**Figure 4.**

*Schematic diagram of vaporizing foil actuator welding.*

#### **2.3 Laser impact welding**

Laser impact welding (LIW) is designed and developed at Ohio State University to join similar and dissimilar material combinations using the energy provided by high-velocity impact.

Joining dissimilar metals for small-scale parts like those used in medical devices and microelectronics could be one of the leading applications. Due to impact welding, the flyer plate collides with the base plate by a high-pressure shock wave created by an intense pulse laser. The basic principle of the technique is that an intense pulse laser beam focused by the lens will generate a specific spot diameter. The absorbent layer then evaporates instantly at a high temperature when exposed to laser irradiation. The vapor absorbs the laser energy, forming high-temperature, high-pressure plasma between the confinement layer and the flyer plate. Between the flyer plate and the confinement layer, the plasma continues to collect laser light and expands faster. As a result of the confinement layer's activity, high surface pressure is created, which propagates through the flyer plate as a shock wave [30].

One notable advantage of LIW over other impact welding processes is that the impact can be confined to a precise spot (sub-micron precision) in a precise time segment (precision of <10-5 seconds). Furthermore, the quantity of energy required for LIW is low, on the order of a few joules. As a result, LIW is an excellent method for generating welds in micro/nano-interface applications. Preliminary studies have been carried out on similar and dissimilar combinations of aluminum, titanium, copper, nickel, and iron. The goal is to understand the underlying bonding mechanisms and to discover if metallurgical reactions that typically lead to embrittlement in dissimilar metal systems (i.e. intermetallic formation) may be avoided. Good bonding can be accomplished in all cases with a typical "wavy" bond contact.

Many investigations on laser impact welding have recently been published. A novel laser high-speed impact spot welding method was proposed by Liu et al., [31]. They used laser impact spot welding to join Ti and Cu metal foils and used scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) to examine the microstructure of the bonding interface. The LIW of aluminum alloy 1100 and low carbon steel 1010 was investigated by Zhang et al., [32]. The welding joint had a relatively gentle curved bonding interface. Wang et al., [33, 34] optimized the flyer plate system in the laser impact welding device (confinement layer, absorption layer, and connecting layer). Wang et al., [35] proposed a parallel laser impact spot welding method. LISW was used to successfully weld dissimilar metal foil plates Cu/Al and Ti/Al. The impact of various welding parameters on welding quality was then carefully examined [35].

LIW process finds applications when thin metal sheets and foils of micron size (at least 25 μm) are to be joined. One distinctive advantage of this approach is that it appears applicable to arbitrarily small foil thicknesses and length scales, and does not rely on the intrinsic electrical conductivity of the flyer. This makes the method well suited for the manufacture and assembly of micro-devices such as micro-electromechanical systems [36, 37]. Wang et al., [34] reported laser impact welding of aluminum foil to titanium which can be used in medical devices, for example, the battery of the heart peacemaker.

#### *2.3.1 Working procedure*

The schematic diagram of LIW is shown in **Figure 5**, and the process involves the following stages:


The benefits and drawbacks of Impact welding are summarized as follows:


**Figure 5.** *Schematic diagram explaining LASER impact welding process.*


The workpieces to join, magnetic pulse welding has various limitations

