Petr Homola

Multilayer composite al99.99/almg3 sheets prepared by accumulative roll bonding

Abstract Accumulative roll bonding (ARB) allows producing ultrafine-grained sheets of high strength and has been successfully used to prepare such materials from aluminium alloys. However, due to intensive work hardening of bond-rolled AlMg3 sheets, significant edge cracking occurred. Cracking was reduced by cladding AlMg3 with aluminium. Lamellar composites were thus prepared. Their structure was studied by electron backscatter diffraction and transmission electron microscopy; hardening was evaluated by microhardness measurements and tensile tests. Composite grain sizes are coarser than in the mono-material sheets. Accordingly, the evolution of strength with ARB-cycling in both materials differs. AlMg3 composite layers exhibit less work hardening than the mono-material; the composite strength is between that of the two mono-material ARB-sheets while the ductility does not differ substantially.

Accumulative roll bonding of AA8006, AA8011 and AA5754 sheets

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.

Thermal Stability of Ultrafine Grains in Al-Fe-Mn-Si Foils Prepared by ARB and Subsequent Rolling

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.

Effect of the Temperature of Accumulative Roll Bonding on the Microstructure and Properties of Twin-Roll Cast AA8006 Alloy

Accumulative Roll Bonding (ARB) does not require any special equipment and enables the production of large amounts of ultra-fine grained (UFG) materials. Grain refinement is thermally stable in materials with finely dispersed particles such as twin-roll cast (TRC) aluminium alloy sheets, favourable materials for manufacturing UFG sheets. The results of a study of the effect of ARB temperature on bonding quality, structure and properties of TRC AA8006 sheet are presented. Examinations by light and transmission electron microscopy, positron annihilation spectroscopy (PAS), hardness and tensile tests were used in the study. After two cycles at 200°C, mean grain size of 0.4 - 0.8 μm is achieved, but areas with extremely fine grains of 0.1 to 0.3 μm in diameter are also observed. Hardness increases significantly after two cycles and it rises a little in subsequent cycles. Processing at higher temperatures (up to 350°C) results in better bonding but it produces smaller increase in hardness. Significant increase of dislocation density is observed by PAS after the first cycle at 250°C but it does not continue during subsequent cycles. Partial recrystallization occurs in samples processed at temperatures above 250°C explaining the smaller increase in hardness. Softening level depends on both ARB temperature and number of cycles. The thermal stability of refined structures produced by ARB at 250°C is better than these formed at higher temperatures.

Annealing Response of Al-0.22Sc-0.13Zr Alloy Processed by Accumulative Roll Bonding

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.

Accumulative Roll-Bonding (ARB) of Sheets of Aluminium and its Commercial Alloys AA8006 and AA5754 at Ambient and Elevated Temperatures

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.

Microstructure, Texture and Property Changes of High Purity Aluminium during Accumulative Roll Bonding and Conventional Rolling

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.

Preparation of ultrafine-grained twin-roll cast AlMg3 sheets by accumulative roll bonding

Abstract Accumulative roll bonding is a method for achieving grain refinement of metallic materials by severe plastic deformation, resulting in strength superior to that of conventionally manufactured sheets. The method has been successfully used by the authors to prepare bulk ultrafine-grained sheets from pure aluminium and several twin-roll cast aluminium alloys. The present paper summarizes the results of a study aimed at monitoring the evolution of microstructure and mechanical properties during the accumulative roll bonding process of a twin-roll cast AlMg3 (AW – 5754) alloy. Moreover, the annealing behaviour of the roll-bonded sheets is compared with the behaviour of conventionally rolled counterparts.

Effect of Processing Parameters on Microstructure and Properties of Continuously Cast Al-Mg Sheets

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.