Thermal hardening of aluminum alloys 6xxx

Solution heat treatment of aluminium profiles on the extrusion press

Cooling rate aluminum profiles – quenching – immediately after exiting the press should be fast enough, to delay the magnesium and silicon in solid solution. This achieves the maximum mechanical property profile of the material due to their isolation in the subsequent hardening of aging.

The required rate of cooling of the solid solution alloying elements – magnesium and silicon in aluminum – to ensure the hardening effect depends on the cross-sectional dimensions of the aluminum profile and the methods of its cooling:

• calm air,
• fans,
• water mist,
• water cooling or spreyernym
• in water stream.

Figure and table shows the minimum allowable cooling rate profiles for various aluminum alloys of the 6xxx series. For aluminum alloy profiles 6060 (aluminum alloy AD31) usually enough cooling in calm air or fans, whereas the profiles of alloy 6061 spreyernoe necessary cooling water or cooling water flow.

Aging aluminum alloys

Aging 6xxx series alloys produced to improve the mechanical properties of aluminum profiles. The degree of this increase depends on the properties of the alloy and aging conditions. These conditions differ for:

• natural aging, that occurs “by itself” at room temperature, and
• artificial aging, is conducted at elevated temperatures, near 160-200 oC.

Strength alloys 6xxx series is directly related to their ability to resist the movement of dislocations during deformation. Upon the application of stress to the material formed in it and move dislocation. With increasing number of stress and dislocation density, moving in the material, increases until, till, finally, the material is not destroyed.

The movement of dislocations is inhibited due to the presence of particles Mg₂Si and thus the strength of aged aluminum alloy increases. The dimensions and density of these particles are controlled aging parameters. A small amount of fine particle β”-Mg₂Si little can be done, to stop the deployment of, which move through the material, but when a lot of them, they hinder the movement of dislocations, and this increases the strength of the material.

If the particles grow too large (β '-Mg₂Si и b-Mg₂Si), they become too small due to the limited content of Mg and Si in the aluminum. In this case, the dislocation easily circumvent these particles and the strength of the material is reduced.

This shows a schematic graph of a typical artificial aging in the figure below.

Peak strength properties is achieved when a large number of fine particles β”-Mg₂Si. For alloys 6xxx typical parameters are the temperature artificial aging 170 ° C when exposed 8 hours or 185 ° C when exposed 6 hours.

T states aluminum alloys

Different parameters correspond to embodiments of aging various designations of the state of aluminum alloys:

• T1 - cooled after pressing to room temperature and naturally aged;
  
• T4 - after pressing, hardened from separate heating and naturally aged;
  
• T5 - cooled after pressing to room temperature and artificially aged to the maximum strength properties;
  
• T6 – hardened after pressing with a separate heating and artificially aged to peak strength properties.

To designate other aging treatments, are specifically designed to obtain mechanical properties, which differ from the maximum strength properties. for instance, state T52 and T591 are applied to aluminum profiles, which are subject to bending, T7 and the state - for profiles, which are applied at elevated temperatures.

Stretching and maturing this mass profiles

The usual practice of manufacturing aluminum extrusions involves stretching them from 0,5 % to 3 % and then aging with a delay of one day of artificial aging for profiles made of low-alloy 6xxx alloys (no more 0,9 % Mg₂Si), for example, Aluminum alloys AD31, 6060 and 6063. This contributes to the achievement of optimum mechanical properties after aging profiles.

However, this delay for higher strength aluminum alloys (content Mg₂Si more 0,9 %), for example, 6061, It may lead to reduced mechanical properties of the material of aluminum profiles. These alloys contain copper in an amount of not less 0,1 %, which counteracts the influence of delay artificial aging on the final mechanical properties of thermally toughened aluminum profiles.