**1. Introduction**

Welding technique has wide applications in the areas of aerospace, automobiles, shipbuilding, pressure vessels, and bridges. It is one of the important manufacturing methods in industries. Based on the state of materials during the welding process, welding techniques are categorized as solid-state welding and fusion welding [1]. In fusion welding, metallurgical bonding occurs during the solidification of materials. In solid-state welding, metallurgical bonding occurs below the melting point of materials. Therefore, defects such as solidification cracking, distortion, and porosity [2], which appear in fusion welding as a result of the liquid phase, can be avoided in solid-state welding. Solid-state welding has a long history that predates the invention of arc welding. Ancients used hammers to weld gold earlier than 1000 B.C., which today is called forge welding. Since the early 20th century, the development of solid-state welding has been limited due to arc welding being easier than forge welding and having a higher efficiency.

The advantages of joining dissimilar and other specific materials, such as Al 7075 alloy, titanium, and zinc-coated sheet steels [3], brought solid-state welding back onto the stage. For example, the joining of steel and copper provides good electric conductivity and mechanical properties; the joining of steel and aluminum reduces the weight of automobile [4]. Dissimilar materials usually have different thermal conductivity, thermal expansion coefficients, and melting points, which may result in defects in fusion welding [5–7]. Furthermore, some specific alloys are sensitive to heat. Both Al 7075 alloy and titanium are important materials in aerospace and aircraft. However, Al 7075 alloy is susceptible to hot cracking [8,9], and titanium is chemically active at high temperature [10–12]. Zinc-coated steel is an important structural material in automobiles. However, during the fusion-welding process, zinc vaporizes, and porosity forms when zinc vapor is trapped.

Various solid-state welding methods have been developed recently, for example, friction stir welding, explosive welding (EXW), friction welding, magnetic pulse welding (MPW), cold welding, ultrasonic welding, roll welding, pressure gas welding, resistance welding, vaporizing foil actuator welding (VFAW), gas gun welding (GGW), and laser impact welding (LIW). Among those solid-state welding methods—GGW, EXW, MPW, VFAW, and LIW—are five kinds of high-velocity impact-welding methods. They shared the same welding mechanism, but have different welding energy sources indicated by their names: high-speed gas [13], explosive [14,15], capacitor bank energy (MPW and VFAW) [16], and laser [17], respectively.

High-velocity impact welding is characterized by a low welding temperature and fast welding speed. The process is conducted at room temperature. Furthermore, there is no external heat input during the welding process. Figure 1 is a schematic transient state in the welding process. After the transient force is applied on the external surface of the flyer, the flyer moves toward the target at the velocity of several hundred meters per second [18]. When the flyer collides on the target, a jet is generated at the collision point, which contains contaminants, oxide layers, and a thin layer of metals. As a result, the "clean" metals (no contaminants, oxide layers) are exposed to each other. With the transient force, they are brought within atomic distance, where the atomic bond is formed. This process is usually takes several to dozens of microseconds based on different welding processes. For example, in LIW, it usually takes less than one microsecond. In other high-velocity impact-welding processes, it takes longer than that.

**Figure 1.** A transient state of bonding interface in high-velocity impact welding, reproduced from [19], with permission from Roral Society, 1934.

In this review, five high-velocity impact-welding methods and the corresponding welding mechanisms are reviewed in Section 2. In Section 3, the macro-characteristics and micro-characteristics of the bonding interface associated with weld quality are summarized. In Section 4, the welding parameters that may affect the welding quality are discussed, such as for example, the material properties, impact velocity, impact angle, and surface preparation. At the end, conclusions and future work are addressed based on the discussion of welding mechanisms, bonding interfaces, and welding parameters.

## **2. Welding Methods and Jet Phenomenon**

## *2.1. Overview of High-Velocity Impact-Welding Methods*

Nowadays, five high-velocity impact-welding methods have been developed, which are GGW, EXW, MPW, VFAW, and LIW. GGW was limited to the lab research on welding parameters and welding mechanisms. EXW experienced decent studies, and has been widely applied in manufacturing all over the world. MPW has limited application in automobiles, and is under development all over the world. VFAW studies have been mainly conducted by researchers from Ohio State University, and are in the transition state from lab research to industrial application. LIW was proposed recently, and is under lab research.

