Joint designs with aluminium
extrusions
Göran Olsson, Sapa Profiler
Rapport nr 1337
Utveckling av Fogningsmetoder för kombination av Olika material till
Hybridlösningar – Ett LIGHTer initiativ [UFoH]
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
Summary The purpose of this document is to show some examples of how aluminium
extrusions can be joined to each other and to other materials.
The material shown here is by no means a complete survey of all existing joint
types, but the intention is rather to be an “appetizer” to show the possibilities and
limitations when joining parts with aluminium extrusions.
To keep the document reasonably short no detailed design solutions are shown.
To get more information and design assistance, please contact Sapa Profiler.
2
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
Contents 1 BACKGROUND ...................................................................................................................... 3 2 JOINT DESIGNS ..................................................................................................................... 4 2.1 PROFILE TO PROFILE ......................................................................................................... 4 2.2 ALUMINIUM TO ALUMINIUM ................................................................................................ 5 2.3 PROFILE TO WOOD ............................................................................................................... 7 2.4 PROFILE TO PLASTIC ............................................................................................................ 7 3 CORROSION ......................................................................................................................... 10 3.1 GENERAL .......................................................................................................................... 10 3.1.1 Corrosion types ....................................................................................................... 12 4 EXTRUSION ALLOYS ...................................................................................................... 14 5 AKNOWLEDGEMENTS ............................... FEL! BOKMÄRKET ÄR INTE DEFINIERAT. 1 Background Aluminium is the most common metal on this planet, meaning that we are not
likely to have material shortage in foreseeable time. The low density, relatively
good strength and very good corrosion resistance for most of its alloys means that
aluminium is a main factor for reducing weight in vehicles etc.
Aluminium (and its alloys) are produced mainly as rolled sheet, castings and as
extruded profiles. This document will focus on the use of extruded profiles.
Although aluminium extrusions are widely used in many different applications,
the knowledge of the possibilities in profile design is not widely spread. Some
design tips can be found in the literature, but most of the design solutions are
customer specific and therefore difficult to find and reuse.
The examples shown in this document comes from a large variety of applications
with completely different technical and aesthetical requirements. The intent is to
not only show the design, but also explain the background for choosing this
design solution.
As the aluminum alloys have a very good corrosion resistance, most of the
material used in a design can be recycled when its technical life is over.
Publik
Rapport nr 1337
3
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
2 Joint designs 2.1 Profile to profile Profiles can be joined using mechanical fasteners (screws, rivets etc.) but also
directly, using features that are integrated in the profile (snap-fits, hinges etc.)
Example 1 Hinge
This all-profile hinge is made from two different extrusions. By punching a square
hole in the bigger profile, the smaller profile can slide in place. Once the smaller
profile is rotated it is locked from sideways movement.
This solution is simple and cheap, the limitation is mainly if the wear resistance of
the moving parts. If it is to be opened many times, other solutions should be used.
Example 2 Hinge with stainless pin
To increase the wear resistance and also the mechanical strength of the hinge, a
stainless steel pin is used.
4
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
2.2 Aluminium to aluminium
Example 3 Friction Stir Welding of profiles
Sapa have been using FSW in serial production since 1997. We use it to join
profiles to large panels but also to join components. The joining method means
that the parts are mechanically joined (material never melts) resulting in far less
heat input and therefore better strength and shape for the joined components.
Characteristics for panels are excellent shape tolerances and high and consistent
joint quality. As no material is added, surface disturbances is avoided.
Typical applications for FSW panels are Sides, Roof and Floor panels for train car
bodies, ship decks ship super structure etc., but they can be used wherever a high
quality joint of profiles is required.
Typical Bodyside panel for a train
As the joining method results in extremely high-quality joints it is often also used
to create water coolers in various applications.
It is perfectly possible to mix components of different Al alloys, even rolled plate
to profiles or cast parts to profiles. As no material is added to the joint, the
