In recent years, requirements of lightweight, high integration and high performance have made structure of castings more and more complex, and a large number of castings with very large wall thickness differences have appeared by die casting. Due to large difference in wall thickness, this type of casting cannot solidify sequentially, and often has defects such as shrinkage holes or shrinkage in local area with large wall thickness, which often leads to air leakage in casting. In order to eliminate shrinkage holes and shrinkage defects, local pressurization technology is widely used in die casting production. Traditional local pressurization process is to apply pressure to pressurization rod at thick wall position of casting during solidification process of casting, achieve purpose of eliminating shrinkage holes and shrinkage defects in local position by forced shrinkage compensation. However, since pressurization speed cannot be adjusted, pressurization is completed instantly during solidification process of casting, resulting in unstable pressurization effect.
When a certain engine rocker chamber casting is produced using traditional local pressurization process, oil channel leakage rate is as high as 18.7% due to internal shrinkage. To solve leakage problem, a new generation of local pressurization technology is used. It can change pressurization speed by controlling flow in hydraulic circuit through speed regulating valve, so that pressurization pin slowly moves forward while metal liquid solidifies to maintain continuous pressurization throughout solidification process. By adopting this new local pressurization technology, leakage rate of casting was finally reduced to 0.6%, and stable production can be maintained.
Figure 1 is a structural diagram of engine rocker chamber casting. Blank weight is 2.3kg, and material is ADC12 alloy. Basic outline size of casting is 248mm*156mm*68mm, main wall thickness is 7mm, maximum wall thickness is 45mm, and wall thickness is extremely uneven. Casting has two intersecting oil channels, and oil channel leakage test requires that leakage volume is less than 4mL/min under a pressure of 0.5MPa.
Through water inspection test, it was found that location where casting leaked was two threaded holes above oil channel. Leaking location was cut, and it was found that there were shrinkage cavities and shrinkage pores of varying degrees in the middle of two threaded holes, as shown in Figure 2. Leaking location is location of maximum wall thickness of casting. During solidification process, since surrounding molten metal has been solidified, this thick wall becomes an isolated liquid phase area and cannot be compensated under pressure, thus forming shrinkage cavities in casting, which is cause of oil channel leakage. In the early design of scheme, it was also evaluated that shrinkage cavity here is likely to be high and it is very easy to leak. Local pressurization was designed on mold to reduce shrinkage cavities, but it did not achieve expected effect, and leakage rate here is still very high.

 

Picture 1 Rocker chamber structure

 

Picture 2 Dissection of leaking parts

 

Picture 3 Schematic diagram of pressurization of traditional local pressurization process

 

Picture 4 Schematic diagram of pressurization of new local pressurization process

 

Picture 5 Relationship between pressurization time and entire injection stroke in two local pressurization technologies

 

Table 1 Debugging parameter combination

 

Picture 6 Sectional surface under two sets of parameters
Process debugging of new local pressurization technology is different from that of traditional local pressurization technology. In debugging of traditional local pressurization process, the most important thing is setting of extrusion delay. If delay is too short, there is no pressurization effect and shrinkage cavity cannot be eliminated; if delay is too long, molten metal has solidified and resistance is too large to squeeze in during pressurization. In process debugging of new local pressurization technology, choosing right extrusion speed is very critical, which directly determines quality of casting. If speed is too slow, pinhole buckles will appear and extrusion cannot be squeezed in; if speed is too fast, it may also lead to poor extrusion effect and shrinkage cavity cannot be completely eliminated. Extrusion speed is controlled by adjusting speed control valve opening to control flow of circuit. The larger speed control valve opening, the faster speed; the smaller opening, the slower speed. New local pressurization technology can maintain pressurization action continuously during the entire solidification process of molten metal, so casting has better density.