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Influence of Vacuum Heat Treatments on aluminum alloy processing

Vacuum annealing and vacuum quenching are the basic heat treatment forms of aluminum alloys. Vacuum annealing is a softening process. Its purpose is to make the alloy uniform and stable in composition and structure, eliminate work hardening, and restore the plasticity of the alloy. Vacuum quenching is a strengthening heat treatment with the purpose of improving the strength of the alloy. It is mainly used in aluminum alloys that can be strengthened by heat treatment.

Vacuum annealing
According to different production needs, aluminum alloy annealing can be divided into several forms: ingot homogenization annealing, billet annealing, intermediate annealing and finished product annealing.
1. Homogenization annealing of ingot
Under rapid condensation and non-equilibrium crystallization conditions, the ingot must have uneven composition and structure, as well as large internal stress. In order to change this situation and improve the thermal processing technology of the ingot, homogenization annealing is generally required.
In order to promote atomic diffusion, a higher annealing temperature should be selected for homogenization annealing, but it should not exceed the low melting point eutectic melting point of the alloy. Generally, the homogenization annealing temperature is 5~40℃ lower than the melting point, and the annealing time is usually between 12~24h. .
2. Blank annealing
Blank annealing refers to annealing before the first cold deformation during pressure processing. The purpose is to ensure that the blank has a balanced structure and maximum plastic deformation capability. For example, the rolling end temperature of aluminum alloy hot-rolled slabs is 280~330°C. After rapid cooling at room temperature, the work hardening phenomenon cannot be completely eliminated. Especially for aluminum alloys strengthened by vacuum heat treatment, after rapid cooling, the recrystallization process has not ended, the supersaturated solid solution has not yet completely decomposed, and part of the work hardening and quenching effects are still retained. It is difficult to cold roll directly without vacuum annealing, so billet annealing is required. For non-heat treatment-strengthened aluminum alloys, such as LF3, the vacuum annealing temperature is 370~470°C, and the temperature is maintained for 1.5~2.5H and then air-cooled. The annealing temperature of the blank used for cold drawn tube processing should be appropriately higher, and the upper limit temperature can be selected. For aluminum alloys that can be heat-treated and strengthened, such as LY11 and LY12, the billet is annealed at a temperature of 390~450℃, held for 1~3H, and then cooled in the furnace at a speed of no more than 30℃/h to below 270℃ before being air-cooled out of the furnace.
3. Intermediate annealing
Intermediate annealing refers to the annealing between cold deformation processes. Its purpose is to eliminate work hardening and facilitate continued cold working deformation. Generally speaking, after the material has been annealed by the billet, it will be difficult to continue cold working without intermediate annealing after enduring 45~85% cold deformation.
The process system of intermediate annealing is basically the same as that of billet annealing. According to the requirements for the degree of cold deformation, intermediate annealing can be divided into three types: complete annealing (total deformation ε≈60~70%), simple annealing (ε≤50%) and slight annealing (ε≈30~40%). The first two annealing systems are the same as the billet annealing, and the latter is heating at 320~350°C for 1.5~2 hours and then air cooling.
4. Annealing of finished products
Finished product annealing is the final heat treatment that gives the material a certain structure and mechanical properties according to the requirements of the product’s technical conditions.
Finished product annealing can be divided into two types: high-temperature annealing (to produce soft products) and low-temperature annealing (to produce semi-hard products in different states). High temperature annealing should ensure that complete recrystallization structure and good plasticity can be obtained. Under the condition of ensuring that the material obtains good structure and performance, the holding time should not be too long. For aluminum alloys that can be strengthened by vacuum heat treatment, in order to prevent the air-cooling quenching effect, the cooling rate should be strictly controlled.
Low temperature annealing includes internal stress elimination annealing and partial softening annealing, which is mainly used for pure aluminum and non-heat treatment strengthened aluminum alloys. Formulating a low-temperature annealing system is a very complicated task. Not only the annealing temperature and holding time must be considered, but also the effects of impurities, alloying degree, cold deformation amount, intermediate annealing temperature and hot deformation temperature. To formulate a low-temperature annealing system, one must measure the change curve between the annealing temperature and mechanical properties, and then determine the annealing temperature range based on the performance indicators specified by the technical conditions.

Vacuum quenching
Vacuum quenching of aluminum alloys, through high-temperature heating, dissolves as many alloying elements in the form of the second phase in the metal into the solid solution as possible, followed by rapid cooling to suppress the precipitation of the second phase, thereby obtaining a supersaturated Aluminum-based α solid solution prepares the structure for the next step of aging treatment.
The prerequisite for obtaining a supersaturated α solid solution is that the solubility of the second phase in the alloy in aluminum should increase significantly with the increase of temperature. Otherwise, the purpose of solid solution treatment will not be achieved. Most alloying elements in aluminum can form a eutectic phase diagram with this characteristic. Taking Al-Cu alloy as an example, the eutectic temperature is 548°C, and the room temperature solubility of copper in aluminum is less than 0.1%. When heated to 548°C, its solubility increases to 5.6%. Therefore, Al containing copper below 5.6% -Cu alloy, after the heating temperature exceeds its solid solution line, enters the α single-phase region, that is, the second phase CuAl2 is completely dissolved into the matrix, and a single supersaturated α solid solution can be obtained after quenching.
Quenching is the most important and demanding heat treatment operation for aluminum alloys. The key is to select the appropriate quenching heating temperature and ensure a sufficient quenching cooling rate, and to strictly control the furnace temperature to reduce quenching deformation.
The principle of selecting the quenching temperature is to increase the quenching heating temperature as much as possible while ensuring that the aluminum alloy does not over-burn or the grains grow too much, so as to increase the supersaturation of the α solid solution and the strength after aging treatment. Generally, aluminum alloy heating furnaces require furnace temperature control accuracy within ±3°C, and at the same time, the air in the furnace is forced to circulate to ensure the uniformity of the furnace temperature.
Overburning of aluminum alloys is caused by the local melting of low melting point components within the metal, such as binary or multicomponent eutectics. Over-burning not only reduces the mechanical properties, but also has a serious impact on the corrosion resistance of the alloy. Therefore, once overburning occurs in aluminum alloys, it cannot be eliminated, and alloy products should be scrapped. The actual over-burning temperature of aluminum alloys mainly depends on the alloy composition and impurity content, and is also related to the processing state of the alloy. The over-burning temperature of products processed by plastic deformation is higher than that of castings. The greater the amount of deformation processing, the more non-equilibrium low melting point compositions. The easier it is to dissolve into the matrix when heated, so the actual overburning temperature increases.
The cooling rate during quenching of aluminum alloys has a significant impact on the aging strengthening ability and corrosion resistance of the alloy. During the quenching process of LY12 and LC4, it must be ensured that the α solid solution does not decompose, especially in the temperature sensitive zone of 290~420°C, it must be large enough cooling rate. It is usually stipulated that the cooling rate should be above 50℃/s, and for LC4 alloy, it should reach or exceed 170℃/s.
The most commonly used quenching medium for aluminum alloys is water. Production practice shows that the greater the cooling rate during quenching, the greater the residual stress and residual deformation of the quenched material or workpiece. Therefore, for small workpieces with simple shapes, the water temperature can be slightly lower, generally 10~30℃, and should not exceed 40℃. For workpieces with complex shapes and widely varying wall thicknesses, in order to reduce quenching deformation and cracking, the water temperature can sometimes be increased to 80°C. However, it must be pointed out that as the water temperature of the quenching tank increases, generally speaking, the strength and corrosion resistance of the material also decrease accordingly.