3.1.3. Interface Behavior

Figure 9 shows the micromorphology and EDS of the interface of the keyhole-free FSSW joint at the center and edge areas. In Figure 9a, the metal interlayer with discontinuous length and uneven thickness can be observed at the Al/steel interface. The metal interlayer is a transition layer composed of IMCs [11], which can be observed by local magnification of the metal interlayer region, as shown in the upper left corner in Figure 9a. The elements and their content were quantitatively analyzed by spot scanning the IMC layer, as shown in the upper right corner in Figure 9a. The IMC layer mainly contained Al, Fe, and Zn elements, and their contents were approximately 71.9%, 20.5%, and 7.1%, respectively. It can be concluded that the IMCs were mainly composed of Al, Fe, and Zn elements. There was an IMC layer at the interface at the edge of the keyhole-free FSSW joint, as shown in Figure 9b. It can be seen that the IMCs were not filled sufficiently at the interface gap, as shown in the lower right corner in Figure 8b. The interfacial bonding was poor and some of them became unbound [37,38]. The element diffusion was qualitatively analyzed by line scanning at the interface, as shown in the EDS at the interface in Figure 9b. It can be observed that the Fe, Al, and Mg elements linearly attenuated at the interface. However, the Zn element increased significantly at the interface. The zinc coating of DP600 galvanized steel participated in the interfacial reaction, resulting in the diffusion and increase in Zn in the interfacial layer [13].

**Figure 9.** Micromorphology and energy-dispersive spectrometer (EDS) of the interface of the keyhole-free FSSW joint: (**a**) center area and (**b**) edge area.

The transition layer was further magnified and analyzed by EDS in Figure 10. Figure 10a shows the microstructure and EDS from the spot scans of the interface layer in the keyhole-free FSSW joint in the center area. The color of the interface transition layer is different from that of the matrix, and the thickness was approximately 15 μm [11]. The elements of the three atlases at points 1, 2, and 3 were quantitatively analyzed, as shown in the lower left corner of Figure 10a. The elements of the three spots in 1, 2, and 3 were mainly Al, Fe, and Zn. As the boundary of the interface became closer, the content of Zn increased, and the content of Fe decreased [13]. This indicates that the composition of the interface layer changed, and the transformed IMCs were also different [13,37–40]. Figure 10b shows the microstructure and EDS of the line-scanning analysis of the interface layer of the keyhole-free FSSW joint at the edge area. It can be found that the Fe, Al, Mg and Zn elements were staggered in the interface layer. This composition difference led to different IMCs, which had a non-uniform composition and content [13,37–40].

**Figure 10.** Micromorphology and EDS of the transition layer of the keyhole-free FSSW joint: (**a**) center area and (**b**) edge area.

Figure 11 shows the micromorphology and EDS of the Al/steel interface in WNZ. The cloud cluster-like chaotic microstructure at the Al/steel interface can be found in the stirring zone. The elements of Fe and Al are distributed alternately with streamlined lines [10]. Meanwhile, the elements of Mg, Cu, Si and Zn were also unevenly distributed. This indicates the stratified intersecting and inhomogeneous mixing of the Al alloy and steel plates [10,41]. The stirring of the pin was uneven for the material in the welding process. The material only reached the thermoplastic state during the short welding time and fast cooling rate, which led to the mechanical mixing of Al and steel components.

**Figure 11.** Micromorphology and EDS of the cloud cluster-like microstructure.

Figure 12 shows the XRD spectrum of the cross-section of the dissimilar Al/steel keyhole-free FSSW joint. It can be found that IMC FeAl3 is formed at the Al/steel interface, as analyzed by the above elemental diffusion. Fereiduni et al. [38] also obtained an IMC with a similar atomic ratio at the interface layer. Due to the diffusion of the Zn element in the galvanized coating, the Zn element dissolved into the Al matrix to form phase AlZn*x*, and form IMC FeAl3Zn*<sup>x</sup>* with IMC FeAl3 [13,37,38]. The Zn element diffused to the lattice space, and its elemental content was rather unstable [4]. At the same time, The IMC FeAl3 formed under different welding parameters is the same, but the content is different. With the increase of rotation speed, the welding temperature becomes higher, and the stirring of joints becomes more sufficient; the content of IMC FeAl3 and the thickness of the transition layers formed at a high temperature also increase.

**Figure 12.** XRD spectrum of a cross-section of the dissimilar Al/steel keyhole-free FSSW joint.
