The effect of the stress-strain state during rotary forging on the microstructure and properties of the Ti–39Nb–7Zr titanium alloy

The paper studies the effect of the stress-strain state of a hot-rolled Ti–39Nb–7Zr titanium alloy bar on its microstructure and properties during rotary forging. Rotary forging is considered as a promising method for severe plastic deformation, which provides the formation of an ultrafine-grained structure, uniform distribution of plastic strain, and improvement of the alloy properties. To determine the stress-strain state, a finite element model is developed; namely, the workpiece geometry is completely reconstructed, the materials and their properties are determined, a finite element grid is generated, the model solver is tuned, the boundary conditions and loads are assigned. The simulation is carried out by the finite element method enabling us to take into account complex three-dimensional tool trajectories and strain distribution during rotary forging. The mechanical properties of the material are determined experimentally and used to construct a model of hardening. The simulation takes into account the material behavior under pre-deformation heating to 450 °C. The simulation results show that the maximum strains in the rotary-forged bar reach 955 MPa in the tool–bar contact zone. The analysis of the specimen cross-section reveals concentric zones with a uniform stress distribution and residual longitudinal compressive stresses 0yy = 200 MPa. The longitudinal stress distribution demonstrates high stresses in the tool–bar contact zone and a stress gradient from the contact zone to the specimen periphery. The study of the alloy microstructure after rotary forging discovers the presence of significant plastic strains and a high dislocation density in the surface zone. The material microhardness increases to 350 HV in the surface zone, as compared to 250 HV in the central part of the specimen. Rotary forging forms a texture and the anisotropy of the mechanical properties, this being supported by the measurements of the elastic modulus varying from 70 to 90 GPa through the bar cross-section. The study aims at developing a multicomponent dynamic 3D model designed to simulate rotary forging of a Ti–39Nb–7Zr titanium alloy bar using the Ansys Mechanical software. A bar made from the Ti–39Nb–7Zr biocompatible alloy by hot rolling in the β region at the VSMPO-AVISMA Corporation is used as the test material.