Tensile deformation behavior of high-strength TA18 titanium alloy tube under warm forming conditions and constitutive model based on dislocation density

Understanding the deformation behavior of high-strength TA18 titanium alloy tube (HS-TA18 tube) under warm forming conditions and the accurate description of flow stress is the basis of tube bending forming research. To this end, in this study, the HS-TA18 tube of 18 mm x 1.5 mm (out diameter x wall thickness) as the objective, uniaxial tensile tests were conducted under a wide temperature range (25 degrees C-500 degrees C). Then the macroscopic mechanical behavior and microstructure evolution were analyzed. The results show that the flow stress decreases with the increase of temperature. In the temperature range of 200-450 degrees C, the degree of decline slows down. Moreover, the effect of strain rate on flow stress is not obvious, which indicates that dynamic strain aging (DSA) occurs in this region. Dislocation slip is the dominant mechanism of plastic deformation in the studied temperature range. Although dynamic recrystallization at 500 degrees C leads to softening trend of flow stress, dynamic recovery (DRV) is still the main softening mechanism during deformation. Then, a constitutive model based on dislocation density is established, which includes common thermal and athermal stresses. In addition, back stresses associated with geometrically necessary dislocations (GNDs) and additional stress caused by DSA are also taken into account. The model is used to predict the flow stress of the HS-TA18 tube under warm forming conditions. The prediction results are in good agreement with the experimental results, indicating that the established model has excellent prediction ability.