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Effects of Bi and Ca addition on the characteristics of eutectic Si particles in Sr-modified 319 alloys

Al-Si-Cu (319-type) alloys containing 0.04 and 0.4 wt% Mg were used in this study. The alloys were modified with ∼80 ppm strontium and various concentrations of bismuth (50–9000 ppm) and calcium (50–200 ppm) were added. The alloy melts were poured into a preheated graphite mould (600 °C, cooling rate ∼0.8 °C/s) that was also used to conduct thermal analysis experiments to monitor the effects of the Bi and Ca additions on the Al-Si eutectic reaction. Microstructures were examined using optical microscopy, image analysis, and electron probe microanalysis (EPMA), coupled with EDX/WDS facilities. The effects of Bi and Ca on the microstructural changes of the modified 319 alloys were studied in terms of: a) the changes in the Al-Si eutectic temperature with trace element additions,b) the Si particle characterisation (particle length, area, roundness, and density), andc) the identification of new phases occurring during solidification. The results showed that the modifying effect of Sr continuously diminished with the addition of Bi up to ∼3000 ppm, after which the modifying action of Bi commenced, with a further increase in its concentration. Bismuth was found to precipitate in the form of oxides containing a few ppm of Sr and an appreciable amount of Mg (∼5%). It was found that Ca additions of 50 ppm and higher coarsened the Si particles, due to the formation of Al-Si-Ca-Sr compounds, the form (rod or plate) and chemical composition (i.e., Al7(Ca, Sr)Si7 or Al2(Ca, Sr)Si2) of which were strongly related to the Mg concentration in the alloy.