materials Article Modification of Surface and Subsurface Properties of AA1050 Alloy by Shot Peening Yasemin Yıldıran Avcu 1 , Berkay Gönül 1 , Okan Yetik 1 , Fikret Sönmez 2 , Abdulkadir Cengiz 3 , Mert Guney 4, * and Egemen Avcu 1,5,6   Citation: Avcu,Y.Y.; Gönül, B.; Yetik, O.; Sönmez, F.; Cengiz, A.; Guney, M.; Avcu, E. Modification of Surface and Subsurface Properties of AA1050 Alloy by Shot Peening. Materials 2021, 14, 6575. https://doi.org/10.3390/ ma14216575 Received: 23 September 2021 Accepted: 27 October 2021 Published: 1 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Department of Mechanical Engineering, Kocaeli University, Kocaeli 41001, Turkey; yaseminyildiran89@gmail.com (Y.Y.A.); berkaygonul@gmail.com (B.G.); oknyetik@gmail.com (O.Y.); avcuegemen@gmail.com (E.A.) 2 Department of Mechanical Engineering, Hasan Ferdi Turgutlu Faculty of Technology, Manisa Celal Bayar University, Manisa 45140, Turkey; sonmezfikret@gmail.com 3 Department of Automotive Engineering, Kocaeli University, Kocaeli 41001, Turkey; akcngz@gmail.com 4 Department of Civil and Environmental Engineering, The Environment and Resource Efficiency Cluster (EREC), Nazarbayev University, Nur-Sultan 010000, Kazakhstan 5 Ford Otosan Ihsaniye Automotive Vocational School, Kocaeli University, Kocaeli 41650, Turkey 6 School of Materials, The University of Manchester, Manchester M13 9PL, UK * Correspondence: mert.guney@nu.edu.kz; Tel.: +77-1-7270-4553 Abstract: AA1050 Al alloy samples were shot-peened using stainless-steel shots at shot peening (SP) pressures of 0.1 and 0.5 MPa and surface cover rates of 100% and 1000% using a custom-designed SP system. The hardness of shot-peened samples was around twice that of unpeened samples. Hardness increased with peening pressure, whereas the higher cover rate did not lead to hardness improvement. Micro-crack formation and embedment of shots occurred by SP, while average surface roughness increased up to 9 μm at the higher peening pressure and cover rate, indicating surface deterioration. The areal coverage of the embedded shots ranged from 1% to 5% depending on the peening parameters, and the number and the mean size of the embedded shots increased at the higher SP pressure and cover rate. As evidenced and discussed through the surface and cross-sectional SEM images, the main deformation mechanisms during SP were schematically described as crater formation, folding, micro-crack formation, and material removal. Overall, shot-peened samples demonstrated improved mechanical properties, whereas sample surface integrity only deteriorated notably during SP at the higher pressure, suggesting that selecting optimal peening parameters is key to the safe use of SP. The implemented methodology can be used to modify similar soft alloys within confined compromises in surface features. Keywords: crack growth; mechanical behaviour; plastic deformation; shot peening; soft metals 1. Introduction Most pure metals exhibit corrosion resistance [1–3], high specific strength [2,4], and good electrical conductivity [2,5] while showing poor mechanical properties [6,7] com- pared to their alloys. The industrial use of pure metals is strongly dependent on their mechanical properties such as σ y , elastic modulus, and hardness [8]. However, most commercial pure metals have relatively poor σ y [2,9], σ u [10], surface hardness [8], and wear resistance [8], which may restrict their usage, particularly in applications requiring moderate mechanical performance [2,4,6,11]. Specifically, their surface hardness could be very low as available strengthening mechanisms within their microstructure are limited due to a lack of alloying elements [4,6,7,12,13], which may restrict their widespread use in surface-related applications. Commercially pure Al, including AA1050 Al alloy, has been widely used in various ap- plications such as household items [11], food containers [3], chemical plant equipment [3], light reflectors [3,14], rivets [15], heat exchangers [14], and electrical wiring applications [16]. Materials 2021, 14, 6575. https://doi.org/10.3390/ma14216575 https://www.mdpi.com/journal/materials