Color Metallography of A356 Aluminum Alloy Castings using Week's Reagent

摘要

A color etching method using Week's reagent (3.81mol%KMnO_4, 0.15mol%NaOH water solution) was developed for both the optical microscopy and micro-segregation characterization of A356 aluminum alloy (Al-7wt%Si-0.35wt%Mg) castings. The coloring mechanism of Week's reagent was also elucidated. In this study, colored microstructures of A3 56 alloy (with 0.1 wt% Ti added) were compared with the solute distributions measured by EPMA analyses to make clear the relationship between color and segregation. It was also evidenced that the tracking the distribution of Ti in Al phase during the spheroidization of Al phase at semi-solid state made possible. These examples prove that color etching with Week's reagent is an efficient way of characterizing micro-segregations in Al alloy castings. The accurate evaluation of solid fraction of A356 alloy slurry using Week's reagent was also achieved. With the help of Week's reagent, the grain growth happened during rapid cooling of the alloy from semi-solid state to room temperature was clearly visualized. Thus the overestimation of solid fraction by conventional method was avoided by excluding the area of grain growth. Additionally, the spheroidization process of dendritic Al phase was observed in detail using Week's reagent and EBSD analysis and the spheroidization mechanism was provided. Furthermore, the etched specimen surface was characterized by various observation and analysis methods to study the coloring mechanism of Week's reagent.