The role of various elements in aluminum alloys
When the aluminum rich part of aluminum copper alloy is 548, the maximum solubility of copper in aluminum is 5.65%, and when the temperature drops to 302, the solubility of copper is 0.45%. Copper is an important alloy element, which has a certain solution strengthening effect. In addition, CuAl2 precipitated by aging has a significant aging strengthening effect. The copper content of aluminum alloy is usually 2.5%~5%, and the strengthening effect is best when the copper content is 4%~6.8%, so the copper content of most duralumin alloys is in this range. Aluminum copper alloys can contain less silicon, magnesium, manganese, chromium, zinc, iron and other elements.
The maximum solubility of silicon in solid solution of Al Si alloy system is 1.65% at eutectic temperature 577. Although the solubility decreases with decreasing temperature, such alloys cannot be strengthened by heat treatment generally. Al Si alloy has excellent casting performance and corrosion resistance. If magnesium and silicon are added to aluminum at the same time to form 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 Al Mg Si alloy system, the content of magnesium and silicon shall be configured on the matrix according to this ratio. Some Al Mg Si alloys, in order to improve the strength, add an appropriate amount of copper, and at the same time add an appropriate amount of chromium to offset the adverse effect of copper on corrosion resistance.
The maximum solubility of Mg2Si in aluminum in the aluminum rich part of the equilibrium phase diagram of Al Mg2Si alloy system is 1.85%, and the deceleration is small with the decrease of temperature. In deformed aluminum alloy, silicon added to aluminum alone is limited to welding materials, and silicon added to aluminum also has a certain strengthening effect.
Although the solubility curve of the aluminum rich part of the equilibrium phase diagram of the Al Mg alloy system shows that the solubility of magnesium in aluminum decreases greatly with temperature, the magnesium content in most industrial wrought aluminum alloys is less than 6%, and the silicon content is also low. This kind of alloy cannot be strengthened by heat treatment, but it has good weldability, corrosion resistance and medium strength. The strengthening of magnesium on aluminum is obvious, and the tensile strength increases by about 34MPa for every 1% increase of magnesium. If less than 1% manganese is added, the strengthening effect may be supplemented. Therefore, adding manganese can reduce magnesium content and hot cracking tendency. In addition, manganese can make Mg5Al8 compound precipitate uniformly, improving corrosion resistance and welding performance.
At the eutectic temperature of 658, the maximum solubility of manganese in the solid solution of the horizontal equilibrium phase diagram of Al Mn alloy system is 1.82%. The strength of the alloy increases with the increase of solubility. When the manganese content is 0.8%, the elongation reaches the maximum. 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 prevented by the dispersed particles of MnAl6 compound. Another function of MnAl6 is to dissolve impurity iron and form (Fe, Mn) Al6, reducing the harmful effect of iron. Manganese is an important element of aluminum alloy, which can be added alone to form an Al Mn binary alloy, more often together with other alloy elements. Therefore, most aluminum alloys contain manganese.
The solubility of zinc in aluminum is 31.6% when the aluminum rich part of the equilibrium phase diagram of Al Zn alloy system is 275, and it decreases to 5.6% when it is 125. When zinc is added to aluminum alone, the strength of aluminum alloy is limited under deformation conditions, and there is a tendency of stress corrosion cracking, which limits its application. The strengthening phase Mg/Zn2 is formed by adding zinc and magnesium to aluminum at the same time, which has an obvious strengthening effect on the alloy. When the content of Mg/Zn2 increases from 0.5% to 12%, the tensile strength and yield strength can be significantly increased. When the content of magnesium exceeds that required for forming Mg/Zn2 phase, and the proportion of zinc and magnesium is controlled at about 2.7, the stress corrosion cracking resistance is the largest. For example, copper element is added on the basis of Al Zn Mg to form Al Zn Mg Cu series alloy. The base strengthening effect is the largest among all aluminum alloys, and it is also an important aluminum alloy material in aerospace, aviation and power industries.
Iron and silicon
Iron in Al Cu Mg Ni Fe forged aluminum alloys, silicon in Al Mg Si forged aluminum and in Al Si electrodes and Al Si casting alloys are all added as alloy elements. In other aluminum alloys, silicon and iron are common impurities, which have a significant impact on the alloy properties. They mainly exist as FeCl3 and free silicon. When silicon is greater than iron β- FeSiAl3 (or Fe2Si2Al9) phase is formed when iron is larger than silicon α- Fe2SiAl8 (or Fe3Si2Al12). When the proportion of iron and silicon is not appropriate, it will cause cracks in the casting, and when the iron content in the cast aluminum is too high, it will make the casting brittle.
Titanium and boron
Titanium is a commonly used addition element in aluminum alloy, which is added in the form of Al Ti or Al-Ti-B master alloy. Titanium and aluminum form TiAl2 phase, which becomes the non spontaneous core during crystallization and plays a role in refining the casting structure and weld structure. The critical content of titanium is about 0.15% when Al Ti alloy undergoes ladle reaction, and the deceleration is reduced to 0.01% if there is boron.
