1. Effect of alloying elements
When the aluminum-copper alloy is rich in aluminum portion 548, the maximum solubility of copper in aluminum is 5.65%, and when the temperature is lowered 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 by aging has obvious aging strengthening effect. The copper content in aluminum alloys is usually between 2.5% and 5%, and the copper content is best at 4% to 6.8%, so the copper content of most hard aluminum alloys is in this range.
The aluminum-copper alloy may contain less elements such as silicon, magnesium, manganese, chromium, zinc, and iron.
When the Al-Si alloy is rich in aluminum at a eutectic temperature of 577, the maximum solubility of silicon in the solid solution is 1.65%. Although the solubility decreases with decreasing temperature, such alloys are generally not heat treatable. Aluminum-silicon 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 alloy composition is designed, the contents of magnesium and silicon are arranged in this ratio on the substrate. In some Al-Mg-Si alloys, in order to increase the strength, 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.
Al-Mg2Si alloy alloy equilibrium phase diagram The maximum solubility of Mg2Si in aluminum is 1.85%, and the deceleration is small with decreasing temperature.
In the deformed aluminum alloy, silicon is added to aluminum alone and is limited to the welding material, and silicon is also added to aluminum to have a certain strengthening effect.
The equilibrium phase diagram of the Al-Mg alloy is rich in aluminum. Although the solubility curve indicates that the solubility of magnesium in aluminum is greatly reduced with temperature, in most industrially deformed aluminum alloys, the content of magnesium is less than 6%. The silicon content is also low, and these alloys are not heat-treated, but have good weldability, good corrosion resistance, and moderate strength.
The strengthening of magnesium by aluminum is obvious. For every 1% increase in magnesium, the tensile strength is about 34 MPa. If less than 1% of manganese is added, it may supplement the strengthening. Therefore, the addition of manganese can reduce the magnesium content, and at the same time reduce the tendency of hot cracking. In addition, manganese can uniformly precipitate the Mg5Al8 compound to improve the corrosion resistance and the welding performance.
In the equilibrium phase diagram of the Al-Mn alloy system, the maximum solubility of manganese in the solid solution was 1.82% at the eutectic temperature of 658. The strength of the alloy increases with increasing solubility. When the manganese content is 0.8%, the elongation reaches a maximum. The Al-Mn alloy is a non-age hardening alloy, that is, it cannot be heat-treated.
Manganese can prevent the recrystallization process of aluminum alloys, increase the recrystallization temperature, and refine the recrystallized grains remarkably. The refinement of recrystallized grains is mainly caused by the diffusion of particles of MnAl6 compound to hinder the growth of recrystallized grains. Another function of MnAl6 is to dissolve the impurity iron and form (Fe, Mn)Al6 to reduce the harmful effects of iron.
Manganese is an important element of aluminum alloy, and it can be added separately to form an Al-Mn binary alloy, and more is added together with other alloying elements, so that most of the aluminum alloys contain manganese.
Al-Zn alloy equilibrium phase diagram The solubility of zinc in aluminum is 31.6% in the aluminum-rich fraction 275, while the solubility decreases to 5.6% at 125.
Zinc is added to aluminum alone. Under the deformation condition, the strength of the aluminum alloy is very limited, and there is a tendency of stress corrosion cracking, which limits its application.
Simultaneous addition of zinc and magnesium in aluminum forms a strengthening phase Mg/Zn2, which has a significant strengthening effect on the alloy. When the Mg/Zn2 content is increased from 0.5% to 12%, the tensile strength and yield strength can be significantly increased. The content of magnesium exceeds that of the superhard aluminum alloy required to form the Mg/Zn2 phase. When the ratio of zinc to magnesium is controlled at about 2.7, the stress corrosion cracking resistance is the largest.
For example, by adding copper element to Al-Zn-Mg to form an Al-Zn-Mg-Cu alloy, the base strengthening effect is the largest among all aluminum alloys, and is also an important aluminum alloy material in the aerospace, aviation industry, and electric power industries.
2. The influence of trace elements
Iron and silicon
In the Al-Cu-Mg-Ni-Fe-based forged aluminum alloy, silicon is added as an alloying element in Al-Mg-Si-based forged aluminum and in Al-Si-based welding rods and aluminum-silicon casting alloys. Among the aluminum alloys, silicon and iron are common impurity elements and have a significant effect on the properties of the alloy. They are mainly present as FeCl3 and free silicon. When silicon is larger than iron, a β-FeSiAl3 (or Fe2Si2Al9) phase is formed, and when iron is larger than silicon, α-Fe2SiAl8 (or Fe3Si2Al12) is formed. When the ratio of iron to silicon is not good, it will cause cracks in the casting. If the iron content in the cast aluminum is too high, the casting will be brittle.
Titanium and boron
Titanium is an additive element commonly used in aluminum alloys and is added in the form of an Al-Ti or Al-Ti-B master alloy. Titanium forms an TiAl2 phase with aluminum, which becomes a non-spontaneous core during crystallization and acts to refine the cast structure and weld microstructure. When the Al-Ti alloy is subjected to a package reaction, the critical content of titanium is about 0.15%, and if boron is present, the deceleration is as small as 0.01%.
