Basic Types of Heat Treatment of Aluminum Alloys

Annealing and quenching and aging are the basic heat treatment types of aluminum alloys. Annealing is a softening treatment, the purpose of which is to make the alloy uniform and stable in composition and structure, eliminate work hardening, and restore the plasticity of the alloy. Quenching and aging is a strengthening heat treatment, the purpose of which is to improve the strength of the alloy, and is mainly used for aluminum alloys that can be strengthened by heat treatment.

1 Annealing

According to different production requirements, aluminum alloy annealing is divided into several forms: ingot homogenization annealing, billet annealing, intermediate annealing and finished product annealing.

1.1 Ingot homogenization annealing

Under the conditions of rapid condensation and non-equilibrium crystallization, the ingot must have uneven composition and structure, and also have great internal stress. In order to change this situation and improve the hot working processability of the ingot, homogenization annealing is generally required.

In order to promote atomic diffusion, a higher temperature should be selected for homogenization annealing, but it must 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 mostly between 12~24h.

1.2 Billet annealing

Billet annealing refers to the annealing before the first cold deformation during the pressure processing. The purpose is to make the billet obtain a balanced structure and have the maximum plastic deformation capacity. For example, the rolling end temperature of the hot-rolled aluminum alloy slab is 280~330℃. After rapid cooling at room temperature, the work hardening phenomenon cannot be completely eliminated. In particular, for heat-treated strengthened aluminum alloys, after rapid cooling, the recrystallization process has not ended, and the supersaturated solid solution has not been completely decomposed, and a part of the work hardening and quenching effect is still retained. It is difficult to cold roll directly without annealing, so billet annealing is required. For non-heat-treated strengthened aluminum alloys, such as LF3, the annealing temperature is 370~470℃, and air cooling is performed after keeping warm for 1.5~2.5h. The billet and annealing temperature used for cold-drawn tube processing should be appropriately higher, 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℃, kept at this temperature for 1~3h, then cooled in the furnace to below 270℃ at a rate of no more than 30℃/h and then air-cooled out of the furnace.

1.3 Intermediate annealing

Intermediate annealing refers to annealing between cold deformation processes, the purpose of which is to eliminate work hardening to facilitate continued cold deformation. Generally speaking, after the material has been annealed, it will be difficult to continue cold working without intermediate annealing after undergoing 45~85% cold deformation.

The process system of intermediate annealing is basically the same as that of billet annealing. According to the requirements of cold deformation degree, 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 billet annealing, and the latter is heated at 320~350℃ for 1.5~2h and then air cooled.

1.4. Finished product annealing

Finished product annealing is the final heat treatment that gives the material certain organizational and mechanical properties according to the requirements of product technical conditions.

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). High temperature annealing should ensure that a 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 heat treatment, in order to prevent the air cooling quenching effect, the cooling rate should be strictly controlled.

Low temperature annealing includes stress relief annealing and partial softening annealing, which are mainly used for pure aluminum and non-heat treatment strengthened aluminum alloys. Formulating a low temperature annealing system is a very complicated task, which not only needs to consider the annealing temperature and holding time, but also needs to consider the influence of impurities, alloying degree, cold deformation, intermediate annealing temperature and hot deformation temperature. To formulate a low temperature annealing system, it is necessary to measure the change curve between annealing temperature and mechanical properties, and then determine the annealing temperature range according to the performance indicators specified in the technical conditions.

2 Quenching

The quenching of aluminum alloy is also called solution treatment, which is to dissolve as much alloying elements in the metal as a second phase into the solid solution as possible through high-temperature heating, followed by rapid cooling to inhibit the precipitation of the second phase, thereby obtaining a supersaturated aluminum-based α solid solution, which is well prepared for the next aging treatment.

The premise of 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 cannot 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℃, and the room temperature solubility of copper in aluminum is less than 0.1%. When heated to 548℃, its solubility increases to 5.6%. Therefore, Al-Cu alloys containing less than 5.6% copper enter the α single phase region after the heating temperature exceeds its solvus line, that is, the second phase CuAl2 is completely dissolved in the matrix, and a single supersaturated α solid solution can be obtained after quenching.

Quenching is the most important and most demanding heat treatment operation for aluminum alloys. The key is to select the appropriate quenching heating temperature and ensure sufficient quenching cooling rate, and to 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 while ensuring that the aluminum alloy does not overburn or the grains grow excessively, so as to increase the supersaturation of the α solid solution and the strength after aging treatment. Generally, the aluminum alloy heating furnace requires the furnace temperature control accuracy to be within ±3℃, and the air in the furnace is forced to circulate to ensure the uniformity of the furnace temperature.

