Characteristics of shell die castings were analyzed, casting simulation software ProCAST was used to analyze flow filling, heat balance and temperature field of die casting mold, which provided verification for rationality of design of casting system and cooling system of mold. Actual production of die casting shows that mold design is reasonable and die castings are of high quality.
With development of industries such as automobiles, aerospace and electrical appliances, in order to improve quality of die castings and achieve design requirements such as energy saving and pollution reduction, application of aluminum alloy die castings is becoming more and more extensive. At present, die casting has become one of the most widely used processes in the forming of aluminum alloys for automobiles. As a forming method of final shape and near final shape, die casting can obtain thin-walled, complex structure, clear contour, dense organization, high strength castings, and high production efficiency, which is suitable for mass production. Die-casting molds are main process equipment for die-casting production, which directly affects quality, cost and production efficiency of die-casting parts. This paper conducts simulation analysis based on ProCAST for complex shell parts, providing support for die-casting mold design.

Die-casting material is A380, which is used as valve body of oil pipeline, as shown in Figure 1. Part structure is relatively complex, with many rib structures. After die-casting, three end openings on valve body need to be threaded to connect oil pipe parts. In order to ensure strength of threaded connection and meet requirements of air tightness of valve body, these open thin walls must be of good quality, dense and free of pores. Shell weight is 2.952 kg, the overall thickness of shell parts is relatively thin, with main wall thickness of about 5 mm, but wall thickness varies greatly, with minimum wall thickness of 2.5 mm and maximum wall thickness of 9.4 mm.

 

Figure 1 Shell parts

According to structural characteristics of shell die casting, parting surface is selected at the largest opening of shell. Pouring system not only plays an important role in controlling flow direction and state of molten metal in mold cavity, exhaust conditions, and pressure transmission of mold, but also can adjust filling speed, filling time and temperature distribution of mold. Shell is a cylindrical casting, and six inner gates are used at the end of the largest cylinder, so that molten metal enters cavity along wall to avoid direct impact on core, and first fills bottom of cavity, which is conducive to exhausting gas. Pouring system is shown in Figure 2.

 

Figure 2 Shell casting pouring system

Layout of cooling system is of decisive significance for forming and deformation of product. In order to achieve cooling effect, a combination of spot cooling and cooling water channels is adopted. Figure 3 shows distribution of cooling water channels inside die casting mold. Each insert has an independent cooling water channel or spot cooling. These water channels are distributed in the center of each thin-wall opening of casting to enhance cooling of each thin-walled part.

 

Figure 3 Cooling water channel and spot cooling layout

This secondary mold is relatively large, length and width of mold frame are 990 mm and 910 mm respectively. Mold core structure is shown in Figure 4. Fixed mold is a whole, and movable mold contains an insert 5; openings around shell are formed by 4 sliders, and core pulling movement is realized by 4 hydraulic cylinders; replaceable long pin cores are made in the 4 deep hole positions. Mold diagram is shown in Figure 5.

 

Figure 4 Moving and fixed molds and inserts

 

Figure 5 Mold diagram

Use HyperMESH to divide surface mesh of casting and mold, then input high-quality surface mesh model into MeshCAST of ProCAST. After checking, generate tetrahedral meshes. Number of meshes of die casting and mold is 4.45 million. Set boundary conditions in PreCAST and perform simulation calculations.

Casting material is A380, and mold material is H13 steel. Pouring temperature of aluminum liquid is 650 ℃, mold preheating temperature is 220 ℃, gate speed is 3 m/s, and water cooling temperature is 20 ℃. According to relevant literature, heat transfer coefficient between mold and casting is set to 20 000 W/(㎡·K), heat transfer coefficient between movable mold and fixed mold is set to 1 000 W/(㎡·K), heat transfer coefficient between mold and air is set to 100 W/(㎡·K), heat transfer coefficient between release agent and mold is set to 100 W/(㎡·K). Diameter of cooling water channel is 10 mm, cooling water flow rate is 1 m/s, heat transfer coefficient between cooling water and mold is calculated to be 5 000 W/(㎡·K).
Die casting production cycle can be divided into four stages: ① metal liquid filling, pressure holding and solidification; ② mold opening and casting removal; ③ mold release agent spraying; ④ mold closing. Four stage times are 40 s, 15 s, 5 s, and 10 s, respectively, and the total time of one cycle is 70 s. Pouring speed of gate is set to 3 m/s, temperature of release agent and air are both set to 20 ℃.

Figure 6 shows flow field distribution of molten metal at different times during the filling process. The entire filling process is 0.08 s. At the beginning, molten metal enters cavity along wall and flows smoothly. Flow rates of six streams of molten metal are not much different. Molten metal on the left and right sides of casting is filled sufficiently and evenly. Filling process is promoted as a whole, avoiding "eddy current" formed by molten metal flowing back along wall cavity. Last position filled with molten metal is overflow groove farthest from gate, which basically realizes filling order.

 

Figure 6 Flow field of shell casting filling process

In die casting cycle, main source of mold heat is poured high-temperature molten metal, heat dissipated by mold is dissipated to air and flowing cooling water takes away part of heat. If heat absorbed and dissipated by mold are equal in unit time, reaching an equilibrium state, it is called thermal balance of mold.
Take a point on cavity surface of movable mold, fixed mold and casting, as shown in points 1, 2 and 3 in Figure 7. Draw temperature-time curve, as shown in Figure 8. It can be seen that after 10 die-castings, mold reaches thermal equilibrium.

 

Figure 7 Selected points on mold and casting

 

Figure 8 Temperature-time curves of three points

Temperature field of 11th cycle when mold reaches thermal equilibrium is selected for analysis. Figure 9 shows temperature field changes of movable mold, fixed mold and insert 5 in one cycle. Three representative moments are selected for analysis, namely 0th, 4.98th and 55th seconds in a cycle, that is, before filling, filling and pressure holding and spraying release agent. Before filling, temperature field of mold is evenly distributed, with an average temperature of about 370 ℃; when metal liquid is filled, surface temperature of mold cavity rises sharply; in pressure holding stage, mold gradually cools down by dissipating heat to air and flowing cooling water takes away part of heat. When mold is opened and parts are taken out, mold release agent is then sprayed. Under effect of mold release agent and air quenching, surface temperature of cavity drops rapidly, and surface temperature of most mold cavities drops below 420 ℃.

 

Figure 9 Temperature field of movable mold, fixed mold and insert 5 at different times in one cycle
As shown in Figure 9, temperature distribution of movable mold and each insert is relatively uniform, and temperature gradient of cavity surface changes little, indicating that cooling system is reasonably designed.

Mold has been used in production of castings in subject. Die casting after machining is shown in Figure 10. There are no pores in three thin walls that need to be processed with threads, and thin wall quality is good; threaded part is complete and has no pores, which meets strength of threaded connection and air tightness requirements of valve body.

 

Figure 10 Shell parts after processing

Based on CAE analysis, die-casting mold design of aluminum alloy shell parts was carried out. CAE analysis results verified rationality of mold casting system and cooling system. Actual production of mold shows that three thin walls that need to be processed are dense and have no pores, which can meet requirements of thread connection strength and valve body air tightness, and mold design is reasonable.