Basic requirements for die casting alloys
The performance of die-casting alloys directly determines the quality and application range of die-casting parts. Therefore, there are clear basic requirements for them, covering fluidity, shrinkage, mechanical properties, heat resistance, corrosion resistance and other aspects. These requirements must be matched with the characteristics of the die-casting process and the product usage environment to achieve efficient and stable production and reliable service performance.
Good fluidity is a primary requirement for die-casting alloys, ensuring that the molten metal can fill complex cavities under high pressure and high speed. Fluidity is typically measured by the length of the spiral (the distance the molten metal flows in a standard mold): ≥ 600mm for aluminum alloys, ≥ 800mm for zinc alloys , and ≥ 500mm for magnesium alloys . Key factors influencing fluidity include melting point, specific heat capacity, and viscosity. Low-melting-point alloys (such as zinc alloys, melting point 380-420 °C) have better fluidity than high-melting-point alloys (such as copper alloys, melting point 900-1100 °C). Alloys with low specific heat capacity (such as magnesium alloys, specific heat capacity 1.02 kJ/(kg・K) ) heat up faster and have better fluidity. Alloy composition also significantly affects fluidity. For example, aluminum alloys have optimal fluidity when the silicon content is 7%-12%. Lower or higher silicon content results in decreased fluidity. In actual production, fluidity can be improved by increasing the pouring temperature (fluidity increases by 5%~8% for every 10°C increase) and the mold temperature (fluidity increases by 3%~5% for every 20°C increase), but a balance must be struck between energy consumption and mold life.
The alloy’s shrinkage must be stable and minimal to minimize dimensional deviation and deformation in die-cast parts. Shrinkage includes liquid, solidification, and solid-state shrinkage. The total shrinkage for aluminum alloys is 0.8%-1.2%, for zinc alloys 0.5%-0.8%, and for magnesium alloys 1.0%-1.5%. Unstable shrinkage can lead to large dimensional fluctuations in die-cast parts. For example, when the magnesium content in an aluminum alloy exceeds 1.5%, shrinkage can fluctuate by as much as ±0.3%, necessitating strict composition control. For large, complex parts, alloys with low shrinkage are preferred. For example, zinc alloy die-castings offer superior dimensional stability compared to aluminum alloys, making them suitable for precision instrumentation. Adding refiners (such as titanium and boron to aluminum alloys) can refine grain size, reduce differential shrinkage, and keep deformation within 0.1 mm/m. Furthermore, the alloy’s bulk and linear shrinkage must be matched to avoid shrinkage cavities and cracks. For example, the bulk shrinkage of magnesium alloys should be controlled between 4%-6% and 1.0%-1.2%, maintaining a ratio of approximately 4:1.
Mechanical properties are the core indicators for die-cast alloys to meet application requirements, including strength, hardness, and plasticity. In terms of tensile strength, aluminum alloys for structural parts require ≥200 MPa, while load-bearing parts require ≥250 MPa. Zinc alloys are generally ≥180 MPa and are suitable for lightly loaded parts. Magnesium alloys are ≥220 MPa, offering a balance of lightweight and high strength. Hardness is determined based on processing requirements. Aluminum alloys with a hardness of 50-100 HBW after die-casting facilitate machining. If higher hardness is required, heat treatment (such as T6 treatment) can be used to increase the hardness to 100-150 HBW. Plasticity (elongation) reflects the alloy’s impact resistance. Alloys for automotive safety components require an elongation of ≥8% to prevent brittle fracture, while decorative parts require lower plasticity (≥3% is sufficient). A balanced mechanical property profile is essential. For example, 6061 aluminum alloy, through composition optimization, can achieve a well-balanced tensile strength of 300 MPa and elongation of 12% , making it suitable for complex load-bearing parts. In addition, the fatigue strength of the alloy is also very important. For example, the fatigue strength of the alloy used for engine parts must be ≥ 100MPa ( 10⁷ cycles) to ensure long-term safety.
Die-cast alloys must possess excellent heat and corrosion resistance to adapt to diverse service environments. Heat resistance requires that the alloy maintain strength at operating temperatures. Aluminum alloys used in automotive engine parts must maintain a tensile strength retention rate of ≥80% at temperatures between 150°C and 250°C. Copper and nickel alloys (such as 2014 aluminum alloy) are suitable. For high-temperature environments (300°C to 400°C), copper alloys (such as tin bronze) are suitable, as their high-temperature strength is 2-3 times that of aluminum alloys. Corrosion resistance is crucial for parts exposed to outdoor environments and media. Die-cast parts used in marine environments require alloys resistant to salt spray corrosion. For example, aluminum-magnesium alloys (5-series aluminum alloys) must pass the salt spray test (5% NaCl solution) for 1000 hours without red rust. The food industry requires lead-free zinc alloys (lead content ≤ 0.004%) that are RoHS-compliant. Surface treatments (such as anodizing and electroplating) can further enhance corrosion resistance. For example, hard anodizing of aluminum alloys can reduce the corrosion rate by over 90%.
The alloy’s processing properties must be adapted to the characteristics of die-casting production, including melting performance, mold release, and weldability. Melting performance requires the alloy to exhibit low gas absorption and low oxidation during melting. Aluminum alloys must have a hydrogen content of ≤0.2mL/100g to avoid porosity. Zinc alloys should be melted between 420°C and 450°C to minimize zinc volatilization losses. Alloys with good mold release properties can reduce mold wear and sticking. Zinc alloys offer better mold release than aluminum alloys due to their low affinity for mold steel. Adding a small amount of silicon (5%-7% in aluminum alloys) can improve mold release and reduce the amount of release agent required. For parts requiring assembly, the alloy must be weldable. Aluminum alloys can be welded using argon arc welding, achieving weld strength exceeding 80% of the matrix strength. Magnesium alloys require specialized welding wire to prevent weld cracking. Furthermore, the alloy’s recyclability is crucial. Aluminum and zinc alloys have recycling rates exceeding 95%, meeting green manufacturing requirements and reducing production costs.
