3.3.2. Influence of Overlap

The residual stress measurement results of the experiments with overlap are presented in Figure 8. The overlap rate was chosen to be 50 percent in one direction, as schematically shown in Figure 1d,e,i,j. An overlap of 50 percent, no matter if the overlap was parallel or orthogonal to the chosen advancing direction, led to a stronger non-equibiaxial residual stress state. The compressive stresses measured parallel to the advancing direction as well as the stress values at 1 mm depth were not influenced. The residual stresses orthogonal to the advancing direction were significantly higher for a peening with overlap than without. However, the reason for this observation is that the overall energy input the area receives is higher for peening with overlap, leading to increased residual stresses. The observed phenomenon agrees with the findings by Toparli and Fitzpatrick [13]. In this study, the effect of the overlap is stronger for a peening parallel to the rolling direction of the specimens (Figure 8b) than for peening orthogonal to the rolling direction (Figure 8c). The phenomenon that the non-equibiaxiality is more pronounced for peening parallel to the rolling direction was observed for all performed peening experiments. An overlap seems to intensify this effect. However, it should be noted that the measured residual stress values exceed 80 percent of the material's yield strength. Consequently, the results can only be interpreted qualitatively; otherwise correction methods, such as those developed by Chupakhin et al. [22] for equibiaxial residual stress profiles, need to be adapted for the special application case.

### 3.3.3. Influence of Order of Different Shot Patterns

In this study, peening with two sequences means that the peening area is peened twice by advancing directions orthogonal to each other as schematically illustrated in Figure 1g,l. Figure 4c shows the residual stress profile for a specimen which was peened via an advancing direction parallel to the rolling direction and afterwards via an advancing direction orthogonal to the rolling direction. An equibiaxial stress profile was observed. In contrast, for a peening strategy where, first, the advancing direction is orthogonal and then the second one is parallel to the rolling direction, the stress state was non-equibiaxial (see Figure 4d). The stresses orthogonal to the rolling direction were higher compared to the ones parallel to the rolling direction. This is in contrast to the residual stress distribution only after the first sequence (see Figure 4a,b).

In accordance with the expectations, specimens peened with two sequences showed higher maximum residual stresses in comparison to one sequence peening. Comparing the stress profiles after two peening sequences (see Figure 4c,d) with the ones after one peening sequence (see Figure 4a,b), it was found that the compressive residual stresses orthogonal to the advancing direction of the second sequence were more significantly affected by the application of the second sequence than the parallel stresses. Therefore, the application of a second peening sequence influenced the stresses orthogonal to the chosen advancing direction, whereas the parallel stresses were hardly changed.

Overall, the measured residual stress profiles show that the order in which the advancing directions are applied on the surface is very important. The choice of peening pattern can control

the stress value, the direction in which the highest stresses are induced, and by that, the difference between both in-plane stress components.

**Figure 8.** Residual stress profiles of AA2024-T3clad after LP for different advancing directions by varying the overlap, parallel or orthogonal to the advancing direction of the peening: No overlap, as well as 50 percent overlap are considered. Advancing direction either parallel ((**a**) *σxx*; (**b**) *<sup>σ</sup>yy*) or orthogonal ((**c**) *σxx*; (**d**) *<sup>σ</sup>yy*) to rolling direction. The gray area indicates the clad layer.

### *3.4. Influence of Laser Pulse Energy*

In order to study the influence of the laser energy, first, single shots were investigated for AA2024-T3 and AA2024-T3clad specimens for laser pulse energies of 1 J, 1.5 J, and 3 J via light optical microscope (Leica DMI 5000M, Leica Microsystems GmbH, Wetzlar, Germany). The apparent influenced surface area by one single shot is significantly larger than the applied focus size of 1 mm × 1 mm, as illustrated by micrographs from the light microscope in Figure 9. For one shot with 1 J, the apparent influenced area is approximately 3.07 mm<sup>2</sup> for AA2024-T3 and approximately 3.70 mm<sup>2</sup> for AA2024-T3clad, indicating that the effect is slightly more significant for the cladded specimen. The micrograph obtained from scanning electron microscope (SEM) of the peened area for AA2024-T3clad (Figure 10) indicates that the affected surface area most probably represents molten surface material. The higher the applied laser energy, the larger is the apparent influenced area on the specimens' surface by one single shot (see Figure 9). The shape of the influenced area is no longer a square. With increasing laser pulse energy, the influenced area of the single shot becomes more circular. Assuming that the influenced area correlates with the pressure affected area, this leads to overlapping effects even if the laser spots are positioned without overlap. Consequently, for higher laser energies, the overlap effect becomes more significant. This might be one reason for the increasing difference between both in-plane stress components (see Figure 11). Overall, higher laser pulse energies lead to higher compressive residual stresses at the depth of one millimeter, whereas the maximum value of compressive stresses does not seem to be affected significantly. In previous studies [8,10,11], it was observed that an increased energy density leads to deeper residual stresses which agrees with the results obtained by varying the energy input from 1 J to 3 J. However, the increase to 3 J did not lead to increased maximum residual stresses for the performed experiments. One aspect to consider in this regard is that the residual stresses induced by an energy input of 1 J are already close to the material's yield strength, which is approximately 345 MPa for AA2024-T3 [26].

**Figure 9.** Light optical micrographs for AA2024-T3clad and AA2024-T3 for single LP shots with a 1 mm × 1 mm squared laser focus with three laser energies: 1 J, 1.5 J, and 3 J. The size of the affected area on the specimens' surface is indicated.

**Figure 10.** SEM micrographs of an AA2024-T3clad peened area applying two sequences by using a laser focus size of 1 mm × 1 mm and a laser pulse energy of 1.5 J. For the first sequence, the advancing direction was chosen to be orthogonal and, in the second sequence, parallel to the rolling direction (see Figure 1l). The surface area indicates molten surface material for the peened area.

**Figure 11.** Residual stress profiles for AA2024-T3clad specimens with the advancing direction chosen orthogonal to the rolling direction: (**a**) 1 J and (**b**) 3 J. The gray area indicates the clad layer.

Experiments with increased energy input and experiments with overlap both showed the similar phenomenon of an increased difference between both in-plane stress components. The experiments with adjusted shot overlap of 50 percent qualitatively showed that the overlap rate is related to the non-equibiaxial stress profile, which agrees with the findings by Toparli and Fitzpatrick [13]. Therefore, a potential increase of the surface pressure-affected area related to the laser power density may influence the difference between both in-plane stress components. Increased laser pulse energy leads to a larger affected area, which might correlate with a different spatial pressure distribution. However, this effect is accompanied by an increased maximum pressure due to the increased pulse energy. These two effects superimpose for LP with increased pulse energies.

Besides the difference between both in-plane stress components, measurements for a higher laser power density show an increase in stress value at a 1 mm depth. This cannot be observed for the experiments with overlap in this study. We assume that the shape of the apparent influenced area is more circular at high energy densities and that the pressure distribution might not be uniform all over the affected area. Therefore, an increase in energy density could only be interpreted as a weak form of peening with overlap. In addition, as the experiments show a possible dependency of the difference between the stress components depending on the rolling direction, a possible anisotropic material behavior might be a mechanism to consider in explaining the difference between both in-plane stress components. Further investigation is needed for a substantiated interpretation of peening with high energy densities as a type of peening with overlap.

In summary, the overall energy input an area receives depends on pulse energy, overlap and number of sequences. Each aspect is crucial for the resulting residual stress distribution.
