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# Magnetic pulse welding of Al-5754 with Al-7075 and MARS 380: Weldability windows and ballistic testing

Benoit Lagain, Thomas Heuzé, Guillaume Racineux, Michel Arrigoni

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Benoit Lagain, Thomas Heuzé, Guillaume Racineux, Michel Arrigoni. Magnetic pulse welding of Al-5754 with Al-7075 and MARS 380: Weldability windows and ballistic testing. Defence Technology, 2025, 10.1016/j.dt.2025.07.024. hal-05316548

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# Magnetic pulse welding of Al-5754 with Al-7075 and MARS 380: Weldability windows and ballistic testing

Benoit Lagain<sup>a</sup>, Thomas Heuzé<sup>b, *</sup>, Guillaume Racineux<sup>a</sup>, Michel Arrigoni<sup>c</sup>

## ARTICLE INFO

**Article history:**

Received 25 April 2025

Received in revised form

22 June 2025

Accepted 25 July 2025

Available online xxx

**Keywords:**

Magnetic pulse welding (MPW)

Dissimilar material joining

Weldability windows

Impact welding

Ballistic testing

## ABSTRACT

Joining dissimilar materials encounters significant engineering challenges due to the contrast in material properties that makes conventional welding not feasible. Magnetic Pulse Welding (MPW) offers a solid-state joining technique that overcomes these issues by using impact to create strong bonds without melting the substrate materials. This study investigates the weldability of aluminum alloy Al-5754 with Al-7075 and MARS 380 steel, used in armouring solutions of defense systems, by the use of MPW. In this work, weldability windows are investigated by varying standoff distances between the coating material and its substrate (0.25–4.5 mm) and discharge energies (5–13 kJ) with both O-shape and U-shape inductors. Mechanical strength of the welded joints were assessed through single lap shear tests, identifying optimal welding parameters. Then, the velocity profiles of the flyer plates were measured using heterodyne velocimetry to understand the dynamics of the impact. Then, substructures assembled with the optimal welding conditions were subjected to ballistic testing using 7.62 mm × 51 mm NATO and 9 mm × 19 mm Parabellum munitions to evaluate the resilience of the welds under ballistic impact. The outcomes demonstrate that MPW effectively joins Al-5754 with both Al-7075 and MARS 380, producing robust welds capable of withstanding ballistic impacts under certain conditions. This research advances the application of MPW in lightweight ballistic protection of defense systems, contributing to the development of more resilient and lighter protective structures.

© 2025 China Ordnance Society. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Performance in ballistic protection is commonly achieved by combining complementary material properties, such as hardness, lightness, and the ability to deform without breaking. These combinations are usually achieved by adhesive bonding, allowing the association of dissimilar materials, but also time-consuming process that is limited in its use at high temperatures. The integration of dissimilar materials in various assemblies presents thus a critical challenge in modern engineering enhancing the structure's performance across a wide range of applications. Impact welding has emerged as a promising solution to answer this challenge.

Impact welding relies on the collision of two metallic surfaces at high speed, facilitating a bond through kinetic energy rather

than relying on thermal fusion in the bulk of a material. This "cold" welding process bypasses the challenges associated with traditional fusion welding, such as thermal distortion and metallurgical issues, enabling the successful joining of materials with different thermal properties [1]. A critical aspect of the process is the formation of a high-velocity jet of metallic particles at the point of collision. This jet, composed of surface contaminants and oxide layers, acts as a cleaning agent, removing impurities and enabling the two metals to bond as shown in Fig. 1 [2–5]. The generation of this jet, alongside careful control of impact angle and velocity, is essential to achieve a high-quality weld [6,7].

The morphology of the welded interface is another crucial factor in the determination of the bond quality. Observations show that the interface can be either flat or wavy, with the latter offering a more robust bond. One explanation amongst others found in the literature of the formation of wavy interfaces is that they are formed from Kelvin-Helmholtz instabilities, a phenomenon that occurs when two materials in relative motion develop instabilities at their interface [8,9]. These instabilities, driven by differences in

<footnote><sup>a</sup>Research Institute in Civil and Mechanical Engineering, UMR CNRS 6183, Ecole Centrale de Nantes, Nantes, F-44321, France</footnote>

<footnote><sup>b</sup>Bretagne INP, Institut de Recherche Dupuy de Lôme, UMR CNRS 6027, Brest, F-29200, France</footnote>

<footnote><sup>c</sup>ENSTA, Institut de Recherche Dupuy de Lôme, UMR CNRS 6027, Brest, 29806, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: thomas.heuze@enib.fr (T. Heuzé).</footnote>

<footnote>Peer review under the responsibility of China Ordnance Society.</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. High velocity impact welding process.</figcaption></figure>

velocity and density between the materials, lead to the formation of a wavy interface that enhances mechanical interlocking and bond strength [10–12].

