With the rise of integrated die casting, more and more automotive structural components are being produced using die cast aluminum alloys. One of the biggest challenges is the need to achieve high elongation, especially in the field of high-strength, tough, and heat-treated materials. There are many factors that affect the mechanical properties of aluminum alloy castings, which can be divided into three categories according to the process sequence: ① alloy composition, adjusting material properties by changing material phase composition, microstructure, grain size, etc.; ② The casting process includes multiple stages such as casting method, mold design, melting, pouring method, aluminum material temperature, solidification, and cooling, which affect the performance of castings by adjusting internal qualities such as inclusions, pores, and cracks The heat treatment process, different solid solution and aging temperatures, time, and cooling methods have a significant impact on the hardness, strength, and toughness of castings.
Research has found that even with the same material and casting process, there may be significant differences in the mechanical properties of the same product in different regions. For areas with poor performance, it is difficult to significantly improve their mechanical properties by adjusting the casting process. For the same material, there are still differences in the measured mechanical properties on different castings, even if samples are taken in the same area. This study analyzes the factors that affect the mechanical properties of die cast parts, summarizes the differences between the mechanical properties of high-strength and tough die cast aluminum alloy castings and the ideal mechanical properties of the material, and focuses on the influence of elongation during the performance testing process of high vacuum die cast structural parts. Through in-depth analysis of the mechanical properties of flat plate molds, test bar molds, and actual structural parts, the effect of material composition on the strength and toughness of heat-treated alloys is explored, with a focus on the influence of sampling methods and casting structural design on the mechanical properties of castings. An analysis was conducted on key factors such as specimen type and gauge length size in the testing process of high-strength and tough aluminum alloys, providing testing methods and recommendations for obtaining ideal material properties. In addition, the system compared the fluctuation deviation between the traditional AlSi10MnMg heat-treated material and the high toughness heat-treated material in the sampling of the casting body and the ideal properties of the material. Intended to provide reference for improving the mechanical properties and testing methods of high-strength and heat-resistant aluminum alloy castings.

Graphic and textual results

Four types of high-strength and tough alloys were selected, and their design compositions are shown in Table 1. These materials are used for die-casting flat specimens and test bars, with gauge lengths of 25 mm and 50 mm. A small shock absorber tower casting was produced using AlSi10MnMg alloy and independently produced heat free HT11 alloy die-casting. The T7 heat treatment process of AlSi10MnMg alloy is carried out on a roller conveyor heat treatment production line, with the specific process being: 460 ℃ × 2.5 h solution treatment+3.5 min air quenching treatment+210 ℃ × 156 min aging treatment.
Figure 1 shows the test bar, flat plate (the first mock examination with two cavities) and shock absorber tower (the first mock examination with two cavities) casting models used for die casting of test materials, which are produced by 4 500, 12 500 and 25 000 kN die-casting machines respectively. After deburring the test bar, it can be directly subjected to tensile testing. The tensile test pieces of the flat plate and shock absorber tower castings are prepared by wire cutting processing, with a gauge length of 25 mm and dimensions shown in Figure 2. The thickness of the flat plate sample is 3 mm, and the thickness of the casting sample depends on the casting. Before the tensile test, the side of the test piece is sanded with fine sandpaper until the surface roughness reaches Ra ≤ 5 μ m. The tensile test was conducted on the CMT5205 tensile testing machine at a rate of 1 mm/min. Use 400, 800, 1500, and 2000 grit sandpaper to polish the metallographic specimens in sequence, and use SiC suspension agent for polishing treatment. Use MDS400 metallographic microscope to observe and analyze the microstructure of the specimens.

Select AlSi10MnMg plate and study the effect of sampling method on its tensile mechanical properties. The test pieces are divided into two groups: test pieces A, B, and C are perpendicular to the main filling direction of the flat plate, and test pieces D, E, and F are parallel to the main filling direction of the flat plate. The sampling positions include near the water inlet, in the middle of the flat plate, and at the tail of the water. Figure 3 shows a schematic diagram of six different sampling methods, and their stress-strain curves are shown in Figure 4. Figure 5 shows the average elongation of AlSi10MnMg flat castings with different sampling methods. It can be seen that the sampling method has no significant effect on the strength of the sample. The change in elongation is more significant compared to strength. At the middle position in the parallel filling direction (Group E), the highest elongation rate was achieved, reaching 9.42%; At the water tail position in the parallel filling direction (Group F), the elongation rate is the lowest, at 6.84%.

