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

Excimer surface treatment was conducted on 7075-T651 aluminium alloy with the aim of improving the alloy's resistance to stress corrosion cracking (SCC) when tested in a NaCl solution. Using a pulse energy of 8 J/cm2, 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.