Outer dimensions of hollow subframe are 1161mm*877mm*310mm, basic wall thickness is 4mm, weight is about 19kg, and material is AL⁃Si7Mg-T6. Compared with gravity casting, low-pressure casting solidifies and crystallizes under pressure, can obtain castings with dense structure and good mechanical properties, which is suitable for production of such large thin-walled parts.

According to shape analysis of subframe, it is divided into upper and lower molds. After mold splitting, it is found that half of shape is complex and the other half is relatively flat. According to low-pressure casting process, casting needs to be left in upper mold and upper top plate is pushed out, then part is taken out. Therefore, complex shape surface is set in upper mold, and flat surface is set in lower mold, as shown in Figure 1.

 

Figure 1 Subframe

This subframe is formed by only one sand core. According to sand outlet of casting, core head is led out. Positioning of bottom plane of sand core needs to be considered to prevent deformation and ensure its stability. Core heads on four sides are connected into one, positioned in left and right directions, and a plane support is designed on the bottom plane of four sand outlets on bottom surface. Core head of sand outlet on upper plane of casting is not positioned, only positioning and sand core exhaust outlet channel are set in height direction.

Considering characteristics of large volume, thin wall thickness, multiple and scattered hot spots of casting, multi-gate casting is designed. Inner gates are opened on each cross runner for shrinkage compensation at hot spots generated by thick ribs; insulation risers are added for shrinkage compensation at thick and large bosses that cannot be compensated by gate. After casting system is completed, it is analyzed and optimized by CAE simulation software, and the simulation shrinkage results are shown in Figure 2. For hot spots that are difficult to avoid in CAE simulation, they can be solved by later pressure compensation and strong cooling.

 

Figure 2 Shrinkage simulation analysis

Four circular shafts and mounting screw hole areas are all directly stressed parts, mechanical properties are required to be high. If thick and large areas are not properly cooled or compensated, shrinkage or thermal cracking will occur. Basic wall thickness of casting is 4mm. It will solidify faster by relying on heat dissipation of mold itself. Thick and large area at gate position is forced to be quenched after filling is completed to ensure sequence of solidification from position far away from gate to gate. Mold is cooled by water, and mold structure is shown in Figure 3.

 

1. Ceramic sprue sleeve 2. Lower mold 3. Water-cooled insert 4. Lower mold 5. Upper mold 6. Riser 7. Exhaust pipe 8. Heater push plate 10. Upper mold
Figure 3 Mold structure

Since low-pressure casting metal cavity is basically closed, whether exhaust is smooth affects metal liquid filling process and casting quality. In last filling area of casting, top surface of rib and blind riser must be equipped with exhaust. Exhaust grooves can be opened on insert and push rod, and exhaust plugs can be added. In addition to exhaust in mold cavity, exhaust of sand core is also important. First, sand core needs to be dried to reduce gas emission of sand core. An exhaust plug is arranged at head of sand core, and a negative pressure exhaust is added at head of upper mold core to strengthen exhaust of sand core.

Subframe is large in size and thin in wall thickness. Although multiple gates are arranged, height difference of casting shape is large. From shrinkage results of mold flow analysis, defects at the highest point of casting are scattered. Main reason is that the highest end is at the end of filling. Temperature of molten metal drops due to too long flow, resulting in cold shut. Based on this problem, external environment is changed, that is, temperature of molten metal and mold temperature are increased to improve filling capacity of molten metal, so as to solve problems of cold shut and insufficient pouring. A gas heating pipe and a protective plate are added at corresponding position on the back of upper mold of mold to protect surrounding cooling pipes, push rods to prevent push rods from being bent, as shown in Figure 4.

 

1. Protective plate 2. Gas heating pipe 3. Push rod protective pipe
Figure 4 Gas heating pipe layout

Mold is a structure with two upper and lower molds. Materials of upper and lower molds are both hot-working die steel, such as H13. Its working process is as follows: dry air is introduced into a sealed crucible furnace. Under action of gas pressure, molten metal rises along riser tube and smoothly enters mold cavity through gate. After pressurization, pressure maintenance, and pressure relief, upper and lower molds are opened, casting is brought to upper mold, receiving plate is turned in, casting is pushed out to receiving plate by push plate, and receiving plate is removed, completing a working cycle. According to characteristics of casting, temperature of molten metal is controlled at 720~730℃, and temperature of mold is controlled at 350~400℃.