Effect of Temperature and Deformation Speed on Formability of IN718 Sheets: Experimentation and Modelling

Abstract

The objective of present work is to get insight into the formability behavior of mill annealed 1mm thick Inconel 718 (IN718) sheets under different elevated temperatures and deformation speeds. The preliminary study consisted of uniaxial tensile testing within a temperature range of 500°C – 700°C at an interval of 50°C and crosshead velocities of 0.03,0.3, 3, and 30mm/s. It was observed that the total elongation improved by approximately 18% with the increase in temperature from 500°C to 700°C. However, a considerable reduction in total elongation with increase in flow stress was found with the increase in deformation speed at elevated temperatures. Based on the true stress-strain responses, the Johnson-Cook (JC) constitutive equation was developed to capture the material flow behavior. The correlation coefficient (R), average absolute error ( Δ ) and standard deviation (SDA) were found to be 0.97, 10.8 and 6.8 respectively. Further, stretch forming behavior at isothermal elevated temperatures of 500°C and 650°C were analyzed by performing laboratory scale limiting dome height (LDH) tests. A 20mm width Hasek specimen was deformed at punch speeds of 0.3mm/s and 30mm/s using a sub-sized hemispherical punch of ϕ50 mm. The LDH was found to be higher at 650°C, and the maximum LDH was achieved at lower punch speed of 0.3mm/s. The higher deformation speed decreased the LDH by 7.4% and 12% at 500°C and 650°C respectively. The thermo-mechanical finite element modeling of isothermal LDH tests was developed successfully by incorporating JC model. The predicted punch load, thickness and surface strain distributions were validated with experimental data. It was established that the predicted peak loads were within nominal 8.6% error highlighting the suitability of JC constitutive equation in FE model.