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Ultrasonic shot peening (USP) is a surface treatment technology used in the mechanical properties strengthening of the engineering material and components during manufacturing. TC4 titanium alloy is a commonly used engineering material in the aerospace industry. In this study, a gradient nanostructured surface layer was successfully fabricated on the TC4 titanium alloy via USP technology at room temperature. The microstructure evolution of TC4 titanium alloy during USP was investigated. The surface microhardness was elevated from 330 HV to 438 HV with a penetrating depth of around 900 μm after USP with the duration of 8 min. EBSD characterization results confirmed the presence of high-density grain boundaries within the gradient structure in the region of 0–200 μm, accompanied by proliferation of dislocation density. TEM characterization indicated a substantial amount of nanograin with an average size of 74.58 nm. Furthermore, the USP process was also investigated by the finite element method to evaluate the surface-strengthening effect. The calculated maximum residual stress reached 973 MPa after multi-ball impact. The impact behavior of the shots during the USP process was studied. The effect of the parameters on the USP strengthening intensity was explored based on the validated model. This work provided a clearer understanding of the USP strengthening process of TC4 titanium alloy through an effective method of evaluating the process parameters.
Yi et al. (Tue,) studied this question.