1. Preface
     Ultra low speed die casting is a new type of die casting process that fills the mold cavity with aluminum alloy liquid in a laminar flow manner at a lower injection rate. It can effectively solve the problem of traditional die casting metal liquid jet entrainment, thereby improving mechanical properties. However, during casting, due to its low injection rate, the aluminum alloy liquid stays in the pressure chamber for a long time, and the contact area between the aluminum liquid and the pressure chamber quickly solidifies into a shell
The temperature distribution of aluminum entering the barrel is shown in Figure 1. During injection, as the punch advances, the cooling layer continuously breaks and eventually enters the mold cavity through a larger internal gate, remaining as an inclusion inside the casting.

     As shown in Figure 2, there is a macroscopic image of the internal solidification plate of the casting. This defect damages the continuity of the casting and greatly reduces its mechanical properties。

The influence of solidification plate on the mechanical properties of castings is shown in Figure 3

    This article introduces the problem that during the process testing of a certain model of our company's product according to the original die-casting process conditions, the proportion of inclusions in the solidification plates inside the product during spot checks reached over 90%, which did not meet the requirements for mass production, and the corresponding countermeasures in the future
2. Improvement and validation
According to relevant literature, the formation of solidified flakes is related to the pre crystallization structure in the pressure chamber. Adjusting the temperature of the aluminum liquid and reducing the residence time of the aluminum liquid in the pressure chamber are countermeasures to reduce the pre crystallization structure. Therefore, three sets of experiments with single casting condition changes and other process conditions unchanged are designed for verification:
① By adjusting the pressure chamber temperature, the temperature drop of aluminum water in the pressure chamber is reduced, thereby reducing the number of pre crystallized structures in the pressure chamber;
② Adjust the temperature of the molten aluminum to also reduce the temperature drop;
③ Adjust the low-speed to reduce the residence time of aluminum in the pressure chamber and reduce the number of pre crystallized structures
2.1 Adjusting the pressure chamber temperature
As shown in Figure 4, the temperature of the pressure chamber is temporarily increased by external heating with natural gas. A handheld infrared thermometer is used to detect the temperature reaching the preset range, and a certain number of products are continuously cast for tensile damage inspection

The result is shown in Figure 5

     From the results of four sets of experiments, it can be seen that without heating in the pressure chamber, the proportion of inclusions in the solidified sheet is close to 90%. However, with increasing heating, the number of inclusions in the solidified sheet decreases as the temperature increases. When the pressure chamber temperature reaches 210-230 ℃, the proportion of inclusions in the solidified sheet decreases to 5%. However, due to the high pressure chamber temperature, the phenomenon of sticking during injection occurs. Therefore, this improvement method can reduce but cannot completely eliminate the defects in the solidified sheet
2.2 Adjusting the temperature of molten aluminum
On the basis of adjusting the temperature of the pressure chamber in Experiment 1, the aluminum liquid temperature was adjusted as a temperature gradient every 10 ℃ to verify the solidification of the sheet under different temperature conditions. The results are shown in Figure 6

     Under the condition of maintaining a pressure chamber temperature of 170 ℃ -180 ℃, as the temperature of the aluminum liquid increases, the number of solidified sheets tends to decrease, but the proportion is still relatively high. When the temperature of the aluminum liquid reaches 700 ℃ or above, problems such as aluminum sticking to the mold and bursting at the sprue may occur
2.3 Adjusting Low Speed
Low speed affects the residence time of aluminum liquid in the pressure chamber. Three different sets of low speed were selected for verification, as shown in Figure 7

    From the results of the low-speed adjustment test, it can be seen that as the low-speed increases, the number of solidified sheets decreases. When the low-speed reaches 60 or more, 40 pieces are checked for fracture, and the number of solidified sheets is 0
3. Result
    Through the experimental verification of the above three sets of process conditions, the suitable die-casting process conditions for this product have been preliminarily determined. Under these process conditions, small-scale production verification has been carried out, and no internal defects of solidified sheets have been found. The product has successfully entered mass production
4. Conclusion
1) The proportion of inclusions in the internal solidification plate of the product decreases with the increase of pressure chamber temperature, decreases with the increase of aluminum liquid temperature, and decreases with the increase of low-speed speed
2) The optimal ultra low speed die casting process conditions for this model of product are: chamber temperature of 170 ℃ -180 ℃, aluminum liquid temperature of 670 ℃ -690 ℃, and low speed of 60-70 ℃
Text: He Zhiyong and Wei Gang from Chongqing Wansi Metal Special Forming Co., Ltd