Visualization of Micro-Segregations in A356 Aluminum Alloy by a Color Etching Method

Abstract

An A356 aluminum alloy billet which has a dendritic microstructure was compressed and then partially re-melted to semi-solid state before water quenching, by which the spheroidization of Al grains was realized. A color etchant called Weck's reagent was used to characterize both the dendritic and spheroidal microstructure. In both cases, distinct color differences were observed inside Al grains by normal optical microscopy. Interestingly, a dendritic-shaped structure inside the spheroidal Al grains was visualized, which should be the reflection of the original dendrite before heating and partial re-melting. Also, the grain growth during water quenching could be clearly visualized after etching with this reagent. As a result, solid fractions could be evaluated more precisely by excluding the grain growth when measuring the area of solid phase in 2-D micrographs. In order to investigate the coloring mechanism, electron probe micro-analyses were carried out to characterize the micro-segregations inside an Al grain. Results showed that the micro-segregation of Ti had a strong correlation with the color difference. Detailed investigation found that the micro-segregation of Ti could be preserved after heating and partial re-melting due to the extremely low diffusion rate of Ti in Al.