Excimer laser surface modification of high strength aluminium alloys

Abstract

An excimer laser (KrF) operating at a wavelength of 248nm was used to modify the surface microstructure of aluminium alloys 6013-T65l and 7075-T651. The aim was to improve both the corrosion resistance and the corrosion fatigue resistance of these alloys by means of laser surface melting (LSM). Laser surface treatment was performed under two different gas environments, air and nitrogen. The microstructure, phase and composition of the modified surface structure were analysed using TEM, XPS and EDX, and the electrochemical and the intergranular corrosion cracking behaviours of the untreated and the laser-treated specimens were evaluated by electrochemical polarization test and immersion test respectively. The corrosion fatigue properties of the 7075 alloy were studied by using a rotating bending fatigue machine. A microscopical examination and the transmission electron microscopy (TEM) study revealed that LSM caused a reduction both in the number and the size of constituent particles and a refinement of the grain structure within the laser melted zone. As a result, the corrosion resistance of the aluminium alloy was improved. Both of the results of the potentiodynamic anodic polarization test and the immersion test have shown that after LSM, the corrosion resistance of the two aluminium alloys has been significantly improved. For aluminium 6013, the highest improvement in intergranular corrosion cracking resistance was obtained for the N2-treated condition where a reduction in corrosion current density of three orders of magnitude was obtained as compared to the untreated condition. In general, the improvement is considered to be primarily due to the reduction of large harmful second phase particles and the absence of PFZ and Cu-bearing precipitates at the grain boundaries. The superior corrosion resistance of the N2-treated material is attributed to the presence of the chemically stable AIN phase in the surface. For A1-7075 alloy, a six-fold reduction in corrosion current was obtained for the air-treated specimen while comparing to the untreated specimen; this was accompanied by a significant reduction of corrosion pitting sites. The fatigue test results showed that under dry fatigue conditions, the total fatigue life of the laser treated specimens, in which the crack initiation period is of considerable significance, was lower than that of the untreated specimens. However, after shot peening , the fatigue lives of the laser treated specimens were recovered. This was primarily attributed to the elimination of surface defects, but could also be in part, due to the surface hardening effect. The fatigue resistance of the shot peered laser-treated specimens, tested in 3.5wt% NaCl solution with 48hrs prior immersion, was greater than the untreated specimens with an increase of two orders of magnitude in fatigue life. This was primarily due to the elimination of surface defects and the reduction of corrosion pits.