The most typical super hard aluminum 7075 aluminum block

7075 aluminum block is the most typical aviation super-hard aluminum. It is not only an "old" AL-Zn-Mg-Cu alloy, but also a young alloy with "energy". It is said to be "ancient" because it is the first one. The stereotyped super-hard aluminum was finalized in 1944. At that time, it was used to manufacture aircraft in the United States, Japan, and the former Soviet Union. It is only 74 "years old" and is still in the "middle-aged" stage. It is "young" because it is now It is also the main material for key structures of aerospace vehicles. It seems that only when composite materials can be industrialized in large quantities and the price is reduced to be competitive with super duralumin, super duralumin will withdraw from the stage of history and give way to composite materials.

After more than 70 years of development, 7075 aluminum block has formed a fairly large family. Now the commonly used alloys are 7075/7175/7475 alloy. 7275 alloy has become a very useful alloy due to its unsatisfactory performance, and 7375 alloy has been completely eliminated. , never use it again.

From the data in Table 1, it can be seen that the 7175 alloy is cleaner than the 7075 aluminum block, while the 7475 alloy is cleaner, that is, their impurity content is lower than the other one. The 7075 aluminum block is 0.30% smaller, so the 7475 alloy has higher toughness and better overall performance.

7075 aluminum block is a high-strength deformed aluminum alloy of aluminum-zinc-magnesium-copper series that can be strengthened by heat treatment. It is a veteran of super-hard aluminum, but it is not a big guy, because there are still stronger ones, and it can be used to process all Semi-finished products, used to manufacture various structural parts, especially aerospace structural parts, are the most widely used high-strength aluminum alloys today, ranging from C919 large aircraft front fuselage long stringers, passenger observation window frame forgings, medium and high-strength aluminum alloys. Fuselage sheet bending and girder, keel beam web, floor turning beam, middle and rear fuselage girder, passenger observation window frame forgings, cargo door frame, front pressure frame edge head, cabin door frame, etc.; small to mobile phone shell . It is available in various heat treatment states: T6, T73, T76, T77. The T6 state material has the highest mechanical strength properties, but the fracture toughness is low, sensitive to stress corrosion, and its toughness decreases with the decrease of temperature. Therefore, the T6 state material is not suitable for manufacturing low-temperature structural parts and workpieces; T73 material has the lowest strength , but has quite high fracture toughness, good stress crack resistance and anti-exfoliation corrosion resistance; T76 material has higher strength than T73 material and higher stress corrosion cracking resistance than T6 material. The static strength properties of the 7075 aluminum block are higher than those of the 2024 and 2124 alloys, and the fatigue properties are comparable. O-state and W-state materials have good room temperature formability.

7075 aluminum block can be resistance welded, and the long-term working temperature of the workpiece is ≤125℃. For incomplete annealing of materials or workpieces, the specification is 290°C~320°C, and after holding for 2h~4h, the furnace is air-cooled; the specification for complete annealing is (390~430)°C/(0.5~1.5)h, with a cooling rate not greater than 30°C/h Cool to ≤200℃, then air-cool.

The solution treatment temperature of 7075 aluminum block sheet is 460℃~490℃, in which the treatment temperature of aluminum sheet should be close to the lower limit, and the number of repeated treatments should not be more than 2 times, so as to prevent alloy elements from diffusing through the aluminum cladding layer and reducing the corrosion resistance of the material Properties; the heating temperature of the extruded material is 460℃~471℃. Quenching in room temperature water or warm water or other suitable cooling medium after heat preservation, the transfer time should be less than 15s.

Thick plates and extruded profiles will generate a lot of internal stress after quenching, and they must be stretched before aging treatment to reduce them to the allowable range. Therefore, this kind of stretching is called pre-stretching. The amount is 1.5%~3%, otherwise serious deformation will occur.

7075 aluminum block is usually smelted in gas furnace or electric furnace, cast by vertical semi-continuous method, and cast into flat ingot or extruded ingot suitable for rolling. The melting temperature of the alloy is 477℃~638℃, the melting temperature is 700℃~745℃, and the casting temperature is 690℃~710℃. Casting small-sized workpieces can also be smelted with a crucible furnace.

The thermal conductivity of the material is related to its state. At 25°C, the thermal conductivity of the T6 material is 124W/(m·°C), while that of the T73 material is higher, which is 156W/(m·°C). The specific heat capacity (at 25°C) of the 7075-T6 alloy is 796 J/(kg·°C). The density of 7075 aluminum block is 2.8g/cm3, the resistivity at room temperature is 57.4nΩ·m, and the electrical conductivity is related to the state of the material, T6 material is 17.7MS/m~20.6MS/m, T73, T76 material is 22.0 MS/m ~ 24.4 MS/m.

Like other aluminum alloys, 7075 aluminum block has strong oxidation resistance, with the exception of stress corrosion, the resistance to general corrosion is comparable to 2024 alloy. The 7075-T6 alloy is extremely sensitive to stress corrosion cracking, especially in the transverse direction (ST), while this property of T73 and T76 materials is greatly improved. T73 material has the highest resistance to stress corrosion cracking, while T76 material has the highest resistance to stress corrosion cracking. The material not only has high strength, but also has high resistance to stress corrosion cracking and exfoliation corrosion.

The structure of 7075 aluminum block is composed of α-Al solid solution and the second phase, which can be divided into three categories according to the formation mechanism of the second phase:

The primary intermetallic compounds, Al7FeCr, Al3Fe, Mg2Si, etc., are formed when the ingot is solidified. They are large in size and appear as blocks under the optical microscope. They are broken during the pressure processing and are often arranged in strings, and their size is 0.5μm~ 10 μm, insoluble in solid solution during the subsequent heating process, and will not change during heat treatment, reducing the fracture toughness of the material.

Chromium-containing disperse particles Al12Mg2Cr and Al18Mg3Cr2 (E phase) are precipitated from solid solution during alloy homogenization, heating before hot working and solution heat treatment, and they have obvious effects on the recrystallization process and grain growth of the alloy. hindering effect.

Aging strengthens particles. They are integrated into the solid solution during the solution treatment of the alloy and precipitate out of the solid solution. Their shape and size are determined by the aging temperature and have a significant impact on the properties of the alloy. The strengthening particles of T6 material are mainly GP regions with a size of ≤1μm; the strengthening particles of T76 material are basically the transition phase of 5nm~6nm; the main strengthening phase of T73 material is the transition phase of 8nm~12nm and the η phase of 20nm~80nm Particles.

The formability of 7075-O material is not as good as that of 2024-O alloy, but it has good formability at 180℃~370℃, and the holding time in this temperature range should be more than 2h. The formability of 7075-W alloy is comparable to that of 2024-W alloy. At room temperature, the parking time after quenching <30min has the best formability, and this good formability can be maintained for 4h. The room temperature formability of 7075-T6 alloy is poor, but its formability can be significantly improved if heated to 150℃~220℃.

7075 aluminum block is not suitable for fusion welding, and the adaptability of resistance welding is not as good as that of 2024 alloy. The heat treatment process of 7075 aluminum block parts is carried out according to GJB1694, and the solution treatment holding time should be ≥ 25min, otherwise satisfactory strength will not be obtained, although the parking time between solid solution and artificial aging affects the mechanical properties of various products of the alloy and various heat treatment states The effect is not large, but it is better to carry out artificial aging at any time after 4h or 48h after solution treatment.

The surface treatment process of 7075 aluminum block is the same as that of other aluminum alloys, and the alloy has good cutting and grinding performance under heat treatment.