Material Flow and Fracture Modes of Friction Stir Lap Welds of Alclad 2024 Aluminum Alloy Sheets

Alclad aluminum is an aluminum sheet laminated with an extremely thin layer of pure aluminum onto each side for corrosion resistance. Typical application of Alclad is aircraft skin or components. Friction stir lap welding has been developed for the manufacturing of sheet metal parts in aerospace industry.

The material flow patterns in the FS lap weld and especially the flow patterns of the thin pure aluminum layer are crucial to the quality and strength of the weld. We found that the thin aluminum layer was a relative intractable layer to some probe shapes. It remained a horizontal layer in the stir zone despite the penetration of the stir probe into sheets and a sound friction stir lap weld was achieved when using a cylindrical threaded or an eccentric triangular probe. Welding parameters had insignificant influence on the flow patterns. By using tools of triangular threaded or partial cylindrical threaded probes was it then possible to cause material flow in the stir zone having vertical movement and to stir the thin aluminum layer into distorted or broken shapes. Threaded cylindrical or triangular Probes caused the thin aluminum layer became a bowed layer.

Corresponding to the material flow modes, four types of failure modes were observed after tensile test. The fracture occurring at the narrowest effective thickness on the upper sheet due to bowed aluminum layer revealed a lowest strength while the horizontal flow of aluminum layer caused a horizontal shear with improved strength. The distorted or broken interface resulted in a fracture at stir zone. Well stirred aluminum layer achieved by using a partial cylindrical threaded probe corresponded to a tensile fracture at base metal. The latter two modes enjoyed a substantial higher strength.

Keywords: Friction stir lap welding, Alclad aluminum sheet, FSW-tools, material flow, fracture mode

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