Modeling the Hot Forging of Nickel‐Based Superalloys: IN718 and Alloy 718Plus( ® )

Abstract

An internal state variable material model is used to describe the rate– and temperature– dependent large deformation response of the nickel-based superalloys Inconel 718 and Inconel 718 Plus. The current version of the material model describes the elastic-plastic and thermal deformation of metals, having two internal state variables whose evolution equations account for dislocation hardening and static /dynamic recovery processes. Other microstructural features such as recrystallization and grain growth are currenlty being added to the model. Experimental data from mechanical characterization tests of cylindrical and double cone compression specimens are used, respectively, to calibrate the material model and to validate its predictive capability. In general, the calibrated model predicts well the experimental stress/load levels as well as the rate and temperature dependence of the mechanical response of these superalloys. Introduction The superalloys IN718 and IN718Plus have been very well characterized in terms of their chemistry, microstructure (precipitation phases), manufacturing processing and mechanical properties [1,2,3,4] It is well known that the use of the correct chemical components together with an adequate thermo-mechanical processing (deformation processing, heat treatment, aging) develop in these materials the desirable microstructure (precipitate phases) that give these alloys their good elevated temperature strentgh, thermal stability, and hot workability, characteristics needed for the long–term high–temperature environments typical of aircraft engine turbine parts [5]. However, there is still a demand to understand the mechanical behavior at the final stages of the manufacturing process, which is typically a multi–step hot forging process [6]. During hot forging processes, the microstructure and mechanical behavior of these superalloys tpically change by metallurgical transformations. Microstructural features such as dislocation structures, annealing phenomena (recovery, recrystallization and grain growth), and precipitate phases are mainly responsible for the final mechanical properties of the material [7], and hence for the performance of the manufactured part during service. In this context, much research has been directed at understanding the mechanisms and phenomenology of microstructure evolution during hot deformation. Microstructural processes such as dynamic, metadynamic and static recrystallization as well as grain growth in metals [8], and their relation to the hot processing parameters has been studied and modeled in these superalloys, with the bulk of the study mainly concentrated on IN718, an alloy invented almost a half-century ago [9,10,11,12,13,14,15]. Modeling the hot deformation of superalloys during