Through Process Characterization and Modelling of Forged Titanium Components

Summary

By through process characterization and modelling of forged titanium components (in the special case Ti-6Al-4V) the whole production and in-service chain can be computed by means of microstructure simulation, followed by microstructure dependent static and fatigue computations. As this can be done in virtual life once the verification is executed by component tests, an interdisciplinary thermo-mechanical process and component optimization of forgings is executable in order to deliver economic and safe aerospace components.

Abstract

The aim was to develop appropriate models for fatigue-proof and damage tolerant lightweight design of forged Ti-6Al-4V components under consideration of microstructural variation resulting from thermo-mechanical processing. For that purpose, specimens were taken from open- and closed-die forgings with different subsequent heat treatments and therefore varying microstructures. The fatigue and crack growth behavior was thoroughly analyzed with respect to microstructure. Based on these results, phenomenological models were developed to link fatigue and fracture behavior with microstructural parameters. Extensive fatigue tests were additionally performed for the development of fatigue models regarding mean stress and notch sensitivity, damage tolerance (influence of pre-existing flaws), influence of multiaxial loading, operating temperature and surface state.

All developed models for the lifetime estimation of Ti-6Al-4V forgings were implemented by Böhler Schmiedetechnik GmbH & Co KG in Fortran in a postprocessor. Results of the forging simulation (microstructural parameters) and finite element stress analyses (stress tensors) are thereby used as input. Local variations of microstructure, owing to different local cooling rates or the like, can hence be considered. Postprocessing results are the distributions of damage and maximum allowable crack lengths in a component for a given load spectrum. The developed models for the lifetime estimation of Ti-6Al-4V components contribute in many respects to the approach of lightweight design. On the one hand, they enable an optimized utilization of the material capability of Ti-6Al-4V owing to improved dimensioning of a component (inter alia in combination with topology and shape optimization); on the other hand, they are the basis for a simulation-based optimization of the whole forging process. The result is an optimized component performance under service conditions.