Achieving low-temperature Zr alloying for microstructural refinement of sand-cast Mg-Gd-Y alloy by employing zirconium tetrachloride

Abstract

The large size and uneven distribution of Zr particles always lead to a high alloying temperature and low Zr yield when MgZr master alloy is used for grain refinement of sand-cast Mg-RE alloys. In this research, the feasibility of Zr alloying by applying ZrCl4 in sand-cast GW83 alloy at a low temperature (720 °C) was investigated. The interaction procedure between ZrCl4 and Mg melt, and corresponding grain refinement mechanisms were systematically studied by thermodynamic calculation, thermal analysis, and microstructure characterization. The results showed that the yield of Zr by applying ZrCl4 was higher than that of the MgZr master alloy. Zr can be generated via the in-suit reaction between ZrCl4 and Mg. Both the nucleation temperature and nucleation undercooling increased with the increasing addition of ZrCl4. The average grain size decreased from 832 μm to 229 μm with 2.606 wt% ZrCl4 addition due to the heterogeneous nuclei of Zr particles and the constitutional supercooling provided by solute Zr. A corresponding physical model depicting the solidification behavior and grain refinement mechanism was proposed. However, the ZrCl4 addition of 4.343 wt% led to a large amount of RECl3 inclusions generated during the Zr alloying process, deteriorating the mechanical properties of the alloy.