## 2.1.1. Explosive Welding and Gas Gun Welding

EXW is the first high-velocity impact-welding method. It was observed when a projectile was fired by explosives and collided on another metal surface [20]. However, there was no record of EXW until the First World War. Its rapid development and wide application in industries was in the middle of the 20th century. The first United States (U.S.) patent filed by Philipchuk et al. in 1962 [21]. As its name suggests, explosives detonation is implemented to provide the impact force for the flyer. It was reported that more than 200 material combinations have been successfully welded by EXW [22], for example, Fe/Ti [23–25], Zr/Fe [26], Al/Ti [27], Ti/Ni [28], and Al/Fe [29,30]. For plate welding, two generally used experimental setups are the inclined mode and parallel mode, as shown in Figure 2. Buffer was used to avoid severe damage to the flyer by explosives. The inclined setup came from the idea of hollow charge (which is introduced later); it was used earlier than parallel setup. α was the initial angle in the inclined mode. Since it was not capable of handling large sheet metal, the parallel setup was developed later, and was mainly used to weld large plates with a pre-determined standoff distance. In the parallel setup, the impact angle varies with different standoffs. EXW was also used for the cladding of tubes [15]. From a two-dimensional observation, the experimental setup for tube cladding or ring to tube welding involves the parallel mode for plate welding curved into a circle. In EXW, it is difficult to measure the impact velocity directly, if it is at all possible. Usually, the explosive detonation velocity is measured [31].

**Figure 2.** Schematic experimental setups of explosive welding (EXW): (**a**) inclined; (**b**) parallel.

Since EXW is not easy to conduct within the laboratory, GGW is usually used to study the welding parameters and welding mechanism for EXW. In GGW, the flyer is accelerated by high speed and high-pressure gas to collide on the target. Botros and Groves provided an impact force for the flyer with high-pressure gas from burning gunpowder in a 76-mm powder cannon gun [32]. Later on, compressed helium was used since it is clean and easy to control. In GGW, the initial angle is preset either on the flyer or target. The impact velocity can be measured with a high-speed camera [32] and electrical circuit [33]. The resolution of the high-speed camera was not enough to record the welding process accurately, because the welding process is usually done within several microseconds. In the electrical circuit method, there was a corresponding voltage change in this electrical circuit when wires were cut successively by the flyer. The time between the voltage change was then recorded to calculate the impact velocity [34]. The effect of the wires' transient block on impact velocity and impact angle has not ye<sup>t</sup> been investigated.

EXW has been applied in industries all over the world. The Dynamic Materials Corporation is a world-leading explosive metalworking business. EXW has been successfully applied in the energy area. Its main application includes heat exchangers, as well as upstream and downstream products, in the oil and gas industries.

## 2.1.2. Magnetic Pulse Welding

MPW was used in Russia in the 1960s for the first time to weld an end closure for nuclear fuel rod holders [35,36]. The application of MPW is mainly tubular, for example, joining between tubes or tubes and cylinders. Currently, the most common application of MPW in industry is driveshaft production in the company of Pulsar and Dana [37]. MPW didn't experience a rapid development and wide application as EXW until recently due to the slow development of equipment [37]. Metal combinations of Cu/Cu [38], Cu/brass [39], Cu/Al [40], Cu/Fe [41], Al/Al [18], Al/brass [39], Al/Mg [42–44], Al/Fe [36,45–48], Al/ bulk metallic glass [49], and plastic/Al [50] etc. have been investigated. The general weld configurations of MPW are the lap joint and butt joint. MPW doesn't require a skilled operator. In addition, the welding parameters of MPW are controlled separately to permit fully exploring the effect of welding parameters over a wide range.

In MPW, the flyer is driven by electromagnetic force to collide on the target. The MPW system includes a capacitor bank, coil actuator, the flyer, and the target. The capacitor bank is the energy source of MPW. It is an open electrical circuit of inductance, resistance, and capacitance. The closed circuit was formed by the connection between the capacitor bank and coil actuator, which is made of electrical conductive materials. With the high-speed switch on, the capacitors begin to charge. The primary current passes through the actuator; thus, a changing magnetic field is around the actuator that interacts with any metals within it. Consequently, a secondary current with the opposite direction of the primary current is induced in the flyer. Simultaneously, the flyer is expelled by the electromagnetic force to collide on the target. A typical primary current and the flyer velocity, as measured by a Rogowski coil and photon Doppler velocimetry, are shown in Figure 3. The rise time is an important factor to relate impact velocity. The shorter the rise time, the higher the impact velocity will be.

