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Frequently Asked Questions

Die line streaking can typically be observed as a darker or lighter continuous line, of uniform thickness, that runs in a longitudinal direction along the surface of an extruded profile. A die line streak typically appears at junction locations, at locations where the wall thickness changes or were welding has occurred during the formation of a hollow section. Die streak lines are often not visible until etching and anodising treatments have been undertaken and are caused by a variation in the metallurgical structure of the aluminium. A die streak line is not the same as a die line, which is a longitudinal depression or protrusion formed on the surface of an extruded profile. 

So what causes the difference in the metallurgical structure of aluminium that creates the die line streak?

Aluminium, like other metals, has a polycrystalline structure. The crystalline structures are formed by clusters of atoms that develop when aluminium transitions from a molten (or high temperature) state to a cold solid state. As the temperature transitions crystalline structures start to form randomly throughout the metal in random orientations. The crystalline structures expand out until they reach other crystalline structures that prevent further growth. A fully grown crystalline structure is called a grain and where the grains meet a grain boundary is formed.  The grains in aluminium are typically small and uniform in size and they can’t normally be seen with the naked eye.

It is possible however to see the grains of aluminium if the surface is polished and then etched in acid as illustrated below. The grains appear to have a different appearance, but this is only due to their origination and the consequential way they reflect light. 

 

 

During the extrusion process the flow of aluminium through the die can result in higher localised  temperatures. Where this occurs the temperature difference can be sufficient that the grain structures that develop during the cooling phase are able to grow slightly larger than the grain structures that grow in the adjacent area. The etching and anodising process is able to accentuate the visual impact of the larger grain structures, which is why die line streaking is often not visible until after etching and anodising has taken place.

A good die design is able to assist in the even flow of aluminium though the die or situate welding zones to non-visible areas, however a good die design cannot eliminate different temperatures occurring during the extrusion process, particularly on more complex profile shapes. When an extrusion is powder coated a die line streak is covered by the coating and is not visible, however when an extrusion is anodised a die line streak is accentuated and visible.  When the surface area of an anodised finish is of critical importance it is important that AMS are informed so that the die design can take this into account and specimen samples should be obtained where die streaks cannot be avoided. 

How is it possible to extrude a hollow profile with aluminium?

Aluminium is the most widely used metal after iron and is the most widespread metal available on Earth. With an excellent strength to mass ratio, it offers a lightweight alternative to other metals and is highly malleable and resistant to corrosion. The high malleability of aluminium enables it to be cost effectively extruded and the ability to extrude provides the key to being able to fully unlock the unique properties that aluminium offers.

  • Cost effective and high precision manufacturing. The extrusion process enables both simple and complex shapes to be formed economically and with a high degree of accuracy. 
  • Optimised weight to profile strength. The extrusion process enables precise shapes to be developed that can optimise profile strength to weight, maximising the low-density strength advantage of aluminium.
  • Extremely low material wastage. The most common form of manufacturing is subtractive, which is extremely wasteful i.e., you start with a block of metal and then machine away metal to get to the shape that is required. In contrast the extrusion process forms a profile shape with no wastage. Wastage only occurs on cut offs and the wastage is 100% recycled.
  • Low-cost post processing. An aluminium profile is easy to post process and the extrusion shape can incorporate items such as screw ports to make it easy to connect profiles.

So, how is it possible to extrude a hollow profile?

Let’s first consider the basics of the extrusion process. An aluminium extrusion is only possible due to the malleable properties of aluminium, which enable it to be deformed and re-welded whilst in a solid state.  Prior to being extruded the aluminium is heated so that it is softened, but still solid. When heated sufficiently the aluminium is then forced through a die under high pressure to produce the desired profile shape. The process can produce both simple and complex shapes that fall under three generic categories:

  • Solid Dies (no enclosed voids or openings, i.e., solid bars, angles, and channels
  • Hollow Dies (a shape that has one or more voids, i.e., a tube)
  • Semi-hollow Dies (similar to a hollow shape, but without enclosed voids)

The ability to extrude a hollow section is made possible by using two separate mating parts that are attached to each other to form the die. The mating parts consist of a die plate, which is used to form the outer shape of the profile and a mandrel, which forms the inside shape of the profile. The die plate and mandrel below were used to create a simple 110mm x32mm x 6mm rectangular hollow box section.

 

 

When the two mating parts are attached the aluminium billet is forced through the four ports on the mandrel die, into the welding chamber and then though the die plate where the finished hollow profile shape then emerges.  

