Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Abstract

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.

A new multi-zone model for porosity distribution in Al–Si alloy castings

“…When there is a sufficient thermal gradient, Na-modified eutectic cells grow with a relatively coherent, planar front [10][11][12][13][14]. This is not the case for Sr-modified eutectic cells.…”

“…Al-Si eutectic cells nucleate in the interdendritic regions (from the surface of Al dendrites in unmodified alloys and independently in modified alloys [10,11,14,15,17]) and then grow into the surrounding intergranular areas, the extent depending on the scale of the eutectic cells. In Figure 1 the eutectic cell boundaries are distinguished by the variation in shade which is a result of variations in the size and shape of the eutectic Si particles, as discussed in Tiedje et al [16] Close to the casting surface the eutectic cells appear to grow in a densely-packed structure (i.e.…”

“…However Sr has a much greater effect on nucleation than Na. Undercooling prior to nucleation is larger in Sr-modified alloys and the eutectic cell size is approximately 10 times greater than in unmodified alloys, see Table 3 and References [11,12]. Therefore Sr-modified alloys do not easily form a solid shell because eutectic nucleation is postponed until a large part of the casting is undercooled.…”

“…In recent years it has been shown that modification not only changes the morphology of the eutectic silicon but also changes the mode of eutectic solidification [10][11][12][13][14][15]. Both Sr and Na lower the temperature at which the eutectic grows.…”

“…Strontium creates an initial undercooling followed by recalescence during which the growth temperature of the eutectic increases gradually to reach a maximum slightly below the unmodified eutectic growth temperature. Towards the end of solidification, the undercooling increases again as the final melt pools trapped between the eutectic cells 1 solidify [11]. The varying growth conditions 1 To avoid confusion, in this paper, the term "cell" (cell size, cell shape/morphology, etc.)…”