Effect of Si and P Content on the Si Phase and Solidification Behavior of Al-40Zn-xSi Alloys Using Synchrotron Radiation Real-Time Imaging

In this paper, the effect of Si (1, 3, 5, 6 wt.%) and P (5, 10, 15, 20, 30 ppm) content on the Si phase and solidification behavior of Al-40Zn-xSi alloys are investigated via phase diagram calculation, scanning electron microscopy (SEM) and synchrotron radiation real-time imaging. It is found that Si content has a great influence on the microstructures of Al-40Zn-xSi alloys, and two distinct primary phases were separated by a critical composition at 5.49 wt.% Si in the calculated phase diagram. The microstructure contained not only needle-like eutectic Si, but also blocky primary Si particles in “hypoeutectic” commercial-purity Al-40Zn-5Si alloys. However, the synchrotron radiation real-time imaging experiment showed that the primary phase was α-Al in commercial-purity Al-40Zn-5Si alloys. The effect of P on the Si phase of the high-purity Al-40Zn-5Si alloy was also studied, showing that the number and size of the blocky primary Si particles in the high-purity “hypoeutectic” Al-40Zn-5Si alloys were mainly related to the P impurity content. A small minority of primary Si particles were found during the whole solidification process of the high-purity Al-40Zn-5Si alloy without P impurities, while a large number of primary Si particles were precipitated continuously at the front of α-Al dendrites in the high-purity Al-40Zn-5Si alloy with the addition of 20ppm P, which may have been caused by the fact that AlP particles can act as the heterogeneous nucleation sites of primary Si particles by a small undercooling in P-impurity-containing alloys.