Friction stir butt-lap welding of A7N01P-T4 aluminum alloy: Efforts to mitigate defects and the resultant mechanical properties

Butt-lap configuration of aluminum alloys was friction stir welded under various tool offsets, rotational speeds, and tool geometries. The tool offset was introduced to mitigate the inherent retreating side hook. As the probe was placed inwards, the retreating side hook was mitigated, but a severe S-line was formed. Since the inherent retreating side hook was difficult to further mitigate, the S-line was selected to be mitigated by increasing the rotational speed. But an excessive rotational speed (2000 r/min) could cause a severe upward-bending retreating side hook, which decreased the effective plate thickness. Subsequently, two-section stepped probes were employed to mitigate the defects. The S-line and retreating side hook were both mitigated and its ultimate tensile strength (369 MPa) reached 91% of the base material.