Engine cylinder block is a fundamental component of engine structure. Main bearing cap structure of cylinder block bears alternating loads imposed by reciprocating motion of crankshaft-connecting rod mechanism. To ensure sufficient rigidity and lightweight, main bearing cap castings are generally produced using aluminum alloy die-casting, with bearing shells embedded in bearing area. Traditional main bearing cap functional design simply machined semi-circular end surface of bearing shell and assembled flywheel assembly. With growing demand for lightweight vehicles, continuous optimization of functional and structural design is being implemented, with aluminum covering side of bearing shell removed to reduce weight. Due to close integration of aluminum and iron, gap separation (abbreviated as "aluminum-iron separation") can occur at inlay joint after machining. This study analyzes causes of aluminum-iron separation in main bearing cap castings and proposes corresponding solutions, aiming to provide a reference for resolving similar issues.
Figure 1 shows fixed and movable molds for main bearing cap casting. Its outline dimensions are 295 mm * 145 mm * 85 mm, weighing 2.65 kg. Casting is made of ADC12 alloy with an average wall thickness of 3 mm. Casting is inlaid with four bearing shells, each weighing 0.4 kg and measuring 100 mm * 53.5 mm * 18 mm. Bearing shells are made of ductile iron. Based on performance requirements of main bearing cap casting, surface finish separation standard for aluminum and iron is a gap of ≤0.2 mm. As shown in Figure 1, main bearing cap casting is rectangular, with four bearing shells inlaid on crossbeam. During initial trial production phase, statistically analyzed aluminum-iron separation defects in castings were as high as 40%, with maximum gap reaching 0.8 mm (see Figure 2). On-site analysis shows that adhesion between aluminum and iron, two materials of different properties, is affected by multiple factors. Therefore, various die-casting process analyses are planned to improve adhesion between molten aluminum and bearing shells.

 

Figure 1 Macromorphology of fixed and movable mold sides of main bearing cap casting

 

Figure 2 Machining results and gap separation of main bearing cap casting
Main bearing cap is cast together with bearing shell. To ensure a tight fit between two different materials, mold temperature, bearing shell preheating temperature, molten aluminum temperature management design were tested and verified.
Runner system design was limited by casting structure and space, resulting in a dual-sided multiple runner layout. When molten aluminum fills through ingate, it directly impacts bearing shell. This is affected and blocked by room-temperature bearing shell, causing molten aluminum filling temperature to drop rapidly. Filling at a low temperature at the end can easily result in poor forming or cold shuts, directly affecting fit between molten aluminum and bearing shell. To compensate for this temperature drop, pouring temperature was increased from 660℃ to 690℃ to ensure fluidity.
Increasing pouring temperature is also crucial, as is raising mold temperature. Main bearing cap casting is designed with eight filling runners. Due to long runners, some heat is lost during flow process. Furthermore, release agent sprayed on movable and fixed mold surfaces evaporates, dissipating some heat and causing a rapid drop in mold surface temperature, which in turn affects filling efficiency of molten aluminum. To address this issue, mold was cooled with oil instead of water. Figure 3 shows mold temperature before and after fixed mold was sprayed. Mold temperature controller was set to 230℃ to ensure a rapid rise in mold surface temperature after release agent was applied, minimizing heat loss and improving fit between molten aluminum and bearing.
Bearing is cast from ductile iron sand molds. Upon contact between room-temperature bearing surface and hot molten aluminum, a chill layer quickly forms. As molten aluminum solidifies and shrinks, significant internal stress promotes separation between aluminum and iron, forming a gap. To mitigate rapid temperature drop during molten aluminum filling process, a bearing preheating device (see Figure 4) is used. This device includes a positioning block, a feed push rod, and a heating wire. High-frequency heating quickly raises temperature to 230℃. A robot then picks up bearing and places it on fixed mold positioning pins. The entire process is quick, efficient, and minimizes heat loss.

 

Figure 3: Mold temperature before and after fixed mold spraying

 

Figure 4: Bearing preheating device

 

Figure 5: Consistent surface temperature of fixed mold bearing
Bearing is sand-cast from ductile iron, which is prone to rust. To ensure surface quality, it is coated with anti-rust oil. Analysis of aluminum-iron separation in casting revealed that oil stains can cause problems with adhesion between molten aluminum and bearing surface. After molten aluminum fills mold cavity, anti-rust oil on bearing surface is encapsulated by molten aluminum. When heated, it vaporizes and cannot escape, resulting in a gap between aluminum skin and bearing. When aluminum skin of casting is machined, exposing bearing, gas escapes through gap, forming a gap and causing aluminum-iron separation.
To verify impact of oil stains, bearing was shot-blasted and sand-blasted, respectively, and compared with a normal casting (without any surface treatment). Results for shot blasting times of 5 and 10 minutes, sand blasting times of 30 and 60 seconds, are shown in Figure 6. Number of bearing gap locations is compared with a normal casting (without any surface treatment), as shown in Figure 7. Number of gap locations in bearings shot blasted for 5 and 10 minutes is similar to that in normal castings, indicating that surface roughness of bearings shot blasted for 5 and 10 minutes does not improve adhesion between molten aluminum and bearing surface. Number of gap locations in bearings shot blasted for 30 and 60 seconds is slightly less than that in normal castings, but improvement is not significant. Therefore, increasing surface roughness of bearing surface through shot blasting and sand blasting does not significantly improve adhesion with molten aluminum or alleviate separation.

 

Figure 6: Shot blasting and sand blasting of bearings

 

Figure 7: Effect of bearing surface treatment on aluminum-iron separation

 

Figure 8: Effect of high speed on aluminum-iron separation
By analyzing percentage of aluminum-iron gap sizes, it was found that 90% of separation occurs at bearing R corner. Analysis indicates that thinner wall thickness at the corners of castings leads to solidification and shrinkage after molten aluminum is filled. During cooling process, subtle stress separation occurs on contact surface with bearing at this location. Furthermore, molten aluminum swirls around this corner, easily forming cold slugs. Based on this analysis, a modification to bearing structure was proposed: transition radius R9 at corners was changed to R20 for die-casting trials. This was intended to improve flow of molten aluminum and collection of cold slugs. Figures 9 and 10 show aluminum-iron separation gap statistics for machining process and after modified process. It can be seen that separation gap dimensions for transition radius R9 and R20 of 24 castings were <0.2 mm in 94.8% and 97.9%, respectively, while those >0.2 mm were 5.20% and 2.10%, respectively. Data for transition radius R20 were slightly higher, indicating that bearing filter radius structure had little effect on separation gap. By adjusting and optimizing main bearing cover mold temperature, bearing shell temperature and die-casting process parameters, main influencing factors were identified, and main bearing cover aluminum-iron separation problem was fundamentally solved. Mass production qualification rate of main bearing cover parts reached 99.5%.

 

Figure 9 Bearing shell processing status

 

Figure 10 Statistical data on size ratio of aluminum-iron separation gap
Conclusion
(1) Combination of two different materials, aluminum and iron, is greatly affected by their inherent shrinkage rate, which easily causes surface separation of aluminum skin and bearing shell inlay joint, forming a gap.
(2) Optimization of bearing shell structure and addition of surface coating slightly improved fit between aluminum liquid and bearing shell, but improvement effect was not obvious.
(3) Increasing mold and bearing shell temperature and optimizing die-casting process can significantly improve die-casting defect of aluminum-iron separation in casting.