The equiaxed-banded microstructural transition during low pressure plasma spraying

The microstructure of low pressure plasma spray deposited Al-12%Si has been investigated as a function of temperature during spray deposition. concentrating on deposit homogeneity, porosity, microstructure and microhardness. The deposit microstructure was determined by the temperature during spray deposition. Low temperatures promoted inhomogeneous banded microstructures, incorporating partially solidified and unmelted particles, with high levels of intersplat porosity, extended Si solubility in the alpha-Al matrix, amorphous and nano-sized Si precipitates and relatively high microhardness. High temperatures promoted homogeneous equiaxed micro structures. with low porosity levels, low Si solubility in the alpha-Al matrix, micro-sized Si precipitates, and relatively low microhardness. The measured critical transition temperature was in the range 190-345 degreesC, in reasonable agreement with Cantor et al.'s intermixine model of microstructure formation during spray deposition, which predicted a critical transition temperature of 328 degreesC. The corresponding critical intersplat time was estimated to be 1.4 x 10(-4) s, indicating intermediate cooling conditions for splatting droplets arriving at the deposit surface. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.