Grain refinement mechanisms of hypoeutectic AlSi alloys

Abstract

The general concept of the grain refinement mechanism in hypoeutectic AlSi alloys has been adopted from that in wrought alloys without consideration of the influence of the silicon content. In the present investigation the commercial grain refinement practice in AlSi foundry alloys has been experimentally simulated by introducing synthetic TiB2 crystals directly into AlSi melts. To explore the possible use of a new generation of AlTiC master alloys in these alloys, the behaviour of synthetic TiC has also been studied. Experimental findings indicate that in the presence of Si, TiC crystals are highly unstable in aluminium melts. TiB2 crystals alone do not nucleate the α-phase, whereas in the presence of dissolved Ti a ternary AlTiSi interfacial layer is formed on the TiB2 which subsequently nucleates the α-Al, via a peritectic reaction. However, due to a drastic decrease in the peritectic temperature, the efficiency of grain refinement is greatly reduced. A theoretical analysis indicates that when boron is added in excess of TiB2 stoichiometry, or simply as AlB2, grain refinement occurs by a eutectic reaction.