Die casting has characteristics of high production efficiency and product dimensional accuracy, and is often used to produce products with relatively uniform wall thickness. As product integration continues to improve, product performance requirements are getting higher and higher, more and more products with uneven wall thickness are produced using die-casting process. However, in die-casting process design, factors such as long flow channels, thin inner gates, and thin-walled locations that are cooled first reduce effect of pressure transmission by punch in pressure chamber to a certain extent, causing shrinkage defects in die-casting parts with large hot spots. To solve above problems, local pressure measures can be taken.
Partial pressurization is also called secondary pressurization or partial pressurization. In order to achieve local pressurization of castings, a local pressurization mechanism driven by hydraulic pressure needs to be added to mold design. There are many factors that affect local extrusion effect, such as extrusion delay time, extrusion pin pressure value, extrusion pin movement speed, extrusion pin stroke and extrusion pin duration time, etc. These become key factors affecting local pressurization effect. Using numerical simulation analysis method, effect of local pressurization on shrinkage cavities and shrinkage porosity in die castings was studied. AnyCasting software was used to analyze extrusion effect under parameters such as different delay times and different extrusion speeds, and simulation results were compared with actual results. Results show that local pressurization of die castings under appropriate extrusion parameters can effectively reduce shrinkage cavities and shrinkage porosity defects in die castings. Simulation results of shrinkage cavities and shrinkage porosity in castings before and after local pressurization are basically consistent with actual results.
Graphical results
In order to study influencing factors of local extrusion effect, simulation tests and verification tests were conducted on automobile driven shaft bracket through local extrusion module of AnyCasting software in order to determine optimal local extrusion parameters. Test model and its dimensional parameters are shown in Figure 1. Among them, A, B and C respectively represent different positions of casting. Table 1 shows delay time and extrusion pin movement speed parameters.

 

Figure 1 Schematic diagram of test model

Table 1 Delay time and extrusion pin movement speed parameters
Use local extrusion module to study extrusion effect under different delay times and different extrusion speeds. In software entity attribute settings, set entity attribute of extrusion pin as an accessory, and at the same time input parameters such as extrusion pin movement direction, extrusion pressure, extrusion delay time, holding time, extrusion pin movement speed, extrusion pin maximum movement distance, alloy Young's modulus, Poisson's ratio and other parameters into the software. It should be pointed out that factors such as extrusion pin speed of mold, extrusion pressure, delay time, dynamic friction and opening of equipment's hydraulic valve are affected. In AnyCasting software, input extrusion pin speed value is used as driving condition for extrusion pin movement. This condition can be obtained from time/movement distance information derived from local extrusion equipment, but extrusion pin speed used is a hypothetical value, and its purpose is to evaluate rationality of set extrusion delay time by obtaining maximum displacement that extrusion pin can achieve at a certain initial speed. Distribution of shrinkage before and after local extrusion is shown in Figures 2 and 3.

 

Figure 2 Shrinkage and porosity prediction without local extrusion

 

Figure 3 Prediction of shrinkage after local extrusion

 

Figure 4 Simulation of shrinkage and porosity at different solidification times when delay time is 3s

 

Figure 5 Simulation of shrinkage and porosity at different solidification times when delay time is 4s

 

Figure 6 Simulation of shrinkage and porosity at different solidification times when delay time is 5s

 

Figure 7 Relationship between delay time and shrinkage volume fraction at positions A, B, and C
Driven shaft bracket has an important impact on automobile driving safety. Wall thickness of shaft sleeve of this product is relatively thick, but it requires higher casting density. In order to analyze effect of local pressurization on position of stent sleeve, software was used to simulate and analyze local pressurization effect, conduct experimental verification. Schematic design diagram of bracket casting process system is shown in Figure 8, various parameters are shown in Table 2, and test results are shown in Figure 9. Under parameters in Table 2, with extrusion pin stroke between 9 and 11 mm as constraint, find minimum value of shrinkage volume fraction within this range.

 

Figure 8 Schematic diagram of bracket casting process system design

Table 2 Bracket casting process parameter table

 

Figure 9 Simulation results of bracket case

 

Figure 10 Comparison of shrinkage defects in bushing position under different local extrusion schemes
In conclusion
(1) Local extrusion device has its local pressurization influence range. When location of shrinkage defect is not within local extrusion action range, local extrusion will not be effective.
(2) For driven shaft bracket die-casting part, set extrusion pin speed to 0.2cm/s, delay time to 1s, and locally pressurize shaft sleeve position, which can effectively reduce shrinkage of shaft sleeve position.