Effect Of Impurities In Aluminum On Properties

- Aug 30, 2019-

1. Effect of alloying elements


Copper element


When aluminum-copper alloy is rich in aluminum for some 548, the greater 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 alloying element and has a certain solid solution strengthening effect. In addition, the time-dependent CuAl2 has a significant age strengthening effect. The copper content in aluminum alloy is generally 2.5%~5%, and the strengthening effect is better when the copper content is 4%~6.8%. Therefore, the copper content of some hard aluminum alloys is at this scale.


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


Silicon element


Al-Si alloys are rich in aluminum. At a eutectic temperature of 577, the greater solubility of silicon in solid solution is 1.65%. Although the solubility decreases as the temperature decreases, such alloys are generally not heat treatable. The aluminum-silicon alloy has excellent forging function and corrosion resistance.


If magnesium and silicon participate in aluminum to form an aluminum-magnesium-silicon-based alloy, the strengthening phase is MgSi. The mass ratio of magnesium to silicon is 1.73:1. When the composition of the Al-Mg-Si alloy is planned, the content of magnesium and silicon is provided on the substrate in this proportion. Some Al-Mg-Si alloys, in order to improve the strength, participate in appropriate copper, together with appropriate chromium to offset the impact of copper on the eccentricity of the helium.


Al-Mg2Si Alloy Alloy Equilibrium Phase Diagram Aluminum-rich Some Mg2Si has a large solubility of 1.85% in aluminum and decelerates with decreasing temperature.


Among the deformed aluminum alloys, silicon alone participates in aluminum and is limited to welding materials. Silicon also has a certain strengthening effect in aluminum.


Magnesium


The equilibrium phase diagram of Al-Mg alloy is rich in aluminum. Although the solubility curve indicates that the solubility of magnesium in aluminum is greatly reduced with the decrease of temperature, in some industrial 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 they have excellent weldability, corrosion resistance, and moderate strength.


The strengthening of magnesium to aluminum is remarkable. For every 1% increase in magnesium, the tensile strength is about 34 MPa. If you participate in manganese below 1%, you can make up for the strengthening effect. Therefore, after adding manganese, the magnesium content can be lowered, and the thermal cracking tendency can be lowered together, and other manganese can uniformly precipitate the Mg5Al8 compound to improve the corrosion resistance and the welding function.


Manganese element


The equilibrium phase diagram of the Al-Mn alloy system is somewhat at a eutectic temperature of 658, and the greater solubility of manganese in the solid solution is 1.82%. The strength of the alloy increases with increasing solubility. When the manganese content is 0.8%, the elongation reaches a large value. The Al-Mn alloy is a right-time hardening alloy, that is, it cannot be heat-treated.


Manganese can hinder the recrystallization process of aluminum alloys, improve the recrystallization temperature, and refine the recrystallized grains remarkably. The refinement of the recrystallized grains is primarily due to the effect of the diffusion of the MnAl6 compound on the growth of recrystallized grains. Another effect of MnAl6 is to dissolve the impurity iron to form (Fe, Mn)Al6, reducing the harmful effects of iron.


Manganese is an important element of aluminum alloys, and it can participate in the formation of Al-Mn binary alloys alone, and more often participates in other alloying elements, so that most of the aluminum alloys are rich in manganese.


Zinc element


The equilibrium phase diagram of the Al-Zn alloy is rich in aluminum. At some 275, the solubility of zinc in aluminum is 31.6%, while at 125, the solubility decreases to 5.6%.


Zinc alone participates in aluminum, and the progress of the strength of aluminum alloy under deformation conditions is very limited, and there is a tendency of stress corrosion cracking, which constrains its application.


Participating in zinc and magnesium together in aluminum constitutes a strengthening phase of 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 the 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 proportion of zinc and magnesium is manipulated at 2.7, the stress corrosion cracking resistance is large.


For example, the Al-Zn-Mg-Cu alloy is formed on the basis of Al-Zn-Mg, and the base strengthening effect is larger among all aluminum alloys. It is also an important aluminum alloy material in the aerospace, aviation industry and power industry. .


2. The influence of trace elements


Iron and silicon


In the Al-Cu-Mg-Ni-Fe system forging aluminum alloy, silicon is added as an alloying element in Al-Mg-Si-based forged aluminum and in Al-Si-based welding rod and aluminum-silicon forged alloy. Among the base metals, silicon and iron are common impurity elements and have a significant effect on the function of the alloy. They are primarily present in FeCl3 and free silicon. When silicon is larger than iron, it constitutes a β-FeSiAl3 (or Fe2Si2Al9) phase, and when iron is larger than silicon, it constitutes α-Fe2SiAl8 (or Fe3Si2Al12). When the proportion of iron and 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 attended by Al-Ti or Al-Ti-B center alloys. Titanium and aluminum form the TiAl2 phase, which becomes the non-conscious center of crystallization, which serves to refine the forging arrangement and the weld arrangement. 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%.


chromium


An additive element common to 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 prevents the nucleation and growth process of recrystallization, has a certain strengthening effect on the alloy, and improves the alloy resistance and the stress corrosion cracking sensitivity. . However, the site increased the quenching sensitivity and made the anodized film yellow.


