Bulk metal formed parts for power plants
Final Report Abstract
The growing demand for green energy in the world supports the development of highly productive and large wind power plants. To increase the power/weight efficiency of the nacelle of the power plant, there are some great potentials concerning the drive systems and components. Amongst this the conventionally cast hollow shaft for connecting the rotor with the gearbox could provide up to 60 % weight reduction. With a new production route open-die forging, the mechanical strength can be increased by grain refinement. Additionally by this a better ultrasonic testing is possible, which further lowers the factor of safety and thus a smaller shaft can be applied. At the beginning of the project open die forging of hollow shafts with the intention of elongation of the part was not known by literature. Based upon the requirements for open die forged products (homogeneous and fine microstructure) a hollow shaft forging process was development based on numerical simulation, microstructure calculation and process optimization. In the first project period the basic kinematic of the process was evaluated and a microstructure model for the applied steel 42CrMo4 was developed. In the second period a realistic example geometry was chosen and a numerical simulation model was developed. The main difficulty was to implement the realistic tool behavior to the FE software. The workpiece is forged between an upper die, a lower die and a mandrel which holds the workpiece in the center hole. So two forming zones are forged at the same time and thus afford a loose fixation of the mandrel. This and the more than hundred individual forming steps caused long setup and simulation times. To calculate the microstructure, the material model for 42CrMo4 was transferred to the FE software Transvalor Forge and an improved grain growth model was implemented. The process consists of several heats and it was found that reheating is mainly influencing the final grain size due to grain growth. A validation of the microstructure simulation was performed based on forging of a hollow shaft with four steps. In principle the results from the microstructure calculation were in coherence with the simulated values. Based on the validation an optimization of the process parameters concerning a homogeneous and high equivalent plastic strain and a preferably low and homogeneous grain size was performed. As a result a bite ratio of 0.7 and a relative height reduction of 20 % should be chosen. These results were transferred to the simulation of a 20 kg hollow shaft, forging and simulation of a 200 kg hollow shaft and the simulation of a 20,000 kg hollow shaft, which represented the main goal of this research. The investigation of scaling effects showed that the 20,000 kg workpiece could be forged within two heats. The grain size evolution showed the same tendency of the 200 kg part. It was in average smaller but more homogeneous. Finally the complexity of the process was increased by using a contoured mandrel to successfully forge a hollow shaft with an inner contour both in simulation and experiment.
Publications
- „Bulk metal formed parts for power plants“, In: Proceedings of the 14th Forging International Conference Brazil, Porto Alegre, 2010, S. 81- 92 (ISSN 2178-3365)
Schäfer, D.; Franzke, M.; Hirt, G.
- “Parametric modelling of complex open die forging processes”, In: Proceedings of the 15th Forging International Conference Brazil, Porto Alegre, 2011, S. 130-140 (ISSN 2178-3365)
Schäfer, D., Medeiros, G., Colombo, T., Hirt, G., Schaeffer,L.
- „Hohlwellen für Windkraftanlagen – Prozessauslegung anhand von FEM-Simulationen“, Industrie Management 27, GITO Verlag 2011, S. 73-76
Hirt, G.; Schäfer, D.; Franzke, M.
- (2013). Forged hollow shafts for wind power drives. 1. Conference for Wind Power Drives. Aachen: 199-214
Recker, D., M. Franzke and G. Hirt
- (2013). Incremental Forging of Hollow Shafts – Process Design Evaluation by Numerical Simulation. SENAFOR. Porto Alegre, Brazil
Hirt, G., D. Rosenstock, D. Recker and L. Gao
- (2013). Lightweight construction by open die forging of hollow generator shafts for wind power plants. International Conference: New Developments in Forging Technology. Fellbach
Recker, D., D. Rosenstock, M. Franzke and G. Hirt
- (2014). Implementation of an Open-Die Forging Process for Hollow Shafts with Respect to an Optimized Microstructure. Material Science and Engineering, 2014. Darmstadt
Wolfgarten, M., D. Rosenstock, G. Hirt and L. Schaeffer
