The role of various elements in aluminum alloys

The role of various elements in aluminum alloys

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The role of various elements in aluminum alloys

1. Influence of alloying elements

Copper element Cu

When the aluminum-rich part of the aluminum-copper alloy is 548, the maximum solubility of copper in aluminum is 5.65%. When the temperature drops to 302, the solubility of copper is 0.45%. Copper is an important alloying element and has a certain solid solution strengthening effect. In addition, the CuAl2 precipitated by aging has an obvious aging strengthening effect. The copper content in aluminum alloys is usually 2.5% to 5%. The strengthening effect is best when the copper content is 4% to 6.8%, so the copper content of most hard aluminum alloys is in this range.

Aluminum-copper alloys can contain less silicon, magnesium, manganese, chromium, zinc, iron and other elements.

Silicon element Si

When the aluminum-rich part of the Al-Si alloy is at the eutectic temperature of 577, the maximum solubility of silicon in the solid solution is 1.65%. Although the solubility decreases with decreasing temperature, this type of alloy generally cannot be heat-treated and strengthened. Aluminum-silicon alloys have excellent casting properties and corrosion resistance.

If magnesium and silicon are added to aluminum at the same time to form an aluminum-magnesium-silicon alloy, the strengthening phase is MgSi. The mass ratio of magnesium to silicon is 1.73:1. When designing the composition of the Al-Mg-Si alloy, the content of magnesium and silicon is configured in this ratio on the matrix. In order to improve the strength of some Al-Mg-Si alloys, an appropriate amount of copper is added, and an appropriate amount of chromium is added to offset the adverse effect of copper on corrosion resistance.

The maximum solubility of Mg2Si in the aluminum-rich part of the Al-Mg2Si alloy equilibrium phase diagram in aluminum is 1.85%, and the deceleration is small as the temperature decreases.

In deformed aluminum alloys, silicon added alone to aluminum is limited to welding materials, and silicon added to aluminum also has a certain strengthening effect.

Magnesium Element Mg

Al-Mg alloy system equilibrium phase diagram aluminum-rich part Although the solubility curve shows that the solubility of magnesium in aluminum decreases greatly with decreasing temperature, in most industrial deformed aluminum alloys, the magnesium content is less than 6%, and the silicon content is also low. This type of alloy cannot be heat-treated and strengthened, but it has good weldability, good corrosion resistance, and medium strength.

The strengthening of aluminum by magnesium is obvious. For every 1% increase in magnesium, the tensile strength increases by about 34MPa. If less than 1% manganese is added, it may supplement the strengthening effect. Therefore, adding manganese can reduce the magnesium content and the tendency of hot cracking. In addition, manganese can also make Mg5Al8 compounds uniformly precipitate, improve corrosion resistance and welding performance.

Manganese Element Mn

In the equilibrium phase diagram of the Al-Mn alloy system, at the eutectic temperature of 658, the maximum solubility of manganese in the solid solution is 1.82%. The alloy strength increases with the increase of solubility. When the manganese content is 0.8%, the elongation reaches the maximum value. Al-Mn alloy is a non-age hardening alloy, that is, it cannot be strengthened by heat treatment.

Manganese can prevent the recrystallization process of aluminum alloy, increase the recrystallization temperature, and significantly refine the recrystallized grains. The refinement of recrystallized grains is mainly through the MnAl6 compound dispersed particles to hinder the growth of recrystallized grains. Another function of MnAl6 is to dissolve impure iron to form (Fe, Mn)Al6, reducing the harmful effects of iron.

Manganese is an important element in aluminum alloys. It can be added alone to form Al-Mn binary alloys, and more often added with other alloying elements. Therefore, most aluminum alloys contain manganese.

Zinc element Zn

The solubility of zinc in aluminum is 31.6% at 275 in the aluminum-rich part of the Al-Zn alloy system equilibrium phase diagram, and its solubility drops to 5.6% at 125.

When zinc is added alone to aluminum, the improvement of the strength of aluminum alloy under deformation conditions is very limited, and there is a tendency to stress corrosion cracking, which limits its application.

