Introduction

Various drivers have pushed for the development and implementation of new joining processes in lightweight metal structures. One technology that has shown significant potential is FSW given the solid-state nature of the joining process. However, when welding large shell structures such as aeronautical fuselage panels, gaps may arise between the abutting faces to join leading to significant degradation of the joint mechanical properties when butt-welding.1 Overlap FSW can overcome this limitation, but the edge of the weld in the advancing side forms a hook like defect, which diminishes the joint strength.2,3 To improve strength in this joint configuration it has been proposed to use multiple pass welding.3 By employing this method, the out of plane bending and peel load at these unwelded tips is reduced. This method, however, comes with disadvantages of its own. To accommodate the shoulder diameter and clamping, the overlaps must be larger, diminishing the weight savings gains. Lead time is also increased, along with tool wear, making the process less economically viable. Another way of improving the strength of overlap FSW joints is by combining it with another joining method such as adhesive bonding, resulting in FS weld-bonding. This way the adhesive layer at the edges increases effective overlap and reduces peel stress at the weld edges. This method was used in magnesium-to-aluminum friction stir spot welded joints, resulting in increased quasi-static and fatigue strength.4 Similarly this method was proposed for continuous joints of AA2024-T3, resulting in improved quasi-static and fatigue strength.5
This study covers hybrid overlap FSW and adhesive bonding of an Al-Mg alloy (AA6082-T6), which is a common multi-purpose alloy. Characterization of the adhesive used in the study is first presented, including the effect of curing temperature in mechanical performance. This is relevant given that the adhesive in FS weld-bonded joints will not cure all at the same temperature. The degradation temperature is also measured through thermogravimetric analysis, to assess if it is bellow temperatures achieved during welding. FSW process parameters for FSW and FS weld-bonded joints were then studied to find a set that results in sound quality joints. Finally, the joints were subjected to cyclic loading at R=0.1 to assess the fatigue performance.