STRUCTURE CHARACTERIZATION AND PRECIPITATION IN TWO Al-Mg-Si-Mn CASTING ALLOYS

The as-cast and heat treated structure of permanent mould and high pressure die castings of the AlMg5Si2Mn alloy has been investigated by differential scanning calorimetry, microhardness measurements, transmission electron microscopy and energy dispersive X-ray analysis. Inside the α-Al grains curved plate-like precipitates were detected for both alloys. Examination of these precipitates revealed a number of features, such as: (i) the composition of the precipitates is very close to the stochiometric Mg2Si compound; (ii) precipitates are aligned along dislocations; (iii) the precipitate density is much higher for the high pressure die castings where the α-Al matrix contains more dislocations than in permanent mould castings; (vi) precipitates lie inside the α-Al grains where they are randomly distributed. Between the Mg2Si lamellas precipitates were not observed; (v) homogenization of the alloy results in complete dissolution of the precipitates and during artificial aging new needle-shaped precipitates are formed