Near-Surface Microstructure on Twin-Roll Cast 8906 Aluminum Alloy
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INTRODUCTION
TWIN-ROLL CASTING (TRC) is widely used for the production of aluminum foil stock. Compared to direct-chill (DC) casting, the TRC process allows aluminum alloys to be continuously cast at about 6 mm gage, reducing the downstream production process, e.g., with less rolling stages.[1] This provides a number of advantages such as lower capital investment cost, energy saving, and footprint saving.[2–4] Furthermore, in contrast to DC cast ingots, no scalping process is applied to TRC strip, reducing material consumption in the production process.[2] Consequently, material of the cast surface of the TRC strip is retained. Recent investigations of hot-rolled aluminum alloy surfaces have shown near-surface layers, which have a unique ultrafine-grained microstructure with a high population density of oxide particles decorating grain boundaries.[5–12] The electrochemical properties, corrosion behavior, optical properties, joining, and welding behavior are all significantly influenced by the nearsurface layer.[6,9,13–22] The formation of the near-surface layer is the result of the high levels of shear deformation in the surface/near-surface region during rolling.[5,8,10–12] The TRC process is basically a combination of casting and hot rolling, as the solidified alloy strip undergoes hot deformation between the caster rolls.[2] Therefore, it JUNJIE WANG, Research Associate, XIAORONG ZHOU, Reader, and GEORGE E. THOMPSON, Professor, are with the School of Materials, The University of Manchester, Manchester, M13 9PL, UK. Contact e-mail: [email protected] JOHN A. HUNTER, Group Leader, and YUDIE YUAN, Surface Scientist, are with the Novelis Global Research and Technology Center, Kennesaw, GA 30144. Manuscript submitted July 25, 2014. Article published online April 2, 2015 2688—VOLUME 46A, JUNE 2015
is reasonable to expect the formation of a near-surface layer on TRC aluminum alloy strip. Thus, an understanding of the surface/near-surface microstructure of TRC aluminum alloy strip is of considerable significance for control of the surface properties of the products. For example, the authors’ previous work showed that a near-surface layer is present on cold-rolled commercial foil stock AlFeSi alloy (8906 aluminum alloy) which is used for producing semi-rigid containers. The nearsurface layer has a significant impact on the tribological properties and wear characteristics of the alloy and, consequently, on the surface quality of the final products after a subsequent sheet forming process.[23] Further, the presence of the near-surface layer can also increase the susceptibility to cosmetic corrosion[13–15] and influence the optical property.[12] In the present work, the upstream TRC strip of foil stock AlFeSi (8906) alloy was investigated using optical microscopy and scanning and transmission electron microscopy. The aim is to understand the cast microstructure, particularly, in the surface/near-surface region of the TRC strip, providing further information for the quality control in the aluminum foil industry.
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