【Casting process】Research on vacuum suction casting process for thin-walled precision castings of melting model(1)

The pouring system and exhaust system in the vacuum suction casting process of the thin-walled feature parts of the melting model under study were designed, and the filling process and possible casting defects of the thin-walled parts were simulated using numerical simulation software. The results show that: vacuum casting forming method can better solve the casting of the casting easily encountered filling difficulties, spattering and air holding and other technical difficulties, can obtain a complete contour, internal and external quality of the qualified castings; vertical single-channel pouring program is better than the horizontal two-channel pouring program, so that the alloy liquid flow filling process smooth and smooth, to avoid the liquid metal convergence and impact phenomenon, reduce the production of casting defects.

01 Process Programming

In the casting process of thin-walled castings, solving the problem of alloy melt filling is the key to obtain high-quality castings. Surface tension and Laplace force are important factors affecting the alloy liquid filling the cavity of thin-walled castings, and it is generally difficult to accurately form using traditional gravity casting. Want to obtain a complete profile, accurate size and quality of castings, which alloy chemical composition, melting process, shell properties and casting process play a very important role, especially casting process and shell properties are important factors affecting the casting to obtain the final organization and performance. According to the casting structure characteristics and technical requirements, the choice of silica-sol casting materials and vacuum suction casting anti-gravity casting process, pouring process in the homemade integration of vacuum suction casting machine to complete.

1.1 Characterization of casting structure

Application of Pro / E modeling software to complete the casting three-dimensional solid modeling, as shown in Figure. Structural characteristics: the casting internal cavity state, the maximum outer diameter of 46 mm, the inner diameter of 42 mm, the profile height of 60 mm, the presence of a number of closed or semi-closed openings in the wall. As can be seen in section 1b, the casting is thin and of uneven thickness, with a minimum wall thickness of 1.8 mm and a maximum wall thickness of 8.6 mm, and there is a gradual change in thickness from 4 to 8.6 mm. It can be seen that the part belongs to the typical miniaturization, lightweight, thin-walled, complex, high-precision near-net-forming components.

Through the structural analysis, it can be seen that the casting of this part is the biggest difficulty is uneven wall thickness, large cross-sectional variation, alloy liquid convergence points, there are certain difficulties in filling, if the gravity casting can not guarantee the integrity of the molding and casting quality, so the use of vacuum suction casting anti-gravity casting in order to meet the product in the dimensional accuracy and surface quality of the process requirements.

 

1.2 Structural design of pouring system

According to the characteristics of the anti-gravity casting process, the pouring system takes care of the two functions of mold filling and solidification and shrinkage at the same time, i.e., it is the flow channel for the alloy liquid to fill the mold smoothly, and it is also the pressure channel for the solidification and shrinkage. Ideal pouring system can not only realize the alloy liquid from the bottom to the top of the sequence of smooth filling casting, but also can ensure that the solidification process in accordance with the sequence of solidification.
Therefore, from the point of view of realizing sequential filling and solidification and shrinkage, the design of the pouring system as a whole adopts the tree-mold structure where the straight sprue and the inner sprue are connected. According to the structural characteristics of the casting, this test is proposed to design the two pouring systems shown in Fig.

  • Option 1: Horizontal dual-channel pouring system, with a square straight pouring channel with a cross-section size of 45 mm × 45 mm and a length of 340 mm, and a rectangular inner channel with a cross-section size of 25 mm × 8 mm and a length of 16 mm, which is connected to the ring part of the casting with a larger wall thickness. Three layers are distributed in the straight channel with a spacing of 80 mm and four castings are placed in each layer. The advantage of this design is that it increases the flow of alloy liquid filling and shrinkage, which helps to improve the casting densities.
  • Option 2: Vertical single-pass pouring system with the same dimensions of the straight sprue as in Option 1 and the same mold setup, with the difference that the rectangular inner sprue is placed on the outside of the ring part of the casting where the wall thickness is larger. The program changed the liquid flow filling direction, the use of bottom injection type pouring, so that the filling is more stable.



1.3 Exhaust system design

The fusion mold shell used in this project is a closed shell with silicone sol as the adhesive, which has poor permeability compared with sand and gypsum mold shells. Therefore, it is not conducive to the smooth discharge of the original gas in the cavity and the gas released from the heat of the shell and core during rapid mold filling, thus affecting the mold filling process and process stability. In view of this situation, a venting hole with a cross-section size of 50 mm × 50 mm was opened at the top of the straight channel to increase the permeability of the shell. The exhaust system designed in this way can not only play a good role of exhaust and slag collection, but also increase the local heat dissipation area, accelerate the solidification rate of the straight channel, and promote the sequential solidification.