Effect of Sr on primary α-Fe phase in liquid Al-11.5Si-0.4Mg cast alloy

The effect of Sr on the sedimentation of primary alpha-Fe phase has been investigated at superheating temperatures of 760 and 900degreesC. A convection free experimental technique was used to aid in quantifying the primary a-Fe particles sedimented at 600degreesC for 4 h. Sr marginally decreases the formed particle weight at normal melting temperature (760degreesC), but greatly increases the particle weight at the superheating temperature of 900degreesC. Sr refines primary a-Fe particles at both temperatures, causing the increase of the number of primary alpha-Fe particles, especially when superheated at 900degreesC. In addition, Sr causes a decrease in the particle volume fraction but an increase in the number of the sedimented particles, and a great increase in the depth of the sediment in the mould at higher superheating temperatures. These observations are interpreted in terms of the precipitation of Fe rich particles on suspended oxide films, and the rigidisation of suspended films at higher temperatures, possibly as a result of their change in chemistry, but especially as a result of inflation of their doubled-over form by hydrogen, and the effect this has on the multiplication of growing iron rich precipitates as a result of their mechanically unstable substrate.