corrosion properties are equal to the parts that are joined.
Publik
Rapport nr 1337
5
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
Example 4 Window/Door corner
A V-shaped joiner part is pushed into a corresponding square hole in the two
frame profiles. The parts are locked by adhesive, screw, rivet or by punching the
parts together.
As all parts are anodized, from the same alloy and with no crevices, the corrosion
resistance is extremely good. The life expectancy of a window or door frame more
than equals the life expectancy of the building.
Example 5 Traffic sign
In this case the entire installation is designed using Al profiles.
6
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
2.3 Profile to wood
This solution can be used for most wood materials, but also for plastics and other
relatively soft materials. The “X-mas tree” shape of the profile grips into the
wood and secure it firmly, also sideways. As the profile can be anodized in
different colours or even covered with veneer, it can be designed to blend in very
well.
This design has been used for end caps for benches and tables but also for
covering vertical end sides.
The groove on the underside of the profile is designed to fit the head of a hexagon
screw head, allowing a simple screw attachment to surrounding parts.
2.4 Profile to plastic
Example 1 End cap to profile
When combining aluminium profiles with plastic parts, it is tempting to use snap
fit solutions. The above example shows one way of using guiding tabs in the
upper part of the end cap + snap fits in the lower part that grips into corresponding
grooves in the profile. Depending on the detail design of the snap fits the end cap
can be possible to remove or permanently fixed.
Publik
Rapport nr 1337
7
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
This type of solution is mainly done for indoor applications due to the durability
of the plastic material.
Example 2 Lengthwise connection of lamp cover
The shown solution combines two different profiles and an extruded plastic cover
that all are snap fit together. This product is for indoor application, but the snap fit
solution can also be used for outdoor applications. A consideration when using
snap fit between profiles is that the profiles should be anodized, otherwise the
friction at assembly might be too high.
Example 3 Snapfit plastic to Al + Cu tube inserts
This is an interesting design of a radiator from several perspectives.
A copper tube is inserted into the Al profile and is locked by deforming the profile.
The snap fit function of the plastic part is used to assemble the two Al profiles.
8
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
Example 4
Bonding to profile
In this case a plastic part is attached to the Al profile with double sided adhesive
tape. The tape gives a strong bond and (if a foamed carrier is used) will also compensate for surface or shape defects. It will also act as a seal for the joint.
The plastic part in this picture can of course also be any other material (Al plate,
composite panel, glass etc.).
A limitation with all kinds of bonded joints is if the joint is subjected to a constant
load, as this will in time cause the bond to slip. It is however ideal for taking large
peak loads. For this reason, a bonded joint is often combined with mechanical
fasteners.
In order to secure the adhesion to the Al profile over time, especially in corrosive
environments, the profile should be anodized.
Please also note the channels in the profile for attaching screws and bolts, this
gives a very flexible way of attaching other components.
Example 5
Publik
Bolted end caps
Rapport nr 1337
9
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
Here, end caps made from injection moulded plastic are bolted to the aluminium
profile using self-threading screws into screw channels in the profile. In designs
where you have vibration it may be useful to use self locking screws like Taptite®.
3 Corrosion 3.1 General
Corrosion is “a natural but undesired breakdown of materials”. All metals
corrode, though at different speed, and we can only influence the speed of the
breakdown.
If we compare the corrosion rate of a few common metals (below) we can see that
aluminium is corroding relatively slowly.
Most aluminium alloys generally have a good resistance to corrosion.
Atmospheric circumstances that influence the corrosion life are mainly the PH
value, the presence of corrosive substances and moisture that act as an electrolyte
(especially in combination with dissimilar metals).
Aluminium is stable between PH 4 and 8 due to the natural oxide layer that
protects the surface. This oxide layer can be strengthened by anodization, where
the oxide layer is artificially made thicker.
10
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
The choice of aluminium alloy also makes a difference in corrosion resistance.
The picture below shows the relative corrosion resistance of some common alloys.
The best corrosion resistance is found in the pure or almost pure Al, the least good
in the highly alloyed Cu content alloys. The 6000 alloy group usually used for
extrusions (AlMgSi) has a very good corrosion resistance.
Publik
Rapport nr 1337
11
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
3.1.1 Corrosion types
There are several types of corrosion to consider for aluminium alloys:

