Joining by Forming of Sheet Metals

Abstract

Sheet metal is the frequently used component geometry in industries, and the joining of sheets is inevitable. There exist numerous conventional processes for joining sheet metals, but the diverse needs of today’s industries necessitate further research into alternative joining technologies. Joining by forming methods to join similar and dissimilar sheet metals has a great potential to reach current industrial requirements. The process involves plastic deformation of at least one constituent part. Among several techniques that evolved in recent times, friction-based welding, impact welding, and roll bonding are the three solid-state welding methods, often known as joining by forming. The present chapter starts with a brief overview of the various aspects of joining sheet metals by forming methods. The working principle, procedure, and the consequences of the impact-based methods such as vapor foil actuator welding, electromagnetic welding, and laser impact welding, along with the roll bonding process are discussed.

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.

Figure 1.

Conventional methods of joining sheet metals.

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), 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.

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 solid-state 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.

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.

Figure 2.

Shrapnel stuck to armored tank.

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 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 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).

Figure 3.

Electromagnetic welding process.

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 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.

Figure 4.

Schematic diagram of vaporizing foil actuator welding.

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.

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-electro-mechanical 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:

1. Excitation stage: The ablation layer is vaporized into plasma as the laser irradiates it through the confinement layer. The reaction force of the plasma drives the flyer to emit due to the constraint of the confinement layer.

2. Flight phase: The flyer passes a pre-determined flight distance or standoff distance before colliding with the target at a specific speed and angle.

3. Welding stage: From the onset of the metallurgical bonding until the end position, the flyer and the target collide at a specific angle to complete welding

Figure 5.

Schematic diagram explaining LASER impact welding process.

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

• Compared to traditional thermal welding procedures, the technology offers significant advantages because it uses pressure rather than heat to achieve the bond.

• Quick and cost-effective joining of sheet metals typically dissimilar material joints.

• Manufacturing sheet metal products that were previously difficult to process using traditional joining methods.

• Impact welding such as magnetic pulse welding is referred to as a “cold” joining technique. Because the temperature increase is localized (on the order of 50 m), the work pieces only reach 30–50°C at the outside surfaces. So pieces can be unloaded and dealt with using simple equipment immediately after welding.

• One can achieve consistent joint quality; high repeatability.

• It is possible to achieve a high production rate.

• Contact-free: no forming tool marks, coatings, or sensitive materials can be processed.

• When compared to traditional welding processes, the technology has a substantially reduced negative environmental impact and is far more environmentally friendly:

• There is no heat, radiation, gas, or smoke thus the operator is protected by a shielding gas. Machines can execute the joining process in adverse situations, reducing further expenses in operator safety.

• Because the technology is environmentally friendly, it is feasible to improve the working conditions of the welder or operator.

• The magnetic pulse welding process is energy-efficient.

The workpieces to join, magnetic pulse welding has various limitations

• In EMPW one of the sheet materials must be a good electrical conductor; otherwise, a conductive ‘driving’ material should be utilized to enhance impact velocity.

• In the impact welding process, one workpiece must endure the impact of the other, a mandrel or support is required to prevent distortion.

• Due to the size of the welding equipment, it’s done in a workshop; nevertheless, this is not necessarily a drawback when considering the prospective applications, which are largely factory-made parts.

3. Roll bonding

Cold roll bonding (CRB) is a solid phase method of bonding similar or dissimilar metal sheets by rolling at room temperature, which has been widely employed in the production of large multilayer composite sheets and foils. Cold pressure welding by rolling (CRB) is a sort of pressure welding or solid-state welding technology in which bonding is created by joint plastic deformation of the metals to be welded, suggesting that the degree of deformation is one of the major criteria [7]. CRB can be applied to a wide range of materials, cold bonding also works effectively with materials that cannot be fused by standard fusion. Rolling generates the high interfacial pressure required for bonding between two metal parts. Tensile shear test, slide shear test, multistep shear test, peeling test, and T-peel test is some of the procedures used to assess the bond strength of layered materials. Many investigations on the parameters influencing bonding have been conducted to understand the mechanisms of the complicated nature of bonding and the process conditions that have been established empirically [38]. The roll bonding parameters to be considered are, the type of metal under consideration, the amount of deformation, the bonding temperature, the amount of pressure applied, the bonding time, the metal purity, the lattice structure, the surface preparation conditions, the geometry of the deformation zone (shape factor), the stacking sequence, the number of layers, the layer thickness, and the type of post-heat treatment, which have been reported in the literature. The removal of contaminated layers from the surface using chemical and mechanical treatments is critical in the CRB process (surface preparation). This usually entails washing and prepping surfaces to eliminate any impurities (bonding barriers) from the surfaces of the two metals to be bonded.

The complicated nature of the bonding mechanism involved in the roll bonding process has been the subject of numerous recent research studies [39]. Due to significant work-hardening by scratch-brushing, the metal surface gets hardened to a depth [40]. The cover layer fracture when exposed to sufficient surface expansions. Fracturing due to high interface pressure opens up new crack surfaces. Extrusion begins when the normal pressure is high enough and the surface expansion (or crack width) is significant. A connection is established when the asperities of the decovered virgin material of the two opposing surfaces make contact. This is the first bonding process mentioned [41]. The materials like aluminum, copper is easy to cold roll bond. Gold, silver, and platinum are other FCC metals that can be cold-bonded easily [7]. The bonding characteristics of hexagonal metals, such as magnesium, cadmium, and zirconium, are inferior to those of FCC materials.

Some of the applications of roll bonded sheet metal parts are, Al-Cu roll bonded sheets used in cooking utensils, heat exchangers, roof and wall plates. Al-Fe bonded sheets are used for electric heater reflectors, automobile silencers. Al roll bonded to stainless steel is used in automobile trims, Al-Steel-Al sheets used in the automobile exhaust system. Ti-Stainless steel-Ni found applications in the bipolar electrode in the fuel cell, Cu to Stainless steel bonded sheets have been widely used in communicator plate, and so on.

3.1 Working procedure

The schematic explaining the process of roll bonding is shown in Figure 6. The strips to be roll bonded are initially cleaned thoroughly by acetone to remove the surface contaminate layer. The cleaned strips are scratch brushed using a wire brush. The wire brushing on any one side produces a rough surface which will help in bonding the sample in two ways. One strain hardens the surface forming a hard brittle cover layer, and the other is it brings the fresh virgin metal surface to open up. Now, the two scratched samples are stacked and clamped as shown in the Figure 6. The stacked samples are passed between the pair of rollers to produce bonding. The bonding between the samples is possible only if the reduction given will be more than the threshold reduction [42]. It is feasible to heat the sheets near or above their recrystallization temperature to get improved bond strength.

Figure 6.

Schematic diagram of roll bonding process.

4. Conclusion

The chapter elucidates the importance of sheet metal joining by forming (plastic deformation). The technique developed so far have immense potential and significance to join similar and dissimilar metal sheets without melting, which allows achieving substantially greater joint efficiency than conventional sheet joining methods. The impact-based solid-state techniques, EMW, VFAW, and LIW share a similar mechanism to join sheet metal, but they differ in the welding energy sources they utilize as indicated by their names. In the roll bonding, the overlapped sheets are joined by joint plastic deformation; the squeezing action of rollers creates sufficient pressure at the interface to create a bond.

The joining by forming methods provide consistent joint quality, high repeatability, and higher production rates; which are the prerequisite factors that industries look for. The processes are environmentally friendly, as there is no emission of heat, radiation, or smoke during the process. Hence, it is feasible to use and develop these forming methods to join sheet metals.

Conflict of interest

The authors declare no conflict of interest.