Steel maker Corus has developed ‘fluxless laser brazing’, a new and robust process for joining steel to aluminium during automotive production line assembly.

The new technique was developed in Holland to support the automotive industry’s increasing use of multi-materials in new car design and manufacture, as a way of reducing weight and improving CO2 emissions.

Although reducing weight to improve CO2 emissions remains one of the main drivers in the global automotive industry, the need to lighten weight is balanced against the need to keep manufacturing costs down. Thanks to its performance and cost benefits, there is little doubt that steel will remain the material of choice for main vehicle body structures in volume car production, Corus said, noting that designers are substituting steel for aluminium for specific applications, such as the bonnet and roof, where advantages beyond weight reduction, such as weight distribution for optimum ride and handling, can be achieved.

Until now though, the welding of steel to aluminium has been a complicated process and one that poses specific challenges to vehicle manufacturers. For example, when the two different materials are welded together, it results in a thick inter-metallic layer, which can result in a brittle and weak weld joint, clearly not suitable for any automotive application. Controlling the heat input using laser based welding techniques has been one way of reducing the thickness of this inter-metallic layer. However, traditional laser welding methods require the use of a salt-based flux to enable a joint to be created. This is not without problems, as a salt-based flux is more prone to expose the joint to corrosion.

The new process removes the need to apply a flux during the welding process which, Corus thinks, will help take significant costs and time out of the manufacturing process. In addition, removing the salt-based flux also removes the potential source of weld corrosion.

The fluxless laser brazing process works by applying an aluminium-based filler wire to create a weld on the aluminium side of the joint. The molten filler metal then ‘wets’ the steel creating a ‘brazed joint’ on the steel side of the weld. Importantly, tests have shown that this new technique creates only a very thin inter-metallic layer making it possible to create joints that are, in some cases, stronger than the base materials themselves.

A Corus spokesman said: “We believe our new welding process is ideally suited to joining aluminium components, for example a roof, into a steel body-in-white, offering vehicle manufacturers further options for saving weight beyond the use of advanced high strength steels.”