Laser pre-treatment of aluminum - An investigation of the adhesive bonding after surface modification

Abstract

This project focused on the laser pre-treatment of aluminum sheet and extruded aluminum within the 6xxx-series, applied on battery pack applications at Volvo Cars. Laser parameters were evaluated by surface characterization and lap shear testing, correlating adhesion to surface properties. Four different adhesives were investigated together with three different laser parameters. The results were compared towards untreated surfaces as well as plasma treated surfaces. Results showed that laser effectively cleaned the treated surfaces, and that it altered the surface structure. Clean and structured surfaces was shown to improve adhesion by increasing the strength, and giving a more favorable failure mode of the adhesives. After tropical ageing, three of the adhesives increased in strength due to post curing, whereas one degraded below the desired strength. XPS measurements showed that laser treatment induced thermal oxide growth, and confirmed cleanliness results, both enabling stronger bonding. The oxide layer thickness of untreated aluminum was 3–4 nm, while laser treated surfaces showed oxide thickness of 25–55 nm. SEM-EDX was not applicable to study the oxide layers due to insufficient resolution. Laser parameter T2 showed the greatest potential, however new sets of laser parameters need to be studied, with a particular emphasis on a high %overlap scan.