The effective fabrication of welded aluminium products demands a substantive level of meticulousness and competence on the part of the welder, as compared with steel fabrication, owing to aluminium’s composition, says Aluminium Federation of South Africa (Afsa) aluminium specialist Dr Tony Paterson.

He comments that the appropriate fabrication and welding procedures must be observed during the welding of aluminium to ensure the formation of competent joints, which will be able to withstand the pressures to which they are subjected, and prevent failure.

Aluminium has a seminal role in industries such as transportation because it enjoys a higher strength:mass ratio compared with steel. The appro- priate use of aluminium can, therefore, assist in enhancing payload and corrosion-resistance efficiency, as well as reduce costs and fuel consumption, says Paterson.

However, he points out that errors in the welding of the aluminium used in high-risk applications, such as for fuel tankers, can result in the possible threat to human life.

“As the consequence of failure increases, so does the need for competent welding,” says Afsa executive director Mark Krieg. He adds that the efficacy of the weld in resisting relevant stresses contributes to the areas of risk assessment and management when designing structures.
Paterson says that unlike steel, aluminium is intolerant of poor fabrication. This means that more time is required during the fabrication of aluminium, prior to the welding process to ensure a competent weld, than is the case with other materials. He adds that the welding of aluminium, therefore, requires a greater measure of patience and attention to detail compared with the welding of steels.

He explains that the welding differs from that of steel in that aluminium’s ability to conduct heat is three to five times greater than that of steel. This conduction property, for instance, supports the use of aluminium in the major material used for overhead conductors. However, this conductivity requires welders of aluminium to work at a higher current and greater speed than would otherwise be required to limit the effects of differential heat loss and distortion, particularly on thinner sheeting.

As aluminium expands at about twice the rate of steel, the heat spread may result in distortion as the weld cools. Higher currents and faster speeds are required during the welding process to lessen the effects of distortion, because there is less time for the heat to dissipate, comments Paterson.

Unlike steel, aluminium does not change in colour as it becomes hotter. This results in there being no visual indicator as fusion temperature is approached. Given that alumi- nium can burn through quickly, elementally rich fillers are recommended for use, as these melt earlier than the parent material. These fillers, therefore, provide a visual indication of the temperature of the weld bead.

A significant aspect of aluminium welding is the chemical composition of the weld bead, which is a combination of the parent material and filler compositions. As some combinations are more sensitive to hot cracking, it is impera- tive that the correct fillers be selected for the joining of specific alloys, comments Paterson.

The surface of aluminium has a high affinity for oxygen. Aluminium quickly acquires an oxide layer, which melts at a faster rate than the metal itself, and can compromise the effectiveness of the weld if not removed. The surface preparation of aluminium prior to welding is a more complicated process than that required for mild steels, says Paterson. This process involves the removal of hydrocarbons or oils from the surface of the aluminium, followed by the removal of the aluminium oxide layer.

He cites the transfer of knowledge as a critical issue and, because a different approach is required, the training of competent alumi- nium welders is imperative.

South Africa produces about 5% of the world’s aluminium, and the bulk of this produc- tion is exported. Paterson says, as a result of the economic downturn, the commodity value of primary aluminium has fallen by about half in proportion to other materials.

However, aluminium is a cost-effective metal because it is 100% recyclable and versatile, with the capacity to be formed into varying complicated hollow and solid shapes, concludes Krieg.

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