3D primary grain shapes resulting from semi-solid metal processing

Abstract

The issue regarding globular grain shape and size has been a topic for semi-solid processing since the discovery of the technique. Semi-solid rheo-processing takes advantage of cooling a liquid metal alloy to the solid + liquid phase field, while it is subjected to some form of turbulence during cooling. It has been suggested that the grains grow into spherical globules by observations of 2D microstructures. It is also known that reducing the melt super-heat has the effect of reducing the globule (grain) size. Are these 2D globules also spherical in shape in 3D or are these 2D shapes remnants of the 3D shapes after sectioning along planes? An Al-Si-Mg alloy is semi-solid processed using a patented processing coil that induces contactless stirring while simultaneously being cooled. Primary aluminium grains are extracted by an etch technique from a sample volume of the casting. The grain size distribution and shapes are analysed. The 3D particles are pictured with scanning electron microscopy. Various interesting particle shapes are observed, from simple to complex. The particle geometry in 3D is compared to 2D optical light microscopy micrographs. It is found that the 2D globules are remnant shapes from the 3D particles after sectioning along random planes.