Citation: Kosturek, R.; ´ Sl˛ ezak, T.; Torzewski, J.; Bucior, M.; Zielecki, W.; ´ Snie ˙ zek, L.; S ˛ ep, J. Effect of Shot Peening on the Low-Cycle Fatigue Behavior of an AA2519-T62 Friction-Stir-Welded Butt Joint. Materials 2023, 16, 7131. https:// doi.org/10.3390/ma16227131 Academic Editor: Davide Palumbo Received: 18 October 2023 Revised: 5 November 2023 Accepted: 9 November 2023 Published: 11 November 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Effect of Shot Peening on the Low-Cycle Fatigue Behavior of an AA2519-T62 Friction-Stir-Welded Butt Joint Robert Kosturek 1, * , Tomasz ´ Sl˛ ezak 1 , Janusz Torzewski 1 , Magdalena Bucior 2 , Wladyslaw Zielecki 2 , Lucjan ´ Snie˙ zek 1 and Jaroslaw S ˛ ep 2 1 Faculty of Mechanical Engineering,Military University of Technology, 2 Gen. S. Kaliskiego Str., 00-908 Warsaw, Poland;tomasz.slezak@wat.edu.pl (T. ´ S.); janusz.torzewski@wat.edu.pl (J.T.); lucjan.sniezek@wat.edu.pl (L. ´ S.) 2 Department of Manufacturing Processes and Production Engineering, Rzeszow University of Technology, 8 Powsta ´ nców Warszawy Str., 35-959 Rzeszow, Poland; magdabucior@prz.edu.pl (M.B.); wzktmiop@prz.edu.pl (W.Z.); jsztmiop@prz.edu.pl (J.S.) * Correspondence: robert.kosturek@wat.edu.pl Abstract: In this investigation, an AA2519-T62 FSW butt joint was subjected to shot peening with an air pressure of p = 0.6 MPa, a processing time of t = 10 min (per side), and a steel ball diameter of d k = 1.5 mm. In order to evaluate the impact of shot peening on the low-cycle behavior, the samples were tested with coefficient R = 0.1 at total strain amplitudes of 0.35%, 0.4%, and 0.5%. The shot-peened welds are characterized by a higher value of stress amplitude, a lower value of plastic strain amplitude, and their fatigue life increased slightly. The cyclic strength coefficient and the cyclic strain hardening exponent were reduced by 45% and 55%, respectively, as the result of the surface layer hardening. The shot peening process had no noticeable effect on the character of crack initiation and propagation. Almost in all cases, the cracking started in the area under the weld face, located close to the boundary between the thermo-mechanically affected zone and the stir zone at the advancing side. Only at the heaviest loadings (ε ac = 0.5%) were cracks initiated in the heat-affected zone at the retreating side. Despite the introduction of small cracks in the stir zone, their presence did not affect the decohesion character of the welded joint. Overall, it was observed that there is a minimal, positive impact of shot peening on the properties of the investigated joints. Keywords: friction stir welding; aluminum; shot peening; mechanical properties; fatigue; fracture 1. Introduction Friction stir welding (FSW) is a highly effective technique for joining precipitation- hardened aluminum alloys, also providing good mechanical properties for the obtained joints in conditions of cyclic loading [1–3]. In recent years, many efforts focused on in- creasing the fatigue performance of the FSW joints, mostly concerning 2XXX aluminum alloys [4–7]. The first solution involves, predominantly, a reduction in heat at the stage of the welding process by increasing tool traverse velocity or applying additional cooling, which limits the unfavorable heat-activated evolutions of strengthening phases (dissolution, overaging, etc.) [1,5,8]. Generally, the increase in welding velocity produces positive effects, e.g., a decrease in plastic strain amplitude in low-cycle fatigue (LCF) testing, as has been reported by Xu et al. [1]. A similar outcome can be achieved by applying additional cooling (e.g., water, liquid nitrogen, or carbon dioxide) directly after the passage of the welding tool [1,4,8]. A comprehensive review of the so-called rapid cooling techniques in friction stir welding and processing was written by Iwaszko [9]. The second solution is based on post-weld processing, mainly surface cold-working processes [10–12]. It has to be mentioned that, in conventional welding of precipitation- hardened aluminum alloys, post-weld heat treatment is the most efficient way of improving the mechanical properties of welded joints, but in the case of the FSW joints it gives a highly Materials 2023, 16, 7131. https://doi.org/10.3390/ma16227131 https://www.mdpi.com/journal/materials