Three-roll four-stand hot longitudinal rolling of superelastic Ti-Zr-Nb alloy bars for orthopedic implants: Finite element modeling and experimental study

Finite element modeling and experimental study of the three-roll four-stand hot longitudinal rolling deformation process were conducted for a Ti-Zr-Nb shape memory alloy. The stress, strain, strain rate, temperature, and triaxiality fields were analyzed in detail. During the process, bar stock predominantly experienced compressive stresses of up to 780 MPa, while the tensile stresses remained below 100 MPa. Triangular calibers generated higher compressive stresses than their circular counterparts, and the stress triaxiality remained below a critical threshold for the defect formation, thus ensuring process stability. The equivalent strain and strain rate distributions were strongly non-uniform, exhibiting six surface-localized maxima that led to the related microstructure and hardness gradients. The strain rate exceeded 50 s⁻¹ on the surface and decreased to 15–25 s⁻¹ in the core of the bar stock, while the deformation-induced heating raised the billet temperature from 700 °C to approximately 850 °C. Microstructural observations confirmed the grain refinement and anisotropy reduction after the final deformation stand. The processed alloy demonstrated a favorable combination of the mechanical and functional properties (UTS = 728 MPa, εrsemax = 3.6%, E = 57 GPa), thus proving that the three-roll longitudinal rolling process is an effective method for producing superelastic Ti-alloy bar stock for biomedical applications.