Influence of strontium modification and solidification history on the microstructure of cast Al-Si alloys

A relatively high solidification rate as well as the chemical modification of molten metal play a key role in Aluminium foundry, influencing the microstructural scale and the morphology and distribution of eutectic silicon. In this work, the effects of the solidification conditions and the strontium (Sr) addition on the modification level of eutectic silicon have been analysed in three different AlSiMg(Ti) foundry alloys. The same alloys have been modified by sodium (Na) addition and the results compared. The modification level has been studied by means of thermal analysis and subsequent metallographic examination of the solid samples. Different arrangements have been set up in thermal analysis experiments to change the cooling rate during solidification. At low solidification rate, the EN AB-42000 (AlSi7MgTi) alloy shows a continuous improvement of the modification level with increasing Sr content and the optimum is reached in a range of 100-200 ppm Sr. The EN AB-43300 (AlSi9MgTi) and 44000 (AlSi11MgTi) alloys, with higher silicon content, do not present similar results, showing a partially modified microstructure, although a higher Sr amount has been added. Further, specimens with the highest Sr additions show the presence of particles Al2Si2Sr type, which do not contribute to the modifying effect. These intermetallics are located in the zones where the eutectic silicon is weakly modified. On the contrary, all the analysed alloys show a good modification level after Na addition. The eutectic depression is higher than 7°C and the metallographic investigation confirms a well modified microstructure with a fine and fibrous silicon eutectic. No recalescence effect is evaluated after Na addition. The modification level of the EN AB-43300 and 44000 alloys, after chemical modification with Sr, significantly improves with increasing solidification rate, as confirmed by the reduction of the eutectic growth temperature. The microstructural analysis shows the combined effect of the elevated heat transfer rate and chemical modification on the improvement of silicon morphology and size. After a first test step based on a thermal analysis approach, the study has turned to gravity cast wheels, where several solidification conditions are present. The preliminary results support the microstructural analysis carried out on cast wheels. The EN AB-42000 e 44000 alloys have been used. A batch of castings has been modified with Sr and another one with Na. The wheels, cast with the EN AB-42000 alloy and modified with 200 ppm Sr or 50 ppm Na, show a well modified microstructure both in the hub and in the front flange. In spite the different cooling conditions of the analysed regions, the modification level is good and the eutectic silicon is fine and fibrous. The wheels, cast with the EN AB-44000 alloy and modified with 400 ppm Sr, show a coarser microstructure, with eutectic islands of unmodified silicon in the hub. At the same time, the front flange presents a well modified microstructure. The cast wheels, modified with 50 ppm Na, show a fibrous microstructure with very few eutectic islands of unmodified eutectic silicon in the hub, and a well modified microstructure in the front flange. Therefore, while a high solidification rate is not needed in an aluminium alloy with low silicon content, this seems to be required for alloys with higher amount of silicon. The consequences are important for real shaped castings where different solidification conditions are present, due to different heat transfer rates. Controlling the heat transfer and the type and amount of chemical modifier, it is possible to obtain, to a large degree, an homogeneous microstructure and an improvement in the final quality of castings.