Zhanglong Zhao

A study on a 3D FE simulation method of the NC bending process of thin-walled tube

Abstract Based on the rigid–plastic finite element method (FEM) principle, a three-dimensional (3D) rigid–plastic FE simulation system named TBS-3D (tube bending simulation by 3D FEM) for the NC bending process of thin-walled tube has been developed, and a reasonable FEM model has been established. By the use of this FE simulation system, an NC bending process of thin-walled tube has been simulated, and deformed meshes under different bending stages, the stress distribution along the bending direction, and the relationship between the maximal wall thickness changing ratio and the bending angle have been obtained. Some forming laws of NC tube bending obtained are as follows: (1) the NC bending process make tube elongate to some extent; (2) the characteristic of the stress distribution is that the outer area is undergoing tensile stress, the inner area is undergoing compression stress, and the stress neutral layer moves close to the inner area, which is in good accordance with practice; (3) the maximal wall thinning ratio in the outer tensile area changes only a little with increase of the bending angle, and the maximal wall thickening ratio in the inner compression area increases linearly with bending angle. The above results show that 3D FE simulation is an important and valid tool for analyzing the NC bending process of tube, this research being beneficial for the practical tube bending process, and may serve as a significant guide to the practice of the relevant processes.

Microstructure characterization and mechanical properties of TC4-DT titanium alloy after thermomechanical treatment

Influence of thermomechanical treatments (mill annealing, duplex annealing, solution treatment plus aging and triple annealing) on microstructures and mechanical properties of TC4-DT titanium alloy was investigated. Results showed that thermomechanical treatments had a significant influence on the microstructure parameters and higher annealing and aging temperature and lower cooling rate led to the decrease of the volume fraction of primary α and the size of prior-β and the increase of the width of grain boundary α and secondary α. The highest strength was obtained by solution treatment and aging due to a large amount of transformed β and finer grain boundary α and secondary α at the expense of slight decrease of elongation and the ultimate strength, yield strength, elongation, reduction of area were 1100 MPa, 1030 MPa, 13% and 53% separately. A good combination of strength and ductility has been obtained by duplex annealing with the above values 940 MPa, 887.5 MPa, 15% and 51% respectively. Analysis between microstructure parameters and tensile properties showed that with the volume fraction of transformed β phase and the prior-β grain size increasing, the ultimate strength, yield strength and reduction of area increased, but the elongation decreased. While the width of grain boundary α and secondary α showed a contrary effect on the tensile properties. Elimination of grain boundary α as well as small prior-β grain size can also improve ductility.

Dependence of α-phase size on flow stress during dynamic recrystallization steady state in Ti60 alloy

The dependence of α-phase size on flow stress was characterized by a proposed kinetic model during dynamic recrystallization (DRX) steady state in Ti60 alloy. According to the isothermal compression tests, the influence of deformation parameters on the steady-state flow stress was analyzed and the constitutive equation was established to predict the steady-state flow stress under different deformation temperatures and strain rates. A power-law relationship between the DRX average grain size and steady-state flow stress with an exponent of −2 is obtained from the dynamic balance during DRX steady state. The effect of deformation parameters on α-phase size was observed through the microstructure after deformation, and the applicability of the model for Ti60 alloy was verified by the comparison between predicted and experimental data.