| As the lightest metallic structural material, Mg alloys have many advantages, such low density, high specific strength and stiffness, good electromagnetic shielding performance, easy to recycle, etc, which is in line with the current social demand for lightweight material and green environmental protection. Therefore, Mg alloys have shown broad application prospect and strong momentum of development in the fields of aviation, aerospace, automotive and electronic. Alloying Mg-Al alloys with Ca is an important direction in the development of low cost heat-resistant Mg alloys. The addition of Ca can significantly improve the heat-resistant behavior of Mg-Al alloys, as well as their ignition-proof behavior. However, the addition of Ca into Mg-Al alloys simultaneously deteriorates the mechanical properties of Mg-Al alloys at ambient temperature, which hinders the practical application of Mg-Al-Ca alloys. Therefore, it is necessary to solve the contradiction between heat-resistant behavior, ignition-proof behavior and mechanical properties at ambient temperature by improving the mechanical properties of Mg-Al-Ca alloys at ambient temperature.Squeeze casting is an advanced liquid molding technology, in which grain refinement and elimination of casting defects such as shrinkage porosity can be achieved. Semi-solid metal process is considered to be one of the most promising metal forming technology in the 21 st century, which takes both advantages of liquid molding plastic processing. As a combined technology of semi-solid rheo-forming and squeeze casting, rheo-squeeze casting is a promising semi-solid metal process, which can significantly improve the mechanical properties of products. However, the existing researches of rheo-squeeze casting focus on Al alloys. Till now, there is still no research work on rheo-squeeze casting of Mg alloys. Study on rheo-squeeze casting of Mg-Al-Ca alloys is a promising way to improve the mechanical properties of Mg-Al-Ca alloys and provide theoretical and experimental basis for semi-solid rheoforming of Mg alloys.By using Differential Scanning Calorimetry(DSC), X-ray diffractometer(XRD), optical microscopy(OM), scanning electron microscopy(SEM) with energy dispersive X-ray analysis(EDX) and transmission electron microscopy(TEM), the present work investaigated the preparation of AZ91-2wt.%Ca(AZX912) semi-solid slurry via gas bubbling process for the first time. Then, microstructure and mechanical properties of squeeze casting AZ91-1wt.%Ca(AZX911) and rheo-squeeze casting AZX912 alloys were studied systematically. Elevated temperature mechanical behavior of AZX912 alloy prepared by different casting processes was studied comparatively. Influence of heat treatment on rheo-squeeze casting AZX912 alloy was studied further. The mechanisms of grain refinement, strengthening and compressive creep, as well as the foremation mechanism of primary phase, were discussed theoretically.Through study on microstructure and tensile properties of squeeze casting AZX911 alloy prepared under different applied pressure and pouring temperature, the influence rules of squeeze casting processing parameters were indentified. The grain refinement mechanism of AZX911 alloy under applied pressure is clarified. On one hand, increase of applied pressure increased the cooling rate via improving the heat transfer during solidification. On the other hand, increase of applied pressure promoted the nucleation via increasing the liquidus of AZX911 alloy. The increase of second phase volume fraction was due to the movement of eutectic point. At certain temperature, heterogeneous nucleation activation energy (?) first increased and then decreased and reached the peak when. The formation mechanism of rosette-like α-Mg phase in near-liquidus casting was revealed. During low superheat pouring, the whole melt was undercooled and numerous nuclei formed. Nearby grains interacted with each other and dendritic growth was restricted.In order to achieve the fast and suffient preparation of semi-solid slurry, the preparation of AZX912 alloy semi-solid slurry via gas bubbling process was proposed for the first time and experiment device was developed. Results proved that, semi-solid slurry of AZX912 alloy with average primary α-Mg particle size < 50μm and average shape factor > 0.7 could be obtained within 30 s. Nucleation and growth mechanism of primary α-Mg particles during gas bubbling was clarified. Primary α-Mg particles nucleated in the melt directly and their morphology depended on gas flow rate. With the increase of gas flow rate, shear and turbulence intensity enhanced accordingly and the morphology of primary α-Mg particles changed from dendritic to rosette-like and then spherical. Since Ar gas acted as both stirring and cooling medium simultaneously, the relationship between shape factor fi and gas flow rate G was proposed as , in which R0 was initial solidification rate without gas bubbling process and A and B were constants. In present work, the relationship between shape factor F and gas flow rate G was , which was in good accordance with experiment data.Prepared AZX912 semi-solid slurry was shaped by rheo-squeeze casting successfully. Effects of semi-solid slurry quality and applied pressure on microstructure and tensile properties at ambient temperature of rheo-squeeze casting AZX912 alloy were indentified and optimized processing parameters were gained. When gas flow rate, pouring temperature and applied pressure were 596℃, 6L/min and 120 MPa respectively, yield strength(YS), ultimate tensile strength(UTS) and elongation to failure(Ef) of AZX912 alloy reached 110.2MPa, 177.9MPa and 3.3% respectively. Model of composite material structure was introduced to illuminate the influence of semi-solid slurry quality on tensile properties of rheo-squeeze casting samples. Primary α-Mg particles and solidified residual melt were treated as soft strengthening phase and solid matrix respectively. Solidification behavior of AZX912 alloy during rheo-squeeze casting was expounded. Solidification process consisted of two discontinuous parts. First part was the formation and evolution of primary α-Mg particles and second part was fast solidification of semi-solid slurry in low temperature mold under applied pressure.Mechanical behavior of gravity casting, squeeze casting and rheo-squeeze casting AZX912 alloy at elevated temperature was studied comparatively. In the range of 25~200℃, rheo-squeeze casting sample exhibited the best tensile properties and gravity casting casting sample exhibited the worst. In the range of 150~200℃ and 50~100MPa, squeeze casting sample exhibited the best compressive creep properties and rheo-squeeze casting was the worst. Grain boundary sliding was easy to take place in rheo-squeeze casting sample due to its fine grain size. Compared with gravity casting, higher volume fraction of second phases in squeeze casting sample provided more obstacle to dislocation annihilation and grain boundary sliding or migration. Compressive creep mechanism was revealed. In the stress range of 50~100MPa and temperature of 200℃, stress exponent values were 7.9~8.4, implying that it was controlled by non-basal dislocation slip. In the temperature range of 150~200℃ and stress of 100 MPa, average activation energy were 118~145k J/mol, implying creep mechanism transferred from dislocation climbing to non-basal dislocation slip.In order to further improve the properties of rheo-squeeze casting AZX912 alloy, effects of heat treatment on microstructure and tensile properties were indentified. During solid solution at 410℃, β-Mg17Al12 phase dissolved into the matrix completely while Al2 Ca phase was modified. After solid solution, Ef of AZX912 alloy was improved significantly. During aging at 225℃, β-Mg17Al12 phase precipitated in the matrix and reached the peak-aging at 96 h. Continuous precipitation of β-Mg17Al12 phase was dominant. Precipitated Mg17Al12 phase had good strengthening effect but it was harmful to ductility. |
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