Superheat treatment of Al-7Si-0.55Mg melt and its influences on the solidification structures and the mechanical properties

Superheat treatment on Al-7Si-0.55Mg melt and its influences on the solidification structure both in the micro- and macroscales, as well as mechanical properties, are studied experimentally. The alloys were first melted and superheated for similar to30 minutes at temperatures between 720 degreesC and 950 degreesC. The melts were then cooled slowly to 720 degreesC before casting into the specimens for microstructural analyses and mechanical property tests. The results show the following. (1) A superheating process above 850 degreesC for 30 minutes modifies the interdendritic eutectic and is comparable to that achieved by a Sr addition. A high cooling rate is necessary to ensure the modification efficiency. (2) Through superheat treatment, the morphology of Fe-rich phases can range from needlelike to skeleton. This effect is not observed in the case of Sr additions. (3) Superheat treatment reduces slightly the secondary dendrite arm spacing and coarsens the primary grain size. (4) The strength of specimens that underwent a superheat treatment during the melting process can be even 10 MPa higher than those modified by Sr addition.