In-situ investigation of a heat-treated AlSi7Cu3Mg cast alloy: influence of solidification rate on the cyclic fatigue behaviour

The study investigates the influence of the solidification rate on both crack initiation and propagation on a heat-treated AlSi7Cu3Mg cast alloy under cyclic loading conditions. Three different cooling speeds were applied to the alloy in a Bridgman furnace to enhance controlled fine to coarse microstructures, and the same post-solidification heat treatment parameters were adopted. Optical microscopy, scanning electron microscopy and energy dispersive x-ray analyses were performed for the microstructural characterisation, which also concerned quantitative analyses on Si particles to deepen the effect of the solidification rates on their geometrical features. In-situ fatigue tests in a scanning electron microscope and electron backscattered diffraction analyses were carried out to study the two-dimensional fatigue behaviour of the alloy depending on the coarseness of the microstructural features. Focused ion beam slices and three-dimensional tomography were performed ahead of the crack tips, revealing micro-cracks closer to the intermetallic particles with the decreasing of the microstructural coarseness. EBSD analyses also supported results correlating the effect of different solidification rates on the transgranular/intergranular propagation paths. In contrast, tensile and microhardness tests were also conducted to better understand the mechanical properties of the alloy. The results showed that the solidification rate and the heat treatment have a synergic effect in controlling the fatigue crack initiation and the subsequent propagation path. Specifically, higher solidification rates lead to a finer microstructure with smaller, rounder Si particles, increased hardness of the α-Al matrix, and concentrated fatigue crack initiation and propagation in eutectic areas. This contrasts with lower solidification rates, where cracks are more likely to initiate and propagate in the softer α-Al matrix or near larger Si and intermetallic particles.