| With the enhancement of overall national strength, our country begins to increase the research on the material of critical item and core part of aerospace, including the technology about preparation, forming and processing. Superalloy is the important guarantee of aero engines and industrial gas turbine development and it is an important material in fields of aerospace, energy, transportation and chemical industry. It is also the key material of modern national defence construction and national economy development. U720Li superalloy is a y’[Ni3(Al, Ti)] precipitation-hardened superalloy with superior mechanical properties and particularly high resistance in hot stress corrosion cracking, it is mainly applied on the high temperature driveline component of turbine disk and aeroplane engine.In this work, a series of palstic compress experiments for U720Li alloy were carried out on Gleeble-1500thermal simulating tester. Its microstructures and mechanical properties at elevated temperature compression deformation were obtained. The experimental microstructures were analyzed. At the same time, its characteristics of stress versus strain curves and the effect of strain rate, deformation temperature etc hot deformation parameters on flow stress were discussed and the m values (strain rate sensitivity exponent) and deformation activation energies were calculated and analyzed. On this basis, Arrhenius type constitutive equation was established. The data were trained and predicted by BP artificial neural network (ANN) and processing maps based on dynamic material models were established after calculating the power dissipation factor and flow instability value for ANN trained data. Above research work has important practical significance and theoretical value for the set-down of processing schedule and theoretical simulation of U720Li alloy. Specific research conclusions are reached as follows:(1) The microstructures at different temperatures and at different strain rates are compared by micro-structural observation of deformed specimens. During deformation, dynamic recrystallization is the primary softening mechanism of U720Li alloy. As deformation increases, the recrystallization grain size are decreased; with temperature increases, the recrystallization grains grow significantly; as the strain rate increases, the recrystallization grains are more fine. The slower the strain rate is, the more sufficient the dynamic recrystallization proceeds. The proportion of y+y’phase decreases and the proportion of y’phase increases during phase transformation.(2) U720Li alloy is sensitive to the variety of temperature and strain rate and the flow curve exhibited a peak stress followed by flow softening. The flow stress of U720Li alloy during hot compression deformation decreases with the inereasing of temperature and the decreasing of strain rate.(3) According to the curves of true stress-strain, strain rate sensitivity exponent m and activation energy Q are calculated.It shows that m is greater than0.2in the range of1000℃～1150℃, indicating that U720Li alloy exhibits definite high temperature plasticity. The activation energy Q values are in the range from477kJ/mol to830kJ/mol under different deformation temperatures and strain rates.(4) Based on Arrhenius equation, flow-stress model is established by cubic polynomial fitting and multivariate linear fitting. The model is as follows:(5) Based on the Dynamic Material Mode, the power dissipation factor and flow instability value are calculated. The processing map of U720Li alloy is established. Appropriate processing region and flow instability region can be seen in the constructed processing maps, which provide sufficient theoretical basis for understanding practical behavior of U720Li alloy. |
PDF Full Text Request