Annealing and quenching aging are the basic heat treatment forms of aluminum alloy. Annealing is a softening process. Its purpose is to make the alloy tend to be uniform and stable in composition and structure, eliminate work hardening, and restore the plasticity of the alloy. Quenching aging is a strengthening heat treatment, the purpose is to improve the strength of the alloy, mainly used in heat-treatable aluminum alloy.

一.annealing
According to the different production demand, aluminum alloy annealing ingot homogenization annealing, blank annealing, intermediate annealing and finished annealing several forms.
First, ingot homogenization annealing
In the condition of rapid condensation and non-equilibrium crystallization, ingot must have non-uniformity in composition and structure, and there is also a great internal stress. In order to change this situation and improve the thermal processing property of the ingot, it is generally necessary to carry out homogenization annealing. In order to promote the diffusion of atoms, homogenization annealing should choose a higher annealing temperature, but not more than the alloy low melting point eutectic melting point, the general homogenization annealing temperature is lower than the melting point 5~40℃, the annealing time is more between 12~24h.

二. blank annealing
Blank annealing refers to the annealing before the first cold deformation in the process of pressure machining. The aim is to give the billet a balanced structure and maximum plastic deformation capacity. For example, the final rolling temperature of aluminum alloy hot rolled slab is 280~330℃, and the work hardening phenomenon can not be completely eliminated after rapid cooling at room temperature. Especially for heat-treated aluminum alloy, after fast cooling, the recrystallization process does not end, and the supersaturated solid solution is not completely decomposed, and some work hardening and quenching effects are still retained. It is difficult to cold roll without annealing, so billet annealing is required. For non-heat-treated strengthened aluminum alloys, such as LF3, the annealing temperature is 370~470℃, the air cooling after holding 1.5~2.5H, the billet and annealing temperature for cold-drawn tube processing should be appropriately high, and the upper limit temperature can be selected. For aluminum alloys that can be strengthened by heat treatment, such as LY11 and LY12, the billet annealing temperature is 390~450 ° C, the heat preservation is 1~3H, and then the furnace is cooled to 270 ° C below the speed of no more than 30 ° C /h.

三.intermediate annealing
Intermediate annealing refers to the annealing between cold deformation processes, the purpose of which is to eliminate work hardening in order to facilitate the continuation of cold working deformation. In general, after the billet annealing of the material, after enduring 45 to 85% cold deformation, if not intermediate annealing and continue cold working will be difficult. The process of intermediate annealing is basically the same as that of blank annealing. According to the requirements of the degree of cold deformation, the intermediate annealing can be divided into 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 blank annealing, and the latter one is air cooling after heating at 320~350℃ for 1.5~2h.

四.the finished product annealing
Annealing of finished product is the final heat treatment that gives the material certain microstructure and mechanical properties according to the requirements of the technical conditions of the product
Finished product annealing can be divided into high temperature annealing (production of soft products) and low temperature annealing (production of semi-hard products in different states) two kinds of high temperature annealing should ensure that complete recrystallization structure and good plasticity. Under the condition of ensuring good organization and performance of the material, the holding time should not be too long. For the aluminum alloy that can be strengthened by heat treatment, in order to prevent the air cooling quenching effect, the cooling rate should be strictly controlled.
Low temperature annealing includes two kinds of internal stress relief annealing and partial softening annealing, which are mainly used for pure aluminum and non-heat treated aluminum alloy strengthening. The formulation of low temperature annealing system is a very complicated work, which should not only consider the annealing temperature and holding time, but also consider the influence of impurities, alloying degree, cold deformation amount, intermediate annealing temperature and hot deformation temperature. The development of low temperature annealing system must be to measure the change curve between the annealing temperature and mechanical properties, and then determine the annealing temperature range according to the performance indicators specified in the technical conditions.

The second quenching
Quenching of aluminum alloy is also known as solid solution treatment, that is, through high temperature heating, so that the alloy elements in the form of the second phase of the metal as much as possible into the solid solution, and then rapid cooling to inhibit the precipitation of the second phase, so as to obtain a susaturated aluminum-based α solid solution, for the next step of aging treatment to prepare the organization.
The premise of obtaining supersaturated alpha solid solution is that the solubility of the second phase of the alloy in aluminum should be significantly increased with the increase of temperature, otherwise, the purpose of solid solution treatment can not be reached. Most of the alloying elements in aluminum can form a eutectic phase diagram with this characteristic. Taking AI-Cu alloy as an example, the eutectic temperature is 548℃, the solubility of copper in aluminum at room temperature is less than 0.1%, heated to 548℃, its solubility is increased to 5.6%, therefore, the A-Cu alloy containing copper in 5.6%, the heating temperature exceeds its solid solution line, into the α single-phase zone, that is, the second phase CuAI2 all dissolve into the matrix, after quenching A single supersaturated alpha solid solution can be obtained.

Quenching is the most important and demanding heat treatment operation of aluminum alloy, among which the key is to choose the appropriate quenching heating temperature and ensure sufficient quenching cooling speed, and can strictly control the furnace temperature and reduce quenching deformation.
The principle of selecting the quenching temperature is to increase the quenching heating temperature as much as possible in order to increase the supersaturation of the alpha solid solution and the strength after aging treatment in order to ensure that the aluminum alloy does not overburn or the grain is over-grown. Generally, aluminum alloy heating furnaces require the temperature control accuracy to be within +3 ° C, and the air in the furnace is forced to circulate to ensure the uniformity of the furnace temperature.

The overburning of aluminum alloys is caused by the local melting of low melting point components inside the metal, such as binary or multicomponent eutectic. Overburning not only reduces the mechanical properties, but also has a serious effect on the corrosion resistance of the alloy. Therefore, once the aluminum alloy is burned, it will not be eliminated, and the alloy products should be scrapped. The actual burning temperature of aluminum alloy is mainly determined by the alloy composition, impurity content, and the processing state of the alloy is also related to the burning temperature of the products processed by plastic deformation is higher than that of the casting, the larger the deformation processing amount, the more easily the non-equilibrium low-melting point components dissolve into the matrix when heated, so the actual burning temperature rises.

The cooling rate of aluminum alloy quenching has a significant impact on the aging strengthening ability and corrosion resistance of the alloy,LY12 and LC4 quenching process must ensure that the α solid solution does not decompose, especially in the temperature sensitive area of 290~420℃, to have a large enough cooling rate. Generally, the cooling rate should be above 50 ° C/s, and LC4 alloys should reach or exceed 170 ° C/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 to 30 ° C, and should not exceed 40 ° C. For the workpiece with complex shape and large difference in wall thickness, the water temperature can sometimes be increased to 80℃ in order to reduce quenching deformation and cracking. However, it must be pointed out that with the increase of the water temperature of the quenching tank, in general, the strength and corrosion resistance of the material are correspondingly reduced.