**2. Impact welding**

In the 1940s, Carl proposed explosives can be used to drive metal and metal collisions for metallurgical bonding, which he termed explosive welding [11]. People nowadays deploy chemical energy, electromagnetic field energy, high-energy-density light energy, high-pressure gas, and other driving sources to produce various forms of impact welding by releasing high energy transiently and driving high-speed collision of welding parts.

During the Great War, engineers noticed shrapnel unusually attached to armored tanks, not simply inserted into the tank's side (**Figure 2**). The tank material and shrapnel fused due to the force of the collision. That is, the impact resulted in the formation of a weld.

In the process of impact welding, two or more metal sheets collide at high speed. The impact begins in the vicinity at the speed of sound in the air [12]. The workpieces experience severe plastic deformation at the contact area during impact by transforming kinetic energy into plastic deformation energy. The superficial (oxide) layers are broken apart by this plastic deformation. The required angle between the workpieces causes a line-shaped contact zone to go over the surface of the workpieces as the space between them closes. The high-pressure contact substantially removes gaps and results in a metallurgical bond between the materials. The impact welding process successfully welds dissimilar metal and does not produce a heat-affected zone. A common drawback of traditional welding processes is that the processes alter the material properties by the local temperature condition, often resulting in a softer region around the weld (heat-affected zone). Joints are by impact, the properties of the base metal are intact at the joint. The resulting interface may or may not be as robust as the base metals.

Different welding technologies, such as electromagnetic pulse welding, vaporizing foil actuator welding, and laser impact welding, are all part of the impact welding family. Although the primary working principle of these processes is a high-velocity collision between a flyer and a target, the method of accelerating the flyer differs. These methods also have a wide range of length scales, giving the impact welding

**Figure 2.** *Shrapnel stuck to armored tank.*

group a wide range of applications. Impact welding can drive the development of numerous scientific investigations, which are necessary for optimizing current production processes by developing new welding techniques and solutions.

### **2.1 Electromagnetic welding**

Electromagnetic welding (EMW) is a solid-state welding process that primarily joins conductive materials using high-speed electromagnetic force. The technique efficiently joins similar or dissimilar metals, as well as metals and non-metals [13]. The process utilizes very high velocity and strain rate to join several materials. The impulsive Lorenz force, produced by repelling magnetic fields due to pulse current, accelerates one or both joining materials, resulting in a high-velocity collision and joint formation. The weld contact does not melt, unlike traditional joining techniques, preserving the material characteristics.

Electromagnetic pulse welding was used by Yu et al., [14] to create aluminum-covered steel tubes. The findings suggest that the proposed EMW technique can generate strong cladding bonds to make a tubular clad component with a long axial length. P. Q. Wang et al., [15] successfully welded Al/Cu dissimilar sheet metal by EMW. Many process parameters define the mechanical performance and interfacial morphology of EMP welded joints. The investigation by C. Li et al., [16] reported a relationship between process parameters such as discharge current frequency, the Lorentz force, and the displacement in the base metals. Shaoluo Wang et al., [17] reported that to improve the weldability range by EMW, discharge energy, and the wieldable standoff distance range would be enhanced as the ascent of discharge energy could make the flyer plate have a higher collision velocity. The process has a significant effect on the microstructural changes at the bond interface as well. The interface morphology generally includes the formation of a dislocation network, mechanically induced dissolution of precipitates, and recrystallization [18]. There is little evidence one can find in the melting of base materials. In addition, it is worth expecting the significant enhancement in the strength for the post-weld heat-treatable alloys by thermomechanical processing. During service, dissimilar metal junctions will invariably experience corrosion, which results in premature failure of the welded joints. Galvanic corrosion at the joints of hybrid structures accelerates corrosion. Corrosion characteristics of EMP welded galvanized steel/aluminum sheets are reported in [19].

The recent advancement in the technique led to the commercialization of the process with many welding applications. T. Aizawa et al., [20] used EMW to provide successful metallurgical and electrical bonds between flexible printed circuit boards (FPCB). The applications are suited for tubular assembly, regular or irregular shapes, and flat shape connections. The EMW is widely used to manufacture crimped gearbox parts, crimped Al/Steel tube instrumental panel beam, hemming of aluminum pressure vessel, Aluminum lid for the pharmaceutical glass bottle etc [21].

