Casting process performance

Magnesium alloy has the characteristics of low density, high specific strength, and high elastic modulus, and can be applied in industries such as electronic communication, transportation, and aerospace. however However, due to the poor plasticity of magnesium alloys at room temperature, their absolute strength is low and their high-temperature performance is insufficient. Compared with materials such as steel and aluminum alloys, the application range of magnesium alloys is limited. In recent years, Mg Zn-Y alloys composed of icosahedral structure (I phase) have received widespread attention due to their excellent mechanical properties at room and high temperatures. However, when Mg-Zn-Y alloy is melted in an electric furnace and slowly cooled in air, the quasi crystalline phase exists in the form of icosahedra and large intermetallic compounds, and aggregates at grain boundaries, which is not conducive to the improvement of mechanical properties. In order to break through the coarse I-phase, hot extrusion, hot rolling, or other thermoplastic forming processes are usually carried out. However, the α – Mg crystal structure in magnesium alloys is hcp structure with fewer slip systems, making deformation more difficult. Therefore, adopting a new casting forming process to regulate the morphology and distribution of quasicrystals in magnesium alloys, refining the solidification structure of the alloy before thermoplastic processing, can simplify the process, reduce costs, and further promote the application and development of quasicrystal reinforced magnesium alloys. Unlike thermoplastic forming processes, casting processes are widely used to produce high-quality complex castings, such as gravity casting, squeeze casting, and rheological squeeze casting. Among them, rheological squeeze casting combines the advantages of traditional die casting and forging processes. In the process of rheological squeeze casting, semi-solid slurry is usually prepared by mechanical stirring, electromagnetic stirring, or ultrasonic vibration, and then solidified and crystallized under pressure to achieve fine and uniform distribution of the structure. In the above-mentioned pulping process, ultrasonic vibration has high efficiency, low cost, and no pollution. The cavitation and sound flow caused by it have a significant effect on the refinement of primary grains. CHEN X R et al. refined the grain size of AZ80 alloy from 679-1454 μ m to 150-241 μ m using ultrasound, significantly improving the mechanical properties of the alloy. The yield strength and tensile strength were increased by 32.4% and 39.4%, respectively. In summary, ultrasonic vibration is a simple and effective method for refining alloy grains, which can improve the mechanical properties of alloys. Using different casting processes (squeeze casting and rheological squeeze casting) to prepare Mg-12Zn-1Y alloy, in order to refine the grain size and change the morphology and distribution of the I phase. On this basis, the influence of casting process on the microstructure and mechanical properties of Mg-12Zn-1Y alloy is systematically studied, aiming to provide reference for its application.

The microstructure of Mg-12Zn-1Y alloy by gravity casting and squeeze casting. It can be seen that the primary α – Mg in gravity cast alloys exists in a typical dendritic shape, and after applying 100 MPa extrusion pressure, the morphology of α – Mg in the alloy undergoes significant changes. The coarse dendrites are broken and the grains transform from dendritic to nearly spherical, with an average grain size of 39 µ m. From Figure 1c, it can be seen that the microstructure of gravity cast alloy is composed of coarse α – Mg and gray and layered phases. The gray phase in the squeeze casting sample decreases, while the content of α – Mg and layered eutectic increases. For magnesium alloys, applying pressure can cause an increase in their melting point. When the crystallization temperature of the alloy under pressure is higher than the actual temperature of the melt, undercooling occurs in the melt, which promotes nucleation. At the same time, the pressure almost eliminates the air gap at the mold interface, accelerates heat transfer between molds, and significantly improves the cooling rate. Research has shown that once pressure is applied, the interfacial heat transfer coefficient (IHTC) between the casting and the mold increases sharply. The higher the cooling rate of the melt, the smaller the microstructure produced under higher undercooling, and the original defects such as casting pores in the casting can also be effectively solved.