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, strengthen the alloy to a certain extent, improve the toughness of the alloy and reduce the susceptibility to stress corrosion cracking. However, the quenching sensitivity of the venue is increased, making the anodic oxide film yellow. The addition of chromium in aluminum alloy generally does not exceed 0.35%, and decreases with the increase of transition elements in the alloy.
Strontium is a surface active element. In crystallography, strontium can change the behavior of intermetallic compounds. Therefore, modification with strontium can improve the plastic workability and final product quality of the alloy. In recent years, strontium has replaced sodium in Al Si casting alloys due to its long modification effective time, good effect and reproducibility. 0.015%~0.03% strontium is added to aluminum alloy for extrusion β- AlFeSi phase becomes Chinese character α- AlFeSi phase reduces the homogenization time of the ingot by 60%~70%, and improves the mechanical properties and plastic processability of the material; Improve the surface roughness of products. For high silicon (10%~13%) wrought aluminum alloy, the addition of 0.02%~0.07% strontium can reduce the initial crystal to the minimum, significantly improve the mechanical properties and tensile strength б B Increase from 233MPa to 236MPa, yield strength б 0.2 from 204MPa to 210MPa, elongation б 5 From 9% to 12%. Adding strontium into hypereutectic Al Si alloys can reduce the size of primary silicon particles, improve the plastic workability, and can be successfully hot rolled and cold rolled.
Zirconium is also a common additive for aluminum alloys. Generally, the addition amount of aluminum alloy is 0.1%~0.3%. Zirconium and aluminum form ZrAl3 compound, which can hinder the recrystallization process and refine recrystallized grains. Zirconium can also refine the casting structure, but its effect is smaller than that of titanium. The presence of zirconium will reduce the grain refinement effect of titanium and boron. In Al Zn Mg Cu alloys, zirconium has less effect on quenching sensitivity than chromium and manganese, so zirconium should be used instead of chromium and manganese to refine recrystallization structure.
rare earth element
The addition of rare earth elements into aluminum alloy can increase the composition supercooling, 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 and increase the fluidity, which is conducive to casting into ingots and has a significant impact on the technological properties. It is better to add 0.1% at% rare earth. The addition of mixed rare earth (La Ce Pr Nd, etc.) makes Al-0.65% Mg-0.61% Si alloy age G? The critical temperature for the formation of P zone decreases. Aluminum alloys containing magnesium can stimulate the modification of rare earth elements.
Vanadium forms VAl11 refractory compound in aluminum alloy, which plays a role in refining grain during melting and casting, but has less effect than titanium and zirconium. Vanadium can also refine the recrystallization structure and increase the recrystallization temperature.
Calcium has very low solid solubility in aluminum alloys, forming CaAl4 compounds with aluminum. Calcium is also a superplastic element of aluminum alloys. About 5% calcium and 5% manganese aluminum alloys have superplasticity. Calcium and silicon form CaSi, which is insoluble in aluminum. As the solid solution amount of silicon is reduced, the conductivity of industrial pure aluminum can be slightly improved. Calcium can improve the machinability of aluminum alloy. CaSi2 can not strengthen the heat treatment of aluminum alloy. Trace calcium is beneficial to the removal of hydrogen from aluminum solution.
Lead, tin and bismuth are low melting point metals. Their solid solubility in aluminum is not large, which slightly reduces the strength of the alloy, but can improve the cutting performance. Bismuth expands during solidification, which is beneficial to feeding. The addition of bismuth to high magnesium alloys can prevent sodium embrittlement.
Antimony is mainly used as a modifier in cast aluminum alloys, while wrought aluminum alloys are rarely used. Only replace bismuth in Al Mg wrought aluminum alloys to prevent sodium embrittlement. Antimony is added to some Al Zn Mg Cu alloys to improve the hot pressing and cold pressing properties.
Beryllium in wrought aluminum alloys can improve the structure of oxide film and reduce burning loss and inclusion during melting and casting. Beryllium is a toxic element, which can cause allergic poisoning. Therefore, aluminum alloys that come into contact with food and beverages should not contain beryllium. Beryllium content in welding materials is usually controlled at 8 μ Below g/ml. The content of beryllium in aluminum alloy used as welding base should also be controlled.
Sodium is almost insoluble in aluminum. The maximum solid solubility is less than 0.0025%. The melting point of sodium is low (97.8 ℃). When sodium is present in the alloy, it will be adsorbed on the dendrite surface or grain boundary during solidification. During hot working, the sodium on the grain boundary will form a liquid adsorption layer. When brittle cracking occurs, NaAlSi compounds will be formed. There is no free sodium and no "sodium brittleness" will occur. When the magnesium content exceeds 2%, the magnesium seizes the silicon and precipitates free sodium, resulting in "sodium brittleness". Therefore, sodium salt flux is not allowed for high magnesium aluminum alloy. The method to prevent "sodium brittleness" is chlorination, which makes sodium form NaCl and discharge it into slag, and bismuth is added to make it generate Na2Bi and enter the metal matrix; Adding antimony to generate Na3Sb or adding rare earth can also play the same role.