Abstract
Industrial in-series aluminum castings contain a wide range of microstructural heterogeneities like differences in secondary dendrite arm spacing (SDAS), eutectic silicon and intermetallic precipitates of varying morphologies and diverse-shaped and-sized porosity. Regarding to technical and economic limitations, the complete elimination of them is hard to achieve, which requires conservative design, i.e., increased wall thicknesses to accommodate the failure tolerance. To improve the performance of cast aluminum products concerning safety and fatigue properties, the present work deals with the significance of such structures with respect to the threshold for crack propagation ΔKI,th under pure bending and the fatigue behaviour in the high-and very-high-cycle-fatigue regime (HCF and VHCF). Therefore, two automotive cast alloys taken from engine blocks (AlSi8Cu3) and cylinder heads (AlSi7Cu0.5Mg) and a gravity die cast set (AlSi7Mg0.3), either T6 conditioned or additionally hot isostatic pressed (HIP), were used. For in-series castings, two positions of maximal difference in cooling rate and respective microstructure were extracted. With this set of specimens, the significance of SDAS in interaction with (i) eutectic silicon regions, (ii) intermetallic precipitates in varying occurrence, (iii) the crystallographic orientation, and (iv) the porosity in correlation with the fatigue threshold is shown and compared with first results of fatigue damaging mechanisms in quasi pore-free material.
Highlights
Cast aluminum alloys are frequently used materials in the automotive sector if a good strength to weight ratio is required
The crystallographic orientation in interaction with the morphology and size of the eutectic was found to have an appreciable influence on the crack-propagation rate for low stress intensity factor range (SIF) ranges near the threshold ∆KI,th
Lower secondary dendrite arm spacing (SDAS), fine and only line-shaped eutectic regions between secondary dendrite arms and a higher amount of porosity fraction lead to lower threshold values ∆KI,th for technical fatigue crack propagation
Summary
Cast aluminum alloys are frequently used materials in the automotive sector if a good strength to weight ratio is required. When considering the extreme complex casting geometries of, e.g., the highly-loaded drive train, engine blocks or cylinder heads, with wide ranges in wall thickness, defect-free castings are technically hard to realize and a compensation by respective conservative design guidelines is required. Since these design guidelines are correlated with high safety factors the full potential of aluminum for lightweight and downsizing concepts cannot be used. This was realized by in-situ high-resolution thermography observation of a specimen equipped with a shallownotch during fatigue tests
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