The effect of prior adhesive bonding on the corrosion behavior of AA2024 FSWed single lap joints

Abstract

Friction stir welding (FSW), when in butt-joint configuration, was found to produce high-mechanically performing joints. However, when in overlap configuration, stress concentration is noticed at the weld edges that significantly reduces the mechanical performance of the joints. Additionally, the welding regions tend to enhance localized corrosion attack due to microstructural changes. In the present work, a new hybrid joint - that is a combination of friction stir welding with a prior step of adhesive bonding of the overlap surface - was investigated. Friction stir welded, adhesive bonded and hybrid single lap joints were exposed to exfoliation corrosion solution - for different exposure times - and mechanically tested to investigate the effect of adhesive bonding on the mechanical behavior of pre-corroded joints. The hybrid joint (adhesive bonding + friction stir welding) showed excellent tensile mechanical performance in terms of deformation, exhibiting approximately 100 % increase when compared against the respective values of adhesive bonded and/or friction stir welded joints. This was attributed to the elimination of the ‘hook defect’ in the friction stir welded area as well as to the out-of-plane displacement restriction of the bonded area. The corrosion-induced degradation mechanisms of the hybrid joint were also investigated and compared against the friction stir welded and the adhesive bonded joints for various exposure times. The hybrid joints outperform the friction stir welded and adhesive bonded joints for all the investigated corrosion exposure times; after very long corrosion exposure times (e.g. 48 h), the hybrid joints maintained their maximum tensile strength by almost 90 %, while the adhesive bonded and the friction stir welded joints retained 85 % and 59 %, respectively. Almost double fracture deformation was noticed for the hybrid joints for all the investigated corrosion exposure times. This superiority was attributed to the dual protection of the intermediate adhesive film; initially the sealing effect protects the lap joint at the primary corrosion stages until debonding takes place and additionally protects the friction stir welded area in-between the lapped sheets from localized corrosion on the interface between the thermo-mechanically affected and heat affected zones.