Relationship between laser-generated micro- and nanostructures and the long-term stability of bonded epoxy-aluminum joints

To improve the mechanical performance and to address current shortcomings of adhesive bonds such as bond degradation due to aging, a pulsed laser surface pretreatment of the metal surfaces of aluminum AW 6082-T6 joints with epoxy adhesive E320 is investigated. The surface treatment of the specimens resulted in increased single-lap shear (SLS) strengths before and after hydrothermal aging in 80°C hot water compared to nonpretreated reference specimens. In order to reveal the correlations of laser parameters, resulting surface morphologies and the SLS strength, differently laser pretreated surfaces were characterized at the micro- and nanoscale using optical and scanning electron microscopies. The surface enlargement was quantified with a digital image analysis of cross-sections prepared from the joint interfaces. An analysis of variances (ANOVA) of the SLS results indicated that the laser parameters power and pulse frequency were most critical for obtaining high SLS strengths. Pretreated joint surfaces with a high micro- and nano-surface enlargement and deep solidification structures provide high SLS strengths of up to 50 MPa and almost negligible aging losses of merely 4%. Undercut structures on the pretreated surfaces were found to be beneficial for the mechanical and aging properties when only limited micro- and nanostructuring was applied.