Integrated modeling for the manufacture of Ni-based superalloy discs from solidification to final heat treatment

Abstract

Process models of the various stages of gas-turbine disc manufacture have been integrated to simulate the physical and microstructural transformations occurring within a nickel-based superalloy throughout the entire manufacturing route. Production of these critical rotating structural components requires several distinct processing stages: vacuum-induction melting (VIM), vacuum-arc remelting (VAR), homogenization heat treatment, cogging, forging, final heat treatment, and machining. During the course of these consecutive manufacturing stages, the various thermal and thermomechanical processes lead to significant changes in both the microstructural characteristics and internal stresses in the alloy. Although separate models have previously been developed to simulate the individual processing stages, this article describes how these models, which are explicitly expressed in terms of the initial and evolving microstructure, can be integrated to simulate the entire manufacturing process from secondary melting through to the final forging and heat treatment. The grain structure predicted for one stage is explicitly transferred as the initial conditions for the model, which simulates the grain evolution during the next step. This information also provides the basis of microstructure-explicit constitutive equations describing the material behavior. Industrial-scale manufacturing trials associated with the production of an INCONEL alloy 718 aeroengine turbine disc were used to validate the integrated model for grain size. The model also allows intrinsic or extrinsic defects entrained within the material during the initial solidification stage to be tracked through the subsequent processes. This provides a basis for calculating the areas of discs likely to be vulnerable to such defects and whether they might be removed during machining. It was shown that the microstructure of the alloy changes significantly throughout the process chain, the final microstructure and defect distribution at each stage being related to those formed in the previous stages. There was good agreement between the model predictions and experimental observations for both the intermediate and final processing stages. The implications of such integrated modeling for quality assurance through process control are discussed.

Residual stresses in a quenched superalloy turbine disc: Measurements and modeling

“…Following a commercial processing route, [5,6] the forged disc was subjected to a standard solution heat treatment, during which the ␦ precipitates were partially returned into solution. The disc was then rapidly cooled by quenching in water.…”

“…To assist with quality control and cost savings, computational simulations of the various processing steps are often utilized to optimize the processing parameters (e.g., References 1 through 4), and significant progress has now been made with the development of integrated models covering several manufacturing stages. [5,6] Validation of such models has concentrated mainly on microstructural changes occurring during each stage of the manufacturing process. On the other hand, proper assessment of deep residual stresses created during postforging heat treatment of superalloys has proven rather more elusive.…”

Simulating the Residual Stress in an A356 Automotive Wheel and Its Impact on Fatigue Life

“…One way of doing this is to adopt a through process modeling methodology linking the microstructural features and defects to the local stress state arising from both residual stresses and in-service cyclic loads. Although this concept has been proposed previously by the current authors [4,6,7] and others, [8] these prior studies have not determined the sensitivity of the final fatigue life of A356 components to the accuracy of the residual stress calculations, nor were these calculations validated. This article will focus on these aspects.…”

Prediction of Mean Grain Size Evolution during the Forging of IN718 Turbine Disc

“…Therefore, the defects of coarse grain and duplex grain always appear in the forgings. Figure 1 shows the manufacturing of IN718 turbine disc [3]. At first, the ascast structure has been changed to as-forged structure during the cogging, and a big forged bar with the diameter lager than 200 mm has been gained.…”