Friction stir welding characteristics of 2017-T351 aluminum alloy sheet

Heat-treatable aluminum alloys are difficult to fusion weld because of easy formation of some welding defects such as crack and porosity in the weld [1]. Friction stir welding (FSW) is a solid state welding process in which the crack and porosity often associated with fusion welding processes are eliminated [1, 2]. Therefore, the FSW process is being studied to weld heat-treatable aluminum alloys in order to obtain high-quality joints [3–10]. However, some studies have indicated that FSW gives rise to the softening of heat-treatable aluminum alloys, thus resulting in the degradation of the mechanical properties of the joints. The degradation extent is related not only to the alloy type [9–11], but also to the alloy thickness [12–16]. 2017-T351 aluminum alloy is one of the 2xxx-series heat-treatable aluminum alloys, and a 5-mm thick 2017-T351 plate has been friction stir welded to examine the tensile properties and fraction locations of the joints [9]. This letter aims to further demonstrate the FSW characteristics of a 3mm thick 2017-T351 sheet to comprehend the effect of alloy thickness. The base material used in this study was a 3-mm thick 2017-T351 aluminum alloy sheet with the chemical compositions and mechanical properties listed in Table I. The sheet was cut and machined into rectangular welding samples, 300 mm long by 80 mm wide, and they were longitudinally butt-welded using an FSW machine. The designated welding tool size and welding parameters are listed in Table II, in which the revolutionary pitch (RP) is defined as the travel speed divided by the rotation speed. After welding, the joints were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests. The crosssections of the metallographic specimens were polished with an alumina suspension, etched with Keller’s reagent and observed by optical microscopy. The configuration and size of the transverse tensile specimens were prepared according to Fig. 1, in which RS and AS denote the retreating side and advancing side of the joint, respectively. Prior to the tensile tests, the Vickers hardness profiles across the weld nugget (WN), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and partial base material were measured along the centerlines of the cross-sections of the tensile specimens under a load of 0.98 N for 10 s, and the