1. Overview of high-speed filling process
In the aluminum alloy die casting process, high-speed filling is a key feature. When liquid aluminum alloy is injected into the mold cavity at high speed, it will flow rapidly within the cavity and fill every corner. This process is usually accompanied by high pressure, allowing the aluminum alloy to complete the filling action in a very short time. For example, the filling speed can reach tens of meters per second or even higher. This high-speed filling can ensure that complex-shaped castings are completely formed and avoid defects such as insufficient pouring. At the same time, the high-speed flowing aluminum alloy interacts strongly with the surface of the mold cavity and the internal gas.
2. Effect on grain morphology
High-speed filling has a significant impact on the grain morphology in the casting microstructure. Due to the rapid cooling rate and strong shearing effect, the grain growth of aluminum alloy is inhibited and deformation occurs. The coarse equiaxed crystal structure that may originally be formed will be transformed into fine equiaxed crystals or columnar crystals. The shear force generated by high-speed flow promotes the formation of a large number of crystal nuclei and limits the space and time for the growth of crystal grains. In the area close to the mold wall, due to the faster cooling rate and greater shear force, a layer of fine chilled grains is often formed. These grains are arranged more closely, providing the casting with better surface hardness and wear resistance. sex. Inside the casting, the grains are relatively fine and uniform, which is beneficial to improving the comprehensive mechanical properties of the casting, such as strength and toughness.
3. Effect on element distribution
The distribution of various alloying elements in aluminum alloys will also change during the high-speed filling process. The high-speed flowing liquid metal prevents elements from fully diffusing during the solidification process, resulting in obvious element segregation. Some alloying elements may be concentrated in certain areas, forming microscopic compositional inhomogeneities. For example, for aluminum alloys containing elements such as copper and silicon, under high-speed filling conditions, the copper element may segregate at the grain boundaries. This may increase the strength of the grain boundaries, but on the other hand, if the segregation is excessive, it may Causes grain boundary embrittlement and affects the quality of castings. Reasonable control of high-speed filling parameters can improve the uniformity of element distribution to a certain extent. For example, by optimizing filling speed and pressure, alloy elements have appropriate diffusion time and space during the solidification process and reduce the occurrence of harmful segregation.
4. Relationship to defect formation
High-speed filling characteristics are closely related to defect formation in the casting microstructure. On the one hand, if high-speed filling is not properly controlled, gas can easily be involved and cause pore defects. Due to the fast filling speed, the gas in the cavity does not have time to be completely discharged, so it is drawn into the liquid metal and trapped inside the casting during solidification. On the other hand, high-speed filling may lead to local thermal stress concentration, causing thermal cracking during solidification. For example, in parts of castings where the wall thickness changes greatly, high-speed flowing aluminum alloys can easily produce tensile stress due to uneven shrinkage during rapid cooling. When the tensile stress exceeds the tensile strength of the material, hot cracks will form. Therefore, in the aluminum alloy die casting process, it is necessary to accurately control the high-speed filling characteristics to reduce the occurrence of defects and optimize the microstructure of the casting, thereby obtaining high-quality aluminum alloy die casting parts.
