1. Analysis of sand casting and selection of sand casting process
Figure 1 shows the chain chamber cover of passenger car engine. The sand casting is designed according to the high-pressure casting process during batch production, and the outline size of sand casting is 380mm × 260mm × 90mm, the basic wall thickness is 3mm, and the weight of sand casting is 1.9kg. The structure of sand casting is relatively simple, but the thin wall area is large. According to the structural characteristics of the chain chamber cover, in order to obtain sand castings with high shape quality and dense structure, the gypsum mold precision casting process is selected in the trial production stage, the wax mold is manufactured by laser selective sintering PS powder process, and the alloy is alsi9cu2m.
2. Design of gating system and sand casting process parameters
Two process schemes of horizontal pouring and vertical pouring are designed as shown in Figure 2. Through simulation analysis, due to the large projection area of sand castings, the gas in the mold is not easy to be discharged orderly during pouring, which is easy to produce suffocation. During vertical casting, the projection area of the sand casting is small, the gas can be concentrated upward above the sand casting, and the gas can be discharged from the sand casting by designing a large riser, which is also conducive to the floating of impurities in the molten aluminum. Therefore, the vertical casting process is finally selected.
The gating system of vertical casting process is selected, and the simulation and process optimization are carried out by using magma software. Fig. 3 shows the simulation results of hot spot, shrinkage and porosity of the optimized process.
According to the simulation results shown in Fig. 4, the air pressure values in many areas of sand casting are at a high level at the end of mold filling. Therefore, the process of vacuum pouring and pressure solidification is selected for the pouring of this sand casting. According to the simulation analysis, the vacuum degree in the tank before pouring is -0.06mpa and the solidification pressure is 0.5MPa.
3. Mold manufacturing process design
The wax mold prototype of chain chamber cover parts selects the wax infiltration process after laser selective sintering PS powder possessed by our company. Due to the thin wall thickness and large plane size of the chain chamber cover, support is added in the prototype design stage to reduce its deformation. The three-dimensional of the part after the support is added is shown in Figure 5. After the wax infiltration cooling is completed, the supported sheets are removed by hand. In order to save the processing time of laser selective sintering, the gating system is not designed on the parts and is assembled by bonding with forming wax in the later stage.
The initial setting time of gypsum casting powder is 12.5min, the final setting time is 13min, and the expansion coefficient is 0.2%. In order to reduce the gas content in the slurry and improve the filling capacity of gypsum slurry, vacuum mixing grouting equipment is used for grouting. The roasting design of gypsum type is divided into three stages. The first stage is 200 ℃ insulation, the second stage is 700 ℃ insulation at the highest temperature, and the third stage is insulation and waiting for pouring after cooling to 250 ℃. The specific roasting process curve is shown in Figure 6.
