Effect Of Cooling Rate On Microstructure Of Mg-Gd-Y-Zr Alloy In Permanent Mold Casting

Mg-Gd-Y-Zr alloys are used in aerospace industry for manufacturing lightweight structural components.In casting process of the components,remarkable variation in cooling rate arises when the casting has a complicated geometry with thickness varying from part to part in the component,resulting in significant effect on the microstructure,mechanical properties and even the consequent heat treatment process of the components.Therefore,it is important to understand the effect of cooling rate on the solidification process and microstructure of Mg-Gd-Y-Zr alloy.In the present study,the effect of cooling rates on the solidification process and microstructure of Mg-Gd-Y-Zr alloy was studied,where a casting containing five steps with thickness of 10-50 mm was produced,in which cooling rate ranging from 2.5?/s to 11.0?/s was created.A computer-aided cooling curve analysis method was proposed,in which the difference in latent heat release of primary and secondary phases was taken into account.The results showed that with the increasing of the cooling rate,the grain size of the alloy decreases from 75?m to 45?m.The size of secondary phase decreases and the fraction of the secondary phase decreases from 12.5% to 4.4%.The relationship between solid fraction and temperature under various cooling rates were obtained via the proposed computer-aided cooling curve analysis method,which shows that with the increasing of the cooling rate,the nucleation undercooling of the alloy increases,and the values of temperature and solid fraction of the primary-secondary phase transition point in the solid fraction-temperature curve increase as well.Based on the experimental results and classic nucleation theory,a model for computing the nucleation rate was established in which the effect of cooling rate and the content of Zr was considered.In addition,an equivalent solute distribution coefficient was introduced to deal with the solute trapping effect during solidification at high cooling rates.The nucleation model and the equivalent solute distribution coefficient were applied to cellular automaton simulation,achieving an accurate simulation of the microstructure of Mg-Gd-Y-Zr alloy.