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The University of Southampton

Research project: In-situ characterization of microstructure and fatigue performance of Al-Si piston alloys

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Light vehicle engine pistons are commonly manufactured from Al-Si alloys of near eutectic composition because of these alloys' excellent castability and high strength to weight ratio, amongst other desirable properties. Elements such as Cu, Ni and Mg are also added in differing quantities usually to improve the high temperature performance of pistons whose operating temperatures are currently upwards of 400oC for some light vehicle diesel engines. Depending on composition and process conditions, piston alloys have defects such as porosity and oxide films as well as an array of typical microstructure features such as eutectic and primary Si and various intermetallics such as Fe-, Ni- and Cu- bearing phases. The amount, size, shape and spatial distribution of these phases significantly influences their mechanical performance.

Al-Si in 3D
Al-Si in 3D

This study has used optical microscopy, SEM and X-ray tomography to assess the 2D and 3D distribution of porosity, oxides and microstructure features of selected model Al-Si piston alloys with varying composition and processing. This has been linked to their creep and fatigue performance at different environments, temperatures and frequency.

Cracks in Al-Si
Cracks in Al-Si

In particular, the initiation, short crack growth and long crack growth behaviour of these materials has been investigated using conventional methods and also via in-situ x-ray tomography techniques to observe closure and failure ahead of the crack. The 3D microstructure data from x-ray tomography has been be used to investigate load transfer in the microstructure and to assess how this affects crack initiation and growth.

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