*3.1. Macroscopic Morphology of Laser Impact Welding Interface*

The typical macroscopic morphology of the laser impact welding interface is shown in Figure 7a. The welding joint area has a ring shape, and the middle is the collision rebound area. As shown in Figure 7c,e, a certain amount of damage was produced on the top surface of the flyer-copper foil irradiated by the laser, and a convex area was produced on the back of the target. For thicker targets, this problem does not exist. Additionally, the generation, propagation, rebound and superposition of stress waves may cause tearing between the flyer and the target [27,28,30]. The center rebound zone of the flat-top laser is larger than that of the Gaussian laser, which may be affected by the impact angle factor mentioned above. The huge rebound zone seriously affects the industrial application and joint performance of laser impact welding (LIW), so eliminating the rebound zone is the primary task of current process optimization.

Liu et al. [31] performed Cu-Al-Cu three-layer impact welding of weldments, that is, using Cu as the flyer, first impact the middle layer Al, and finally, under the impact of the impact, the middle layer accelerates the impact to the Cu of the bottom target, completing three-layer impact welding. Due to the first impact welding of the flyer and the middle layer, impact velocity and impact angle are adjusted, thus reducing the springback of the middle layer Al and the bottom target Cu during the second impact welding process, but the springback of the first flyer and the middle layer still not be controlled. Sadeh et al. [32] found in experiments that the use of black tape between the target and the fixed plate reduces the rebound of the flyer. They used a black tape buffer layer to eliminate the center spring back phenomenon, greatly increasing the area of the weldment area. Convert the weldment area from a ring to a dot and outer ring shape.

Three-layer impact welding and the use of black tape have better eliminated the center spring back and obtained an ideal circular solder joint. They confirmed the possibility of laser impact welding to eliminate the center spring back and made a great contribution to the application of the process. These two experiments jointly pointed out that "buffering" is the key factor for laser impact welding to eliminate center spring back.

**Figure 7.** (**a**) Cross-section of weld interface at 1550 mJ energy (reprodueced from [27], with permission of Elsevier 2019); (**b**–**e**) Weld spot between molybdenum and copper (reproduced from [28], with permission of Elsevier 2019).
