Modelling Texturised Cast Structures in the Forging of Superalloys

Jan Terhaar,Dieter Bokelmann,Hendrik Schafstall,Nikolaus Blaes

doi:10.4028/www.scientific.net/amr.278.210

Abstract

The main objective of remelting processes commonly used in the production of super¬alloys is to obtain a columnar dendritic solidification structure throughout the whole ingot. Besides reduced microsegregation, this cast structure features a preferred orientation, which is depending on the primary dendrites’ growth direction and therefore closely related to the ingot’s pool shape. As a result, non-isotropic material behaviour can be observed during initial forging operations. Since the correct prediction of material flow is a prerequisite for the further analysis of forging processes by means of numerical simulation, the solidification texture’s influence on plastic flow was accounted for by the application of an anisotropic material model. The model according to Barlat was used to scale the flow stress with respect to the crystal orientations observed in the examination of vacuum arc remelted alloy 718, thereby considering the flow stress’ dependency on strain, strain rate and temperature. The parameters defining the material's anisotropy could be determined by the upsetting of cylindrical specimen from a remelted ingot.

Highlights

The possibilities to optimise bulk metal forming processes with the help of process simulation are limited by the fact that the material commonly is modelled as a homogeneous, isotropic continuum
Since the correct prediction of material flow is a prerequisite for the further analysis of forging processes by means of numerical simulation, the solidification texture’s influence on plastic flow was accounted for by the application of an anisotropic material model
The model according to Barlat was used to scale the flow stress with respect to the crystal orientations observed in the examination of vacuum arc remelted alloy 718, thereby considering the flow stress’ dependency on strain, strain rate and temperature

Summary

Introduction

The possibilities to optimise bulk metal forming processes with the help of process simulation are limited by the fact that the material commonly is modelled as a homogeneous, isotropic continuum. It is likely that the existing solidification texture, if it was accounted for in the numerical simulation, would lead to a different stress-strain state in the material, as compared to the isotropic case. Concerning superalloys, this issue is of particular importance since these alloys react comparably sensitive on process conditions. The mentioned anisotropic flow behaviour has already been observed and commented on by other authors [1, 2], when plastic properties of remelted superalloys were examined by using material in the as-cast or homogenized state, i.e. when the samples have not been subjected to prior deformation.

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Results

Conclusion