Directional solidification of gas-saturated melts

Directional solidification of gas-saturated melts

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The GASAR process exploits that most metal-hydrogen equilibria have phase diagrams similar to the one shown in Figure 1, with a eutectic decomposition where a molten metal saturated with hydrogen will form a solid metal and hydrogen gas on cooling. The aluminium-hydrogen system is the most commonly used.

A metal is melted in a furnace contained within a pressure vessel. As the solubility of hydrogen in molten metals increases with applied pressure, a high external pressure of hydrogen causes gas to dissolve into the melt. The gas-saturated melt is then transferred to another crucible with a cooled base, and the pressure is controlled while the melt gradually solidifies from the cooled base of the crucible upwards, at a rate between 0.05 and 5 mm s^-1. As the solubility of the gas in metal below the eutectic temperature is negligible, it is expelled from the metal on solidification, to form elongated parallel pores aligned in the direction of cooling.

Figure 1: Schematic phase diagram where cooling the saturated melt from 1 to 2 causes simultaneous formation of solid metal and a gaseous phase.

Figure 2: The GASAR process: Metal is melted in the upper crucible and then let into the lower crucible, where a chilled base promotes directional solidification. The entire system is surrounded by gas at a controlled pressure.

The shape and size of the pores is controlled by the pressure, the rate and direction of cooling, the gas pressure, the metal or alloy used, the presence of non-wetted inclusions in the melt, and the melt temperature. Foams have been produced using aluminium, nickel, copper, iron, magnesium and various alloys, with relative porosities between 5 and 75% and pore sizes between 10 痠 and 10 mm. The pores are cylindrical, and can have aspect ratios of up to 300. The uniformity of the pores is variable - tending to be better in the first metal to solidify (near the chilled base of the crucible) than in later stages.

Figure 3: Fracture surface of an aluminium foam produced using the GASAR process. The fracture surface is approximately perpendicular to the direction in which the pores are elongated.

This process produces foams of reasonable quality, but the maximum porosity levels are not particularly high, net-shape results can only be produced for simple shapes, production rates are slow and difficult to scale up, and the process is complex and expensive.