| The squeeze-cast process, combining the advantages of the casting and forging processes, has been widely used to produce quality castings. Because of the high pressure applied during solidification, porosities caused by both gas and shrinkage can be prevented or eliminated. The cooling rate of the casting can be increased by applied high pressure during solidification, since heat transfer between the casting and the die is improved by pressurization, which results in the formation of fine-grained structures. Although squeeze cast has gained more and more attention and application., limit researches have been done about squeeze-cast magnesium alloy, and these researches mainly focus on Mg-Al system and rarely on Mg-RE system (RE: rare earth elements). According to the existing research results of squeeze-cast Mg-Al series, it is still not clear why the volume fraction of Mg17Al12 increases with the increment of applied pressure, which goes against the squeeze-cast alloy design. Moreover, it has been demonstrated that rare earth is the most effective element to improve the properties of magnesium alloys especially at elevated temperatures, and the resources of magnesium and rare earth are abundant in our country. Therefore, it is necessary to discuss the change of solid solubility with applied pressure in order to provide the theory to the squeeze-cast alloy design and do systematic research on the squeeze cast Mg-RE alloy.By scanning electron microscopy (SEM), optical microscopy (OM), X-ray diffractometer (XRD) and positron annihilation technique (PAT), the effect of applied pressure on the solid solubility is investigated by changing the volume fraction of eutectic phase and lattice constant of magnesium element on base of Mg-9Al alloy, Mg-5Zn alloy and pure magnesium. According to the discussion of the change in partial molar volume on solidification of solute in dilute solutions, it is found that the change of atomic radius may be the basic reason for the variation of solid solubility under applied pressure. And the empirical formula for the change trend of solid solubility with applied pressure is obtained. And the solid solubility of Nd in Mg is predicted and verified.Based on the study of solid solubility, Mg-2.5wt%Nd is selected as material for investigation here. By OM, SEM, transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and tensile test, the effects of process parameters and nucleating agents on the macrostructure, microstructure and mechanical properties of Mg-Nd alloy are studied and the effects of applied pressure on the nucleation and growth of Mg-Nd alloy are especially focused on. The main results can be summarized as follows:Pouring temperature has a decisive effect on the macrostructure of squeeze-cast Mg-2.5Nd alloy. When the pouring temperatures are 725°C and 750°C, there are no obviously differences among the grain structure and the grain structures mainly consist of a band of thick columnar grains surrounding some large equiaxed grains in the center, but the grains tend to be coarse with the increment of applied pressure. However, when the pouring temperature is 700°C, the average grain sizes of the samples are decreased with the increment of applied pressure. When the applied pressure is 120MPa, the grains structure changes from coarse columnar and equiaxed grains into fine grains. On the other side, die temperature has small effect on the squeeze-cast Mg-Nd alloy, and can not change the grain structure, but low die temperature decreases the grain size. It is the practical melt temperature at the time of a pressure application that decides the relationship between grain size and applied pressure. When the practical melt temperature is above the melting point under applied pressure, the applied pressure leads to grain coarsening; while it is below the melting point under applied pressure, grains are refined by applied pressure. Further analysis of the fine grains, it is found that the microstructure of squeeze-cast Mg-2.5Nd alloy exhibits distinct regions of fine and coarse structure (Bi-modal structure). As a result, decreasing die temperature as well as pouring temperature and increasing applied pressure can improve the mechanical properties, especially elongation and tensile strength.The microstructure of solution-treated Mg-Nd alloy is composed ofα-Mg and NdH2. The NdH2 particles are located at or near the grain boundaries, which worsens the alloy's mechanical property, especially the cracking and fracture behaviors. Also, the NdH2 particles become another defects of hydrogen in magnesium alloy except of hydrogen embrittlement and pores. However, in the case of solution-treated Mg-Nd-Zr alloy, there are no NdH2 particles at or near the grain boundaries. While, some small particles, which might be ZrH2 phase, are distributed in grains.In squeeze casting process, the nucleation of Mg-Nd alloy is still dominated by heterogeneous nucleation when Zr element exists. When process parameters of squeeze cast are same, the grain size decreases with the increment of Zr content; however, applied pressure has no obvious effect on the grain size when the Zr content is 0.46%.The freezing curves show that the solidification temperature is about 645~646 oC, 5 oC higher than that of the sample by gravity cast under the condition of pouring temperature at 725 oC and applied pressure at 120MPa, while the solidification temperature is the same as that of the sample by gravity case when the pouring temperature is 700 oC and applied pressure is 120MPa. Study on nucleation in squeeze cast shows that if the practical melt temperature at the time of pressure application is above the melting point under applied pressure, the increment of applied pressure enhances the critical nucleation free energy and reduces the nucleation rate. On the contrary, if the practical melt temperature at the time of pressure application is below the melting point under applied pressure, the increment of applied pressure can decrease the critical nucleation free energy and enhance the nucleation rate. The study on growth of a formed nucleus shows that the increment of applied pressure can increase the speed of atoms attaching to solid-liquid interface, and decrease the unstable wavelength of a growing global grain, resulting in the increment of solidification rate.The study of this thesis provides a new approach for the squeeze-cast alloy design, reliable data and theoretical analysis for the process parameters of squeeze-cast Mg-RE alloy. Microstructure can be controlled through adjusting applied pressure, die and pouring temperatures, which then results in the high density, fine and uniform microstructure to improve the mechanical properties. |
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