Grain Refinement and Mechanical Properties Enhancement in Cross Roll Rolled Pure Copper
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Grain Refinement and Mechanical Properties Enhancement in Cross Roll Rolled Pure Copper Kuk Hyun Song, Han Sol Kim and Won Yong Kim Korea Institute of Industrial Technology, 7-47, Songdo-Dong, Yeonsu-gu, Incheon, 406-840, Korea. Email: [email protected] ABSTRACT To evaluate the microstructures and mechanical properties in cross-roll rolled pure copper, comparing with conventionally rolled materials, this work was carried out. Pure copper (99.99 mass%) sheets with thickness of 5 mm were cold rolled to 90% thickness reduction by cross-roll rolling (CRR) and subsequently annealed at 400 oC for 30 min. Also, to analyze the grain boundary character distributions (GBCDs), electron back-scattered diffraction (EBSD) technique was employed. As a result, the cold rolled and annealed materials consisted of significantly refined grains than that of the initial material (100 Pm). Especially, the CRR processed material showed more refined grain size (6.5 Pm) in average than that (9.8 Pm) of conventional rolling (CR). These grain refinements directly affected an increase in mechanical properties. Furthermore, the texture development in CRR processed material, in which grains were densely distributed in the normal direction (ND), was more effective to enhance the yield strength. Keywords: Cu, grain size, microstructure, hardness, strength. INTRODUCTION Cross-roll rolling (CRR), i.e., a new rolling process in which the roll axes are tilted against the transverse direction (TD) in the rolling direction (RD)–TD plane, was proposed by Chino et al [1]. Owing to the shear strain generated in this process, it can impose the higher effective strain on rolled sheets than conventional rolling (CR). Lately, SPD processes, such as equal channel angular pressing (ECAP), high pressure torsion (HPT) and high speed ratio differential speed rolling (HRDSR), have been studied with regard to obtaining of high strength, resulted from the grain refining [2-4]. These metals having a refined grain size, fabricated by SPD, attract considerable attention due to their attractive mechanical properties [5, 6]. Generally, the effective strain imposed by SPD directly affects the grain refinement during heat treatment. In other words, higher effective strain can increase stored energy in material, resulted in more nuclei sites during heat-treatment [7]. However, researches on CRR have been rarely reported until now. Therefore, this study was carried out to evaluate the microstructure and mechanical properties of CRR processed pure copper, comparing with CR processed material. EXPERIMENTAL DETAILS The material used in this study was pure Cu sheets with size of 30 mm × 50 mm × 5 mm, and CR and CRR as SPD methods were introduced on this material. Cold rolling was carried out to 90% thickness reduction using both cold rolling methods, particularly, CRR was carried out
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with retaining the roll mill condition in which roll axes were tilted by 5° from the transverse direction (TD) in RD-TD plane. Furthermore, to impose the more severe deformation, cold rolling
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