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Four-point bending fatigue tests were performed on semi-solid Al–7Si–Mg castings with varying magnesium contents and heat treatment conditions. Additionally, the effect of anodising on the fatigue resistance of semi-solid Al–7Si–Mg castings was evaluated. Fracture surface and microstructure analysis showed that fatigue crack initiation occurred mainly at the periphery of regions of positive macrosegregation at the casting surface, resulting most likely from exudation. The microstructure of these macrosegregation regions was mostly eutectic and was frequently found surrounded by a layer of oxides. This layer of oxides promoted weak bonding between the macrosegregation region and the surrounding material and acted as a crack initiation site. In this study, primary α-Al globule agglomerates at the casting surface and surrounded by a layer of oxides also promoted fatigue crack initiation. Fatigue resistance of semi-solid Al–7Si–Mg castings in the T5 and T6 conditions increased with the increase in the magnesium content of the alloy from 0.3 to 0.45 wt.% due to the higher precipitation hardening response. However, the increase in the magnesium content from 0.45 to 0.6 wt.% resulted in a slight decrease in the fatigue resistance. The oxide layer formed during anodising had no significant effect on the fatigue resistance of the semi-solid Al–7Si–Mg castings in this study due to the dominant effect of the macrosegregation regions on fatigue crack initiation. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the effect of Si-content and microstructure on mechanical properties of Al-Si-Mg alloys with the purpose of to develop high-strength cast aluminum alloys that are anodizable. The Si-content was 2.5, 3.5 and 5.5wt% Si, and both Sr-modified and unmodified conditions were used. The samples were produced using the gradient solidification furnace to generate awell-controlled microstructure. The resulting secondary dendrite armspacingwas 10 and 20μm. Themicrostructural features were evaluated by employing SEM/EDS and optical microscopy. Furthermore, computed tomography (CT) scan technology was used to provide a 3D view of high-density phases in the microstructure. The mechanical properties of these alloys were studied by means of tensile and hardness testing where the latter was performed on the macrolevel and microlevel. The results demonstrate clearly how the Si growth in the microstructure is restricted by increased cooling rate and modification and its role in strength development in Al-Si alloys. Additionally, the CT scan visualized the morphology of intermetallics and supported in identifying the oxide layer growth as a result of the anodizing process. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the mechanisms of formation and growth of the anodised surface layer on Al-Si castings by applying different analytical techniques such as optical microscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and X-ray computer tomography (X-ray CT) scanning. Three different Al alloys with various Si content (2.43%, 3.53% and 5.45%) were investigated. Si particle morphological modification by Sr addition, as well as directional solidification, was used to vary the microstructural coarseness in a controlled manner to study the influence of these parameters on the growth behaviour of the oxide layer. This study observed residual unanodised Al phases trapped beneath or between Si particles in the oxide layer. It was found, depending on the geometry and morphology of Si particles, that Al can be shielded by Si particles and prevented from oxidising. [ABSTRACT FROM AUTHOR]