Effect of grain refining and Sr modification on Prefil measurement sensitivity in 356 alloys using electron probe microanalysis technique

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To obtain the best possible Ai–Si alloy casting, the quality of the molten alloy must be optimised, i.e. the molten metal must be free of inclusions, impurities and dissolved gases during the casting process. The cleanliness of the molten metal can be monitored by means of various filtration techniques such as the Prefil (pressure filtration) technique used in the present study. Two melt treatment processes also generally applied to Al–Si alloys to improve the cast microstructure and alloy properties are grain refining and modification. However, the addition of grain refiners (in the form of Al–Ti–B master alloys) or modifiers (in the form of Al–Sr master alloys) to the melt can affect the behaviour of the inclusion measurement system. The present work was carried out on A356 alloys to study the effect of such additions when using the Prefil technique (additions of Ti, B or Sr can skew the Prefil filtration curve and make it less effective). The Prefil experiments were carried out using two types of ceramic filters: standard permeability (SP) filters with an average filter pore size of ~90 μm, and high permeability (HP) filters with an average filter pore size of ~130 μm. The results show that the Prefil curves obtained from the high permeability filters are more sensitive to the presence of oxides (film or particles) than the standard filters. The filtration rate is greatly affected by the amount of grain refiner added. Best results are obtained for the alloy without grain refiner addition. The filtration rate progressively decreases with increasing amount of grain refiner. When the total amount of boron added to the melt (through the Al–5Ti–1B grain refiner used) reaches 60 ppm, the filtration rate is approximately nil. In the case of strontium modification, a reasonably good correlation between the amount of Sr in the molten metal and the Prefil curve (or 'footprint') has been established. However, more experiments are still needed for a better understanding of the effect. Oxidation of Sr is possible, even for concentrations as low as 30 ppm. The SrO oxide particles (white, round shaped) are always seen to be associated with Al2O3 oxide particles (observed in the form of dark short rods). The curvature of the Prefil footprint is found to be sensitive to the amount of oxides present.