*Article* **Residual Stress, Microstructure and Mechanical Properties in Thick 6005A-T6 Aluminium Alloy Friction Stir Welds**

**Xiaolong Liu 1,**†**, Pu Xie 2,**†**, Robert Wimpory 3, Wenya Li 4, Ruilin Lai 5, Meijuan Li 1, Dongfeng Chen 1, Yuntao Liu <sup>1</sup> and Haiyan Zhao 6,\***


Received: 3 July 2019; Accepted: 18 July 2019; Published: 21 July 2019

**Abstract:** Plates (37 mm thick) of 6005A-T6 aluminum alloy were butt joined by a single-sided and double-sided friction stir welding (FSW). The 3D residual stresses in the joints were determined using neutron diffraction. The microstructures were characterized by a transmission electron microscope (TEM) and electron backscatter diffraction (EBSD). In the single-sided FSW specimen, there were acceptable mechanical properties with a tensile strength of 74.4% of base metal (BM) and low residual stresses with peak magnitudes of approximately 37.5% yield strength of BM were achieved. The hardness is related to the grain size of the nugget zone (NZ), and in this study, precipitations were dissolved due to the high heat input. In the double-sided FSW specimen, there were good mechanical properties with a tensile strength of 80.8% of BM, but high residual stresses with peak magnitudes of approximately 70% yield strength of BM were obtained. The heat input by the second pass provided an aging environment for the first-pass weld zone where the dissolved phases were precipitated and residual stresses were relaxed.

**Keywords:** residual stresses; friction stir welding; neutron diffraction; aluminium alloys; hardness; precipitation
