Peter Skeldon

Microstructure Analysis and Corrosion Study of Excimer Laser Modified AA2024-T351

Laser surface melting (LSM) of aluminum alloys with high power continuous wave CO2 and Nd:YAG lasers (CW lasers) has been shown to produce dendritic/cellular microstructures with refined second phase particles distributed along the dendritic boundaries (1-5). Although refinement of the microstructure and extension of the solute solubility in the α-Al matrix can be achieved, the refined second phase particles still act as preferential sites to initiate localized corrosion (2, 3). In contrast to a CW laser, an excimer laser with a UV wavelength and pulse width in the range of nanoseconds, resulting in extremely high cooling rate, is expected to generate a further refining of the near-surface microstructure and thus, improved corrosion performance. The excimer LSM treatment has been reported to dissolve intermetallic particles within the melted layer, thus limiting compositional variations in the layer (6, 7). In this project, a Lumonics IPEX 848 KrF excimer laser, wavelength of 248 nm and pulse width of 13 ns, has been used for surface melting of an AA2024-T351 alloy. The aim is to investigate in detail the microstructure and the resultant corrosion behavior of the laser treated surface. Microstructure characterization and composition analysis have been performed by FEG-SEM/EDS, TEM/EDS and XRD to disclose solidification phenomena and phase transformations. The corrosion behavior has been evaluated by EXCO testing (ASTM G34-01). Figure 1 shows a SEM back-scattered image of a cross section of the LSM layer on the AA2024 alloy, revealing a melt layer of thickness ∼5μm. The layer composition appears homogeneous relative to that of the bulk alloy. The relatively fine dispersoids of the originally alloy matrix have been dissolved, while the large intermetallic particles at the interface between the matrix and the laser-treated layer have been partially melted. The solute rich liquid from melted particles was not fully dispersed in the layer (arrowed in Fig.1), possibly due to insufficient time for diffusion during excimer laser processing. However, large intermetallic particles were consistently absent from the melted layer, which was further confirmed by XRD results (results not shown). Figure 2 shows a transmission electron micrograph of the interface between the matrix and the laser-melted layer. Cu-Mn-containing particles are observed in the interface region. Notably, a copper rich layer of thickness ∼15 nm is present there as well (arrowed in Fig. 2), with the enhancement of copper evident in EDS elemental line profiles (results not shown). EXCO immersion testing clearly shows that the untreated samples exhibited severe pitting corrosion and intergranular corrosion, while considerably reduced corrosion was observed at the surface of the laser treated specimen. However, delamination of an intact laser melted layer from the matrix was evident after immersion for 6 hours (Fig. 3). Further work is in progress to understand the delamination behavior; the absence of significant corrosion product may suggest a stress-related mechanism. Acknowledgements The authors are grateful to EPSRC for support of this work, which was funded through a Portfolio Award for Light Alloys for Environmentally Sustainable Transport.