Abstract
In this study, the isothermal die forging process of high Nb–TiAl (Ti-44Al-8Nb-0.2W-0.2B-Y, at.%) alloy blades was simulated using the ABAQUS V6.11 software and the blades were fabricated successfully. The influence of a low forging temperature (lower than 1000 °C) and strain rate on the distributions of effective strain and stress were analyzed. The results indicate that the effective strain exhibits negative temperature sensitivity and positive strain rate sensitivity. The stress exponent (n = 3.02) and the apparent activation energy (Q = 293.381 kJ/mol) of the present alloy suggests that this as-forged high Nb–TiAl alloy exhibits good deformability at low temperatures. With the reduction in strain rate and the increase in forging temperature, the effective stress decreases. Finally, high-quality high Nb–TiAl alloy blades were fabricated using an isothermal die forging technology at a rate of 0.01 mm/s and temperature of 950 °C, chosen on the basis of the simulations results. Scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD) results indicated that the center of the TiAl alloy blade possessed a duplex microstructure, consisting of remnant lamellar colonies and recrystallized γ/B2 grains. The refined α2 laths showed a typical forging flow line feature in the edge position, whereas the γ laths had broken down and recrystallized.
Highlights
TiAl alloys have been regarded as a new generation of light-weight alloys for high temperature structural applications in the aerospace and automotive industries due to their low density, high specific strength and modulus, excellent mechanical properties and good high temperature oxidation resistance [1,2,3,4,5].In particular, since Nb plays a critical role in increasing the high temperature strength along with creep and oxidation resistance, high Nb–TiAl alloys have been identified as promising materials to substitute nickel-based superalloys in the rotating blades of high-pressure compressors (HPC) and the low pressure turbines (LPT) of aircraft engines [6,7]
Since Nb plays a critical role in increasing the high temperature strength along with creep and oxidation resistance, high Nb–TiAl alloys have been identified as promising materials to substitute nickel-based superalloys in the rotating blades of high-pressure compressors (HPC) and the low pressure turbines (LPT) of aircraft engines [6,7]
It can be seen that the peak stress decreases with the increase in test temperature and the decrease in strain rates, indicating as-forged high Nb–TiAl alloy exhibits negative temperature sensitivity and positive strain rate sensitivity at a low temperature
Summary
TiAl alloys have been regarded as a new generation of light-weight alloys for high temperature structural applications in the aerospace and automotive industries due to their low density, high specific strength and modulus, excellent mechanical properties and good high temperature oxidation resistance [1,2,3,4,5]. It was declared that isothermally forged TiAl blades had been used for the engine PW1134G as power unit for the Airbus A320neo and made a successful maiden flight in 2014 [6] These results suggest that isothermal die forging could be a key technology for the fabrication of TiAl alloy components. Zhang et al researched the hot deformation behavior and quasi-isothermal forging processing simulation of high Nb–TiAl alloys above 1150 ◦ C [20,21,22]; there are few reports about the fabrication of blades by isothermal die forging using forged or extruded high Nb–TiAl alloy, especially at temperatures lower than 1000 ◦ C due to their sensitivity to hot-working temperatures. As an experimental validation of the simulation results, high Nb–TiAl alloy blades were forged at a rate of 0.01 mm/s at 950 ◦ C
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