(1) Characteristics of 7xxx series aluminum alloys
7XXx series aluminum alloys are aluminum alloys with Zn as the main alloying element and are heat-treatable aluminum alloys. When Mg is added to the alloy, it becomes Al-Zn-Mg alloy. The alloy has good thermal deformation properties and a wide quenching range. Under appropriate heat treatment conditions, it can obtain high strength and good welding properties. It generally has good corrosion resistance and a certain tendency to stress corrosion. It is a high-strength weldable aluminum alloy. Al-Zn-Mg-Cu alloy is developed on the basis of Al-Zn-Mg alloy by adding Cu. Its strength is higher than that of 2X series aluminum alloys. It is generally called ultra-high-strength aluminum alloy. The yield strength of the alloy is close to the tensile strength, the yield strength ratio is high, and the specific strength is also high, but the plasticity and high-temperature strength are low. It is suitable for load-bearing structural parts used at room temperature and below 120°C. The alloy is easy to process and has good corrosion resistance and high toughness. This series of alloys are widely used in the fields of aviation and aerospace, and have become one of the most important structural materials in this field.
(2) Alloying elements and impurity elements and their functions
① Al-Zn-Mg alloy Zn and Mg are the main alloying elements in Al-Zn-Mg alloy, and their content is generally not more than 7.5%.
Zn and Mg: As the content of Zn and Mg in the alloy increases, its tensile strength and heat treatment effect generally increase accordingly. The stress corrosion tendency of the alloy is related to the sum of the content of Zn and Mg. For alloys with high Mg and low Zn or high Zn and low Mg, as long as the sum of the content of Zn and Mg is not more than 7%, the alloy has good stress corrosion resistance. The welding crack tendency of the alloy decreases with the increase of Mg content.
Trace amounts of added elements in Al-Zn-Mg alloys include Mn, Cr, Cu, Zr and Ti, and the main impurities include Fe and Si.
Mn and Cr: Adding Mn and Cr can improve the stress corrosion resistance of the alloy. The Mn content is 0.2%~
At 0.4%, the effect is significant. The effect of adding Cr is greater than adding Mn. If Mn and Cr are added at the same time, the effect of reducing stress corrosion tendency will be better. The appropriate amount of Cr added is 0.1%~0.2%.
Zr: Zr can significantly improve the weldability of A1-Zn-Mg alloys. When 0.2% Zr is added to AlZn5Mg3Cu0.35Cr0.35 alloy, welding cracks are significantly reduced. Zr can also increase the final recrystallization temperature of the alloy. In the AlZn4.5Mg1.8Mn0.6 alloy, when the Zr content is higher than 0.2%, the final recrystallization temperature of the alloy is above 500°C. Therefore, the material still retains its strength after quenching. Deformed tissue. Adding 0.1% to 0.2% Zr to Al-Zn-Mg alloys containing Mn can also improve the stress corrosion resistance of the alloy, but Zr has a lower effect than Cr.
Ti: Adding Ti to the alloy can refine the alloy's grain size in the cast state and improve the alloy's weldability, but its effect is lower than that of Zr. If Ti and Zr are added at the same time, the effect is better. In the AlZn5Mg3Cr0.3Cu0.3 alloy with a Ti content of 0.12%, when the Zr content exceeds 0.15%, the alloy has good weldability and elongation, and can achieve the same effect as when more than 0.2% Zr is added alone. Ti can also increase the alloy's recrystallization temperature.
Cu: Adding a small amount of Cu to Al-Zn-Mg alloys can improve stress corrosion resistance and tensile strength. However, the alloy's weldability is reduced.
Fe: Fe can reduce the alloy's corrosion resistance and mechanical properties, especially for alloys with a high Mn content. Therefore, the Fe content should be as low as possible, and its content should be limited to less than 0.3%.
Si: Si can reduce the strength of the alloy, slightly reduce the bending performance, and increase the tendency of welding cracks. The content of Si in the alloy should be limited to less than 0.3%.
② Al-Zn-Mg-Cu alloy Al-Zn-Mg-Cu alloy is a heat-treatable alloy. The main strengthening elements are Zn and Mg. Cu also has a certain strengthening effect, but its main function is to improve the corrosion resistance of the material.
