materials Review Manufacturing Parameters, Materials, and Welds Properties of Butt Friction Stir Welded Joints–Overview Aleksandra Laska * and Marek Szkodo Department of Materials Engineering and Bonding, Faculty of Mechanical Engineering, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-698-071-526 Received: 2 October 2020; Accepted: 31 October 2020; Published: 3 November 2020 Abstract: The modern and eco-friendly friction stir welding (FSW) method allows the combination of even such materials that are considered to be non-weldable. The development of FSW technology in recent years has allowed a rapid increase in the understanding of the mechanism of this process and made it possible to perform the first welding trials of modern polymeric and composite materials, the joining of which was previously a challenge. The following review work focuses on presenting the current state of the art on applying this method to particular groups of materials. The paper has been divided into subchapters focusing on the most frequently used construction materials, with particular emphasis on their properties, applications, and usage of the FSW method for these materials. Mechanisms of joint creation are discussed, and the microstructure of joints and the influence of material characteristics on the welding process are described. The biggest problems observed during FSW of these materials and potential causes of their occurrence are quoted. The influence of particular parameters on the properties of manufactured joints for each group of materials is discussed on the basis of a wide literature review. Keywords: friction stir welding; FSW; solid type welding; mechanical properties; weld strength 1. Introduction Friction stir welding (FSW) is a method invented at the Welding Institute of the United Kingdom and patented by Wayne Thomas in 1991 [1]. It is considered to be one of the most prospective material joining developments in the last 30 years. Primarily, this method was dedicated to joining aluminum and its alloys, but today it is widely used for titanium and its alloys, magnesium and its alloys, steel and ferrous alloys, and copper, but also polymers and composites. The FSW process is defined as a solid-state method. Materials to be joined do not melt during the process. Since the melting point is not reached, typical problems of fusion welding techniques are eliminated. These problems are usually related to a change of state, such as changes of volume and solubility of gases, and these effects are not observed during friction stir welding process [2–4]. During the process, a specially designed tool is put into linear movement along a joint line, rotating at the same time. The kinetic energy of the tool is transformed into thermal energy, generated by the friction on the interface between the tool and the components. The heated material is plasticized by a tool and extruded around the pin in a backward direction of a tool moving along the edge of a contact line. The FSW method is usually used to produce butt welds, but it also allows the fabrication of joints of other types, such as corner welds, T-welds, lap welds, and fillet welds [5–8]. A schematic illustration of a friction stir welded butt joint is shown in Figure1. Nowadays, the FSW method is widely used in many industrial areas, such as aerospace (wings, fuel and cryogenic tanks, Materials 2020, 13, 4940; doi:10.3390/ma13214940 www.mdpi.com/journal/materials Materials 2020, 13, 4940 2 of 46 Materials 2020, 13, x FOR PEER REVIEW 2 of 55 fuselages) [9–12], railways (underground carriages, wagons, container bodies) [13–15], marine and fuselages) [9–12], railways (underground carriages, wagons, container bodies) [13–15], marine and shipbuilding (deck panels, hulls, booms, masts, offshore accommodation) [14,16], construction industry shipbuilding (deck panels, hulls, booms, masts, offshore accommodation) [14,16], construction (frames, bridges, pipelines) [17,18] and land transportation (wheel rims, mobile cranes, tail lifts) [19]. industry (frames, bridges, pipelines) [17,18] and land transportation (wheel rims, mobile cranes, tail The FSW technology is also applied in sectors such as machinery equipment, electronics, metalworking, lifts) [19]. The FSW technology is also applied in sectors such as machinery equipment, electronics, and the R&D sector [20]. metalworking, and the R&D sector [20]. Figure 1. Schematic illustration of a friction stir welding process. Figure 1. Schematic illustration of a friction stir welding process. In the cross-section of the friction stir welded joint, a specific microstructure is observed. Due to the solid-stateIn the cross-section nature of of the the process, friction