Alloys of aluminum profiles
Production of extruded aluminum profiles from 6xxx series alloys (the most technologically advanced and therefore popular of them – Aluminium alloy 6060 according to EN and ISO standards (its partial analogue – Aluminum alloy AD31 GOST 4784-97) is accompanied by a complex combination of several thermal processes. Therefore, without a basic understanding of these steel alloys can hardly achieve consistently high quality aluminum profiles.
The role of magnesium and silicon in the alloys 6060, 6063 and AD31
- Aluminium alloy 6060, and
- Aluminium alloy 6063 and
- their analogue – Aluminum alloy AD31
are typical thermally hardenable aluminum alloys. They reach their strength due to heat treatment, instead of strain hardening. The key elements of these aluminum alloys are magnesium (Mg) and silicon (Si), particles which form Mg2Si. These particles may take several forms, which are usually grouped into three main categories:
- b "-Mg2Si - the smallest particles Mg2Si, which have a rod shape and make the main contribution to the strength properties, when they have a high distribution density;
- b-Mg2Si - enlarged version of the rod-shaped discharge, which grow from β "-category. These particles make a negligible contribution to the increase in strength properties;
- b-Mg2Si - the largest particles Mg2Si, which have a cubic shape and because of its large size do not contribute to increasing the strength properties.
Most of the aluminum alloys 6xxx series, which includes alloys 6060, 6063 and AD31, "Designed" so, to have balanced ("stoichiometric") magnesium and silicon contents, that is, in the proportions, in which they are contained in the magnesium silicide Mg2Si. The figure below shows the boundary line for a balanced chemical composition.
Excess silicon is preferable, than excess magnesium for the following reasons:
- excess magnesium does not increase the strength properties of the finished product;
- excess magnesium increases the flow stress of the alloy and makes extrusion of the alloy more difficult;
- excess silicon increases the efficiency of artificial aging and thus increases the strength properties of the finished product
Effect of alloying
Iron is always present in aluminum alloys and often forms in combination with silicon and aluminum AlFeSi intermetallics. These intermetallic compounds do not affect the mechanical properties of the alloy, but if not heat treated alloy may adversely affect its moldability. Accurate control of the iron content is important for anodizing profiles. Various iron content may produce differences in shades of color or haze degree anodized surface.
Manganese is added to the 6xxx alloys for several reasons.
- Manganese homogenization reduces the duration, helping to accelerate the conversion of β-AlFeSi particles into α- ALFEU.
- It prevents the growth of coarse grains during the thermal treatments with separate heating of high-strength alloys, such as, 6061 and 6082.
- Another is the use of manganese is, Additives that increase its viscous properties of the alloy, as it helps to prevent the loss of free silicon at the grain boundaries.
The most durable aluminum alloy 6xxx series, which have a high content of manganese (more 0,1 %), it reduces the compressibility, and increases sensitivity to quenching.
Chromium acts much like manganese, but its effect on the quench sensitivity is more significant.
- Copper additions help to improve the quality of machining (cutting, milling, etc.) extruded profiles.
- A small copper content reduces the negative effect of the time delay of the operation of artificial aging to the level of strength properties of high strength alloys, for example, 6061.
- When the copper content is more 0,2 % decreases the corrosion resistance of alloys 6xxx series.
- Zinc does not have any adverse effect on the mechanical properties of the alloys of the 6xxx series.
- However, when the content of a 0,03 % Zinc may be the cause of the defect, consisting in selective (differentiated) etching of the surface of the profiles during their anodization.
Thermal transformations in alloys 6060, 6063 and AD31
The 6xxx series alloys achieve maximum strength through the following heat treatment sequence:
- heating during pressing until all particles are completely dissolved in aluminum or separate heating for the same purpose;
- cooling at a rate, which depends on the chemical composition of the alloy;
- aging hardening due to the release of the excess phase particles.
Maximum mechanical properties of aluminum profiles achieved with strict control of temperature during the whole of their production. It is dependent on the temperature characteristics of the particles Mg2Si. The figure 2 shows the temperature history of the production process of aluminum extrusions of alloys 6xxx. All structural transformations occur with magnesium and silicon.