Observations on the formation of β-Al₅FeSi phase in 319 type Al-Si alloys

Abstract

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The β-Al₅FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy. The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied. The volume fraction of β-phase formed was seen to increase with the decrease in cooling rate (i.e. with increasing distance from the chill) in the untreated alloy. Sympathetic (preferential) nucleation of the β-iron needles was also observed, in which the branching of β-needles from a parent needle resulted in the formation of large β-needle entities that can cover distances of ∼ 1200 μm across the matrix surface. The beneficial effect of modification, i.e. strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the β-needles. The strontium “poisons” the sites where sympathetic nucleation takes place. Dissolution was accelerated with increasing strontium content, the optimum level being ∼ 300 p.p.m. Grain refining, on the other hand, negated the beneficial effect of modification, in that the β-needles underwent thickening and the sympathetic nucleation/branching also occurred. The modified alloy was found to possess the lowest volume fraction of β-Al₅FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys.