Abstract
The purpose of this study is to obtain a constitutive equation of high-accuracy flow stress in superalloy 718, which allows fabrication of highly reliable disks for gas turbine engines. Hot compression tests using superalloy 718 at deformation temperatures from 850 to 1100 °C, a 67% height reduction, and strain rates of 1, 10, and 25 s−1 were performed to investigate the flow stress behavior, which excludes environmental effects during hot working by inverse analysis. The effects of dynamic recrystallization and strain-induced dynamic precipitation on the flow stress were also investigated. The dynamically precipitated δ phases deformed at 1050 °C and γ″ phases deformed at 950 °C might affect the increase in the plastic modulus F1 and the decrease in the critical strain εc, deteriorating the accuracy of regression in terms of, for example, the strain rate sensitivity m and the temperature sensitivity A. A constitutive equation for a generalized flow curve for superalloy 718 is proposed by considering these effects.
Highlights
As an aircraft engine component, gas turbine disks with a gear shape are exposed to temperatures up to 650 ◦C at the outer periphery, a high rotation speed of about 10,500 rpm, and a load of more than 1000 MPa at the inner periphery at takeoff
The flow curves obtained by experiment are calculated using Equation (10) and inverse analysis (IA) results are summarized in Table 2, which are used to calculate the flow stress: σ=
At strain rates from 1 to 25 s−1, a small difference between both flow curves obtained by experiment and IA is observed at deformation temperatures above 1000 ◦C and strain rates below 10 s−1
Summary
As an aircraft engine component, gas turbine disks with a gear shape are exposed to temperatures up to 650 ◦C at the outer periphery, a high rotation speed of about 10,500 rpm, and a load of more than 1000 MPa at the inner periphery at takeoff. The disks should have high performance characteristics such as tensile strength at elevated temperatures, creep properties, low-cycle fatigue, and crack propagation resistance [1]. To satisfy these critical characteristics, the hot forging and heat treatment in Ni-based superalloys such as superalloy 718 have been applied in manufacturing gas turbine disks [2,3,4]. Various experiments have been carried out to obtain the constitutive equation of flow stress in Ni-based superalloys at elevated temperatures and various strain rates [6,7,8,9,10]. To achieve a uniaxial flow stress, for which these affects are compensated, a variety of studies have been carried out by this approach using Cr–Mo–V [13], carbon [14,15] and stainless [16,17] steels, an aluminum alloy [18,19], and a nickel-based superalloy [20]
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