**3. Description of the MPW process**

requirement of being consumable and eco-efficiency [18]. This method simply uses a standard electrical source intermittently and a magnetic coil. The welding test does not require either a surface treatment or a long experimental preparation, and is performed in a very short period of time, i.e. it only takes less than a few hundred microseconds to produce a joint. This is a precise joining method that has been successfully applied on several similar and dissimilar metallic combinations for different configurations such as overlap, half lap, cross lap, end lap etc. This joining method is also suitable for various geometrical components including tubular assembly, plates or any specific shape. It is possible to generate a complex distribution of a magnetic pulse force due to the strong flexibility of electromagnetic welding tool design [19]. Current potentialities of the EMPT are depicted by Kapil et al. [18]. The authors addressed a comprehensive review of successful applications, where some are being industrialized, and the growing interest given to the process in several industrial sectors such as in automobile, aerospace, nuclear, electrical and microelectromechanical systems (MEMS), ordinance and packaging [18]. Although the pragmatic results are numerous, concisely its applications are well suited to any tubular assembly, regular or irregular shapes, as well as to any flat shape connections (**Figure 1**). EMPT is successfully implemented to perform various manufacturing tasks using semi and fully automated lines by "PSTproducts GmbH" that also offers engi‐ neering and industrial solutions including a robotic arms to effectively handle the portability of the unit in industrial welding cases (**Figure 2**) [20]. In addition, the process covers a broad range of material combinations including Cu/Zr-based metallic glass [21], Al/metallic glass [22], Cu/Manganin [23], flexible circuit boards [24], Cu/Brass, Cu/steel, Cu/Al, Al/steel, Al/Mg, Al/Ni, Al/Fe, Al/Ti and Ti/Ni [25–27]. With all these aforementioned benefits, the EMPT is continuously explored and progressively optimized to bring new potential advancements for

**Figure 1.** EMPT for industrial applications implemented by "PSTproducts" (a) Al/Cu electric bus bar [www.pstprod‐ ucts.com], (b) EMPT crimped gear box part [www.pstproducts.com] (c) EMPT welded Al pressure vessel for air condi‐ tioning system [28], (d) EMPT welded Al/steel crash box [www.pstproducts.com], (e) EMPT welded Al/Cu cooling plate [www.pstproducts.com] (f) EMPT crimped Al/steel tube instrumental panel beam [28], (g) EMPT for hemming of a Al pressure vessel [28], (h) EMPT crimped Al lid on a pharmaceutical glass bottle [29], (i) EMPT crimped drive shaft

effective industrial implementation.

[28] and (j) EMPT crimped air suspension [28].

246 Joining Technologies

In general, the MPW process is a user-friendly joining method. The working principle of the process is simple and the welding procedure is fast, easy, and viable. This section briefly explains the general principle of the process including the architecture of the welding machine and the welding parameters. Interactions between process and welding parameters are provided including the specifications of their controllable and measurable natures. This gives a holistic understanding of the process principle with different variables involved in the selection of the welding parameters.

