Papers by Author: Michael Modigell

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Authors: Michael Modigell, Lars Pape
Abstract: When analysing semi-solid metal alloys or any other suspension in rheometers a major problem is related to wall slip effects which lead to an underestimation of the suspensions viscosity. Using smooth rotating rods in rotational rheometry for analysing suspensions a thin particle free layer of liquid is formed at the surface of the rod due to near-wall segregation. Thus, mainly the viscosity of this layer is measured which is much lower than the one of the bulk phase. Kiljanski [1] proposed a method for rotational rheometers to recalculate such measurements to get viscosity values without slip effects. A device-related change to minimize wall slip is the use of rotating rods with grooved surfaces. Therewith near-wall mixing is produced to destroy the particle free layer. Alternatively, the use of multiple blade vanes is proposed in the literature to avoid wall slip. The differences of the two measuring devices (grooved, vane) are discussed at the example of measurements for a low melting tin-lead alloy in the semi-solid state. It is figured out whether a vane geometry is also suitable to be applied for rheological measurements on metallic suspensions.
Authors: Andreas Bührig-Polaczek, C. Afrath, Michael Modigell, Lars Pape
Abstract: Alloys of a specific microstructure exhibit a shear-rate history-dependent flow behaviour in the semi-solid state. Rheological studies have been carried out using various experimental techniques such as concentric cylinder rheometers of Couette or Searle type as well as capillary rheometers and parallel plate compression tests. With the Searle type rheometer and the capillary rheometer two different approaches of rheological measurement techniques are discussed and compared regarding advantages, potentials and its distinct characteristics. Experiments using the Searle type rheometer have been conducted under steady-state or oscillating conditions and have been used to verify time-dependency of step responses due to discontinuous or linear variances of the shear rate. These experiments provide all types of flow curve parameters as implemented in the numerical simulation. However the variances of the commonly used Couette and Searle type rheometers are limited to shear-rates which are significantly lower than that appearing in the technical thixo- or rheocasting processes. For that reason an additional experimental set-up with a horizontal slot capillary integrated in a shot-controlled high-pressure die-casting machine is used in order to examine flow properties at shear rate conditions similar to those in real thixocasting processes. After an initial examination of low temperature model alloys, such as PbSn15, the experimental set-up is then adjusted in order to obtain rheological data for light metal alloys such as Aluminium AlSi7Mg0.3. The Aluminium alloy is processed within each rheometer either as partially solidified or partially remelted in order to represent conditions of both, the conventional thixocasting and the lately introduced rheocasting process. Finally the appearance of physical phenomena which affects the accuracy of rheological experiments such as wall slip is considered for both types of rheometers.
Authors: Michael Modigell, Lars Pape, Ksenija Vasilic
Abstract: The direct analysis and the inverse parameter fitting are methods for determination of unknown material parameters in rheometry. In this paper the efficiency of the both methods is examined on the case of a semi-solid alloy. The difference between the obtained material parameters is studied and the necessity of inverse parameter fitting is discussed.
Authors: Michael Modigell, M. Hufschmidt
Abstract: Oscillation experiments, creep tests and shear stress ramps have been performed to analyze the yield stress and its time dependency. It has to be distinguished between iso-structural, dynamic and static yield stress. The iso-structural yield stress occurs immediately after shearing. Since the slurry structure remains unchanged, it is equivalent to the structure during shearing. At rest an internal structure builds up, this leads to an increase of the yield stress, which is referred to as the dynamic yield stress. It increases until its maximum value, the static yield stress, is reached.
