In recent years, the requirements for lightweight, high integration, and high performance have made the structure of castings increasingly complex, resulting in a large number of castings formed by die-casting with very large wall thickness differences. Due to significant differences in wall thickness, these types of castings cannot be solidified sequentially, often resulting in defects such as shrinkage or porosity in areas with high wall thickness, which often leads to air leakage in the castings. In order to eliminate shrinkage and porosity defects, local pressure technology is widely used in die-casting production. The traditional local pressure process is to apply pressure to the pressure rod at the thick wall position of the casting during the solidification process, and achieve the goal of eliminating local shrinkage and porosity defects through forced shrinkage. However, due to the non adjustable pressurization speed, the pressurization is completed instantly during the solidification process of the casting, resulting in unstable pressurization effect.
During the production of a certain engine rocker arm chamber casting using traditional local pressurization technology, the oil passage leakage rate reached as high as 18.7% due to internal shrinkage. To solve the problem of air leakage, a new generation of local pressurization technology has been adopted. It can change the pressurization speed by controlling the flow rate in the hydraulic circuit through a speed regulating valve, so that the pressurization pin slowly advances while the metal liquid solidifies, to maintain continuous pressurization throughout the solidification process. By adopting this new local pressurization technology, the gas leakage rate of the casting was ultimately reduced to 0.6%, and stable production could be maintained.
Graphic and textual results
Figure 1 shows the structural diagram of the engine rocker arm chamber casting. The raw material weighs 2.3kg and is made of ADC12 alloy. The basic contour dimensions of the casting are 248mm × 156mm × 68mm, with a main body wall thickness of 7mm and a maximum wall thickness of 45mm. The wall thickness is extremely uneven. The casting has two intersecting oil channels, and the leakage inspection of the oil channels requires a leakage rate of less than 4mL/min under a pressure of 0.5MPa.
Through water testing, it was found that the leakage occurred in two threaded holes above the oil passage of the casting. Cutting through the leakage location, it was found that there were varying degrees of shrinkage and looseness in the middle of these two threaded holes, as shown in Figure 2. The leakage location is at the maximum wall thickness of the casting. During the solidification process, due to the completion of solidification of the surrounding metal liquid, this thick wall becomes an isolated liquid phase zone that cannot be filled under pressure, resulting in the formation of shrinkage holes in the casting, which is the cause of oil passage leakage. During the preliminary design of the plan, it was also assessed that there was a high possibility of shrinkage in this area, which could easily lead to air leakage. Local pressure was designed on the mold to reduce shrinkage, but the expected effect was not achieved, and the air leakage rate at this location was still very high.
The process debugging of the new local pressurization technology is different from the debugging method of the traditional local pressurization technology. The most important aspect of traditional local pressurization process debugging is the setting of extrusion delay. The delay is too short, there is no pressure effect, and the shrinkage cannot be eliminated; The delay is too long, and the metal liquid has already solidified, making it difficult to squeeze in due to high resistance when pressurized. In the process debugging of the new local pressure technology, choosing the appropriate extrusion speed is crucial, as it directly determines the quality of the castings. If the speed is too slow, there may be pinholes and scratches that cannot be squeezed in; If the speed is too fast, it may also lead to poor extrusion effect and incomplete elimination of shrinkage holes. The extrusion speed is controlled by adjusting the flow rate of the control circuit through the opening of the speed regulating valve. The larger the opening of the speed regulating valve, the higher the speed; The smaller the opening, the slower the speed. The new local pressurization technology has better casting density due to its adjustable pressurization speed, which can maintain the pressurization action continuously throughout the solidification process of the molten metal.
Author of this article:
Huang Zhiyuan, Zhang Yulong, Zhao Wei, Hong An, Zhao Yong
This article comes from: "Special Casting and Nonferrous Alloys" magazine, "Die Casting Weekly" strategic partner.
