*3.1. Macrocharacteristics*

In high-velocity impact welding, regarding the geometric shape of the bonding interface, straight interface and wave interface (with or without vortices) have been observed [4,71,72], as shown in Figure 6. The wave interface was thought to be not only one of the typical characteristics, but also the sign of metallurgical bonding in EXW for a long time: since the invention of EXW. Recently, Behcet Gulenc [73] also believed that metallurgical bonding should be with a wave interface based on experimental results in which welded samples were characterized by a wave interface, while welded samples with a straight interface were not. However, the observation of a straight interface in some of the explosive-welded materials [71,72,74] demonstrated that a wave interface was not necessary in order for metallurgical bonding to occur. Furthermore, it has been established that the morphology of the bonding interface changed in the order of a straight interface, wave interface without vortices, and wave interface with vortices with the increase of impact pressure/impact velocity [4,14,74–77] in high-velocity impact welding. In addition, the wavelength and amplitude of the wave interface increased with the increase of the impact pressure/impact velocity once the wave interface formed [73,78,79]. The wavelength and amplitude of the wave interface were also related to the impact angle [62]. They also reported that the wavelength and amplitude of the wave interface was affected the flyer thickness. The bonding interface characteristics were also related to the base material properties [31]. In high-velocity impact welding, a collision interface characterized by a wave with vortices is usually accompanied by a melting phenomenon [80]. Previous studies have concluded that a wave is not essential in order for metallurgical bonding to occur in high-velocity impact welding.

**Figure 6.** Bonding interface in high-velocity impact welding: (**a**) straight interface, reproduced from [81], with permission from Elsevier, 2016; (**b**) wave interface with vortices, reproduced from [18], with permission from Elsevier, 2011.

Wave interface and straight interface were also studied in relation to aspects of the mechanical properties, such as hardness and tensile strength. In one study that compared a straight interface and wave interface (Al/mild steel bonding interface), a higher hardness was detected near the bonding interface for both types of interfaces. By tensile test, it was demonstrated that a straight interface was stronger than a wave interface [71]. For welded samples with straight interfaces, fracture occurred at the Al side, and for welded samples with wave interfaces, fracture occurred at the welded zone. However, for welded samples with wave interfaces, the fracture mode was brittle. The brittle fracture of the welded sample with a wave interface indicated in a tensile test that there might be continuously distributed brittle phases along the bonding interface, which caused the fracture at the welded zone. Furthermore, some other researchers believed that a wave interface was better than a straight interface because of interlocking or mechanical locking [82,83]. In order to figure out which one is better, experiments should be done very carefully to find out the transition from a straight interface to a wave interface. The maximum strength should be within the transition range in which there is no continuous melting along the collision interface and interlocking played a role to increase the weld strength.

## *3.2. Microstructure at Bonding Interface*

In high-velocity impact welding, two other types of interfaces were observed based on the composition variation across the bonding interface: a sharp interface [4,18,74,84] and a transition layer interface [34,36,40,43,49,75,85–87]. These both appeared periodically along the same wave interface with vortices. The transition layer interface was present at vortices where there was melting, and the sharp interface was present at other places along the bonding interface. It is generally accepted that a sharp interface is the result of a solid-state bond. However, it is not clear whether the transition layer interface is a solid-state bond or fusion bond. At the transition layer interface, defects (microvoids) and a crystal structure change (amorphous material) were also observed in some of the welded samples. Therefore, some researchers stated that a transition layer interface was the result of melting and solidification. However, others have argued that welding occurs by a solid-state bond at the transition layer interface where only compositional change happened, such as in cold-pressure welding. Previous studies have indicated that the transition layer interface could be the result of a solid-state bond and a fusion bond, or a mixture of both along the same collision interface, which relies on the welding parameters.

## 3.2.1. Grain Refinement

Grain refinement was reported along both types of metallurgical bonding interface [74,75,88,89]. In the study of the Al/Al metallurgical bonding interface, both fine and elongated grains were observed by Zhang et al. with the electron backscattered diffraction method (EBSD) [83]. They believed that grain refinement was the result of plastic deformation. They argued if it resulted from melting and solidification, grains with epitaxial orientation should be observed. However, Grignon et al. observed microvoids at the bonding interface of Al/Al beside grain refinement. They stated that grain refinement was the result of melting and solidification [90]. Therefore, both mechanisms for grain refinement are possible in high-velocity impact welding. In this specific experiment, the grain refinement mechanism should be based on the conversion of the kinetic energy of the flyer to heat. However, different mechanisms resulted in different mechanical properties. Severe plastic resulted in grain refinement that brought a high-stress concentration along the welding interface, whereas melting resulted in grain refinement that relieved the stress concentration.

