Casting structure and development difficulties of automobile engine main bearing cap bracket are introduced. By analyzing filling and solidification shrinkage principles of molten metal during die-casting process, it is determined that key factors affecting internal quality of thick-walled part of casting are casting pressure, injection speed and pouring temperature. Orthogonal experiments are used to optimize key die-casting parameters.
Graphical results
Main bearing cap bracket is a component of automobile engine. It is made of ADC12 aluminum alloy high-pressure casting. Maximum outer contour size of casting is 410mm*184mm*87mm. The overall structure of main bearing cap bracket is relatively complex (see Figure 1). Periphery of casting is surrounded by special-shaped cross-reinforced ribs with a wall thickness of 3mm. There are 5 thick bearing seats in the middle. Thickness of bearing seat is 22~24mm, and there is a semicircular hole of φ55mm in the center. Bearing bush is assembled after being processed together with cylinder body. Therefore, in addition to ensuring machining accuracy, it is also required that machined surface be free of casting defects such as pores and shrinkage holes. Function of bearing seat is to support crankshaft, ensure normal operation and power output of crankshaft. Therefore, internal quality of bearing seat must be ensured to meet required tensile strength and fatigue life of bearing seat.

 

Figure 1 Main bearing cap bracket
Requirement of die-casting process for structure of die-casting part is that wall thickness is as uniform as possible. When wall thickness is uneven, die-casting part will produce defects such as internal shrinkage holes and pores in thick parts due to different solidification rates. Based on analysis of structure and function of casting, it was determined that difficulty in developing this product was to ensure internal quality of five bearing seats.
Mold adopts a two-way filling pouring system, see Figure 2. Inner gate is selected at thick wall of bearing seat, and thickness is set to 4mm, which is beneficial to quickly filling cavity and ensuring effective transmission of boost pressure. Large-volume slag collection bags are designed on both sides of each bearing seat, which are used to collect bidirectionally filled molten metal. They can also store cold dirt mixed with gas, play a certain role in balancing mold temperature and feeding. Four pairs of internal gates are opened, and a slag bag and a vacuum exhaust channel are set up at upper end. Die-casting process uses vacuum die-casting, which can effectively reduce occurrence of pores inside casting.
Based on reasonable design of pouring system, overflow system and cooling system of mold, a simulation analysis of casting plan is carried out, as shown in Figure 3. It was found that solidification time of casting was 20 seconds, and there was an isolated liquidus area at bearing seat, which was location where casting defects occurred. In actual die-casting production, how to optimize process parameters and minimize internal defects of bearing seat is a key issue to be solved in product development of main bearing cap bracket.

 

Figure 2 Casting plan

 

Figure 3 Simulated solidification analysis
In die-casting process, filling of molten metal is divided into three stages. In the first stage, molten metal passes through inner gate, and under action of kinetic energy, it shoots directly and rapidly, expands in all directions along surface of cavity. At this time, metal flow is a jet flow. In the second stage, kinetic energy maintained by jet stream weakens, and injection filling pushes molten metal to continue filling mold until mold cavity is filled. Metal flow in this process is a pressure flow. The third stage is when molten metal fills mold cavity until casting solidifies. Boosting pressure is transmitted through metal that has not yet solidified, and casting is pressurized and fed. At this time, metal flow is a feeding metal flow. It can be seen that process parameters that have the greatest impact on quality of die castings are casting pressure and injection speed, and impact on internal quality of thick-walled die castings is more obvious.
Main bearing cap bracket is made of ADC12 aluminum alloy and complies with JIS H5302 standard. It has good casting performance and mechanical properties, is suitable for die-casting production of cylinder blocks, shells, brackets and other products. Based on calculation of its projected area and clamping force, this product is produced on a 9000kN die-casting machine. Die-casting process uses a self-made vacuum system with a vacuum degree of 10 to 20kPa. The key process parameters that affect internal quality of main bearing cap bracket: casting pressure, injection speed and pouring temperature are taken as influencing factors. Each factor takes 4 levels, numerical range of test factor is determined based on analysis of casting plan simulation software and empirical data of similar products. Internal quality of castings is evaluated by density value, which is used as an evaluation index. Orthogonal test factors and levels are shown in Table 1.

Table 1 Orthogonal test factor table
Through simulation analysis, location of shrinkage cavities in castings is concentrated at bearing seat. Schematic diagram of density detection sampling is shown in Figure 4. Use a sawing machine to cut a sample between two bolt mounting holes for density value testing. Sampling width is approximately 60mm. The higher sample density, the fewer shrinkage cavities and shrinkage defects in casting. Since five bearing seats are arranged in a stepped manner in mold, even if it is same die-casting part, internal quality of five bearing seats is different. In actual production, X-ray inspection is carried out first, poor internal quality is tested during sampling.

 

Figure 4 Sampling location

Table 2 Orthogonal test results

Table 3 Range value analysis table

Table 4 Die casting process parameters
Through data analysis in Table 2 and Table 3, it can be seen that pouring temperature of molten aluminum is an important factor affecting density. Order of influence of each test factor on density value of main bearing cap bracket is aluminum liquid pouring temperature > injection speed > casting pressure. Combined with analysis of density mean value ki, when casting pressure is 110MPa, injection speed is 3.5m/s, and aluminum liquid pouring temperature is 655℃, density value of main bearing cap bracket is the largest and internal quality is the best. Based on above analysis of orthogonal test results, main bearing cap bracket is batch-die-casted on a 9000kN die-casting machine. Die-casting process parameters are shown in Table 4. Conduct small batch production verification according to this parameter, and continuously select 30 pieces for density value testing. Density values are all greater than 2.65g/cm3. The first mass-produced main bearing cap brackets are inspected daily for density value and cross-section, and all meet quality requirements. All products produced in batches are subject to X-ray inspection. There is no obvious shrinkage and porosity in internal quality of spindle seat. Internal quality meets product requirements, and the first-time pass rate reaches more than 95%.