*4.2. Interface Hardness of Laser Impact Welding*

In addition to the bonding force, the nanoindentation experiment is used to test the hardness of the tiny interface area to characterize its strengthening under the high-speed impact. As shown in Figure 17, the laser impact welding joint is impact-strengthened, and the hardness is higher. The hardness of the matrix on both sides of the joint gradually decreases from the interface. However, the hardening effect is different for LIW (Laser Impact Welding) and MPW (Magnetic Pulse Welding) due to the different input energy. The higher hardness at the same position is affected by the more energy input [33].

Laser impact welding is a transient, high-temperature, and high-pressure process. The microstructure of the material undergoes abrupt changes, such as the increase in dislocations, the refinement of grains, and the formation of cellular structures, which change the properties of the material. On the one hand, the welded joint produces high-rate strain plastic deformation strengthening under impact; on the other hand, the rapid plastic deformation during the impact process will generate a lot of heat and cool down in a short period, which can be regarded as a quenching process [28,33,55,56].

However, it should be noted that the formation of brittle intermetallic compounds will also significantly increase its hardness. Therefore, to evaluate its performance, the bonding force of the welded joint and its hardening degree should be considered together.

**Figure 17.** Hardness of weld interface with MPW (Magnetic Pulse Welding) and LIW (Laser Impact Welding) (reproduced from [33], with permission of Elsevier 2011).
