23 Gasreleasing particle decomposition in the melt

Metal alloys can be foamed by mixing into them a foaming agent that releases gas when heated. The widely used foaming agent titanium hydride (TiH2) begins to decompose into Ti and gaseous H2 when heated above about 465°C. By adding titanium hydride particles to an aluminum melt, large volumes of hydrogen gas are rapidly produced, creating bubbles that can lead to a closed-cell foam, provided foam drainage is sufficiently slow, which requires a high melt viscosity. The Shinko Wire Company has developed an aluminum foam trade named Alporas using this approach (Figure 2.3).

The process begins by melting aluminum and stabilizing the melt temperature between 670 and 690°C. Its viscosity is then raised by adding 1-2% of calcium which rapidly oxidizes and forms finely dispersed CaO and CaAl2O4 particles. The melt is then aggressively stirred and 1-2% of TiH2 is added in the form of 5-20 ^m diameter particles. As soon as these are dispersed in the melt, the stirring system is withdrawn, and a foam is allowed to form above the melt. Control of the process is achieved by adjusting the overpressure, temperature and time. It takes, typically, about ten minutes to totally decompose the titanium hydride. When foaming is complete the melt is cooled to solidify the foam before the hydrogen escapes and the bubbles coalesce or collapse.

The volume fraction of calcium and titanium hydride added to the melt ultimately determines the relative density and, in combination with cooling conditions, the cell size. The cell size can be varied from 0.5 to 5 mm by changing the TiH2 content, and the foaming and cooling conditions. Relative densities from 0.2 to as low as 0.07 can be manufactured. As produced, the Alporas foam has predominantly closed cells, though a subsequent rolling treatment can be used to fracture many of the cell walls in order to increase their acoustic damping. A significant manufacturing capacity now exists in Japan. Although only small volume fractions of expensive calcium and titanium hydride are used, the process is likely to be more costly than gas-injection

PARTICLE DECOMPOSITION IN LIQUID

a) Viscosity modification

Aluminum alloy + 1- 2% Calcium"

Calcium b) Foaming agent addition

Thickened aluminum alloy + 1 - 2% TiH2 (670 - 690 C)

b) Foaming agent addition

Thickened aluminum alloy + 1 - 2% TiH2 (670 - 690 C)

Foaming agent

Foaming agent

c) Isothermal foaming

Metal drainage

Metal drainage

Foaming aluminum

Figure 2.3 The process steps used in the manufacture of aluminum foams by gas-releasing particle decomposition in the melt (Alporas process)

methods because it is a batch process. Today, only aluminum alloys are made in this way because hydrogen embrittles many metals and because the decomposition of TiH2 occurs too quickly in higher melting point alloys. Research using alternative foaming agents (carbonates, nitrates) with higher decomposition temperatures offers the prospect of using this method to foam iron, steels and nickel-based alloys.

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