Abstract: Taking an automobile engine crankcase as an example, leakage location of aluminum alloy die castings is introduced. Through sealing test "bubble" test and section inspection of defective parts of casting, it is determined that cause of leakage is through shrinkage and shrinkage inside casting. Root cause of shrinkage is analyzed by using cause analysis table, and shrinkage defects of crankcase are effectively solved through local pressurization, increased core cooling, and heat dissipation mesh processing on cavity forming surface, so as to improve first-time qualified rate of aluminum alloy die-casting products.
Die casting is process of filling mold cavity with molten metal at high pressure and high speed, cooling and forming under high pressure to obtain product. With rapid development of automobile industry, demand for die castings is also increasing. Most of parts such as automobile engine cylinder block, oil pan, cylinder head cover, front and rear end covers, clutch housing and transmission housing of gearbox are made of aluminum silicon alloy through high-pressure casting to obtain blanks. Blanks obtained by high-pressure casting are near-net forming processes, and final product is obtained through only a small number of CNC machining processes.

Figure 1 is a crankcase of a certain automobile engine. Material is ADC12, casting outline size is 442mm×358mm×173mm, and weight is about 4.5kg. This product has a complex structure, with a general wall thickness of 3.0mm and a wall thickness of 15-20mm in local functional area. Five faces of case need to be CNC machined to meet product's dimensional accuracy and assembly requirements. Sealing performance requirements of product are: cavity detection pressure is 100kPa, and leakage is less than 20cm3/min; there is a cross oil channel hole with a diameter of 12mm in 90° direction in area A. Oil channel hole requires a detection pressure of 600kPa and a leakage of less than 15cm3/min.

 

In actual production, crankcase is tested for sealing after processing, and there is no leakage in cavity. Under pressure of 600kPa, 12% of parts of high-pressure oil channel are scrapped due to excessive leakage. Leakage of leaking parts is statistically analyzed, and leakage value is distributed in 15-30cm3/min. To solve leakage problem, specific leakage location must be determined first. "Bubble" test is performed on parts. It is found that there are small bubbles seeping out from M8 hole in K direction shown in Figure 1, which is determined to be leakage point of high-pressure oil channel.

In order to investigate cause of crankcase oil channel leakage, Φ12mm oil channel hole and M8 bolt hole are cut in direction of connection line at the side of leakage part. It is found that there are small shrinkage holes and shrinkage in thick wall of defective part. Defect state is shown in Figure 2. Due to shrinkage phenomenon inside casting, when oil channel hole and M8 bolt hole of casting are machined in subsequent process, dense chill layer on the surface of die casting is destroyed, and micro leakage occurs along M8 threaded hole under a pressure of 600kPa.

 

The key elements of die casting production are die casting machine, die casting mold, and die casting alloy. Die casting process combines three elements organically. Therefore, factors affecting quality of die castings include die casting machine, die casting mold, die casting alloy, die casting process and die casting structure. Defect cause comparison table is used to analyze causes of shrinkage and shrinkage in crankcase from above five factors. Cause comparison table of shrinkage defects is shown in Table 1. The ones with significant influence are position of inner runner, boost pressure, mold temperature and wall thickness of casting.
Table 1 Comparison table of causes of shrinkage defects in aluminum alloy die castings

Note: ○ no effect, ● slightly effect, ●● some effect, ●●● significant effect.
Combined with analysis results of cause table, design of mold pouring system is re-evaluated, and it is confirmed that there is a branch runner at defective part, which can meet requirements of metal liquid filling and transmission of boost pressure. Simulation software was used to simulate and analyze different die-casting parameters. As a result, there was always an isolated liquid phase area in defective part, as shown in A-A section in Figure 3.

 

Above analysis confirmed that root cause of crankcase oil channel leakage is large local wall thickness of casting. There is an isolated liquid phase area in cooling process of casting after die-casting. Isolated liquid phase area cannot be pressurized and compensated during cooling process, finally shrinkage and shrinkage are formed.

According to above analysis of root cause of crankcase leakage, die-casting mold adopts optimization design of adding local supercharging, M8 pre-casting holes and other lateral small cores to increase cooling, "reticulation" on lateral slider forming surface in defective part of casting to solve shrinkage defect at oil channel hole. Structural scheme layout is shown in Figure 4.