**Figure 3.** Typical primary current and flyer velocity in magnetic pulse welding (MPW).

Figure 4 shows schematic experimental setups for MPW with different coil actuators. A solenoid coil actuator is used for tube-to-tube welding (Figure 4a). Figure 4b shows the experimental setup with a uniform pressure actuator (UPA) for plate-to-plate welding. The UPA was initially designed for magnetic pulse forming at Ohio State University [51]. The wires between the flyer and the target form an impact angle during the welding process. The three types of bar actuators are I-shaped, U-shaped, and E-shaped (Figure 4c–e); they were developed by Kore et al. in India [52,53], Zhang et al. in the U.S. [54] and Aizawa et al. in Japan [45], separately. The thinner bar is used to push the flyer for U-shaped and E-shaped actuators. If an I-shaped bar and the thinner bars have the same dimension, an E-shaped actuator brings more inductance into the circuit. The rise time becomes longer. Currently, MPW is limited to lap joints. Experimental setups for other types of welding should be developed in the future.

**Figure 4.** Schematic experimental setups for magnetic pulse welding (MPW) with various actuators: (**a**) solenoid coil actuator for tube-to-tube welding; (**b**) uniform pressure actuator for plate-to-plate welding; (**<sup>c</sup>**–**<sup>e</sup>**) I-shaped actuator, U-shaped actuator, and E-shaped actuator for plate-to-plate welding.

## 2.1.3. Laser Impact Welding

The first U.S. LIW patent was filed in 2009 by Daehn and Lippold from Ohio State University [55]. The welding system consists of a laser system, confinement layer, ablative layer, flyer, and target [17]. A laser beam ablates the ablative layer into plasma. With the confinement layer, the expansion of the plasma pushes the flyer to collide on the target. Materials such as copper, aluminum, steel, and titanium were welded with the LIW method [18,56,57]. The current experimental setup is similar to that for EXW, as shown in Figure 2. The possible industry application setup was proposed by Wang et al. [17]. In their study, the effect of the confinement layer, ablative layer, and the connection between the flyer and confinement layer were investigated for industrial application. The current weld configuration is limited to spot welding. In the study of Liu et al., they proposed the flyer movement with a Gaussian laser beam for laser spot welding. The central part of the spot was not welded, while welding occurred around the circumferential direction. The same phenomenon was also observed by Wang et al. [58], who also simulated the welding process with the smooth particle hydrodynamics (SPH) method [59] and reproduced the spalling and rebound phenomena by the simulation. For LIW, the spalling and rebound phenomena should be further studied in order to propose ways for the procedures to avoid those two behaviors. The welding area within the laser spot should be further increased. One of the possible future applications of LIW is the welding between Al and Ti in the heart pacemaker.

## 2.1.4. Vaporizing Foil Actuator Welding

VFAW utilizes the expanding plasma from the vaporization of thin foils to push the flyer to move toward the target. The development of VFAW before 2013 was summarized in [60]. In recent studies, the energy for the vaporization of thin foils is from the capacitor bank. The foil was designed in a dog-bone shape in order to concentrate the transient current from the capacitor bank to vaporize the

foil. Vivek pointed out that the aluminum foil provided better mechanical impulse than copper foil through its rapid vaporization. Materials such as steel, aluminum, titanium, copper, magnesium, bulk metallic glass, etc., have been successfully welded with VFAW [61–65]. The original experimental setups in VFAW were similar to those used in EXW. The issue with those experimental setups was that the central part of the impacted region was not welded due to the rebound behavior of the flyer. The welds are very narrow. With this experimental setup, the impact angle was formed due to the standoff between the flyer and the target. Upon collision, the edge of the flyer was kept static, while the central part of the flyer moved toward the target. The impact angle was formed when the flyer collided on the target. However, unlike in EXW, in which the flyer gradually contacted with the target from one end to the other, in VFAW, the collision between the flyer and the target were at the same time. Thus, non-continuous metallurgical bonding resulted on the collision interface. By studying the effect of the impact angle on metallurgical bonding, the target was designed with grooves, which have different angles. The angle range for wave formation along the collision interface was proposed as 8–24◦ for 3003 Al and 4130 steel [64], and 16–24◦ for 3003 Al and pure Ti [61]. The recorded maximum impact velocity for VFAW was 900 m/s [66]. As for the numerical simulation, the finite element method with Eulerian formalism is a suitable way to predict the morphology of the collision interface in high-velocity impact welding [67]. The main possible application of the VFAW process is in the automobile industries. Researchers at Ohio State University are investigating its application in the car frame.

