Refining Mechanism of Al-Fe-Si Compounds in Al-Si Alloy Die Castings Containing High Iron

Abstract

  Generally, high iron contents in Al-Si system alloys tend to coarsen Al-Fe-Si compounds. However, in the preliminary experiments of this study, the compounds in the die castings made of Al-17.8%Si-4.4%Cu-0.5%Mg-1.9%Fe-0.9%Mn-0.01%P alloy were found to be fine. Gravity casting experiments were conducted using high iron containing hypereutectic Al-Si alloys whose trace elements (phosphorus and strontium) were controlled to change the melt undercooling during the crystallization of the primary silicon. As a result of microstructure observations and thermal analysis, it was found that the compounds tended to be finer as the degree of melt undercooling increased. The refining mechanism of the compounds is discussed as follows. (1) It is assumed that when a primary silicon crystal solidifies in a highly undercooled melt, the concentration of silicon decreases and that of iron and manganese increases in the melt in the front of the primary silicon surface. (2) According to the assumption (1), the crystallization temperature of the compound is thought to rise near the surface of the primary silicon. (3) These two factors (namely, the melt undercooling during the crystallization of the primary silicon and the rise of the crystallization temperature of the compound) are thought to cause the constitutional undercooling that serves as the driving force refining the compounds.