Application of Color Metallography on As-cast Al-Mg-Si-Cu-Mn Alloy during Heat Treatment

Abstract

Color etching was performed to characterize microstructure evolution and phase transformation on as-cast and homogenized Al-Mg-Si-Cu-Mn alloys. Initial intermetallic phases in microstructure of as-cast Al-Mg-Si-Cu-Mn alloy are mainly composed of primary Mg2Si, α-Al(FeMn)Si and quaternary Q phase. Post-etching microstructure of as-cast alloy presents a typical dendritic morphology with distinctive intermetallic distributions. Micro-segregation of solute elements are visualized as color difference within grains. Q phase is mainly located between the dendrite arms, while Mg2Si and α-Al(FeMn)Si are arranged along the grain boundaries. Heterogeneous nucleation sites provided by Al3Ti during solidification are also observed and identified at the core of grains. The distribution of alloying elements and intermetallic exhibited by color micrograph presents a good agreement with outcomes of EPMA. The color metallography of etched sample reveals the distribution of α-Al(MnCr)Si dispersoids and dispersoids free zone (DFZ), after the alloy is subjected to two-step homogenization. Micro-segregation of solute elements within solid solution is dramatically eliminated, associated with reduction of color difference. The dissolution of primary Mg2Si and Q phase during two-step homogenization are also directly detected. Therefore, color etching is an effective and reliable auxiliary approach to reveal microstructural evolution and phase transformation of as-cast Al-Mg-Si-Cu-Mn alloy during homogenization process.