*3.1. Microstructure and Mechanical Properties of the FSW Joints and the Base Metal*

The mechanical properties of the base metal and the joints are listed in Table 2. The tensile strength and hardness of the base metal were 1162 MPa and 380 HV, respectively, showing toughness values of 4.6 J and elongation of 9.4%. After FSW, both the tensile strength and hardness decreased down to 461 MPa and 162 HV, respectively. However, the toughness and elongation showed significant corresponding increases of 92.2 J and 53%. It was revealed that FSW has a considerable effect on the mechanical properties of beryllium-copper alloys implying that microstructural changes occur in the welding region during FSW. An optical micrograph and a TEM bright-field image of the base metal are shown in Figure 5a,b, respectively. From Figure 5a, it can be seen that the equiaxed α matrix ranged from 40 to 150 μm. In Figure 5b, a dense γ (CuBe) needle-like precipitates can be observed. Figure 6 shows the overall cross-section of the FSW joints. It can easily be identified from Figure 6a that the welding zone of the FSW joint is divided into three regions: the stir zone (SZ), the thermo-mechanical-affected zone (TMAZ) and the heat-affected zone (HAZ). The coarse grain structure in the base metal (BM) is replaced by introducing the SZ containing refined grains (as shown in Figure 6b). This is attributed to the dynamic recrystallization caused by severe thermal plastic deformation in the SZ. A mixture of coarsened, refined and elongated grains in the TMAZ (a transitional region between the SZ and the HAZ) was typical due to the incomplete dynamic recovery and recrystallization in this zone (Figure 6c). The grain size in the HAZ (Figure 6d) was similar than that of the BM (Figure 5a). Figure 7 shows the microstructures of the BM and SZ. Compared with the BM (Figure 7a) and the SZ (Figure 7c) under low magnification, it is apparent that the grains are significantly refined due to the dynamic recrystallization stemming from the frictional heat and plastic flow of the softened material during FSW. It could also be observed that the microstructure consisted of discontinuous precipitation (DP) cells (γ) located at the grain boundaries of α matrix, as presented in Figure 7b. As shown in Figure 7d, the grain boundaries in the SZ do not have DP cells due to their dissolution into the BM during FSW. Figure 8 shows a TEM bright-field image of the SZ. The microstructure consists of only α phase, without γ precipitates. This suggests that the strengthening γ precipitates dissolved into the BM during the FSW heat cycle. Such dissolution of the γ precipitates can be related to decreases in the hardness and tensile strength of the SZ.


**Table 2.** Mechanical properties of the base metal and the FSW joints.

**Figure 5.** Microstructural analysis of the base metal: (**a**) optical microscopy, (**b**) transmission electron microscopy.

**Figure 6.** Cross-sectional macrograph of (**a**) the FSW region and (**b**) micrographs of the stir zone (SZ), (**c**) thermo-mechanical-affected zone (TMAZ) and (**d**) heat-affected zone (HAZ).

**Figure 7.** Scanning electron microscope (SEM) Images of the base metal: (**a**) 100×, (**b**) 3000× and stir zone (**c**) 3000×, and (**d**) 7000×.

**Figure 8.** Transmission electron microscope (TEM) bright-field images in the stir zone.