As the aluminum liquid gradually fills the mold cavity, the temperature of the aluminum liquid will gradually decrease and solidify. Due to the high and uniform temperature at the nozzle position, the internal quality of castings is usually better than other positions when there is no curling or hot spots; The lower the temperature, the more prone it is to defects such as shrinkage and cold insulation, and the internal quality is usually poor, resulting in lower performance. However, as shown in Figure 5, the designed tablet does not fully follow this rule. The optimal position for elongation occurs in the middle of the plate, not at the water outlet. This phenomenon is closely related to specific factors in tablet design. Figure 6 shows the shape of the tablet, X-ray detection, and mode flow analysis. From Figure 6a, it can be seen that the inner gate of the test plate has undergone thinning treatment, resulting in the aluminum liquid being able to fill forward at a higher speed after entering the inner gate. Although there is not much difference in solidification time between different parts of the flat plate (see Figure 6c), there is a difference in the filling sequence of the aluminum liquid near the nozzle (see Figure 6d), which results in uneven filling of the aluminum liquid in the vertical filling direction, leading to relatively poor elongation. At the tail position, the elongation in the vertical direction is actually higher, indicating that the rule of vertical direction being inferior to parallel direction is not fixed and unchanging. The solidification process of aluminum liquid does not strictly follow the sequential solidification law. The inconsistency of aluminum liquid temperature in the mold cavity can still lead to differences in solidification, and the impact on the internal quality of the casting is not significant, as shown in Figure 6b. However, it has a significant effect on the mechanical properties, especially the elongation rate.

able 2 shows the mechanical properties of two types of test bars. It can be seen that the change in gauge length has little effect on tensile strength and yield strength, but the longer the gauge length of the die cast test bar, the lower the elongation rate. This can be explained by the material plastic deformation model, as shown in Figure 8. Due to the fact that the stretching conditions of the material during the testing process are not ideal static stretching, the plastic deformation of the test bar is not ideal uniform deformation, but the deformation near the fracture point is greater, while the deformation far away from the fracture point shows relatively smaller deformation. Especially in test bars with longer gauge lengths (i.e. parallel segments), the area with smaller deformation accounts for a larger proportion, resulting in a decrease in the average value of overall elongation. The strength test results of the test bar are only related to the force at fracture and the cross-sectional area of the fracture, so the influence of gauge length on tensile strength and yield strength is relatively limited.

conclusion

1) The sampling method includes position and direction, which have a significant impact on the elongation of the casting body during sampling, but have a relatively small impact on strength. In the flat test, the elongation measured by the worst performing sampling method was 72.61% of that measured by the best sampling method. In areas with poor casting quality, better results can be obtained by optimizing the sampling direction.
(2) The sampling method includes section type and gauge size, which affect the final results of mechanical property testing of castings. Samples with shorter gauge lengths have higher elongation rates, circular section die cast specimens exhibit higher elongation rates and lower strength, while rectangular section die cast specimens have lower elongation rates and higher strength. In addition, the mechanical properties of the die-casting plate are uneven, and the positions with better performance are more representative of the ideal performance level of the material's die-casting parts.
(3) There is a significant difference in elongation between the ideal performance of the same material and the performance of the die-casting body. The mechanical performance standards of die-casting parts are usually lower than the ideal performance level of the material, especially compared to test data at the specimen level. The standard value of elongation of castings is set as the minimum value when the die-casting process reaches its optimal state, that is, when the casting has no obvious defects such as pores or cold material, usually 50%~70% of the ideal performance.
(4) For high-strength and tough die cast aluminum alloy materials, it is feasible to achieve an average elongation rate of 10% in the as cast state. However, considering the overall performance of the actual casting, it is more reasonable to set the target elongation rate to be around ≥ 8%. At the same time, according to the actual connection requirements of different positions of the casting, the elongation requirements of high strength, toughness, and heat-treated materials can be adjusted regionally。

Author of this article:
Huang Hua, Zhu Yulin, Yun Zhao Shaoliang, Deng Xingjian
Huang Xitai and Lin Yufei travel thousands of miles
Guangdong Hongtu Technology Co., Ltd
Huang Hua, Chen Shiming, Zhang Lei, Zhong Yu, Yi Wan Li
Guangdong Hongtu Automotive Parts Co., Ltd
This article is from the magazine "Special Casting and Nonferrous Alloys",
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