Study On Microstructure Evolution And Number Simulation Of The High-temperature Deformation Of TiAl Alloy

TiAl based alloy is one of the most potential high-temperature structure materials in aerospace and auto engine because of its excellent properties. However, the poor workability severely limits its practical application. In order to optimize the process for TiAl alloy and obtain a idea microstructure which has good mechanical properties. This study is to simulated the hot forging and rolling process on TiAl alloy through DEFORM numberical analysis software, trying to find a more optimized process parameters to improve the success rate of TiAl alloy deformation. Heat treatments on as-rolled TiAl sheets would be applied to adjust its microstructure which would be characterized by SEM, OM, TEM and XRD analysis detection means. Finally, the tensile tests would be utilized to characterize the basic mechanical properties of the alloy. The main conclusions are presented as follows:(1) The temperature distribution became more uniform with the thickness of the pack increasing during forging process. At the same time, the side pressure in the TiAl alloy billet would increase with the increasing of the thickness. This would help to decreas the damage of the billet. Established a force balance equation after analyzing all forces in the billet during forging process and obtained an ideal thickness of the pack.(2) During the rolling process, rolling force decreased with the elevation of rolling temperature and the reduction of rolling rate and pass reduction. Since the cooling rate of the sheet edges is faster with lower temperature, it is easy to generate stress concentrated in edge region during the deformation process. The greater the reduction is and the greater the damage is. In summary, in order to ensure the success of TiAl based alloy sheet rolling, it should be used at higher temperatures, lower rolling reduction rate and slower rolling rate.(3) The y-phase volume fraction reduced with the temperature increasing when annealing at different temperatures in (α+γ) two-phase zone. However, the grain size of the lamellar grans has a continuous growth during the same experience. After gamma grains dissolved in a phase completely, the sheets were cooled to obtain fully lamellar microstructure. The size of lamellar grain became small with the decline of annealing temperature and annealing time. Analyzing of the thermodynamics and kinetics of TiAl based alloy which had been soaked in a phase zone for2min-120min. And found that the grain growth exponent of the sheet is about0.2, and the activation energy of grain boundary migration (Qm) is about225KJ/mol.(4) The as-rolled TiAl alloy sheets have good high temperature mechanical properties. The ultimate tensile strength of this kind of alloy can reach450MPa with a29.8%elongation at800℃and10-4s-1strain rate.