Factors affecting the properties of friction stir welded aluminum lap joints

Friction stir welding (FSW) is a solid-state joining process invented at The Welding Institute (TWI) in 1991. The ability to produce high-quality welds in high-strength aluminum alloys sets FSW apart from typical fusion welding techniques. The process has mainly been used for making butt joints in aluminum alloys. Development of FSW for use in lap joint production would expand the number of applications that could benefit from the technique. In this study, an extensive investigation was carried out on FSW lap joints, including interface morphology and mechanical properties. Two materials, Alclad 2024-T3 and Al7075-T6, sheet materials commonly used in the aerospace industry, were joined. Welding variables included welding speed, rotational speed and, of particular importance, tool dimensions. Examination of metallographic cross sections and failure locations showed a critical sheet interface present in all welds. Consequently, a second weld pass was added to eliminate the critical sheet interface. Results indicated FSW lap joints may, on the basis of strength, potentially replace other joining processes like resistance spot welding and riveting.