Stress-strain curves and room temperature mechanical properties of Mg-12Zn-1Y alloy prepared by different casting processes. It can be seen that the yield strength, tensile strength, and elongation of gravity cast specimens are 98 MPa, 138 MPa, and 2.3%, respectively. The yield strength, tensile strength, and elongation of squeeze cast specimens are 137 MPa, 221 MPa, and 2.8%, respectively. Compared with the gravity casting sample, it increased by 39.8%, 60.1%, and 21.7%, respectively. In the rheological squeeze casting process, when the ultrasonic power is 1600 W, the mechanical properties of the alloy are the best. Its yield strength, tensile strength, and elongation are 185 MPa, 276 MPa, and 6.8%, respectively, which are increased by 35.0%, 24.9%, and 142.9% compared to the squeeze casting sample. When the ultrasonic power increases to 2400 W, the yield strength, tensile strength, and elongation slightly decrease. Therefore, it can be inferred that the optimal ultrasonic power for rheological squeeze casting of Mg-12Zn-1Y alloy is 1600 W. SEM morphology of tensile fracture surface of Mg-12Zn-1Y alloy prepared by different casting processes. It can be seen that for gravity cast tensile specimens, the fracture surface is mainly composed of large-sized cleavage planes, exhibiting the characteristics of cleavage fracture. After applying a pressure of 100 MPa, the size of the cleavage surface decreases and some ductile dimples appear. For rheological squeeze casting alloys, as the ultrasonic power increases from 800 W to 1600 W, the large ductile dimples are gradually replaced by small ductile dimples. However, as the ultrasound power continued to increase to 2400 W, the number of small dimples decreased relatively.

The dense hexagonal structure of magnesium alloy generates fewer slip systems, and there is a significant difference in critical shear stress between basal slip systems and non basal slip systems. Before the activation of non basal slip systems, cracks often occur and propagate. In this study, the size and distribution of phase I were positively correlated with mechanical properties, and the rheological squeeze casting process inhibited the formation of the second phase. During severe plastic deformation, a sufficiently rigid atomic bond is formed between the I-phase and the hexagonal structure. Therefore, the sliding of dislocations around grain boundaries is suppressed, thereby promoting an increase in yield strength and tensile strength. Meanwhile, for Mg-12Zn-1Y alloy, most of the W phase can be transformed into the I phase, resulting in the I phase presenting a plate-like eutectic morphology. In addition, the effects of ultrasonic cavitation and acoustic flow can significantly refine and spheroidize the α – Mg grains. During the solidification process, the extensive refinement of the primary α – Mg helps to refine the W phase, which eventually transforms into the I phase after the peritectic reaction. Due to the refinement of the W phase, the I phase is also refined, and the spacing of the eutectic structure is reduced.

(1) The microstructure of the gravity cast Mg-12Zn-1Y sample consists of coarse α – Mg, Mg7Zn3 phases, and layered eutectic (α – Mg and I phases). High pressure in squeeze casting increases the cooling rate of the alloy melt, resulting in a decrease in the spacing between eutectic structures and a decrease in the content of Mg7Zn3 phase.
(2) During the slurry stage of Mg-12Zn-1Y alloy in rheological squeeze casting, ultrasonic action causes α – Mg to transform from coarse dendrites to nearly spherical crystals Granules. As the ultrasound power increased from 800 W to 1600 W, the size of α – Mg grains in the rheological formed microstructure gradually decreased, and the distribution of layered eutectic along grain boundaries became more uniform. In addition, small quasi crystalline I-phase particles appeared in some α – Mg matrices. However, as the ultrasound power continued to increase to 2400 W, there was a certain degree of enrichment in the grain boundary eutectic structure.
(3) The yield strength, tensile strength, and elongation of squeeze cast Mg-12Zn-1Y specimens were 137 MPa, 221 MPa, and 2.8%, respectively, which were 39.8%, 60.1%, and 27.1% higher than those of gravity cast specimens. For rheological squeeze casting, when the ultrasonic power is 1600 W, the yield strength, tensile strength, and elongation are optimal, reaching 185 MPa, 276 MPa, and 6.8%, respectively.

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