Papers by Author: Miroslav Cieslar

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Authors: Margarita Slámová, Petr Homola, P. Sláma, Miroslav Karlík, Miroslav Cieslar, Yoshitatsu Ohara, Nobuhiro Tsuji
Abstract: Accumulative Roll Bonding (ARB) is a technique of grain refinement by severe plastic deformation, which involves multiple repetitions of surface treatment, stacking, rolling, and cutting. The rolling with 50% reduction in thickness bonds the sheets. After several cycles, ultrafine-grained (UFG) materials are produced. Since ARB enables the production of large amounts of UFG materials, its adoption into industrial practice is favoured. ARB has been successfully used for preparation of UFG sheets from different ingot cast aluminium alloys. Twin-roll casting (TRC) is a cost and energy effective method for manufacturing aluminium sheets. Fine particles and small grain size are intrinsic for TRC sheets making them good starting materials for ARB. The paper presents the results of a research aimed at investigating the feasibility of ARB processing of three TRC alloys, AA8006, AA8011 and AA5754, at ambient temperature. The microstructure and properties of the ARB were investigated by means of light and transmission electron microscopy and hardness measurements. AA8006 specimens were ARB processed without any problems. Sound sheets of AA8011 alloy were also obtained even after 8 cycles of ARB. The AA5754 alloy suffered from severe edge and notch cracking since the first cycle. The work hardening of AA8006 alloy saturated after the 3rd cycle, whereas the hardness of AA5754 alloy increased steadily up to the 5th cycle. Monotonous increase in strength up to 280 MPa was observed in the ARB processed AA8011 alloy.
Authors: Miroslav Karlík, Margarita Slámová, Petr Homola, P. Sláma, Miroslav Cieslar
Abstract: Mechanical properties and microstructure of twin-roll cast (TRC) pure aluminium, Al-Fe-Mn-Si (AA8006) and Al-Mg (AA5754) alloy sheets ARB processed at ambient and elevated temperatures (200, 250, 300 and 350°C) were investigated. Processing at elevated temperatures results in better bonding but it produces smaller increases in hardness. AA8006 specimens were processed without any problems up to 7 cycles. The alloy AA5754 suffered from severe edge and notch cracking since the first cycle. The strength was evaluated from tensile test and microhardness measurements; the microstructure was examined using light microscopy, and transmission electron microscopy. The microstructure was compared to that of conventionally cold rolled (CCR) specimens with true strain ε of 0.8, 1.6, 2.4 and 3.2 corresponding to the strain induced by 1 to 4 ARB cycles. The work hardening of alloy AA8006 saturated after the 3rd cycle, whereas the hardness of alloy AA5754 increased steadily up to the 5th cycle. Very fine grain structure with large fraction of high angle boundaries was observed in both alloys after two cycles of ARB. The grains were refined to submicrometre and nanometre size (down to 90 nm in alloy AA5754). Intensive post-dynamic recovery was observed in AA8006 specimens. The recovery is less pronounced in the AA5754 alloy with high concentration of solute atoms in solid solution.
Authors: Petr Homola, Margarita Slámová, Vladivoj Očenášek, J. Uhlíř, Miroslav Cieslar
Abstract: Ultra-fine grained (UFG) materials can be produced by several techniques involving severe plastic deformation (SPD). Accumulative Roll Bonding (ARB) is one of the SPD methods that enable the production of large amounts of UFG sheets. UFG sheets were prepared by up to six cycles of ARB at ambient temperature from an Al-0.22Sc-0.13Zr alloy in two states: a non-agehardened and a peak-aged. The effect of Al3(Sc1-xZrx) precipitates on the thermal stability of the UFG structures produced by ARB was investigated by isochronal annealing at temperatures between 200 and 550 °C. Additionally, the non-age-hardened ARB material was peak-aged prior to annealing and annealed together with both as-ARB-processed materials. The changes of microstructure and hardness due to annealing were studied. Annealing at 300 °C induces an additional strengthening in both non-pre-aged ARB materials that may be ascribed to precipitation and growth of coherent Al3(Sc1-xZrx) particles. This result suggests that the hardness decrease introduced by ARB in the peak-aged specimen is due to dissolution of precipitates during deformation. The annealing response of the materials above 300 °C does not depend on their thermal pre-treatment. However, the finely dispersed Al3(Sc1-xZrx) precipitates stabilise the refined deformed microstructure suitable for superplastic forming up to relatively high temperatures.
