At present, the production of large aluminum alloy wheels is mostly forging, and the production cost is relatively high. If low pressure casting is used, the cost can be saved. However, the mold in the low-pressure casting equipment forms a closed cavity, the internal flow field and temperature field are difficult to observe and study, and the large size and uneven wall thickness of the wheel are prone to defects.
In order to be able to deeply understand the changes in the flow field and temperature field of the casting filling and solidification process inside the mold, and to optimize the production process to improve the quality of the casting, a computer numerical simulation method is used [1], according to different casting production conditions and process requirements Optimize the gating system to achieve the purpose of improving the quality of castings.
1. Mathematical model of solidification process
1.1 Material and initial conditions The hub material is A356 alloy, and the mold material is cast iron. If the pouring temperature is too high, it is easy to cause defects such as local oxidation, thermal cracking, and fire running; if the pouring temperature is too low, the molten aluminum will solidify at the riser and block the pouring channel. According to the production requirements, the pouring temperature of the casting is set to 670. For the determination of the mold temperature, it is necessary to consider that if the mold temperature is too high, mold sticking will occur, the casting will be deformed, and the production cycle will be longer; if the mold temperature is too low, the molten metal is not full, which reduces the life of the mold and the casting cracks. So the mold temperature used: the initial temperature of the upper mold is 370, the temperature of the side mold and the lower mold are both 400.
1.2 Determination of the heat transfer coefficient The calculation formula of the boundary conditions is: xTxnx+yTyny+zTznz=qs(x,y,z) (1) where qs is the external input heat source per unit area; nx, ny, nz are the boundaries, respectively The cosine of the outer normal direction. According to formula (1), the heat transfer coefficient between different contact surfaces is calculated
[3]: The heat transfer coefficient between the mold and the natural cooling medium is 25W/(m2K); the heat transfer coefficient between the mold and the compressed air is 500W/(m2K); the heat transfer coefficient between the mold and the cooling water is 8000W/(m2K). During the low-pressure casting process, when the molten aluminum is solidified from the beginning to the demolding period, the molten aluminum generates a gap between the mold and the casting due to solidification and shrinkage, which reduces the heat transfer capacity between the mold and the casting. In order to better reflect the heat transfer of the interface between the mold and the casting, the heat transfer coefficient between the mold and the casting changes according to the temperature.
[4], at 25, 540, 560, 700, the heat transfer coefficient is 50, 50, 270, 270W/(m2K) respectively. 2 Mathematical model of casting filling process The flow process of molten metal obeys the relationship of conservation of mass, momentum and energy. For incompressible fluids, the continuity equation (the law of conservation of mass) is xx+yy+zz=0 (2) where x, y and z are the components of the velocity on x, y, and z respectively. Momentum conservation law equation (NS equation for short): xt+xxx+yyy+zzz=Fx-1px+v2x(3)yt+xxx+yyy+zzz=Fy-1py+v2y(4)xz+xxx+yyy+zzz= Fz-1pz+v2z
(5) In the formula, Fx, Fy and Fz are the volume force components per unit mass; p is the pressure of the fluid; 2=2x2+2y2+2z2, called Laplace operator. The conservation of energy is also observed during fluid movement. When the fluid is incompressible, it is expressed as et+pddt(1)=1div[grad(T)] in the three-dimensional coordinate system.
(6) In the formula, e is the internal energy of the fluid; is the fluid density; is the dissipation function; / is the heat generated by the internal surface stress on the fluid; is the fluid thermal conductivity; grad(T) is the temperature change rate . 3 Numerical simulation and experimental verification of solidification process The ProCAST software is used to simulate the filling and solidification process of castings [5~8], and the temperature field changes during the solidification process of castings are mainly observed. For thick and large castings, it is reasonable to assume that the numerical simulation of the solidification process is based on "instantaneous filling", but for thin-walled castings, this assumption will have larger errors. Because the simulated large wheel hub is a thick casting, in order to save the simulation time, the instantaneous filling method is used to simulate the solidification process of the casting. The size is an 18 inch wheel (145.72cm). Due to the symmetry of the casting, only 1/4 of the computer simulation diagram is taken for observation.
The temperature field distribution diagram at different moments in the filling and solidification process of a large aluminum alloy wheel. It can be seen that the temperature field of the hub does not show a stepped distribution during the solidification process, indicating that the solidification rate of the thicker part of the hub is relatively slow.The spokes that solidify first prevent the central gate aluminum alloy liquid from supplying the thicker part of the hub, so this part Shrinkage porosity, shrinkage cavity, cracks and other defects are most likely to occur.
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