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.