Iron-Bearing Compounds in Al-Si Diecasting Alloys: Morphology and Conditions Under Which They Form

摘要

Relatively large amounts of iron (up to 2.0 percent) are usually added to Al-Si diecasting alloys, mainly to mitigate their tendency to stick to the die components. Since iron has a low solubility in Al (less than 0.05 percent at 655 C), Fe-bearing phases invariably form in these alloys. The Fe-bearing phases may appear in a variety of shapes, depending on the alloy chemistry, the casting conditions and the melt treatment. Typical morphologies of Fe-bearing compounds include large and small needles, Chinese script, star-like shapes and polyhedral particles. Since the properties of aluminum alloys depend largely on the amount and morphology of their Fe-bearing phases, a systematic study was conducted to elucidate the effect of alloying elements on the formation and the morphology of these phases. It was found that the ratio of Fe:Mn:Cr has a significant effect on the morphology and size of the Fe-bearing phase. At low Mn and Cr levels, the Fe-bearing phase is in the form of needles. With increasing levels of Mn and Cr, the needles are replaced by Chinese script and/or polyhedral and star-like particles. The addition of Mn favors the formation of Chinese script, and the addition of Cr favors the formation of polyhedral and star-like particles. The effect of cooling rate on the morphology of the Fe-bearing phase is also illustrated and the temperature range over which the various morphologies form is determined.