| Counter-gravity casting method was considered to be an effective technique to improve the formability of large complex thin-walled castings due to its advantage in smoothly filling process. However, there is no general rule for the counter-gravity casting, and some defects such as shrinkage cavity, shrinkage porosity and misrun are detected in the large castings, leading to a low yield production. In this thesis, the hydrodynamics of mold filling for counter-gravity was analyzed, and the impacts of pressurizing speed and section variation of cavity on filling velocity and stability of liquid surface were also researched. Accordingly, interms of the hydrodynamics of mold filling in low pressure casting, the effect of pressurizing speed and variation of the cross-section of the mold on gate velocity, and the influence of the gate velocity on filling behavior and stability of mold filling were investigated in the paper. the entrainment mechanism of oxide film during the filling process was defined and corresponding solving plans were raised. Meanwhile, the effect of gating system on stability of liquid surface was conducted, and a new design principle for gating system was put forward.The counter-gravity casting model, which is similar with its working process, was established based on the two-phase model using the volume of fluid(VOF) method. The effect of equipment parameters, technological parameters and structure of mold on gate velocity and stability of mold filling and the filling behavior of traditional runner system in counter gravity casting were studied by the numerical calculation method. Water simulation and aluminum alloy casting experiments were carried out and the numerical calcultion results were proved to be ture by the experimentation.The results show that the gate velocity first increased dramatically, then changed with the depressurizing speed: the gate velocity increased slowly under relatively high depressurizing speed; when under reasonable depressurizing speed, the gate velocity kept unchanged; while under lower depressurizing speed, the gate velocity decreased firstly and then kept unchanged.Compared with the case of sudden expansion section, the filling process of gradual expansion section has relatively shorter spreading time and smaller increment of gate velocity. In the filling stage of gradual expansion section, the gate velocity keeps increasing and the higher pressurizing speed, the larger acceleration. However, the acceleration is always smaller than the spreading stage. The character of filling in sudden contraction section is that the gate velocity decreases drastically first and then keeping damping oscillations. While in the case of gradual contraction section, gate velocity decreases slowly and also appears the oscillations feature. And the higher pressurizing speed induces faster descent velocity and smaller velocity amplitude. In the filling process of counter gravity casting, pressurizing speed and variation of cavity section show the equal impact on the gate velocity.The filling behavior and velocity field shows that the oxide film will adhere to raiser tube wall when the average speed of raising stage is smaller than 0.12m/s. Otherwise, the oxide film will be cracked near the tube wall and floated on the liquid surface, involved into the melt during the mold filling process. The gate velocity will increase dramatically due to the horizontal flow which resulting from the growth cross-section area in the spreading stage of sudden expansion structure or gradual expansion structure. During the filling stage, effective pressure head increases with the pressurizing speed, leading to a higher gate velocity and jet velocity. The critical gate velocity is 0.5m/s for both sudden and gradual expansion sections. When the critical velocity is higher than 0.5m/s, the free surface of melt will raised due to the jet, and then fall back resulting in entrainment of oxide film under gravity. Thus a new design rule for avoiding entrainment of oxide film was established in sudden and gradual expansion sections. In sudden contraction structure, the velocity of the liquid surface increased rapidly under the inertia effect due to the decrease of cross-section area, and he liquid surface will flow back to crucible due to pressure insufficient to support the current melt level when the pressurizing speed was unchanged in bottom vessel. A thin layer shell will form on the sand mold wall and will be folded, leading to the formation of surface crack after the liquid surface rise again. The surface crack defects can be eliminated by increasing pressurizing speed in sudden and gradual contraction sections.Air entrapment and back flow can be easily formed in traditional runner structure under counter-gravity casting during the filling process. These problems could be avoid by increasing the height of the runner and decreasing the pressurizing speed, which still has some limitation in practical application. The structure, cross-sectional area does not change as much as possible in the vertical direction, suitable for the counter-gravity casting. Obviously, Y-shaped structure is good for filling in the counter-gravity casting. Its gate velocity is not affected by the length and width, and just relates to the pressurizing speed. Moreover, the stepped structure is more convenient in modeling using a layered approach and the overlapping area between mold sand and runner should be less than 1/2. Pouring basin with liquid hovering technology can smooth filling process, while the linear pressure process will result in a strong fluctuation of melt surface in the pouring basin. When the metal liquid flows into pouring basin, the fluctuation of melt surface will be reduced by adopting nonlinear pressure process and the pressurizing speed decreased slowly to zero. For this purpose, an intelligent controller with predictive factor of slope based on fuzzy self-tuning PID control was designed, and a nonlinear pressure system of liquid surface of counter-gravity casting was achieved. |
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