Precipitation of primary intermetallic compounds in liquid Al 11.5Si 0.4Mg alloy

The precipitation of primary intermetallic compounds from melts is often encountered in the aluminium casting industry, so that the formation, growth and sedimentation of intermetallics is usually viewed as having only a harmful influence on casting quality and production. However, such precipitation may remove both oxide films and harmful primary intermetallics from aluminium melts. Precipitation, therefore, may provide a new approach to purify liquid metals. However, little has been known about the precipitation of primary intermetallics in liquid aluminium-silcon alloy containing Fe, Mn and Ti. This research has investigated the mechanism of precipitation of primary intermetallic compounds in liquid Al 11.5Si 0.4Mg alloy. Precipitation was carried out at 600 degreesC for 10 minutes with two alloys; a base alloy with no added titanium (0.01%Ti on analysis) and an alloy with 0.17%Ti. The precipitating particles were examined and analysed using scanning electron microscopy. For the base alloy, only one precipitating particle, the primary iron-rich phase Al-15(FeMn)(3)Si-2, was found. For the higher level of Ti, in addition to the iron-rich phase, TiAl3 and Ti(AlSi)(2) were separately formed. The iron-rich phase was generally polyhedral or star-like in form (occasionally, a dendritic morphology of the primary iron-rich phase was also found) whereas TiAl3 formed as thick plates, and Ti(AlSi)(2) as thin platelets. The primary intermetallic compounds appeared to precipitate onto the wetted sides of the double oxide films.