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


strontium


Rhodium is an externally active element that crystallizes the action of the intermetallic phase. Therefore, the enthalpy treatment with bismuth element can improve the plastic workability of the alloy and the final product quality. In recent years, the use of sodium has been replaced by Al-Si casting alloys because of the effective time, effect and reproducibility of the enthalpy. In the aluminum alloy for kneading, 0.015% to 0.03% of yttrium is used to make the β-AlFeSi phase in the ingot into a Chinese-shaped α-AlFeSi phase, which reduces the homogenization time of the ingot by 60% to 70%, and improves the mechanical function and plasticity of the data. Processability; improve the appearance of finished products. For high-silicon (10%~13%) deformed aluminum alloys, 0.02%~0.07% bismuth element can reduce the primary crystal to the lower limit, and the mechanical function is also improved. The tensile strength σb is improved from 233MPa to 236MPa. The yield strength σ0.2 was increased from 204 MPa to 210 MPa, and the elongation б5 was increased from 9% to 12%. Participation in ruthenium in the hypereutectic Al-Si alloy can reduce the size of the primary silicon particles, improve the plastic working function, and smoothly perform hot rolling and cold rolling.


Zirconium


Zirconium is also a commonly used additive for aluminum alloys. Generally, the amount of participation in the aluminum alloy is 0.1% to 0.3%, and zirconium and aluminum constitute a ZrAl3 compound, which prevents the recrystallization process and refines the recrystallized grains. Zirconium also refines the forging arrangement 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 the quenching sensitivity than chromium and manganese, it is preferable to use zirconium instead of the chromium and manganese recrystallization recrystallization arrangement.


Impurity element


The rare earth element participates in the aluminum alloy, so that when the aluminum alloy is cast, the composition is supercooled, the crystal grains are refined, the secondary crystal distance is reduced, the gas and the doping in the alloy are reduced, and the doped phase tends to be spheroidized. 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 function. It is preferred that the amount of various rare earths is about 0.1% at%. The increase of the mixed rare earth (La-Ce-Pr-Nd and the like) causes the critical temperature of the aging G?P region of the Al-0.65% Mg-0.61% Si alloy to decrease. Magnesium-containing aluminum alloy can stimulate the enthalpy effect of rare earth elements.


3. The influence of impurity elements


Vanadium forms a VAl11 refractory compound in an aluminum alloy, which refines the grain effect during the casting process, but has less effect than titanium and zirconium. Vanadium also has the effect of refining the recrystallization arrangement and improving the recrystallization temperature.


Calcium has a very low solid solubility in aluminum alloys, and forms a CaAl4 compound with aluminum. Calcium is a superplastic element of aluminum alloy, and aluminum alloy of about 5% calcium and 5% manganese has superplasticity. Calcium and silicon constitute CaSi, which is insoluble in aluminum. Because the amount of solid solution of silicon is reduced, the conductive function of industrial pure aluminum can be slightly improved. Calcium improves the cutting function of aluminum alloys. CaSi2 does not heat-treat the aluminum alloy. Traces of calcium help to remove hydrogen from the aluminum solution.


The lead, tin and antimony elements are low melting point metals. They have little solid solubility in aluminum and slightly lower the strength of the alloy, but can improve the cutting function. The swells in the process of coagulation, which is beneficial to the feeding. Participation in high magnesium alloys can avoid sodium brittleness.


锑 is primarily used as a mutating agent in forged aluminum alloys, and deformed aluminum alloys are rarely used. It is only used in Al-Mg deformed aluminum alloys to avoid sodium brittleness. Niobium elements participate in certain Al-Zn-Mg-Cu alloys to improve the hot pressing and cold pressing process functions.


铍In the deformed aluminum alloy, the structure of the oxide film can be improved, and the burning and noisy during the casting can be reduced. Earthworms are toxic elements that can cause allergic poisoning. Thus, aluminum alloys that touch food and beverages are not rich in strontium. The bismuth content in the welding guess is generally controlled below 8 μg/ml. The aluminum alloy used as the 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 is adsorbed on the dendrite surface or grain boundary during the coagulation process. The sodium on the boundary constitutes a liquid adsorption layer, and when brittle cracking occurs, a NaAlSi compound is formed, and no free sodium exists, and "sodium brittleness" does not occur. When the magnesium content exceeds 2%, the magnesium is extracted from silicon, and the free sodium is separated to cause "sodium brittleness". Therefore, high-magnesium aluminum alloys are not allowed to use sodium salt flux. The method of avoiding "sodium brittleness" is a chlorination method, in which sodium is formed into NaCl and discharged into the slag, and the ruthenium is added to form Na2Bi into the metal matrix; the addition of Na3Sb or the addition of rare earth can also have the same effect.