Low-pressure casting aluminum alloy castings have excellent performance. At present, most of aluminum wheels of cars at home and abroad are produced by low-pressure casting technology. Low-pressure casting can not only produce some high-quality complex precision thin-walled castings, but also does not need to design risers and carry out subsequent machining. It has become an advanced near-net shape casting technology.
For aluminum alloy box joint, low-pressure casting process and mold of this part are designed, and solidification process of low-pressure casting is simulated by using ProCAST software, simulation analysis is carried out from aspects of mold parting surface, cavity size, mold wall thickness, exhaust system and core pulling method, causes of casting defects and preventive measures are analyzed, rationality of low-pressure casting process and mold structure design is verified.

Joint parts of aluminum alloy box are shown in Figure 1. Its material is AC3A aluminum alloy, its mass is 1kg, and its chemical composition (mass fraction) is: 12% Si, 1.0% Cu, 1.0% Mg, 0.2% Ti, 1.3% Ni, 0.2% Zn, balance is Al. Outline size of part is 295mm*78mm*128mm, and minimum wall thickness is 2mm. Thickness of the two bosses is the highest, and connection corner between surfaces is relatively thick. Internal space of part is large, so a large core is required, and the two cores must be connected to each other. Airtightness requirement of casting is to maintain no leakage for 15s under pressure of 0.3MPa. Casting adopts T4 heat treatment process. It is planned to be produced by low-pressure casting process.

 

Figure 1 Aluminum alloy box joint parts

 

Figure 2 Schematic diagram of gating system

 

Figure 3 Parting surface selection diagram

 

Figure 4 Die structure diagram

1. Half form of lower formwork 2. Left half form of upper formwork 3. Moving formwork 4. Right half form of upper formwork 5. Gate sleeve 6. Gate screw 7. Guide pillar 8. Guide sleeve 9. Hanging ring 10. Inner mold 11. Upper mold

 

Figure 5 Exhaust grooves on the core

 

Figure 6 Metal core structure
Using ProCAST software for numerical simulation, the pouring temperature is 670℃, preheating temperature of upper mold is 300℃, preheating temperature of lower mold is 350℃, and filling pressure is 5kPa. Since upper mold and lower mold, upper mold and metal core, lower mold and metal core all belong to metal-metal interface, heat transfer coefficient is taken as 5000 W/(m2 K), heat transfer coefficient of mold and air interface is 10W/( m2·K). Surface of mold is set to be air-cooled, and a heat source is set at ingate.

 

Figure 7 Filling process at different times

 

Fig.8 Distribution of solidification time

 

Figure 9 Defect distribution and size
Low-pressure casting process and mold structure of aluminum alloy box joints were developed by computer-aided design method. Gating system adopts a single-liter pipe with an open center structure. Mold structure is designed into three parts: upper mold, lower mold and metal core insert. Lower mold is fixed, upper mold is opened up and down, main core insert is opened left and right, forming a mold with two metal cores. At the same time, core-pulling mechanism and exhaust system of mold are designed. Low-pressure casting process of castings was simulated by ProCAST software, results showed that liquid metal filled mold smoothly, could realize sequential solidification, and had a good shrinkage effect. Rationality of low pressure casting process scheme and mold structure is verified.