Casting Process Design And Trial Production Of Aluminum Alloy Pressure Shell Castings

1
Structural characteristics and requirements of castings

The basic size of the pressure shell casting is 157.8 mm×151.1 mm×64.6 mm, the material is AlSi7Mg0.3, and the weight is 0.91 kg. The three-dimensional model and main dimensions of the casting are shown in Figure 1, which is a small and complex part. Its technical requirements include: unfilled casting tolerances should comply with BS6615/ISO 8062 and engineering standard E3-20, defects should comply with engineering specification E4-05-006, and castings should meet relevant quality requirements in material specifications. The minimum wall thickness of the casting is 4 mm, and flashes, burrs and protrusions are not allowed on multiple surfaces, so the pouring position and parting surface need to be considered; in addition, the maximum cross-section of the casting is not on the same plane, so Reasonably design the parting surface.

Fig.1 Three-dimensional model and outline size of casting

The wall thickness of the middle part and the vertical flange of the pressure shell casting is relatively large, which is easy to form hot spots and shrinkage defects during the solidification process. Therefore, risers are added at these two positions for feeding.

2
Casting Process Design

2.1 Pouring position and parting surface

First, determine its pouring position and parting surface. Since the digital moldless casting precision forming machine is used for molding, it is necessary to consider whether the machine can mill out the corresponding sand mold and sand core when designing the pouring position and parting surface. After consulting the casting process manual and comparing the advantages and disadvantages of different pouring positions, choose the pouring position and parting surface as shown in Figure 2. The advantage is that the place with the largest wall thickness is located in the upper part, which is conducive to the realization of sequential solidification, and the measurement reference plane is located in the lower part. It is easy to ensure the casting quality; the parting surface is generally flat, and partly curved surface parting, the shape is relatively simple, and there is no plane that affects the technical requirements of the drawing (flash, burrs and protrusions are not allowed for multiple surface requirements).

Figure 2 Pouring position and parting surface

2.2 Sand core and gating system design

According to the casting structure, two sand cores are designed, as shown in Figure 3.

Fig.3 Three-dimensional model diagram of two sand cores

Since the piece is a small aluminum casting, after checking the casting manual and related information, it is known that the aluminum casting mainly adopts an open pouring system. The open gating system can ensure the filling liquid flow rate is low when filling the mold, so that the filling is stable, and it can also avoid turbulent flow and oxide film involvement, so as to ensure that the molten metal entering the cavity is relatively pure, so this process adopts For an open gating system, the section ratio of each runner is ΣA straight: ΣA horizontal: ΣA inner = 1:1.4:1.3. Moreover, using the center pouring pouring system, the molten metal flows into the cavity from the middle. Since the structure of the part is a turbine shell structure, it is easy to generate air entrainment during the filling process. Therefore, the side injection type inrunner part is filled along the tangential direction, which can reduce air

Figure 4 gating system design (a) 1 # sand core

For the pouring time of aluminum alloy, the following empirical formula can be used to calculate:

In the formula: t is the pouring time, s; s is the coefficient, generally s=3; G is the total weight of molten metal in the mold, kg.

From this, the pouring time is calculated as t=3.27s. In order to control the pouring time in actual pouring, the pouring time is taken as 3 s.

Consult the aluminum alloy manual, and use the formula (2) to calculate the cross-sectional area of the sprue:

In the formula: ΣA is the total cross-sectional area of the narrowest part of the sprue, cm²; η is a coefficient, usually 0.04~0.07; H is the average pressure head, cm, and its calculation formula is shown in formula (3).

In the formula: H0 is the height of the sprue above the inner runner, cm; h0 is the height of the casting above the inner runner, cm; h is the height of the casting, cm.

The calculated cross-sectional area of the sprue is 2 cm². The cross-section and dimensions of each part of the gating system finally obtained through calculation are shown in Figure 5.

Figure 5 Cross-section and dimensions of each part of the gating system

3

Preliminary Simulation and Analysis of Casting Process

The preliminary process of the pressure shell casting was simulated by ProCAST software, and the simulation results of the filling and solidification process are shown in Figure 6 and Figure 7.

Fig.6 Filling velocity field of casting at time 1.1 s

Fig.7 Filling and solidification temperature field of casting at 3 s

It can be seen from the figure that the molten metal is injected along the inner runner against the cavity wall and then gradually fills the cavity. During the entire filling process, the molten metal flows relatively smoothly without obvious gas entrainment. The simulation results of shrinkage porosity and shrinkage cavity of the pressure shell casting are shown in Fig. 8 .

