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Welding of Aluminium

Much of the successful art of aluminium welding is to be found in careful thought and preparation even before welding is started. Through correct preparation, it is easier to avoid the pitfalls that can trap the unwary. Therefore, below are a few facts on base metals, welding methods, types of joint and filler metals.

Base Metals

Aluminium and its alloys can, in general terms, be divided up into three groups:

  • aluminium (pure aluminium)
  • non-heat-treatable alloys
  • heat-treatable alloys.

Aluminiums occur in various degrees of purity. The most common commercial grades contain 99.7, 99.5 or 99.0% aluminium. Non-hardenable alloys, i.e. not suitable for heat treatment, contain small amounts of manganese (Mn) or magnesium (Mg). Al-Mn alloys are often made up of between 1.0 and 1.2% Mn, while Al-Mg alloys with up to 5% are quite common. Al-Mg-Mn alloys are also used.

The hardenable alloys contain copper (Cu), magnesium and silicon (Mg + Si), or zinc and magnesium (Zn + Mg).

Aluminium and most of the non-heat-treatable and heat-treatable alloys possess good weldability. In the case of hardenable alloys with copper and lead additives, there is a risk of hot cracking and therefore they are difficult to weld. Many casting alloys are also suitable for welding except in the case of those which have high content of copper or magnesium which are unsuitable for welding.

Welding Methods

Aluminium is an easily welded metal. However, consideration must be given to both the welding method, the type of joint and the filler material. The two dominating welding methods are MIG and TIG welding, but also gas, plasma, and resistance welding are used as well as welding with covered electrodes.

Whether to use MIG or TIG welding depends on numerous factors. The TIG method is better for thin light-gauge materials, when there is a need for good surface finish and when welding from one side, that is to say when the root side is not accessible such as when welding pipes, and when repairing castings. TIG welding of aluminium is generally done with alternating current.

The MIG method is used primarily in the case of thicker or heavy-gauge materials when high welding speed is a priority and also for long, continuous welds. Due to the lower heat input, MIG welding results in less distortion in the welding zone. For consistent, reliable feeding of soft aluminium filler wire, the push-pull type of equipment gives the best results.

Pulse-Arc welding with MIG is an interesting technology in the context of aluminium welding and is quickly paining in popularity. One of the main benefits of this method is that it gives better control of the molten pool in the case of thin material, and the arc is more stable and there is less spatter. The risk of welding imperfections is also lower.

Types of Joint

The type of joint chosen for aluminium welding depends on the thickness of the material and the type and shape of the workpiece. As a rule, no preparation of the joint is required for thinner materials. An ordinary Tee joint is recommended for one sided TIG welding up to 4mm thickness, and two-sided welding for thicknesses, a 50 V-joint with a 2 to 3mm unbevelled edge is recommended or, alternatively, a 90 double V-joint.

Good joint fit-up makes the welding easier, saves shielding gas and filler material and also contributes to a higher quality of welding. If jigs are not used, the weldments should be tack welded in the correct position prior to final welding. Where it is possible, use of a root support or backing is recommended. The backing can be either copper or steel.

A special characteristic of aluminium is the higher melting point of oxide that forms on its surface, which is mixed into the molten pool, can cause welding defects. The joint surfaces must therefore be scraped or brushed clean using stainless steel wire brush immediately prior to welding. Aluminium is also sensitive to other impurities such a soil, grease, paint and general contamination, all of which can cause pores in the weld. Cleaning of the joint surfaces, as well as the root and top surfaces bordering on the joint is recommended.

Bear in mind that welding causes greater deformation in aluminium than it does in steel. It is therefore essential to give good consideration to the welding sequence, the need of pre-setting to allow for contraction, use of fixtures, and so on.

Filler Metals

When welding aluminium, the choice of filler metal must be determined by the composition of the base metal and the demands which will be made on the end product.

Generally speaking, aluminium and non-heat-treatable alloys should be welded with a filler metal of the same type of alloy as the base metal. Alloys which are suitable for hardening should be welded with a filler metal with a high content of silicon (Si) or magnesium (Mg) in order to avoid the risk of hot cracking.

If there is need of a good match in colour between the welded joint and the base material after anodic treatment, a suitable filler metal should be used. As in the case of the base metal, care must be taken to keep the filler metal clean. On no account must it be contaminated by oil, grease or dust and therefore it must be kept well protected and stored in warm and dry conditions.

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