*3.1. The Interface Microstructure of the Explosive Welded Plate*

The typical interface microstructure of explosive welded TA1/Q235B composite plate is shown in Figure 3. It exhibits a typical periodic wave and the wavelength is about 400 μm (Figure 3a). The wave interfaces of the TA1/Q235B composite plate are divided into two categories: The vortex region is shown in Figure 3b and the transition layer as shown in Figure 3c. The wave expands along the effective bonding area of titanium and steel hinders crack propagation, which is an ideal microstructure of a welded joint [24]. The vortex region is composed of titanium and steel, and the transition layer is composed of compounds [25,26].

**Figure 3.** (**a**) SEM of the TA1/Q235B interface: (**b**) the vortex region and (**c**) transition layer.

Figure 4a shows the details of vortex region at higher magnifications. The Ti and Fe element distributions measured by EDS mapping are displayed in Figure 4b,c, respectively. The Ti element is marked in green, and the red represents Fe element. It can be seen that the vortex region is a mixture of the two melted elements. The segregation of Fe (red) can be observed in the center of the melted vortex region, while Ti (green) is uniformly distributed in the red areas (Fe element). In addition, a wide red layer can be found in the crest, revealing the liquid steel flow from the high-pressure part to the low-pressure part. When titanium and carbon steel collide at high temperatures, pressures and speed, the accumulated heat does not have enough time to transfer outwards, causing the carbon steel and part of the titanium to melt at 1811 K. The most deformed steel melts first at the bonding interface, while the titanium does not directly melt. Only when more liquid steel flows to the titanium part and is mixed with the molten titanium can the vortex region be formed; this can explain why Fe dominates the central vortex region. Finally, under the action of relatively low wave-front pressure, the mixture of liquid steel and titanium accumulates gradually and flows towards the vortex region along the direction of the wave crest.

**Figure 4.** (**a**) SEM microstructure of melted vortex regions of TA1/Q235B welded plate (**b**) EDS map for Ti (Green) and (**c**) Fe (Red).

Another interface is a transition layer. As seen from Figure 5a, the large proportion of microscopic interface is widely distributed at the joint surface of the two metals. They tend to present behind the waveform. The energy dispersive spectroscopy (EDS) element line distribution in Figure 5b also shows the concentration gradient distributions of the two elements in the transition layer, indicating that the diffusion of titanium and carbon steel occurred in the transition layer. Additionally, because the pressure behind the wave is lower than at the peak, the metal in the transition layer is less likely to melt compared to the vortex region. When the two metals explosive welding, the heat generated by the collision is difficult to spread and accumulated at the back of the wave [27]. Although the metal in the transition layer did not melt, high temperature and friction region during the explosion were caused by collision, more importantly, which also caused the atoms of two metals to diffuse rapidly, creating a concentration gradient.

**Figure 5.** (**a**) SEM microstructure of transition layer, and (**b**) EDS element line distribution for Ti (Green) and Fe (Red) in transition layer of explosive welded TA1/Q235B plate.
