Metallurgy of Extrusion and Rolling Aluminum Alloys

The real difference is not only the alloy number. It is the interaction between
chemical composition, phases, grain structure, deformation and heat treatment.

This is why alloys such as 6061 and 6082 can be used for both extruded and rolled products,
while 6063 is strongly associated with extrusion and 7075 is much more common in
plate, sheet and forgings.

Usually 6xxx alloys are described as heat-treatable alloys based mainly on magnesium and silicon, while 7xxx alloys use zinc as the main alloying element and can reach much higher strength.

Alloying Elements Are Only Part of the Story

Aluminum is relatively soft in its pure form. Alloying elements change its behavior
through several metallurgical mechanisms:

• solid-solution strengthening;
• precipitation hardening;
• grain refinement;
• work hardening;
• dispersion strengthening.

For heat-treatable 6xxx and 7xxx alloys, precipitation hardening is especially important.
After solution treatment and quenching, the alloy contains a supersaturated solid solution.
During aging, very fine precipitates form and restrict dislocation movement.
Their size, number and distribution strongly influence the final strength.

This is the basic metallurgical reason why an aluminum alloy can be relatively soft
during processing but become much stronger after heat treatment.

Why 6063 Is Such a Good Extrusion Alloy

6063 belongs to the Al-Mg-Si family. Its relatively moderate alloy content gives it
a useful combination of low hot deformation resistance, good surface quality and
good response to heat treatment.

The important precipitation system is associated with Mg-Si and Mg₂Si.

For extrusion, this is highly advantageous. The billet can deform relatively easily
and fill a complex die without the very high deformation resistance found in many
high-strength alloys.

This is one reason why 6063 became a standard alloy for architectural and general-purpose
extrusions. The Aluminum Association identifies 6xxx extrusion products as a major choice for architectural and structural applications.

The metallurgical compromise is clear: 6063 gives excellent processing characteristics,
but its maximum strength is not comparable with high-strength 7xxx alloys.

6061 and 6082: The Transition Zone

6061 and 6082 are particularly interesting because they demonstrate why
“extrusion alloy” and “rolling alloy” are not mutually exclusive categories.

6061 contains Mg and Si together with Cu and other elements. It offers higher structural
strength than 6063 while retaining useful extrudability.

6082 goes further. It contains higher levels of alloying elements and, importantly, Mn.

Mn forms dispersoids during homogenization. These small particles influence grain structure
and can restrict recrystallization during hot working.

European Aluminium describes the role of Mn-containing dispersoids in controlling recrystallization in
6xxx alloys.

This is an important metallurgical point:

Mn is not simply added to increase strength. It also changes how the alloy’s
microstructure develops during processing.

Dispersoids and Precipitates Are Not the Same

This distinction is important for engineers.

A precipitate, such as the strengthening phases in 6xxx or 7xxx alloys,
is formed mainly to produce precipitation hardening.

A dispersoid, often containing Mn, Cr or Zr, has another important role.
It can control recrystallization and grain growth.

In simple terms:

The real metallurgy is more complicated, but this distinction is very useful when
thinking about alloy design.

Why 7xxx Alloys Are Different

The 7xxx family moves into the Al-Zn-Mg system.

With suitable additions of Cu and other elements, alloys such as 7075 can achieve
very high strength through precipitation hardening.

The price is increased metallurgical complexity.

The alloy becomes more sensitive to:

• homogenization;
• billet heating;
• deformation temperature;
• cooling rate;
• solution treatment;
• quenching;
• aging.

This makes high-strength 7xxx alloys more demanding to process.

ASM International discusses the increased deformation resistance and processing
challenges of high-alloy aluminum extrusion materials in its
Extrusion of Hard Alloys.

Homogenization Is a Critical Step

The metallurgical story starts before extrusion or rolling.

A DC-cast aluminum billet or slab is not chemically uniform at the microscopic level.
Solidification creates segregation and a complex network of secondary phases.

Homogenization changes this structure.

1. Segregation is reduced.
2. Some phases dissolve.
3. Other phases become more stable and finely distributed.
4. Dispersoids can form.
5. The subsequent deformation behavior changes.

Research on 6xxx and 7xxx billets shows that homogenization can modify the interdendritic
structure, reduce microsegregation and create dispersoids that influence recrystallization.

Therefore, billet chemistry alone does not define extrusion behavior.

Two billets with nominally the same alloy designation can behave differently if their
casting structure and homogenization history are different.

Extrusion Versus Rolling: The Main Metallurgical Difference

During extrusion, deformation is concentrated through a die. The material experiences
strong shear and very large local strains.

During rolling, deformation is distributed through repeated reductions in thickness.

Both processes produce:

• deformation texture;
• dislocations;
• subgrains;
• changes in precipitates;
• possible recrystallization.

However, the final microstructure can be very different.

In extrusion, controlling surface recrystallization and the grain structure across
the profile is particularly important.

In rolling, texture, sheet anisotropy, intermediate annealing and the final grain
structure become especially important.

The Real Boundary Between Extrusion and Rolling

The most useful way to think about aluminum alloys is therefore not:

extrusion alloys vs. rolling alloys

but rather:

alloy chemistry → casting structure → homogenization → deformation
→ recrystallization → heat treatment → final properties

The same alloy can move through this chain in different ways.

The Engineer’s View

The key lesson is simple:

An aluminum alloy is not defined only by its chemical composition.
Its industrial performance is defined by its microstructure and by the thermal
and deformation history used to create that microstructure.

This is why two products made from the same nominal alloy can have different properties.
The difference may come from casting, homogenization, deformation, cooling, solution
treatment or aging.

For engineers, this is often more important than the alloy designation itself.

The real connection between chemistry and industrial performance is
aluminum alloy metallurgy.

References

• The Aluminum Association — Aluminum Alloys and Standards
  
• European Aluminium — Wrought Materials Production
  
• ASM International — Extrusion of Hard Alloys
  
• NIST — Extrusion of Aluminum Alloys