#### **3.1. Magnetic pulse welding architecture**

**Figure 3** shows typical magnetic pulse welding architecture with overlap configuration used to weld a core clad combination. The MPW is sufficiently flexible to weld various shapes of components for different joint configurations such as half lap, overlap, cross lap and end lap (Section 2.2). Basically, a MPW setup consists of a pulse generator, a coil and an optional field shaper. The generator contains a transformer which transforms a low-voltage power supply into a high voltage charge in the range of kilo-Volts stored in a capacitor bank. This generator set, connected to an inductive coil through a control switch, delivers a high discharge current in the range of a few hundred kilo-Ampere. The electric discharge flowing through the coil generates a magnetic field which creates significantly large Lorentz force within the external tube (the flyer) in the case of a tubular assembly. Thus, the flyer tube undergoes a high strain rate plastic deformation and collides onto the fixed inner rod to produce a high velocity collision. The discharge pulse frequency depends on the parameters of the electromagnetic circuit (Equation 1) and which lies in between 10–200 kHz, but usual operational frequencies are in between 10–20 kHz during the applications. The inductive multi turn coil can be used with a field shaper that concentrates the magnetic field in the working area while increasing the magnetic field intensity. Moreover, one can also control the electromagnetic field and consequent deformation of the material by utilizing various geometries of the field shaper for the same coil [30]. The process can also be performed without the field shaper depending on the required process parameters, or the coil itself can be manufactured which includes the field shaper geometry, especially suitable for single turn coils. However, using a multi turn coil with a single turn field shaper is the notable practice in the current manufacturing that reduces the replacement cost in case of damage of either tool. **Figure 3** shows a MPW configuration including a coil with a separate field shaper and also indicates the direction of the electromagnetic forces on the external tube where the compressive Lorentz force facilitates the deformation of the flyer tube.

**Figure 3.** A typical architecture of the MPW and crimping of tubular assemblies used in overlap configuration.

The circuit frequency *f* can be determined by other parameters using Equation (1).

$$f = \frac{1}{2\pi} \sqrt{\frac{1}{LC}}\tag{1}$$

the inserted lengths of both the rod and tube inside the working area in an assembly. The MPW user prepares this installation where the initial air gap is one of the main geometrical parameter affecting the collision condition, and thus the weld formation. A command console allows for setting up the discharge voltage that is the main adjustable parameter controlling the welding test. The discharge voltage value is indicated as it is or in terms of discharge energy (*E*) given

Magnetic Pulse Welding: An Innovative Joining Technology for Similar and Dissimilar Metal Pairs

1 <sup>2</sup> 2

Where *E* is the discharge energy (J), *C* the used total capacity of the bank capacitor (F) and *U* the discharge voltage (V). Generally, the discharge voltage and the initial air gap provide a set of controllable parameters denoted as (*U, g*). They are indicated for each corresponding welding test. However, the discharge pulse frequency also becomes a crucial process param‐ eter since it decides the penetration of the magnetic field from the coil through the thickness of the flyer. This creates an eddy current in the external region of the flyer that penetrates in accordance with the skin depth defined by Eq. (3). The interaction of the eddy current with the magnetic field from the coil produces the Lorentz force that accelerates the flyer till its onset of collision with the inner rod. Hence, without the skin depth effect, the motion of the flyer is impossible. Therefore, for an effective collision, the discharge pulse frequency should be selected to generate a skin depth lower than the wall thickness of flyer. Eq. (3) also provides an indication of the influence of skin depth on the discharge pulse frequency. More details of

> *<sup>o</sup> f* r

Where *δ, ρ, f* and *μo* respectively denote the thickness of the zone affected by the skin depth effect (m), the electrical resistivity of the flyer (Ωm), discharge current frequency (Hz) and

High frequency current is generally recommended for high resistive metals. One should also carefully consider the discharge pulse frequency of a setup requirement for a generator that also modifies the capacity of a welding machine. In addition, increase in discharge frequency reduces the impulse duration that subsequently increases the strain rate of the flyer and the consequent dynamic response of the material and collision conditions. Those interrelated effects can make difficult in the selection of a suitable discharge frequency that is conducive to provide expected results. Generally, the discharge voltage and the air gap are the most adjustable parameters for a MPW test. The discharge pulse frequency value is indicated by an

p m

d

*E CU* = (2)

http://dx.doi.org/10.5772/63525

249

<sup>=</sup> (3)

by Equation (2).

the skin depth could be found elsewhere [31].

magnetic permeability in free space.

experimental measurement using a Rogowski probe.

Where *L* is the circuit total inductance (H) and *C* the capacitance of the generator (F).

