Aluminium encyclopaedia

Joining

Adhesive bonding

Aluminium alloys are suitable for using adhesive bonding technologies. The most important design consideration is that the adhesive-bonded joint should only be subjected to shear forces. One should avoid peel-off forces or tensile stresses. Adhesive-bonded joints are thus mainly overlapping or push-in connections (length of overlap approx. ten times material thickness). Depending on the demands placed on the strength of the joint, one can increase the roughness of the surface, and thus its suitability for adhesive bonding, either by roughening it, thoroughly degreasing it, pickling it or anodising it without compaction. No less effort is required to produce a good adhesive joint than for other joining techniques. The advantages of adhesive bonding are the favourable stress distribution, the fact that the material of the parts being joined is not affected by heat, or only insignificantly so, and the fact that anodised parts can be joined with adhesive without adversely affecting their appearance or their surface protection.

Joining by forming

Every method of forming can also be used to make a joint, whereby essentially one differentiates between manual skills, such as folding, crimping and beading, and techniques used on an industrial scale, like expanding, flaring, rolling in, encapsulating and clinching. Some of the techniques are particularly suitable for sheet (e.g. folding and clinching); the others are better for tube and profiles. These joints cannot be undone non-destructively. The need for plastic deformation means that certain material properties are required do not use hard or age-hardened tempers; when joining different materials together, attention should be given to avoid possible crevice or contact corrosion. The figure below shows typical joints produced by forming thin-gauge components.

Snap connections

With this type of fastener, which is typical for extruded aluminium profiles, use is made of the springiness of the material over the range where its behaviour is elastic. In most cases, the joints can be undone again. Depending on the environment, consideration might have to be given to crevice and contact corrosion. Some types of snap connection are compared with other typical ways of joining profiles below.

Thermal joining of materials by "unifying" them

 With thermal joining one differentiates in principle between processes in which the base material is partially melted (fusion welding) or plasticised (pressure or friction welding) and those where this does not occur (for example soldering). The thermal energy required for the joining process is provided by means of an electric arc, a gas flame, a beam (laser or electron) or by frictional heat and introduced into the workpiece.

The processes that are mainly used are listed below.

Welding

Welding falls into the category of "joining materials by unifying them", as for example do soldering and brazing. An important prerequisite for a good welded joint between aluminium components is the removal of the oxide layer which forms immediately when aluminium surfaces come into contact with the oxygen in the air. This can be achieved in various ways: chemically, using a flux, by means of an arcing effect when welding under a protective atmosphere, by increasing the surface area while excluding air as in pressure welding (e.g. in overlaps) or by squeezing out the original surface zone as with butt welds.

With fusion welding, one differentiates between two basic types:

  • tungsten inert-gas arc welding (TIG)
  • metal inert-gas arc-welding (MIG).


In addition, some of the following special processes are widely used, whereby the last two processes listed are not used for aluminium or only so in special cases:

  • laser beam welding
  • electron beam welding
  • pressure or friction welding
  • metal"arc welding with rod electrode
  • gas welding.

 

Soldering and brazing

With aluminium and its alloys, one uses the term "brazing" if the melting temperature of the braze metal is above 450 °C (approx. 600 °C when using the L-AlSi12) and "soldering" or "soft soldering" when the melting temperature of the solder is below 450 °C.