## *2.2. Jet Phenomenon*

Jet is the essential reason for welding to occur in the high-velocity impact-welding process. In high-velocity impact welding, a jet is generated by the high-velocity oblique impact of the flyer and target, and then rapidly flies away. Without the impact angle, the jet would be trapped between the collision interfaces. Therefore, the impact angle is one of the significant parameters in order for metallurgical bonding to occur. It is commonly believed that the jet consists of thin metal layers, oxide layers, and other contaminants from the colliding surfaces of the flyer and the target [1]. After the jet flies away, clean virgin metal surfaces are generated. The surfaces are then brought together to a distance within the atomic scale by a transient high-impact pressure. As a result, the atomic bond is produced between the contact area of the flyer and the target. The jet phenomenon has been studied by several researchers [68,69]. In EXW, some welding parameters are applied to predict the jet generation. There is continuous jet formation in EXW due to the collision between the flyer and the target moving from one end to the other end continuously with the detonation of explosives. In other high-velocity impact-welding processes, the continuous generation of the jet along the collision interface is an issue to be resolved by the optimization of an experimental setup design.

The phenomenon in which explosives with a cavity in contact with a steel plate can make a deeper hole in the steel ahead of it than explosives without a cavity has been known for more than 200 years. This phenomenon is called hollow charge. Regarding the study of hollow charge, the phenomenon that explosives with a cavity lined with thin metal layers have enormous penetration ability was discovered around 1948. Birkhoff et al. studied the powerful penetrating and cutting phenomenon of hollow conical liners and hollow wedge-shaped liners (cross-section shown in Figure 5), respectively. As shown in Figure 5b, a high pressure from the detonation of explosives was applied on the outer wall of the wedge, causing it to collapse inward. Figure 5c shows the geometry of the collapse process of the metal liner: α is the initial angle that was set before the experiment; β (β = 0) is the dynamic angle (impact angle) that was formed during the experiment; *V*0 is the velocity of the metal liner (impact velocity); *V*1 (*Vw*) is the welding velocity in explosive welding; *Vd* is the detonation velocity of the explosives. The metal liner became two parts at the collision: slug and jet. The jet was from the inside of the metal liners, and the slug was from the outside of the metal liners. Birkhoff et al. [68] proved that the jet was the one with strong penetration ability. This is the first time that the jet phenomenon was studied in detail. It proved that the oblique collision between materials resulted in jet formation on the collision interfaces.

**Figure 5.** Hollow charge with wedge-shaped metal liner: (**a**) prior to the detonation, and (**b**) during the detonation; and (**c**) the geometry of the collapse process of the metal liner.

The geometry of the collapse process of the wedge-shaped metal line was adopted in EXW to build the math relationship between the welding parameters later on. The mathematic model to predict the velocity and the mass of the jet was built when *V*2 is subsonic [19,70]. Cowan and Holtzman studied the limiting conditions for jet formation when *V*2 is supersonic, and pointed out that a minimum impact angle should be satisfied for jet formation when *V*2 is supersonic [69]. *V*2 is the main entrance jet velocity under the assumption that the metal collision is the fluid-like flow.

However, the limitation of their model should be discussed. The pre-condition for their model is the fluid-like flow metal behavior. In their study, the metal was treated as fluid, but they didn't quantitatively clarify what the requirement was for that pre-condition. They assumed that the shear strength of the metals was negligible compared with the impact pressure from the explosives. Furthermore, in the study of Birkhoff et al. [68], a jet was always generated regardless of the other conditions. Actually, the pressure at the collision point should be high enough for the deformation of materials into a jet [22]. In other words, their model couldn't predict whether a jet will happen or not. Although the mechanism of jet generation has not been fully understood, the existence of a jet and the role of a jet in high-velocity impact welding have been generally accepted. As stated earlier, the penetration phenomenon of explosives with a cavity with a metal liner on the armor plate or concrete walls is direct evidence for the existence of a jet. The existence of a jet was also proved using radiographs according to Birkhoff et al. [68]. The existence of a jet was also verified by gas gun welding in which mass of the flyer and target can be measured accurately. Mass comparison before and after the welding has shown that the loss of mass should equal the mass of the jet. In the recent studies regarding the welding mechanism, the research has focused on the effect of the welding interface characteristics, weldability window, and welding parameters on the weld quality. However, jet formation should be studied further due to the continuous metallurgical bonding along the collision interface relying on the continuous generation of the jet, especially for MPW, VFAW, and LIW.

## **3. Bonding Interface Characteristics**

In welding engineering, a welded zone with high strength, good toughness, and enough hardness is desirable. However, in reality, the welded zone is usually weakest welded part, since the microstructure at the welding zone keeps evolving during the welding process. Some defects may appear at the welded zone during the welding process. Therefore, it is necessary to study the bonding interface characteristics in high-velocity impact welding.