Several techniques exist to perform impact welding of electrically conductive materials, among which magnetic pulse welding (MPW), relying on electromagnetic forces to accelerate the flyer, which is the purpose of this study. In MPW, the electrical discharge of a capacitor bank within a coil or any kind of inductor creates a time-varying electrical current of high-intensity, involving time-varying magnetic field that induces Lorentz forces in the conductive flyer. These forces quickly accelerate the flyer towards the substrate material, resulting in an impact welding. Controlling the time evolution of the current, or parameters such as the impact velocity and the impact angle, is crucial to achieve successful welds [13,14]. The flight velocity of the flyer is mainly influenced by the initial current peak [7]. Moreover, the design of the electromagnetic coil, which generates the magnetic field, directly affects the quality of the weld [15]. MPW can be adapted for welding a large variety of geometries, such as tubes or sheets, depending on the specific requirements of the assembly.

As part of this study, we plan to carry out dissimilar welding between aluminum and steel. The challenge of this type of assembly lies both in the very different physical properties of the materials (melting temperature and coefficient of thermal expansion) and in the formation of intermetallic compounds at the welding interface, which are very brittle. Fusion-based welding processes lead not only to significant deformation but also to the formation of thick intermetallic layers. So-called "cold" welding processes help mitigate these issues. Among these processes, Magnetic Pulse Welding (MPW) is the most effective, as the very high welding speed combined with the very low welding temperature helps limit — or even prevent — the formation of intermetallic. As a result, the strength of these welds under dynamic loading is significantly improved [16].

This study aims to advance the understanding of MPW by applying the technique to dissimilar material combinations used in ballistic protection systems. It consists in a coverage on essentially experimental considerations with applied investigations of a MPW panel subjected to real ballistic impacts. The proposed research defines weldability windows for various configurations, beginning with the welding of an aluminum 5754 flyer onto an aluminum 7075 base plate using an O-shape inductor. Subsequent modifications will involve replacing the base plate by an armouring plate of MARS 380 steel or transitioning to a U-shape inductor. The experimental approach describes how to construct and characterize a weldability window for materials commonly used in ballistic protection of defense systems, that particularly combines lightweight and armouring of vehicles. The transition to a new assembly technology, such as magnetic pulse welding, aims at reducing the vehicle's overall weight by eliminating bolts, which are traditionally used for fastening, and associating lighter materials with harder materials that sustain ballistic impacts.

Removing bolts not only leads to a lighter structure but also has the potential to improve structural resilience. In addition, bolts and rivets are often considered weak points in a vehicle's design due to stress concentration, they are time consuming to be fastened and require a regular checking. By adopting alternative assembly methods, such as magnetic pulse welding, these weaknesses can potentially be reinforced, resulting in a more resilient and durable protective shield. Ultimately, one configuration will be selected for ballistic testing to assess the weld's durability under impact conditions.

## 2. Method and material

### 2.1. Experimental protocol

The experimental methodology of this study is structured into five distinct steps, outlined as follows:

1. **Construction of the weldability window:** The experiment begins by establishing a weldability window through a series of assembly tests by impact welding. In these tests, the influence of two key parameters is studied: the standoff distance *e*, ranging from 0.25 to 4.5 mm with increments of 0.25 mm, and the discharge energy *E*, ranging from 5 to 13 kJ with increments of 0.5 kJ. This range is selected to comprehensively cover potential welding conditions and ensure the identification of the parameters that yield to the best configurations. The selection of the ranges of distance and discharge energy relies on previous experimental [17], numerical [18], and analytical [19] studies. Indeed, the reasons why magnetic pulse welding results in successful bonding remains to be clearly established. Although numerical simulations can reproduce the wavy morphology observed on welded surfaces, it remains impossible to predict a *priori* whether the experimental conditions will lead to actual welding [20]. Indeed, the presence of a wavy interface is not a necessary condition for bonding to occur. Therefore, experimental work is essential to identify weldability windows.

