Modification of Al–Si alloys with Ba, Ca, Y and Yb
Introduction
Modification of the eutectic in hypoeutectic Al–Si alloys is carried out extensively in industry to improve the mechanical properties, particularly ductility. The eutectic consists of a hard, brittle silicon phase in a softer aluminium matrix which is the reason why most of the mechanical properties of castings are determined by the eutectic microstructure. The most common aluminium foundry alloys contain between 5 and 12 wt% silicon and, therefore, the volume fraction eutectic in the alloy is between 40% and 100%. Silicon is a faceted phase and makes the Al–Si eutectic an irregular eutectic [1], [2].
Modification of the Al–Si eutectic from a flake-like to a fine fibrous silicon structure can be achieved in two different ways; by addition of certain elements (chemical modification) or with a rapid cooling rate (quench modification). Several elements are known to cause chemical modification. The most common elements used in industry today are Sr and Na, which changes silicon from coarse plate-like to a fine fibrous structure, and Sb which only causes a refinement in the plate-like silicon structure. Addition of other alkali, alkaline earth and rare earth metals have also been reported to cause modification, although very limited data is available in the literature [1], [2], [3], [4], [5], [6].
Thermal analysis has been used extensively to characterise the solidification characteristics of aluminium alloys. In parallel with metallographic studies, the technique has been used to determine the precipitation events in commercial alloys [7]. The technique has also been shown to be a good method to obtain information about the nucleation characteristics and grain refinement of the primary aluminium crystals in the early stages of solidification.
It is well-established that modification of the Al–Si eutectic is accompanied by a depression in the eutectic growth temperature [1], [2], [7]. It has recently been suggested that modification level in an alloy can directly, and automatically, be determined by analysis of the cooling curve [8]. However, the work only considers the impact of strontium and other elements may significantly impact on the thermal characteristics of the eutectic arrest as well as the degree of modification produced. Most of the previous characterisation of the eutectic arrest temperature has been performed with the most widely used modifiers, i.e. Na and Sr, and also a limited amount of work on calcium [1], [2], [6], [7], [8].
The aim of the present paper was therefore to investigate the effects of Ba, Ca, Y and Yb on the eutectic microstructure in an A356.0 alloy (Al–7%Si–Mg). Furthermore, the study of the effects of these additions on the thermal characteristics of the eutectic arrest, particularly nucleation and growth temperatures and recalescence, could provide further information about the mechanism causing chemical modification in hypoeutectic Al–Si alloys.
Section snippets
Experimental
The base alloy used in the experiments was an A356.0 alloy (Al–7%Si–Mg). The composition of the base alloy is given in Table 1.
The base alloy was melted in a 20 kW Inductotherm induction furnace and the temperature of the melt was kept at 730 °C. The melt was degassed with high purity argon gas for 10 min through a graphite rod immersed into the melt to ensure low hydrogen content. The surface of the melt was skimmed periodically. The modifying agent was added after degassing was completed.
Results
The yield, dissolution rate and fading behaviour were quite different for the four elements. The best result was achieved with ytterbium which dissolved quickly and with close to 100% yield. Only about 20% of the added calcium was dissolved into the A356.0 melt, and much of the calcium was observed to react at the surface of the melt. The dissolution rate was also observed to be relatively long, about 20 min. Fading was observed for extended holding times. The experience with barium was similar
Discussion
A modified eutectic structure is obtained with barium addition to the A356.0 alloy. The modification increases with increasing level of Ba and a fully modified eutectic structure was obtained with 890 and 1010 ppm Ba. A fully modified, fine and fibrous eutectic is also produced with calcium addition. The results show that the eutectic was modified at 36 ppm Ca, and the modification level increased with increasing Ca content. The best modification was found at 210 ppm Ca, but it is expected that
Conclusions
The modifying action of the elements Ba, Ca, Y or Yb in an A356.0 (Al–7%Si–Mg) alloy has been investigated. Samples were produced at a cooling rate of about 1 K/s just prior to nucleation of the primary phase. The results showed that all elements modify the eutectic to different degrees. Addition of Ba resulted in a very fine fibrous structure and the best modification among the elements and composition ranges studied in this work. Ca also caused modification of Si to a fine, fibrous silicon
Acknowledgements
This research is sponsored by a Large Grant from the Australian Research Council. Anne Knuutinen would like to thank the CRC for Cast Metals Manufacturing (CAST) for hosting her exchange from Jönköping University (Sweden) to the University of Queensland. The authors would like to thank John Taylor for input on this manuscript.
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