#### **3.2. Description of the influencing parameters and working conditions**

Basically, a MPW test consists of a simple procedure. The workpieces are placed inside the working section for a tubular assembly. Diameters of both flyer and inner rod determine the air gap, which lies in between the inner surface of the flyer and the outer surface of the rod. The formation of the joint in the overlap configuration decides the weld length depending on the inserted lengths of both the rod and tube inside the working area in an assembly. The MPW user prepares this installation where the initial air gap is one of the main geometrical parameter affecting the collision condition, and thus the weld formation. A command console allows for setting up the discharge voltage that is the main adjustable parameter controlling the welding test. The discharge voltage value is indicated as it is or in terms of discharge energy (*E*) given by Equation (2).

with a field shaper that concentrates the magnetic field in the working area while increasing the magnetic field intensity. Moreover, one can also control the electromagnetic field and consequent deformation of the material by utilizing various geometries of the field shaper for the same coil [30]. The process can also be performed without the field shaper depending on the required process parameters, or the coil itself can be manufactured which includes the field shaper geometry, especially suitable for single turn coils. However, using a multi turn coil with a single turn field shaper is the notable practice in the current manufacturing that reduces the replacement cost in case of damage of either tool. **Figure 3** shows a MPW configuration including a coil with a separate field shaper and also indicates the direction of the electromagnetic forces on the external tube where the compressive Lorentz force facilitates the

**Figure 3.** A typical architecture of the MPW and crimping of tubular assemblies used in overlap configuration.

1 1

Basically, a MPW test consists of a simple procedure. The workpieces are placed inside the working section for a tubular assembly. Diameters of both flyer and inner rod determine the air gap, which lies in between the inner surface of the flyer and the outer surface of the rod. The formation of the joint in the overlap configuration decides the weld length depending on

*LC* <sup>=</sup> (1)

The circuit frequency *f* can be determined by other parameters using Equation (1).

<sup>2</sup> *<sup>f</sup>* p

**3.2. Description of the influencing parameters and working conditions**

Where *L* is the circuit total inductance (H) and *C* the capacitance of the generator (F).

deformation of the flyer tube.

248 Joining Technologies

$$E = \frac{1}{2}CU^2\tag{2}$$

Where *E* is the discharge energy (J), *C* the used total capacity of the bank capacitor (F) and *U* the discharge voltage (V). Generally, the discharge voltage and the initial air gap provide a set of controllable parameters denoted as (*U, g*). They are indicated for each corresponding welding test. However, the discharge pulse frequency also becomes a crucial process param‐ eter since it decides the penetration of the magnetic field from the coil through the thickness of the flyer. This creates an eddy current in the external region of the flyer that penetrates in accordance with the skin depth defined by Eq. (3). The interaction of the eddy current with the magnetic field from the coil produces the Lorentz force that accelerates the flyer till its onset of collision with the inner rod. Hence, without the skin depth effect, the motion of the flyer is impossible. Therefore, for an effective collision, the discharge pulse frequency should be selected to generate a skin depth lower than the wall thickness of flyer. Eq. (3) also provides an indication of the influence of skin depth on the discharge pulse frequency. More details of the skin depth could be found elsewhere [31].

$$\mathcal{S} = \sqrt{\frac{\rho}{\pi f \,\mu\_\circ}}\tag{3}$$

Where *δ, ρ, f* and *μo* respectively denote the thickness of the zone affected by the skin depth effect (m), the electrical resistivity of the flyer (Ωm), discharge current frequency (Hz) and magnetic permeability in free space.

High frequency current is generally recommended for high resistive metals. One should also carefully consider the discharge pulse frequency of a setup requirement for a generator that also modifies the capacity of a welding machine. In addition, increase in discharge frequency reduces the impulse duration that subsequently increases the strain rate of the flyer and the consequent dynamic response of the material and collision conditions. Those interrelated effects can make difficult in the selection of a suitable discharge frequency that is conducive to provide expected results. Generally, the discharge voltage and the air gap are the most adjustable parameters for a MPW test. The discharge pulse frequency value is indicated by an experimental measurement using a Rogowski probe.
