Process performance requirements of die-casting alloys
The process performance requirements of die-casting alloys are crucial for ensuring smooth die-casting production and high-quality castings. These requirements encompass multiple aspects, including fluidity, filling properties, solidification characteristics, and thermal stability, directly impacting the stability of the die-casting process and the final quality of the casting. First and foremost, the die-casting alloy must possess excellent fluidity—the ability of the molten metal to flow within the mold cavity—which is essential for ensuring complete filling of the casting. Alloys with poor fluidity are prone to defects such as underfill and cold shuts due to rapid cooling during the filling process. Sufficient fluidity is particularly crucial for thin-walled, complex castings. Alloy fluidity is generally related to its melting point, viscosity, and crystallization temperature range. Alloys with lower melting points, lower viscosities, and narrower crystallization temperature ranges exhibit better fluidity. For example, zinc alloys have a lower melting point (approximately 420°C) and better fluidity than aluminum alloys, making them more suitable for producing thin-walled precision castings. Copper alloys, on the other hand, have a higher melting point (approximately 1083°C) and poorer fluidity, placing higher demands on the die-casting process.
Secondly, the die-casting alloy must have good filling properties, that is, the ability to fill every corner of the mold cavity under a certain injection pressure and speed. Filling properties are not only related to fluidity, but also to the solidification rate and shrinkage rate of the alloy. Alloys with slow solidification rates have longer filling times and better filling properties; alloys with low shrinkage rates can reduce volume shrinkage after filling, reducing the risk of shrinkage porosity and shrinkage cavities. For example, magnesium alloys have a narrow crystallization temperature range, a relatively slow solidification rate, and good filling properties, making them suitable for the production of complex cavity castings; while some aluminum alloys (such as Al-Si-Cu series) have a wide crystallization temperature range and a fast solidification rate, resulting in relatively poor filling properties, which need to be compensated by increasing the pouring temperature and injection speed.
The solidification characteristics of die-cast alloys significantly impact the quality of castings, requiring small and uniform solidification shrinkage to reduce internal stress and deformation in the casting. Solidification shrinkage of alloys includes liquid shrinkage, solidification shrinkage, and solid-state shrinkage, and the total shrinkage must be controlled within a reasonable range. Generally, the total shrinkage of aluminum alloys is 1.0%-1.5%, that of zinc alloys is 0.5%-1.0%, and that of magnesium alloys is 1.2%-1.8%. Excessive or uneven shrinkage can lead to shrinkage porosity, shrinkage cavities, deformation, and even cracks in the casting. For example, zinc alloys have a small and uniform shrinkage rate, resulting in high dimensional accuracy in castings, making them suitable for the production of demanding precision parts. However, certain high-silicon aluminum alloys are prone to deformation due to uneven shrinkage, requiring control through reasonable mold design and adjustment of process parameters.
Die-casting alloys must exhibit excellent thermal stability, meaning their mechanical properties and microstructure remain stable and exhibit no significant changes during the high-temperature cycles of the die-casting process. Alloys with poor thermal stability may develop problems such as coarsening of grains and precipitation phase changes during repeated heating and cooling, leading to a decrease in the mechanical properties of the casting. For example, aluminum alloys typically require aging treatment after die-casting to stabilize their structure and properties. Zinc alloys, on the other hand, have relatively poor thermal stability and are not suitable for use in high-temperature environments, as they are prone to softening and deformation. Therefore, it is crucial to select an alloy with appropriate thermal stability for the casting’s intended use. For example, die-cast parts in automotive engine compartments require aluminum alloys with high thermal stability (such as the Al-Si-Mg series).
Die-cast alloys must also exhibit excellent machinability and surface treatment properties to meet subsequent processing and usage requirements. Machinability refers to the ease with which an alloy can be machined, such as cutting, drilling, and grinding. This is related to the alloy’s hardness and toughness. Alloys with moderate hardness (e.g., aluminum alloys with a hardness of HB60-100) exhibit good machinability. Excessively hard or soft alloys can lead to rapid tool wear or rough machined surfaces. Surface treatment properties refer to the alloy’s suitability for surface treatments such as electroplating, anodizing, and painting to enhance corrosion resistance and decorative properties. For example, aluminum alloys exhibit excellent anodizing properties, which can form a dense oxide film through oxidation, significantly improving corrosion resistance. Zinc alloys, on the other hand, exhibit excellent electroplating properties, and treatments such as zinc and chromium plating can enhance surface hardness and aesthetics. Furthermore, die-cast alloys must exhibit low gas absorption and oxidation propensity to minimize the formation of pores and oxide inclusions during the die-casting process. For example, magnesium alloys are susceptible to oxidation, and die-casting requires a protective gas atmosphere (such as SF6) to prevent oxidation combustion. When a die-casting company was producing an aluminum alloy connector, it selected Al-Si alloy with low gas absorption and optimized the smelting process. The porosity of the casting was reduced from 5% to 1%, and the qualified rate after surface treatment was increased to 98%.
