Annealing aluminum: full, partial, stabilization
Aluminum rolled sheet may require further reduction of thickness. In this case, it can be rolled at room temperature in a rolling mill with a single mill stand or a multi-roll mill stand (tandem mill). The general principles of cold and hot rolling are very similar..
Aluminum annealing: full and partial
To, after or between cold rolling operations, the aluminum sheet for various reasons may require full or partial annealing - heating and slow cooling - to soften it and remove the condition, which the material has already received.
If annealing is performed before or between cold rolling operations, then annealing prepares the metal for further cold rolling.
Partial annealing can be used after the completion of cold rolling of the sheet to stabilize the mechanical properties of the material..
Full or partial annealing can be used to prepare a sheet for subsequent mechanical forming in the manufacture of aluminum products from it.
Moreover, the annealed state of the aluminum sheet can be, at the request of the customer, the state of its delivery.
Intermediate annealing of aluminum sheets
Aluminum alloys can be hardened by hot or cold rolling operations to such an extent, which is not needed by the final product, or which will prevent further rolling and the achievement of a given state of the material.
Heat-hardenable alloys can obtain significant heating and cooling during hot rolling., as a result of which they can undergo partial hardening and strengthening due to the precipitation of secondary phases.
Cold rolling, on the other hand, lengthens grains and causes residual stresses and deformations. These changes create resistance to further deformation: they say, that the cold-rolled sheet is "riveted" or "received full auto-work".
Before further rolling, undesirable heat hardening or work hardening is removed by intermediate annealing.. Intermediate annealing can be carried out at any stage of the rolling process.
Figure 1 – Intermediate annealing in the process of manufacturing aluminum flat products [2]
Complete annealing of aluminum alloys
When fully annealed, the alloy is heated "hot" enough and long enough, to completely soften the product - to achieve complete recrystallization.
"Recrystallization temperature" is not an accurate term. Recrystallization does not occur instantaneously at a precisely set temperature. Instead of this, it starts gradually, when the temperature reaches some effective range and continues to complete for a long time. The effective recrystallization temperature depends on the alloy and on deformation and processing, to which he was previously exposed, and also on the exposure time at the annealing temperature.
Full annealing transforms both thermally hardened, and thermally unhardenable wrought aluminum alloys to their softest, the most plastic state. In this state, the alloy has maximum possibilities for plastic deformation.. This state of aluminum alloys is indicated by the letter "O" in the international classification.
For complete annealing of heat-hardened aluminum alloys, the metal is usually held for about 2 hours at a temperature in the range 335-370 ° C for peening or 400-425 ° С to remove thermal hardening. Then the metal is slowly cooled at a rate, which depends on the type of alloy.
Thermally unhardened aluminum alloys are annealed by heating for half an hour to two hours (most often within an hour) at a temperature in the range 335-405 ° C. Then they are cooled at a controlled rate..
Full annealing also provides conditions for relieving residual stresses and can be carried out specifically for this purpose..
The state of aluminum alloys H1 and H3 is obtained by applying a certain amount of strain hardening to a completely annealed metal. These states are sometimes called so - "rolled to a given state".
Table 1 – Typical full annealing conditions for some popular wrought aluminum alloys [1]
Symbols in the table 1:
(a) Oven time must not be longer, than is necessary for all parts of the charge to reach the annealing temperature. Cooling speed doesn't matter.
(b) These annealing modes are designed to remove the effects of heating on the solid solution and include cooling from the annealing temperature to 260 ºС at a speed of approx. 30 ºC per hour. subsequent cooling rate does not matter.
Annealing at 345 ºС followed by uncontrolled cooling can be used to remove the effects of cold deformation processing or partial removal of the effects of heat treatment.
(c) These annealing modes are designed to remove the effects of heating on the solid solution and include cooling at an uncontrolled rate up to 205 ºС or lower, followed by reheating to 230 ° C within 4 hours.
Annealing at 345 ºС followed by uncontrolled cooling can be used to remove the effects of cold working or to, to partially remove the effects of thermal
(d) Cooling rate up to 205 ºС or lower no more 30 ºC per hour.
Partial annealing of aluminum alloys
As the name suggests this type of annealing, partial annealing is part of full annealing. This type of annealing applies only to thermally unhardened wrought aluminum alloys.. Its task is to subject the strain-hardened - work-hardened - work-hardened metal to such heating and with such an exposure, to obtain the specified mechanical properties between completely soft and fully work-hardened states.
These intermediate states are referred to as "H2X" and are called "cold-worked and partially annealed".
The quality of a partially annealed product requires careful technological control.
The figure 2 shows the change in the yield strength depending on the temperature and duration of annealing for two thermally non-hardenable alloys (1100 and 5052). These alloys were originally cold worked to H18 temper.. From these graphs it is clear, that by choosing an appropriate combination of temperature and annealing time, mechanical properties can be obtained, which are between the values for the work-hardened state and the state after complete annealing. Also seen, that the yield strength is more dependent on the annealing temperature, than on its duration.
Figure 2 – Graphs of isothermal annealing of alloys 1100-H18 and 5052-H18 [1]
Stabilizing annealing of aluminum alloys
Some thermally non-hardenable aluminum-magnesium alloys, such as 5052, 5456, 5083 and 5086, and Alloy 3004 achieve high strength due to internal stresses, which they get as a result of rolling. However, due to the tendency of magnesium to precipitate from solid solution, the initial state of these alloys is unstable and they can suffer from the so-called "softening aging" – gradual loss of some strength over time, and at room temperature. such alloys, if not stabilized, may also be subject to unpredictable dimensional changes.
To prevent such phenomena in these alloys, they are subjected to stabilization annealing.. This annealing consists in heating the alloys to a relatively low temperature - usually about 180 From. Once stabilized, depravity, hardness and dimensions do not change at room temperature.
Annealing methods for aluminum sheets
Annealing is carried out in convective annealing furnaces. To avoid oxidation of grease residues or the formation of magnesium oxide on magnesium-containing alloys, annealing can be carried out dry, inert atmosphere, such as nitrogen gas.
Annealing methods can be divided into two general approaches - set annealing and continuous annealing..
Saddle annealing
Charge annealing means charging the charge into the furnace, usually batches of bays, and keeping them there until the end of the process. When cage annealing, heat, which flows from the atmosphere of the furnace to the outer layers of the bay, should extend to the inner layers of the bay. Therefore, to achieve annealing with all layers of the coil, a considerable time is required..
Continuous annealing
With continuous annealing, the uncoiled sheet passes through the furnace so, that the entire surface of the sheet is heated by an oven atmosphere. This heating is fast, which also makes it possible to form a more dispersed grain structure of the alloy.
