4.3.1 Effect of roll bending cycle on stress
It can be seen from the evolution of the stress and strain in the ring that the stress and strain in the roll bending process are uniform. In order to examine the influence of roll bending on the uniformity of stress distribution of ring parts, four rings with the same quenching process were subjected to continuous roll bending for 1 to 4 times respectively. Eight points was uniformly distributed on the end faces of ring parts as shown in Fig. 2, and the surface stress distribution at each point of the four rings after roll bending was compared. According to the characteristics of residual stress distribution of 2219 aluminum alloy rings, the circumferential stress and axial stress of 2219 aluminum alloy rings at different points were compared and analyzed by drawing a radar chart[18]. The closer the position of eight points to the uniform octagon, the more homogeneous the residual stress of the ring is, and the more consistent the anisotropic structure is.
It can be seen from Fig. 12 that, the residual stress at each analysis point of the ring after different number of roll bending turns is far less than the original quenching residual stress. After the first roll bending, the change of residual stress of the ring is the largest, but the distribution of residual stress is uneven. With the increase of the number of roll bends, the residual stress changes little, but the residual stress tends to be uniform.
In the figure, the distribution of stress in all directions appears a "sharp Angle" phenomenon, that is, residual stress at 1 ~ 2 test points is significantly higher or lower than other test points. After the second roll bending, the stress reduction value decreases, the axial stress is on the brink of the normal octagon, but the circumferential stress still has the phenomenon of "sharp Angle". After 3 rolls bending, the residual stress in each direction tends to be a uniform octagon. There is no changes in stress after 4 rolls bending.
In order to quantify the influence of the number of turns in the roll bending process on the stress inhomogeneity, stress standard deviation S is used to evaluate and describe the distribution uniformity of residual stress on the above points. The formula for standard deviation is as following:
It can be seen from the standard deviation analysis shown in Fig. 13 and Fig. 14 that, there are great differences in the distribution uniformity of ring residual stress after different number of roll bending. After one roll bending, although the stress reduction is very large, the distribution of stress at each part of the ring is also quite different, so the standard deviation of stress at each part is much larger than that in the quenched state, with the standard deviation of stress distribution of 17.35MPa, the inhomogeneity increased by 71.6%. After 2 turns of roll bending, the standard deviation of circumferential stress decreases and reaches 12.2MPa. Meanwhile the sharp Angle phenomenon is partially improved, but the fluctuation is still large, and the unevenness increases by 59.5%. After 3 turns of roll bending, the circumferential stress homogeneity decreases and reaches 2.21MPa, and the homogeneity is enhanced by 55.1%, showing good uniformity. After 4 rolls, due to the local deformation of the roll bending process, excessive number of turns will not result in the decrease of stress uniformity.
As can be observed in Fig. 14, the distribution uniformity of residual stress in the third roll bending circle has the most significant difference, reaching 1.27MPa, indicating good stress uniformity. The standard deviation of axial stress has decreased by 75.7%. Therefore, it can be assumed that the uniformity of the roll bending to the third turn is better.
4.3.2 Effect of roll bending cycle on strain
According to the strain distribution law, the maximum deformation occurred at the outer contour of the ring, while the minimum plastic deformation occurred at the core of the ring. In order to analyze the influence of different number of roll bending turns on the strain uniformity, the ratio E of the minimum strain at the core of the large ring section (aforementioned S1) and the maximum strain at the surface was used to represent the strain uniformity
The strain nonuniformity coefficient e was used to evaluate the distribution uniformity of plastic deformation in the ring. After adopting different roll bending processes, E ratio is shown in Table 5. In the process of roll bending with different roll bending turns, E gradually decreases. The e value of the first circle is very large, reaching 2.9%, and the strain is not uniform, but the stress reduction effect is very good. In the second lap, E gradually decreases to 1.7%, the uniformity increases, but the stress reduction effect decreases. From the third turn, E reaches 1.3%, and the strain coefficient tends to be stable after that, so it can be considered that the uniformity is good at the third cycles.
Table 5. Strain nonuniformity coefficient under different cycles