Overburning of aluminum alloy is caused by the partial melting of low-melting-point components inside the metal, such as binary or multi-element eutectics. Overburning not only causes the reduction of mechanical properties, but also has a serious impact on the corrosion resistance of the alloy. Therefore, once an aluminum alloy is overburned, it cannot be eliminated and the alloy product should be scrapped. The actual overburning temperature of aluminum alloy is mainly determined by the alloy composition and impurity content, and is also related to the alloy processing state. The overburning temperature of products that have undergone plastic deformation processing is higher than that of castings. The greater the deformation processing, the easier it is for non-equilibrium low-melting-point components to dissolve into the matrix when heated, so the actual overburning temperature increases.

The cooling rate during quenching of aluminum alloy has a significant impact on the aging strengthening ability and corrosion resistance of the alloy. During the quenching process of LY12 and LC4, it is necessary to ensure that the α solid solution does not decompose, especially in the temperature sensitive area of 290~420℃, and a sufficiently large cooling rate is required. 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 large differences in wall thickness, in order to reduce quenching deformation and cracking, the water temperature can sometimes be increased to 80℃. However, it must be pointed out that as the water temperature of the quenching tank increases, the strength and corrosion resistance of the material also decrease accordingly.

3. Aging

3.1 Organizational transformation and performance changes during aging

The supersaturated α solid solution obtained by quenching is an unstable structure. When heated, it will decompose and transform into an equilibrium structure. Taking Al-4Cu alloy as an example, its equilibrium structure should be α+CuAl2 (θ phase). When the single-phase supersaturated α solid solution after quenching is heated for aging, if the temperature is high enough, the θ phase will be precipitated directly. Otherwise, it will be carried out in stages, that is, after some intermediate transition stages, the final equilibrium phase CuAl2 can be reached. The figure below illustrates the crystal structure characteristics of each precipitation stage during the aging process of Al-Cu alloy. Figure a. is the crystal lattice structure in the quenched state. At this time, it is a single-phase α supersaturated solid solution, and copper atoms (black dots) are evenly and randomly distributed in the aluminum (white dots) matrix lattice. Figure b. shows the lattice structure in the early stage of precipitation. Copper atoms begin to concentrate in certain areas of the matrix lattice to form a Guinier-Preston area, called the GP area. The GP zone is extremely small and disc-shaped, with a diameter of about 5~10μm and a thickness of 0.4~0.6nm. The number of GP zones in the matrix is extremely large, and the distribution density can reach 101?~101?cm-3?. The crystal structure of the GP zone is still the same as that of the matrix, both are face-centered cubic, and it maintains a coherent interface with the matrix. However, because the size of copper atoms is smaller than that of aluminum atoms, the enrichment of copper atoms will cause the crystal lattice near the region to shrink, which causes lattice distortion.

Schematic diagram of the crystal structure changes of Al-Cu alloy during aging

Figure a. Quenched state, a single-phase α solid solution, copper atoms (black dots) are evenly distributed;

Figure b. In the early stage of aging, the GP zone is formed;

Figure c. In the late stage of aging, a semi-coherent transition phase is formed;

Figure d. High temperature aging, precipitation of incoherent equilibrium phase

The GP zone is the first pre-precipitation product that appears during the aging process of aluminum alloys. Extending the aging time, especially increasing the aging temperature, will also form other intermediate transition phases. In the Al-4Cu alloy, there are θ” and θ' phases after the GP zone, and finally the equilibrium phase CuAl2 is reached. θ” and θ' are both transition phases of the θ phase, and the crystal structure is a square lattice, but the lattice constant is different. The size of θ is larger than that of the GP zone, still disc-shaped, with a diameter of about 15~40nm and a thickness of 0.8~2.0nm. It continues to maintain a coherent interface with the matrix, but the degree of lattice distortion is more intense. When transitioning from θ” to θ' phase, the size has grown to 20~600nm, the thickness is 10~15nm, and the coherent interface is also partially destroyed, becoming a semi-coherent interface, as shown in Figure c. The final product of aging precipitation is the equilibrium phase θ (CuAl2), at which time the coherent interface is completely destroyed and becomes a non-coherent interface, as shown in Figure d.

According to the above situation, the aging precipitation order of Al-Cu alloy is αs→α+GP zone→α+θ”→α+θ'→α+θ. The stage of aging structure depends on the alloy composition and aging specification. There are often more than one aging product in the same state. The higher the aging temperature, the closer to the equilibrium structure.

