The role of various elements in aluminum alloys

[China Aluminum Network] I. Effect of alloying elements

Copper element Cu

When the aluminum-copper alloy-rich aluminum portion 548, the copper in the aluminum in the greater solubility of 5.65%, the temperature dropped 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, CuAl2 precipitated during ageing has a significant aging strengthening effect. The content of copper in aluminum alloy is usually between 2.5% and 5%, and the copper content is between 4% and 6.8%. Therefore, the copper content of most hard aluminum alloys is within this range.

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

Silicon Si

The Al-Si alloy-rich Al-rich part at the eutectic temperature of 577, the greater the solubility of silicon in the solid solution is 1.65%. Although the solubility decreases with decreasing temperature, such alloys are generally not heat-treatable. Al-Si alloys have excellent casting properties and corrosion resistance.

If magnesium and silicon are simultaneously added to aluminum to form an aluminum-magnesium silicon alloy, the strengthening phase is MgSi. The mass ratio of magnesium to silicon is 1.73:1. When the Al-Mg-Si-based alloy composition is designed, the content of magnesium and silicon is configured on the substrate in this ratio. In some Al-Mg-Si alloys, in order to increase the strength, an appropriate amount of copper is added while adding an appropriate amount of chromium to offset the adverse effect of copper corrosion resistance.

The equilibrium phase diagram of the Al-Mg2Si alloy system has a relatively large solubility of 1.85% in the Al-rich Mg2Si and decreases with decreasing temperature.

In the deformed aluminum alloy, silicon alone is added to the aluminum and is limited to the welding material. The addition of silicon to the aluminum also has a certain strengthening effect.

Magnesium Mg

Al-Mg alloy equilibrium phase diagram Al rich part Although the solubility curve shows that the solubility of magnesium in aluminum is greatly reduced as the temperature decreases, but in most industrial deformable aluminum alloys, the magnesium content is less than 6%, The silicon content is also low. Such alloys cannot be heat-treated but have good weldability, good corrosion resistance, and moderate strength.

The enhancement of magnesium by aluminum is obvious. With every 1% increase in magnesium, the tensile strength increases by approximately 34MPa. If you add 1% or less of manganese, you may supplement the strengthening effect. Therefore, after adding manganese, the content of magnesium can be reduced, and the hot cracking tendency can be reduced. In addition, manganese can evenly precipitate the Mg5Al8 compound and improve the corrosion resistance and welding performance.

Manganese element Mn

In the Al-Mn alloy flat equilibrium phase diagram, at the eutectic temperature of 658, the larger solubility of manganese in the solid solution was 1.82%. The alloy strength increases with the increase of solubility, and when the manganese content is 0.8%, the elongation reaches a large value. The Al-Mn alloy is a non-aging hardening alloy, ie, it is not heat-treatable.

Manganese can prevent the recrystallization process of aluminum alloys, increase the recrystallization temperature, and can significantly refine the recrystallized grains. The refinement of the recrystallized grains is mainly hindered by the growth of recrystallized grains by the dispersed particles of the MnAl6 compound. Another role of MnAl6 is to dissolve the impurity iron and form (Fe, Mn)Al6, reducing the harmful effects of iron.

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

Zinc Zn

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

Zinc alone added to aluminum, under the deformation conditions of the aluminum alloy strength is very limited, while there is stress corrosion cracking, tend to limit its application.

When zinc and magnesium are added simultaneously in the aluminum, a strengthening phase Mg/Zn2 is formed, which has a significant strengthening effect on the alloy. When the content of Mg/Zn2 is increased from 0.5% to 12%, the tensile strength and yield strength can be significantly increased. Magnesium content exceeds the superhard aluminum alloy required for Mg/Zn2 phase formation. When the ratio of zinc and magnesium is controlled to about 2.7, the stress corrosion cracking resistance is large.

If Al-Zn-Mg is added on the basis of copper to form an Al-Zn-Mg-Cu alloy, the base strengthening effect is larger in all aluminum alloys, and it is also an important aluminum alloy material in the aerospace, aviation and power industries. .

Second, the influence of trace elements

Iron and silicon Fe-Si

Iron in Al-Cu-Mg-Ni-Fe forged aluminum alloys, silicon is added as an alloying element in Al-Mg-Si series forged aluminum and in Al-Si based electrodes and aluminum-silicon casting alloys. Among the aluminum alloys, silicon and iron are common impurity elements, which have a significant effect on the properties of the alloy. They mainly exist as FeCl3 and free silicon. When silicon is larger than iron, β-FeSiAl3 (or Fe2Si2Al9) phases are formed, and when iron is larger than silicon, α-Fe2SiAl8 (or Fe3Si2Al12) is formed. When the proportion of iron and silicon is not appropriate, cracks may be caused in the casting. When the iron content in the cast aluminum is too high, the casting may be brittle.

Titanium and boron Ti-B

Titanium is a commonly used additive element in aluminum alloys and is added in the form of Al-Ti or Al-Ti-B master alloys. Titanium forms a TiAl2 phase with aluminum and becomes a non-spontaneous core during crystallization, which functions to refine the cast structure and weld microstructure. When the Al-Ti alloy generates a package reaction, the critical content of titanium is about 0.15%, and if there is boron, the deceleration is as small as 0.01%.

