3.3.1. Interface with and without Transition Layer

Figure 12 shows two typical interface structures in impact welding revealed by electron microscopy: welding interface with transition layer and welding interface without transition layer [44,45]. For the welding interface with a transition layer, the transition layer is a new phase different from the base material produced after melting and solidification of the interface metals, which belongs to the "rapid melting-solidification" interface bonding mechanism [46]. For the interface without a transition layer, there are currently two opposing views: on the one hand, scholars represented by Stern [47] believe that the interface combination is attributed to the "mechanical mixing" effect and there is no melting. The high plastic deformation of the interface leads to the rapid refinement of crystals and the formation of an intermediate thin layer. On the other hand, scholars represented by Marya [48] believe that under high-speed impact conditions, the temperature increase in the impact interface is inevitable. This type of interface is also formed by the "rapid melting-solidification" of the thin metal layer. The formation of the transition layer is related to the input energy.

For the foil vaporization welding with higher input energy than LIW, Sridharan N et al. [49] used the latest TEM and APT techniques to observe the structure of the weld interface without transition layers from the nanoscale. As shown in Figure 12a, they found that there is a nano-scale amorphous layer at the interface. The amorphous layer contains elements of the two weldments, confirming the diffusion of interface atoms during the impact. In this regard, they proposed a "liquid film" diffusion mechanism. That is, the metal atoms with the lower melting point are first melted into a liquid film during the impact welding process, and the higher melting point atoms on the other side will enter the liquid film for diffusion. Their study indicated that the interface reaction in impact welding is complex and exhibited different phenomena at a different scale.

**Figure 12.** Two welding interfaces of impact welding (**a**) Transition zone interface (reproduced from [49], with permission of Elsevier 2019); (**b**) Interface without transition zone (reproduced from [24], with permission of Laser Institute of America 2016).

Under SEM observation, the laser impact welding interface mostly belongs to the interface without the transition layer. Wang H. et al. [50] used EBSD to confirm that the grains in the vicinity of the joint are significantly refined, and there are nanocrystalline ribbons like the vaporized foil, but no obvious continuous transition layer is found on the welded joints of dissimilar metals. However, the impact is a rapid process of energy accumulation and release. Especially in the second half of the cycle, as shown below, the impact will abnormally increase the energy to form a discontinuous intermetallic compound, and finally form a mixed interface without a transition layer and a transition layer. At present, the mixed interface formed by laser impact welding with such low energy input has not been explored. Exploring the formation and distribution of these two interface structures is of

great significance to reveal the order of the formation of the impact welding interface. In particular, the influence of discontinuous intermetallic compounds on the brittleness of the bonding surface has a certain significance for the improvement of process performance.
