Cross Flow During Twist Extrusion: Theory, Experiment, and Application
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is growing interest in scaling up severe plastic deformation (SPD) techniques ever since bulk ultrafine grained materials processed by these techniques have been found to possess unique properties.[1] Various SPD processes are now available, including high-pressure torsion,[2] equal channel angular pressing or extrusion,[3,4] multidirectional forging,[5] accumulative roll-bonding,[6] repetitive corrugation and strengthening,[7] twist extrusion (TE),[8] shear extrusion,[9] and torsion extrusion.[10] All of these methods have their own advantages that attract interest from both academia and industry. Several SPD techniques, such as high-pressure torsion, equal channel angular pressing, twist extrusion (TE), shear extrusion, torsion extrusion, employ simple shear as the main deformation mode. Simple shear was found quite distinct from the other deformation modes in terms of microstructural formation.[11—13] Along with other ROMAN KULAGIN, Junior Researcher, YAN BEYGELZIMER, Principal Researcher, and VICTOR VARYUKHIN, Director, are with the Donetsk Physics & Engineering, Institute of the National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine. Contact e-mail: [email protected] MARAT I. LATYPOV, Ph.D. Candidate, is with the Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea. HYOUNG SEOP KIM, Professor, is with Department of Materials Science and Engineering, POSTECH, and also with the Center for Advanced Aerospace Materials, POSTECH, Pohang 790-784, Republic of Korea. Roman Kulagin and Marat I. Latypov contributed equally to this work. Manuscript submitted August 7, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
popular SPD methods, TE has received considerable attention in the recent years, which is best illustrated by the advent of new SPD techniques inspired by and based on TE.[14—16] The interest in TE is attributed to a peculiar plastic flow of the material, which can be utilized for microstructural formation at different scales.[17—21] The principle of TE is to subject a bulk billet to torsion when pressing it through a die having the socalled twist zone (Figure 1). The surface of the twist zone is formed by ‘‘sweeping’’ the transverse* profile of *Throughout the current article, «transverse» and «cross» mean ‘‘normal to the extrusion direction.’’
the die along a helix line, which results in a ‘‘twist angle’’ between the inlet and outlet zones. A wide range of shapes for the transverse profile of the die, and thus the billet, can be used: square, rectangle, polygon, ellipse, etc.[8] The surface of the die channel is such that the transverse profile is the same along the extrusion axis. Therefore, the billet ideally preserves its initial geometry after a TE pass, which allows repeating the process and accumulating large plastic deformation in the billet. The mechanics of TE were analyzed in an introductory article.[8] It was shown that the material flow during TE can be decomposed into a helical flow and deviations from the helical flow. Herein, the helical flow means idea
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