Chrome Cr

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

Chromium forms intermetallic compounds such as (CrFe)Al7 and (CrMn)Al12 in aluminum, hinders the nucleation and growth of recrystallization, strengthens the alloy, and improves the toughness of the alloy and reduces the stress corrosion cracking susceptibility. . However, the meeting site increases the quench sensitivity, making the anodic oxide film yellow.

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

锶Sr

Ruthenium is a surface-active element. Crystallographically, ruthenium can change the behavior of intermetallic phases. Therefore, deteriorating with niobium can improve the plasticity of the alloy and the quality of the final product. Due to the advantages of long life, good effect and reproducibility, the use of sodium has been replaced in Al-Si casting alloys in recent years. The addition of 0.015% to 0.03% niobium to the aluminum alloy for extrusion results in the transformation of β-AlFeSi phase into a Chinese character α-AlFeSi phase, which reduces the ingot homogenization time by 60% to 70% and improves the mechanical properties of the material. Plastic processing; Improve the surface roughness of products. The addition of 0.02% to 0.07% niobium to high silicon (10% to 13%) deformed aluminum alloys can reduce the primary crystal to a lower limit, the mechanical properties are also significantly improved, and the tensile strength бb is increased from 233 MPa to 236 MPa. The yield strength б0.2 increased from 204 MPa to 210 MPa, and the elongation б5 increased from 9% to 12%. Adding niobium 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 added in the amount of aluminum alloy is 0.1% ~ 0.3%, zirconium and aluminum to form ZrAl3 compounds, can hinder the recrystallization process, refine the recrystallized grains. Zirconium can also refine the cast structure, but it is less effective than titanium. The presence of zirconium reduces the effect of titanium and boron grain refinement. In Al-Zn-Mg-Cu alloys, since zirconium has less effect on the quenching sensitivity than chromium and manganese, zirconium is preferred instead of chromium and manganese to refine the recrystallized structure.

Impurity element Re

The addition of rare earth elements to the aluminum alloy will increase the composition of the aluminum alloy during the casting and will cause excessive cooling, refine the crystal grains, reduce the secondary crystal spacing, reduce the gas and inclusions in the alloy, and tend to spherodize the inclusions. It can also reduce the surface tension of the melt, increase the fluidity, and facilitate the casting into ingots, which has a significant effect on the process performance. It is good to add about 0.1% of various rare earths. The addition of mixed rare earths (such as La-Ce-Pr-Nd mixed) made the critical temperature of the Ag-0.65%Mg-0.61%Si alloy aged in the G?P region decrease. Magnesium-containing aluminum alloys can stimulate the metamorphism of rare earth elements.

Third, the impact of impurity elements

Vanadium forms VAl11 refractory compounds in aluminum alloys, which play a role in grain refinement during the casting process, but is less effective than titanium and zirconium. Vanadium also has the effect of refining the recrystallized structure and increasing the recrystallization temperature.

Calcium has a very low solid solubility in aluminum alloys and forms CaAl4 compounds with aluminum, which is a superplastic element of aluminum alloys. Aluminum alloys with approximately 5% calcium and 5% manganese have superplasticity. Calcium and silicon form CaSi, which is insoluble in aluminum. Since the amount of solid solution of silicon is reduced, the conductive properties of industrial pure aluminum can be slightly improved. Calcium can improve the cutting performance of aluminum alloys. CaSi2 cannot strengthen the aluminum alloy by heat treatment. The trace amount of calcium facilitates the removal of hydrogen from the aluminum bath.

Lead, tin, and bismuth are low-melting metals. Their solid solubility in aluminum is small, slightly reducing the strength of the alloy, but it can improve cutting performance. Swelling during solidification favors feeding. The addition of niobium to high magnesium alloys can prevent sodium embrittlement.

Germanium is mainly used as a modifier in cast aluminum alloys, and deformed aluminum alloys are rarely used. Replacement of niobium in Al-Mg deformed aluminum alloys alone prevents sodium embrittlement. The niobium element is added to certain Al-Zn-Mg-Cu alloys to improve hot-pressing and cold-pressing process performance.

Niobium in the deformed aluminum alloy can improve the structure of the oxide film and reduce the burning loss and inclusion during the casting. Earthworms are toxic elements that can cause allergic poisoning. Therefore, aluminum alloys that come in contact with foods and beverages cannot contain barium. The germanium content in the solder material is usually controlled below 8 μg/ml. Aluminum alloy used as a welding base should also control the content of niobium.

Sodium is almost insoluble in aluminum, with a large solid solubility of less than 0.0025%, and a low melting point of sodium (97.8°C). When sodium is present in the alloy, it adsorbs to the dendrite surface or grain boundary during solidification, and is crystallized during hot working. The sodium on the boundary forms a liquid adsorption layer, and when brittle cracking occurs, a NaAlSi compound is formed, no free sodium is present, and no "sodium embrittlement" occurs. When magnesium content exceeds 2%, magnesium captures silicon and free sodium precipitates, resulting in "sodium embrittlement". Therefore, high magnesium aluminum alloys are not allowed to use sodium salt flux. The method of preventing "sodium embrittlement" is chlorination, so that sodium forms NaCl into the slag, adding cerium to form Na2Bi into the metal matrix; adding cerium to generate Na3Sb or adding rare earth can also play the same role.

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