Pulsed Laser Pre-treatment of Aluminium Adhesive Bonded Joints: Process Parameter Selection for Improved Fatigue Resistance

Pulsed fibre laser pre-treated aluminium alloy double cantilever beam (DCB) adhesive bonded joints are subject to mode I crack growth fatigue resistance tests. The effects of nanosecond pulsed laser irradiation at 5, 50 and 500 mm/s on crack growth rate are investigated and compared with degreased and grit-blasted samples. A scanning electron microscope and optical profiler are utilised to analyse the treated surfaces, revealing deep valleys from laser treatment at 5 and 50 mm/s, shallow undulations from laser treatment at 500 mm/s and irregular pits from grit blasting. Best fatigue resistance is achieved with laser treatment at 500 mm/s and with grit blasting, resulting in crack growth rates two orders of magnitude below that of the degreased sample. Analysis of the resulting fracture surfaces reveals porosity in specimens subject to laser treatment at 5 and 50 mm/s, leading to marginally lower fatigue resistance than with laser exposure at 500 mm/s. With correct parameter selection, nanosecond pulsed laser irradiation is confirmed as a valid pre-treatment method for adhesive-bonded joints subject to cyclic loading, avoiding repeatability, handling and environmental issues associated with mechanical and chemical surface treatments.