The development of integrated die casting technology requires aluminum alloy materials without heat treatment to have high strength and toughness. However, during the forming process, large die castings may have defects such as pre crystallized structure (ESC) and pores, leading to a decrease in their performance. In the die casting process, the lower the vacuum degree, the higher the porosity content caused by gas entrapment during the liquid flow filling process; The higher the content of pre crystallized microstructure, the easier it is to generate an ESC dendrite network in the center of the casting, and the content of large-sized shrinkage porosity also increases. Although increasing the vacuum pumping time can improve the vacuum effect, prolonged residence of the melt in the pressure chamber will increase the ESC content. In this study, the flow channel was designed to regulate the velocity of the liquid during the filling process, achieving the goal of crushing and dispersing ESC, reducing or avoiding the formation of large-sized ESC dendrite networks in the casting, and reducing the porosity in the casting. The results showed that under the high vacuum die-casting process with bent runners, ESC achieved uniform distribution, and the porosity in the test bar was significantly reduced, The large-sized pores have been eliminated. This research result can provide new ideas for regulating the large-sized ESC and pores in die castings.

At present, integrated body forming technology can greatly reduce the production cost of automobiles, and the selection of body materials is a key challenge that restricts integrated body technology. Therefore, the research and development of integrated heat free body materials has become a hot topic in the die-casting industry in recent years. Hypoeutectic Al-10% Si (mass fraction,%) alloy is widely used in automotive components due to its low density (about one-third of steel), good formability, and high specific strength. In addition, the solidification range of Al-10% Si alloy is narrow, so this type of aluminum silicon alloy has good flow performance and is not prone to solidification shrinkage defects, which can make automotive structural components have good mechanical properties. Research has found that Al-10% Si-1.2% Cu-0.7% Mn has excellent tensile properties, with a yield strength of 206MPa, a tensile strength of 331MPa, and an elongation of 10%.

High pressure casting (HPDC) is one of the most commonly used methods for producing complex thin-walled components, with high production efficiency and good applicability. However, during the die-casting process, high-speed filling and rapid solidification can lead to the formation of specific microstructures inside the casting. Among them, pre crystallized microstructure (ESC) is usually formed along the pressure chamber wall during the low-speed stage and grows continuously with the movement of the punch, ultimately remaining in the casting as the melt fills the mold. At the same time, typical defect band structures may also appear inside the castings. In addition, there are also a large number of pores and shrinkage porosity in the castings, which are caused by air entrainment and solidification shrinkage, respectively, which can greatly harm the mechanical properties of the castings. Designing the runner can regulate the speed and direction of liquid flow, achieving the goal of regulating the structure and defects of die castings. Therefore, in this study, the flow velocity was controlled by designing a bent flow channel and adding ESC collection blocks to achieve uniform distribution of ESC and reduce the overall porosity of the casting.

1. Experimental plan

1.1 Alloy composition

  In this experiment, hypoeutectic AlSi10MnMg alloy was used, and its composition is listed in Table 1. The ingots used have no major defects such as oxidation inclusions and are of qualified quality.

Table 1 Chemical composition (mass fraction) of AlSi10MnMg alloy

1.2 Die casting conditions

Figure 1 (a) shows a newly designed die casting. The die casting includes a standard tensile test bar, tensile test piece, hot crack insert, step insert, and flow insert above the step from left to right. It can not only test the standard mechanical properties of the alloy, but also test the alloy's resistance to hot cracking and flowability.

(a) - Die cast castings; (b) - Traditional pouring channels; (c) - Improve the sprue; (d) - Traditional sprue; (e) - Bent straight sprue.

Figures 1 (b) and (c) respectively show two types of sprue inserts: one is a traditional sprue; Another type is the improved sprue. Figures 1 (d) and (e) show the straight sprue parts in Figures 1 (b) and (c), respectively. One type is the traditional straight sprue; Another type is the improved bent sprue. Among them, the design of the angle and cross-sectional area of the bent sprue can be found in the literature (Xiong Shoumei, Jiao Xiangyi, Wang Jun. Flow channels for crushing and collecting pre crystallized tissue in the crushing and collecting chamber). The purpose of this design is to achieve the crushing and collection of ESC. In this experiment, standard test rods were used to study the microstructure and porosity.

