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what is the role of alloying elements in cast aluminum alloys

Pure aluminum has good plasticity, with an elongation rate of about 35% and a cross-sectional shrinkage rate of 80%, making it suitable for cold working forming. However, its tensile strength is relatively low (around 80 MPa), making it difficult to meet the strength requirements of structural components. Therefore, cold work hardening can only be used to improve strength. At the same time, the die-casting performance of pure aluminum is poor, mainly manifested in the following three aspects: (1) the viscosity of the molten metal is high and the flowability is poor; (2) During the solidification process, there is a significant amount of shrinkage, which can easily lead to the formation of shrinkage cavities and porosity inside the casting; (3) During the die-casting process, it has a strong affinity for iron and is prone to sticking to the mold. Therefore, in order to improve the strength and die-casting performance of pure aluminum, it is often necessary to add other alloying elements, and the elements added to die-casting aluminum alloys include Si, Mg, Zn, Mn, etc.

Si element

The purpose of adding Si is to improve the fluidity, strength, and hardness of aluminum alloys. Due to the higher latent heat of solidification of Si compared to Al, the addition of Si can greatly improve the fluidity of the alloy. Combined with the Al Si binary alloy phase, it can be seen that aluminum silicon alloy undergoes eutectic reaction at 577 ℃, and the mass fraction of eutectic point Si is 12.6%. When the Si content is less than 12.6%, with the increase of Si content, the solid-liquid two-phase interval of the alloy narrows, the latent heat of solidification increases, the melt flowability improves, and the tendency to form shrinkage and hot cracking decreases, resulting in better casting performance. However, when the Si content exceeds 12.6%, the cutting performance of the die casting deteriorates, which is not conducive to subsequent precision cnc machining.

Si has high hardness and good chemical stability. Increasing the Si content in the alloy will enhance its hardness and strength, improve the wear resistance of die castings, but strip eutectic Si and block shaped primary Si will reduce the plasticity of the alloy. In addition, the thermal expansion coefficient of Si is relatively low. As the Si content increases, the thermal expansion coefficient of aluminum alloys decreases. When the Si mass fraction reaches 25%, the alloy can even prevent volume shrinkage during die-casting. Therefore, for some die-casting parts with special requirements for wear resistance and low expansion, such as pistons, high silicon aluminum alloys are generally used.

Mg element

The addition of Mg can significantly enhance the strength of the alloy, improve its fluidity, reduce the tendency to stick to the mold, and lower the surface roughness of the die-casting parts, but it is prone to thermal cracking. The maximum solid solubility of Mg in Al reaches 17.4%. Therefore, the magnesium element in aluminum alloy is generally solid dissolved in the matrix, which has a good solid solution strengthening effect. At this time, the alloy has good strength, toughness, and corrosion resistance.

When the Mg content is high, Mg and Al will form Al3Mg2 phase, and the morphology and distribution of this phase have a significant impact on the mechanical properties and corrosion resistance of the alloy. When a small amount of Al3Mg2 is dispersed in the grain boundaries and grains, it can improve the strength of the alloy without damaging its plasticity and corrosion resistance. However, when the number of Al3Mg2 phases is too large, it will form a network distribution along the grain boundaries, and the die casting is prone to intergranular corrosion and stress corrosion cracking, resulting in a sharp decrease in plasticity. The simultaneous addition of Mg and Si can generate Mg2Si phase, which can significantly improve the strength of the alloy. The second phase strengthening effect of Mg2Si is better than that of Mg solid solution strengthening.

Cu element

The addition of Cu can improve the high-temperature mechanical properties of the alloy, but it will reduce the elongation and corrosion resistance. When Cu is solid dissolved in the alloy matrix or exists as small and round granular second phases, the alloy strength can be significantly improved, and the elongation is only slightly reduced. However, when there are a large number of second phases formed by Cu, it is easy to form a network that wraps around the grains, which will seriously reduce the plasticity of the alloy. This is similar to the influence of Mg element. However, the maximum solid solubility of Cu in Al is lower than that of Mg, so the amount of Cu added in Al is lower than that of Mg, generally controlled at 1-4%. At the same time, the addition of Cu reduces the corrosion resistance of the alloy, as Al has a lower potential than Cu, making it prone to intergranular corrosion in the alloy, especially when Cu exists in the form of a second phase, the corrosion resistance of the alloy is even worse.

Fe element

The addition of Fe element is mainly to suppress mold sticking, and to a certain extent, it can also improve the fluidity of the alloy, refine the grain size, and generate iron rich phases (Al3Fe, α - Al8Fe2Si, β - Al5FeSi), which cause the oxide film on the contact surface between the casting and the mold to lose continuity, which is conducive to the smooth demolding of the die casting. However, when the iron content is too high, the second phase containing iron will split the matrix in the form of needle or lath, which will easily lead to stress concentration at the interface between the matrix and the iron containing phase and seriously reduce the alloy plasticity. Therefore, it is necessary to choose a reasonable Fe content for die-casting aluminum alloys.

Zn element

The maximum solid solubility of Zn in Al is the highest among all commonly used alloying elements. Under rapid cooling conditions such as die casting, Zn is generally solid soluble in the alloy matrix. The addition of Zn alone has little effect on the mechanical properties of die cast aluminum alloys, and it also increases the tendency of hot cracking in the castings. Therefore, Zn is generally added together with other elements to strengthen the alloy by forming a second phase.

RE element

Rare earth elements can easily combine with dissolved gases and non-metallic oxides in the melt to form high melting point compounds, which can reduce impurities in the metal melt and minimize defects during alloy forming. Rare earth elements have lively chemical properties, which can reduce the surface tension of the melt and improve the fluidity of the molten metal. Rare earth elements have a good modification effect on Al Si alloys, which can transform strip eutectic silicon into round grains and reduce the size of primary silicon.

Cr element

Cr is a commonly used additive element in Al Mg Si alloys, which has a similar effect to Mn, can suppress mold sticking, improve the morphology of Fe containing phases, and increase alloy toughness. The solid solubility of Cr in Al is very low, and it generally combines with Al to form intermetallic compounds, which inhibit the nucleation and growth of recrystallized grains, but increase the quenching sensitivity of castings.

Sr element

Sr is mainly used as the modifier of Al Si alloy, which can transform strip eutectic Si into coral, refine eutectic structure and improve the toughness of the alloy. Due to its good modification effect, long effective time, and good reproducibility, Sr has gradually replaced Na as the main modifier for aluminum silicon alloys. However, the burning loss of Sr is relatively large, and it is generally added in the form of Al-10Sr intermediate alloy. Sr is also prone to increase the gas absorption of the melt, and it needs to be degassed during the melting process to avoid shrinkage porosity inside the casting.

In summary, the influence of each element on the mechanical and casting properties of aluminum alloys is both independent and interactive. The amount of each alloy element added in die cast aluminum alloys should be selected according to the requirements of the die casting. Automotive structural components require high strength and toughness of aluminum alloys, but the improvement of alloy strength generally requires sacrificing its toughness. When the tensile strength of the alloy reaches over 300MPa, its elongation is generally low. Therefore, it is necessary to optimize the types and contents of alloy elements and develop compressible cast aluminum alloys with both high strength and toughness.