materials Article Investigation of Mechanical and Microstructural Properties of Welded Specimens of AA6061-T6 Alloy with Friction Stir Welding and Parallel-Friction Stir Welding Methods Amir Ghiasvand 1 , Mohammad Mahdi Yavari 2 , Jacek Tomków 3 , John William Grimaldo Guerrero 4 , Hasan Kheradmandan 5 , Aleskei Dorofeev 6 , Shabbir Memon 7 and Hesamoddin Aghajani Derazkola 8, *   Citation: Ghiasvand, A.; Yavari, M.M.; Tomków, J.; Grimaldo Guerrero, J.W.; Kheradmandan, H.; Dorofeev, A.; Memon, S.; Derazkola, H.A. Investigation of Mechanical and Microstructural Properties of Welded Specimens of AA6061-T6 Alloy with Friction Stir Welding and Parallel-Friction Stir Welding Methods. Materials 2021, 14, 6003. https://doi.org/10.3390/ma14206003 Academic Editors: Eduardo Garcia, Alberto Murillo-Marrodán, Hamed Aghajani Derazkola and Tomasz Trzepieci´ nski Received: 7 September 2021 Accepted: 11 October 2021 Published: 12 October 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, University of Tabriz, Tabriz 5166616471, Iran; Amir.ghiasvand@tabrizu.ac.ir 2 Department of Mechanical Engineering, Kermanshah Branch, Islamic Azad University, Kermanshah 67189-97551, Iran; Mmahdiyavarisep@gmail.com 3 Institute of Machines and Materials Technology, Faculty of Mechanical Engineering and Ship Technology, Gda ´ nsk University of Technology, Gabriela Narutowicza Street 11/12, 80-233 Gda´ nsk, Poland; jacek.tomkow@pg.edu.pl 4 Departamento de Energía, Univesidad de la Costa, Barranquilla 55-66, Colombia; jgrimald1@cuc.edu.co 5 Department of Mechanical Engineering, Arak Branch, Islamic Azad University, Arak 38361-1-9131, Iran; kheradmandan.hasan@gmail.com 6 Department of Propaedeutics of Dental Diseases, Sechenov First Moscow State Medical University, Moscow 119991, Russia; sfmsmu@mail.ru 7 Department of Mechanical Engineering, Wichita State University, Wichita, KS 67260-133, USA; sxmemon@shockers.wichita.edu 8 Department of Mechanical Engineering, Islamic Azad University of Nour Branch, Nour 21655432, Iran * Correspondence: h.aghajany@live.com Abstract: The present study investigates the effect of two parameters of process type and tool offset on tensile, microhardness, and microstructure properties of AA6061-T6 aluminum alloy joints. Three methods of Friction Stir Welding (FSW), Advancing Parallel-Friction Stir Welding (AP-FSW), and Retreating Parallel-Friction Stir Welding (RP-FSW) were used. In addition, four modes of 0.5, 1, 1.5, and 2 mm of tool offset were used in two welding passes in AP-FSW and RP-FSW processes. Based on the results, it was found that the mechanical properties of welded specimens with AP-FSW and RP-FSW techniques experience significant increments compared to FSW specimens. The best mechanical and microstructural properties were observed in the samples welded by RP-FSW, AP- FSW, and FSW methods, respectively. Welded specimens with the RP-FSW technique had better mechanical properties than other specimens due to the concentration of material flow in the weld nugget and proper microstructure refinement. In both AP-FSW and RP-FSW processes, by increasing the tool offset to 1.5 mm, joint efficiency increased significantly. The highest weld strength was found for welded specimens by RP-FSW and AP-FSW processes with a 1.5 mm tool offset. The peak sample of the RP-FSW process (1.5 mm offset) had the closest mechanical properties to the base metal, in which the Yield Stress (YS), ultimate tensile strength (UTS), and elongation percentage (E%) were 76.4%, 86.5%, and 70% of base metal, respectively. In the welding area, RP-FSW specimens had smaller average grain size and higher hardness values than AP-FSW specimens. Keywords: parallel-friction stir welding; tool offset; mechanical properties; aluminum alloy 1. Introduction Among solid-state joining techniques, Friction Stir Welding (FSW) is a relatively new and useful technique used in various industries such as aerospace, marine, and automotive industries [1–4]. In the FSW process, a non-consumable rotating tool heats the two pieces due to the contact and intense friction between the two pieces [5–7]. Many factors affect the FSW joint, which are classified into two general categories of process parameters and Materials 2021, 14, 6003. https://doi.org/10.3390/ma14206003 https://www.mdpi.com/journal/materials