Inhomogeneous Gradient Microstructure and Mechanical Properties of Thick Copper Plate via Friction Stir Welding

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JMEPEG https://doi.org/10.1007/s11665-020-05296-6

Inhomogeneous Gradient Microstructure and Mechanical Properties of Thick Copper Plate via Friction Stir Welding Dongyao Wang, Quanqing Zeng, Diqiu He, and Kefu Gan Submitted: 10 April 2020 / Revised: 5 October 2020 / Accepted: 26 October 2020 Inhomogeneous microstructural gradient of the friction stir welding (FSW) joint in T2 thick copper plates was studied in the present work. Microstructures along the thickness direction of the nugget zone (NZ) were systematically characterized via using electron back-scattering diffraction (EBSD) method. The corresponding mechanical properties of different layers in the NZ, involving tensile performance and microhardness, were examined as well. Through our characterization results of each layer in the NZ, it is found that a signiﬁcant gradient microstructure is generated due to the temperature and strain gradient produced by FSW process. Reﬁned equiaxed grains dominate the NZÕs lower areas, while coarse grains with a mass of twins occupy the upper regions. Along the thickness direction from the top to bottom, the mean grain size of the NZ gradually declines from 15.56 ± 8.79 to 5.74 ± 3.06 lm. Such reﬁned microstructure in the lower layer of the NZ leads to the enhancement of mechanical properties of the specimens due to the Hall–Petch relationship. Particularly, the strength and hardness of the lowest layer are found to be comparable with the cold-rolled base metals. The result further uncovers the underlying mechanism about how the microstructural gradient forms on FSW and how it affects the mechanical performance of the weld joint. The work also inspires that, through controlling the heat input of the present FSW method, the gradient microstructure of NZ can be rationally adjusted to improve the welding quality. Keywords

copper, friction stir welding, hardness, microstructure, mechanical properties

1. Introduction Copper and copper alloys have been widely applied in the manufacturing industries because of its excellent electrical/ thermal conductivity, desirable ductility as well as outstanding corrosion resistance. Particularly, in the ﬁeld of nuclear energy, copper and copper alloys are commonly used for manufacturing of divertor, reactor vacuum vessel and nuclear fuel disposal canister (Ref 1, 2). Therefore, a safe and reliable weld joint of copper components is of great importance. However, due to the high melting point and thermal conductivity of copper, traditional fusion welding technology can hardly ensure the quality of the weld joint of copper and copper alloys, to satisfy the requirements of industrial applications. Friction stir welding (FSW) is a green and efﬁcient solidstate welding technique developed by Welding Institution of UK, which have widely utilized for on welding nonferrous metals (Ref 3). Till now, a mass of studies have been conducted Dongyao Wang, College of Mechanical and Electrical Engineering, Central South University, Changsha 410083 Hunan, China; Quanqing Zeng, Research Institute of Light All

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