Thesis
An experimental and numerical investigation of refill friction stir spot welding of aerospace aluminium alloys
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2024
- Thesis identifier
- T17113
- Person Identifier (Local)
- 202164614
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Refill friction stir spot welding is a solid-state joining process that offers exceptional joint strength and quality in high strength aluminium alloys. However, industrial adoption has not been forthcoming, in part due to the impractically long welding times required by the process that make it uncompetitive with conventional techniques. The research presented in this thesis addresses the issue of long welding times by investigating the microstructural characteristics and mechanical properties of AA2024-T3 lap joints produced at welding times significantly below those previously reported in the literature. Various microscopy techniques were used to assess weld microstructure and defect evolution, while mechanical properties were determined through hardness measurements and tensile lap shear testing. A series of novel welding tool shoulders were developed to improve material flow and counteract the formation of defects as welding time was reduced. The influence of shoulder design on weld microstructure, defect evolution, material flow, and mechanical properties was assessed. It was found that implementing a chamfer on the inner rim of the shoulder tip improved joint quality at short welding times by promoting material flow towards critical joining interfaces during the refill stage of the process. The formation of voids in the region of the weld periphery was eliminated and tensile lap shear strength of the welded joints was increased by 27%. However, large tunnel defects remained, and fractography of the failed joints revealed that strengthening of the weld periphery came at the cost of weakening across the lap interface. To better understand material flow behaviour during welding, and the mechanism by which defects form, a fully coupled thermomechanical model was developed using commercial finite element analysis software. The model was validated against experimental temperature measurements and observations of microstructure, material flow, and defect characteristics. By observing the simulated flow of material, the formation process of the commonly observed tunnel defect was identified; the model was then used to predict the optimal tool volume ratio to eliminate the defects. With the knowledge gained through the above outlined experimental and numerical research, a second generation of chamfered shoulders was developed and explored experimentally. Improvements to the path of the welding tools were investigated by implementing the optimal volume ratio predicted numerically, as well as modifying the end position of the shoulder and probe. Microstructural and mechanical testing of the resulting welds revealed that defect free joints can be produced in approximately half the welding time, and with a higher lap shear strength, than previously reported in the literature for a comparable alloy.
- Advisor / supervisor
- Galloway, Alex
- Toumpis, Athanasios
- Resource Type
- Note
- This thesis was previously held under moratorium from 01/10/2024 to 01/10/2026.
- DOI
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