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

Abstract

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 non-pretreated 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.

Transferability of the Structure–Property Relationships from Laser-Pretreated Metal–Polymer Joints to Aluminum–CFRP Hybrid Joints

“…The laser surface treatment was described in more detail in reference [17], where the same pulsed Nd: YAG laser was used for the pretreatment of AW 6082-T6 adherents prior to adhesive bonding and mechanical testing. Here, different laser parameter sets were used to create different surface micro-and nanostructures that influenced the mechanical strength of the adhesive joints.…”

“…Here, different laser parameter sets were used to create different surface micro-and nanostructures that influenced the mechanical strength of the adhesive joints. In a similar fashion, the parameters of laser power, frequency, laser spot overlap, and the number of subsequent scans of the same spot are varied in this study to create the same varying surface structures as in the previous case [17]. These parameter sets are ranked according to the performance of the specimens in the initial state and after seven days of hydrothermal aging in 80 • C deionized water.…”

“…These specimens then exhibit a joint or overlap area of 10 × 5 mm 2 (Figure 1b), similar to the metal-polymer SLS samples in ref. [17]. Six specimens are produced with each laser parameter set as well as six additional untreated specimens.…”

“…The laser-induced surface enlargement on the micro-and nanoscale, as well as the depth of the ablated craters on the surface, were determined through a digital image analysis approach for scanning electron microscope (SEM) images and a laser scanning microscope (LSM). It was concluded that surface structures leading to a high micro-and nano-surface enlargement, combined with deep melt craters and undercut structures, promote high SLS strengths and a high resistance against hydrothermal aging [17].…”

“…In order to investigate the influence of the surface morphology on the mechanical performance and aging resistance of AW 6082-T6-CFRP hybrid joints, the SLS strengths of hybrid specimens before and after 7 days of hydrothermal aging are determined. The AW 6082-T6 surfaces are pretreated with the same laser parameter sets that have been investigated in the previous study [17] so that the influence of the surface enlargement, crater depth, and undercut structures on the SLS strength of hybrid specimens can be directly compared with AW 6082-T6-E320 adhesive joints. Furthermore, stereomicroscopic and SEM analyses of the fracture surfaces are performed in order to investigate the influence of the laser-generated surface structures on the failure patterns of hybrid and adhesively bonded metal SLS specimens.…”