Classification of die cast aluminum alloy

In addition to having good die-casting process properties and better mechanical properties, die-casting aluminum alloy also needs to have the following process properties:

(1) Good thermoplastic rheology, in the superheat is not high and the liquid, solid phase line temperature should have good thermoplastic rheology, in order to achieve the filling of complex cavity, the formation of a good casting surface, to avoid the production of shrinkage hole defects.

(2) Small line shrinkage, avoid cracks and deformation in the die casting process, and improve the dimensional accuracy of the product.

(3) Small solidification temperature interval, easy to achieve rapid simultaneous solidification, reduce the number of internal shrinkage holes and other defects.

(4) Good high temperature thermal strength to avoid hot cracking or serious deformation when opening the mold.

(5) Good casting/casting interface performance, no chemical reaction with die casting mold, small affinity, to avoid alloying reaction between sticky and casting/casting interface.

(6) Good physical and chemical properties, it is not easy to inhale and oxidize at high temperature melting state to meet the needs of long-term heat preservation in the die casting process.

According to the alloy composition, die casting aluminum alloy can be divided into Al-Si (Al-Si-Cu, Al-Si-Mg), Al-Cu, Al-Mg and Al-Zn four series, Al-Si die-casting alloy is widely used because of small crystallization temperature interval, large latent heat and specific heat capacity of silicon phase solidification, small linear shrinkage, good flow performance, filling performance and small thermal cracking and porosity tendency. Although Al-Cu die-casting alloy has high mechanical and mechanical properties, the addition of Cu element reduces the corrosion resistance of the material, and the service life of die casting products is greatly reduced, and the die casting process is easy to produce segregation and cracking, so the application range is small.

Compared with Al-Si die-casting alloys, Al-Mg die-casting alloys have poor casting properties, fluctuating mechanical properties, large wall thickness effects, easy cracking during die casting, and greater stress corrosion tendency. Although Al-Zn die-casting alloy can obtain better mechanical properties after natural aging, its corrosion resistance is poor, it is easy to appear hot cracking and stress corrosion, and it is rarely used. Table 1 shows the brands, chemical compositions and mechanical properties of commonly used die-cast aluminum alloys, among which Al-SI-Cu alloys are most widely used, such as AlSi9Cu3 (A380) and AlSi11Cu3 (ADC12). As shown in Table 1, die-cast aluminum alloys generally add Si, Cu, Mg, Mn, Fe, Ni and rare earth elements.

Because the latent heat of solidification of Si element is much greater than that of Al element, adding an appropriate amount of Si can improve the process flow performance of the alloy, reduce the thermal cracking tendency, improve the material's air tightness, corrosion resistance and thermal conductivity, when adding 25% Si element, the solidification shrinkage rate of the die-casting alloy can be reduced to 0, which can be used to manufacture die-casting products with extremely high dimensional stability requirements such as pistons. Fe element can improve the stripping performance, generally add more than 0.6%, excessive Fe element is easy to form a needle or sheet β-AlFeSi phase, reduce the plasticity of the alloy, can increase the content of Fe element added at the same time add an appropriate amount of Mn element (≤0.5%), improve the stripping performance of the alloy. The addition of Cu element can enhance the corrosion resistance, mechanical strength and thermal conductivity of the alloy, improve the process flow, creep resistance, fatigue strength and machining performance of the alloy. When Cu is dissolved in aluminum matrix as a strengthened phase or exists in the form of granular compounds, the strength and hardness of the alloy can be obviously increased, but the elongation is slightly decreased. When Cu formed a network compound, the strength and elongation of the alloy decreased significantly.

The Mg element can improve the corrosion resistance and mechanical strength of the alloy, and the mucous membrane tendency will also be reduced, and the mechanical processing property of the alloy will be improved. Al-Si-Cu die-casting alloy materials usually add 0.5-1.0% Sm element to refine the size of eutectic silicon phase, reduce the secondary dendrite arm spacing, and improve the strength and plasticity of the alloy. The addition of appropriate Sr element (0.02 ~ 0.08%) can improve the morphology of eutectic silicon, achieve grain refinement, and effectively improve the comprehensive mechanical properties of die-casting alloy. The addition of 1 ~ 1.5% Ni can improve the strength, hardness and corrosion resistance of the alloy, and reduce the damage of the alloy to the die. At present, by increasing the content of Si, Mn and Mg, reducing the content of Fe and Cu, adding 0.04%Sr and 0.05%Be elements, the elongation of the most commonly used die-cast aluminum alloy A380 can be increased to more than 5.0%.

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