T. M. Yue

Excimer laser surface treatment of Ti-6Al-4V alloy for corrosion resistance enhancement

Abstract Excimer laser surface treatment of Ti–6Al–4V alloy was conducted with the aim of improving the pitting corrosion resistance of the alloy. Laser surface treatment was performed under two different gas environments, argon and nitrogen. The microstructure, phase and composition of the modified surface structure were analysed using TEM, XRD and EDX; the electrochemical behaviours of the untreated and the laser-treated specimens were evaluated by electrochemical polarization tests. Excimer laser surface treatment significantly increased the pitting potential of the Ti alloy, especially when the material was treated in argon gas, while a seven-fold reduction in corrosion current was obtained when the material was treated in nitrogen gas. These improvements are considered to be primarily due to the reduction of solute partitioning effect of detrimental Al segregated to the α phase. The N 2 -treated specimen, although it had the lowest corrosion current, had a pitting potential some 100 mV lower than that of the Ar-treated specimen. This is considered to be due to TiN precipitates acting as galvanic cathodes at high corrosion potentials.

Excimer laser surface treatment of aluminum alloy AA7075 to improve corrosion resistance

Abstract Excimer laser surface treatment was found to be an effective method for improving the pitting corrosion resistance of the aluminum alloy 7075. The results of the TEM study showed that laser surface melting of the alloy at an intensity of 10.3 J/cm2 resulted in the elimination of coarse second-phase particles in the laser-melted zone. More importantly, two compact layers containing aluminum oxide were formed on top of the laser-melted surface. Potentiodynamic polarization tests showed that as a result of the laser treatment, the corrosion current can be reduced by as much as six times, and a passive region was obtained. Besides, the analysis of the electrochemical impedance measurements showed that at an open-circuit potential (OCP), the polarization resistance and double-layer capacitance of the film/electrolyte interface of the laser-treated specimen were one order of magnitude higher and six times lower than that of the untreated specimen, respectively. Furthermore, when tested at OCP+50 mV, the untreated specimen suffered serious pitting corrosion, while a passive film had formed on the laser-treated specimen, which served as an effective barrier for reducing anodic dissolution.

The effect of excimer laser surface treatment on pitting corrosion resistance of 316LS stainless steel

Excimer laser surface treatment of 316LS bio-grade stainless steel (ASTM-F138) was conducted with the aim of enhancing the pitting corrosion resistance of the material. The experiment was performed under two different gas environments: air and nitrogen. The microstructure, phase and compositional changes after laser treatment were characterized by means of XRD, XPS and EDX; the resulting pitting corrosion resistance was evaluated by electrochemical polarization tests. The results show that excimer laser surface melting can effectively eliminate carbides and second phases alike, and also serves the function of homogenizing the microstructure. Nitrogen induced into the laser-treated surface could promote new precipitates and as a result lowered the corrosion resistance. On the other hand, laser treatment in a low partial pressure of nitrogen could enhance the corrosion resistance of 316LS stainless steel in that the active corrosion current was reduced and the passive range was widened.

The effect of laser surface treatment on stress corrosion cracking behaviour of 7075 aluminium alloy

Abstract Excimer surface treatment was conducted on 7075-T651 aluminium alloy with the aim of improving the alloys resistance to stress corrosion cracking (SCC) when tested in a NaCl solution. Using a pulse energy of 8 J/cm 2 , a laser-melted layer of about 8 μm thick was formed. Within the re-solidified laser-melted layer, the original grain boundaries of the wrought structure and most of the coarse constituent particles were removed. The grain boundaries of the re-solidified structure were found to be free from precipitates. The results of the SCC tests showed that after the 30-day immersion test, the untreated specimen was severely attacked with intergranular cracks that reached deep into the specimen; by contrast, no intergranular cracks were found in the laser-treated specimen. The form of corrosion attack in the laser-treated specimen took the form of corrosion pits instead. The electrochemical impedance measurements taken during the SCC test showed that the film resistance of the laser-treated specimen was always higher than that of the untreated specimen.

The effect of excimer laser surface treatment on the pitting corrosion fatigue behaviour of aluminium alloy 7075

An excimer laser (KrF) operating at a wavelength of 248 nm was used to modify the surface microstructure of 7075-T651 aluminium alloy. The aim was to improve both the corrosion resistance and the pitting corrosion fatigue resistance of the alloy by means of laser surface melting (LSM). The microstructure and the phases of the modified surface structure were analysed, and the corrosion behaviour of the untreated and the laser-treated specimens were evaluated by immersion test. The fatigue resistance of the 7075 alloy has been presented in the form of S/N curves.A microscopical examination and the transmission electron microscopy (TEM) study revealed that LSM caused a reduction both in number and 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. There was a significant reduction in the number of corrosion pits and shallow attack occurred. 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 life of the laser treated specimens was recovered. This was primarily attributed to the elimination of surface defects, but also be in part, due to the introduction of compressive residual stresses in the surface layer of the specimen. The fatigue resistance of the shot peened laser-treated specimens, tested in 3.5 wt% NaCl solution with 48 hrs 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.

Effect of excimer laser surface melting on intergranular corrosion cracking of the aluminum–lithium alloy 8090

Excimer laser surface melting of the aluminum alloy 8090 has been conducted with the aim of improving the intergranular corrosion resistance of the alloy. Both the results of the electrochemical and the intergranular corrosion cracking immersion tests show that the laser treatment significantly increases the intergranular corrosion resistance of the alloy, especially when the material is treated in nitrogen gas. The improvement in intergranular corrosion resistance is considered to be primarily due to the elimination of grain boundary Cu enriched precipitates in the laser-melted layer, which has effectively prohibited intergranular corrosion cracking. Besides, the reduction of Cu and Fe enriched constituent particles increases the pitting corrosion resistance of the alloy. As a result, a reduction in corrosion current density of one order of magnitude has been obtained for the laser-treated material. Moreover, a strong passivation has been obtained for the N2-treated specimen. This is attributed to the pr...

Some observations on cleavage fracture in high-strength aluminium alloy 7010

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