**1. Introduction**

Aluminium alloy 6005A, a β-precipitation (Mg2Si) strengthened heat treatable alloy, is widely used in rail transportation industries due to its excellent corrosion resistance and extrusion characteristics [1,2]. The welding joints to connect the extruded sheets and plates are usually the weak regions since the welding heat input introduces the microstructural changes deteriorating the mechanical properties and residual stresses causing fatigue cracks and stress-induced corrosion [3,4]. Therefore, sound welding is required to join extruded aluminium alloy sheets and plates.

Friction stir welding is a solid-state joining technique, which involves both plastic and thermal deformations [5,6]. In this process, process parameters, such as tool rotation and traverse speeds, need to be optimized to get good quality joints. The effect of tool rotation and traverse speeds, on microstructural changes and residual stresses of friction stir welding (FSW) aluminium alloys, have been studied widely [1–4,7–34]. Simar et al. reported that the β" originally present in the base metal

(BM) fully dissolved in the nugget zone (NZ) and coarsened in the heat affected zone (HAZ) of 6005A-T6 aluminium FSW joint [1]. They also explained the softened region around the weld center. Dong et al. studied the effect of welding speed on microstructures and the mechanical properties of 6005A-T6FSW joints, reporting an increased tendency of tensile properties with increasing the welding speed [4]. Wang et al. concluded that the 6061-T6 aluminum FSW joints made at low welding speed exhibited lower residual stress, due to a change in microstructure and stress relaxation that occurred as a result of the longer heating time associated with the low welding speed [8]. Therefore, microstructure, mechanical properties and residual stresses should be considered together to obtain the optimized FSW process. However, most studies have focused on the effects of welding speed and rotation speed on either microstructures and mechanical properties, or residual stresses in aluminium sheets and thin plates.

Nonetheless, FSW has been applied to the production of large prefabricated aluminium panels in high speed railcars, which helps to reduce the weight and improve the integrity of aluminium sheets. To join thick 6005A aluminium plates using FSW, a better understanding of residual stresses and microstructure is required. However, the residual stresses and microstructures have not been studied in thick 6005A-T6 aluminium alloy plates joined by single-sided and double-sided FSW.

Among the methods of residual stress measurement, neutron diffraction can nondestructively characterize the 3D residual stress distribution of engineering materials [35,36]. The deep penetration capability of neutrons into most metallic materials makes neutron diffraction a powerful tool for determining residual stresses through welds. The E3 residual stress diffractometer at HZB is one such high-performance neutron residual stress instrument, capable of experimental measurement of 3D residual stress distributions [37]. This instrument was chosen to characterize the residual stresses in thick 6005A-T6 aluminium alloy FSWs.

In this work, thick 6005A-T6 aluminium alloy plates (37 mm thick) were studied to determine the single-sided and double-sided FSW residual stress distributions using neutron diffraction. In addition, microstructures and mechanical properties were studied to characterize welding behavior. This experimental study was carried out to understand the 3D residual stress distributions and microstructural changes. This study can be helpful in the design of welding strategies.
