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Treatments

Depending on the types of treatment used to harden aluminium, they can be classified into two groups:

A_Non heat-treatable alloys, with hardening by work hardening. 
B_Treatable alloys, with structural hardening.

NON HEAT-TREATABLE ALLOYS WITH HARDENING BY WORK HARDENING

These are alloys whose mechanical properties depend on the different forms of rolling or stretching and intermediate or final annealing, if necessary. Their hardness is characterised by state H and they correspond to the following families: 1000 (Pure aluminium), 3000 (Aluminium-Manganese), 4000 (Aluminium-Silicon) and 5000 (Aluminium-Magnesium).

Work hardening is hardening obtained by cold plastic deformation that produces an increase in the mechanical properties and the hardness of the material. It simultaneously produces a decrease in its deformation capacity and a loss of malleability. This effect is much more marked the greater the deformation undergone or the higher the rate of work hardening. It also depends on the composition of the metal.

Thus alloy 5083 (Magnealtok 45), which contains between 4 and 4.9% Magnesium, has higher mechanical properties but a more limited deformation capacity than alloy 5754 (Magnealtok 30), which contains between 2.6 and 3.6% Magnesium.

Work hardening is a phenomenon that occurs in any of the deformation methods used: Rolling, stretching, bending, hammering, curving, drawing, notching, etc.

After work hardening it is possible to recover or restore the deformation capacity of a work hardened metal by an "annealing" treatment. This treatment is carried out at a temperature higher than 300ºC. The hardness and mechanical properties of this metal begin to decrease slowly, this is the "restoration" of the material to finally obtain a minimum value corresponding to the mechanical properties of the metal in its natural state.

In the annealing, modification of the texture and grain size of the metal takes place that can be observed by optical microscope with 50 magnification. The texture evolves from a laminar structure to completely crystallised one.

During the recrystallisation phase and at the time of annealing, the grain size is likely to grow. This effect becomes apparent during shaping, for example, bending, by the orange peel appearance of the metal surface.

The increase in grain size, above a value of around 100 microns, reduces the deformation capacity of aluminium alloys

To avoid grain growth and preserve a fine grain structure of the annealed metal, the following conditions must be met:

  1. Ensure that the metal has undergone a sufficient deformation ratio, corresponding to a relative decrease of the section of at least 15%. Work hardening is critical, if this condition is not met, it is necessary merely perform an annealing treatment without allowing recrystallisation
  2. Adopt a rapid temperature increase rate: from 20ºC to 60ºC per hour.
  3. Limit the temperature level to the range of 350ºC to 380ºC.
  4. Limit the duration of maintenance at temperature to two hours maximum.

For the alloys from the 5000 family (Aluminum-Magnesium), 5005, 5050, 5251, 5052, 5754, 5454, 5086, 5083 and 5056, the annealing usually takes place in a range between 345°C and 380°C, with a duration of 30 to 120 minutes.

TREATABLE ALLOYS WITH STRUCTURAL HARDENING

Structural hardening alloys are those whose mechanical properties depend on heat treatments such as solvation (or solubilisation), quenching and maturation (natural or artificial). The 2000 families (Aluminium-Copper), 6000 (Aluminium-Magnesium-Silicon), 7000 (Aluminium-Zinc) and 8000 (Aluminium-Other) belong to this group. By way of structural hardening, these allows are obtained in accordance with the sequence of the following heat treatments:

  • Solvation.
  • Quenching.
  • Maturation (natural or artificial)

In certain cases they may be completed with cold stretching in a certain phase of the treatment.

SOLUBILISATION

This takes place at a high temperature of around 530°C for alloys from the 6000 family (Simagaltok/Al-Mg-Si). This temperature is higher when the alloy is charged with the alloying elements; magnesium and silicon. The time for which the temperature is maintained depends on the thickness of the products.

When subjected to high temperatures for a prolonged period of time, the intermetallic compounds type Mg2Si for the 6000 series alloys and type Al2Cu for those from the 2000 family are redissolved and the alloy then forms a homogeneous solid solution.

The solvation temperature of aluminium structural hardening alloys must be regulated with precision so as not to reach that of eutectic ones. With eutectic temperatures, a local fusion of intermetallic and eutectic compounds (alloys with a low melting point) occurs. The metal is then unsuitable for use. Depending on its composition, this temperature is between 555º and 620ºC for alloys from the 6000 family.

QUENCHING

This is very rapid cooling of the metal that is normally carried out by immersion or showering in cold water at the exit of the furnace in rolling or in the extrusion press, when quenching is carried out at the exit of the extrusion die. The sudden cooling of the metal has the effect of preventing precipitation of the intermetallic compounds.

It is immediately after quenching when structural hardening alloys (AlCu-AlMgSi-AlZn) are easily deformable. The quenching speed is a very important parameter on which certain properties depend, such as the mechanical traction properties, tenacity, corrosion response ..., for each alloy there is a critical quenching speed for which it must not go below the threshold.

To obtain maximum tenacity, the quenching speed must be three times faster than the critical quenching speed. Note: quenching is liable to produce internal stresses, especially in parts with complex shapes or large sections.

The stresses can be reduced with controlled plastic deformation, for example, traction with 2% extension after quenching and before maturation whether natural (T451) or artificial (T651).

TEMPERING OR MATURATION

After quenching the supersaturated solid solution is in a metastable state. The return to equilibrium, i.e. precipitation of the intermetallic compounds that cause the structural hardening, can be carried out in two ways:

  • By maturation at room temperature (natural maturation). After resting for several hours, which depends on the alloys, the hardness and mechanical properties do not increase. Precipitation and structural hardening have ended. This is state T4.
  • By tempering, i.e. heating for several hours to between 160 and 180º for the 6000 series. Tempering (artificial maturation) accelerates precipitation. It is carried out immediately after quenching. The conditions of tempering depend on the alloys.

QUENCHING IN PRESS OF SERIES 6000 ALLOYS

The alloys of the 6000 family, such as 6005 A, 6060, 6061, 6063, 6082, 6101, 6351, can be quenched immediately upon exiting the press by cooling with blown air or by a water shower. At the exit of the press (around 530ºC approx.) the profiles are at a temperature higher than that of precipitation. The products thus quenched can be used in the state designated as T1 or undergo tempering after quenching on the press.

This procedure has several advantages:

  • It eliminates heating for solvation.
  • It reduces the risk of formation of grain size in the cortical area, which is significantly corroded by extrusion.
  • It retains a non-recrystallisable texture and therefore better mechanical properties are achieved.
  • It avoids geometric deformations.

The quenching conditions in the press must be adapted to the critical quenching temperatures of the alloys, the thickness and the geometry of the product. The maximum thickness depends on the alloy.

REPRESENTATION OF STRUCTURAL HARDENING TREATMENT

This treatment involves the following sequences:

  • Solvation
  • Quenching
  • Natural maturation (at room temperature)
  • Tempering or artificial maturation

Solvation treatment. | The solid solvation treatment consists of dissolving in the base metal, for maintenance at high temperature, the elements of the alloy that are in separate phases. By sudden cooling of the solid solution thus obtained, a quenched state is obtained. The optimum solvation temperature depends on the chemical composition of the alloy. The temperature must be respected by around ± 5ºC, the duration of the treatment depends on the chemical composition, type of product, the rates of work hardening before quenching, etc.