Zn and Mg: Zn and Mg are the main strengthening elements. When they coexist, they will form η (MgZn2) and T (Al2Mg2Zn3) phases. The η phase and T phase have a large solubility in AI and change dramatically with the rise and fall of temperature. The solubility of MgZn₂ at the eutectic temperature is 28%, which is reduced to 4%~5% at room temperature. It has a strong aging strengthening effect. The increase in Zn and Mg content can greatly improve the strength and hardness, but it will reduce plasticity, stress corrosion resistance and fracture toughness.
Cu: When Zn/Mg is greater than 2.2 and the Cu content is greater than Mg, Cu and other elements can produce a strengthened S (CuMgAlz) phase to increase the strength of the alloy, but in the opposite case, the possibility of the existence of the S phase is very small. Cu can reduce the potential difference between the grain boundary and the intragranular, and can also change the precipitate phase structure and refine the grain boundary precipitate phase, but it has little effect on the width of the grain boundary non-precipitation zone. It can inhibit the tendency of intergranular cracking, thereby improving the stress corrosion resistance of the alloy. However, when the Cu content is greater than 3%, the corrosion resistance of the alloy deteriorates. Cu can increase the supersaturation of the alloy, accelerate the artificial aging process of the alloy between 100 and 200 ° C, expand the stable temperature range of the GP zone, and improve the tensile strength, plasticity and fatigue strength. In the range where the Cu content is not too high, the cyclic strain fatigue resistance and fracture toughness are increased with the increase of the Cu content, and the crack growth rate is reduced in the corrosive medium, but the addition of Cu has a tendency to produce intergranular corrosion and pitting corrosion. The effect of Cu on fracture toughness is related to the Zn/Mg ratio. When the ratio is small, the higher the Cu content, the worse the toughness; when the ratio is large, even if the Cu content is high, the toughness is still very good.
There are also a small amount of trace elements such as Mn, Cr, Zr, V, Ti, B in the alloy. Fe and Si are harmful impurities in the alloy, and their interactions are as follows.
Mn, Cr: Adding a small amount of transition elements such as Mn and Cr has a significant effect on the structure and properties of the alloy. These elements can produce dispersed particles during the homogenization annealing of the ingot, prevent the migration of dislocations and grain boundaries, thereby increasing the recrystallization temperature, effectively preventing the growth of grains, refining grains, and ensuring that the structure remains unrecrystallized or partially recrystallized after hot working and heat treatment, so that the strength is improved while having better stress corrosion resistance. In terms of improving stress corrosion resistance, adding Cr is better than adding Mn.
Zr: Recently, there has been a trend to replace Cr and Mn with Zr. Zr can greatly increase the recrystallization temperature of the alloy. Whether it is hot deformation or cold deformation, non-recrystallized structure can be obtained after heat treatment. Zr can also improve the alloy's hardenability, weldability, fracture toughness, stress corrosion resistance, etc. Zr is a very promising trace additive element in Al-Zn-Mg-Cu alloys.
Ti and B: Ti and B can refine the alloy's grains in the cast state and increase the alloy's recrystallization temperature.
Fe and Si: Fe and Si are unavoidable harmful impurities in 7XxX aluminum alloys, which mainly come from raw materials, as well as tools and equipment used in smelting and casting. These impurities mainly exist in the form of hard and brittle FeAl: and free Si. These impurities can also form coarse compounds such as (FeMn)Als, (FeMn)Si2Als, Al(FeMnCr) with Mn and Cr. FeAl3 has the effect of refining grains, but has a greater impact on corrosion resistance. As the insoluble phase content increases, the volume fraction of the insoluble phase also increases. These insoluble second phases will break and elongate during deformation, resulting in a banded structure, and the particles are arranged in a straight line along the deformation direction. Since the impurity particles are distributed inside the grains or on the grain boundaries, during plastic deformation, pores occur on some particle-matrix boundaries, resulting in microcracks, which become the origin of macro cracks. In addition, it has a great influence on the growth rate of fatigue cracks. It has a certain effect of reducing local plasticity during destruction. The increase in the number of impurities shortens the distance between particles, thereby reducing the plastic deformation fluidity around the crack tip. Because the phase containing Fe and Si is difficult to dissolve at room temperature, it plays a role of notch and easily becomes a crack source, causing the material to break, which has a very adverse effect on the elongation, especially the fracture toughness of the alloy. Therefore, when designing and producing new alloys, the content of Fe and Si is strictly controlled. In addition to using high-purity metal raw materials, some measures are also taken during the melting and casting process to prevent these two elements from mixing into the alloy.