Finite Element Analysis of the Stress–Strain State of the Deformation Zone of a Workpiece from UFG Grade 4 Ti Subjected to Abrasive-Free Ultrasonic Finishing

Abstract

An effective approach to increasing the fatigue resistance of metal products is to create residual compressive stresses on the surface of the product using surface plastic deformation (SPD) processing SPD. In the present study, with the help of the finite element analysis, one of the effective SPD methods, the process of abrasive-free ultrasonic finishing (AFUF), is studied. Another well-known approach to improving mechanical characteristics, including the fatigue resistance, is the creation of an ultrafine-grained (UFG) structural state in the product. This study is devoted to investigation of the stress–strain state of a UFG workpiece subjected to SPD by the AFUF method using the finite element analysis. Commercially pure Grade 4 titanium in the UFG state obtained by the equal channel angular pressing “conform” method (ECAP-C) is chosen as the workpiece material. In the course of the study, the stress–strain state of the deformation zone after a single impact of an indenter with subsequent unloading is analyzed in the elastoplastic formulation of the problem. The effect of the oscillation amplitude and geometrical characteristics of the indenter on residual radial stresses, including their depth of occurrence, average normal stress, and the accumulated effective strain, has been analyzed. It has been established that, with an increase in the indenter radius, the value of the accumulated effective strain (e) decreases. The behavior of distribution of the e parameter shows a gradient character with its values decreasing from the surface to the center of the workpiece. An analysis of the simulation results shows that the residual radial stresses in the region of the deformation zone are predominantly compressive stresses and, accordingly, allow increasing the fatigue resistance of the final product. It has been established that, with an increase in the indenter oscillation amplitude, the values of residual radial stresses also rise, with their maximum achieving 540 MPa at the amplitude of 75 µm and the depth of occurrence of these stresses reaching 0.3 mm. Increasing the indenter radius, or, in other words, in fact, the contact surface area, leads to an increase in the residual radial compressive stresses, which turns out to be an almost linear increase.