Adding zinc and magnesium to aluminum at the same time forms a strengthening phase Mg/Zn2, which has a significant strengthening effect on the alloy. When the Mg/Zn2 content increases from 0.5% to 12%, the tensile strength and yield strength can be significantly increased. When the magnesium content exceeds the super-hard aluminum alloy required to form the Mg/Zn2 phase, the stress corrosion cracking resistance is the greatest when the ratio of zinc to magnesium is controlled at about 2.7.

For example, adding copper to the Al-Zn-Mg base to form an Al-Zn-Mg-Cu alloy has the greatest strengthening effect among all aluminum alloys, and is also an important aluminum alloy material in aerospace, aviation, and power industries.

2. The influence of trace elements

Iron and silicon Fe-Si

Iron is added as alloying elements in Al-Cu-Mg-Ni-Fe forged aluminum alloys, and silicon is added as alloying elements in Al-Mg-Si forged aluminum and in Al-Si welding rods and aluminum-silicon casting alloys. In other aluminum alloys, silicon and iron are common impurity elements, which have a significant effect on alloy properties. They mainly exist as FeCl3 and free silicon. When silicon is greater than iron, β-FeSiAl3 (or Fe2Si2Al9) phase is formed, and when iron is greater than silicon, α-Fe2SiAl8 (or Fe3Si2Al12) is formed. When the ratio of iron to silicon is not appropriate, it will cause cracks in the casting. When the iron content in cast aluminum is too high, the casting will become brittle.

Titanium and Boron Ti-B

Titanium is a commonly used additive element in aluminum alloys, added in the form of Al-Ti or Al-Ti-B intermediate alloys. Titanium forms TiAl2 phase with aluminum, which becomes a non-spontaneous core during crystallization and plays a role in refining the casting structure and weld structure. When Al-Ti alloys produce a package reaction, the critical content of titanium is about 0.15%, and if boron is present, the reduction is reduced to 0.01%.

Chromium Cr

Chromium is a common additive element in Al-Mg-Si, Al-Mg-Zn, and Al-Mg alloys. At 600℃, the solubility of chromium in aluminum is 0.8%, and it is basically insoluble at room temperature.

Chromium forms intermetallic compounds such as (CrFe)Al7 and (CrMn)Al12 in aluminum, which hinder the nucleation and growth process of recrystallization, have a certain strengthening effect on the alloy, and can also improve the toughness of the alloy and reduce the sensitivity to stress corrosion cracking. However, it will increase the sensitivity to quenching and make the anodic oxide film yellow.

The amount of chromium added to aluminum alloys generally does not exceed 0.35%, and decreases with the increase of transition elements in the alloy.

Strontium Sr

Strontium is a surface active element. In crystallography, strontium can change the behavior of the intermetallic compound phase. Therefore, the modification treatment with strontium can improve the plasticity of the alloy and the quality of the final product. Due to the advantages of strontium modification such as long effective time, good effect and reproducibility, it has replaced the use of sodium in Al-Si casting alloys in recent years. Adding 0.015% to 0.03% strontium to the aluminum alloy for extrusion can transform the β-AlFeSi phase in the ingot into the Chinese character α-AlFeSi phase, reduce the homogenization time of the ingot by 60% to 70%, improve the mechanical properties and plastic processing of the material; improve the surface roughness of the product. For high silicon (10% to 13%) deformation aluminum alloy, adding 0.02% to 0.07% strontium can reduce the primary crystal to a minimum, and the mechanical properties are also significantly improved. The tensile strength бb is increased from 233MPa to 236MPa, the yield strength б0.2 is increased from 204MPa to 210MPa, and the elongation б5 is increased from 9% to 12%. Adding strontium to the hypereutectic Al-Si alloy can reduce the size of the primary silicon particles, improve the plastic processing performance, and can be smoothly hot-rolled and cold-rolled.

