Al-Zn-Mg alloy
Zinc and magnesium in Al-Zn-Mg alloy are the main alloying elements, and their mass fraction is generally not more than 7.5%. As the content of zinc and magnesium increases, the tensile strength and heat treatment effect of the alloy generally increase. The stress corrosion tendency of alloy is related to the sum of zinc and magnesium content. For high-magnesium low-zinc or high-zinc low-magnesium alloys, as long as the sum of zinc and magnesium mass fractions is not more than 7%, the alloy has good stress corrosion resistance. The weld cracking tendency of the alloy decreases with the increase of the magnesium content.
The trace addition elements in Al-Zn-Mg series alloys are manganese, chromium, copper, zirconium and titanium, and the main impurities are iron and silicon. The specific functions are as follows:
(1) Manganese and chromium: Adding manganese and chromium can improve the stress corrosion resistance of the alloy. When ω(Mn)=0.2%~0.4%, the effect is significant. The effect of adding chromium is greater than adding manganese. If manganese and chromium are added at the same time, the effect of reducing stress corrosion tendency is better, and ω(Cr)=0.1%~0.2% is appropriate.
(2) Zirconium: Zirconium can significantly improve the weldability of Al-Zn-Mg alloys. When 0.2% Zr is added to AlZn5Mg3Cu0.35Cr0.35 alloy, welding cracks are significantly reduced. Zirconium can also increase the final recrystallization temperature of the alloy. In AlZn4.5Mgl.8Mn0.6 alloy, when ω(Zr)>0.2%, the final recrystallization temperature of the alloy is above 500℃. Therefore, the material remains after quenching. Deformed tissue. The addition of ω(Zr)=0.1%~0.2% to the Al-Zn-Mg alloy containing manganese can also improve the stress corrosion resistance of the alloy, but the effect of zirconium is lower than that of chromium.
(3) Titanium: The addition of titanium to the alloy can refine the crystal grains of the alloy in the as-cast state and improve the weldability of the alloy, but its effect is lower than that of zirconium. If titanium and zirconium are added at the same time, the effect will be better. In AlZn5Mg3Cr0.3Cu0.3 alloy with ω(Ti)=0.12%, when ω(Zr)>0.15%, the alloy has better weldability and elongation, which can be obtained and added separately ω(Zr)>0.2 The same effect as %. Titanium can also increase the recrystallization temperature of the alloy.
(4) Copper: Adding a small amount of copper to the Al-Zn-Mg series alloy can improve the stress corrosion resistance and tensile strength, but the weldability of the alloy is reduced.
(5) Iron: Iron can reduce the corrosion resistance and mechanical properties of alloys, especially for alloys with higher manganese content. Therefore, the iron content should be as low as possible and should limit ω(Fe)<0.3%.
(6) Silicon: Silicon can reduce the strength of the alloy, reduce the bending performance slightly, and increase the tendency of welding cracks. Therefore, ω (Si) should be limited to <0.3%.
Al-Zn-Mg-Cu alloy
Al-Zn-Mg-Cu alloy is a heat-treatable alloy that can be strengthened. The main strengthening elements are zinc and magnesium. Copper also has a certain strengthening effect, but its main function is to improve the corrosion resistance of the material.
(1) Zinc and magnesium: Zinc and magnesium are the main strengthening elements. When they coexist, η (MgZn 2) and T (Al 2 Mg 2 Zn 3) phases are formed. The solubility of η phase and T phase in aluminum is very large, and changes drastically with the rise and fall of temperature. The solubility of MgZn 2 at the eutectic temperature reaches 28%, which is reduced to 4%~5% at room temperature, which has a strong aging strengthening effect. , The increase of zinc and magnesium content can greatly increase the strength and hardness, but it will reduce the plasticity, stress corrosion resistance and fracture toughness.