During the aging process, the GP zone and transition phase precipitated from the matrix are small in size, highly dispersed, and not easily deformed. At the same time, they cause lattice distortion in the matrix and form a stress field, which has a significant hindering effect on the movement of dislocations, thereby increasing the resistance to plastic deformation of the alloy and improving its strength and hardness. This aging hardening phenomenon is called precipitation hardening. The figure below illustrates the hardness change of Al-4Cu alloy during quenching and aging treatment in the form of a curve. Stage I in the figure represents the hardness of the alloy in its original state. Due to different hot working histories, the hardness of the original state will vary, generally HV=30~80. After heating at 500℃ and quenching (stage II), all copper atoms are dissolved into the matrix to form a single-phase supersaturated α solid solution with HV=60, which is twice as hard as the hardness in the annealed state (HV=30). This is the result of solid solution strengthening. After quenching, it is placed at room temperature, and the hardness of the alloy is continuously increased due to the continuous formation of GP zones (stage III). This aging hardening process at room temperature is called natural aging.

I—original state;

II—solid solution state;

III—natural aging (GP zone);

IVa—regression treatment at 150~200℃ (redissolved in GP zone);

IVb—artificial aging (θ”+θ' phase);

V—overaging (θ”+θ' phase)

In stage IV, the alloy is heated to 150°C for aging, and the hardening effect is more obvious than that of natural aging. At this time, the precipitation product is mainly the θ” phase, which has the greatest strengthening effect in Al-Cu alloys. If the aging temperature is further increased, the precipitation phase transitions from the θ” phase to the θ' phase, the hardening effect weakens, and the hardness decreases, entering stage V. Any aging treatment that requires artificial heating is called artificial aging, and stages IV and V belong to this category. If the hardness reaches the maximum hardness value that the alloy can reach after aging (i.e., stage IVb), this aging is called peak aging. If the peak hardness value is not reached, it is called under-aging or incomplete artificial aging. If the peak value is crossed and the hardness decreases, it is called over-aging. Stabilization aging treatment also belongs to over-aging. The GP zone formed during natural aging is very unstable. When rapidly heated to a higher temperature, such as about 200°C, and kept warm for a short time, the GP zone will dissolve back into the α solid solution. If it is rapidly cooled (quenched) before other transition phases such as θ" or θ' precipitate, the alloy can be restored to its original quenched state. This phenomenon is called "regression", which is the hardness drop indicated by the dotted line in stage IVa in the figure. The aluminum alloy that has been regressed still has the same aging hardening ability.

Age hardening is the basis for developing heat-treatable aluminum alloys, and its age hardening ability is directly related to the alloy composition and heat treatment system. Al-Si and Al-Mn binary alloys have no precipitation hardening effect because the equilibrium phase is directly precipitated during the aging process, and are non-heat-treatable aluminum alloys. Although Al-Mg alloys can form GP zones and transition phases β', they only have certain precipitation hardening ability in high-magnesium alloys. Al-Cu, Al-Cu-Mg, Al-Mg-Si and Al-Zn-Mg-Cu alloys have strong precipitation hardening ability in their GP zones and transition phases, and are currently the main alloy systems that can be heat-treatable and strengthened.

3.2 Natural Aging

Generally, aluminum alloys that can be strengthened by heat treatment have natural aging effect after quenching. Natural aging strengthening is caused by GP zone. Natural aging is widely used in Al-Cu and Al-Cu-Mg alloys. The natural aging of Al-Zn-Mg-Cu alloys lasts too long, and it often takes several months to reach a stable stage, so the natural aging system is not used.

Compared with artificial aging, after natural aging, the yield strength of the alloy is lower, but the plasticity and toughness are better, and the corrosion resistance is higher. The situation of super-hard aluminum of Al-Zn-Mg-Cu system is slightly different. The corrosion resistance after artificial aging is often better than that after natural aging.

3.3 Artificial aging

After artificial aging treatment, aluminum alloys can often obtain the highest yield strength (mainly transition phase strengthening) and better organizational stability. Super-hard aluminum, forged aluminum and cast aluminum are mainly artificially aged. Aging temperature and aging time have an important influence on alloy properties. Aging temperature is mostly between 120~190℃, and aging time does not exceed 24h.

In addition to single-stage artificial aging, aluminum alloys can also adopt a graded artificial aging system. That is, heating is performed twice or more at different temperatures. For example, LC4 alloy can be aged at 115~125℃ for 2~4h and then at 160~170℃ for 3~5h. Gradual aging can not only significantly shorten the time, but also improve the microstructure of Al-Zn-Mg and Al-Zn-Mg-Cu alloys, and significantly improve the stress corrosion resistance, fatigue strength and fracture toughness without basically reducing the mechanical properties.