Twin Roll Casting of Al-Mg Alloy with High Added Impurity Content
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THE increasing need to recycle Al[1] is the driving force for researchers to develop and design new alloys with a wide range of impurities[2] and sustainable lowcost casting technologies.[3–6] In our previous studies,[7,8] an alloy containing a high level of impurities was designed based on the composition of a AA5754 alloy from a high level of recycled, cold-rolled aluminum sheet as the postconsumer scrap source. The presence of impurities like Fe, Si, and Cu react with other alloy elements during solidification and result in coarse secondary phases in the final ingot microstructure. During rolling deformation, these insoluble secondary phases break into fine clusters of particles. However, the sheet processed from the high-impurity alloy resulted in poor tensile properties and reduced formability compared with a purer alloy.[7] Twin roll casting (TRC) is a promising route to producing high-quality sheet.[9–15] Recently, researchers used TRC to produce sheet with a high-impurity content, specifically focused on Fe S. KUMAR, Research Fellow, and K.A.Q. O’REILLY, Lecturer, are with the Department of Materials, The EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineering, University of Oxford, Parks Road, Oxford OX1 3PH, U.K. Contact e-mail: [email protected] N. HARI BABU, Lecturer, and G.M. SCAMANS and Z. FAN, Professors, are with the BCAST, The EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineering, Brunel University, BCAST, Uxbridge UB8 3PH, U.K. Manuscript submitted October 8, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
levels.[16–18] In the current study, for the first time, we aimed to produce high-quality sheet from a very highly impure Al-Mg-Mn-Fe-Cu-Si alloy using TRC. Since the downstream-processing conditions (homogenization/rolling/extrusion) of the cast strip are dependent on the as-cast grain size and insoluble secondary phases present,[7] in the current study, an attempt was made to reveal the 3D nature of the secondary phases in the TRC strip and their influence on rolling deformation. A phase extraction technique is used for the first time to facilitate viewing the 3D nature of secondary phases in the as-cast and homogenized strip. In this technique, the secondary phases where extracted by dissolving the Al matrix in anhydrous boiling Butan-1-ol. A systematic study was performed to understand the changes in secondary-phase nature during casting and subsequent downstream processing. In addition, the current study also investigates the microstructural evolution during TRC and subsequent downstream processing. The tensile properties of sheets processed from TRC and book mold casting were compared, and a cost-effective processing route is proposed to produce high-strength and goodquality-formable sheets from highly impure alloys.
II.
EXPERIMENTAL
The composition of the primary (or pure) AA5754 Al alloy (PA) used in the current study is given in Table I. The primary alloy sheet was remelted in a clay graphite
Table I.
The Measured Chemical Composition of the A
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