**1. Introduction**

The improvement of fatigue life of lightweight structures plays an important role in the construction of aircrafts. In order to fulfill the requirement of long-living lightweight structures, advanced materials with innovative processing technologies have to be utilized. Aluminum alloys like AA2024-T3 are frequently used, for example, in commercial aircrafts [1]. AA2024 is characterized by a high strength and a comparatively low density. Despite the advanced development of modern alloys, corrosion and fatigue remain factors limiting aircrafts' working life [2]. Modification technologies like laser peening (LP), also named laser shock peening (LSP), treat the surface of the material locally, leading to residual stresses in the material which can improve corrosion and fatigue resistance [3,4]. The LP process is contact-free, highly controllable and achieves deeper residual stress profiles in comparison to shot peening [5].

To generate tailored residual stress distributions, it is necessary to gain knowledge of the residual stress influencing LP process parameters. The result of the LP process is influenced by laser parameters like the energy input and the size of the laser focus [6,7] as well as the geometry of the laser focus [8]. The achievable maximum residual stress values are limited by the material's yield strength. Multiple investigations [9–11] have observed increased residual stresses for higher energy densities. Multiple shots on one area lead to higher and deeper residual stresses [5,9,12]. Furthermore, as shown by Toparli and Fitzpatrick [13], a shot overlap also leads to higher stresses. The overlap rate may influence the anisotropy of the residual stress profile [13–15]. To sum up, the energy density, directly defined by the focus size and chosen energy input, the number of shots, and the applied overlap rate determine how much energy an area experiences. This is crucial for the residual stress distribution.

The shot pattern describes the strategy for the positioning of the laser pulses on the peening area. Saliminarizi et al. [16] compared a row-wise peening pattern to a spiral peening pattern with regard to the surface properties of the material. As a result, surface roughness was found to be dependent on the peening pattern and overlap rate. Xu et al. [15] have shown that the choice of scanning path has an influence on residual stresses in a 316L stainless steel blade. Correa et al. [14,17] showed that the choice of the advancing direction influences the residual stress profile and the improvement of fatigue attributes. Furthermore, the same research group [18] proposed that randomly applied shots provide a more equibiaxial stress profile than the row-wise application of shots.

The present study relates the applied shot pattern to the residual stresses developing in the material. The systematic investigation showed the influence of the advancing direction as well as the applied overlap on the residual stress profile. Moreover, different shot patterns are combined by peening two sequences with different advancing directions, and possible mechanisms for the resulting residual stress profiles are discussed. The experimental setup for this work as well as material used and measurement technology are presented in the following section. The influence of the clad layer and the use of aluminum foil during peening on the resulting residual stresses is also shown in this study. Moreover, the importance of the choice of the advancing direction for the residual stress state as well as the effect of shot overlap is presented for one peening sequence. Additionally, the residual stresses for peening with two sequences with different shot patterns for each sequence are pointed out. For unpeened materials as well as for materials treated by particular shot patterns, local orientation changes are investigated using electron backscattered diffraction (EBSD). Furthermore, the effect of different energy densities on the material's surface is investigated.
