Aluminium alloys

Aluminium and its alloys are used in fabrications. Because they-

• Have low weight
• Have good corrosion resistance and weldability.

Some of the more complex alloys in low strength, can have mechanical properties equivalent to steels. The various types of aluminium alloy are identified and guidance is given on fabricating components without impairing corrosion and mechanical properties of the material or introducing imperfections into the weld.

Material types

Pure aluminium is relatively soft. So, small amounts of alloying elements are added to produce a range of mechanical properties. The alloys are grouped according to the principal alloying elements, Specific commercial alloys have a four-digit designation according to the international specifications for wrought alloys or the ISO alpha - numeric system.

According to the means by which the alloying elements develop mechanical properties, alloys can be classified

• Non-heat-treatable, or
• Heat-treatable alloys.

Non-heat-treatable alloys
Material strength depends on the effect of work hardening and solid solution hardening of alloy elements such as

• magnesium, and manganese;

the alloying elements are mainly found in the 1xxx, 3xxx and 5xxx series of alloys. When welded, these alloys may lose the effects of work hardening which results in softening of the HAZ adjacent to the weld.

Heat-treatable alloys
Material hardness and strength depend on alloy composition and heat treatment (solution heat treatment and quenching followed by either natural or artificial ageing produces a fine dispersion of the alloying constituents). Principal alloying elements are defined in the 2xxx, 6xxx and 7xxx series. Fusion welding redistributes the hardening constituents in the HAZ which locally reduces material strength.

 Most of the wrought grades in the 1xxx, 3xxx, 5xxx, 6xxx and medium strength 7xxx (e.g. 7020) series can be fusion welded using TIG, MIG and oxyfuel processes. The 5xxx series alloys, in particular, have excellent weldability. High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for fusion welding because they are prone to liquation and solidification cracking.

The technique of Friction Stir Welding is particularly suited to aluminium alloys. It is capable of producing sound welds in many alloys, including those heat treatable alloys which are prone to hot cracking during fusion welding.

Filler alloys
Filler metal composition is determined by

• weldability of the parent metal
• minimum mechanical properties of the weld metal
• corrosion resistance
• anodic coating requirements

Nominally matching filler metals are often employed for non-heat-treatable alloys. However, for alloy-lean materials and heat-treatable alloys, non-matching fillers are used to prevent solidification cracking.

For the various weldable alloys, the choice of filler metal composition is specified in BS EN 1011 Pt 4:2000 for TIG and MIG welding; recommended filler metal compositions for the more commonly used alloys are given in the Table.

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