Concluding Remarks

The combination of FSW and adhesive bonding into friction stir weld-bonding was studied regarding quasi-static and fatigue performance. In order to benchmark the process, it was compared against overlap FSW and adhesive bonding.
The epoxy adhesive used was characterized taking into consideration the curing temperature, showing that even though it cures at room temperature, higher temperature curing will increase strength while reducing ductility. Degradation temperature was also found to be above reported temperatures incurred during the welding procedure.
In the joints quasi-static testing, overlap FSW showed lower strength and ductility than FS weld-bonding joints. Downward force during welding, showed a significant effect in strength and ductility of FSW joints, with both increasing up to 550 kgf. The same trend was not observed in the FS weld-bonded joints, with the highest strength and ductility achieved at 450 kgf, with a joint efficiency of 94.96%. In FS weld-bonded joints the critical weld edges were kept close by the adhesive, leading to increase mechanical performance. As such, adhesive strength and the quality of the surface treatment are more significant to joint strength than downward force during welding. Adhesive bonded joints showed the highest strength and ductility given the relatively large overlap.
In cyclic loading at R=0.1, similar trends to the quasi-static loading were observed. Adhesive bonding achieved the highest fatigue strength followed by FS weld-bonding and FSW showed significantly lower fatigue strength (41.6% strength of adhesive bonded joints at 106 cycles). The hook defect serves as a fatigue crack initiation location and leads to the failure of FSW joints. Adhesive joints fail in an adhesive / cohesive manner, while FS weld-bonding fail through the adhesive immediately followed by cracking through the hook defect.
Acknowledgements
This work was supported by FCT, through IDMEC, under LAETA, project UIDB/50022/2020. Funding provided from NORTE-01-0145-FEDER-000022 SciTech – Science and Technology for Competitive and Sustainable Industries is acknowledged. The authors acknowledge the funding provided by FCT project PTDC/EME-EME/29340/2017 – DisFri.
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