| Grain refinement of cast magnesium alloys by inoculation of the melt with potentnucleant particles is the first effective step towards the goal of achieving a fine, uniformand equiaxed grain structure. This imparts to castings critically needed structuraluniformity and consistency in performance improving strength, ductility and so on.Zirconium is the most powerful refiner for Al-free Mg alloys, and the alloying processis normally by adding a binary Mg-Zr master alloy. In recent years, the light, highstrength and heat-resisting Mg-Gd-Y-Zr Mg alloy has attracted a great deal of attentionin the automotive, aviation, military and communication industry. As the density of Zr(~6.52g/cm~3) is much higher than liquid Mg (~1.58g/cm~3), Zr spontaneously settlesdown to the bottom of the crucible and thus this causes grain coarsening and muchwaste. This is an obstacle to development of grain refinement technology of Mg alloys.Therefore, it is very important to study the fading behavior of Zr in the melt, which isbenifical to improve the casting technique and save the usage of Zr. Moreover, thesettling of Zr raises certain concerns for researchers who pay more attention to theinherent quality of Mg-Zr refiners. Therefore, it is very crucial to investigate the effectof Zr master alloys on the grain refinement, which will help develop a more efficient Zrrefiner. Another thought is to develop a new Zr alloying method, which is cost savingand convenient. Furthermore, these studiesm will be useful for us to understand moreabout the grain refinement mechanism of Mg by Zr, if we incorporate the above issueswith the interaction effect of solute Gd, Y and Zr refiner.In this thesis, the grain refinement technology and mechanism of Mg-Gd-Y alloy byZr was investigated. Based on the settling behavior, the effect of settling of Zr on thefading of grain size was uncovered. The effect of Zr particle distribution in Mg-Zrmaster alloy on grain refinement was explained based on the experiments. Moreover, the refining efficiency of a Mg-30%Zr master alloy was impoved by hot-rolling. Furthermore, in order to develop an anti-settling and cheap alloying method of Zr, a kind of new Zr-rich salt mixture for Mg-Gd-Y alloy was formulated. The grain refining mechanism of Zr salts mixture is analyzed from thermodynamic aspect. Finally, the interaction effects of Gd, Y and Zr elements during grain refinement were clarified, and the grain refinement mechanism of Zr was discussed.The settling experiments show that it has an adverse (fading) effect on Zr content and grain size. Within the temperature range of730℃-780℃, when holding from5mins after stirring to240mins, the Zr content decreases by～33%, and the grain size coarsens by23～27%. The settling calcutaion shows that the particles greater than7μm in size settle out of the sample during the first5mins, which is better for removal of Zr clusters. However, the settling of much finer particles plays crucial role when settling time is from5mins to240mins. The optimal holding time is15mins. The dependence of coarsening rate of grain size on the holding time is established by the equations: ΔC,%=5.21·1nt-8.15(t>5min,730℃), ΔC,%=6.76·1tt-9.42(t>5min,780℃). This relationship is instructive to understand the coarsening of Zr grain refinement.The grain refinement experiments of Mg-Gd-Y alloys by Zr show that the main factor that influences the grain refining effect is the Zr particle size distribution within the microstructure of Mg-Zr master alloy. The master alloy with the largest number density of particles less than5μm in size exhibits the best grain refinement. This is because that those sutiable particles can produce more nucleating sites, when the soluble Zr is of sufficient level. Based on this finding, the particle number density of finer particles within a Mg-30%Zr master alloy has been prepared by hot rolling by8passes from7.5mm to0.5mm. The grain refining efficiency of the Mg-30%Zr is then improved by～30%.Considering settling problem, a new Zr-rich salts mixture for Zr alloying in Mg-Gd-Y alloy has been developed for the first time. The optimal composition of salts mixture is optimized to be [50%K2ZrF6-25%NaC1-25%KCl](with16.1%Zr inside). Compared with Mg-Zr master alloy, the grain refinement is satisfactory and the fading resistance is much better. Thermodynamic calculations show that the grain refinement of Zr salts is mainly due to the strong grain Growth Restriction Effect/Factor (GRF) of in-situ solute Zr produced by the reaction between Mg melt and Zr salts.The inclusions followed by chemical reaction of Zr are identified to be the agglomeration of KF, MgF2and the KSM residuals. Filtration purification by Ceramic Foam Filter (CFF) shows that the mechanical properties is slightly improved by-20MPa using the20ppi (pores per inch) MgO CFF, which indicates the purification effect is obvious. However, a more efficient multistage purification technology needs to be developed in future.The interaction behaviors between Gd, Y and Zr have been disclosed. The results show that the group of alloy without Zr addition has a decreasing grain size with increasing Gd and/or Y contents, and there is a mathematic relationship to show the grain size as following:dgs=3894.3·e-0.113Q, where Q is the value of GRF. The grain size of the group of alloy with an approximately the same Zr content also decreases with the increase in Gd and Y contents, while the increase in Gd and Y content decreases the grain refinement effect of Zr. In addition, the grain size of alloy with certain Gd/Y decreases with the increase in Zr content. The analysis indicates that the nucleating contribution (n,%) of Zr particles to grain refinement decreases with enhancing solute elements (Gd, Y, Zrs), and this effect has been established by the equation:n,%=115.8·e-0.111Q, which has theoretical significance for further investigation of Zr grain refinement mechanism.Combining the analysis of contribution of Zr to grain refinement with the calculation for weight percentage of active Zr nuclei, it is shown that the nucleation frequency decreases with increasing Gd and/or Y contents. This is possibly because some potential nucleating Zr particles are suppressed by the solute diffusion filed. |
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