| The present investigation addresses the characterization of the thermal history of rapid solidification in metal droplets, and its effects on powder microstructure. The main effort was focused on modelling the heat flow during solidification, and relationships were established between the atomization parameters, the growth kinetics, the interface velocity and undercooling, and other important variables. Numerical solutions based on the enthalpy model were developed, and their results compared to the trends predicted from the Newtonian model. The analysis covered situations of isothermal solidification at the melting temperature, as well as those where significant undercoolings are necessary for nucleation and growth. The implications of single vs. multiple nucleation were also discussed.;The concepts developed from the heat flow analysis were coupled to microstructural observations in aluminum alloy powders, mostly in the submicron size range. It was shown that reducing the particle size decreases the extent of segregation, promotes multiple nucleation and the formation of twins during solidification. Homogeneously solidified powders were found in Al-3%Si and 6%Si below 1 (mu)m in diameter; their incidence is enhanced by reducing the solute content and increasing the heat transfer coefficient.;In general, the results indicate that when substantial undercoolings are achieved in the droplet prior to nucleation, the thermal history consists of two distinct solidification regimes. In the first one the interface velocities are high, the droplet absorbs most of the latent heat released, and the external cooling plays usually a minor role. The second regime is one of slower growth, and strongly depends on the heat extraction at the droplet surface. The extent of "rapid solidification", as determined from the fraction of material solidified above a certain critical undercooling, is a function of the nucleation temperature, the particle size, a kinetic parameter and the heat transfer coefficient. Significant departures from the Newtonian model were calculated for Biot numbers as low as 0.0001. |
