Date of Award


Document Type

Campus Access Thesis


Mechanical Engineering

First Advisor

Tony Reynolds


Friction Stir Welding (FSW) is a solid state joining process invented in 1991 by The Welding Institute (TWI, Cambridge, UK). The performance of the resultant welded joint in high strength aluminum alloys sets FSW apart from traditional fusion welding. Rapid process development of FSW joints for Al 7050 to be used in aerospace applications depends on understanding of the mapping between FSW process and joint properties. Ideally, we would like to optimize joint properties by intelligent control of weld parameters. In this study, an extensive investigation was carried out on FSW butt joints. Two weld parameter sets were utilized to provide a range of weld properties for correlation with weld response parameters. Controlled welding parameters included welding speed (translational), rotational speed and in some cases the ambient environment. Auxiliary cooling (vortex air cooling and underwater welding) was used to modify the welding environment. Weld properties and features examined included interface morphology/weld microstructure, mechanical properties, including tensile, hardness, residual stress, and fatigue behavior. Evaluation of mechanical performances and characteristics of the FSW butt joints indicates that increased welding speed increases the hardness minima, tensile strength and fatigue resistance significantly. Measured joint efficiencies showed that increasing vii welding and rotational speed from 2IPM, 90RPM to 16IPM and 540RPM improved joint efficiency from 69% to 96%. Studying the effects of the ambient on the overall weld performance reveals that at lower welding speeds the joint mechanical performance could be enhanced via auxiliary cooling since the quench rate is expected to be affected by the ambient. When applied to the FSW fast welded joint, auxiliary cooling via underwater welding improved the joint efficiency from 96% to 98%. Results indicate that the mechanical properties of FSW joint could be improved via controlled process parameters such as welding speed and ambient which would allow the FSW joints to replace many existing joining techniques such as traditional welding techniques and riveting.