(2) Copper: When ω(Zn):ω(Mg)>2.2 and the copper content is greater than the magnesium content, copper and other elements can produce a strengthening phase S(CuMgAl 2) to increase the strength of the alloy, but on the contrary In the case of S phase, the possibility of existence is very small. Copper can reduce the potential difference between the grain boundary and the intragranular, and can also change the structure of the precipitated phase and refine the grain boundary precipitated phase, but it has little effect on the width of the PFZ; it can inhibit the tendency of intergranular cracking, thereby improving the alloy's stress corrosion resistance performance. However, when ω(CU)>3%, the corrosion resistance of the alloy deteriorates instead. Copper can increase the degree of supersaturation of the alloy, accelerate the artificial aging process of the alloy at 100~200℃, expand the stable temperature range of the GP zone, and improve the tensile strength, plasticity and fatigue strength. In addition, FSLin and others in the United States studied the effect of copper content on the fatigue strength of 7000 series aluminum, and found that the copper content in a range that is not too high increases the fatigue resistance and fracture toughness of the cycle strain with the increase of the copper content, and the corrosion The medium reduces the crack growth rate, but the addition of copper has the tendency to produce intergranular corrosion and pitting corrosion. According to other data, the effect of copper on fracture toughness is related to the value of ω(Zn):ω(Mg). When the ratio is small, the higher the copper content, the worse the toughness; when the ratio is large, the toughness is still higher even if the copper content is higher. very good.
There are also a small amount of trace elements such as manganese, chromium, zirconium, vanadium, titanium, and boron in the alloy. Iron and silicon are harmful impurities in the alloy. Their interaction is as follows:
(1) Manganese and chromium: adding a small amount of transition group elements manganese, chromium, etc. has a significant effect on the structure and properties of the alloy. These elements can produce dispersed particles during homogenization and annealing of the ingot to prevent the migration of dislocations and grain boundaries, thereby increasing the recrystallization temperature and effectively preventing the growth of grains; it can refine the grains and ensure that the structure is hot After processing and heat treatment, the non-recrystallized or partially recrystallized state is maintained, which improves the strength and has better stress corrosion resistance. In improving the stress corrosion resistance, adding chromium has a better effect than adding manganese. The stress corrosion cracking life of adding ω(Cr)=0.45% is dozens of hundreds of times longer than adding the same amount of manganese.
(2) Zirconium: There is a recent trend of replacing chromium and manganese with zirconium. Zirconium can greatly increase the recrystallization temperature of the alloy. Whether it is hot or cold deformation, unrecrystallized structure can be obtained after heat treatment, and zirconium can also increase The alloy's hardenability, weldability, fracture toughness, stress corrosion resistance, etc., are very promising trace additives in Al-Zn-Mg-Cu series alloys.
(3) Titanium and boron: Titanium and boron can refine the crystal grains of the alloy in the as-cast state and increase the recrystallization temperature of the alloy.
(4) Iron and silicon: Iron and silicon are harmful impurities inevitably present in 7 series aluminum alloys, which mainly come from raw materials and tools and equipment used in smelting and casting. These impurities mainly exist in the form of hard and brittle FeAl 3 and free silicon. These impurities also form (FeMn)Al 6, (FeMn)Si 2 Al 5, Al(FeMnCr) and other coarse compounds with manganese and chromium. FeAl 3 has The role of grain refinement, but it has a greater impact on corrosion resistance. With the increase of the insoluble phase content, the volume fraction of the insoluble phase also increases. These insoluble phases will be broken and elongated when deformed, and a band-like structure will appear. , The particles are arranged in a straight line along the deformation direction and are composed of short, unconnected strips. Because the impurity particles are distributed inside the grains or on the grain boundaries, during plastic deformation, pores will occur on part of the grain-matrix boundary, resulting in micro cracks, which become the birthplace of macro cracks. At the same time, it will also promote the premature development of cracks. In addition, it has a greater impact on the growth rate of fatigue cracks. It has a certain effect of reducing local plasticity during failure. This may be due to the increase in the number of impurities that shortens the distance between particles, thereby reducing the flow of plastic deformation around the crack. Sexually related. Because the phases containing iron and silicon are difficult to dissolve at room temperature, they play the role of notches and are likely to become crack sources to cause the material to fracture, which has a very negative effect on the elongation, especially the fracture toughness of the alloy. Therefore, in the design and production of the new alloy, the content of iron and silicon is strictly controlled. In addition to the use of high-purity metal raw materials, some measures have also been taken during the melting and casting process to avoid the mixing of the two elements into the alloy.