The effect of main alloying elements on the physical properties of Al–Si foundry alloys F. Stadler a , H. Antrekowitsch a,n , W. Fragner b , H. Kaufmann c , E.R. Pinatel d , P.J. Uggowitzer e a Institute of Nonferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Straße 18, A-8700 Leoben, Austria b AMAG Casting GmbH, Postfach 32, A-5282 Ranshofen, Austria c AMAG Austria Metall AG, Postfach 32, A-5282 Ranshofen, Austria d Dipartimento di Chimica and NIS, Universit  a degli Studi di Torino, via Giuria 7/9, Torino 10125, Italy e Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, CH-8093 Zurich, Switzerland article info Article history: Received 28 August 2012 Received in revised form 25 September 2012 Accepted 27 September 2012 Available online 3 October 2012 Keywords: Al–Si foundry alloy Compositional variation Thermal conductivity Thermal expansion Thermal shock resistance abstract In this study we describe the effect of the main alloying elements Si, Cu and Ni on the thermal properties of hypoeutectic and near-eutectic Al–Si foundry alloys. By means of systematic variations of the chemical composition, the influence of the amount of ‘second phases’ on the thermal conductivity, thermal expansion coefficient, and thermal shock resistance is evaluated. Thermodynamic calculations predicting the phase formation in multi-component Al–Si cast alloys were carried out and verified using SEM, EDX and XRD analysis. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared to theoretical models for the thermal conductivity and thermal expansion of heterogeneous solids. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Today, power train components of modern transportation vehicles such as engine blocks or gearbox housings are frequently produced from recycled Al–Si foundry alloys on high-pressure die casting machines – a cost-efficient combination of material and process applicable for sustainable mass production. The acceler- ated need for weight reduction, however, leads to higher mechan- ical and thermal loading of these aluminium castings in future vehicles, requiring improved Al–Si foundry alloys. Therefore, in the last couple of years, several investigations were carried out with the objective of improving the mechanical properties of Al–Si foundry alloys at elevated temperatures [1–6]. However, it has taken a longer time to recognise the importance of the physical properties. In addition to high temperature strength, adequate thermal conductivity (TC) as well as low thermal expan- sion are crucial physical properties for alloys used as motor components. In case of pistons, e.g., the heat generated in the course of the compression process has to be removed as quickly as possible to avoid thermal stresses and hot spots on the surface, whereas low thermal expansion prevents the piston from becoming tight and seizing under operation temperature. TC as well as the coefficient of thermal expansion (CTE) can therefore play a major role in deter- mining the life time of certain motor components [7]. TC is a measure of the rate at which heat is transferred through a material. It is mainly governed by electric conductivity, elastic vibrations of the lattice (phonons) and thermal consumption processes (specific heat). If the contribution of phonons is negli- gible (this is the case for pure metals), TC is mainly influenced by the mobility of electrons, i.e. the electric conductivity, s e . It is well known that s e and thus TC is significantly decreased by the addition of alloying elements, whereupon elements in solid solution result in lower values than the same amount of elements forming intermetallic phases. The latter typically reduce thermal conductivity proportionally with increasing volume fraction [8]. The thermal expansion coefficient of alloyed metals often varies according to the expansion coefficients of the solute and the solvent elements. By alloying elements with a low CTE, the coefficient of the alloy can be decreased. Additions of Si, Cu and Ni reduce CTE in approximately a linear manner, whereas Mg or Zn can increase the expansion. Generally, the effects of alloying additions on thermal expansion are additive, following the rule of mixtures [9,10]. Since Si, Cu and Ni are the main alloying elements in Al–Si cast alloys used for high-temperature applications, the understanding of their effect on TC and CTE is fundamental defining reasonable concentrations of these elements in the respective alloys. Conse- quently, this work aims at quantifying the effect of single Si, Cu and Ni additions as well as their combined influence on TC and Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.093 n Corresponding author. Tel.: þ43 3842 4025200; fax: þ43 3842 4025202. E-mail address: helmut.antrekowitsch@unileoben.ac.at (H. Antrekowitsch). Materials Science & Engineering A 560 (2013) 481–491