Abstract
Pinholes in aluminium alloy castings are due to gas, mainly hydrogen, liberated during solidification of the molten metal, and it remains in the metal as small bubbles. The origin of hydrogen in aluminium was shown by many authors to be ascribed to the decomposition of water vapour by the melt. How hydrogen originating from water vapour is absorbed by the melt, forms the subject of the present report, the results obtained being summarised as follows: - (1) Oxide film on the surface of the molten metal forms a remarkable protective barrier against water vapour. Owing to this protective action of the oxide coating, pinholes seldom occur even when steam is blown against the surface of the melt. The moisture in the atomosphere, therefore, seems to have almost no effect upon the melt, provided that it is coated with oxide film. On the other hand, direct contact with steam by removing the oxide film from the melt results in the formation of numerous pinholes. (2) That pinholes in aluminium alloys castings are mainly due to water vapour from the sand mould has been experimentally proved. Thus, when the oxide coating is removed in casting the metal into the mould, the most favourable condition for the absorption of hydrogen is established. (3) Hydrogen originating from water vapour seems to be in nascent state and has been found to be absorbed with great rapidity, … within 10 seconds.
Prolonged contact of water vapour with the molten metal, however, does not appreciably increase the gas content in the metal. (4) Gases other than water vapour, namely, hydrogen, acetylene, carbon monoxide, and carbon dioxide were blown into the melt to examine their degree of pinhole formation. It has been found that hydrogen at a temperature below 800° is feeble, above this temperature its action gradually becomes vigorous; the action of acetylene resembles that of hydrogen, while carbon monoxide and carbon dioxide are almost inactive in creating pinholes. As the results of the above-mentioned tests, water vapour has been found, to be far more effective than the other gases in creating pinholes. (5) Gas absorbed by the melt can easily be removed by remelting, by treating with dry fluxes, or by maintaining the melt at a suitable temperature for approximately 20 minutes.
