Yongquan Ning

Hot deformation behavior of TC11/Ti–22Al–25Nb dual-alloy in isothermal compression

Abstract The high-temperature flow behavior of TC11/Ti–22Al–25Nb electron beam (EB) weldments was investigated by the isothermal compression tests at the temperature of 900–1060 °C and the strain rate of 0.001–10 s −1 . Based on the experimental data, the constitutive equation that describes the flow stress as a function of strain rate and deformation temperature is obtained. The apparent activation energy of deformation is calculated, which decreases with increasing the strain and the value is 334 kJ/mol at strain of 0.90. The efficiency of power dissipation η changes obviously with the variation of deformation conditions. Under the strain rates of 0.01, 0.1 and 1 s −1 , the value of η increases with increasing the true strain for different deformation temperatures. While the value of η decreases with increasing the strain under the strain rates of 0.001 and 10 s −1 . The optimum processing condition is ( t opi =1060 °C, opi =0.1 s −1 ) with the peak efficiency of 0.51. Under this deformation, dynamic recrystallization (DRX) is observed obviously in the microstructure of welding zone. Under the condition of 1060 °C and 0.001 s −1 , the deformation mechanism is dominated by dynamic recovery (DRV) and the value of η decreases sharply ( η =0.02). The flow instability is predicted to occur since the instability parameter ξ () becomes negative. The hot working process can be carried out safely in the domain with the strain rate of 0.001–0.6 s −1 and the temperature of 900–1060 °C.

Effect of hot working on microstructure and mechanical properties of TC11/Ti2AlNb dual-alloy joint welded by electron beam welding process

Abstract The influence of hot working on the microstructures of TC11/Ti2AlNb dual-alloy joints welded by electron beam welding (EBW) process was investigated. The tensile tests were performed at room temperature for specimens before and after thermal exposure. The results show that the fusion zone of TC11/Ti2AlNb dual-alloy joint welded by EBW is mainly composed of β phase. After deformation and heat treatment, the grain boundaries of the as-cast alloy are broken and the fusion zone mainly consists of β, α2 and α phases. The fusion zone performs poor property in the tensile test. Specimens before and after thermal exposure all fail in this area under different deformation conditions. The ultimate tensile strength of specimens after heat treatment is up to 1190 MPa at room temperature. The joints by water quenching after deformation have better plasticity with an elongation up to 4.4%. After thermal exposure at 500 °C for 100 h, the tensile strength of the specimen slightly rises while the ductility changes a little. SEM observation shows that the fracture mechanism is predominantly transgranular under different deformation conditions.

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.