 

 

 

In the previous frequently asked question article the malleability of aluminum alloys was identified as a key attribute to enable aluminium to be extruded. Whilst the malleability of aluminum is a great attribute to facilitate the extrusion process the downside is that malleability is also associated with softness and low yield strengths, in fact aluminum in its unalloyed form is amongst the weakest of all metals.  

How is it possible to turn one of the weakest metals into a metal that is capable of being used for structural applications? The answer to this question arrived by accident at the turn of the 1900’s. A German engineer, Alfred Wilm, started to experiment with aluminium alloys but found no significant strength advantage straight after the alloys were formed. It was discovered by accident however that a significant increase in strength occurred simply by leaving the metal alloy over a weekend at room temperature. Age hardening had been discovered, although it would be some time before the processes that enabled it to occur was understood. 

The age hardening process, also known as precipitation hardening or particle hardening, is enabled by the addition of metals, such as manganese, copper and silicon that have similar (but critically different) sized atoms to aluminium that can disperse and dissolve throughout the aluminum atoms when the alloy is heated. This process is known as Solution Heat Treatment. When the alloy is subsequently rapidly cooled (Quenched) the atoms are frozen into position. Over time at room temperature the atoms of the added metals start to group together forming precipitates that provide the increased strength. The process can be accelerated and better implemented by artificially heating the aluminum alloy, known as Precipitation Heat Treatment.

A small change in chemical composition to create an alloy, in combination with age hardening unlocks the potential of aluminum and enables it to be soft enough to extrude, yet strong enough to be used structurally. As an example of this transformation pure aluminum has a yield strength of 7 to 11 Mpa. The aluminium alloy 6063 has a minimum aluminium content of 97.35 % and when naturally age hardened (T4) it can achieve a yield strength of 69 Mpa. When artificially age hardened (T6) the yield strength goes up to 172 Mpa. 

Subsequent heat treatment, such as welding or prolonged exposure to high temperatures can impact the strength of aluminium alloys as it can disrupt the participates, however this can be mitigated by the extent and inclusion of different metals into the alloy. The extent and use of different metals have enabled many different grades of highly engineered aluminum alloys to be developed that serve different purposes dependent on the intended application. As an example, the inclusion of lead and bismuth provides an aluminium alloy that is more suitable for machining. Due to the extent and wide variety of engineered aluminum alloys that are available they are grouped into a set of series: 

1xxx series – 99% aluminium content (pure) that offers high thermal and electrical conductivity and corrosion resistance.

2xxx series - The main alloying element is copper (4-5%) offering excellent strength over a broad range of temperatures.

3xxx series - The main alloying element is manganese (0.9-1.5%) offering offer good corrosion resistance and moderate strength.

5xxx series - The main alloying element is magnesium (1.7-4.9%) offering good fatigue strength and very good resistance to corrosion, especially in marine environments.

6xxx series – The main alloying elements are manganese and silicon.  Ideal for extruding with medium strength.

7xxx series – The main alloying element is zinc (5.7 to 8.3%) offing the highest strength and stress corrosion resistance.

Typical applications for three of the commonly used grades in the 6xxx series are provided below.

Grade

Key advantage

Trade off

Typical Applications

Maximum weight % of metal content other than aluminium

6063

Ease of extrudability

Lower strength

Window Frames & Doors, Curtain Walling Louvres, Brise Soleil, Fins, Balcony Decking, Balustrading, Bespoke Light Fittings, Heatsinks and Housings, Display Equipment, Partitions, Solar Panels, Shop Fittings and Signage

Manganese (0.1), Iron (0.35), Magnesium (0.9), Silicon (0.6), Zinc (0.1), Titanium (0.1), Copper (0.1), Chromium (0.1)

6082

Structural strength

Lower quality surface finish

Roof trusses, Scaffolding, Bridges, Cranes, Aluminium Beams

Manganese (1.0), Iron (0.5), Magnesium (1.2), Silicon (1.3), Zinc (0.2), Titanium (0.1), Copper (0.1), Chromium (0.25)

6026 (Replacement for 6262)

Machinability

Harder to weld and anodise

Valve Screws, Marine Fittings, Nuts, Hinges and Couplings, Oil Line Fittings, Decorative Hardware, Appliance Fittings

Manganese (1.0), Iron (0.7), Magnesium (1.2), Silicon (1.4), Zinc (0.3), Titanium (0.2), Chromium (0.3), Copper (0.5), Bismuth (1.5), Lead (0.4), Tin (0.05)

 

      

 

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