Papers by Author: P. Sláma

Abstract: Accumulative Roll Bonding (ARB) is a severe plastic deformation process that allows producing ultrafine-grained materials (UFG). UFG sheets exhibit enhanced strength and very fine grain structure. Foils used as fins in heat exchangers have to be very thin but must exhibit high strength combined with relatively high formability. Thus, materials produced using ARB may fulfil the exacting requirements on foil properties for such applications. The thermal stability of Al-Fe- Mn-Si foils produced using ARB and subsequent cold rolling was studied and compared with conventionally cold rolled (CCR) counterparts. The stability was assessed by isothermal annealing in the temperature range from 200 to 450 °C. Electron back scatter diffraction in a scanning electron microscope and transmission electron microscopy examinations of foils microstructure in the deformed and annealed states allowed the monitoring of structural changes. The magnitude of mechanical properties changes due to annealing was evaluated by microhardness measurements. Significant hardness increase was observed after annealing at 200 °C only in the ARB samples and was assigned to an annealing-induced hardening. The CCR foil exhibits higher non-recrystallized fraction and smaller mean lamellae boundary spacing in the temperature interval of 200-250 °C than ARB foils. The annealing at 450 °C results in identical hardness values and fully recrystallized microstructure of all foils, regardless the method used for their manufacturing. However, the ARB samples show higher stability of the refined substructure than their cold rolled counterparts due to continuous recrystallization occurring in the ARB foils.

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.

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.

Abstract: Twin-roll casting (TRC) is an advantageous substitution for Direct-Chill (DC) casting in the manufacturing of rolled aluminium products. The results of a study of the phase transformations and their interaction with recrystallization occurring during the annealing of TRC Al-Mn based alloys are reported. Four alloys with different contents of Mn, Si and Fe were investigated. Precipitation was studied by resistometric measurements in the course of a heating at linear rate. The microstructural processes responsible for the observed changes in resistivity were identified by TEM examinations of quenched specimens. The changes in the microstructure and solute content during homogenisations at 450°C and 610°C were monitored by conductivity and hardness measurements and polarised light microscopy. It was elucidated that the temperature and kinetics of phase transformations are influenced not only by the content of Mn, but also by Si content. In alloys with low Si content, the decomposition of solid solution and the transformation of primary phases occur in much larger temperature range than in the alloys with high Si content. The precipitation of Mn and Si, concurrent to recrystallization, was observed to retard the latter, especially in alloys with high Mn and Si content.

Abstract: It is known that the severe plastic deformation (SPD) induced by Accumulative Roll Bonding (ARB) results in more important grain refinement as compared to conventional rolling. Since ARB enables production of large amounts of ultra-fine grained (UFG) materials, its adoption into industrial practice is favoured. The paper presents the results of a study of high purity aluminium processed by ARB and cold rolling. Microstructure changes induced by both methods were studied by light and transmission electron microscopy. Dislocation density and arrangement were assessed by positron annihilation spectroscopy. Strength evolution was estimated by hardness measurements. Texture measurements were performed by X-ray diffraction. ARB processing results in over twofold increase in hardness. Hardness increases significantly after two ARB cycles and it raises only a little or decreases during subsequent cycles. The increase in hardness induced by conventional rolling is smaller. Positron lifetime measurements reveal a substantial increase of dislocation density at the first ARB cycle and a moderate increase or even a decrease at further cycles. The high fraction of positrons trapped at grain-boundary dislocations gives evidence for substantial grain refinement confirmed by TEM examinations. Grain size of 1.2 􀁐m in the rolling plane and as small as of 90 nm in the normal direction is obtained. The rolled samples have a typical rolling texture (􀁅-fibre). The 􀁅- fibre of the sample ARB processed to strain of 2.4 is weaker as compared to its rolled counterpart and it presents through thickness variations. The surface layers do not have any 􀁅-fibre orientations but they have ND-rotated cube texture formed by the shear strains induced by lubricant-free rolling.

Abstract: Inner panels of modern ecological cars are nowadays manufactured from Al-Mg alloys. Continuously twin-roll cast sheets are a cost effective substitution for direct-chill cast sheets. The effect of composition and down-stream processing on sheet properties should be well understood in order to produce high quality products, which exhibit good formability and high strength. Finegrained microstructure and well-balanced texture are the necessary pre-requisites. Results of the investigation of twin-roll cast AlMg2Mn0.8 and AlMg3 alloys are reported. Sheets of 1.0 mm gauge were prepared using different processing routes. The route involving homogenisation results in grain coarsening and anisotropy, however, deep-drawing ability is affected only a little.