Microstructural Softening of Eutectic and Hyper-eutectic Al-Si-X Continuous Casts by Annealing

Abstract

Softening behavior of eutectic and hyper-eutectic Al-Si-Cu-Mg-(Ni, Fe, Mn) cast billets produced by the new vertical semi-continuous casting process using an adiabatic and rapid cooling mold was examined. Ductile fracture manner fully covered on fracture surface of the specimens fatigued at 523 K, and was responsible for the low fatigue strength. Hyper-eutectic casts containing coarse Si crystals exhibited higher hardness than eutectic ones. However the hyper-eutectic alloys remarkably showed softening behavior above 523 K. The additions of Fe and Mn were effective to increase hardness of the Al-Si-Cu-Mg alloys at high temperature. Coarsening of Al-Cu-Mg compounds and precipitation of acicular compounds enriched Mg-Si-Cu were detected in the matrix of the hyper-eutectic alloy annealed at 523 K, and those may result in the softening behavior.