2. **Mechanical performance evaluation:** Following the assembly tests on coupon materials designed for tensile testing machine, single lap shear tests are conducted on the successfully welded samples. These tests aim to assess the mechanical strength of the welds, providing quantitative data on the weld's performance obtained under various conditions.

3. **Selection of optimal configuration:** Based on the results of the single lap shear tests, a configuration is selected for further investigation. This selection is determined by the weld performance as well as the availability of materials. Once the optimal configuration is identified, an instrumented assembly test is performed to capture the velocity profile of the flyer coating during the welding process. At this stage, the interest is focused on the discharge energy *E*, the characteristics of the flyer, and the inductor's shape while the properties of the base plate are not relevant for this measurement. The flyer's local velocity is measured through the base plate by using a heterodyne velocimetry (HV) probe, also named Photonic Doppler Velocimetry (PDV).

4. **Ballistic testing of substructure elements:** In the final step, the selected configuration is used to assemble substructure elements via magnetic pulse spot welding. These substructure elements are then subjected to ballistics tests of the welded regions. The purpose of these tests is to evaluate the resilience and integrity of the welded joints under real ballistic impact conditions.
## 2.2. Material and instrumentation

### 2.2.1. High power pulse generator

The electrical generator used in this study is a 46 kJ pulse current generator, designed and developed at Ecole Centrale de Nantes (ECN), France. The generator can be represented as a RLC circuit with a capacitance of 408 µF, an inductance of 0.1 µH, and a resistance of 14 mΩ, delivering a maximum voltage of 15 kV and a peak current of 500 kA at a short-circuit frequency of 25 kHz. The time evolution of the discharged current $I(t)$ is expressed as:

$$I(t) = I_0 e^{-\tau} \sin(\omega t) \quad (1)$$

where parameters of the signal ($I_0$, $\tau$, $\omega$) are related to these of the circuit with the usual relations:

$$I_0 = V_0 \sqrt{\frac{C}{L}} \quad (2)$$

$$\tau = \frac{2L}{R} \quad (3)$$

$$\omega = \frac{f}{2\pi} = \frac{1}{\sqrt{LC}} \quad (4)$$

The system's inductance $L$ and resistance $R$ consist of the contributions of both the generator and the coil:

$$L = L_{\text{Generator}} + L_{\text{coil}} \quad (5)$$

$$R = R_{\text{Generator}} + R_{\text{coil}} \quad (6)$$

The electrical energy stored in the capacitor bank is expressed as:

$$E = \frac{1}{2} C V_0^2 \quad (7)$$

and depends on the capacity $C$ and the charging voltage $V_0$. To ensure the longevity of the generator, the peak discharge current is limited to 330 kA. This restriction is set to prevent overloading or applying excessive stress on the generator, thereby preserving its components and ensuring a sustainable operation.

### 2.2.2. Welding configuration and geometry of test specimen

In this study, the weldability window is constructed by considering two key process parameters: the discharge energy ($E$) and the standoff distance ($e$). The study focuses on two material combinations: the first involves Aluminum 5754 as the flyer and Aluminum 7075 as the base plate, while the second consists of Aluminum 5754 as the flyer and MARS 380 armor steel as the base plate. The thickness of the Aluminum 5754 flyer is 0.5 mm in both cases, and the base plates (whether Aluminum 7075 or MARS 380) have a thickness of 9 mm. The geometry of the samples is illustrated in Fig. 2.

### 2.2.3. Inductor shape

Since the coils must withstand very high mechanical stresses at elevated temperatures, their durability remains a significant challenge. In this context, previous studies have been conducted to evaluate the performance of U-shaped and O-shaped coils [17,19].

#### 2.2.3.1. O-shape inductor
The first inductor considered is an O-shape inductor or coil, whose geometry is illustrated in Fig. 3. The coil is subjected to a discharged current $I(t)$ generating a reaction force $F_a$ resulting from the interaction with the flyer. Simultaneously, the coil's structure, influenced by the magnetic fields B

oriented in the y-direction, generates a force $F$, between the different parts of the coil itself, which induces a stress on the coil's structure which might damage it. These forces and fields are represented in Fig. 4.

