A Novel Rolling Approach to Refining the Microstructure and Enhancing the Mechanical Strength of Pure Aluminium
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TRODUCTION, STATE OF ISSUE AND MOTIVATION
NANO-GRAINED (NG) materials are defined as materials with a grain size of less than 100 nm, whereas ultrafine-grained (UFG) materials are defined as materials with a grain size in a range of 100 to 1000 nm. These materials feature an elevated amount of grain boundaries and, in comparison with their coarse-grained counterparts, exhibit superior properties, such as excellent mechanical strength,[1,2] electrical conductivity[3,4] or enhanced corrosion resistance.[5] Nowadays, there are many methods of plastic forming that can be used to produce metals having a refined microstructure, and thereby increased strength. About a dozen such methods are well-known and widely applied. These include: (1) severe plastic deformation—SPD[6] (which fulfills the criteria of inducing heavy strain without any change in the overall dimensions of the workpiece and forming
MARTA ORŁOWSKA, KRZYSZTOF TOPOLSKI, and MAŁGORZATA LEWANDOWSKA, are with the Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland. Contact e-mail: [email protected] Manuscript submitted August 27, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
high-angle grain boundaries) and (2) other methods of plastic forming, which can be described as conventional. Based on numerous data in the literature, it can be concluded that the most important and most developed SPD methods, whose effectiveness in grain refinement has been proved, are: equal channel angular pressing (ECAP),[7] modifications of ECAP (e.g. rotary-die ECAP, parallel channel ECAP, ECAP-Conform, Incremental ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB), multi-axial forging (MF), twist extrusion (TE), cyclic extrusion and compression (CEC), and repetitive corrugation and straightening (RCS).[8–11] The most important conventional methods of plastic forming used for the production of NG and UFG metals include: caliber rolling (CR),[12,13] symmetric flat rolling (SFR),[14,15] asymmetric flat rolling (AFR),[16,17] hydrostatic extrusion (HE),[16] and rotary swaging (RS).[17] As for producing NG and/or UFG pure aluminium using methods that involve rolling, the most popular method seems to be ARB,[18–20] one of the most common SPD techniques. The use of conventional plastic working processes such as SFR[21,22] and AFR[20] have also been reported. The ARB method permits effective grain refinement into the UFG and NG ranges. It is mainly used on aluminium and its alloys. In theory, the rolling can be repeated indefinitely, which makes it possible to obtain virtually unlimited strain and materials having very
small grains. The reported grain refinement for commercial purity aluminium of 99.50 wt pct is about 300 nm,[19,20] which makes the ARB process more efficient than ECAP in terms of grain refinement and mechanical strength improvement, due to the different deformation modes it offers.[21] For example, in Reference 23 AA1050, a sheet with a purity of 99.5 pct was subjected to 9 ARB passes conducted a
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