EVAPORATION RATE OF MOLTEN ALUMINUM IN LOW-TEMPERATURE AR-FLOW AND CHARACTERISTICS OF CONDENSED PARTICLES(O2 GAS)

Molten aluminum was evaporated in a cold gas stream of Ar+O2 mixture i n order to investigate both the evaporation rate of the liquid metal a nd the characteristics of condensed particle in the reactive gas strea m. Experiments were carried out under various oxygen partial pressures (P(O2)), temperatures and total gas flow rates using a levitation mel ting apparatus. The relations between the evaporation rate and P(O2) c ould be classified into three regions; i.e. low, medium and high P(O2) regions. The rate in each P(O2) region was discussed using a counter flux model of aluminum vapor and oxygen in the boundary layer around t he levitated specimen. The results obtained in each P(O2) region were summarized as follows: (1) Low P(O2) region: As most oxygen reacted wi th aluminum particles after condensation of the aluminum vapor, the ev aporation rate was almost the same as the one in the Ar gas stream irr espective of P(O2). The condensed particles were a mixture of aluminum and widely defined gamma-Al2O3. The amount of the gamma-Al2O3 increas ed with an increase of P(O2). (2) Medium P(O2) region: As oxygen diffu sed beyond the site where the aluminum vapor nucleated, it reacted wit h the aluminum vapor. The boundary layer thickness for the aluminum va por decreased with an increase of P(O2), so that the evaporation rate increased linearly. The condensed particles were almost widely defined gamma-Al2O3. As a whole, it could be said that the mean particle diam eter decreased with increasing P(O2) and total gas flow rate and with decreasing temperature. (3) High P(O2) region: As oxygen diffused onto the levitated specimen surface, an oxide film covered on the specimen surface. Accordingly, the vaporization of aluminum ceased and no part icle was obtained.