To ensure the integrity and functionality of the inductor, it is crucial to limit its movement in all directions. As well as damaging the inductor, the internal forces could lead to undesired displacements or deformations which might alter the performance of the process. Therefore, an inductor clamping system is necessary to maintain its stability, ensuring it to remain securely in place after the impact. The design of the inductor clamping system and the associated O-shape inductor was established in a previous study [17]. However, a system has been developed for this study to facilitate the positioning of the specimen and the repeatability of the tests.

The clamping system for the coil is represented in Fig. 5. The coil is labeled as component (4). Component (1), a glass FRPC (fiber reinforced polymer composite) support, supports the coil, while component (2) restricts coil's movement in the positive z-direction as well as in both the positive and negative x-directions. Moreover, component (2) stops the movement in the negative z-direction and is designed to counter forces in the y-direction. Additionally, a hold (3) is integrated within the system to secure the specimen, stopping any undesired displacement. The support (2) and hold (3) of the system, which are bolted together, are subjected to high pressure. Therefore, they are made of steel ensuring their durability and resistance. To minimize the gap between the inductor and the specimen while preventing direct contact, a kapton sheet is used to provide electrical insulation. Besides, the inductance of the coil $L_{coil}$ has been experimentally quantified as $L_{coil} = 0.49L_{Generator}$ [17].

The metallic components of the structure have to be electrically isolated from one another to ensure the efficiency of the process. Fig. 6 illustrates the specimen positioning and the clamping systems for the O-shape inductor. The flyer is first positioned on the inductor, separated by a Kapton sheet to ensure electrical insulation. Next, the desired stand-off distance is adjusted between the flyer and the base plate. Finally, the base plate is placed on top of this assembly and secured in place by the holding mechanism shown in Fig. 5. This ensures that the inductor's active zone aligns precisely with the center of the specimen, ensuring consistency and repeatability in the experiments.

#### 2.2.3.2. U-shape inductor
Then, a U-shape inductor, with linear shape rectangular cross section, will be considered and whose geometry is presented in Fig. 7. In this case, the coil will be subjected to a current $I(t)$ creating repulsive forces between its two side parts due to the opposite currents flowing in them. Additionally, a force is applied downward in the active zone due to the flyer's reaction magnetic forces. These forces are illustrated in Fig. 8.

Hence, similarly to the O-shape inductor, a clamping system, illustrated in Fig. 9, has been designed to prevent the coil from any deformation which could damage it or deteriorate the performances of the process. The support, element (4) of the clamping system, aims to stop the inductor to open by counteracting repulsive forces between its side parts. Element (1), is reused from the clamping system of the O-shape inductor and aims to maintain the inductor and the specimen in y-direction. Finally insulators (2) aims to electrically isolate the different metallic parts of the system which is essential for its functioning. As for O-shape inductor's clamping system, the hold (1) and the support (4) are bolted together and a kapton leaf is used to electrically isolate the specimen from the inductor.

Fig. 10 illustrates the specimen positioning and the clamping systems for the U-shape which is analog to the one used for the O-shape inductor.
<figure><img src="image_5.png" /><figcaption>Fig. 2. Welding configuration and geometries (dimensions are in mm). Parameters ($d_A, d_B, D$) are defined subsequently.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Geometry of the O-shape inductor (lengths are in mm) [21].</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Clamping system for O-shape inductor.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 4. Forces acting on O-shape inductor [21].</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Specimen positioned on O-shape inductor's clamping system.</figcaption></figure>

shape inductor. The assembly is secured in place by the holding mechanism shown in Fig. 9.

### 2.2.4. Single lap shear test setup

To evaluate the strength of the magnetic pulse welded assemblies under quasi-static loading conditions, single lap shear tests

were conducted on successfully welded MPW specimens, which will be detailed in section 3.1. The dimensions of these specimens are described in Fig. 2. These displacement-controlled tests actually expose the weld to a combination of shear and tensile stresses.
<figure><img src="image_10.png" /><figcaption>Fig. 7. U-shape inductor geometry (lengths are in mm) [21].</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Clamping system for U-shape inductor.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. Specimen positioned on U-shape inductor's clamping system.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 8. Forces acting on U-shape inductor [21].</figcaption></figure>

A wedge, visible in Fig. 11, was used to account for the thickness disparity between the base plate and the flyer, enabling a correct pulling of the specimen. The wedge is essential for the tensile tests.