  In this experiment, TOYOBD-350V5 die-casting machine was used, which was equipped with VCSU-15 vacuum equipment in terms of vacuum. Add AlSi10MnMg alloy ingots to the melting furnace and heat them to a temperature range of 700-720 ℃. After melting, it is kept warm for a period of time before carrying out degassing and slag removal. The temperature drops to 680 ℃ and the die-casting experiment begins. Table 2 lists the three process parameters used in this die-casting experiment. In order to prevent the front end of the punch from being affected by the pouring port, a multi-stage low-speed speed is set. When the punch stroke is between 80-270mm, the low-speed of the vacuum die-casting process and the high vacuum die-casting process are set to 0.15m/s and 0.05m/s, respectively, In order to test the effect of improving the sprue on the fragmentation and dispersion of ESC, a lower high-speed speed of 1.5m/s was used in this experiment.

Figure 2 (a) shows a comparison of the travel times of the vacuum die-casting and high vacuum die-casting punches. From the figure, it can be seen that at multiple low speeds, the travel time of the vacuum die-casting punches is 1.83 seconds, and the travel time of the high vacuum die-casting punches is 3.92 seconds, with a difference of 2.09 seconds. This difference indicates that high vacuum die-casting provides more vacuum pumping time, but an increase in the residence time of the liquid in the pressure chamber will lead to an increase in the ESC content.

Figure 2 (b) shows the comparison of effective vacuum degree and effective vacuum time between vacuum die-casting and high vacuum die-casting. The effective vacuum degree refers to the vacuum degree of the cavity before filling the metal liquid, which is the intersection point between the vacuum pressure and the oil cylinder pressure of the die-casting machine. As shown in the figure, the effective vacuum degree of vacuum die-casting is 25kPa, and the effective vacuum time is 0.7s; The effective vacuum degree of high vacuum die-casting is 9.5kPa, and the effective vacuum time is as high as 1.9s, resulting in excellent vacuum effect. In summary, the third process of high vacuum die casting with bent sprue can achieve the goal of maintaining high vacuum.

(a) - Punch movement stroke; (b) - Effective vacuum degree and effective vacuum time.

To observe the porosity in castings, nanoelectronic computed tomography was used for non-destructive testing of the porosity in die castings, and the three-dimensional morphology of the pores was restored. The working voltage and current used in the experiment were set to 100kV and 110, respectively μ A. Resolution set to 3 μ M. First, use water sandpaper labeled 200 #~3000 # to grind the selected die-casting test bar, and then use particle sizes of 2.5-0.1 μ The diamond grinding paste of m was mechanically polished, and after ultrasonic descaling treatment, it was used for metallographic (OM) observation. In addition, Avizo software was used for ESC and porosity statistics, and the three-dimensional morphology of the pores was also presented through Avizo software.

2. Experimental Results and Discussion

2.1 Differences in pre crystallized tissue in compression chamber

Figure 3 shows the circular cross-section of the test bar and the enlarged microstructure along the diameter direction under three different die-casting processes. From Figures 3 (a) and (b), it can be seen that there is a certain amount of pore distribution in the microstructure under vacuum and high vacuum processes, and there are still over 100 pores on the surface layer μ The skin layer of m [see Figure 3 (d) and (e)]. However, under the high vacuum die-casting process with bent flow channels, no large-sized holes were observed on the cross-section of the test bar, and no skin layer was found on the surface [see Figure 3 (c) and (f)]. Table 3 presents a comparison of the area fraction, equivalent diameter, and quantity of ESC under three die-casting processes. Under vacuum die-casting, due to the relatively high low-speed, the area fraction of ESC is lower (4.86%). In the high vacuum die casting process, due to the relatively low low-speed, the area fraction of ESC is higher (11.06%). After adding a bent sprue, the area fraction of ESC was 10.73%. Compared with the high vacuum die casting process, the area fraction of ESC did not show a significant decrease.

Table 3 Comparison of ESC under Three Die Casting Processes

(a) Vacuum die-casting; (b) Low speed reduction+high vacuum die casting; (c) Bending sprue+high vacuum die casting; (d) (f) - Corresponding distribution along the diameter direction.

