**4. Mechanical Properties of Laser Impact Welding Interface**

### *4.1. Laser Impact Welding Interface Strength*

The welding area of laser impact welding is a millimeter-sized ring, and it is difficult to measure its area under mechanical testing in real-time. Therefore, the maximum force (N) that can be achieved in the tensile fracture of the welded joint is usually used to characterize the bonding performance of the welded joint. The test methods mainly include the peeling test and shearing test [24,28], as shown in Figure 15.

**Figure 15.** Test method for bonding force of welded joints (**a**) shearing test and result (reproduced from [28], with permission of Elsevier 2019); (**b**) peeling test and result (reproduced from [24] with permission of Laser Institute of America 2016).

The mechanical properties of laser impact welded joints are generally considered to be related to energy density, welding area, and flyer thickness. The improvement of bonding force is affected by the higher the energy density, the larger the welding area, and the thicker the flying piece. As mentioned

above, the higher the energy density, which leads to more generation of interface waves. It can be seen as increasing the bonding force by increasing the welding area and "mechanical interlocking".

According to the failure location, laser impact welding can be divided into joint damage failures under low energy state and matrix damage failures under high energy state. The joint failure under low input energy is shown in Figure 16a,c,e. The joint bonding force is lower than the strength of the matrix, and the failure location is located in the joint, which is mostly brittle fracture; the matrix fails under high input energy. As shown in Figure 16b,d,f, the strength of the metallurgical joint is higher than the strength of the matrix, and the failure location is in the matrix, which is generally ductile fracture. Therefore, when the input energy is low, the input energy can be increased to increase the joint bonding force, and when the input energy is high, the thickness of the weldment can be increased to directly strengthen the target material and improve the damage resistance [27].

**Figure 16.** Two forms of failure (reproduced from [27], with permission of Elsevier 2019) (**a**,**c**,**e**) Low-energy joint failure; (**b**,**d**,**f**) High-energy base metal failure.
