Microstructure Evolution in Powder Metallurgy Al-Si-Cu Foam during Foaming Process

Abstract:

The microstructure evolution in Al-Si-Cu foam during foaming process was studied using the scanning electron microscope. Through mixing the powder to the composition of Al-6Si-5Cu-2TiH₂-1.5MnO₂ wt%, and using the hot pressure process at 500℃, the Al foam could be foamed homogeneously at 640℃ for 12-20min. The corresponding microstructure analysis showed that the sample before foaming contained spherical Si, Al₂Cu, lump-shaped TiH₂ and flat MnO₂ phase. After foaming, the sample showed the millimeter-scale main hole with the sparse micrometer-scale pores inside the hole wall. The precipitates in hole wall were intragrain small Si and Al₂Cu particles, and grain boundary eutectic Si and Al₂Cu phases, as well as randomly distributed lump-like Al-Mn phase, MnO₂ phase and spherical Ti-rich phase. Therein, the Ti-rich phase, formed by the reaction of liquid Al and TiH₂ after release H₂, exhibited two types of morphology. The first one consisted of a Ti core, a thin layer of τ₂(Al₁₄Ti₃₃Si₅₃) inner shell, and the mixed dendrite-like τ₁(Al₂₀Ti₃₂Si₄₈) and τ₂ phases outer shell. The second type of morphology consisted of Ti core, and a mixed lump-like τ₁and (Al,Si)₃Ti shell. The composition of Si in the first type was much higher than that of the second type. That was mainly because the Si composition in liquid Al was not uniform due to the inheritance of the original microstructure of solid Al and Si, and the two types of Ti-rich phase formed in the Si rich and Si poor regions. The microstructure analysis of foaming process could be the foundation for designing the aging strengthening aluminum foam.