Magnesium alloy has characteristics of light weight, high specific strength, good damping performance, strong electromagnetic shielding ability, and good cutting performance. It is widely used in aerospace, automotive electronics and other fields. At present, the most widely used magnesium alloys in China are ZM5 and ZM6. However, due to addition of some trace elements, high-temperature performance of ZM6 has been significantly improved, giving it more application advantages in aerospace and other fields. ZM6 magnesium alloy grille is intended to be used in a new aeroengine in China. Outline size is 550 mm*300 mm*40 mm, wall thickness is 2.5~3.0 mm, main structure is grid-type, ribs are all curved surfaces, and ends are rounded structures. Dimensional accuracy is required to reach HB6103-CT5 level. It is a typical complex thin-walled magnesium alloy casting and is produced by sand casting. It is extremely difficult to ensure forming, dimensional accuracy and appearance quality, so investment low-pressure casting is used for production..
Low-pressure casting has characteristics of both gravity casting and pressure casting. Microstructure of parts is dense and quality is good. It is especially suitable for casting of complex parts with thin walls, such as aluminum alloys, copper alloys and magnesium alloys. However, since wall thickness of grid body is uniform and thin, and outer dimensions are large, it is not easy to form a single temperature gradient inside mold cavity during pouring process. If casting pouring system, process parameters and other settings are unreasonable, casting defects such as slag inclusion, air entrapment, insufficient pouring, shrinkage porosity, shrinkage cavities, and hot cracking will easily occur in actual production.
This study intends to use slot runner to design pouring system of casting. Use ProCAST software to conduct numerical simulation calculations on casting process of grid casting, dynamically observe the entire process of filling and solidification as well as changes in temperature field, and predict defects. Form position and analyze its causes, so as to optimize design process and achieve purpose of forming high-quality grille castings.
Graphic and text results
BBased on previous experience in process design of thin-walled investment castings, two gating systems were designed according to structural characteristics of grid casting. Horizontal slot runner is used, and molten metal is introduced from the side. Connection position between slot runner and casting body is at flat position of side wall, which will not damage fillet structure of grid casting rib plate, and difficulty of later repair is small. Design scheme is shown in Figure 1.

 

Figure 1 Gating system design plan
Import grid casting pouring system design plan into ProCAST software, use meshing tool MeshCAST to divide it into finite element meshes, then import it into PreCAST to set relevant process parameters (pouring temperature is 750℃, shell preheating temperature is about 300℃, filling pressure difference is 60 kPa, and filling time is about 8 s). Finally, ProCAST software is run to perform coupling calculations on flow filling process and temperature field of model.

 

Figure 2 Scheme 1 filling process simulation

 

Figure 3 Scheme 2 filling process simulation
Comparing Figure 2 and Figure 3, it can be seen that compared with Scheme 1, Scheme 2 reduces filling time by 8.2%. In Scheme 1, filling of horizontal rib plate is formed by confluence of two front-end metals entering from left and right sprues. There is no replenishment of fresh molten metal, and lateral flow distance of molten metal is long, while in Scheme 2, filling of horizontal rib plate is in width direction. It is divided into two parts, and lateral flow distance of molten metal is shortened by 50% compared with Plan 1. Similarly, when mold filling rate reaches 98.8%, unfilled positions in Scheme 1 are distributed at ribs and uppermost end of casting, while unfilled positions in Scheme 2 are only at the uppermost end of casting.

 

Figure 4 Simulation of solidification process in Scheme 1

 

Figure 5 Scheme 2 solidification process simulation
Comparing Figure 4 and Figure 5, it can be seen that compared with Scheme 1, the entire casting in Scheme 2 cools down to below solidus temperature at least 6 s earlier. Temperature gradient between left and right sides of casting and other parts of casting in Scheme 1 is significantly higher than that in Scheme 2. In addition, studies have shown that low-pressure casting process of magnesium alloys may cause stress concentration and cracking due to influence of solidification sequence and cooling rate control.
Figure 6 is a comparison chart of defect prone prediction simulation results of two schemes. It can be seen that both solutions have tendency to produce shrinkage cavity defects at intersection of cross rib plates. Plan 2 has an obvious tendency to produce shrinkage defects at three vertical tubes. In terms of number of defects that tend to occur in casting body, Plan 1 has increased compared to Plan 2; in terms of size of a single defect, Plan 1 has decreased compared to Plan 2.

