Now researchers at Brunel University London have demonstrated a cheaper and greener method of de-gassing aluminium melt, using ultrasound.

Unless hydrogen gas is removed from aluminium alloys before casting, the resulting solid metals can end up being too porous, and therefore cannot be used.

The existing method, known as argon rotary degassing, involves submerging a graphite rotor into the melt from above. This releases a jet of argon gas which is forced through the melt by the rotating shaft, creating bubbles that float to the surface, collecting any dissolved hydrogen.

However, the process is energy intensive and expensive, as it consumes a lot of argon gas, which is simply released into the atmosphere. The technique also produces turbulence in the melt, leading to the creation of “dross” on the surface, or a mixture of metal and oxide, and thereby wasting some of the valuable material, according to Prof Dmitry Eskin, of the Brunel Centre for Advanced Solidification Technology, who led the research.

Previous work has shown that using ultrasound to de-gas aluminium melt is cheaper, more environmentally friendly, and just as efficient.

So the researchers, working as part of a European consortium, set out to demonstrate that they could achieve a continuous process that would allow them to apply ultrasonic de-gassing to large volumes of molten aluminium.

To this end, the researchers used a flat plate, which they submerged into the melt. This plate then vibrates at ultrasonic frequencies, creating cavitation bubbles in the melt that push upwards, drawing hydrogen into them. When the bubbles reach the surface of the melt, the hydrogen is released into the atmosphere.

The researchers are now seeking an engineering partner to optimise the design of the system for industrial-scale operation.