**2. Laser Impact Welding Process**

Laser irradiation on the surface of the material will cause temperature and force effects. According to the order of magnitude of energy input from small to large, the phenomenon of temperature rise, melting, vaporization, and plasma excitation will occur in sequence. While vaporizing and exciting the plasma, an instantaneous stress action is formed on the surface.

In the decades since the 1970s, the stability and high speed of the laser-driven flyer flying were verified, and the Gurney mathematical model of flyer flying speed and its influencing factors were established [16–18]:

$$
\rho \mathbf{x}\_d \mathbf{E} = (\rho/2)(\mathbf{x}\_0 - \mathbf{x}\_d)\mathbf{v}\_0^2 + (\rho/2) \int\_0^{\mathbf{x}\_d} (v\_0 \mathbf{x}/\mathbf{x}\_d)^2 d\mathbf{x} \tag{1}
$$

ρ is the density of ablated material, *xd* is the thickness ablated away, E is called Gurney energy, *x*<sup>0</sup> is the original thickness, and *v*<sup>0</sup> is the final velocity.

The formula is based on the principle of conservation of energy. The left side is the energy released by the ablation layer, and the right side is the kinetic energy. Equation (1) was simplified to obtain the final velocity:

$$v\_0 = \sqrt{\frac{3E}{3x\_0/2x\_d - 1}}\tag{2}$$

In the 1940s, Carl first proposed the use of explosives to drive metal and metal collisions for metallurgical bonding, named explosive welding [19]. Nowadays, people use chemical energy [20], electromagnetic field energy [21], high-energy-density light energy [22], high-pressure gas [23], etc. as

driving sources to achieve various forms of impact welding by the transient release of high energy and drive high-speed collision of welding parts.

The principle of atomic bonding at the impact welding interface is shown in Figure 2. When atoms reach a certain position, interatomic bonding will occur. However, the obstacles on the surface of the flyer and the target such as oxides, oil stains, and surface impurities prevented the atoms from the flyer and the target to get close within the atomic distance. The basic principle of high-speed impact welding is to remove the bonding obstacles by jet flow, and with the help of the transient and huge impact force of the high-speed impact to make the atoms reach a close enough distance to achieve the bond between the atoms [23].

**Figure 2.** Atomic force-distance curve.

Laser impact welding is also an impact welding process with laser as the driving source, which is mainly used in spot welding of millimeter/micron-scale [24,25]. As shown in Figure 3a, the laser impact welding system is divided into two parts: the energy source, namely the laser system, and the weldment support system [26,27]. To prevent non-uniform force caused by continuous energy input and to ensure energy transfer efficiency, a pulsed laser with a pulse width of about 10 ns and a wavelength of 1064 nm is usually used. Since the laser can achieve 0–100% capacity adjustment, the greater the energy that the laser can achieve, the wider the applicability, but generally the minimum energy required to achieve millimeter-level spot welding is about 1 J. The commonly used laser types are flat-top laser and Gaussian laser. The energy distribution of the laser beam is shown in Figure 3b,c respectively. We can change the laser beam diameter by adjusting the position of the convex lens to determine the energy density and solder joint size. The arrangement sequence of the support system from left to right is confinement layer, ablation layer, flyer, standoff, and target. The whole set of equipment is fixed on the stander.

During the impact welding process, due to collision and extrusion, a jet along the welding direction is generated at the collision point to clean the surface, which is a necessary condition for the metallurgical bonding of impact welding. For the flat-top laser-driven flyer, the flat-top light action area on the flyer first collides with the target in parallel, that is, the impact angle is zero degrees, there is no metallurgical bonding in this area, and rebound occurs for a large area after the collision. As the impact collision progresses, the impact angle gradually increases, enters the welding window, and the interface metallurgical bond is formed. Therefore, the solid-state metallurgical bonding area produced by the flat-top laser is a narrow ring shape. However, the metallurgical bonding area produced using Gaussian laser to drive the flyer is a wider circular ring shape.

The parameters of laser impact welding are also categorized into two groups: laser system parameters and weldment combination parameters. The parameters of the laser system are the laser energy and laser spot size, that is, the laser energy and laser spot size acting on the ablation layer. The combination parameters of weldment are complex, as shown in Table 1, including various indexes of the weldment support system, such as confinement layer, binder, ablative layer, the thickness of the flyer, preset flying distance, and so on [26].

**Figure 3.** (**a**) Schematic diagram of laser impact welding system; (**b**) flat-top laser energy distribution; (**c**) Gaussian laser energy distribution.

After starting the laser, laser impact welding can be divided into three stages, as shown in Figure 4 [28]:

**Figure 4.** Schematic diagram of laser impact welding process (reproduced from [28], with permission of Elsevier 2019). (**a**) Excitation stage: The laser irradiates the ablation layer through the confinement layer, and the ablation layer is vaporized into plasma. Due to the limitation of the confinement layer, the reaction force of the plasma drives the flyer to emit; (**b**) Flight phase: the flyer passes the preset flight distance (standoff) and collides with the target at a certain speed and angle; (**c**) Welding stage: The behavior of the impact point is shown in Figure 5 (the welded area on the left). The flyer and the target collide at a certain angle from the starting position of the metallurgical bonding to the end position and the welding is over.

**Figure 5.** Schematic diagram of the impact welding process.



The laser system parameters and weldment combination parameters affect the metallurgical bonding process by controlling the impact velocity Vp and impact angle β in Figure 5 at the collision point. The jet only starts in the shaded area process window as shown in Figure 6. Therefore, studying the effects of various parameters on the impact velocity and angle is very important for optimizing the laser impact welding process. It is worth noting that the jet is also affected by the welding metal itself, and the jet process window of different metals is different [24,29].

**Figure 6.** Generic welding window (reproduced from [10], with permission of Elsevier 2019).
