Prof. Wenya Li’s group (School of Materials Science and Engineering, Northwestern Polytechnical University) recently published a paper entitled “In-depth understanding of material flow behavior and refinement mechanism during bobbin tool friction stir welding” on International Journal of Machine Tools and Manufacture (2021) 171: 103816. In this research, they visualized the material flow behavior during bobbin tool friction stir welding (BT-FSW) of aluminum alloy which was used to explain the defect formation and microstructure evolution, and to reveal the joint forming mechanism, as illustrated in Fig. 1.

Fig. 1. Schematic diagram of joint forming mechanism.

The demand for high loading capacity, high endurance and low energy consumption for aircrafts, spacecrafts and new energy vehicles, makes the application of lightweight materials, such as aluminum alloys, a great attention. Among that, the welding of closed or hollow profiles usually faces big problems, such as root defects, complex rigid support requirements. By introducing additional shoulder on the bottom side instead of the fixed backing plate, BT-FSW can achieve self-supporting and strengthen the thermal effect at the same time. However, one significant challenge is to avoid the formation of internal defects, which requires an in-depth understanding of the underlying physical processes of the welding process. Therefore, they considered how to visually present the flow behavior of materials during BT-FSW. Several interesting features are shown in this work:

a. In the Coupled Eulerian-Lagrangian (CEL) model, tracing particles were specially embedded, which can clarify the overall temperature and strain distribution and intuitively observe the local material migration.

b. The material flow was concentrated in the SZ but non-simultaneous. In the horizontal plane, the materials initially located on the advancing side (AS) occupied the inner side of the shear layer and deposited behind the advancing side to refill the instantaneous cavity. In contrast, the materials originating from the retreating side (RS) flowed around under the direct action of the rotating shoulders or the indirect action of flowing materials inside to accumulate in the weld, with limited material near the shoulders refilling the instantaneous cavity together (Fig. 2).

Fig. 2. Horizontal material flow near: (a) the shoulder, and (b) the pin.

c. The flow behavior in vertical direction was also asynchronous. The material flow was relatively weak with hysteresis before reaching the AS, but it was more stable and presented laminar feature which was broken and deflected toward inside due to the instantaneous cavity. Then, multiple material flows converged near the mid-thickness on the AS under the shear/extrusion action of the rotating tool, eventually forming a funnel-shaped structure (Fig. 3).

Fig. 3. Vertical material flow near: (a) the shoulder, and (b) the pin.

d. In addition to the direct effect of material migration driven by the rotating tool, the confluence zone on the AS also affected the morphology of S-line, of which the extension to the RS further promoted the deflection of S-line under the squeezing effect

Prof. Wenya Li’s group has long engaged in the microstructure evolution and forming mechanism of the friction welding processes. Dr. Qiang Chu is the first author, Prof. Wenya Li and Assoc. Prof. Xiawei Yang are the corresponding authors. This research is funded by Natural Science Foundation of Shaanxi Province and National Natural Science Foundation of China, and this published paper can be viewed by https://authors.elsevier.com/c/1dxIY3AXKs8Tyd.

Author: Wenya Li

Reviewer: Hongqiang Wang，Wei  Liu