Mold filling process refers to the flow process of liquid metal in each component of gating system and mold cavity. In this process, with the continuous heat exchange between high-temperature liquid metal and mold wall in the mold. At the same time, contact with the air in the cavity will oxidize the surface layer of liquid metal, increase the impurity content, decrease the temperature, increase the viscosity and increase the flow resistance. Moreover, the continuous flow process is accompanied by the involvement of oxide film and gas, so that the new surface is exposed and further oxidized. The involved gas and oxide film may develop into various casting defects. The whole flow process is in a changing and unstable state. This makes the filling law of liquid metal more difficult to grasp. Despite many difficulties, researchers have come up with a variety of methods to explore the flow of molten metal, such as hydraulic simulation experiment, X-ray display, sensor positioning and computer simulation.
(1) Hydraulic simulation experiment
According to the similarity principle, the hydraulic simulation experiment uses water as the medium instead of liquid metal to pour into the pre designed transparent mold, and judges the rationality of the gating system design by observing the flow of water. This method provides a certain reference for the theoretical research of gating system design. However, because the surface tension of water is far less than that of various liquid metals, and its critical velocity and critical height of turbulence are also far from that of liquid metals, the simulation accuracy is low by simply using water instead of liquid metal as the simulation medium. Moreover, this method does not consider the temperature change in the filling process of high-temperature metal liquid, nor the viscosity change caused by temperature change, so it can not truly reproduce the filling form of liquid metal in the gating system.
(2) X-ray display
Using X-ray fluorescence technology to directly display the filling process can obtain reliable test results. The application of X-ray real-time observation system makes people see the mold filling process of castings for the first time, which provides the most reliable basis for verifying the rationality of gating system design. This kind of equipment has been used in many foreign universities. At present, the Institute of metals, Chinese Academy of Sciences has also introduced one, but this kind of equipment is expensive and limits the volume of observation targets. At present, it can only be used in scientific research.
(3) Sensor positioning method
The sensor positioning method takes fine copper wire as the sensing element. It is buried in the sand mold, and one end of the fine copper wire is flush with the inner surface of the mold. When the mold is allowed, the position of the pouring liquid metal is displayed with the help of the conductivity of the liquid metal. This method requires hundreds of copper wires, which is not expensive, but the operation is troublesome. In addition, there are similar methods. Resistance simulation method is a method proposed by Beijing University of science and technology to display the filling process. It is simple to operate and low cost. In this method, a resistance wire fixed in the mold cavity is used, and its resistance value has a linear relationship with the length. In the process of mold filling, the resistance wire is gradually covered by liquid metal, and the resistance value of the resistance wire also decreases. So as to determine the rising position of metal liquid level.
(4) Computer simulation
The significance of numerical simulation lies in the numerical simulation calculation of casting filling and solidification process. By simulating the influence of different process parameters on casting quality, the optimization scheme of casting process is put forward and repeated simulation verification is carried out to ensure qualified castings. Because numerical simulation can greatly reduce the cost of casting trial production, it has attracted extensive attention and carried out in-depth research at home and abroad.