Papers by Author: Christof Sommitsch

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: In this paper, the precipitation behaviour of  (Ni3(Nb,Al)) and ’ (Ni3(Al,Ti,Nb)) phases in the nickel-base superalloy ATI Allvac® 718PlusTM, as well as their kinetic interactions are discussed. Important parameters such as volume fraction, mean radius and number density of precipitates are experimentally determined and numerically simulated as a function of the heat treatment parameters time and temperature. To match the experimentally observed kinetics, the predicted interfacial energy of the precipitates, as calculated for a sharp, planar phase boundary, is adjusted to take into account the interfacial curvature and entropic effects of a diffuse interface. Correction functions for the interfacial energies of  as well as ’ precipitates are presented. Using these modified interfacial energies, the calculated results show excellent agreement with the experimental measurements.

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.

Abstract: A physical model based on three types of dislocations and three nucleation sites for recrystallized grain is applied to hot rolling simulation. This model was implemented into a commercial Finite Element (FE) analysis package FORGE 2008 to calculate both the structure evolution during and the recrystallized volume fraction after hot working of aluminium alloy 5083. It is shown that the main nucleation mechanisms in the aluminium alloy are the particle stimulated nucleation (PSN) and nucleation at grain boundaries. Hence the precipitation kinetics during homogenisation was investigated by use of the thermodynamic calculation software MatCalc. To validate the simulation results hot rolling experiments were performed by means of a laboratory mill. The grain structure evolution was analysed by electron backscatter diffraction (EBSD).

Abstract: Process parameters in aluminium extrusion technology are key points that influence product properties. The precipitation hardening aluminium alloy 6082 is investigated according to different process conditions and the influence onto the final microstructure is simulated as well as experimentally verified. A physical microstructure model based on three dislocation types and three nucleation sites for recrystallization is implemented into the commercial Finite Element package FORGE 2008 to calculate both the microstructure evolution during the extrusion process as well as the recrystallized fraction after the process. The precipitation kinetics during homogenization was investigated using the thermodynamic calculation software MatCalc since the main nucleation mechanism for recrystallization is particle stimulated. The experimental validation was done by miniature extrusion tests and the microstructure was investigated metallographically and by EBSD measurements.

Abstract: For the investigation of retained strains during hot forming, which are related to the dislocation structure, single and double hit compression tests were carried out at different temperatures and strain rates for a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations in both the deformed and recrystallized grains, respectively. At a given total amount of deformation in a double hit compression, the retained strains for the as deformed unrecrystallized grains are reduced for a lower deformation in the first hit. For the recrystallized grains the retained strain increases, when comparing double hit with single hit compression.

Abstract: During hot extrusion, tools experience cyclic thermo-mechanical loads that can lead to materials degradation and failure. For a process optimization and study of the occurring damage mechanisms, the finite element method (FEM) is an appropriate means. Local inelastic strains result from the interaction of the applied temperature and stress loading and can be computed by suitable inelastic constitutive equations. Stress amplitudes and dwell times during extrusion result in creepfatigue damage. A lifetime consumption model sums increments of a damage variable over time and defines materials failure as the accumulation of the resulting damage variable to a critical value. The predominant failure mechanism, i.e. creep or fatigue, can be found by the investigation of the damage rate over several cycles. A comparison of both a creep dominated (copper extrusion) and a fatigue controlled (aluminium extrusion) lifetime consumption in an extrusion die is shown with the hot work tool steel Böhler W300 ISOBLOC in comparison with W400 VMR (both ~ EN 1.2343).

Abstract: Double-hit compression tests were carried out at different temperatures and strain rates for a nickel based alloy and a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations. It is shown that at a given total amount of deformation, the retained strain is reduced for the as deformed grains that have not been recrystallized yet, but increased for the recrystallized grains, when comparing double hit with single hit compression tests.

Abstract: In the high temperature deformation window of the nickel base Alloy 80A the lower temperature region during open die forging was examined with regard to the materials formability. For that purpose, hot compression samples were investigated by means of EBSD and TEM in order to look at recrystallization, precipitations and ductile damage as well as their reciprocal effects. Further a microstructure model was used, which calculates the materials strengthening, softening and the particle kinetics. A micro mechanical damage model of the effective stresses was coupled with the grain structure development in order to describe a retarded damage rate due to the ongoing recrystallization.

Abstract: To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.