization and hardness of pure titanium were studied. Significant hardness increases of different degrees were observed, and nanocrystalline (NC) layers of different thicknesses and grain sizes were obtained on the surface of pure titanium after USP treatments with different process parameters, including peening duration, shot diameter, sonotrode amplitude, and peening distance. The influence rules of process parameters on the NC layers and surface hardness of peened pure titanium were analyzed with experimental results measured by Vickers microhardness meter, optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction pattern (XRD), and high-resolution transmission electron microscope (HRTEM). After the influence mechanism of process parameters was discussed, a relationship expression was identified between the surface hardness of pure titanium treated by USP and the process parameters with a correlation coefficient equal to 0.998.