the stir zones welded that joint, are not a specific found inmicrostructure welds produced is observed. by conventional Due to thewelding solid-state methods nature can of be the distinguished. process, the zones Based that on are thermomechanical not found in welds actions produced of the by FSW conventional tool, four weldingdistinct zonesmethods can becan observed: be distinguished. weld nugget Based (stir on zone, thermomechanical SZ), thermo-mechanically actions of aff theected FSW zone tool, (TMAZ), four distinctheat aff ectedzones zone can (HAZ),be observed: and base weld material nugget (una (stiffrected zone, zone, SZ), BM).thermo-mechanically The presence of the affected stir zone zone is a (TMAZ),result of theheat recrystallization affected zone (HAZ), in the middleand base part material of the (unaffected thermo-mechanically zone, BM). aff Theected presence zone. The of the nugget stir zoneis formed is a result by fine of grainthe recrystallization sized metal. The in material the middle of the part SZ of experiences the thermo-mechanically plastic deformation affected due zone. to the Theinteractions nugget withis formed the tool. by Rhodes fine grain et al. [sized21] and metal. Liu et The al. [ 22material] claimed of that the in SZ the experiences recrystallized plastic grains, deformationa low density due of dislocationsto the interactions is observed. with the However, tool. Rhod otheres studieset al. [21] proved and Liu that et theal. [22] recrystallized claimed that grains in theof therecrystallized SZ contain grains, high density a low ofdensity sub-boundaries of dislocations [23], dislocationsis observed. [However,24] and subgrains other studies [25]. Betweenproved thatthe SZthe andrecrystallized the HAZ, agrains unique of transitionthe SZ contain zone, high called density the thermo-mechanically of sub-boundaries [23], affected dislocations zone, can [24] be andobserved. subgrains TMAZ [25]. is Between exposed tothe both SZ temperatureand the HAZ, and a deformationsunique transition during zone, the process.called the Because thermo- of mechanicallyinsufficient deformations, affected zone, strain can recrystallization be observed. isTMAZ not observed. is exposed Exposure to both to hightemperature temperatures and deformationsduring welding during might the cause process. the dissolution Because of of insufficient precipitates deformations, in TMAZ. Beyond strain TMAZ, recrystallization the heat-a ffisected not observed.zone is observed. Exposure In to that high region, temperatures there are during no plastic welding deformations, might cause but the it is dissolution still subjected of precipitates to a thermal incycle. TMAZ. The Beyond alteration TMAZ, of properties the heat-affected in HAZ, compared zone isto observed. the base material,In that region, includes there changes are inno ductility, plastic deformations,toughness, corrosion but it is susceptibility, still subjected and to strength. a therma Thel cycle. changes The in alteration grain size orof chemicalproperties makeup in HAZ, are comparednot observed to the [26 base]. The material, typical cross-sectionincludes changes of the in FSW ductility, joint andtoughness, the microstructures corrosion susceptibility, of different zones and strength.are shown The in Figurechanges2. in grain size or chemical makeup are not observed [26]. The typical cross- section of the FSW joint and the microstructures of different zones are shown in Figure 2. Materials 2020, 13, 4940 3 of 46 Materials 2020, 13, x FOR PEER REVIEW 3 of 55 Figure 2. (a) Typical weld macrostructure of 6061–T6 Al alloy in FSW, (b) microstructure of the joint- Figure 2. (a) Typical weld macrostructure of 6061–T6 Al alloy in FSW, (b) microstructure of the joint-stir stir zone (DXZ, dynamic recrystallized zone), TMAZ, HAZ, and BM taken at yellow square marks zone (DXZ, dynamic recrystallized zone), TMAZ, HAZ, and BM taken at yellow square marks shown shown in (a) [27]. in (a)[27]. The FSW technology isis classifiedclassified asas a green technology.technology. AsAs a melting point is not reached during the process, less energy is used in comparison to fusion welding techniques. Moreover, CO2 emission the process, less energy is used in comparison to fusion welding techniques. Moreover, CO2 emission toto thethe atmosphereatmosphere cancan bebe significantlysignificantly reducedreduced [[28].28]. It is relatively easy to controlcontrol thethe processprocess andand setting optimaloptimal parameters parameters allows allows for for a subsequenta subsequent reduction reduction of necessaryof necessary non-destructive non-destructive testing. testing. 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