Discussion of “On the Refinement Mechanism of Silicon in Al-Si-Cu-Zn Alloy with Addition of Bismuth”*

Abstract

Despite having been proposed many times, with respect, it is not easy to see the logic underlying this approach. The surface tension of the liquid alloy (a liquid/gas interface) bears no clear relation to the mechanisms of the growth of the silicon phase (on a liquid/solid interface). Nor is it clear that any enhanced ‘wetting’ of the liquid alloy on the Al dendrites occurs, and whether this would in fact aid the formation of silicon. The authors seem unaware that a new theory of ‘modification’ of Si in Al-Si alloys has been proposed which appears to have been completely successful to explain all the features observed so far concerning this complex process. [2] It is based on the fact that during the casting of most alloys, the turbulence of the pouring action entrains the surface oxide on the liquid in the form of fragments of doubled-over oxide films, which have been called bifilms. Al-Si alloys are usually full of a suspension of bifilms. X-ray radiographs reveal snowstorm-like conditions. [3] It seems that silicon nuclei, AlP, precipitate on either side of the bifilms, initiating the growth of Si on the bifilms and straightening them during the lateral growth of the silicon flakes. The unbonded central interface in the bifilm forms the familiar crack often seen down the center of silicon flakes, and minor folds in the bifilm create the often-seen transverse cracks. In this way, the silicon particles appear to be brittle, and their content of cracks, incorporated during their growth, contributes to the impairment of properties. This coarse morphology is the so-called ‘unmodified’ structure.