Effect of Sr–Grain Refiner–Si Interactions on the Microstructure Characteristics of Al–Si Hypereutectic Alloys

Abstract

The present study is part of a larger study where the effectiveness of different types of grain refiners in strontium-modified hypoeutectic alloys was evaluated. Silicon was shown to affect the mechanism of nucleation of α-aluminum. To explore this phenomenon, an investigation was carried out on hypereutectic A390.1 alloy containing 17.3%Si to establish the role of primary silicon crystallization on subsequent α-aluminum and silicon eutectic reactions with different grain refiner additions. The alloy was modified with 0.02%Sr as before, and the same three-grain refiners: Al–10%Ti, Al–5%Ti–1%B, and Al–4%B master alloys, were used at controlled addition levels. The liquid metal was held at 750 °C for periods up to 120 min during which the molten metal was continuously stirred. The results show that addition of 0.1%Ti (using Al–10%Ti) to A390.1 alloy modified with 0.02%Sr results in reducing the initial grain size by about 20% only, due to transformation of Al3Ti phase to (Al,Si)2Ti phase which is a poor grain refiner. The stability of the (Al,Si)2Ti phase increases with the increase in the alloy Si content. The formation of (Al,Si)2Ti also reduces the undercooling caused by Sr addition, reducing the degree of morphology modification. Additionally, boron has a strong affinity to react with Sr, forming the compound SrB6, resulting in no grain refining or “poisoning,” regardless of the added amount of B or the holding time. Combined addition of Sr and B, however, improves the modification of the eutectic Si. In general, due to the high Si content of A390.1 alloy, it is difficult to achieve an appreciable degree of grain refining.