1. Introduction
Al-Zn-Mg-Cu
alloys (7xxx series) have been
widely used in fields of aviation, aerospace, and civilian transport
owing to their high strength to weight ratio, good fracture toughness
and corrosion resistance [1-3]. For the preparation of complex
structural parts in applications, the
fusion
welding technologies, e.g. Tungsten Inert Gas process, Gas Metal Arc
Welding are the most used methods to join these high strength aluminum
alloys. However, the joints made by these welding method showed high hot
cracking susceptibility in 7xxx series alloys with high Cu content
[4]. In addition to hot cracks, welding defects like porosity, lack
of fusion and incomplete penetration also frequently took place. In
order to avoid these problems associated with the liquid-solid coupling,
a newly invented process, friction stir welding (FSW), which is a
solid-state welding technique has been wildly used for defect-free and
efficient welding of 7xxx series alloys [5, 6].
Unlike
other conventional fusion welding methods, FSW is a complicated
thermal-mechanical
process. Because of the combined influence of frictional heating and
intense plastic deformation, FSWed joints have generally
inhomogeneous complex spatial
arrangements
of microstructure and mechanical properties [7-9]. In
heat-treatable 2xxx and 7xxx
alloys, recovery/recrystallization of substructure and
re-dissolution/coarsening of hardening precipitates usually occur in
weld joint due to great frictional
heat generated between welding tool shoulder and
workpieces [10], resulting
pronounced degradation in mechanical properties as the absence of
strengthening of dislocations, refinement structure and precipitates
[11, 12]. In order to decrease these adverse effects, one approach
is to reduce frictional heat, such as increasing tool
traverse speed/rotation rate
ratio, cooling FSW joint by water (underwater FSW). However, these low
heat input FSW strategies would raise the possibility of non-welded
groove on the root side of the weld [13, 14]. Another alternative is
to increase the recrystallization resistance of workpiece. It has been
widely proved that trace Sc can improve recrystallization temperature of
aluminum alloys due to the formation of
Al3Sc/Al3(Sc,Zr)
precipitates in matrix [15,
16]. The strength benefits of minor Sc addition have been demonstrated
in 7055 and 7075 alloys [17, 18].
7085 aluminum alloy is a newer 7xxx
alloy designed for aerospace applications. It has been widely used in
many aircraft structural components due to high fracture toughness and
low quench sensitivity [19-21]. Because the Cu content of the 7085
alloy exceeds 0.8 wt.%, there are challenges in welding by using fusion
welding technologies. As the outstanding advantages of FSW technology,
the application of FSW in 7085 alloy aroused widespread concern in
recent years. Xu et al. have investigated the
FSW behaviors of 7085 alloy and
discussed the effect of welding parameters on the microstructure
inhomogeneity and mechanical properties of FSWed joint [5, 22-24].
However, because of the increasing use of 7085 alloy in fatigue critical
applications, fatigue crack growth (FCG) behavior of the FSWed joint
should be highly regarded, but the related appropriate experimental data
is insufficient so far. Moreover, the influence of trace Sc addition on
FSW behaviors of 7085 alloy has also rarely been reported, the effect of
Sc on the microstructural evolution and mechanical properties,
especially FCG in FSWed joint of alloy 7085 is not clear yet.
Therefore, in present work, a
7085-0.25Sc (wt.%)-T6 Al alloy plates were welded by FSW. The
microstructures (sub-structures, precipitates) and mechanical properties
(hardness, tensile property and resistance to fatigue) of base metal
(BM), heat-affected zone (HAZ) and weld nugget zone (WNZ) have been
thoroughly investigated. The
effects of Sc addition on the microstructures and properties, especially
FCG inhomogeneity of FSWed joint were discussed in detail and
elucidated.