 

1. Casting 2. Side slider assembly 3. Side core 4. Local boost mechanism assembly
Figure 4 Mold optimization design

Local supercharging is to implement local extrusion on location of isolated liquid phase area in local thick part of casting during cooling process of casting after die casting. Hydraulic cylinder pushes extrusion rod to squeeze metal liquid in pre-stored space into casting for shrinkage compensation. It is an effective measure to solve shrinkage cavity caused by thick wall thickness. Local supercharging structure diagram of crankcase is shown in Figure 5. According to structural shape of defective part of crankcase, local supercharging mechanism is designed in movable mold, and extrusion cylinder is bolted to rear end of movable mold sleeve. In order to prevent movement wear of extrusion rod and movable mold insert from affecting matching accuracy, extrusion sleeve part is designed. Extrusion sleeve is fixed on movable mold insert to ensure a matching clearance of 0.02mm with extrusion rod. According to volume of isolated liquid phase area, extrusion rod diameter is designed to be 9mm, maximum extrusion stroke is 10mm, and it is flush with bottom surface of cavity after extrusion. According to maximum extrusion pressure of extrusion rod of 4500kg/cm2, diameter of extrusion cylinder should be 50mm.

 

1. Casting 2. Moving mold insert 3. Extrusion sleeve 4. Extrusion rod 5. Moving mold sleeve plate 6. Extrusion cylinder
Figure 5 Local pressurization mechanism

From structure of casting, it can be seen that there are 7 M8 threaded holes in lateral part of casting except oil channel hole. Due to large number of M8 threaded holes, considering influence of position on movable slider, diameter of precast hole core at forming part is the largest, which is 5.6mm, which brings difficulties to cooling of core. Crankcase die-casting mold adopts a split cooling structure for cooling of small core of side slider, and uses high-pressure pure water to force cooling of core at a single point.
Split cooling structure is shown in Figure 6. Lateral core adopts a two-body type. Front end of core is made of SKD61. Diameter of cooling water hole at forming part is 2.8mm. It is processed by an electric spark punching machine. Rear end of core is made of H13 and diameter of cooling water hole is 6mm. Front end core and rear end core are connected by M10×1 thread. Cooling water seal adopts O-ring radial static seal, and material is fluororubber or silicone rubber. Working temperature is required to be 200-250℃, and working pressure is 15 atmospheres without leakage.

 

Cooling water pipe adopts splicing, inner nozzle adopts white steel pipe with an outer diameter of 2.2mm (inner diameter 1.8mm) and an outer diameter of 4mm, and outer pipe adopts 1/8′ galvanized pipe. As shown in Figure 6, during operation, cooling water enters front end of core through inner nozzle from rear end, then returns through gap between outer wall of inner nozzle and core water channel hole. In order to prevent cooling water inside small core from being blocked, 10Bar high-pressure pure water is used to force small core to cool.

During die-casting process, high-temperature molten metal is pressed into cavity for forming and cooling, and cavity is cooled by heat exchange with mold. Die-casting mold absorbs heat brought by high-temperature molten metal and dissipates heat through mold cooling and external spraying, so as to ensure that mold is in a thermal balance state. If mold temperature is too high, it will affect casting quality and mold life. For local specific parts of mold, in addition to strengthening internal cooling of mold, heat dissipation surface can be increased by increasing surface area of cavity to improve heat dissipation efficiency. At oil channel leakage of crankcase, due to large local volume of casting, mold cooling and heat dissipation cannot reach a balanced state. Therefore, heat dissipation mesh is processed in the area with high local temperature on forming surface of side slider. Mesh depth is 0.63mm and arranged crosswise at 90°. Position and cross-sectional shape of heat dissipation mesh are shown in Figure 7. Heat dissipation mesh is lower than mold forming surface and on machining surface of casting. It is removed during subsequent machining of lateral plane without adding additional process processing.

 

After mold is optimized by above three measures, die casting is carried out on original production equipment according to original process parameters. Shrinkage of oil channel hole leakage are significantly reduced, wall chilling layer of M8 threaded hole is thickened, material is dense, casting has no through shrinkage. Mold has been verified by mass production. After product is processed, high-pressure oil channel is tested at a pressure of 600kPa, and product leakage rate is reduced to less than 1%. After mold is optimized, internal quality of high-pressure oil channel hole is optimized, as shown in X-ray comparison picture in Figure 8.

 

(1) Shrinkage cavities and shrinkage defects will occur in thick wall of aluminum alloy crankcase. If shrinkage cavities and shrinkage penetrate into other processing holes, it will cause leakage of castings.
(2) For local thick walls of castings, there is an isolated liquid phase area during cooling. Local pressurization at this location can effectively reduce occurrence of shrinkage cavities and shrinkage.
(3) Small cores can be cooled by split cooling structure. In order to prevent clogging of fine water pipes, high-pressure pure water can be used to cool core.
(4) For specific areas of mold, cavity surface area can be increased by processing cavity with mesh, thereby increasing heat dissipation surface and improving heat dissipation efficiency.