Zirconium element Zr

Zirconium is also a common additive for aluminum alloys. Generally, the amount added to aluminum alloy is 0.1% to 0.3%. Zirconium and aluminum form ZrAl3 compounds, which can hinder the recrystallization process and refine the recrystallized grains. Zirconium can also refine the casting structure, but the effect is smaller than that of titanium. The presence of zirconium will reduce the effect of titanium and boron in refining grains. In Al-Zn-Mg-Cu alloys, since the effect of zirconium on quenching sensitivity is smaller than that of chromium and manganese, zirconium is preferably used to replace chromium and manganese to refine the recrystallized structure.

Impurity elements Re

Rare earth elements are added to aluminum alloys to increase the component supercooling during aluminum alloy casting, refine the grains, reduce the secondary crystal spacing, reduce the gas and inclusions in the alloy, and make the inclusion phase tend to spheroidize. It can also reduce the surface tension of the melt, increase fluidity, facilitate casting into ingots, and have a significant impact on process performance. The addition amount of various rare earths is about 0.1%. The addition of mixed rare earth (La-Ce-Pr-Nd, etc.) reduces the critical temperature of the formation of the G?P zone during aging of Al-0.65%Mg-0.61%Si alloy. Aluminum alloys containing magnesium can stimulate the metamorphic effect of rare earth elements.

3. Influence of impurity elements

Vanadium forms VAl11 refractory compounds in aluminum alloys, which plays a role in refining grains during the melting and casting process, but the effect is smaller than that of titanium and zirconium. Vanadium also has the effect of refining recrystallization structure and increasing recrystallization temperature.

Calcium has a very low solid solubility in aluminum alloys and forms CaAl4 compounds with aluminum. Calcium is also a superplastic element of aluminum alloys. Aluminum alloys with about 5% calcium and 5% manganese have superplasticity. Calcium and silicon form CaSi, which is insoluble in aluminum. Due to the reduction of the solid solution of silicon, the electrical conductivity of industrial pure aluminum can be slightly improved. Calcium can improve the cutting performance of aluminum alloys. CaSi2 cannot strengthen aluminum alloys by heat treatment. Trace amounts of calcium are beneficial for removing hydrogen from aluminum liquid.

Lead, tin and bismuth are low melting point metals. They have low solubility in aluminum, which slightly reduces the strength of the alloy, but can improve cutting performance. Bismuth expands during solidification, which is beneficial for shrinkage compensation. Adding bismuth to high magnesium alloys can prevent sodium embrittlement.

Antimony is mainly used as a modifier in cast aluminum alloys and is rarely used in deformed aluminum alloys. It only replaces bismuth in Al-Mg deformed aluminum alloys to prevent sodium embrittlement. Antimony is added to some Al-Zn-Mg-Cu alloys to improve hot pressing and cold pressing process performance.

Beryllium can improve the structure of the oxide film in deformed aluminum alloys and reduce burning and inclusions during casting. Beryllium is a toxic element that can cause allergic poisoning in people. Therefore, aluminum alloys that come into contact with food and beverages cannot contain beryllium. The beryllium content in welding materials is usually controlled below 8μg/ml. The beryllium content of aluminum alloys used as welding substrates should also be controlled.

Sodium is almost insoluble in aluminum, with a maximum solubility of less than 0.0025%. Sodium has a low melting point (97.8°C). When sodium exists in the alloy, it is adsorbed on the dendrite surface or grain boundary during solidification. During hot working, the sodium on the grain boundary forms a liquid adsorption layer. When brittle cracking occurs, a NaAlSi compound is formed. There is no free sodium, and "sodium brittleness" does not occur. When the magnesium content exceeds 2%, magnesium takes away silicon, precipitates free sodium, and produces "sodium brittleness". Therefore, sodium salt flux is not allowed to be used in high-magnesium aluminum alloys. Methods to prevent "sodium brittleness" include chlorination, which makes sodium form NaCl and discharge it into the slag, and adding bismuth to generate Na2Bi and enter the metal matrix; adding antimony to generate Na3Sb or adding rare earth can also play the same role.