Evolution of interaction domain microstructure during spray deposition

Abstract

An interaction domain, defined in the present article as the region where semisolid, atomized droplets impinge and are collected during spray atomization and deposition, was systematically investigated on the basis of a semisolid metal-forming mechanism. Accordingly, microstructural evolution in the interaction domain was rationalized by quantitative analyses of (1) the solid fraction of semisolid metal, (2) the extent of deformation and deformation strain rate, and (3) the solidification cooling rate. The results demonstrate that the fraction of solid in the interaction domain ranges from 0.5 to 0.8 for the materials studied here: Ni3Al, Al-6 wt pct Si, and Al-6 wt pct Fe. Moreover, the results show that the semisolid material in the interaction domain experiences a severe deformation during deposition with an associated strain rate of up to 106 s-1. As a result of this deformation; the solidification structure is modified, and, in particular, any dendritic structure that is present will undergo extensive fragmentation. The severe deformation that is experienced by the interaction domain and the presence of a solidification cooling rate that is on the order of 10 to 105 Ks-1 were proposed to be critical factors that promote the formation of a spheroidal grain morphology during spray atomization and deposition. Experimental support to this suggestion was provided by microstructural observations on Ni3Al, Al-6Si, and Al-6Fe. In particular, the morphological modification of the primary Si phase that is observed in spray-atomized and spray-deposited Al-6Si was rationalized on the basis of these factors.