Figure 4 shows the radial distribution of ESC area fraction under three different processes. It can be seen from the figure that the ESC content under vacuum die-casting process is much lower than that under high vacuum die-casting process. Under high vacuum conditions, ESC is mainly enriched in the center of the casting, forming an ESC dendrite network at the center. Although the ESC content decreases slightly after adding a bent flow channel, it is distributed very uniformly along the diameter direction, and there is no phenomenon of large-sized ESC dendrite network aggregation.

2.2 Differences in porosity

Figure 5 shows the distribution of pores under three different die-casting processes. From Figures 5 (a) to (c), it can be seen that there are large-sized pores in the center of the specimen slice under vacuum die-casting and high vacuum die-casting processes, while no large-sized pores were found in the slice under the high vacuum die-casting process with added bent runners. From Figures 5 (d) to (f), it can be seen that under vacuum die-casting and high vacuum die-casting processes, large-sized holes penetrate the entire center position of the test bar; Under the high vacuum die-casting process with the addition of bent runners, the porosity at the center of the test bar decreases sharply, and the size of the pores is also smaller.

Table 4 Comparison of Hole Characteristics under Three Die Casting Processes

Table 4 presents a comparison of pore characteristics under three die-casting processes. It can be observed from the table that under vacuum die-casting and high vacuum die-casting processes, the porosity of the specimen is high, the average equivalent diameter of the pores is large, and there are many large-sized pores; Under the high vacuum die-casting process with the addition of a bent runner, the porosity of the specimen decreased from 0.073% to 0.006%, and the average size of the pores also increased from 35.5% μ M has dropped to 25.5 μ m. Over 100 μ The number of large-sized holes in m has also decreased from 15 to 0. Therefore, in the die-casting process, adding a bent runner under high vacuum technology can effectively reduce the porosity in the casting and improve the quality of the casting.

(a) Two dimensional slicing under vacuum die casting; (b) Low speed reduction+high vacuum die casting for two-dimensional slicing; (c) Two dimensional slicing under bending sprue and high vacuum die casting; (d) The three-dimensional pore distribution map corresponding to (f) - (a) - (c)

ombining Table 3 and Figure 4, it can be observed that under the high vacuum die-casting process with the addition of bent runners, the area fraction of ESC is not significantly affected, but it can make the distribution of ESC from the surface to the center more uniform, thereby increasing the filling effect between dendrites. Moreover, under high vacuum conditions, the porosity is significantly reduced (see Figure 5 and Table 4). The reason for these phenomena is that bent runners can change the trajectory of melt movement and regulate the speed of melt movement, Causing a sudden increase or decrease in liquid flow velocity at the location where the cross-sectional area of the sprue changes. In this way, ESC will be subjected to significant shear forces during the filling process, which can achieve the goal of crushing ESC and also make the distribution of ESC uniform. For high vacuum die-casting test bars, due to the high vacuum degree, the porosity content in the alloy will be significantly reduced. However, the increase in the residence time of the melt in the pressure chamber will lead to the appearance of a large number of dendritic ESCs. In previous studies, it was found that ESCs will accumulate in the center of the casting and form a large-sized dendritic network. Due to the difficulty of filling the melt at the interface of the dendritic network, it will cause the formation of large-sized network shrinkage, This becomes the crack source of casting fracture. Bending the runner can reduce the size of the ESC, reduce the proportion of ESC dendrite network, thereby improving the filling effect between dendrites and eliminating large-sized network shrinkage.

3.Conclusion

(1)Designing a bent sprue to assist the high vacuum die casting process can cause ESC in the casting to break and distribute uniformly along the radial direction, effectively reducing the porosity in the casting, eliminating large-sized holes, and greatly improving the quality of the casting.

(2) The bent sprue promotes the uniform distribution of ESC in castings, and uniformly distributed ESC can significantly reduce porosity, reducing it from 0.073% to 0.006% and eliminating large-sized pores.

Jiao Xiangyi, School of Materials Science and Engineering, Northeastern University

Jiao Xiangyi, Key Laboratory of Key Metal Structural Materials for Lightweight Use in Liaoning Province, Northeastern University

Wang Pengyue, Shi Lijun, and Wang Chenggang from FAW Casting Co., Ltd

Liu Yixian and Xiong Shoumei from the School of Materials Science, Tsinghua University