3.3.2. Laser Impact Welding Interface Diffusion and Intermetallic Compounds

At the impact welding interface with the corresponding occurring obvious melting phenomenon, the atom diffusion mechanism is the same as that in the melting and welding process, and intermetallic compounds forms. This kind of regional melting phenomenon is common in explosive welding and magnetic pulse welding/foil vaporization welding interface. Wang X et al. [27] found that laser impact welding also showed a local melting lump as shown in Figure 13 at high welding input energy, and a platform also appeared on the EDS (Energy Dispersive Spectrometer) concentration curve. According to previous studies by Akbari and Behnagh [51] and Zhang et al. [33], it is an intermetallic compound. Therefore, in laser impact welding, with high laser energy, melting at the interface also occurs. EDS can only qualitatively analyze the existing problems of intermetallic compounds. In-depth analysis is still needed for the composition and shape of intermetallic compounds.

**Figure 13.** EDS (Energy Dispersive Spectrometer) curves of impact weld melting block (reproduced from [27], with permission of Elsevier 2019) (**a**) SEM image of aluminum/copper weld interface with fusion block; (**b**,**c**) EDS curves at positions 1 and 2.

For laser impact welding, the interface is generally no obvious melting phenomenon and the intermetallic compounds [24]. For example, Wang et al. [52] studied the laser impact welding of amorphous and crystalline materials. They used XRD to compare and analyze the changes in amorphous materials before and after laser impact welding. As shown in Figure 14, the EDS curve is the same as other materials. It is a continuous curve and no intermetallic compounds were found. In addition, they found that LIW could not cause structural changes in the amorphous matrix.

Chen S et al. [53] and Ning L et al. [54] found that the crystal structure at the interface was destroyed during an impact, resulting in a disordered organization. The two elements diffused and interacted with each other in a disordered structure, then, followed into the interior of the ordered crystal structure. Although the calculated diffusion layer thickness is slightly lower than the experimentally measured diffusion layer thickness, it provides an effective way to study atomic diffusion at the impact welding interface. This numerical simulation calculation result is consistent with "liquid film", that is, the key to the inter-diffusion of LIW across the bonding interface is the disordered layer of atoms produced by the impact. In the future, high-precision characterization technology and numerical simulation technology are expected to reveal the atomic diffusion mechanism of laser impact welding interface.

**Figure 14.** EDS concentration curve for laser impact welding of crystal and amorphous materials [52].
