*3.5. E*ff*ect of Material Flow on Microstructure*

Figure 19 shows a comparison of the simulated temperature field and the metallographic structure of the joint cross section. It can be seen from the observation that the weld nugget zone (WNZ, zone I) in the cross section of the friction stir welding joint is connected with the thermo-mechanically affected zone (TMAZ, zone II). The heat affected zone (HAZ, zone III) is adjacent to the thermo-mechanically affected zone. In the simulation model, the temperature in the contact area with the tool is relatively high and gradually decreases to both sides.

**Figure 19.** Cross-sectional comparison of the joints.

Due to the different material flows and microstructures in each region of the joint, the WNZ of the material forms a clear boundary with the TMAZ, and the material flow direction can be clearly seen from the formation of the microstructure in the TMAZ. As shown in Figure 20b, the microstructure in the WNZ is composed of fine recrystallized grains due to the intense stirring action of the pin. At the same time, it can be observed that the material in the TMAZ also has a plastic flow. The material under TMAZ near WAZ is rotated by the tool pin, and the material has an upward flow trend. Some of the material is squeezed into the WNZ, which is also found in the simulation. The material in TMAZ far away from the WNZ tends to move obliquely upward, and shows a similar rule on the advancing side, as shown in Figure 20a. However, it can be observed that the material flow slope on the advancing side is larger, which also confirms that the material flow on the advancing side is more intense.

**Figure 20.** The boundary and material flow between the weld nugget zone (WNZ) and the thermo-mechanically affected zone (TMAZ). (**a**) The boundary area between the WNZ and the TMAZ on the advancing side; and (**b**) the boundary area between the WNZ and the TMAZ on the retreating side.

Figure 21a shows that the material flows obliquely to the weld nugget region above the TMAZ, and that the material in the tool shoulder action region has a slight flow trend. However, it can be seen from Figure 21b that a triangular region of material flow appears on the upper part of the advancing side due to the joint action of the tool pin and the tool shoulder. Here, the upper part of the weld has a downward flow trend under the extrusion of the tool shoulder. The simulation process also observed the action process of the tool shoulder. The action of the tool shoulder is more intense on the retreating side. The three-directional flows of the TMAZ material, tool shoulder action zone material and the WNZ material meet sharply. At the same time, if the material flow is abnormal in this area, weld defects will easily occur.

**Figure 21.** The material flow intersection of the welded joint. (**a**) The material intersection on the advancing side; and (**b**) the material intersection on the retreating side.
