Speed-up of isothermal forging processes of titanium aluminides by microstructure-adapted control of ram speed

• Applicants: Professor Dr.-Ing. Markus Bambach; Professorin Dr.-Ing. Sabine Weiß
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 325021729

The forging of intermetallic titanium aluminides (TiAl), used e.g. for turbine blades in modern aero engines, currently occurs as a two-stage isothermal forging process, at temperatures of at least 1200°C and process times of several minutes per stroke. This is associated with long process times, high loads on the extremely expensive molybdenum-based dies and high manufacturing costs, which prevents the wider use of TiAl. The aim of this project is to shorten the isothermal forging process of TNM-B1, which industrially takes place with constant ram speed, by targeted control of the ram movement. Results of the first funding period could demonstrate, both, experimentally and simulatively, that the forming process can be accelerated significantly, without causing increased damage to the material, by a previous heat treatment. So far, only the material-specific hardening / softening behavior has been taken into account and not the high variance of the material, the influence of the workpiece geometry or the complex structure and damage development of the material during deformation. In order to include these factors, a process control will be developed that combines a machine learning (ML) algorithm, the existing FEM model and a damage model. FEM is used to simulate the variable material behavior and the different workpiece geometries during the forming process. A Gurson-Tvergaard-Needleman damage model will be adapted to titanium aluminides and coupled with the FEM model. The ML algorithm will then adjust the ram speed according to the workpiece geometry, the material variance and the predicted damage behavior. In order to gain more in-depth knowledge about the microstructure development of TNM-B1 during hot forming, in-situ pressure tests with accompanying structure and phase analysis (DESY - Petra III) will be carried out. The results will provide information about the exact phase composition and the initiation of recrystallization. Details of why the heat treatment results in decreased yield stress, regardless of the test conditions, are also expected. During the heat treatment of larger components, e.g. Turbine blades, a temperature gradient is formed. The areas close to the surface cool faster than the inside of the component, which leads to different globular, lamellar and pearlitic / cellular microstructures over the component cross-section. In order to evaluate the microstructure gradient, a forging blank will pass the entire process chain, both, for constant and for accelerated test conditions. In addition, a post-heat treatment will be developed to achieve the desired final microstructures and mechanical properties.

• DFG Programme: Research Grants