Image icon  Cross-section of an aluminium casting

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Description: Solidification initiates on the mould walls because this is where the melt is coolest and because inhomogeneities promote nucleation. Selective growth then occurs, whereby the grains which have their easy growth direction <100> parallel to the heat flow direction grow fastest, and cut off the paths of less well-aligned grains. This results in a 'columnar zone' which may extend to the centre of the casting but often gives way to an 'equiaxed zone'. The equiaxed grains may result from sedimentation from the free surface, from the detachment of dendrite tips, or simply from heterogeneous nucleation ahead of the advancing front (though the latter is only likely if deliberate 'melt inoculation' has been implemented).
Keywords: alloy • aluminium • cast • casting • chill zone • columnar • equiaxed • metal
Categories: Science approaches > Microstructure
Materials > Metals & alloys > Aluminium & alloys
Processes > Shaping > Casting
Testing, analysis & experimentation > Metallography
Scale > Micro
Created by: Prof T W Clyne, Department of Materials Science and Metallurgy, University of Cambridge
DoITPoMS, University of Cambridge
Published by: DoITPoMS, University of Cambridge
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Date created: 03 October 2002
Date added: 21 August 2009
Resource ID: 1648