Fig.8 Simulation results of shrinkage cavity and shrinkage porosity

It can be seen from Figure 8 that, except in the gating system, the shrinkage cavity and porosity defects are mainly concentrated at the top and the thick part near the middle of the casting, and a small amount of defects also appear at the lower part of the flange and the rear boss of the flange. The main reason for these defects is that the average wall thickness of these parts is relatively large and the heat dissipation is not easy.

4

Optimization of casting process

According to the shape characteristics of pressure shell castings and the above simulation results, the casting process is optimized, and the top riser is designed. The purpose of setting the top riser is to feed the top of the casting, so as to eliminate the thickness of the top and the middle of the casting. Partial shrinkage cavity and shrinkage porosity defects. At the same time, a flat air outlet riser is added on the top of the flange, which can feed the liquid and solidification shrinkage of the flange, and cooperate with a semicircular conformal cold iron placed at the bottom of the flange to adjust the method. The temperature field distribution of the flange part is used to eliminate a small amount of shrinkage cavity and shrinkage porosity defects in the flange part. In addition, 4 circular air outlets are added, mainly to enhance the discharge of gas in the cavity. The optimized casting process scheme is shown in Figure 9. Using ProCAST software to simulate, the simulation results of shrinkage cavity and shrinkage porosity defects are shown in Figure 10.

Figure 9 Optimized casting process scheme

Fig.10 Simulation results of shrinkage cavity and porosity defects of optimized process scheme

It can be seen from Figure 10 that the shrinkage cavity and shrinkage defects in the main body of the pressure shell casting are all eliminated. Therefore, the final casting process scheme shown in Figure 9 was adopted for production trial production.

5

Actual production of pressure shell castings

5.1 Mold processing

Resin sand is used for numerical control machining of upper mold, lower mold and sand core by digital moldless precision casting machine (model: CAMTC-SMM2000). Use the processing module of UG software to generate the processing tool path of sand mold and sand core. The tool used for rough machining is a Φ8 mm flat knife; Feed, cutting between layers.

Paint the alcohol-based zircon powder paint on the processed sand mold and sand core respectively, and the painted sand mold and sand core are shown in Figure 11. The lower mold with assembled sand core is shown in Figure 12.

Figure 11 Sand molds and sand cores painted with paint

Figure 12 Lower mold with sand core assembled

5.2 Melting and pouring of aluminum alloy and post-treatment of castings

Then fit the mold and prepare for pouring. Put the required weight of AlSi7Mg0.3 (ZL101A) casting alloy ingot into the crucible resistance furnace for heating and melting, set the melting temperature to 750 °C, and the pouring temperature to (730±5) °C. Aluminum alloy special degassing and slag removal agent is used for melt treatment, and Al-5Ti-1B is used for modification treatment of aluminum alloy melt.

After the casting was cooled for 1 h, it was boxed and shaken out, and the flash and burrs at the parting surface of the casting were cleaned with tools. The finally obtained casting (with casting process) is shown in Figure 13.

The quality inspection of the castings is carried out, and the size meets the requirements of BS6615/ISO8062 and engineering standard E3-20. It can be seen that the defects meet the requirements of engineering specification E4-05-006, and the chemical composition meets the relevant quality in the national standard specification of "Casting Aluminum Alloy" Require.

Figure 13 The actual casting of the trial production (including the casting process part)

4

in conclusion

(1) According to the structural characteristics of small pressure shell castings, a complete casting process design was carried out, and trial production was carried out using digital moldless precision casting forming technology. The obtained castings met the technical requirements of the drawings, and gravity The casting method lays a technological foundation for the next step of pilot test and large-scale production, and also provides a certain reference for the casting process design of other pressure shell castings.

(2) The yield rate of pressure shell castings produced by the optimized process in this paper is 69%. If it is necessary to further improve the process yield, special casting methods such as metal mold casting or low pressure casting are required.

(3) The digital moldless precision casting forming technology has the characteristics of high processing precision of sand molds and sand cores, curved surface modeling, no need to manufacture molds, and short production cycle. It is especially suitable for small batch single-piece production and casting process development and trial production of castings .

author
Ma Yuqi Liu Ruixiang Zeng Wenwen History Books
Wang Chengyun Zhang Wei Xiang Qingchun
School of Materials Science and Engineering, Shenyang University of Technology

Yu Weiye
Dalian Bobst Motor Co., Ltd.

This article is from: Foundry Magazine