Without it, the thinner aluminum part (compared to the steel part) would bend during clamping, and the shear-tensile test would not yield usable results. The wedge compensates for the thickness difference between the flyer and the base plate, ensuring proper alignment of the specimen during loading. This is particularly important for single lap shear tests, which are widely used to characterize welded or bonded joints, as they aim to apply a predominantly in-plane shear load at the interface. If bending occurs, the load distribution becomes non-uniform, introducing out-of-plane moments and mixed stress states that can lead to premature or misleading failure. The wedge thus helps maintain the validity and reproducibility of the mechanical strength measurements by preserving the intended loading configuration.

## 2.3. Set up to determination of the flyer's impact velocity depending on the standoff distance

The success of high velocity impact welding, especially in the context of magnetic pulse welding (MPW), hinges on two factors for a given couple of materials: the impact velocity of the flyer and the angle at which it strikes. Acknowledging the importance of these parameters, this section is dedicated to an experimental analysis aimed at measuring the flyer's velocity as a function of the standoff distance. This investigation represents the fourth step in the experimental protocol outlined in section 2.1. To accurately measure the velocity of the flyer's center, a heterodyne velocimetry (VH) probe is used as depicted in Fig. 12. This measurement technique relies on Doppler effect. The VH probe focuses a laser beam on a target, which then reflects back to its source. Thus, any movement of the target causes a change in the wavelength of the reflected laser beam, enabling the estimation of the target's velocity [22].
<figure><img src="image_14.png" /><figcaption>Fig. 11. Single lap shear test setup.</figcaption></figure>

However, for the accuracy of this technique, it is necessary that a part of the reflected laser beam precisely returns back to the probe (1). As illustrated in Fig. 12, a hole is drilled into the base plate (2) to enable the measurement of the flyer's backface velocity. Finally, to ensure the optical fiber remains protected from any debris, a transparent polymer (3), placed directly over the hole, serves as a protective window.

## 2.4. Method and test facilities for ballistic testing

### 2.4.1. Substructure

The final step of this study involves the fabrication and ballistic testing of substructures assembled using magnetic pulse spot welding, as outlined in step five of the protocol in section 2.1. The substructure consists of a 50 cm × 50 cm panel, as specified in the NATO standard STANAG 4569 [23], composed of a substrate, a sandwiched material, and a coating. The primary objective of this phase is to evaluate the isolated performance of the weld joint under ballistic impact. The overall ballistic performance of the

<figure><img src="image_15.png" /><figcaption>Fig. 12. System to measure the backface velocity of the flyer.</figcaption></figure>

structure, such as its ability to stop a projectile, is not the focus of this study. To isolate the weld spot's behavior, a lightweight "Isorel®" panel is employed as a spacer material, which establishes a specific "standoff distance". One can note that "Isorel®" is a wood fibered material, and is thus not conductive nor affected by the MPW process, showing that MPW can assemble in a sandwiched structure non electrically conductive materials such as ceramics and composite materials. However, to mimic lightweight vehicle shielding as closely as possible while adhering to the welding limitations, the standoff distance between the flyer and base plate is maximized.

The front material used in this study consists of a thin (0.5 mm) Aluminum 5754 flyer plate, a thicker (3.2 mm) "Isorel®" spacer panel, and a 9 mm thick Aluminum 7075 base plate, as depicted in Fig. 13. These layers are joined via magnetic pulse spot welding using an O-shaped inductor, operating within the optimal welding zone (green region) shown in Fig. 35.

For successful magnetic pulse spot welding, the "Isorel®" plate must be perforated with openings to allow direct interaction between the flyer and base plate. The design parameters for those openings, such as their dimensions and number, are based on the outcomes from the assembly tests on smaller specimens. The distance between the wedges (D) and the post-mortem deformations of the flyer are considered when determining the dimensions of these openings. As a result, the "Isorel®" plate has five openings, each measuring 40 mm × 150 mm, as shown in Fig. 14. To ensure a consistent assembly of the structures, a positioning system is used. The final assembly of the structure is completed by applying five weld points, as illustrated in Fig. 15.

### 2.4.2. Ballistics tests setup

The O-shape inductor welding process results in two weld points. The primary objective of this series of tests is to evaluate the effectiveness of these weld points rather than the overall structural integrity of the assembly. The test campaign tackles three scenarios: (1) shooting exactly at one of the weld point, (2) shooting between the two weld points, and (3) shooting at the "Isorel®" panel near the weld area. In each scenario, the resilience of the weld points in the vicinity of the impact will be assessed.

