Abstract: Due to the fact that the speed parameters of the low-speed injection stage in high-pressure casting of aluminum alloys are basically set based on the experience of technical personnel, the uncertainty of the amount of air brought in by the aluminum liquid in the injection chamber during the injection process results in large fluctuations in the quality of the castings. Taking the die-casting process of a certain automobile oil cooler as an example, the DOE optimization function of Magma software was used to reduce the internal air pressure and entrainment of the casting and improve the filling performance. Multiple sets of parameter virtual tests were conducted on the low-speed injection of the casting. Using computer simulation and result comparison, select the optimal value for low injection speed. After the trial production verification of die-casting, it has been shown that the use of software screened low-speed injection parameters results in good internal quality of the castings, ensuring the stability of the die-casting process and improving economic benefits.
In the process of high-pressure casting of aluminum alloys, if the injection speed is too high during the low-speed stage, it will cause a large amount of air to enter the mold cavity through the pressure chamber, and on the other hand, it will cause the aluminum liquid to churn inside the pressure chamber, resulting in an increase in the gas content of the aluminum liquid and affecting the quality of the die-casting parts; If the injection speed is too low, it will cause cold insulation and poor forming of the casting. The specific requirement for low speed is to slowly push the molten metal at a uniform speed or acceleration within the low speed range (0.1-0.5 m/s), avoiding turbulent flow and gas dissolution, thereby reducing the gas content of the molten metal and facilitating the smooth discharge of gas from the mold cavity. In general, the injection speed during this stage is relatively low, usually within 0.1-0.5 m/s.
At present, the commonly used low-speed calculation formula cannot intuitively analyze the effect of filling and forming, resulting in significant deviations in the calculated results. The production personnel only rely on experience and repeated debugging to determine the speed parameters for the low-speed stage, which on the one hand causes process instability and a large amount of trial production waste, and on the other hand prolongs the development cycle.
Taking the development of an automobile oil cooler as an example, multiple sets of parameters during the low-speed injection stage were simulated using Magma software. Based on the simulation results, select the low-speed die-casting process parameters that meet the optimal objectives. After verification through die-casting trial production, the castings produced using the optimal low-speed parameters have excellent quality, ensuring process stability, shortening the development cycle of castings, and improving economic benefits.
1、 Basic Introduction to Castings
Figure 1 shows the complete casting of an oil cooler with pouring and overflow system for a certain automotive component. The mass of the casting blank is 0.463 kg, the mass of the pouring and overflow system is 0.252 kg, and the material is AlSi9Cu3. The basic dimensions of the casting are 155 mm × 136 mm × 45 mm, with an average wall thickness of 5 mm, of which the maximum wall thickness is 15 mm and the minimum wall thickness is 3 mm. The casting structure is relatively complex, with pre cast oil passage hole A being 81 mm long and pre cast oil passage hole B being 62 mm long, as shown in Figure 2. The surface of the casting should not have cold insulation, bubbles, or peeling, and the quality of internal pores should be in accordance with ASTM E 505 aluminum alloy gas (shrinkage) pore grade 2; The leakage test requires that the leakage value at 620 ± 10 kPa should not exceed 5 mL/min.
2、 Virtual filling selection for optimal low speed die-casting
Due to the difficulty in accurately mastering the low-speed parameters during the injection low speed stage of aluminum alloy high-pressure casting, the trial production process only relies on experience to adjust the parameters, resulting in a large amount of scrap and poor process stability. Combining Magma software to simulate the filling process of aluminum alloy high-pressure casting and determine low-speed parameters can avoid blind adjustment of die-casting process parameters, reduce debugging waste, and stabilize the die-casting process.
1. Virtual experiment process parameter setting and optimization
Using MAGMASOFT to simulate the filling and forming of automotive oil coolers, based on the experience of similar products, constant simulation conditions are set: using a 4000 kN die-casting machine, pouring temperature of 660 ° C, fixed mold temperature of 150 ° C, dynamic mold temperature of 150 ° C, chamber temperature of 180 ° C, effective distance of the chamber of 395 mm, casting pressure of 78 MPa, high-speed position of 237 mm (inner gate), high-speed speed of 4 m/s, spraying for 4 seconds, drying for 3 seconds, injection delay of 2 seconds, and mold closing for 3 seconds.
The optimization objectives are to reduce air pressure, minimize rolled air mass, and improve smooth filling. Smooth filling is a unique simulation result criterion indicator of MAGMASOFT software. Simply put, the smaller the smoothness value of the filling, the better the filling performance.
Set multiple sets of low-speed parameters for optimization simulation, with low-speed values ranging from 0.15 to 0.5 m/s and a step size of 0.05 m/s. The software automatically arranges and combines the input low-speed process parameters to generate 8 schemes and simulation results, as shown in Table 1.
It can be seen that Design 5 (low-speed 0.35m/s), Design 3 (low-speed 0.25m/s), Design 7 (low-speed 0.45m/s), and Design 6 (low-speed 0.40m/s) have the best performance and are alternative solutions.
By analyzing the DOE comprehensive index curve chart (see Figure 3) and the DOE comprehensive index trend chart (see Figure 4), from the observation of reducing air pressure and reducing entrainment indicators, Design 3 (low speed 0.25 m/s), Design 5 (low speed 0.35 m/s), Design 6 (low speed 0.40 m/s), and Design 7 (low speed 0.45 m/s) are relatively close. From the analysis of the simulated air pressure results of the alternative plan (see Figure 5), the highest air pressure of the Design 3 casting is 2.554 MPa, the highest air pressure of the Design 5 casting is 1.405 MPa, the highest air pressure of the Design 6 casting is 11.887 MPa, and the highest air pressure of the Design 7 casting is 1.124 MPa. The air pressure distribution of the Design 6 casting is the worst, and there is little difference between the other plans. From the analysis of the simulated gas entrainment results of the alternative plan (see Figure 6), the highest gas entrainment of the Design 3 casting is 54.35 μ g, the highest gas entrainment of the Design 5 casting is 63.08 μ g, the highest gas entrainment of the Design 6 casting is 757.95 μ g, and the highest gas entrainment of the Design 7 casting is 135.09 μ g. The gas entrainment of the Design 6 casting is the worst, while the gas entrainment of the Design 3 and Design 5 castings is the best. However, considering the filling performance, Design 3 (low speed 0.25 m/s) with the best filling performance (minimum smoothness value) was selected as the low-speed parameter for the injection low-speed stage.
2. Verification of optimal solution
Through the statistical analysis of defect data from 8 low-speed schemes, the identified optimal solution, which is the low-speed 0.25 m/s in the low-speed stage of Design 3 injection, was selected as the casting process parameter. The castings obtained in actual production have good quality with a total scrap rate of 3%, as shown in the defect record tables in Figures 7 and 8. The castings obtained by using other low-speed schemes have varying degrees of porosity defects on the pipe walls of pre cast oil passage holes A and B, as shown in Figure 9.
3、 Conclusion
In high-pressure die casting, the DOE optimization function of MAGMASOFT software is used to conduct virtual experiments on castings, which can simulate filling and forming simultaneously with multiple pouring system schemes and multiple sets of process parameters. The software automatically combines and generates multiple simulation schemes based on optimized conditions, and through the analysis and comparison of simulation results, selects the scheme that meets the optimization objectives.
Author: Huang Zhiyuan, Chen Guoen, An Zhaoyong
Source: "Special Casting and Nonferrous Alloys" Magazine