An additive element commonly found in chromium in Al-Mg-Si systems, Al-Mg-Zn systems, 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, which hinders the nucleation and growth process of recrystallization, has a certain strengthening effect on the alloy, and can improve the toughness of the alloy and reduce the stress corrosion cracking sensitivity. . However, the site increased the quenching sensitivity and made the anodized film yellow.
The addition amount of chromium in the aluminum alloy generally does not exceed 0.35%, and decreases as the transition element in the alloy increases.
Niobium is a surface active element that crystallizes the behavior of the intermetallic phase. Therefore, metamorphism with niobium can improve the plastic workability and final product quality of the alloy. In recent years, the use of sodium has been replaced in Al-Si casting alloys due to the advantages of long decay time, good effect and reproducibility of niobium. Adding 0.015%~0.03% bismuth to the aluminum alloy for extrusion, the β-AlFeSi phase in the ingot becomes a Chinese-shaped α-AlFeSi phase, which reduces the homogenization time of the ingot by 60%~70%, improves the mechanical properties of the material and Plastic workability; improve the surface roughness of the product. For high-silicon (10%~13%) deformed aluminum alloy, adding 0.02%~0.07% antimony element 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, yield strength. Б0.2 is increased from 204 MPa to 210 MPa, and the elongation б5 is increased from 9% to 12%. The addition of yttrium to the hypereutectic Al-Si alloy can reduce the size of the primary silicon particles, improve the plastic working property, and smoothly perform hot rolling and cold rolling.
Zirconium is also a common additive for aluminum alloys. Generally, the amount of addition to the aluminum alloy is 0.1% to 0.3%, and zirconium and aluminum form a ZrAl3 compound, which hinders the recrystallization process and refines the recrystallized grains. Zirconium also refines the cast structure but is less effective than titanium. In the presence of zirconium, the effect of refining grains of titanium and boron is reduced. In the Al-Zn-Mg-Cu alloy, since zirconium has a smaller influence on quenching sensitivity than chromium and manganese, it is preferable to use zirconium instead of chromium and manganese to recrystallize the recrystallized structure.
The rare earth element is added to the aluminum alloy to increase the supercooling of the aluminum alloy during melting, refine the crystal grains, reduce the secondary crystal spacing, reduce the gas and inclusions in the alloy, and tend to spheroidize the inclusion phase. It can also reduce the surface tension of the melt, increase the fluidity, and facilitate the casting into ingots, which has a significant impact on the process performance. It is preferred that the amount of each rare earth added is about 0.1% at%. The addition of mixed rare earth (La-Ce-Pr-Nd or the like) reduces the critical temperature of the formation of the aged G?P region of the Al-0.65% Mg-0.61% Si alloy. Magnesium-containing aluminum alloy can excite the metamorphism of rare earth elements.
3. The influence of impurity elements
Vanadium forms a VAl11 refractory compound in an aluminum alloy, which acts to refine grains during the casting process, but has a smaller effect 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, forms a CaAl4 compound with aluminum, and calcium is a superplastic element of aluminum alloy. The aluminum alloy of about 5% calcium and 5% manganese has superplasticity. Calcium and silicon form CaSi, which is insoluble in aluminum. Since the amount of solid solution of silicon is reduced, the electrical conductivity of industrial pure aluminum can be slightly improved. Calcium can improve the cutting performance of aluminum alloys. CaSi2 does not heat-treat the aluminum alloy. Trace calcium helps to remove hydrogen from the aluminum liquid.
Lead, tin and antimony are low melting point metals. They have a low solid solubility in aluminum, slightly lowering the strength of the alloy, but improving the cutting performance. The swell expands during solidification and is beneficial for feeding. The addition of bismuth to high magnesium alloys prevents sodium brittleness.
Tantalum is mainly used as a modifier in cast aluminum alloys, and deformed aluminum alloys are rarely used. It is only used in the Al-Mg deformed aluminum alloy to prevent sodium brittleness. The lanthanum element is added to some Al-Zn-Mg-Cu alloys to improve the hot and cold pressing process performance.
铍In the deformed aluminum alloy, the structure of the oxide film can be improved, and the burning loss and inclusion during casting can be reduced. Earthworms are toxic elements that can cause allergic poisoning. Therefore, aluminum alloys that come into contact with foods and beverages should not contain barium. The niobium content in the solder material is usually controlled to be 8 μg/ml or less. The aluminum alloy used as the welding base should also control the content of niobium.
Sodium is almost insoluble in aluminum, the maximum solid solubility is less than 0.0025%, and the melting point of sodium is low (97.8 °C). When sodium is present in the alloy, it is adsorbed on the surface or grain boundary of the dendrite during solidification. The upper sodium forms a liquid adsorption layer, and when brittle cracking occurs, a NaAlSi compound is formed, and no free sodium exists, and no "sodium brittleness" is produced. When the magnesium content exceeds 2%, magnesium takes up silicon and precipitates free sodium, resulting in "sodium brittleness". Therefore, high magnesium alloys do not allow the use of sodium salt fluxes. Prevent "sodium brittleness"