Taking an automobile engine crankcase as an example, this paper introduces location where leakage occurs in aluminum alloy die castings. Through sealing test "bubble" test and cross-section inspection of defective parts of casting, it is determined that cause of leakage was penetrating shrinkage cavities and shrinkage porosity inside casting. Use cause analysis table to analyze root causes of shrinkage cavities, and adopt mold optimization solutions such as local pressurization, increased core cooling, and processing heat dissipation textures on cavity forming surface to effectively solve shrinkage cavity defects in crankcase and improve first pass rate of aluminum alloy die-casting products.
Die casting is a process in which molten metal fills mold cavity with high pressure and high speed, then cools and forms product under high pressure. With rapid development of automobile industry, demand for die castings is also increasing. Automotive engine blocks, oil pans, cylinder head covers, front and rear end covers, as well as transmission clutch housings, transmission housings and other parts are mostly made of aluminum-silicon alloy and are obtained by high-pressure casting. Mold base are obtained by high-pressure casting. High-pressure casting method is a near-net shape process, and final product is obtained through only a small number of CNC machining processes.

Figure 1 shows crankcase body of a certain automobile engine. Material is ADC12.Outline dimensions of casting are 442 mm * 358 mm * 173 mm, and weight is about 4.5 kg. This product has a complex structure, with a general wall thickness of 3.0 mm and a wall thickness of 15~20 mm in some functional areas. Five sides of box need to be CNC processed to meet dimensional accuracy and assembly requirements of product. Sealing performance requirements of product: cavity detection pressure is 100 kPa, leakage volume is less than 20 cm³/min; area A has a cross oil channel hole with a diameter of 12 mm in 90° direction. Oil channel hole requires a detection pressure of 600 kPa, and leakage volume is less than 15 cm³/min.
In actual production, crankcase body was tested for sealing after processing. There was no leakage in cavity. Under pressure of 600 kPa in high-pressure oil passage, 12% of parts were scrapped due to excessive leakage. Statistics on leakage of leaking parts showed that leakage values were distributed between 15 and 30cm³/min. In order to solve leakage problem, we must first determine specific leakage location and conduct a "bubble" test on parts. It was found that small bubbles seeped out from M8 hole in K direction as shown in Figure 1, which was determined to be leakage point of high-pressure oil passage.

 

Figure 1 Crankcase shape and leakage location

In order to investigate cause of oil passage leakage in crankcase body, a side section was made at leakage site in direction connecting Φ12 mm oil passage hole and M8 bolt hole. It was found that defective part had small shrinkage holes and shrinkage porosity in thick wall. Defective state is shown in Figure 2. Due to shrinkage phenomenon inside casting, when oil passage hole and M8 bolt hole of casting are machined later, dense chill layer on the surface of die casting is destroyed, and micro leakage occurs along M8 threaded hole under a pressure of 600 kPa.

 

Figure 2 Cutaway picture of defective part

Key elements of die-casting production are die-casting machines, die-casting molds, and die-casting alloys. Die-casting process organically combines these three elements. Therefore, factors that affect quality of die-casting parts include die-casting machines, die-casting molds, die-casting alloys, die-casting processes, and structure of die-casting parts. A defect cause comparison table is used to analyze causes of shrinkage holes and shrinkage porosity in crankcase from above five factors. Comparison table of causes of shrinkage cavity defects is shown in Table 2. Ones that have a significant impact are position of ingate, boost pressure, mold temperature and casting wall thickness.

Note: ○ has no effect, ●Slightly affected, ●●Influence, ●●●Tremendous influence.
Table 2 Comparison table of causes of shrinkage cavities in aluminum alloy die castings
Combined with analysis results of cause table, design of mold pouring system was re-evaluated, and it was confirmed that there were branch gates at defective parts, which could meet requirements of filling molten metal and transmitting boost pressure. Simulation software was used to conduct simulation analysis with different die-casting parameters. Result was that there was always an isolated liquid phase area in defective part, as shown in A-A section in Figure 3.
In summary, analysis confirms that root cause of crankcase oil passage leakage is that local wall thickness of casting is large. There is an isolated liquid phase zone in cooling process of casting after die-casting. Pressure compensation cannot be achieved during continued cooling of isolated liquid phase zone. shrinkage, eventually forming shrinkage cavities and shrinkage porosity.

