Thermophysical properties of Al-Cu and Al-Si liquid alloys

New workable aluminium-based light alloys are a key issue in current materials science. Al-Cu alloys, for instance AlCu4TiMg (A356), are the most utilized casting alloys in the aluminium industry. The distinctive characteristics of these alloys are low density, high melting temperature, good thermal conductivity and excellent oxidation resistance. Al-Cu alloys are ubiquitous in technical applications: they are the main components for screw machine products, truck frames, aircraft structures, jet engine impellers and aircraft engine cylinder heads. Moreover, binary Al-Cu alloys are in the focus of numerous, academic studies which are especially concerned with solidification processes and consider for instance, the columnar-to equiaxed transition, the formation of macrosegregation zones, the visualization of the dendritic growth, the impact of melt convection or the application of electromagnetic stirring. The solidification process of a liquid alloy has a profound impact on the structure and properties of the solid material. Therefore, the knowledge of the physical properties of the molten alloys prior to solidification becomes very important for the development of materials with predetermined characteristics.
Aluminium alloy castings have achieved wide usage in automobile, aerospace and other applications because of their high strength to weight ratio. Al–Si–Mg alloys, a group of heat treatable cast Al–Si alloys, exhibit good castability and corrosion resistance in addition to high strength to weight ratio. However, the pursuit of high quality castings with consistent mechanical properties depends upon the proper processing of the alloy, including grain refinement, modification and precipitation heat treatment, etc.
Among all physical properties, the structure-sensitive characteristics of the liquid phase play a prominent role. The density is directly related to the atomic structure and potential short range order of the liquid; on the other hand, it is a fundamental quantity for all technological applications, since it determines fundamental non-dimensional parameters characterising the fluid flow, such as the Reynolds or the Rayleigh number. Reliable information about the structural heterogeneity of the melt can be also obtained from studies considering the temperature dependence of the electrical conductivity and the viscosity. The knowledge of the viscosity is of particular importance considering the relationship between melt convection and solidification.
In that paper we present data of the thermophysical properties of Al-Cu liquid alloys (Al96Cu4, Al80Cu20, Al70Cu30, and AlCu4TiMg in wt.%) and Al-Si liquid alloys (AlSi7, AlSi7Mg, AlSi8Cu3), namely density, viscosity and electrical conductivity. The measurements cover a wide temperature range above the liquidus. The anomalies with respect to the concentration dependence of the electrical conductivity in some Al-Cu alloys are explained in terms of the s–d hybridization model. A comparison with data and scaling relations available in literature is given.