Alloy Composition and Dendrite Arm Spacing in Al-Si-Cu-Mg-Fe Alloys
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THE microstructure of commercial hypoeutectic AlSi-Cu-Mg alloys consists of primary a-Al solid solution, Al-Si eutectic and various intermetallic phases such as bAl5FeSi, Al2Cu, Mg2Si, Al5Cu2Mg8Si6, and a-Al15 (Fe,Mn)3Si2, the latter when Mn is added to limit the amount of b-phase. The sequences of reaction are well documented[1] and these, along with the amounts of each phase formed, can be predicted with acceptable reliability using current thermodynamic software packages such as ThermoCalc. Silicon is the main constituent in these alloys and as its content is raised the amount of the Al-Si eutectic phase increases, whereas the relative amount of primary a-Al dendrites decreases. The latter also exhibit a changing morphology (from globular, to rosette, dendritic, and fully orthogonal dendritic) as the Si content increases through the range of 1-10 mass pct.[2] Dendritic structures are normally characterized by the secondary dendrite arm spacing (SDAS), k2, which, in multicomponent aluminum alloys[3] and for a given solidification rate,[4] is thought to be controlled through constitutional undercooling effects.[5] k2 is related to the solidification time, tf, according to: THARMALINGAM SIVARUPAN, Doctoral Student, and JOHN A. TAYLOR, Principal Research Fellow, are with the CAST Co-operative Research Centre, Materials Engineering, School of Engineering, The University of Queensland, St Lucia, QLD 4072, Australia. CARLOS H. CACERES, Reader in Casting Technology, is with the Materials Engineering, School of Engineering, The University of Queensland. Contact e-mail: [email protected] Manuscript submitted October 31, 2012. Article published online May 9, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
k2 ¼ Kðtnf Þ
½1
where K and n are the alloy-dependent constants, and tf is the time interval between the liquidus and the solidus, which, for a given heat-extraction rate, depends upon the alloy’s composition.[3,6] The values of n are usually between 0.33 and 0.5, whereas the K-value for an AA357 alloy is about 20.8.[7] An Al-7Si-0.5Mg alloy with/ without treatment showed an n value of 0.3 with different K values.[8–10] For a given alloy composition, the temperature difference between the liquidus and the solidus, Tliq–sol, is constant; Eq. [1] then becomes: k2 ¼ aðRn Þ;
½2
where a ¼ KTnliqsol ; and R is the average cooling rate over tf. In the specific case of the industrially important Al-SiCu-Mg-(Fe/Mn) alloys (the alloy family includes, among others, alloy A319), an increase in solute content has been reported to generally improve the interdendritic feedability, reduce the porosity level, and decrease the SDAS.[3,11–14] The SDAS is also an important parameter when ductility is considered: generally, the smaller the dendrite arms, the finer the intermetallics, (as well as the eutectic Si particles in the nonmodified varieties) and the higher the ductility.[15] Insufficient ductility in the stronger versions of the alloy can be a limiting factor in critical applications. However, detailed studies of the effect of Si, C
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