**3. Results and Discussion**

### *3.1. Residual Stresses in Base Material*

The residual stress profiles for the unpeened AA2024-T3clad and AA2024-T3 (milled clad layer) specimens are given in Figure 2. Near the surface, both specimens show a relatively high standard deviation, whereas the milled specimen shows an even larger variation which might be related to the milling process. With increasing depth, the deviation decreases strongly. For the AA2024-T3clad specimen, compressive residual stresses are measured within the clad layer. Deeper residual stresses are tensile in the order of 20 MPa with less than a 10 MPa difference between both in-plane stress components. The specimen AA2024-T3 without a clad layer shows slightly higher compressive residual stresses directly underneath the material surface, whereas stresses within the material are slightly lower than for AA2024-T3clad. The minimal differences in residual stresses in both unpeened specimens can be related to the milling process.

**Figure 2.** The residual stress profile of unpeened materials: (**a**) AA2024-T3clad, the gray area indicates the clad layer; (**b**) AA2024-T3 (milled).

### *3.2. Influence of Clad Layer and Aluminum Foil*

To investigate the influence of the clad layer, the specimens were peened parallel to the rolling direction. The resulting residual stresses show a non-equibiaxial profile (see Figure 3). The higher compressive residual stresses, *<sup>σ</sup>yy*, are orthogonal to the rolling direction. In comparison to AA2024-T3, AA2024-T3clad shows a different residual stress state near the surface. The clad layer led to an offset in the maximum residual stresses to a deeper depth, which corresponds to the clad layer thickness. The residual stresses within the clad layer are relatively low. Overall, the compressive residual stresses are slightly increased for AA2024-T3clad.

In industrial practice, an Al foil is often used. This leads to a better surface quality because the foil acts as thermal protection. Consequently, the energy input does not melt the surface of the material [23]. Slightly higher compressive residual stresses were measured for peening with Al foil of AA2024-T3 as well as AA2024-T3clad (see Figure 3). Especially for the measurement points in a depth larger than 0.4 mm, the differences are slightly more pronounced. Xu et al. [24] observed that a coating can increase the induced residual stresses, e.g., because of different absorption attributes of the laser energy. However, due to the fact that both the clad layer and foil consist of pure aluminum, no differences in absorption are expected. In this study, the increase detected is marginal compared to the absolute value of the residual stresses so that no significant influence of the Al foil on the residual stresses could be highlighted. Therefore, the effect is not discussed further.

**Figure 3.** Laser Peening (LP) with and without Al foil: The advancing direction is chosen parallel to the rolling direction of the specimens: (**a**) AA2024-T3clad, the gray area indicates the clad layer; (**b**) AA2024-T3 (milled).

### *3.3. Influence of Laser Pulse Pattern*

The measurement results for different patterns and overlap, as presented schematically in Figure 1c–l, are compared to investigate the influence of the chosen laser pulse pattern on the residual stresses. Firstly, the effect of the chosen advancing direction, including row-wise changes, is discussed. This is followed by a discussion on the effect of overlap and, finally, peening with two sequences of orthogonal advancing directions.

### 3.3.1. Influence of Advancing Direction

As already observed from the previous results, the residual stresses are non-equibiaxial for a single peening sequence where the stresses transversing to the advancing direction are larger than the parallel ones. The difference between both in-plane stress components is more pronounced for peening parallel to the rolling direction as compared to peening orthogonal to the rolling direction (see Figure 4a,b). The advancing direction determined which in-plane stress component dominates the residual stress profile. However, the absolute values of the maximum residual stresses as well as the values in a material depth of 1 mm are not significantly different for the two applied advancing directions. Especially for nonsymmetrical peening areas and structures, the choice of advancing direction could have an even more significant effect in this regard, which was seen in previous studies [14,15]. As illustrated in Figure 5, stress profiles for bidirectional peening (see Figure 1f,k) are not significantly different as compared to unidirectional peening (see Figure 1c,h). Therefore, a row-wise change in the advancing direction does not influence the resulting residual stress profile.

In order to correlate the observed residual stress profiles for different advancing directions to possible local orientation effects, EBSD measurements were performed. Unpeened AA2024-T3clad (see Figure 6) exhibits an orientation band between <1 0 1>//[001] and <1 1 2>//[001] whereby the crystal direction <1 0 1>//[001] is characterized by the highest axial intensity (see Figure 7a). In the normal direction, parallel to the material thickness, the crystal direction <0 0 1>//[010] is pronounced, followed by <1 0 1>//[001]. The incident laser beam mainly interacted with the (1 0 0) crystal planes at the beginning of the LP process.

The LP sequence parallel to the rolling direction leads to the formation of crystal directions such as <1 1 1>//[001], <1 0 2>//[001], and <0 0 1>//[001] (see Figure 7b). Furthermore, the crystal direction <0 0 1>//[010] parallel to the sheet normal direction shows a weakening whereas the axial intensity of <1 0 1>//[010] increased after the LP treatment. For instance, the weakening of <0 0 1>//[010] and the gain of <1 0 1>//[010] compared with orientations of the base material means a rotation around the [010] axis within 45◦. The appearance of <1 1 2>//[010] having low axial intensity is the result of a split-up.

**Figure 4.** AA2024-T3clad: (**a**) one peening sequence with the LP advancing direction parallel to the rolling direction of the specimens; (**b**) one peening sequence with the LP advancing direction orthogonal to the rolling direction of the specimens; (**c**) two peening sequences with the first peening sequence advancing direction chosen parallel to the rolling direction and second peening sequence advancing direction chosen orthogonal to the rolling direction; (**d**) two peening sequences with the first peening sequence advancing direction chosen orthogonal to the rolling direction and the second peening sequence advancing direction chosen parallel to the rolling direction. The gray area indicates the clad layer.

**Figure 5.** LP with unidirectional and bidirectional advancing directions for AA2024-T3clad: (**a**) advancing direction parallel to the rolling direction; (**b**) advancing direction orthogonal to the rolling direction. The gray area indicates the clad layer.

**Figure 6.** Electron backscattered diffraction (EBSD) micrographs of unpeened AA2024-T3clad: the TD-direction is defined as the direction of the material's thickness ([010]), and RD-direction is defined as the rolling direction ([100]).

**Figure 7.** Inverse pole figures of unpeened and peened AA2024-T3clad specimens: The [001] direction is orthogonal to the rolling direction; the [010] direction corresponds to the specimens' thickness direction: (**a**) unpeened material; one peening sequence was applied where the advancing direction was parallel to the rolling direction (**b**) or orthogonal to the rolling direction (**c**).

The LP sequence orthogonal to the rolling direction of the AA2024-T3clad sheet results in the formation of an orientation band between <1 1 2>//[001] and <1 0 2>//[001] (Figure 7c). This means a shift of the <1 0 1>//[001] crystal direction to <1 0 2>//[001] as well as an increase of axial intensity of the <1 1 2>//[001] crystal direction within the LP treated region. The <0 0 1>//[010] crystal direction has been retained, whereas the <1 0 1>//[010] showed a weakening compared to the base material and additionally a connection to <1 1 3>//[010].

The comparison to the base material shows that a significant change has taken place due to local plastic deformations induced by the propagating shock waves induced by LP. Kashaev et al. [25] reached a similar conclusion. However, different changes in local orientation were observed between the two applied advancing directions. The material seems to react differently depending on the choice of advancing direction which might be an explanation for the differences in the residual stress profiles. Especially, the weakening of initial axial intensities or pole densities seem to play a significant role. The results indicate that LP induced texture changes can be correlated to the development of misorientations within the microstructure as seen by the angle spreading of axial intensity distributions.