General corrosion. This normally occurs at low pH (<4) or high pH (>8).

Pitting corrosion. This is a local attack and is normally more an aesthetic
problem than a mechanical problem. The corrosion products cover the
corroded spot and thereby slow down the process. This can sometimes be
found on parts that have been subjected to foreign metals like steel or
copper.

Crevice corrosion. This is caused by unfavorable design, where two parts
are joined in such a way that moisture can enter the (small) distance
between the parts. It can only occur in moist environments and is
increased in wet saline environments. To avoid this type of corrosion you
can design to avoid the crevice or to prevent moisture from entering the
crevice (geometry, sealant or corrosion inhibitor).

Galvanic corrosion. This occurs when you have different metals in
contact and in the presence of moisture (electrolyte) and is sped up in
saline environments. The less noble of the materials will act as Anode and
the other as Cathode. The picture below shows a combination of
aluminium and copper, something that obviously should be avoided.
The relative area between Anode and Cathode also makes a difference on
the corrosion speed.
Picture A below shows an example of using a large Al part together with
a nobler rivet (stainless steel) in a high conductivity environment (salt
water). This gives very small attack on either metals.
Picture B shows the same material combination but with a low
conductivity electrolyte. Here we have local corrosion next to the rivet.
Picture C shows what could be a stainless steel part combined with an Al
rivet. In this case the rivet would corrode.
12
Rapport nr 1337
Publik
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
To avoid galvanic corrosion you can either prevent electrical contact
between the parts, prevent moisture from entering the contact area
between the parts or use a sacrificial anode. For Al ships or large
structures an externally applied current source can also be used as
protection.
Comment:
For mechanical connections in corrosive environments, a thin Zn layer only gives a very limited
corrosion protection. Therefore, a stainless steel fastener should be used, in extreme cases insulated
with corrosion inhibitant or sealant.

Stress corrosion. Some (Al) alloys, like the 7XXX series alloys, are
sensitive to Stress Corrosion Cracking. For this to occur the parts must be
subjected to mechanical stress at the same time as a corrosive environment
is present. The stress can be an externally applied force, but often residual
stresses from forming operations or welding is enough to start the process.

Filiform corrosion. This occurs between a paint layer and the painted part
and normally starts at a paint defect or at a cut edge. It is mainly seen in
humid saline environments. To prevent this from occurring, special care
must be taken in the choice of painting system and pre-painting surface
treatment. Otherwise the corrosion will creep under the layer of paint and
gradually make the paint layer come loose. (This is the type of corrosion
you often see around paint damages on cars).
Publik
Rapport nr 1337
13
Utveckling av fogningsmetoder för kombination av olika material till hybridlösningar – Ett LIGHTer initiativ
4 Extrusion alloys The alloys used for extrusions are usually from the 6XXX-series (AlMgSi and
AlMgSiMn) and these have a very good corrosion resistance. The most common
6XXX extrusion alloys in Europe are:

AA6063. It is usually used for general purposes (window and door frames
etc.) where the strength requirements are moderate.
It is well suited for decorative anodization.

AA6060F22.

AA6005A. This is typically used in train car bodies and in some marine
applications. Better mechanical strength than AA6063 and also slightly
better corrosion resistance.
It is not suited for decorative anodization, but a technical anodization will
improve the corrosion resistance.

AA6082. This is the alloy typically used for marine applications. It has the
best corrosion resistance of the three alloys and also the best mechanical
properties.
It is not suited for decorative anodization, but a technical anodization will
improve the corrosion resistance.
The 7XXX alloys contain Zn and are also well suited for extrusion. The advantage
of the alloy is that the mechanical properties are very good, slightly higher than
the AA6082 alloys. They are however sensitive to stress corrosion, why they
should be used with caution. One other disadvantage with this alloy is that the
fatigue life is substantially lower in the transverse direction than in the length
direction, something that must be considered during the design.
The alloy is often used in bumper beams for cars etc.
5 Acknowledgements Information and presentation material about corrosion was provided by Sapa
Technology, Finspong.
Product examples were mainly provided by Sapa Profiles, Vetlanda.
14
Rapport nr 1337
Publik