Equal Channel Angular Pressing of Canned 2124-Al Compacts: Processing, Experiments, and Modeling
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FORMING techniques that result in intense plastic strains that can exceed 400 pct, without significant changes in geometry, are a class of processing techniques generally referred to as severe plastic deformation (SPD). Examples of SPD include high-pressure torsion, twist extrusion, accumulative roll bonding, multidirectional forging, and cyclic extrusion and compression.[1,2] A particularly effective SPD technique is equal channel angular pressing (ECAP). Equal channel angular pressing has been extensively investigated for its capability to significantly improve mechanical properties of metals by effectively refining grain structures to scales ranging from the nano to the micro. Shape preservation of a billet entails applying ECAP processing on a billet repeatedly and accumulating strain levels that would generally yield failure in nonshape-preserving techniques. It is this multipass ECAP process that can transform an initially coarse-grained billet to an ultra-fine-grained structure that can be up to an order of magnitude smaller than that attained from conventional processing techniques. However, it has been experimentally observed that grain refinement induced by ECAP significantly decreases under repeated ECAP application such that, after a certain number of extrusion passes, the grain K.I. ELKHODARY, Graduate Student, formerly with the Department of Mechanical Engineering, The Yousef Jameel Science and Technology Research Center, The American University in Cairo, is with the Department of Mechanical and Aerospace Engineering, North Carolina State University. H.G. SALEM, Associate Professor, is with the Department of Mechanical Engineering, The Yousef Jameel Science and Technology Research Center, The American University in Cairo, Cairo 11511, Egypt. M.A. ZIKRY, Professor, is with the Department of Mechanical and Aerospace Engineering, North Carolina State University 27695-791, Raleigh, NC. Contact e-mail: [email protected] Manuscript submitted September 19, 2007. Article published online May 30, 2008 2184—VOLUME 39A, SEPTEMBER 2008
structure will no longer undergo any significant refinement.[3–5] An alternative approach proposed by several experimental investigations[6–11] has indicated that green or sintered powders can be processed by ECAP, in which it is desired to retain the ultrafine structure of the powder in the consolidated extrudate. The hypothesis is that the intense plastic shear involved with ECAP would be the main mechanism for the consolidation and the densification of the green and sintered powders. These two inter-related mechanisms would then yield the desired bulk properties and allow for the retention of the original fine structure in the powder. It has also been observed that full consolidation of aluminum powders by conventional extrusion was attained at temperatures of 450 C, while full consolidation was attained at 200 C with ECAP.[6,7] Moreover, Xia and co-workers have reported a synthesis of fully dense bulk aluminum compacts at temperatures as low as 100 C by ECAP.[8] Other approaches[8–11
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