Authors: Luiz Eleno, Josef Vezelý, Bo Sundman, Miroslav Cieslar, Jacques Lacaze
Abstract: The present work provides a review of the information available on the Al-rich corner of the Al–Fe–Si system as well as a CALPHAD type assessment making use of the COST 507 database as a starting point. The description of the intermetallic compounds has been modified to account for substitution of Al and Si in the ternary Al-Fe-Si system and to take new experimental information into account.
Authors: Miroslav Cieslar, P. Vostrý, Ivana Stulíková
Authors: Margarita Slámová, Vladivoj Očenášek, Miroslav Cieslar, B. Chalupa, P. Merle
Authors: Margarita Slámová, Miloš Janeček, Miroslav Cieslar, Vladimír Šíma
Abstract: Heat treatable AlMgSi alloys are being used increasingly for automotive skin sheet. AA6016 sheets produced from direct chill (DC) cast ingots are the material of choice in Europe. However, manufacturing of sheets using twin-roll casting (TRC) is a cost effective alternative to DC casting. The effect of processing parameters on the age hardening of TRC AA6016 sheets was assessed. The influence of quenching temperature after solution annealing and subsequent pre-aging at 100 °C was studied. Sheets in the artificially aged condition were prepared using a simulated car-body paint baking procedure (180 °C/30 min). The precipitation kinetics, resulting dispersion of the agehardening phase and their correlations with properties were investigated by differential scanning calorimetry, transmission electron microscopy and tensile tests. It is demonstrated that pre-aging results in increased age hardening effect, especially in the material quenched to 20 °C after solution annealing. The differences in strength are ascribed to the differences in the nucleation rate of β'' precipitates during stabilisation and the formation of stable GP-I zones in naturally aged specimen. The differences in precipitate microstructure inherited from the pre-treatment affect precipitation kinetics during paint baking and result in different precipitate dispersion and sheet strength.
Authors: Miroslav Cieslar, Margarita Slámová, Michal Hájek, Jozef Veselý
Abstract: Fins in automotive heat exchangers are manufactured from Al-Mn-Fe-Si foils. Since continuous twin-roll casting (TRC) reduces energy and work consumption, it is the preferred manufacturing technology. The precipitation processes resulting from annealing of TRC Al-Mn-Fe- Si based alloys were studied by resistometric measurements during linear heating from room temperature up to 620 °C, hardness measurements and transmission electron microscopy examinations. Primary particles and precipitates of the cubic α-Al15(Mn,Fe)3Si2, hexagonal Al8Fe2Si and orthorhombic Al6(Fe,Mn) phases were identified. Phases of different types prevail in different alloys depending on composition. The increase of cold rolling prestrain prior to annealing induces a significant shift to lower temperatures of the start of precipitation. Prestrain accelerates precipitation kinetics by redistributing solute atoms and favoring their segregation on nucleation sites such as dislocations, subgrain and grain boundaries.
Authors: Krystof Turba, Premysl Malek, Edgar F. Rauch, Miroslav Cieslar
Abstract: Equal-channel angular pressing (ECAP) at 443 K was used to introduce an ultra-fine grained (UFG) microstructure to a Zr and Sc modified 7075 aluminum alloy. Using the methods of TEM and EBSD, an average grain size of 0.6 1m was recorded after the pressing. The UFG microstructure remained very stable up to the temperature of 723 K, where the material exhibited high strain rate superplasticity (HSRSP) with elongations to failure of 610 % and 410 % at initial strain rates of 6.4 x 10-2 s-1 and 1 x 10-1 s-1, respectively. A strain rate sensitivity parameter m in the vicinity of 0.45 was observed at temperatures as high as 773 K. At this temperature, the material still reached an elongation to failure of 430 % at 2 x 10-2 s-1. These results confirm the stabilizing effect of the Zr and Sc additions on the UFG microstructure in a 7XXX series aluminum alloy produced by severe plastic deformation.
Authors: Jakub Čížek, Ivan Procházka, Radomír Kužel, František Bečvář, Miroslav Cieslar, Gerhard Brauer, W. Anwand, Reiner Kirchheim, Astrid Pundt
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