Effect of laser spot overlap ratio on surface characteristics and adhesive bonding strength of an Al alloy processed by nanosecond pulsed laser

Abstract

• Nanosecond laser ablation treatment enhances adhesive bonding strength for Al alloy. • Laser-induced surface physiochemical modifications increase with overlap ratio. • Excessive overlap ratio of laser spots reduces the strength of laser-treated joints. • Micropores in adhesive layer caused by insufficient wetting reduce joint strength. • Corrosion resistance of laser-treated substrate drops with increasing overlap ratio. The laser ablation treatment has been validated to be an effective surface treatment method in improving adhesive bonding performance for metal or composite materials. This study was further conducted to investigate the effect of laser spot overlap ratio on surface characteristics and resultant adhesive bonding strength for AA6022-T4 Al alloy. The laser spot overlap ratio was changed by varying the laser processing speed and line spacing between laser scanning lines. Surface characteristics analyses show that the laser ablation treatment at higher overlap ratio produces rougher surface morphology and thicker aluminum oxide layer on substrate surface, which are considered to be more favorable for adhesive bonding strength. Lap-shear strength results show that the laser ablation treatment at overlap ratio of -49 % (corresponding with separated laser spots) improves the joint strength by 6.8 % and 17.1 % before and after water soak exposure compared to as-received condition. Meanwhile, the desired cohesive fracture mode is also achieved which indicates that the joint strength reaches to the saturated value depending on mechanical performance of adhesive. This explains that the continuous increase of laser spot overlap ratio from -49 % to 63 % exhibits negligible improvement on joint strength. Nevertheless, although inducing a higher degree of surface physicochemical modifications on Al substrates, the laser ablation treatment with an overlap ratio beyond 63 % decreases the joint strength. Detailed studies of SEM micrographs verified that the decreased joint strength is attributed to insufficient wetting of adhesive on the rough surfaces of laser-treated substrates resulting in entrapment of air between substrate and adhesive layer, which forms micropores defects in the adhesive layer and consequently weakens the adhesive-bonding joints. In addition, the improved corrosion resistance of adhesive-bonded Al joints by laser ablation treatment is attributed to the barrier effect of laser-produced rough surface morphology to water diffusion along bonding interface.