Aluminum annealing

What is annealing

Heat treatment of aluminum products in order to soften the metal. Occurs by reducing or removing strain hardening, obtained as a result of cold deformation processing and / or the most complete separation from the solid solution, hardening phase dissolved in it.

Includes:

heating metal to a certain temperature
hold at this temperature
controlled or uncontrolled cooling.

Types of annealing

In aluminum metallurgy, the following main types are distinguished annealing [1]:

full annealing
partial annealing
annealing to relieve residual stresses.

Full annealing

After complete annealing of all aluminum alloys – as thermally hardened, so thermally non-hardened - get the state, which is the softest, most ductile and most conducive to plastic deformation. The international designation of this condition the letter "O". sometimes this letter "O" is confused with digit «0».

products, which have been hardened by cold deformation, usually a fully recrystallized structure is obtained in this state.
products, made by hot working may remain uncrystallized.
In the case of heat-hardenable alloys, the dissolved alloying elements precipitate from the solid solution as completely as possible., to prevent hardening by natural aging. For this, an increased heating temperature and additional holding at a lower temperature are usually used..

Fig. 1 – When a large amount of cold work is followed by annealing, new grains are formed by the process of recrystallisation [2]

Fig. 2 – Schematic diagram of the processes that may occur during the annealing of a deformed metallic material.
(a) Deformed grain structure; (b) recovered subgrain structure; (c) partially recrystallized structure; (d) recrystallized structure [3]

Temperature, time, cooling

As for heat-hardened, and for thermally non-hardened alloys, the reduction or removal of work hardening from cold deformation processing is achieved by heating at a temperature of 260 to 440 degrees Celcius. The softening rate is highly temperature dependent.. Time, required to soften a given material at a given hardening degree can vary from a few hours at low temperature to a few seconds at high temperature.

If the purpose of the annealing is simply the removal of strain hardening, then heating to a temperature of about 345 ° C will be sufficient.
If it is necessary to remove the hardening by heat treatment, or just by cooling the hot working temperature, then a special heat treatment is needed to obtain a structure with the release of a hardening phase in the form of large particles. This is usually achieved by holding at temperatures from 415 to 440 °C and slow cooling at a rate of approx. 30 ° C per hour to 260 ° C. In this case, high diffusion rates give the maximum coalescence of the precipitated particles and, as a result, minimum hardness.

When annealing is important to provide, that the desired temperature has been reached in all parts of the cages and at all points of each product. Therefore the usually prescribed duration of soaking at an annealing temperature of not less than 1 o'clock. The maximum annealing temperature is moderately critical: it is recommended not to exceed the temperature 415 ° C due to the possible oxidation and grain growth. The heating rate may be critical, for example, alloy 3003, which usually requires rapid heating to prevent grain growth. Relatively slow cooling at still air or oven is recommended for all alloys to minimize warpage.

Typical Full Annealing Parameters

The following are typical full annealing parameters depending on its purpose and alloy [1].

a) Alloys:

1060, 1100, 1350
3003, 3004, 3105
5005, 5050, 5052, 5083, 5086, 5154, 5182, 5254, 5454, 5456, 5457, 5652

Heating temperature:

345 ° C

Exposure time:

Time in the oven should not be more, than is necessary, that would bring all parts of the cages to an annealing temperature.

Special instructions:

The cooling rate after annealing is irrelevant.

b) Alloys:

2014, 2017, 2024, 2036, 2117, 2124, 2219
6005, 6061, 6060, 6063, 6066
7079, 7050, 7075, 7079, 7178, 7475

Annealing temperature

415 ° C

Holding time at heating temperature

from 2 to 3 hours

Special instructions:

These annealing modes are designed to remove the effect of solution treatment and include cooling at a rate of about 30 ° C per hour from the annealing temperature to 260 ° C. subsequent cooling rate does not matter.
Processing at 345 °C followed by uncontrolled cooling can be used to remove the effect of cold working or partial removal of the effect of heat treatment.

c) Alloys:

7079, 7050, 7075, 7079, 7178, 7475

Annealing temperature

415 ° C

Holding time at heating temperature

from 2 to 3 hours

Special instructions:

These annealing modes are designed to remove the effect of solid solution treatment and include uncontrolled cooling to 205 °C or lower, followed by reheating to 230 ° C. 4 hours. subsequent cooling rate does not matter.
Processing at 345 °C followed by uncontrolled cooling can be used to remove the effect of cold working or partial removal of the effect of heat treatment.

d) Alloy:

7005

Annealing temperature

345 ° C

Holding time at heating temperature

from 2 to 3 hours

Special instructions:

Cooling up to 205 °С or lower at a speed not exceeding 30 °C per hour.

Partial annealing

Partial annealing is called the annealing of cold-worked, thermally non-hardenable wrought alloys to obtain intermediate mechanical properties (type of states – H2) . Partial annealing is also called reduction annealing.. Partial annealing temperatures below those, at which extensive crystallization occurs, and incomplete softening occurs due to substructural changes in the dislocation density and polygonization (Fig.. 3 and Fig.4). An example of the application of intermediate annealing in the production of aluminum foil is shown in Fig. 2.

Fig. 3 – Temperature limit for complete and intermediate annealing for the AlMg3-H18 alloy [4]

Fig. 4 – Evolution of Vickers microhardness HV with annealing
time t as a function of annealing temperature
(Al-1.3% Mn, 97% cold-rolled)[3]

Fig. 5 – The use of intermediate (recovery) annealing in the production of foil [3]

The ability to bend and change shape by plastic deformation in alloys, annealed to the state of the H2 type is usually much higher, than the same alloy with equal strength, but in a state of type H1. State type H1 – this state, which is achieved, when cold working is the last operation. The difference between the states H1x and H2x is shown in Fig. 6.

In Fig. 6 and Fig. 7 shows the change in the yield strength depending on the temperature and annealing time of two thermally non-hardenable alloys 1100 and 5052, which were initially in a heavily cold-worked state H18. These curves show, that by selecting suitable annealing temperatures and holding times, mechanical properties intermediate between the high strength of the work hardened state (H18) and the fully annealed state (O) can be obtained. These curves also show, that the yield strength depends on temperature much more strongly, than on the duration of heating.

Figure 5 – The difference between the types of states H1x and H2x

Figure 6 – Representative isothermal annealing curves for alloy 1100-H18 [1]

Figure 7 – Representative isothermal annealing curves for alloy 5052-H18 [1]

Annealing of aluminum castings

Annealing of aluminum castings is carried out during 2-4 hours at a temperature of 315 to 345 °С provides the most complete removal of residual stresses. Provides maximum dimensional stability when operating at elevated operating temperatures. Designated as “ABOUT” [1].

Sources:

Heat Treating //Aluminum and Aluminum Alloys, AMS International, 1993.
Mechanical Working and Forming of Shapes – TALAT 1251 – 1999
Design of Aluminum Rolling Process for Foil, Sheet and Plate // Encyclopedia of Aluminum and Its Alloys, Two-Volume Set (2019) – Eds. G.E. Totten, M. Tiryakioğlu, and O. Kessler
Materials – Alloy constitution // The Aluminium Automotive Manual – EEE, 2002