**1. Introduction**

Sheet metal is the foundation of most engineering today. It has a wide variety of applications, including automobiles, airplanes, machinery, equipment, home facades, and furniture. Sheet metal is one of the shapes a metal can be formed into by industrial processing. Sheet metal is defined as any metal with a thickness of 0.5–6 mm. When constructing a sheet metal product, engineers will inevitably use sheet metal parts. It's a challenging task to join several sheet metal parts in a cost-effective and secure manner.

The very commonly used conventional methods of joining sheet metals are shown in **Figure 1**; folding/tab-joint, pulling and pressing of a rivet, self-clinching, screw joint, and welding joints.

Folding/tab-joint (**Figure 1a**): Two sheets of metal are connected by folding or bending tabs in the shape of a buckle and a clamping groove. The assembly is straightforward, convenient, and completed quickly. However, full positioning is not guaranteed, and further supplementary positioning is required.

Rivet pulling and pressing (**Figure 1b**): Riveting is done in the holes that correspond to the two pieces, and the rivet gun is used to draw the rivet to expand and deform the outer rivet sleeve, therefore securing the two parts. The resulting connection will be simple, convenient, and quick. Stringers and airframe skins are virtually always joined by rivets. Although millions of rivets are used in aircraft structures (adding weight),

**Figure 1.** *Conventional methods of joining sheet metals.*

the stress concentrations caused by rivet holes quadruple the skin's local stresses. One advantage of rivets is that they are more reliable, if not more efficient.

Self-clinching (**Figure 1c**): Self-clinching is also known as self-riveting, is a method of completing mutual fastening by deformation between the sheet metal. Despite its simplicity, this technology is employed frequently in regions where disassembling is not required.

Screw joint (**Figure 1d**): Self-tapping screws to directly tap the thread on a piece of sheet metal, so the fit is good, which requires disassembling.

Welding (**Figure 1d**): This is a spot or seam welding used to keep a sequence of solder joints on two sheets of metal together. At the welding head, it directly melts the local sheet metal.

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

During the welding of sheet metals, everything happens quickly. As a result of the rapid heating of the materials, distortion like warping gets amplified. Furthermore, the thin materials burn when heated abruptly, resulting in undesired perforations. Mechanical strength is compromised if heat-affected zones are not controlled (HAZ). Welders of thin metal materials must concentrate on reducing the warping, melt-through, and size of HAZ [1]. Selecting the most appropriate welding method, carefully managing the physical setup, creating optimum welding parameters, and precisely completing the weld are necessary steps in overcoming these problems. Precision joint preparation for a tight fit and appropriate clamping to prevent movement during the weld is part of the physical setup. Much percussion are needed to be taken care of during the process [2]. As an example, a copper backup bar is often handy to place beneath the joint. During excessive melting, this bar conducts heat away from the base materials.

Joining by forming a sheet metal involves solid-state joining methods, which are a viable alternative to traditional, fusion-based welding and is effective for combining materials with different melting temperatures or brittle intermetallic compounds [3]. Several existing and emerging solid-state (SS) welding methods can produce sound metallurgical welds across similar and dissimilar metals without melting, yet some localized and isolated pockets of melting may occur. Joining by forming sheet metals is the name given to these methods, and there has been a surge in interest in them in recent years. Solid-state joining allows combining advanced metals and dissimilar metal combinations that are difficult or impossible to join using fusion welding to avoid melting [4]. It is possible to achieve substantially greater joint efficiencies than fasteners and adhesives. Solid-state joining technologies might vary depending on usage, the materials of interest, and the topologies they use. Joining by forming methods are preferable to fusion welding from a metallurgical standpoint since no significant microstructural change occurs and a lesser risk of intermetallic formation or inter-diffusion across the weld interface.

Multi-material components are becoming increasingly important in recent industrial criteria. Conventional welding procedures are unable to meet the manufacturing needs of these components. Forming can be an alternative to joining sheet metal. Among several techniques that evolved in recent times, friction-based welding [5], impact welding [6], and diffusion bonding/roll bonding [7] are the three main solidstate welding procedures, often known as joining by forming. Since the 1950s, explosives used to perform impact welding, in which one metal sheet (called a flyer) is driven to a high velocity by a high-pressure pulse to contact another sheet (called a target) at an angle [8]. The hydrodynamic nature of the metals surfaces causes them to eject as a jet at the moment of impact. Surface oxides and other impurities are present in the jetted material, but clean metallic surfaces in high-pressure contact are left behind. As the space between the two parent sheets closes and the impact point moves closer to the edge of the sheets, a metallurgical bond forms between them. Depending on the material properties, impact angle, and velocity, the weld interface morphology can range from flat to variable degrees of waviness, and may or may not contain pores or a layer of intermetallic compounds [9, 10]. As a result, for a particular material combination, there is a "welding window" of characteristics that generate a strong weld. Explosive welding (EXW) is useful for joining centimeter-thick flyers to targets that are as thick as or thicker than that; but, at lower scales, this method performs poorly. 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.

The following sections explain impact-based sheet metal joining methods such as electromagnetic welding, vaporizing foil actuator welding, and laser impact welding, along with the roll bonding process.
