THE MICROSTRUCTURE OF RAPIDLY SOLIDIFIED AL-FE ALLOYS SUBJECTED TO LASER SURFACE-TREATMENT

Using traverse rates of between 0.1 and 6.0 m/s, laser-melted tracks were produced on Al-Fe alloy samples containing between 0.25 and 8.0 wt% Fe. The local solidification rates were measured by taking a longitudinal section through the centre of the laser trace and the corresponding microstructures were studied quantitatively using transmission electron miscroscopy. Three types of miscrostructures were observed. At low growth rates, cellular/dendritic structures were obtained. At high growth rates, a banded structure was formed which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. At the lower concentrations, precipitate-free structures were obtained at very high solidification rates. A recent model, for dendritic growth under rapid solidification conditions, was compared with the experimental results and a good correlation was found. It was shown that, at rates close to the limit of absolute stability, steady-state planar-front growth was not the preferred growth morphology; but rather a banded structure. It was only at much higher rates that a fully precipitation-free structure, probably involving plane-front growth, developed.