High Strength and Ductility of Ultrathin Laminate Foils Using Accumulative Roll Bonding and Asymmetric Rolling
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INTRODUCTION
PRODUCT miniaturization is a trend for facilitating product usage, enabling product functions to be implemented in microscale geometries, and aimed at reducing product weight, volume, and cost.[1] A number of microchannels and microcups have been produced by sheets/foils microforming.[2,3] The smaller the microparts, the thinner the foils need to be. These foils should be strong enough to maintain the structural stability of the microparts. In addition, the size effects on the behavior of microformed materials have assumed an increasing importance.[4-6] For rolled monolayer foils, the size effect is often evident in drawn cups. Recently, Fu and Chan[7] carried out a review of microforming technologies, and they noted that the fracture strain decreased with a reduction in the workpiece size in tensile tests on sheet metal. It was observed that fracture results from localized shear in individual grains. Thin cold-rolled and annealed copper foils of varying thicknesses with a scaled geometry and comparable microstructure were tested in tension by Simons et al.[8] They HAILIANG YU, Vice-chancellor’s Research Fellow, is with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, Wollongong, NSW 2500, Australia, and also Professor with the School of Mechanical Engineering, Shenyang University, Shenyang 110044, P.R. China. Contact e-mail: [email protected] KIET TIEU, Professor, CHENG LU, Associate Professor, and AJIT GODBOLE, Senior Research Fellow, are with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong. SYAMSUL HADI, Ph.D. Student, is with the School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong and also with Mechanical Engineering Department, State Polytechnic of Malang, P.O. Box 04 Malang 65100, East Java, Indonesia. CHARLIE KONG, Senior Research Officer, is with the Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted May 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
found that when the foil thickness was reduced from 250 to 10 lm, the fracture strain decreased from approximately 0.2 to 0.002 for cold-rolled foils and from 0.35 to 0.15 for annealed foils, respectively. To study the size effects on fracture, Fu and Chan[9] conducted tensile tests on annealed pure copper foils with varying thicknesses and grain sizes. It was found that the flow stress, fracture stress/strain, and the number of microvoids on the fracture surface decreased with decreasing specimen size/grain size ratio. Nanostructured/ultrafinegrained sheets/foils fabricated using Severe Plastic Deformation (SPD) techniques can be used to offset size effects on workpieces during microforming and to improve the structural stability of microparts. Ma et al.[10] used an SPD technique, called equal channel angular pressing, to fabricate ultrafine-grained copper sheets which were subsequently manufactured in a micro-deep drawing process. They found that the ultrafine-grained copper has p
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