Grain structure and homogeneity of pulsed laser treated surfaces on Al-aerospace alloys and FSWs

The effect of pulsed excimer laser surface treatment, on typical Al-aerospace alloys and friction stir weld substrates, has been studied. A thin <10 μm deep surface layer can be achieved, where the majority of the re-solidified material does not exhibit phase separation, and has a higher level of homogeneity relative to the substrate material. However, due to the extremely short melt times, the layer is not as homogeneous as has previously been claimed and melted constituent particles leave solute rich trails throughout the layer. The treated layer also contains a series of second phase bands that are the result of a transient region, caused by the growth front from each pulse initially accelerating through speeds below ultimate stability, combined with the effects of composition variations within the liquid from the solute trails. The layer was further found to develop a grain structure that was very sensitive to that of the substrate and the presence of Al3Zr β′ dispersoids within the treated alloy, which can alter the grain structure from coarse epitaxial, to very fine columnar.