Thermal Analysis for Detection of Zr-Rich Phases in Al–Si–Cu–Mg 354-Type Alloys

The principal aim of the present research study was to investigate the effects of minor additions of zirconium on phase precipitation in 354 alloys under very slow solidification rates. The results showed that reactions corresponding to precipitation of Cu-containing phases are also observed in the 354 alloy at temperatures below 500 °C. Zirconium reacts only with Ti, Si, and Al. The Zr-rich intermetallic phases observed in this study appear in two different forms: the phase (Al,Si)2(Zr,Ti) containing high levels of silicon which is block-like in form, and the phase (Al,Si)3(Zr,Ti) containing high levels of aluminum, which appears in needle-like form. There is no observable poisoning effect on the refinement of grain size after the addition of Zr to the alloys investigated, as was observed from the macrostructures of the thermal analysis samples obtained at high solidification rates, for the alloys containing Zr and Ti. In thermal analysis experiments carried out at the higher solidification rate, namely 4 °C s−1, there is an observable reaction within the temperature range of 667–671 °C which appears to be related to the precipitation of Zr-rich phases; it was not possible to observe this reaction at the lower solidification rate of 0.35 °C s−1 generally used in thermal analysis.