Deformation characteristics of isothermally forged UDIMET 720 nickel-base superalloy

Abstract

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10 Ϫ3 to 1 s Ϫ1 and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above ␥Ј solvus temperature was accompanied with grain boundary separation. The hot working window was determined to be in the interval 1100 °C to 1150 °C. Thermomechanical behavior of the material was modeled using the power-law, the Sellars-Tegart, and an empirical equation. The flow stress values showed a nonlinear dependence of strain rate sensitivity to strain rate. The analysis indicated that the empirical method provides a better constitutive equation for process modeling of this alloy. The apparent activation energy for deformation was calculated and its variations with strain rate and temperature are discussed.

Correcting the Stress-Strain Curve in Hot Compression Process to High Strain Level

“…The average friction coefficient was observed to increase with the increasing strain level. These results did not conform to the conventional viewpoint [8,14] that the friction coefficient as a function of strain level is a constant. There are several reasons existing for the increase in the friction coefficient with an increasing strain level.…”

“…[11,14] In previous research, [8,11,14,15] l was a constant. In this article, l is replaced by the instantaneous friction coefficient obtained from Eq.…”

“…In practice, directly predicting the friction coefficient at a random strain level is difficult because of the difficulty in measuring related parameters of specimens in the working process; researchers often simply assume that the friction coefficient is a constant or independent of the strain level. [5,[8][9][10][11] The details about the evolution behavior of friction coefficients in working processes have not yet been made clear. In this case, the stressstrain curve can only be effectively plotted for a limited strain level (lower than 0.6 or 0.7), [8][9][10][11] because at higher strain levels a great increase in the applied stress occurs due to an abnormal increase in the friction coefficient; this cannot be corrected properly by the assumed average friction coefficient.…”

“…[5,[8][9][10][11] The details about the evolution behavior of friction coefficients in working processes have not yet been made clear. In this case, the stressstrain curve can only be effectively plotted for a limited strain level (lower than 0.6 or 0.7), [8][9][10][11] because at higher strain levels a great increase in the applied stress occurs due to an abnormal increase in the friction coefficient; this cannot be corrected properly by the assumed average friction coefficient. [8,[11][12][13] Therefore, in order to correct the deformation curves in large strain compression processes a basic evolutional model of the friction coefficient as a function of strain needs to be proposed.…”

Microstructural evolution of Inconel 625 during hot rolling

“…The new recrystallized grains nucleate on the deformed grain boundaries and form a necklace around it 7,[11][12][13] , as visible in Figure 5c. It has been reported that during deformation the cooling is slower due to the conversion of deformation energy to heat by adiabatic heating 14 . It is well known that the temperature rise during hot deformation can not only induce the process of DRX, but also lead to a rapid reduction in the work-hardening rate 15 .…”