Thermodynamique des phenomenes de vaporisation et de condensation dans les fours de coulee de superalliages sous vide

摘要

Casting of superalloys is sometimes made by the lost wax process. In this process, the superalloy is cast in a ceramic mould under vacuum. Casting is done at high temperature in graphite resistor furnaces, and the oxide compounds of the mould vaporize. The vacuum vaporization when pumping is ruled out by a congruent vaporization relation because the used oxides are sufficiently stable. The employed method to calculate this steady-state vaporization and the associated pressures is described. This method will be applied at two simple cases : the vaporization of alumina and the one with its impurities, Na_2O for example. In the alumina case, another constraint is considered: the oxygen flow associated to the furnace pumping capacities which is opposed to the evaporated flow. Two vaporization processes can be also distinguished: for sufficiently low furnace pressures, it is alumina which imposes its vaporization conditions and in that case, the oxygen furnace pressure will be that of alumina vaporization. For high oxygen furnace pressure, this one controls the vaporization of alumina. Then, we show how this method can be generalized to more complicated cases such as the pseudo-binary system Al_2O_3-Na_2O for which the evaporated flow composition stays in the pseudo-binary section for these stable oxides. This study shows that the most important pressures are due to the gaseous species Na and O_2 associated even though the Na_2O activity is low since Na_2O is an impurity in Al_2O_3.