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ORIGINAL ARTICLE

Microstructure characteristics and mechanical properties of stationary shoulder friction stir welded 2219-T6 aluminium alloy at high rotation speeds Jiaqing You 1,2 & Yunqiang Zhao 2 & Chunlin Dong 1,2 & Chungui Wang 2 & Shu Miao 2 & Yaoyong Yi 2 & Yunhai Su 1 Received: 25 June 2019 / Accepted: 21 October 2019 # Springer-Verlag London Ltd., part of Springer Nature 2019

Abstract Stationary shoulder friction stir welding has been used to weld 4-mm-thick 2219-T6 aluminium alloy at high rotation speeds. Strain plastic damage was applied to demonstrate the formation mechanism of welding defects at high rotation speeds above 2000 rpm. A three-way converging zone in the joint, in which materials of different microstructure characteristics converged from three directions during high tool rotation speed welding, was found. At the relatively high tool rotation speed, the significant differences in the microstructures would result in weld defects in this zone. It could be attributed to material toughness damage at high strain rate. With increasing tool rotation speed, the tensile strength of the joint constantly decreased. When the tool rotation speed varied from 2000 to 2600 rpm, the tensile strength decreased from 305 MPa (68.2% of the BM) to 238 MPa (53.2% of the BM). Keywords Stationary shoulder friction stir welding . Three-way converging zone . Microstructure . Mechanical properties

1 Introduction Friction stir welding (FSW) is a solid joining technology with the characteristics of high welding efficiency, good joint quality and no pollution. It has been widely applied in many industries, such as aviation, aerospace, rail trains and ships [1–3]. To improve the weld appearance, stationary shoulder friction stir welding (SSFSW) was invented. In the SSFSW, the shoulder and pin are separated. In the course of welding, only the pin is rotated. As a result, flash and fish scales can be eliminated by the stationary shoulder [4–5].

* Yunqiang Zhao [email protected] * Chunlin Dong [email protected] 1

Department of material science and engineering, Shenyang University of Technology, Shenyang 110870, Liaoning, China

2

Guangdong Welding Institute (China-Ukraine E.O. Paton Institute of Welding), Guangdong Provincial Key Laboratory of Advanced Welding Technology, Guangzhou 510651, China

Compared with conventional FSW, the welding heat input for the SSFSW is low because no rotation friction occurs for the stationary shoulder. Therefore, joint softening can be reduced, and the mechanical properties of the joint can be improved [6–8]. Wu et al. [9] investigated the microstructures and mechanical properties of joints welded by FSW and SSFSW. Their results indicated that the heat input of SSFSW was 30% lower than that of FSW, and a narrower heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ) were obtained. However, the correlation between microstructures and mechanical properties was not explained in detail. Sun et al. [10] characterised the residual stress distribution and texture evolution in AA