Evolution of Fe Bearing Intermetallics During DC Casting and Homogenization of an Al-Mg-Si Al Alloy
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UCTION
DUE to their high strength and good formability, AA6xxx Al alloys have found widespread applications as structural materials in the transport and building industries.[1] The alloys are direct chill (DC) cast in billet form suitable for subsequent extrusion. Due to the non-equilibrium solidification conditions in DC casting, most of the solute elements (in this system, Fe, Si, and Mg) segregate into the inter-dendritic regions and grain boundaries of the primary a-Al.[2] This solute-rich liquid leads to the formation of the inter-dendritic secondary phases, such as Fe-rich intermetallic compounds (Fe-IMCs) and strengthening Mg2Si precipitates.[3,4] Previous studies show that ac-AlFeSi and b-AlFeSi are the two dominant Fe-IMCs in 6063 Al alloys.[5–8] 3D analysis of these Fe-IMCs after extraction from the primary a-Al using an Al dissolution approach revealed that ac-Al(FeMn)Si (from now referred to as ac-AlFeSi) had a dendritic-like morphology while b-AlFeSi had a plate-like morphology.[9] Among these IMCs, it has been noted that b-AlFeSi in particular, because of its more planar geometry, reduced the ductility of an Al alloy 6063.[6] Therefore, a post-cast homogenization heat treatment is used commercially to encourage transformation of b to a to allow: (a) more reliable
S. KUMAR, Research Fellow, P.S. GRANT, Professor, and K.A.Q. O´REILLY, Professor, are with the Department of Materials, The EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineering, Parks Road, Oxford, OX1 3PH, UK. Contact e-mail: [email protected] Manuscript submitted October 8, 2015. Article published online April 13, 2016 3000—VOLUME 47A, JUNE 2016
downstream deformation, typically by extrusion, (b) improved mechanical properties, especially toughness and elongation to failure, and (c) improved surface finish.[5,10–16] Despite the apparent maturity of AA6xxx alloys, there is a significant on-going effort to optimize homogenization conditions in terms of properties, while also minimizing the homogenization time. Among the factors that govern the homogenization response, the initial cast microstructure and the alloy chemical composition play key roles. Therefore, in developing new solidification processing routes[15] or advanced solidification technologies[17–20] that seek to promote more favorable as-cast microstructures, it is important to understand better the link between microstructural evolution in casting and the subsequent homogenization response of the secondary phases. For example the cooling rate and solid/liquid growth velocity in DC casting significantly affect the proportions of ac-AlFeSi and b-AlFeSi in the final microstructure.[8,10,21,22] But, since the cooling rate and growth velocity vary from position to position in the billet during DC casting,[2,23] the primary Al grain size and proportion of the Fe-IMCs and Mg2Si across the cross section of the billet also varies, which should be accounted for in homogenization heat treatment optimization. In this paper, we apply an IMC phase extraction technique,
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