The tests will be carried out using two types of munition: the 7.62 mm × 51 mm NATO, impacting at a velocity of approximately 850 m/s, and the 9 mm × 19 mm Parabellum, travelling at around 390 m/s. This ballistic test campaign was conducted at the Douillet Armoury in Plumeliau, France, with the support of OLEA Technology, France. Projectile velocities were measured with a velocimeter at the exit of the barrel.

The experimental setup is shown in Fig. 16. The gun barrel is positioned 8 m from the target. The target, whose dimensions are described in Fig. 14, is illuminated by three spotlights. The event is captured by three Photron Fastcam high-speed cameras, recording at approximately 50,000 frames per second, triggered by a

<figure><img src="image_16.png" /><figcaption>Fig. 13. Substructure components (from left to right: Al 5754 substrate, Isorel intermediate material, Al 7075 skin).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Isorel panel (dimensions are in mm).</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Assembled substructure.</figcaption></figure>

microphone at the moment of firing. These cameras are positioned to capture footage from the front, back, and side of the target. To protect the equipment, a plexiglass transparent safety shield is placed between the cameras and the target to prevent damage from debris. Additionally, a diffusive screen is used to block the glare from the spotlights, ensuring that the cameras are not blinded by the intense light. Figs. 17 and 18 provide further details of the setup, including the gun's adjustment bench and the anti-recoil spring system.

# 3. Results and discussion

## 3.1. Construction of weldability windows

### 3.1.1. Extended weldability windows

Figs. 19–21 present the weldability windows for different material combinations and inductor shapes used during the assembly process. Specifically, Fig. 19 highlights the weldability window for

<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Sketch of the setup used for the ballistic test campaign.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Weapons used in the ballistic test campaign.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. 18.** Photography of the setup used for the ballistic test campaign.</figcaption></figure>

Aluminum 5754 as the coated material, paired with MARS 380 armor steel as the substrate plate, using an O-shape inductor. Similarly, Fig. 20 shows the weldability window for the same flyer material, Aluminum 5754, but paired with Aluminum 7075 as the substrate plate, also assembled with an O-shape inductor. This comparison allows an assessment of the influence of the nature of the substrate material on the bond strength resulting from the welding process for a given coating under similar conditions. Lastly, Fig. 21 illustrates the weldability window for Aluminum 5754 as the flyer and Aluminum 7075 as the substrate plate, but assembled using a U-shape inductor, providing insight into the influence of the inductor shape on the MPW the process.

Within the weldability windows, three distinct zones can be identified, each represented by a different color: red, yellow, and green. The red zones correspond to configurations that did not result in successful welding. The yellow zones represent untested
<figure><img src="image_22.png" /><figcaption>**Fig. 19.** Weldability window of Al5754 and MARS 380 assembled using an O-shape inductor for a 0.5 mm Al 5754 flyer.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Fig. 20.** Weldability window of Al5754 and Al 7075 assembled using an O-shape inductor for a 0.5 mm thick Al 5754 flyer.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>**Fig. 21.** Weldability window of Al5754 and Al 7075 assembled using a U-shape inductor for a 0.5 mm thick Al 5754 flyer.</figcaption></figure>

configurations, while the green zones indicate configurations that resulted in successful welds. The untested yellow zones exist due to the discrete nature of the exploration within the weldability window. Configurations within these zones are likely on the borderline of weldability window which could result in either unsuccessful welds or welds with insufficient bond strength for practical applications. As such, these configurations cannot be considered reliable.

The results of the welding tests are indicated by different symbols: a red cross marks configurations where welding failed, a green dot represents successful welds, and a blue star means welds that were initially successful but broke later, possibly due to residual stresses. In these cases, the bonding was likely due to mechanical interlocking caused by the wavy interface. The samples appeared to be welded, but very slight thermomechanical stresses were generally sufficient to separate the two parts. This typically occurs near the boundary between the weldable and non-weldable zones. The zones are determined as follows: any point below a blue star or red cross is classified as part of the red zone, indicating failure, while points above a green dot fall within the green zone, indicating successful welding. Configurations not in the green or red zones fall into the yellow one.

In Figs. 20 and 21, with aluminum substrate, configurations marked by a blue star (indicating debonding after initial success) may appear within the green zone. This inconsistency could be attributed to variations in the manual sanding and acetone cleaning process, which can affect surface conditions and welding outcomes. The surface quality of the plates being crucial for the quality of the weld.

