Solidification and microstructure evolution during spray atomization and deposition of Ni 3Al

Abstract

The synergism between solidification phenomena and microstructural evolution in spray atomization and deposition was systematically analyzed on the basis of numerical and experimental studies. Accordingly, an integral model, whose formulation was based on fundamental principles of fluid mechanics, heat transfer and solidification, was used to describe the evolution of microstructure during spray atomization and deposition. To keep the problem tractable, the process was divided into two distinct but closely related stages: (a) atomization; and (b) deposition. In the atomization stage, the solidification microstructure was noted to be intimately coupled to the transfer of thermal energy from the droplets to the atomization gas, and the degree of undercooling experienced by the droplets. The heterogeneous nucleation behavior of the droplets was analyzed and a heterogeneous undercooling factor was defined as ƒ h = (1 + Av d ) −1 . In the deposition stage, the various microstructural features present in the spray-deposited material were rationalized on the basis of thermal energy considerations. In particular, the formation of spheroidal grains was proposed to evolve from two distinct mechanisms: (i) the homogenization and coalescence of deformed or fractured dendrite fragments. Support for this suggestion was provided by experimental results and numerical analyses that show that the microstructure is exposed to a high temperature anneal during deposition. Experimental measurement of the temperature demonstrated that the temperature during deposition remained below the solidus temperature, with an average cooling rate of approximately 2.6 K s −1.