Forschungsprojekte der Forschungsgruppe Metallurgische Verarbeitungstechnologie

Laufende Projekte

rotary swaging process

Figure 1: Schematic representation of the rotary swaging process (Source: FELSS Group GmbH).

Robert Kahlenberg, Project start 08/2020

Al-Mg-Cu alloys (2xxx series) are well-established materials in aviation industries, due to their high strength to weight ratio in combination with a decent ductility (fatigue resistance). The main process of interest in this project is the forming of tubular parts via rotary swaging, by which high dimensional accuracy is accomplished. However, many of the process parameters used today are primarily based on experience and several gaps remain with respect to the detailed microstructural evolution during processing.

Therefore, the project focuses on the investigation and the simulation of 2024 during production, using SimpleMSE (MatCalc). Especially the influence of the deformation on precipitation kinetics as well as recrystallization phenomena and the resulting mechanical properties are key aspects.

Diagramme

© E308-03-01

Figure 1: The second ductility minimum and some of its causes   Figure 2: An example for grain boundary segregation

TU Wien / K1-Met / voestalpine Linz / Primetals

Paul Estermann, Project start: 07/2019

Project description:

The problem of surface cracks originating on steels slabs has been a problem since the invention of continuous casting of steels more than 50 years ago. This problem is usually referred to as the second ductility minimum or intermediate temperature embrittlement. Several mechanisms are responsible for this embrittlement between 700 and 900 °C, including precipitates, ferrite formation and segregation. The aim of this project is to identify and characterize these influences using experimental techniques as well as simulations.

The experiments are performed using a Gleeble thermal-mechanical testing machine as well as a Dilatometer and the samples are analyzed using optical and electron microscopes. The simulations will use the materials calculator program MatCalc as well as JMat Pro. Comparing the measurements and the predicted values allows us to optimize the simulation parameters and find out where the particular steel will show brittle behavior as well as at which temperatures.

One of the mechanisms which will be of particular interest for this project is segregation. Of the two types of segregation, namely equilibrium and non-equilibrium segregation, only the second is relevant at the elevated temperatures and relatively short times present during the casting process. The process behind non-equilibrium segregation is the enrichment of solutes at grain boundaries via a flux of solute-vacancy complexes. These complexes form inside the grains because it is energetically favorable for these defects to combine for some solutes. Once the material is quenched, excess vacancies are annihilated at the grain boundaries and the solute-vacancy complexes break up to compensate this loss. As a consequence, a complex concentration gradient forms and the brought-along solute atoms can become enriched at grain boundaries and other vacancy sinks. A similar effect appears when the material is deformed, because certain microstructural structures can create vacancies during mechanical working.

Since the mechanism is based on a non-equilibrium effect, the grain boundary concentration can surpass the maximum equilibrium concentration. As soon as this threshold is reached, the solute atoms desegregate again, but their movement is limited by the speed at which they can diffuse. Consequently, the concentration increases sharply and then decreases slowly again over time. This mechanism can in some cases lead to rapid embrittlement at short holding times and at elevated temperatures, which may be a deciding factor for the ductility behavior of some steels.

 

[1] Caliskanoglu, O., 2016, Hot ductility investigations of continuously cast steels, Dissertation, TU Wien, Vienna (Figure 1)

[2] Li Y.J., Ponge D., Choi P. and Raabe D., 2015, Atomic scale investigation of non-equilibrium segregation of boron in a quenched Mo-free martensitic steel. Ultramicroscopy 159, 240–247. doi:10.1016/j.ultramic.2015.03.009 (Figure 2)

Abgeschlossene Projekte

 Flow curve simulations, Simulation of Mises stresses

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Figure 1: Flow curve simulations at various temperatures   Figure 2: Simulation of Mises stresses after a quenching process

Bernhard Viernstein, Start 01/2018

For the construction of automotive components, rather complex temperature- and deformation steps are necessary. To understand the material’s behavior, microstructural simulations are used beside experimental techniques, such as mechanical tests or microscopical characterizations. The aim of this work is to develop a physically based model, which is able to calculate internal stress responses of complex components in any material state. Therefore, strengthening mechanisms, such as solid solution strengthening, precipitation strengthening and work hardening need to be included. Compression tests at different temperatures are used to calibrate the model. Figure 1 shows exemplary flow curve simulations at various temperatures.

A new Abaqus user hardening (UHARD) subroutine is developed and used to calculate the component’s Mises stresses for any material state. Therefore, the temperature, the strain and the strain rate are the required input parameters. Figure 2 shows simulated Mises stresses after a quenching process. The simulations are experimentally verified in four chosen integration points.