Authors: Annalisa Pola, Roberto Roberti, Michael Modigell, Lars Pape
Abstract: A new aluminum alloy (AlSi5Mg0.5Cu0.3Ag) for semisolid die-casting applications was designed, starting from computational thermodynamics calculations by Computherm Database. The goal was to obtain a combination of good castability and proper concentration of hardening elements for strengthening precipitation treatment. The predicted thixotropic properties were verified by measuring the microstructural conventional parameters, such as globule size and shape factor, and the solidification range, by means of differential scanning calorimetry. To complete the characterization of this new alloy and to evaluate its applicability in industrial production, the shear rate and time-dependent flow behavior of the alloy in the semisolid state was studied in a Searle-type rheometer. A future aim of the present research is to try to use rheology testing as the tool to optimize the chemical composition, in order to design an alloy characterized by good mechanical performances and easy processability. Considering the strong influence of the solid fraction content on semisolid alloy viscosity, the rheology tests were interrupted after a certain time and the alloy was deeply freezed using vaporized liquid nitrogen, in order to fix the microstructure and verify the correctness of the thermodynamic simulation.
Authors: Michael Modigell, Lars Pape, Horst R. Maier
Abstract: The investigation of the flow behavior of semi-solid steel alloys poses a great challenge by reason of high operation temperatures and the need of ceramic components for the measuring system. A high-temperature Couette rheometer has been developed to analyze flow properties of semi-solid alloys up to temperatures of 1500°C. In the present work the alloy under investigation is X210CrW12. Differently performed experiments clearly show its shear-thinning and time-dependent flow properties with a yield stress. This flow behavior is modeled using a Herschel-Bulkley approach where the experimental results achieved serve to identify model parameters.
Authors: Michael Modigell, Siri Harboe, Annalisa Pola, Christoph Zang
Abstract: The improvement of mathematical models for semisolid alloy flow properties requires profound understanding of the underlying physical nature. To date, it is commonly accepted that the shear thinning behaviour of these suspensions is caused by the solid phase microstructure, while the liquid phase is assumed to be Newtonian with a viscosity in the lower mPas-range. Recent measurements however, demonstrate non-Newtonian behaviour of fully liquid metals with pronounced shear thinning and high viscosities (multiple Pas) in the low shear-rate range. By gathering and analysing rheological measurement data of various alloys (Sn14.2%Pb, A356 and X210CrW12), the relevance of the new findings for semisolid metals is investigated. The results indicate that the previously unexamined non-Newtonian flow behaviour of the liquid matrix has, besides the solid fraction, the most dominant influence on the shear thinning behaviour of semisolid alloys. The influences of shear-rate and solid fraction are nearly independent of each other which allow the construction of master-curves; a general flow curve for the suspension where the solid fraction is considered by a scaling factor. Consequently, a modelling approach is suggested in which the dependency of solid fraction is considered independently of the shear-rate.
Authors: Siri Harboe, Michael Modigell
Abstract: An important aspect of the rheological characterization of semi-solid metals is the determination of the materials ability to exhibit yield stress. The yield stress is the stress level at which the material cease from purely elastic to plastic deformation. In semi-solid metal suspensions the yield stress is not a constant value, but depends on the transient state of the material. The investigation of yield stress in semi-solid metal alloys has been initiated in previous works, however, a profound understanding of the influence of the material history on the yield stress is so far lacking. In semi-solid metal suspensions, the physical cause of the yield stress is generally described by the formation of a particle "skeleton" structure which can hold a certain stress level without being subject to irreversible deformation. The yield stress in alloy suspensions can be described as a function of structural properties i.e. the solid fraction and the particle size, form and spatial distribution. It has been observed in previous works that the yield stress of semi-solid metals depends on the duration of resting time. It is generally assumed that this is due that as the material is resting, the particle network build up over time leading to an increase in the yield stress. In addition, we assume that the yield stress is also a function of the shear history as this affects the particle spatial distribution. The goal of the proposed work was to investigate the yield stress in semi-solid alloys including its dependency on the materials shear history and resting time. This was investigated both from a structural and a rheological point of view. The yield stress of the semi-solid alloys was measured with shear stress ramp and oscillation tests. Structural imaging measurements of quenched samples were performed to correlate structural parameters with the value of the yield stress. However, in semi-solid alloys, the experiments performed at a long time scale are influenced by aging of the material (Ostwald ripening). Therefore, rheological experiments were also performed with synthetic suspensions to investigate the long term yield stress dependencies without errors caused by the aging effects.
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