 

Figure 3 Simulated solidification analysis

Based on above analysis of root cause of crankcase leakage, die-casting mold adopts optimized designs such as adding local pressurization in defective parts of casting, lateral small cores such as M8 pre-cast holes to increase cooling, and "reticulation" on lateral slider forming surface to solve shrinkage cavity defect in oil passage hole. Structural plan layout is shown in Figure 4.

 

Figure 4 Mold optimization design

Local supercharging is process of cooling casting after die-casting. It performs local extrusion on location where there is an isolated liquid phase area in thick part of casting. Hydraulic cylinder pushes extrusion rod to squeeze molten metal in pre-stored space into casting for feeding, which is an effective measure to solve shrinkage cavities caused by large wall thickness. Partial supercharging structure diagram of crankcase is shown in Figure 5. According to structural shape of crankcase defect, local supercharging mechanism is designed in movable die, and extrusion cylinder is bolted to rear end of movable die cover plate. In order to prevent movement wear of extrusion rod and movable die insert from affecting matching accuracy, extrusion sleeve parts are designed. Extrusion sleeve is fixed on movable die insert and ensures a matching clearance of 0.02 mm with extrusion rod. According to volume of isolated liquid phase area, diameter of extrusion rod is designed to be 9 mm, maximum extrusion stroke is 10 mm, and it is flush with bottom surface of cavity after extrusion. According to maximum extrusion pressure of extrusion rod of 4 500 kg/cm², diameter of extrusion cylinder should be 50 mm.

 

Figure 5 Local boosting mechanism

It can be seen from casting structure that there are 7 M8 threaded holes in lateral side of casting in addition to oil channel hole. Due to large number of M8 threaded holes, considering influence of position on movable slider, precast hole core forming area has the largest diameter. is 5.6 mm, which brings difficulties to cooling of core. Crankcase die-casting mold adopts a split cooling structure to cool small core of side slider, and uses high-pressure pure water to perform single-point forced cooling of core.
Split cooling structure is shown in Figure 6. Lateral core adopts a two-piece type. Material of front end of core is SKD61. Diameter of cooling water hole at forming place is 2.8 mm. It is processed by an EDM punching machine. Rear end of core is made of H13 and diameter of cooling water hole is 6 mm. Front core and rear core are connected with M10*1 thread. Cooling water seal adopts O-ring radial static sealing, and material is fluorine rubber or silicone rubber. Working temperature is required to be 200~250 ℃, and working pressure must not leak under 15 atmospheres.

 

Figure 6 Lateral core cooling water design
Cooling water pipe adopts splicing type. Inner nozzle pipe is spliced with white steel pipes with an outer diameter of 2.2 mm (inner diameter 1.8 mm) and an outer diameter of 4 mm. Outer pipe is made of 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 clogging, 10 Bar high-pressure pure water is used to force-cool small core.

During die-casting process, high-temperature molten metal is forced into mold cavity to form and cool, and cavity is cooled through 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 equilibrium state. If mold temperature is too high, casting quality and mold life will be affected. For specific parts of mold, in addition to strengthening internal cooling of mold, surface area of cavity can also be increased to increase heat dissipation surface and improve heat dissipation efficiency. At leakage point of crankcase oil passage, due to large local volume of casting, cooling and heat dissipation of mold cannot reach a balanced state. Therefore, a heat dissipation texture is processed in the area where local temperature of side slider forming surface is high. Depth of texture is 0.63 mm and is arranged at a 90° cross. Position and cross-sectional shape of heat dissipation texture are shown in Figure 7. Heat dissipation texture is lower than mold forming surface and is on processing surface of casting. It can be removed during subsequent machining of lateral plane without additional processing steps.

 

Figure 7 Forming surface heat dissipation texture design

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

(1) Aluminum alloy crankcase will produce shrinkage cavities and shrinkage porosity defects at thick wall thicknesses. If shrinkage cavities and shrinkage porosity penetrate into other processed holes, it will cause casting leakage.
(2) For castings with locally thick walls, there are isolated liquid phase areas during cooling. Implementing local pressurization in this area can effectively reduce occurrence of shrinkage cavities and shrinkage porosity.
(3) Small cores can be cooled through a split cooling structure. To prevent clogging of thin water pipes, high-pressure pure water can be used to cool core.
(4) For specific areas of mold, surface area of cavity can be increased through cavity processing and texture, thereby increasing heat dissipation surface and improving heat dissipation efficiency.