Implementation of an open-die forging process for large hollow shafts for wind power plants with respect to an optimized microstructure

  • M. Wolfgarten
  • D. Rosenstock
  • L. Schaeffer
  • G. Hirt


To realize large wind power plants in an economically feasible way, it is necessary to identify potential for
lightweight design of the generator hollow-shafts, which are commonly produced by casting. The weight of
these shafts can significantly be reduced by producing them by open-die forging, since the forming of the
material leads to a higher strength, which allows to reduce the wall thickness noticeable. This paper describes
the development and implementation of a forging process for hollow shafts with respect to an optimized
microstructure. To numerically investigate this process, a realistic finite element simulation model was
developed in a first step. The kinematic of the tools has been implemented authentically to provide a realistic
material flow and process conditions. Additionally, a material model for the steel 42CrMo4 was integrated into
the simulation model to predict the resulting microstructure. Using the implemented FE model, the forging
process was optimized manually to achieve a homogeneous and fine-grained microstructure. The optimization
was based upon a variation of different forging parameters and the sequence of forging steps. In the next
step, a forging on laboratorial scale was performed to validate the simulation model. For this purpose, after
forging, specimens from the hollow shaft were evaluated by metallography to determine the final grain size. A
comparison of the results with the numerical simulation showed a general agreement of the measured grain
size with the numerically calculated grain size. Based upon these results, the process model was transferred
to an FE model with an industrial scale. By this it was possible to analyze the transferability of the used FE
model regarding the assumptions about the kinematics and the sequence of the forging steps. A numerical
investigation of the industrial process proved the scalability of the process to an industrial relevant geometry.