Abstract
In this paper, the hot tearing behaviour in Al-(4.8?6)%Cu sand mould castings was preliminary investigated by means of microstructural examination and image analysis. A dog-bone pattern was employed for the realisation of the castings and three Al-Cu alloys with different Cu and Si contents were used. The effects of the alloy composition and of different pouring temperatures on the hot tearing behaviour of the castings were evaluated. The quantity of the eutectic phase available during solidification is considered a very important parameter for the crack healing phenomenon, in fact the eutectic liquid flows into the hot tear areas and covers parts of the cracks. The hot tear paths and surfaces were observed by means of optical and scanning electron microscopes, which showed that the fracture surfaces were dominated by bridged grain boundaries and the presence of a liquid film, in particular at higher copper concentrations. Several samples were also drawn from the zones characterised by the maximum cross-section variation and the micrographs from the optical microscope were statistically analysed by means of commercially available image analysis software. The quantitative microstructural parameters of percentage, mean area and distribution of the eutectic phase were evaluated and correlated to the capacity of the eutectic liquid to heal open fractures caused by hot tearing for the examined alloys.
Highlights
I n foundry practice the occurrence of solidification defects limits the production and performance of cast components
Novikov and Novik [8,9] reported that the grain boundary sliding is the main mechanism of deformation of a semi-solid body at low strain rates, while Pellini [10] suggested a hot tearing theory based on the strain accumulation in which the most important factor is the total strain in the hot spot region
A liquid penetrant inspection has been performed on the castings to detect surface cracks and several specimens have been drawn from significative zones of the castings for metallographic analysis, carried out by optical microscopy (OM) and scanning electron microscopy (SEM)
Summary
I n foundry practice the occurrence of solidification defects limits the production and performance of cast components. Alloys gradually transform from a liquid to a solid state over a certain temperature interval; the alloy consists of both a liquid and solid for a considerable period of time during casting. The test used for the study of copper alloys that was performed by Couture and Edwards is of greater interest [20,21] and is the one carried out by Rosenberg et al [22], who designed a mould for the realisation of castings composed of bars of different length produced from one single pouring. All data have been collected and plotted in order to understand the possible correlation between the microstructure and the tendency of the sand mould castings, realised with the examined alloys, to a complete crack caused by the hot tearing phenomenon
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