## 3.2.2. Intermetallics, Microvoids, and Amorphous Materials

The characteristics of the Ti/steel metallurgical bonding interface were studied by Inal et al., Nishida et al., and Kahraman et al. [76,82,87]. Higher hardness was detected at the bonding interface by Inal et al. and Kahraman et al. [76,82]. Regarding the tensile test, welded samples failed out of the welded zone in the study of Inal et al. and Kahraman et al., while in the study of Nishida et al., failure occurred within the welded zone [87]. In the studies of Inal et al. and Kahramen et al., neither melting voids nor intermetallics were found along the bonding interface. However, the existence of an amorphous material was proved by a TEM diffraction pattern in the study of Nishida et al. The authors stated that the amorphous materials were the result of melting and rapid solidification. Therefore, those study indicated that melting in the bonding interface lowered the mechanical properties.

Ben-Artzy et al. and Kore et al. studied the bonding interface of Al/Mg [42,43]. Ben-Artzy et al. believed that bonding between aluminum and magnesium was the result of melting and solidification, while Kore et al. thought the bonding between aluminum and magnesium was pure solid state. Both observed a transition layer. In the study of Ben-Artzy et al., extensive microvoids were observed at the bonding interface within the transition layer. Microvoids were also observed by Marya M. and Marya S. in their study of the interfacial microstructures of magnetic pulse-welded Cu and Al [40]. Kore et al. did not observe defects such as intermetallics and microvoids by SEM and XRD. The above observations indicated that the metallurgical bonding as a result of melting and solidification can be avoided by a lower impact pressure input in high-velocity impact welding.

Liu et al. and Watanabe et al. investigated explosively welded Al/metallic glass and magnetic pulse welded Al/metallic glass. respectively [49,84]. In the study of Liu et al., hardness increased at both the Al and metallic glass sides close to the bonding interface, and a TEM bright field image showed a sharp transition at the bonding interface. The TEM diffraction pattern verified that the metallic glass kept its amorphous structure. However, the authors believed that melting happened and the cooling rate was high enough to allow the metallic glass to retain the amorphous structure based on the simulation result. In the study of Watanabe et al., hardness increased only at the Al side, and close to the bonding interface, the hardness of the metallic glass decreased, which was thought to be caused by the crystallization of metallic glass. An SEM backscattered electron image showed a transition layer at the bonding interface. However, TEM did not detect any crystal structure at the metallic glass side. From the above studies, it is hard to tell whether melting and solidification happened during the welding process. Two phenomena in their study could not be explained. Liu et al. thought that melting and solidification happened, but the hardness of the metallic glass increased at the bonding interface. In the study of Watanabe et al., a lower hardness of metallic glass was observed at the bonding interface, but no crystal structure was found at the metallic glass side.

In their investigation of magnetic pulse welded similar and dissimilar materials, Stern and Aizenshtein [86] believed that the flyer and target were bonded by melting and solidification. For combinations of dissimilar materials, compositions that were similar to some intermetallics were detected by EDS at the transition layer interface, such as in the study of Marya M. and Marya S. [40]. However, the determination of intermetallics needs further investigation, such as what Lee et al. did in their study of magnetic pulse welded low-carbon steel to aluminum [36]. In their study, varied composition was also detected by EDS at the bonding interface. However, the new composition, which is different from the flyer and the target, is not consistent with any composition of intermetallics in the Al–Fe phase diagram. So, they did further research using TEM. From the TEM image and diffraction patterns, fine aluminum grains, as well as fine Al–Fe intermetallics grains, were observed within the transition layer. The authors atttributed the higher hardness at the bonding interface to fine grains and possible intermetallics. Continuous intermetallics should be avoided by adjusting the welding parameters [91,92].

## *3.3. Summary*

The straight interface and wave interface (with or without vortices) are two types of bonding interfaces in high-velocity impact welding based on the geometric shape of the bonding interface. Grain refinement and higher hardness were observed for both of them. There are two different explanations regarding the mechanism of grain refinement. One is melting and solidification, and the other is severe plastic deformation at the bonding interface. Both of them are possible mechanisms for grain refinement. The mechanism could depend on the welding parameters. However, melting along the bonding interface lowers the mechanical properties.

At the transition layer interface, compositional change, microvoids, and amorphous materials were observed, although they may not appear at the same time. The mechanism of compositional change with microvoids and amorphous materials at the bonding interface was the result of melting and solidification. The mechanism of compositional change at the bonding interface without defects

may be due to solid-state diffusion. The existence of a sharp interface demonstrates that a bonding interface without defects could be produced in high-velocity impact welding. The formation of intermetallics caused compositional change, but this should be confirmed through detecting its crystal structure.

