22 Melt gas injection air bubbling

Pure liquid metals cannot easily be caused to foam by bubbling a gas into them. Drainage of liquid down the walls of the bubbles usually occurs too quickly to create a foam that remains stable long enough to solidify. However, 10-30% of small, insoluble, or slowly dissolving particles, such as aluminum oxide or silicon carbide, raise the viscosity of the aluminum melt and impede drainage in the bubble membrane, stabilizing the foam. Gas-injection processes are easiest to implement with aluminum alloys because they have a low density and do not excessively oxidize when the melt is exposed to air or other gases containing oxygen. There are several variants of the method, one of which is shown in Figure 2.2. Pure aluminum or an aluminum alloy is melted and 5-15 wt% of the stabilizing ceramic particles are added. These particles, typically 0.5-25 ^m in diameter, can be made of alumina, zirconia, silicon carbide, or titanium diboride.

MELT GAS INJECTION

Melt drainage

Crucible

Stirring paddle & gas injector

Heating

MELT GAS INJECTION

Melt drainage

Crucible

Stirring paddle & gas injector

Heating

Figure 2.2 A schematic illustration of the manufacture of an aluminum foam by the melt gas injection method (CYMAT and HYDRO processes)

Figure 2.2 A schematic illustration of the manufacture of an aluminum foam by the melt gas injection method (CYMAT and HYDRO processes)

A variety of gases can be used to create bubbles within liquid aluminum. Air is most commonly used but carbon dioxide, oxygen, inert gases, and even water can also be injected into liquid aluminum to create bubbles. Bubbles formed by this process float to the melt surface, drain, and then begin to solidify. The thermal gradient in the foam determines how long the foam remains liquid or semi-solid, and thus the extent of drainage. Low relative density, closed-cell foams can be produced by carefully controling the gas-injection process and the cooling rate of the foam.

Various techniques can be used to draw-off the foam and create large (up to 1 m wide and 0.2 m thick slabs containing closed cell pores with diameters between 5 and 20 mm. NORSK-HYDRO and CYMAT (the latter using a process developed by ALCAN in Canada) supply foamed aluminum alloys made this way. This approach is the least costly to implement and results in a foam with relative densities in the range 0.03 to 0.1. It is at present limited to the manufacture of aluminum foams.

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