Research On Vacuum Directional Solidification Process And Crystal Growth Control

In the vacuum directional solidification process, suction casting and solidification occur in the process of evacuating the mold. It can produce complex shapes, high precision thin-walled castings. Vacuum environment helps to reduce the gas in the mold, to avoid casting appeared wrapped gas, shrinkage porosity and other defects. Bottom rapid solidification makes full feeding, and further, more detailed microstructure and mechanical properties improved significantly. The research group has developed an adjustable suction side suction vacuum casting mold(patent number: ZL 2015 2 0274416.2), moreover, directional solidification casting is one of the important features. However, in the experimental scene, physical fields such as velocity field, temperature field and so on in the casting filling and solidification process are more difficult to detect, so the numerical simulation is a very good alternative. Filling and solidification processes of self-developed vacuum directional solidification apparatus is studied with numerical simulation methods in this paper. It is optimized for devices and exploring its microstructure formation process.Firstly, the device casting filling and solidification process were simulated through simulation software based on the introduction of numerical simulation methods on casting filling and solidification process, which analyzed the distribution and variation of casting temperature, velocity and liquid rate. Secondly, it reveals influence of the flow channel, distance between flow channel and the inner mold, the lower mold height and other parameters on the mold filling velocity, temperature distribution, homogeneous distribution and the solidified layer by layer. Finally, crystal growth mode is simulated during solidification.The conclusion shows that: The temperature on the side edge portion of the casting decreased significantly and the overall temperature bottom was higher at the end of filling; The liquid flow rate was greater as it close to the centerline; Casting and mold temperature increased from the bottom layer by layer and temperature gradient smaller and smaller from bottom to top during directional solidification process; When increasing flow channel radius, filling time was reduced, and the trend was approximately inversely. Therefore, in order to ensure that the smaller filling time, the flow channel radius should be provided in more than 5mm, less than10 mm. When hot liquid metal pouring into the mold mouth, uniform distance increased as the flow channel radius increased and reached the extreme value at 7mm. The filling time changed with the distance between inner mold and flow channel unobtrusively; When increasing the distance between the mold and the flow channel, the metal liquid absolutely uniform distance increased and when the distance was equal to 2mm, uniform distance was smaller but when the distance was greater than 7mm, a serious turmoil occurred during the filling; With the increase in the height of the lower mold, maximum temperature difference of the center point on the casting surface increased, and showed a sharp upward trend, when the height was equal to 20 mm, the temperature difference tended to be stable. Microstructure simulation to casting filling and solidification process showed that during solidification, columnar crystals grew along the direction perpendicular to bottom surface of cavity clearly. The distribution of casting columnar crystals decreased from bottom to top gradually and the isometric crystal appeared to increase, which indicated the temperature gradient was more conducive to the formation of columnar crystals.