Evolution of microstructure in spray formed Al–18%Si alloy

Abstract

Spray atomization and deposition process has emerged as an alternative to ingot and powder metallurgy routes. In the present investigation, we study the microstructural control during spray deposition of hypereutectic Al–Si alloy, employing different nozzle to substrate distances. Spray deposition is carried out using convergent–divergent close-coupled nozzle design at deposition distances of 200, 300, 450, and 550 mm. Microstructural characterization of oversprayed powders as well as spray formed deposits was performed. Microstructural features obtained at smaller deposition distance consist of co-existing primary Si phase and needle like eutectic Si. Dendrites of α-Al phase are observed indicating a large undercooling of the liquid pool prior to solidification. A large number of pre-solidified particles with very fine microstructure, embedded in a relatively coarse region, typically characterize those evolved at large deposition distances. However, at intermediate deposition distances, uniform and refined primary Si phases, 3–8 μm size, are observed. An undercooling effect is manifested in both the spray deposits as well as powder particles. These microstructural features have been discussed in light of a proposed model, which describes the presence of two layers (1) solidification layer and (2) interaction layer in the liquid pool.