BANDING PHENOMENA IN AL-FE ALLOYS SUBJECTED TO LASER SURFACE-TREATMENT

Using beam rates of between 0.005 and 18.0 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 microscopy. Two different banded structures were observed: one at slow scanning rates (low-velocity bands) and another at high growth rates (high-velocity bands). The low-velocity bands were shown to depend essentially upon conditions prevailing at the surface, and were attributed to convection (Marangoni) effects. For all of the alloys there was a critical growth rate, at which the cellular-dendritic structure was replaced by a high-velocity banded structure which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. The structure of the dark bands was similar to that of the cellular-dendritic structure prior to the onset of the banded structure. Diffraction patterns from the light bands exhibited spots only of the f.c.c. alpha-Al solid solution and microanalyses showed that, within a light band, the concentration was uniform and equal to the nominal concentration of the alloy. With increasing growth rate, the widths of the dark bands decreased continuously and, since the overall spacing of the bands remained constant, this finally led to the disappearance of the dark bands. A completely precipitation-free structure was then observed at very high growth rates in the more dilute alloys. A phenomenological model, based upon periodic instabilities of the growth rate, was proposed in order to explain the origin of the high-velocity banded structure.