Ultrasonic Degassing

Ultrasonic degassing

in molten aluminium melts

Ultrasonic Degassing

Ultrasonic treatment of metal melts can substantially improve the properties of castings, in particular, their microstructure and mechanical properties. Recently, new refinements to the technique using proprietary ultrasound  technology provide significant benefits to industrial continuous casting, offering an alternative to argon degassing, replacement of standard master alloy additives, and substantial improvements to microstructure. Here we will look at the mechanisms that underlie ultrasonic liquid metal interactions, in particular how they apply to ultrasonic degassing and grain refinement. Finally, we will look briefly at our ultrasonic technology as applied to industrial continuous casting.

Cavitation

Applied to liquids including water and liquid metals, ultrasonic energy can induce cavitation: the formation of microbubbles of vapour caused by rapid energy changes. Such bubbles, or voids, occur when the pressure is reduced to below the saturated vapour pressure of the liquid phase, then rapidly collapse or implode under high pressure, producing a shock wave and dissipating considerable energy. Cavitation is enhanced when nucleating centres are present; typically these are microbubbles and impurities. When casting aluminium and its alloys, the presence of hydrogen, usually in its atomic form,  can lead to porosity problems in the final product. To overcome this, a degassing process is often employed. In one approach, an inert gas such as argon is injected into the melt forming bubbles into which hydrogen diffuses forming molecular hydrogen.

 

The gas bubbled rise to the surface and are expelled. An alternative approach involves subjecting the melt to reduced pressure, but both processes have environmental and economic drawbacks.

During ultrasonic degassing, the microbubbles that form during the low-pressure cycle provide nuclei for the formation of hydrogen bubbles. Essentially, hydrogen diffuses to these bubbles.  Assisted by ultrasonically induced acoustic flow and streaming, the hydrogen bubbles rise to the melt surface and are expelled.

This process has many advantages over the traditional methods, including reduced environmental cost and improved efficiency. Effectively ultrasonic degassing reduces the porosity of the cast increasing both strength and ductility.

Results ultrasonic degassing molten aluminium:

    • Improved metal homogenization, grain refinement and mixing of new alloys.
    • Excellent ultrasonic degassing results (defragments and wetted inclusions)
    • Excellent results after vibrating ‘in sump’ of a vertical Wagstaff DC caster.
    • Excellent results on a Bruno Presezzi continuous casting line.
    • Improved micro crystallization and alloy characteristics in casting.
    • Friction reduction between a tool (e.g. casting, drawing, extruding, molding).
    • Improved surface finish.