Laser surface melting of aluminium alloy 6013 for improving stress corrosion and corrosion fatigue resistance

Abstract

Laser surface treatment of aluminium alloy 6013, a relatively new high strength aluminium alloy, was conducted with the aim of improving the alloy's resistance to stress corrosion cracking and corrosion fatigue. In the first phase of this research, laser surface melting (LSM) of the alloy was conducted using an excimer laser. The microstructural changes induced by the laser treatment were studied in detail and characterised. The results showed that excimer LSM produced a relatively thin, non-dentritic planar re-melted layer which is largely free of coarse constituent particles and precipitates. The planar growth phenomenon was explained using the high velocity and high temperature gradient absolute stability criteria. The structure of the oxide and/or the nitride bearing film at the outmost surface of the re-melted layer was also characterised. The results of the electrochemical tests showed that the pitting corrosion resistance of the alloy could be greatly increased by excimer laser melting, especially when the alloy was treated in nitrogen gas: the corrosion current density of the N2-treated specimen was some two orders of magnitude lower than that of the air-treated specimen which was one order of magnitude lower than that of the untreated specimen. The effect of the outer surface oxide and/or nitride bearing film per se on pitting corrosion resistance was determined. The results of a Mott-Schottky analysis strongly suggest that the outer surface film, which exhibited the nature of an n-type semiconductor was responsible for the significant improvement of the corrosion resistance of the laser-treated material. Furthermore, the corrosion response of the surface film was modelled using equivalent circuits. Based on the results of the slow strain rate tensile (SSRT) and corrosion fatigue tests, the stress corrosion cracking and pitting corrosion fatigue behaviour of the excimer laser treated material was evaluated. The results of the SSRT test showed that, in terms of percentage elongation and corrosion current density, the resistance to stress corrosion cracking improved after excimer laser treatment. While the corrosion fatigue results showed that the total fatigue life of the alloy increased noticeably after laser surface treatment. However, a higher overall fatigue crack propagation rate occurred in the laser-treated material. The effect of post-treatment ageing and residual stress on fatigue strength was also investigated; the results showed that they did not have a significant influence. Recognising that the depth of the excimer laser melted layer was very shallow, typically in the range of a few micrometers, this when compared to the melt depth in the hundred micrometer range for Nd-YAG laser treatment may disadvantage excimer laser treatment. As a consequence, a 2kW high power Nd:YAG laser was employed for the LSM experiment during the second phase of this research. It was found that a totally different re-melt microstructure to that of excimer laser melting resulted from YAG laser melting: the depth of the melt pool was in the range of a few hundreds micrometers, and instead of the planar growth morphology that occurred in excimer laser melting, a fine dendritic/cellular structure was produced. Although, an A1N film was produced at the outer surface of the re-melt layer which was similar to that of excimer laser melting, the YAG laser formed film was much thicker with larger crystallines of the nitride phase. With regard to pitting corrosion resistance, although improvement was obtained after YAG laser melting, the improvement was not as significant as that which can be achieved by excimer laser melting. This is mainly because the second phase particles at the dendrite boundaries behaved as cathodic sites while the dendrite cells acted as anodic sites. The results of the equivalent circuit modelling showed that the polarisation resistance of the YAG laser formed surface film was one order of magnitude lower than that of the excimer laser formed surface film. When comparing the stress corrosion cracking resistance of the excimer laser and YAG laser treated specimens in terms of elongation reduction, the latter was found to be superior. This is considered to be caused by the tortuous fracture path which occurred in the YAG laser re-melted layer. This has the advantage of reducing the effective stress intensity at the crack front, and thus delays the fracture process. However, it must be recognised that a drop in tensile strength occurred after YAG laser treatment. This drop is primarily considered to be the result of the formation of a relatively thick and soft re-melted and heat affected layer. Considering the pitting corrosion fatigue, though, similar to the case of excimer laser melting, the initiation of fatigue cracks can be effectively retarded after YAG laser treatment, the mode of crack propagation was different for the two situations. Crack propagation within the YAG laser re-melt layer primarily followed interdendritic boundaries, and resulted in a rough fracture surface. A tortuous interdendritic crack path may result in a relatively low fatigue growth rate. By contrast, a relatively flat fracture surface was produced when excimer laser treatment was used. Finally, it was found that the electrochemical noise method is a promising means for in-situ monitoring of pitting corrosion fatigue damage of the laser-melted layer both for excimer and Nd:YAG laser treatments. The results showed that fatigue crack initiation and propagation phenomena could be correlated to the patterns of electrochemical potential and current noise.