| A novel rheocasting process, termed as Low Superheat Pouring with a Shear Field (LSPSF) has been developed, by which sound semi-solid metal slurry can be manufactured for rheoforming. And a LSPSF machine was self-designed and built. A series of experiments on casting aluminum alloys and wrought aluminum alloys were carried out to investigate the effects of LSPSF parameters on the semi-solid microstructure. The primary phase evolution behavior, formation mechanism of non-dendritic structures, rheo-diecasting process for secondary diecasting alloy YL112 and rheo-squeeze casting process for wrought aluminum alloys were investigated. The main work and results are as follows:High quality semi-solid metal slurries of a series of aluminum alloys were manufactured by LSPSF process, such as casting alloy A356, high strength alloy 201, secondary diecasting alloy YL112 and wrought alloy 2024, 6082 and 7075. The results show that LSPSF process can produce 1500g sound semi-solid slurries of experimental alloys within 15-25s, in which the primary a-A1 presents spherical or particle-like, small and homogeneous distribution, especially with zero-entrapped liquid.Affecting rules of parameters in LSPSF on the semi-solid metal microstructures were discovered. Pouring temperature, rotation speed of barrel and inclined degree of barrel determine the stirring-mixing intensity and cooling ability applied by the rotational barrel on the molten alloy, and finally determine the outlet temperature of molten alloy. It can be concluded that the outlet temperature of molten alloy is bellow its liquidus temperature and sound semi-solid metal slurry can be produced under the appreciate combination of pouring temperature, rotation speed of barrel and inclined degree of barrel. Processing conditions to produce sound semi-solid slurry of above mentioned 6 kinds of alloys were presented. If both rotation speed of barrel and inclined degree of barrel are in the effective range, with the decreasing of pouring temperature, the primaryα-Al undergoes a transition of the growth morphology from coarse dendritic to rosette and to spherical.Under Sn-cooling copper mould casting, rheo-deicasting and V shaped steel mould casting, the process of primaryα-Al formation in Al-20wt. %Cu alloy and A1Si9Cu3 alloy, the interface stability of spherical primaryα-Al in Al-20wt. %Cu alloy were investigated. The experimental results show that: (1) the morphology of primaryα-Al is determined by both the number of free crystals in semi-solid slurry and the cooling intensity of semi-solid slurry. Increasing the number of free crystals and decreasing the cooling intensity of semi-solid slurry can enhance the formation of spherical primaryα-Al. (2) The first observable primaryα-Al of Al-20wt. %Cu alloy and AlSi9Cu3 alloy is spherical shape, and grows spherically in the afterwards slower cooling. This indicates that the spherical structure can be directly formed from the molten alloy under LSPSF process. (3) The interface stability of spherical primaryα-Al is influenced by solid fraction-cooling rate combination. Semi-solid metal slurry with higher solid fraction is able to maintain the interface stability at higher cooling rate than primaryα-Al in semi-solid slurry with lower solid fraction.Based on the simultaneous nucleation theory and the separation theory, the nucleation behavior and the free crystal formation under LSPSF process were analyzed. Low superheat pouring combined with localized rapid cooling and vigorous mixing during the initial stage of solidification can enhance wall nucleation, crystal separation and crystal survival. Moreover the effective nucleation rate is significantly affected by the combined effects of pouring temperature, rotation speed of barrel and inclined degree of barrel.The exact formation and evolution mechanism of spherical primary phase under LSPSF process is determined as controlled nucleation and limited growth, but not dendrite fragmentation. A Cellular Automaton (CA) model was developed to numerically simulate the microstructure formation in rheocasting, by which effects of grain density and cooling intensity on the morphology evolution of primaryα-Al in rheocasting were investigated. The results from simulations show good agreement with the experimental results. Based on M-S stability theory and results from experiments and numerical simulations, the "Diffusion field overlapping" principle was proposed: the overlapping of solution diffusion fields from adjacent growing crystals has a significant influence on the morphology of crystal by affecting the concentration gradients that can cause instabilities at the solid-liquid interfaces. It can be seen that, under combined effects of high grain density and a much slower cooling, the overlapping solution diffusion fields from adjacent growing crystals increases the stability of sphere interface, and the sphere can stably grow.Combined the LSPSF technique with the conventional cold chamber diecasting process (HPDC), a rheo-diecasting process (RDC) has been developed for producing components of Al-alloys, such as YL112. The quantitatively supply of semi-solid slurry and the transportation of semi-solid slurry from the LSPSF machine to the HPDC machine were investigated. Compared with HPDC, RDC can improve the microstructures and mechanical properties, especially for the elongation. Heat treatments can be used to further improve the mechanical properties of RDC samples.Rheo-squeeze casting for wrought alloy 2024 and 7075. Microstructure characteristics, defects and liquid segregation and filling ability of semi-solid slurry were analyzed. Solution treatment and mechanical properties of 2024 alloy were investigated. Compared with conventional squeeze casting, rheo-squeeze casting can improve the microstructures and mechanical properties. Solution treatment can significantly reduce the micro-segregation. T6 heat treatment for 2024 alloy (Solution treatment: 12h at 500℃→quenching→aging: 12h at 170℃) can improve the mechanical properties that can be close to the level of thixoforged 2024 alloy, but still below the level of standard forged 2024 alloy.
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