Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Potential of process modelling to forecast residual stresses in titanium alloy forgings and linear friction welded titanium alloy joints

Residual stresses are omnipresent meaning that their existence is normal and not an exception. Therefore the residual stress state of a part and/or a specific location in a part is important to know as the residual stresses and the stresses imposed by the loading must be superimposed and the sum of both stresses can lead locally to higher stresses than expected with possible premature failure of the component.

Within the project the modelling/forecast of two kinds of residual stresses were investigated and compared to some extend with measured residual stresses:
- The residual stresses in the forged, heat treated and pre-machined raw part of the disk-part of the demonstrator, resulting from the different hot-forming operations and various heat treatments. These play an important role during manufacture as well as operation of complex shaped structures. An understanding of the residual stress evolution during the manufacturing process from the starting ingot to the final part allows predicting possible machining problems and distortions that can occur. Additionally it could be very important input information for further processes and process-modelling, like the LFW-process.

- The residual stresses induced by the linear friction welding process into the blade, as these will be superimposed to any loading induced by normal operation (=aerodynamic loading).

Böhler Schmiedetechnik (BSTG) determined the necessary elasto-plastic material data and calculated the residual stress state in different stages of a beta-forged Ti6246 compressor disk during the production process. The software package used was DEFORM"-2D. and the forecast made for the pre-machined raw part was compared to residual stress measurements made by MTU using the hole drilling method. The comparison showed good agreement between modelling and measurements.
MTU’s sub-contractor FSU (Friedrich Schiller University in Jena, Germany) had the job to model the residual stresses for the two weld-combinations -Ti6246<>-Ti6246 and Ti6246<>-Ti6246 based on MTU linear friction welding equipment and the long time existing bilateral experience. During the first iteration and the comparison of the first results of the two weld-combinations it was discovered that the existing information on relevant physical properties were not sufficient accurate enough.

Therefore additional data like young’s modulus, heat capacity, density, coefficient of linear friction where determined by MTU and FSU up to the -Transus temperature of Ti6246. The results of the forecast of the residual stresses and the measurements using the hole drilling method show big differences, reasons could be on both sides. The major result regarding these process related residual stresses is, that the modelling is much more challenging than anticipated and a lot of work has to be done.

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Ergebnis in Kürze

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MTU Aero Engines GmbH
Dachauer Strasse 665
80995 Munich
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