## **4. Welding Parameters**

## *4.1. Welding Parameters Selection*

A welded zone with acceptable weld quality should be as strong as the weaker part of the two welded parts, as determined by a mechanical test such as the tensile test, bending test, or hardness test. Additionally, damage to parent materials, such as spalling, should be avoided. To obtain acceptable weld quality, proper welding parameters should be selected.

It is generally accepted that a jet is essential in order for welding to occur in high-velocity impact welding. It has been shown from the "jet phenomenon" section that jet formation was related to the impact angle, impact velocity, and impact pressure. A large impact pressure, such as spalling, caused damage to the flyer and target [83]. The impact pressure at the collision point should have a maximum magnitude. The excessive kinetic energy of the flyer results in melting and continuous intermetallics at the bonding interface; thus, there is an upper limit for kinetic energy. The following parameters can be used to describe high-velocity impact welding: kinetic energy (*Ek*), impact pressure (*P*), impact velocity ( *Vp*), and impact angle (*β*). Certainly, the properties of materials also affect the weld quality [93], density (*ρ*), and thickness of the flyer plate (*t*)). For convenience, the kinetic energy, impact pressure, impact velocity, impact angle, and materials properties are regarded as basic parameters, and others—such as for example the standoff distance (*L*), explosive properties [15,81] (explosive ratio, thickness, and detonation velocity ( *Vd*)), and initial angle (*α*) in explosive welding, capacitor bank energy in magnetic pulse welding, and laser properties in laser impact welding—are process parameters. Basic parameters are determined by the process parameters. In this literature review, the discussion of welding parameters was confined to the basic parameters and standoff distance.

A map called the weldability window was proposed by Wittman et al. [1] in EXW, as shown in Figure 7. *Vw* is the welding velocity (see Figure 5c). The dynamic angle (impact angle) of obliquity is the angle between the flyer and the target at the collision point. On this map, the upper limit and lower limit of the welding velocity were included. On the right side of the map where the supersonic region is located, the critical angle for jet formation was defined. The transition velocity from a straight interface to a wave interface was also included. The experimental results demonstrated that there was a transition zone for the transition from a straight interface to a wave interface within the welding range [1]. Regarding this map, the following issues should be pointed out. Firstly, this map varies depending on materials' properties [94]. Secondly, welding velocity is not appropriate to be used in this map, except for EXW, as it is not one of the basic parameters that directly determines the weld quality. Thirdly, it is not appropriate for the transition velocity to be regarded as a constant. However, the weldability window guided people in the right direction in the research of welding parameters in high-velocity impact welding. Once it is built, it provides the manual for the application of high-velocity impact welding in industry.

**Figure 7.** Weldability window. Velocity of welding (*Vw*) is represented by *V*1 in Figure 5c. The dynamic angle of obliquity is the impact angle. When the combination of parameters is within the welding range, welding will take place.

## *4.2. Effect of Welding Parameters on Weld Quality*

The welding parameters that have been investigated in different papers vary; for example, they have included the impact angle, detonation velocity, impact velocity, standoff distance, and discharge energy.

## 4.2.1. Effect of Welding Parameters on Wave Formation

In EXW, wave formation was believed to be a sign of strong metallurgical bonding between the flyer and the target. Therefore, early work on EXW focused on establishing critical parameters for wave formation. Deribas et al. studied the effect of detonation velocity, standoff distance, and initial angle on wave formation [95]. They pointed out that with a high detonation velocity, there was a critical angle below which there was no wave. With a low detonation velocity, there was no critical angle, but the wave dimension (amplitude and wavelength) would increase with the initial angle and standoff distance. Furthermore, for each fixed initial angle, there was a critical impact velocity for wave formation, and the impact velocity increased with the initial angle. However, they couldn't establish the direct relationship between the wave dimension and the initial angle, impact velocity, and standoff distance, since in each serial experiment, there were more than two parameters varying at the same time. Their conclusion was consistent with the jet formation regimes in which there was a critical initial angle that was required for supersonic flow, whereas there was no requirement for subsonic flow [19].

Acarer et al. [14] studied the effect of explosive loading and standoff distance on the wave dimension. The experimental results showed that the wave dimension increased with standoff distance and explosive loading. Durgutlu et al. [96] also found a similar effect of standoff on wave dimension; they also observed that the bonding interface was straight with a lower standoff distance, while with a higher standoff, the bonding interface had a wavy feature. However, they didn't build the quantitative relationship between the welding parameters and the wave dimension.

