## 3 Results and Discussions
### The Results of the Stress-Strain Curves
#### 3.2.2 The Influence of Different Forming Directions on Dynamic Impact Shear Properties  
The dynamic shear stress-strain curves of aluminum alloy 7050-T7451 in three strain rates at different forming directions were shown in Figure 9. The shear modulus of material obtained by Figure 9 at different directions in the stage of elastic deformation were shown in Table 4. From Table 4, it can be seen that the shear modulus of different directions had little difference when the strain rate was 2.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\), which showed that the material at this strain rate tended to isotropic in the elastic deformation stage. With the increase of strain rate, that is, the strain rate was greater than 3.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\), there were significant differences in shear modulus in different directions, which showed that the material at this strain rate range had obvious anisotropy in the elastic deformation stage. Figure 9: Dynamic stress-strain curves of different strain rates: (a) 2.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\); (b) 3.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\); (c) 4.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\). In the stage of plastic deformation, in general, at the strain rate of \(2.5\times 10^{4}\) s\({}^{-1}\), the shear stress in ND was the largest, and the shear stress in RD was higher than that in TD. However, when strain was in the interval of 40-60%, the shear stress of TD increased sharply and then decreased, as shown in point B of Figure 9. That was due to the critical deformation of grain in TD was reached in the strain interval of 40-60%, and a large number of dynamic recrystallization and twin phenomenon had occurred [23; 24]. This can be confirmed from the results of later microstructural analysis. At point B, the difference value of shear stress between TD and RD was about 50 MPa, which showed a significant difference in dynamic mechanical properties of different directions. At the strain rate of \(3.5\times 10^{4}\) s\({}^{-1}\), the difference of shear stress in all directions was not obvious, however, it can still be seen that the shear stress in ND n was the largest, and there was little difference of shear stress between RD and TD. At the strain rate of \(4.5\times 10^{4}\) s\({}^{-1}\), the shear stress in ND was the largest, and there was no obvious difference of shear stress between RD and TD. With the increase of strain rate, the difference of shear stress in different directions had decreased, which showed the dynamic mechanical properties tended to isotropic. One of the noteworthy points in Figure 9 was that, when strain was about 50-55%, the shear stress at the same strain rate tended to be consistent in different directions, which was probably that the same resultant force in the internal structure of three directions under the strain, thus the sensitivity of material to the forming directions was reduced [19]. The strain of TD direction in three directions was the least, which showed a more sensitive characteristic of thermal softening. The results showed that [25] the main reason for the above results of aluminum alloy 7050-T7451 plate was that the strong plate texture parallel to the rolling plate was formed in the rolling process or other deformations. The preferred orientation, basal slip Schmid value, grain size and misorientation distribution were different in different directions. The microstructure of aluminum alloy 7050-T7451 in different directions, as shown in the following Figures. The grain size and orientation of ND can be clearly seen from Figure 10. As shown in Figure 10a, there were many deformed grains which were obviously compressed and elongated, and most of them were long axial grain. The direction of grain lengthening was the same as the rolling direction. As shown in Figure 10b,c, with the larger magnification of EBSD and the observation of SEM, the change rule of grain can be seen more clearly, the grain deformation was not uniform, and the distances of each grain decreased in ND and increased in RD.