JMC has developed an automobile engine cylinder group frame, which is made of AlSi9Cu3 (Fe) alloy, and its chemical composition is shown in Table 1. Casting has a contour size of 485 mm*364 mm*148 mm, a total weight of about 7.1 kg, a complex structure, a wall thickness of 4.5 mm, a large number of bosses and holes around it, resulting in excessive wall thickness in some parts of casting. Internal pores of casting are implemented in accordance with "ASTM E505" Level 2 specification, with maximum hole less than 1.5 mm; sealing surface is required to be less than 0.4 mm. When testing sealing of casting, cavity pressure is 0.1 MPa, leakage is less than 10 mL/min; oil channel pressure is 0.3 MPa, and leakage is less than 10 mL/min.

 

Figure 1 Cylinder frame
Table 1 Chemical composition of AlSi9Cu3 (Fe) alloy

According to structural characteristics of cylinder frame, in order to ensure internal quality of thicker parts of functional area around cylinder frame, casting adopts a U-shaped pouring system, which can quickly fill casting and effectively compensate for thick parts on both sides. U-shaped runner of cylinder frame is shown in Figure 2.

 

Figure 2 Cylinder frame U-shaped runner
Mold structure is shown in Figure 3, which consists of movable and fixed mold inserts, 4 sliders and static mold oblique core pulling. This mold is a large die-casting mold. In order to prevent mold from affecting normal guiding accuracy due to thermal expansion during production process, a square guide column and guide sleeve structure are used, which are arranged at four corners of mold. Slider core pulling adopts hydraulic cylinder core pulling, mold design is simple, stable and reliable.

 

1. Moving mold sleeve 2. Moving mold insert 3. Upper core pulling cylinder assembly 4. Upper slider 5. Square guide sleeve 6. Vacuum valve plate 7. Right core pulling cylinder assembly 8. Right slider 9. Lower slider 10, Lower core pulling cylinder assembly 11, Left slider 12. Left core pulling cylinder assembly
Figure 3 Structure diagram of die casting mold for cylinder frame (movable mold)

During production process, casting was tested for sealing after processing, and a micro-leakage was found in M6 threaded hole on filter installation surface. Leakage position is shown in Figure 4, and scrap rate reached 8.3%. X-ray inspection of leaking parts revealed shrinkage cavities and shrinkage pores inside casting. Defective parts were sectioned and it was found that there was overheated adhesive on the wall of blank hole. Cause of leakage was shrinkage cavities and shrinkage pores inside. After machining, small shrinkage cavities inside connected oil channel holes with bolt holes, resulting in a leakage of more than 10 mL/min when oil channel was pressurized to 0.3 MPa.

 

Figure 4 Casting leakage position
Shrinkage cavities in casting exist at filter installation site. Local wall thickness in this area is too thick and there are heat nodes. Cooling speed of casting is slow during solidification, resulting in shrinkage cavities here. Therefore, it is necessary to increase local cooling of mold, increase local cooling speed, balance the overall temperature of mold to effectively reduce shrinkage cavities.

Defect occurs on left slider. Mold slider adopts serial water cooling. Cooling water channel is far away from local wall thickness hot spot of casting, single-point independent forced water cooling cannot be achieved, and cooling effect is not good. Heat generated locally on installation surface of casting machine filter is calculated, local point cooling is re-optimized and added to ensure that left slider is in a reasonable mold temperature range during continuous production.
Original mold design plan is that left slider forming part is an integral structure, and cooling water is a serial structure, which cannot achieve local single-point cooling; M6 bolts are pre-adopted with a core with a diameter of ∅4.8 mm and no cooling. Mold is optimized by changing structure and cooling method of slider forming part. Optimization plan is shown in Figure 5.

 

Figure 5 Structure diagram of left slider optimization plan
Special-shaped part adopts an insert structure, and a Φ7 mm cooling water channel is added inside. It adopts nozzle-type independent water cooling, with a built-in stainless steel nozzle Φ4 mm, water inlet in inner hole, and water return on outer wall, see Figure 6.

 

Figure 6 Left slider special-shaped insert
Small and fine cores are equipped with cooling water to force cooling of core. Core is connected to core nozzle assembly at rear end by thread and sealed by high-temperature resistant O-ring, which can realize quick replacement without removing mold. Front end of core is punched with a water channel hole with a diameter of Φ2 mm by an electric spark puncher. Core adopts independent water cooling of nozzle type, with a built-in stainless steel nozzle of Φ1.2 mm, water entering inner hole and returning water to outer wall. Structural diagram is shown in Figure 7.

 

(a) Cooling structure of small core

 

(b) Small core
Figure 7 Cooling scheme of small core of left slider
Core and insert adopt independent point cooling method, and mold temperature can be controlled by adjusting flow rate of cooling water at a single point. Because diameter of built-in nozzle is too small, pure water is used for cooling, and cooling is carried out under a high pressure of 1 to 1.5 MPa to force cooling of small core.
After calculation, mold cooling capacity Q2' at hot spot of casting after optimization is greater than heat that mold water channel should take away. During use, flow of cooling water can be adjusted to achieve thermal balance of mold.

Through optimization and improvement of mold, local temperature of mold is effectively controlled to avoid defects such as shrinkage, sticking, and strain caused by local overheating of core and cavity at hot spot of casting. Rejection rate of sealing test after machining was reduced from 8.3% to 0.9%. Figure 8 is a comparison of X-ray inspection near filter installation surface before and after improvement.

 

Figure 8 Comparison of X-ray inspection near filter installation surface before and after improvement