materials Article Influence of Welding Speed on Fracture Toughness of Friction Stir Welded AA2024-T351 Joints Miodrag Milˇ ci´ c 1, * , Dragan Milˇ ci´ c 1 , Tomaž Vuherer 2 , Ljubica Radovi´ c 3 , Igor Radisavljevi´ c 3 and Aleksija Ðuri´ c 4   Citation: Milˇ ci´ c, M.; Milˇ ci´ c, D.; Vuherer, T.; Radovi´ c, L.; Radisavljevi´ c, I.; Ðuri´ c, A. Influence of Welding Speed on Fracture Toughness of Friction Stir Welded AA2024-T351 Joints. Materials 2021, 14, 1561. https://doi.org/10.3390/ma14061561 Academic Editor: Shinichi Tashiro Received: 20 February 2021 Accepted: 16 March 2021 Published: 22 March 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 Faculty of Mechanical Engineering, University of Nis, Aleksandra Medvedeva 14, 18000 Niš, Serbia; dragan.milcic@masfak.ni.ac.rs 2 Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia; tomaz.vuherer@um.si 3 Military Technical Institute, Ratka Resanovi´ ca 1, 11000 Belgrade, Serbia; ljubica.radovic@vti.vs.rs (L.R.); igor.radisavljevic@vti.vs.rs (I.R.) 4 Faculty of Mechanical Engineering, University of East Sarajevo, Vuka Karadži´ ca 30, 71123 East Sarajevo, Bosnia and Herzegovina; aleksija.djuric@ues.rs.ba * Correspondence: miodrag.milcic@masfak.ni.ac.rs Abstract: In order to ensure a quality welded joint, and thus safe operation and high reliability of the welded part or structure achieved by friction stir welding, it is necessary to select the optimal welding parameters. The parameters of friction stir welding significantly affect the structure of the welded joint, and thus the mechanical properties of the welded joint. Investigation of the influence of friction stir welding parameters was performed on 6-mm thick plates of aluminum alloy AA2024 T351. The quality of the welded joint is predominantly influenced by the tool rotation speed n and the welding speed v. In this research, constant tool rotation speed was adopted n = 750 rpm, and the welding speed was varied (v = 73, 116 and 150 mm/min). By the visual method and radiographic examination, imperfections of the face and roots of the welded specimens were not found. This paper presents the performed experimental tests of the macro and microstructure of welded joints, followed by tests of micro hardness and fracture behavior of Friction Stir Welded AA2024-T351 joints. It can be concluded that the welding speed of v = 116 mm/min is favorable with regard to the fracture behavior of the analysed FSW-joint. Keywords: friction stir welding; process parameters; welding speed; tool rotation speed; AA-2024-T351; fracture toughness 1. Introduction Aluminum is one of the most common metals on earth and is widely used for en- gineering structures and components in many industries such as aerospace, automotive, rail vehicles, and shipbuilding. Aluminum and aluminum alloys have low density, high corrosion resistance, excellent processing properties and excellent thermal and electri- cal properties [1]. Aluminum is intrinsically sustainable because once produced, it can be recycled repeatedly without any loss in quality and reused in the manufacturing of consumer and industrial products. The fabrication of light structures from Al alloys is usually done by welding the parts. Various welding processes are used, most often electric arc welding (Metal Inert Gas—MIG and Tungsten Inert Gas—TIG), friction stir welding (FSW), friction joining. Many aluminum alloys, such as the 2XXX and 7XXX series, are known for their low welding capabilities and are often classified as non-weldable materials when electric arc processes are used [2]. The problem of non-weldable aluminum alloys of the 2XXX and 7XXX series has conditioned the research of new welding technologies in the last thirty years. The friction stir welding (FSW) process is a relatively new joining technique [3]. Friction stir welding (FSW) is a welding process developed and patented Materials 2021, 14, 1561. https://doi.org/10.3390/ma14061561 https://www.mdpi.com/journal/materials