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.