Effect of Ultrasonic Melt-Treatment on the Eutectic Silicon and Iron Intermetallic Phases in Al-Si Cast Alloys

The ultrasonic melt treatment is highly efficient in controlling the size and morphology of the alpha-aluminum phase in the Al-Si cast alloys. However, the influence of this treatment on the other phases, namely: the eutectic Si and the Fe-intermetallic phases, has not been thoroughly investigated. This study was undertaken to investigate the effect of ultrasonic melt treatment on the morphology and size of these phases. Four Al-Si cast alloys, 384, 380, 356 and 356 (with 0.8% Fe), were considered in this study. The treatment temperatures were varied from about 100oC above the liquidus temperatures down to the Al-Si eutectic temperature, for different treatment times (4 to 54 s). The results showed that the Si particles are only affected by the ultrasonic vibrations when the eutectic reaction takes place under the ultrasonic field, where more compacted and large Si particles are formed. This limited effect was observed only near to the ultrasonic horn. For other investigated conditions, including treatment in the liquid and semisolid states, the Si particles were obviously unaffected by the ultrasonic treatment. On the other hand, the iron intermetallic phases changed their morphology from large plate-like particles to a more compacted globular form, by the application of ultrasonic vibrations at temperatures up to 10oC above liquidus. Treatments at higher temperatures have limited effect on the morphology of the Fe-intermetallic phases. These observations are general and apply to all the studied alloys.