Fatigue behavior of A356/357 aluminum cast alloys. Part II – Effect of microstructural constituents

A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (SDAS), eutectic structure, matrix strength, and fatigue behavior of two Al–7% Si–Mg casting alloys with magnesium contents of 0.4% and 0.7%, respectively, has been conducted. In the absence of casting defects, the influence of microstructural features on the fatigue performance becomes more pronounced. The degree and rate of microdamage (microcracking) is strongly affected by the strength of the matrix, and especially by the eutectic particle size, morphology, and distribution (clustering). A soft matrix (under-aged alloys) will generate more local microdamage compared to a peak-aged one. Large and elongated eutectic particles present in unmodified alloys result in lower fatigue lives. A decrease of fatigue life with increasing Mg and Fe content is observed, mainly due to the increased sizes of Fe-rich intermetallic particles. Microstructures with similar eutectic particle size and morphology as the Sr-modified ones, show a minimum fatigue life at intermediate SDAS values (∼60 μm), which is related to the continuity of particles on the dendrite cell boundaries. For coarser microstructures (SDAS>60μm), the increase in fatigue life is attributed to the reduced damage rate along the cell boundaries.