*2.1. Laser Peening*

During the LP process, the pulsed laser is focused on the material's surface. The energy input of a laser pulse leads to vaporization of the material at the surface. Thermal expansion of the developing plasma induces a shock wave propagating into the material. Local plastic deformation occurs and causes residual stresses. A transparent overlay increases the pressure and the duration of the plasma and, thus, the efficiency of the process [3]. In the present work, water is used as a transparent overlay. An Nd:YAG laser (wavelength 1064 nm) with the full width at half maximum (FWHM) of 20 ns and a frequency of 10 Hz was employed. A squared laser focus of 1 mm × 1 mm with 1.5 J of energy is used to study the effect of the laser sequence. Additionally, the energy input is changed to 1 J and 3 J for investigating the influence of the energy density. For each specimen, two peening areas which had no interacting effects on the residual stress profile are used. Possible interacting effects were tested for quadratic specimens (40 mm × 40 mm) with only one peening area, which did not show variation in the residual stress profile in comparison to a specimen (40 mm × 80 mm) peened with the same process parameters in two different areas.

For the experimental investigation of the process, AA2024-T3clad specimens (40 mm × 80 mm) with thicknesses of 4.8 mm are investigated (see Figure 1a,b). The specimens feature a clad layer of approximately 0.15 mm thickness on both sides. For some specimens, the aluminum clad layer on one side is removed by milling 0.2 mm, leading to a specimen thickness of 4.6 mm. This allows the investigation of the effect of the clad layer on residual stresses. The milled specimens are named AA2024-T3 in the following. Influences on residual stresses due to the reduced thickness of the milled specimens are assumed negligible. The dimension of the peening areas on the specimens are

16 mm × 17 mm for the investigation of the laser sequence. For particular experiments, an adhesive Al foil of approximately 0.05 mm thickness is applied to the untreated specimen surface with special attention to avoid any air inclusions between the foil and specimen. After peening, the adhesive foil is removed to allow residual stress measurements of the specimen only.

An overview of the different applied peening strategies in this study is given in Figure 1c–l. The two basic peening strategies are characterized by the choice of the advancing direction in relation to the rolling direction (see Figure 1c,h). For the investigation of the influence of shot overlap, the overlap is chosen to be 50 percent only in one direction. Consequently, an overlap can be applied parallel or orthogonal to the chosen advancing direction. The generation of the overlap is schematically shown in Figure 1d,e,i,j. The two basic patterns are also peened with a row-wise change in direction (see Figure 1f,k). The applied strategies for peening with two sequences are presented in Figure 1g,l.

**Figure 1.** (**a**) An illustration of the specimen dimensions with a schematic peening area; (**b**) a peened specimen after residual stress measurements; (**<sup>c</sup>**–**l**) the schematic shot patterns used in this study: (**<sup>c</sup>**–**f**) one peening sequence with an advancing direction parallel to the rolling direction: (**c**); (**d**) overlap in the advancing direction; (**e**) overlap orthogonal to the advancing direction; (**f**) a row-wise change in direction; (**g**) the application of two orthogonal shot patterns, first parallel and second orthogonal to the rolling direction; (**h**–**k**) one peening sequence with an advancing direction orthogonal to the rolling direction: (**h**); (**i**) overlap in the advancing direction; (**j**) overlap orthogonal to the advancing direction; (**k**) a row-wise change in direction; (**l**) the application of two orthogonal shot pattern, first orthogonal and second parallel to the rolling direction.

### *2.2. Residual Stress Measurement via Incremental Hole Drilling Technique*

The PRISM system by Stresstech (Rennerod, Germany) was used for the residual stress measurements. The basis of the system is the incremental hole drilling technique and the electronic speckle pattern interferometry (ESPI). The procedure can be divided into three steps [19]:


All three steps were performed for each increment of the hole, leading to a residual stress profile over the material depth. Coherent light illuminates the surface around the hole, and the light reflection results in a shift of each pixel depending on the roughness of the surface. The superposition of reflected light and reference beam lead to the speckle pattern. The phase shift after the reflection defines the pixel intensity. The comparison of intensity before and after each drilling increment is an indicator for the displacement [20]. Accordingly, thousands of pixels are considered for determining the displacement using the ESPI technique. Subsequently, the integral method is applied. The hole drilling technique assumes constant residual stresses parallel to the material's surface as well as purely elastic deformations during the drilling. Restrictions and necessary assumptions for the incremental hole drilling method using ESPI are explained in detail [21]. In this regard, Chupakhin et al. [22] presented a correction method for equibiaxial residual stress profiles based on an artificial neural network to account for possible plasticity effects.

For all performed measurements, holes of 2 mm diameter and 1 mm depth were drilled incrementally. One hole was drilled in 19 increments. Increments close to the surface were smaller than below, according to the expected residual stress gradient. The position of the holes in the peened area is exemplarily shown in Figure 1b. For most investigated combinations of shot patterns, eight measurements were performed to have sufficient statistics. Four measurements are performed for the peening experiments with Al foil, with overlap, with a 1 J pulse energy as well as the peening experiment where the advancing direction is orthogonal to the rolling direction with a row-wise change in direction. Therefore, the average value as well as the minimum and maximum measured values are shown for these results.

### *2.3. Determining Local Orientation via EBSD*

The crystal orientations of rolled AA2024-T3clad within unpeened materials as well as LP treated regions close to the sheet surface are investigated using a scanning electron microscope (SEM) (JSM-6490LV with EBSD by EDAX, Jeol Ltd., Tokyo, Japan) combined with EBSD in order to clarify the question of how the crystal orientations of AA2024-T3clad have been changed by the LP treatment. The EBSD analysis is performed for an unpeened material as well as for two areas treated with one LP sequence at 1.5 J, where one was peened with the advancing direction parallel and the other one orthogonal to the rolling direction (RD). The specimens are prepared by means of multi-stage grinding and subsequent final polishing whereby the prepared plane was defined by the rolling direction and the direction of material thickness. The LP-treated samples are cut in the center of the peened areas. The specimens are analyzed at 30 kV, a beam current of 0.25 nA, an emission current of 78 μA, a magnification of 300×, a working distance of 14 mm, a step size of 0.70 μm, and a sample tilt of 70◦. The area directly below the clad layer is analyzed in order to generate the inverse pole figures. Consequently, the clad layer is not included in the analysis with the inverse pole figures. The calculation of [001] and [010] inverse pole figures to determine the crystal directions was conducted on the basis of the generalized spherical harmonic expansion (GSHE) method and an assumed triclinic sample symmetry. [001] corresponds to the transverse direction (TD) and [010] to the thickness direction, the normal direction of the rolled AA2024-T3clad sheet.