#### *2.1.1 Working procedure*

An AC power supply charges the capacitor bank. After storing the appropriate quantity of energy in the capacitors, it is released into a coil instantly (**Figure 3**). The discharge current creates a strong transient magnetic field inside the coil, which causes eddy currents to form in the work piece. Eddy currents prevent the magnetic field from diffusing through the outer work piece, resulting in a difference in the magnitude of the magnetic field on both sides of the work piece. The magnetic field

**Figure 3.** *Electromagnetic welding process.*

causes the outer work piece to collide with the inner one. The collision of the work pieces causes bonding via a variety of mechanisms. The bonding will be intact as the distance between the atoms becomes smaller than the range of mutually attractive forces. In this instance, electrons are shared between the two materials, resulting in an intermetallic phase (potentially high hardness).

The actual bonding procedure takes less than 30 seconds. There are no shielding gases, fillers, or any auxiliary materials utilized. The electromagnetic pulse welding procedure is like a cold joining method, with very little heat generated. As a result, there is no heat-affected zone, and materials retain their qualities. The lack of heat and solid-state nature of the method allows for the joining of different materials. Aluminum to copper, aluminum to steel, and copper to brass are examples. The process EMW can weld the sheet metals with cross-sections comparable to that of an explosive weld. Since there is limited intermetallic phase generation at the interfaces, EMW produces a robust metallurgical bonded structure. EMW finds applications in tube forming, sheet metal forming, crimping, welding, and metal cutting with good results in highly conducting metals such as aluminum, copper, steel, and others.

### **2.2 Vaporizing foil actuator welding**

Vaporizing foil actuator welding is an impact welding technique without chemical explosives. In this process, welding is achieved through a high-speed, oblique impact between the welding materials. At small-scale length scales and with similar driving pressures as explosive welding, vaporizing foil actuator welding can weld a wide range of advanced and dissimilar metal combinations. The fabrication of nano-sized particles and the structuring of high current pulses are two examples of outstanding achievements. Until the recent work by Vivek et al., [16] vaporizing foil actuators were not explored much. VFAW uses the same machinery as EMW, but instead of

### *Joining by Forming of Sheet Metals DOI: http://dx.doi.org/10.5772/intechopen.102098*

vaporizing a thin foil by the discharge current, the capacitor quickly vaporizes a thin layer to launch the metal flyer plate, as shown in **Figure 4**. Thermal deformation does not occur due to the low heat generated during the operation, and the properties of base metal do not deteriorate in the weld. Daehn et al., [22] developed the novel collision welding method called vaporizing foil actuator welding (VFAW). Many experimental [23, 24] and numerical simulations [25] suggest that VFAW can be a cost-effective, high-performance technology to join similar and dissimilar metal sheets. According to Hahn et al., [26] VFAW is a competitor technology to the MPW. Vivek et al., [27] successfully joined bulk metallic glass and copper sheets by VFAW. Suhani Chen et al., successfully welded Al −3003 and pure titanium by VFAW. Shuhai Chen et al., [23] reported microstructures, interfacial morphology, and mechanical property of dissimilar metals joint by the VFAW and investigated the influence of processing parameters on the anti-shear capacity of the joint produced by VFAW.

The process can be used to join steel sheets with aluminum, aluminum alloy sheets with different grade aluminum, and many dissimilar metal combinations [28, 29]. This could solve problems that automotive industries are facing in improving fuel efficiency by weight minimization.

## *2.2.1 Working procedure*

The vaporizing foil actuator is placed against the flyer and supported by an anvil in VFAW, to direct the driving pressure toward the flyer sheet. (**Figure 4**). VFAW works by sending a strong electrical pulse into a foil. The foil is sandwiched between an anvil and a flyer. The flyer is forced into another base material called the target. An electrical pulse travels from the capacitor bank to the foil (actuator), inducing enough energy in the foil to cause it to vaporize, transforming it from a solid to a plasma gas in an instant that strains the system. The flyer placed over the foil moves a short distance before impacting the target. The flyer and target may be overlapping portions (as in a lap joint) or base metals overlapping in a different arrangement (as in a flange weld). A modest standoff distance exists between the base metals, which is crucial.

For the required pressure distribution of the flyer, a suitably shaped foil is required. A minor constriction in the foil can create a spot pressure for developing an impact spot weld. The foil is cut into variable weld shape, a longitudinal seam, stitch geometry, or even many weld places in a limited region. The quality of the weld depends on how foil explodes when a large amount of energy is transferred into it. A few kilojoules (up to 10) from a capacitor bank are deposited in 10 microseconds and directed to a small location.
