Effects of the Mn/Fe Ratio and Cooling Rate on the Modification of Fe Intermetallic Compounds in Cast A356 Based Alloy with Different Fe Contents

In cast Al-Si alloys, the platelet beta type Fe intermetallic compound has been considered as most detrimental Fe compounds to their mechanical properties, especially ductility. When the Fe is inevitable, to enhance the properties of the cast aluminum alloys, the modification of Fe compounds from the platelet beta type Fe compound to a more compact and less harmful alpha type Fe compound having the Chinese script morphology becomes the efficient and important way. This work mainly investigated how did three factors, Fe content, Mn/Fe ratio and cooling rate combined influence this modification process in cast A356 (JIS, AC4C) based alloys. The results revealed that with increasing the Fe content in the alloy without Mn addition, the size of the platelet beta type Fe compound increases significantly under the same cooling rate. The Mn addition was effective to modify the platelet beta type Fe compound to more compact Chinese script and/or polyhedral shape alpha type Fe compound. The ratio of these two kinds of alpha type Fe compounds mainly depends on the Mn/Fe ratio and the cooling rate in a given Fe level. The further research work about the solidification sequence of Fe compounds during casting revealed that in the alloys without Mn addition, the Fe compounds only appeared as a platelet shape. With Mn addition, the Fe compound crystallized as the platelet beta type compound at the beginning of the solidification. With the solidification proceeding, the Chinese script alpha type Fe compound became the dominant one. The formation of the polyhedral shape alpha type Fe compound mainly came from the growth of the Chinese script alpha type Fe compound. The ratio of these three Fe compounds was greatly influenced by the Mn/Fe ratio and cooling rate in a given Fe content.