With rapid development of automobile industry, automobiles are gradually developing towards high performance, low energy consumption, and low pollution, which has led to increasing demands for body weight reduction and material lightweighting. Aluminum alloy has excellent specific strength and corrosion resistance, and is increasingly used in automobile body, chassis and powertrain structures. Common automotive aluminum alloy engine castings include cylinder blocks, cylinder heads, covers, oil pans, etc., which use high-pressure casting to meet dimensional accuracy, mechanical properties and sealing requirements of mass production.
Shrinkage is a common defect in aluminum alloy die castings. Die castings are generally allowed to have a small amount of shrinkage cavities, but when they appear in functional areas of product (such as finishing surfaces, high-pressure oil passage accessories, sealing surfaces, etc.), they will affect its performance, and in severe cases, it will directly lead to scrapping of castings. Methods for improving common shrinkage defects in aluminum alloy die-casting parts have been reported, but there are few reports on relationship between die-casting quality and casting process parameters in fully automated die-casting production. Taking an aluminum alloy cylinder head die-casting as an example, taking shrinkage problem that occurs in fully automated production process as starting point, combined with basic principles of real-time control system of die-casting machine, through investigation and analysis of feeding-related process parameters during abnormal quality period, relationship between real-time monitoring pressure of die-casting machine and casting pressure, impact of injection chamber status on casting pressure and casting quality were discussed.

Figure 1 shows AlSi9Cu3 alloy die-cast cylinder cover. Its outline size is 156mm*260mm*428mm, average wall thickness is about 3.6mm, and weight is 2.9kg. The overall structure of casting is complex and wall thickness is uneven. The thickest part is 8mm and the thinnest part is 2.5mm. A 16000kN cold chamber die-casting machine is used, equipped with a vacuum system, ABB pick-up robot, automatic spraying robot and edge trimming machine. The entire casting cycle runs in a fully automated state, die-casting machine can provide real-time monitoring and feedback on process parameters of die-casting process, such as speed, oil pressure, filling time, and cake thickness. A mold temperature controller and cooling circulating water are used to control mold temperature field. Different cooling circulation channels are designed according to potential failure points of casting quality. Through optimization of casting process, an attempt is made to solve shrinkage and porosity problem in key hot zone areas.

 

Figure 1 AlSi9Cu3 alloy die-cast cylinder cover

 

Figure 2 Product structure analysis on high-pressure oil pump side

 

Figure 3 High-pressure oil pump side runner structure

 

Figure 4 Cooling cycle in camshaft core
In order to achieve purpose of lightweighting car, engine has undergone a large number of component structural optimizations. After optimization, engine is 14.7kg lighter than old engine. For example, high-pressure oil pump seat, camshaft bearing shell and cylinder head are integrated. After integration, intersection area between parts forms a wall thickness area. For example, wall thickness near high-pressure oil passage of cylinder head reaches about 8mm, which is much higher than average wall thickness of casting. It has become a hot spot, making process control more difficult. Shrinkage and porosity near oil passage will cause air tightness test to fail. Figure 2 shows cylinder head wall thickness analysis and location of high-pressure oil passage. In order to ensure that position of high-pressure oil passage is effectively fed during casting process, mold development was optimized in two directions. First, a branch gate with a large cross-sectional area is designed to ensure aluminum liquid filling flow of complex structure on high-pressure oil pump side of mold and feeding effect on hot joint, as shown in Figure 3; second, a cooling water path is designed inside camshaft core formed at high-pressure oil passage, and a mold temperature machine is used to control core temperature, as shown in Figure 4. Failure of cooling and feeding in high-pressure oil passage will cause shrinkage and porosity in high-pressure oil passage. Therefore, it is necessary to find all influencing factors and ensure that process is controlled.

 

Figure 5 X-ray inspection of high-pressure oil passage holes and surrounding defects

Table 1 Potential failure points and control methods of shrinkage and porosity in high-pressure oil passages

 

Figure 6 Surface condition of inner cavity of aluminum-glued pressure chamber

 

Figure 7 X-ray of die casting after pressure chamber replacement
Combined with hydraulic principle of die-casting machine, it can be seen that pressure sensor placed can detect oil pressure changes in injection cylinder of die-casting machine, and data will be fed back to die-casting machine control system in real time for process control. Among them, maximum oil pressure Pm³ in rodless cavity of injection cylinder after supercharging is completed is a key monitoring item for process control. That is, maximum value measured by pressure gauge is shown in "01M01" in Figure 8. It can effectively monitor "sealing and storage of injection cylinder." "Energy accumulator oil pressure and boosting action" are operating normally, but it cannot reflect actual casting pressure acting on molten aluminum during pressure maintaining process. During filling and feeding process of aluminum liquid in cavity, punch encounters greater forward resistance when running in aluminum-stick pressure chamber, and extrusion of punch by material cake is energy source for runner feeding. Decline in pressure transmission effect will significantly affect die-casting process, causing internal shrinkage and porosity of die-casting parts.

 

Figure 8 Schematic diagram of injection end structure and oil pressure detection points

Table 2 Pressure chamber life improvement and refined management plan

To sum up, quality of injection chamber has a significant impact on casting feeding process. Aluminum stuck in injection chamber creates resistance to punch operation, which will weaken pressure transmission and cause casting pressure to drop, affecting internal quality of die casting. In addition, real-time monitoring of die-casting machine cannot accurately feedback impact of changes in resistance on feeding process when punch is running. Manual inspection and confirmation of quality of injection chamber must be added to process control. At the same time, injection chamber must be included in key spare parts management system to ensure that injection chamber operates within its effective life. Only in this way can continuous and stable quality of die castings be ensured.