Grain Refiner Settling and Its Effect on the Melt Quality of Aluminum Casting Alloys

Grain refiner particles, which are intended to induce the formation of fine equiaxed grain structure during the solidification of aluminum alloys, are prone to settling during the holding of the liquid metal, which phenomenon can affect not only the grain size but the spatial distribution of the double oxide films in the melt. In this study, the settling of Al₃Ti inoculant particles, as well as its effects on melt quality and grain refinement, were studied. During the experiments, the Ti-concentration of a liquid Al-Si-Mg-Cu alloy was increased to 0.3 wt.% by the addition of Al-10%Ti master alloy at different melt temperatures. Particle settling and grain size evolution were studied by quantitative metallography, while the interactions of grain refiners and bifilms were investigated by scanning electron microscopy (SEM). The evolution of melt quality was assessed by the computed tomographic (CT) analysis of reduced pressure test (RPT) samples. It was found that effective grain refinement was only realized when the introduced blocky Al₃Ti particles were dissolved and re-precipitated in the form of (Al,Si)₃Ti at a lower temperature. Without dissolving at higher holding temperatures, Al₃Ti particle settling has taken place within 10 min. The settling of (Al,Si)₃Ti particles improved melt quality by the aided sedimentation of bifilms in the melt.