 

Figure 6 Defect tendency prediction simulation results

 

Figure 7 Radiographic flaw detection results at ribs of casting in Scheme 1
Since castings are poured according to plan 1, it is easy to repair later. Based on simulation results of filling and solidification process of plan 1 and plan 2, as well as prediction results of defect tendencies of two plans, plan 1 was given priority for production trial production. After casting is poured, in horizontal direction, there are a large number of insufficient pouring defects in the middle rib plate, and casting cannot be repaired through later repair welding. Filling of plan 1 horizontal rib plate is formed by confluence of two front-end metals entering from left and right ingates. On the one hand, high-speed filling of molten metal will increase friction resistance; on the other hand, longer filling distance will lead to a long lateral flow distance of molten metal during mold filling. In addition, horizontal middle rib is far away from gating system and lacks replenishment of fresh molten metal. These reasons all lead to insufficient pouring defects in horizontal middle ribs. Radiographic flaw detection was carried out at intersection of ribs, and no shrinkage porosity or shrinkage cavity defects as shown in Figure 6a were found, as shown in Figure 7. This may be due to thin wall thickness of casting, rapid cooling rate, and similar temperatures of all parts of casting, which solidified almost at the same time. Therefore, no shrinkage cavities and porosity defects occurred during actual trial production process.
After using Plan 2 for trial production, the overall casting was completely formed and there were no under-casting defects. This is because compared to Scheme 1, Plan 2 adds a vertical tube, which greatly increases area of slot runner. At the same time, slot runner changes from side covered with horizontal ribs to the front covered with vertical ribs. Increase in number of vertical tubes and change in direction of gap runner coverage have optimized pouring system. On the one hand, gap runner has changed from covering horizontal ribs on the side to covering vertical ribs on the front, which reduces distance that molten metal flows in horizontal runner, and molten metal can enter casting cavity earlier, thereby reducing heat loss of molten metal; on the other hand, gap runner added in the middle reduces lateral flow distance of molten metal during filling process by half, greatly reducing risk of insufficient pouring defects in casting. Trial production was carried out according to Plan 2, and purpose of casting casting was achieved. However, finishing of casting after runner was removed was difficult and time-consuming. Finished casting is shown in Figure 8a. Casting was subjected to radiographic inspection and no shrinkage porosity or shrinkage cavities were found, as shown in Figure 8b. Reason for absence of shrinkage porosity and shrinkage cavities is same as mentioned above.

 

(a) Finished grille castings

 

(b) Radiographic flaw detection results at ribs
Figure 8 Casting trial production results of Plan 2
Figure 9 Metallographic structure at different locations of casting
After first trial production of Plan 2 was successful, 6 grid castings were continuously produced in small batches, and all were completely formed. By customizing a special waist-shaped grinding head to solve problem of grinding fillet of rib plate, and using a small diameter grinding head to solve problem of grinding cross section of rib plate, problem of later correction of casting is completely solved, and efficiency and quality of dressing are significantly improved. In summary, Option 2 can meet requirements for mass production of grille castings.
Figure 9 shows metallographic structure of castings obtained by two schemes. It can be seen that there are no obvious shrinkage porosity and shrinkage cavity defects in as-cast structure as a whole. Grains are chrysanthemum-shaped and have obvious dendritic morphology. According to Mg-Nd binary phase diagram, it can be seen that dendrite matrix is composed of α-Mg phase, while grain boundaries are eutectic phases. By comparison, it can be found that grain size at horizontal middle rib corresponding to Figure 9a is significantly smaller than grain size at vertical tube runner corresponding to Figure 9b and Figure 9c, which indicates that middle rib has a greater cooling rate. Due to extremely fast cooling rate, size of desolvated phase in α-Mg phase is very small, so it is not observed in metallographic structure.
In conclusion
(1) Adopting hydraulic principles and increasing contact surface area between slot runner and casting as well as number of vertical cylinders can significantly improve mold filling effect and reduce temperature gradient throughout solidification process of casting.
(2) Optimized gating system design of Scheme 2 is used to realize forming of grille castings. It is used with a special grinding head for later casting trimming, which can meet requirements for mass production of grille castings.