Deformation Geometry and Through-Thickness Strain Gradients in Asymmetric Rolling
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ASYMMETRIC conditions are introduced during rolling by making the velocities or diameters of the two work rolls different. While rolling under asymmetric conditions has consequences macroscopically on rolling loads (reduced in asymmetric rolling) and sheet flatness, more recent interest has shifted toward the possibility of using asymmetric rolling to control microstructure and texture (e.g., References 1 through 5). The control of microstructure and texture via asymmetric rolling relies on the ability to modify the through thickness strain distribution compared to that obtained via conventional rolling. In particular, asymmetric rolling can significantly modify shear strains arising from the contact conditions between the rolls and the sheet surfaces.[6] Several studies have shown that it is possible to obtain a quasi-constant shear strain distribution through the thickness of materials.[1,2,7] This is most directly interpreted based on the way in which the asymmetric rolling conditions modify the positions of the neutral points in the roll gap (Figure 1).[8] In the case of symmetric rolling, both neutral points (the position where the sheet velocity equals the roll velocity) are located at the same position within the roll gap. At this point, the direction of the contact friction reverses, assisting the movement of material into the roll gap on the entry side and opposing it on the exit side. This leads to the characteristic shear strain profiles, varying from positive on one surface to negative on the other, observed in conventional symmetric rolling. R. ROUMINA, Graduate Student, and C.W. SINCLAIR, Assistant Professor, are with the Department of Materials Engineering, The University of British Columbia, V6T 1Z4, Vancouver, BC, Canada. Contact e-mail: [email protected] Manuscript submitted September 5, 2007. Article published online July 17, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
In asymmetric rolling using different roll velocities, the neutral points associated with the two rolls are no longer located at the same position. The neutral point associated with the slow roll is shifted toward the entrance of the roll gap, while the neutral point associated with the fast roll is moved toward the exit of the roll gap. In the region between the neutral points, friction acts in the same sense on both surfaces and thus is often considered to induce a more nearly constant shear strain distribution through thickness. The clearest experimental evidence of the effect of asymmetric rolling on modifying strains in sheet metals is in the form of through thickness texture measurements. Studies on steels[2] and aluminum alloys[1,3,5,9] have reported similar results, showing that, when compared with symmetrically rolled sheet, asymmetrically rolled sheets exhibit textures that appear rotated about the transverse direction (TD). The degree of rotation of the textures is typically observed to vary through thickness and is sensitive to the rolling conditions (e.g., References 1 and 2). Such texture modifications c
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