Papers by Author: Daniel Huber

Abstract: Gamma titanium aluminides are promising alloys for low-pressure turbine blades. A significant disadvantage of such intermetallic alloys is failure induced during forming processes due to ductile damage and flow instabilities. Previous investigations on a gamma titanium aluminide alloy (TNM), have shown ductile damage due to tensile stress components and instabilities such as shear bands, pores and micro-cracks at low temperatures and high strain rates. The main part of the current work is to delineate damage and unstable regions in the low temperature region. Hot deformation experiments are conducted on a Gleeble®3800 thermomechanical treatment simulator to obtain flow curves to be implemented in a finite element method (FEM) code. Instabilities in the material are described by existing instability criteria as proposed by Semiatin and Jonas and implemented into FEM code DEFORM^TM 2D. Predictions of ductile damage models and the instability parameter are validated through detailed microstructural studies of deformed specimens analysed by light optical- and scanning electron microscopy.

Abstract: Gamma titanium aluminides are innovative materials for high temperature and light weight applications [1]. On the other hand, their hot workability can be limited by failure during hot deformation processes. The prediction of ductile damage in metallic materials can be performed by macromechanical ductile damage criteria [2-4]. If the calculated damage D parameter exceeds a critical value D_c, the material fails. Some macromechanical ductile damage criteria are shown in Table 1, with σ as effective stress, ε as effective strain, σ_max as maximum principal stress, σ_m as hydrostatic stress (mean stress) and εf as equivalent fracture strain. The damage responds to strain localization and thus, to multiaxial stress concentration that increases fracture probability.

Abstract: The efficiency of gas turbine engines can improved by an increase of the working temperature. As a consequence Allvac® 718Plus™ was developed to enhance the high temperature properties. Since the performance of this alloy is strongly related to the microstructure the knowledge of the softening processes is important to develop precise microstructure evolution models. Specimens were deformed at different temperatures (950°-1050°C) and strain rate (0.1s^-1 – 10s^-1) to strains of 0.2-1.5. The microstructures obtained were analyzed by electron backscatter diffraction (EBSD) in the scanning electron microscope to investigate the softening mechanisms at the respective forming conditions.

Abstract: Aerospace gas turbine disks operate in an environment of relatively high stresses caused by centrifugal forces and elevated temperatures. Because of the strong mechanical requirements and narrow specifications of such parts not only a correct, defect free final geometry is necessary, but also a defined microstructure. Even though the microstructure evolution during thermo-mechanical processing is well studied and understood for superalloys like IN718, the influences cannot easily be described analytically. Thus simulation tools are used to assure process stability and to optimize design parameters to meet the tough requirements in aerospace industries. Microstructure simulation of IN718 (and other materials) is well established at Bohler Schmiedetechnik GmbH & Co KG and appreciated by its customers. The advent of the newly developed nickel-base superalloy ATI Allvac® 718PlusTM led to extensive investigations and the development of an adapted microstructure model by Bohler Schmiedetechnik GmbH & Co KG and its research partners. Aim of this paper is a comparison of the microstructure evolution in IN718 and ATI Allvac® 718PlusTM during the thermo-mechanical treatment of turbine disks. Influences of process temperature, strain and strain rate on the final grain size are discussed by finite element simulations with a coupled grain structure model. Experimental results from trial forgings are compared with the outcome of the microstructure simulations.

Abstract: During hot working of the recently developed nickel based alloy Allvac 718PlusTM softening kinetics like dynamic, metadynamic as well as static recrystallization govern the microstructure evolution during and after hot forming and hence the final mechanical properties. In this work the metadynamic recrystallization was investigated using double hit compression tests. The classical methodology of offset stress comparison was not usable because of discontinuous yielding of this material. Thus a new method to describe the softening during metadynamic recrystallization was developed, which compares the deformation energy at equal strains, i.e. the area beneath the true strain vs. true stress curve, of the first and second hit as well as at steady state.