## 4.2.2. Effect of Welding Parameters on Mechanical Properties

The weld quality is usually evaluated with a tensile test and a peeling test; the weld quality is compared by measuring the fracture strength and observing the fracture location. When the fracture location is at the weld interface, the weld quality is ordered by the fracture strength. In some other cases, the fracture location is on the base metals, which indicated that the weld quality is better than the basemetalstrength.Severalresearchersinvestigatedtheeffectoftheparametersontheweldquality.

Kore et al. studied the welding parameters in MPW. Al with a thickness of one mm was welded with a discharge energy of 3.6 kJ. The bonding strength increased with increasing discharge energy.

They also found that there was an optimum standoff distance in magnetic pulse welding, which was also concluded by Hokari et al. [39]. This observation was not applicable to EXW. In MPW, the highest impact velocity occurs at the peak primary current. Therefore, before the impact velocity gets to its highest value, the flyer travels a specific distance, which is determined by the impact velocity and the rise time of the primary current. So, a specific standoff distance is needed for a specific MPW process.

In a study of the bonding interface of explosively welded steel to steel, microhardness increased at the bonding interface, but decreased far from the bonding interface [14]. However, Gulenc [73] observed that microhardness increased both at the bonding interface and far from the bonding interface. Microhardness should increase both at the bonding interface and the outer surface of the welded samples, which is caused by work hardening. The outer surface of the flyer is work-hardened by the explosives, and the outer surface of the target is work-hardened by the interaction with anvil. The decrease of hardness far from the bonding interface may be caused by softening.

Chizari and Barrett studied impact velocity with an aluminum flyer that was two-mm thick [33]. They found that there was no bonding when the impact velocity was lower than 250 m/s, and that a wave interface began to appear when the impact velocity was 340 m/s. Besides the impact velocity, they also studied the effect of two flyers on the weld quality. In their experiment, two pieces of parallel flyers with a separate distance were used. Their experimental results showed that the weld quality between the first flyer and the target was better than that between the two flyers. They attributed the poor bond between the two flyers to the increased roughness of the back surface of the first flyer during the colliding process. However, it should be noted that during the colliding process, the roughness of the target also increased. So, roughness was not causing the poor bond between the two flyers.

To summarize the research about welding parameters, welding parameters vary in different research papers, and some of them are dependent on specific techniques, and even specific experimental setups. For example, the flyer will be applied with a different impact force despite using the same capacitor bank energy and different actuators. Therefore, the basic parameters should be studied extensively. In former studies, the impact angle and impact velocity were chosen as the factors that can determine the weld quality. However, the jet formation is much more reliant on the impact pressure and impact angle. Kinetic energy is also very critical in determining the weld quality in order to avoid melting and solidification. Therefore, welding parameters need to be carefully investigated in order to figure out which ones could be selected to build the weldability window.

In explosive welding, the fatigue life of welded joints was also studied. Karolczuk et al. investigated the fatigue phenomena of explosively welded Fe/Ti [24]. They studied two types of bonding interface: flat interface and wavy interface. They concluded that a flat interface has a higher fatigue life than a wavy interface. Through the study of Szachogluchowicz et al. [27], it was found that the lower fatigue life of the wavy interface may be due to the stress concentration. In their study, heat treatment improved the fatigue life by stress relaxation and the elimination of microvoids. Prazmowski ˙ et al. [26] studied the fatigue life of explosively welded Zr/Fe, and concluded that the remelted layer in the bonding zone increased the fatigue life. This phenomena was also due to the stress relaxation. The corrosion behavior of explosively welded Al/Fe was studied by Kaya [29] with neutral salt spray (NSS) tests. Their results showed that the corrosion occurred on the steel side, while no corrosion behavior was observed on the Al side. Therefore, a cladded Al layer played a significant role in the protection of steel.

## **5. Conclusion and Future Work**

High-velocity impact welding is suitable to join dissimilar materials and other specific materials, and has potential applications in industries. The conclusions are listed below.

1. In high-velocity impact welding, the weld configuration was limited by the experimental setup. The current general welding configurations are lap joint (cladding), spot weld, and tubular weld. More welding configurations should be developed for wider applications of high-velocity impact welding.


**Author Contributions:** Conceptualization, H.W. and Y.W.; Literature search, H.W.; Literature summarization, H.W. and Y.W.; writing—original draft preparation, H.W.; writing—review and editing, Y.W.

**Funding:** This research was funded by the Fundamental Research Funds for the Central Universities, gran<sup>t</sup> number 06500107 and the APC was funded by 06500107.

**Acknowledgments:** Thanks for lab members from The Ohio State University for their valuable discussions and instructions.

**Conflicts of Interest:** The authors declare no conflict of interest.