The weldability windows for the O-shape inductor, shown in Figs. 19 and 20, reveal minimum standoff distances of 1 mm and 2 mm, respectively. Below these thresholds, the flyer does not have enough acceleration time to reach the required welding velocity. These figures also display a gradual transition at the right/bottom boundary, reflecting the effective-to-ineffective welding transition caused by the decreasing impact velocity, as discussed in section 3.2. In contrast, Fig. 21, which shows results using a U-shape inductor, does not exhibit a distinct left boundary or a clear stepped right/bottom boundary, indicating different dynamics for this shape of inductor.

It is important to note that, in all cases, the used pulse generator is unable to reach the theoretical upper bounds of the weldability window beyond which the materials would fuse together [7]. This limitation emphasizes that the experimental setup cannot explore the entire theoretical weldability window under the current conditions. A more powerful generator could potentially reach stronger weld configurations. Nevertheless, the weldability windows presented here are specific to the materials, inductor types, generator, and geometric configurations used in this study (including the standoff distance *e* and wedge distance *D*). Any alteration in these parameters would need a new weldability window design. Despite these constraints, this investigation confirms the feasibility of welding dissimilar materials, particularly those used in light weight shielding using magnetic pulse welding technology.

### 3.1.2. Post mortem observations after single lap shear test

In the Al5754-MARS 380 configurations welded with an O-shape inductor, post-mortem observations reveal two distinct welding points circled in red in Fig. 22. Typically, one weld point would fail before the other; however, in some specimens, both points failed simultaneously. In four specific cases, the flyer material tore around one or two of the welding points, as shown in
Fig. 23. The affected specimens include {e = 1.5; E = 11}, {e = 1; E = 11}, {e = 2; E = 9}, and {e = 2; E = 11}; with e in mm and E in kJ. This suggests that the weld strength exceeds that of the flyer material, indicating good weld quality. Additionally, the base plates show the presence of aluminum dust, circled in blue in Fig. 22, which is likely a sign of the jetting phenomenon associated with impact welding.

For the Al5754-Al7075 configurations welded using an O-shape inductor, post-mortem analysis reveals a clear welding line, as seen in Fig. 24. This observation is supported by the results of single lap shear tests, which show a straight fracture pattern, as depicted in Fig. 32.

The deformation kinematics of the flyer with the U-shaped coil [18] differs from that observed with the O-shaped coil, leading to distinct behaviors and observations. In configurations involving a U-shape inductor, the welds exhibit various patterns, as shown in Fig. 26. These welds can appear as either welding spots (highlighted in red) or welding lines (circled in green), or a combination of both. Notably, configurations featuring welding spots tend to exhibit greater strength than those without. This suggests that the presence of welding spots significantly enhances the structural resilience of the welds by increasing the welded area.

For both the Al5754-MARS 380 and Al5754-Al7075 pairings, regardless of whether a U-shape or O-shape inductor was used, the flyer deformation corresponds to the geometry of the inductor's shape. Notably, areas of the flyer outside the inductor's active region show no deformation, even without the presence of a physical wedge. This observation leads to the concept of a "virtual wedge" effect, highlighted in green in Figs. 24 and 25, and in blue in Fig. 27. The virtual wedge effect implies that the physical distance between wedges, denoted by D, could be reduced without affecting the welding quality. This phenomenon is likely due to the dynamic effects during the welding process.

### 3.1.3. Analysis of maximal tensile forces, and reduced weldability windows

The three-dimensional plots shown in Figs. 28–30 provide a comprehensive visualization of the correlation between the maximal tensile force reached during single lap shear tests and the welding process parameters for various material combinations. Across all material combinations and regardless of the shape of the inductor, configurations exposed to higher energy levels (E) produced stronger welds, highlighting the direct link between the stored energy and the impact velocity. However, the effect of standoff distance (e) on weld strength depends on the material combination. For the Al5754 - MARS 380 pair, increasing the standoff distance correlates with reduced weld strength. It emphasizes the fact that a soft material impacting a hard material tends to rebound. In contrast, for the Al5754-Al7075 pair, greater standoff distances result in stronger welds, highlighting the influence of the substrate material on the welded structures.

For configurations welded with the U-shaped inductor, the strongest bonds are observed at a standoff distance of

<figure><img src="image_25.png" /><figcaption>Fig. 23. Tore flyer around welding points. An O-shape inductor was used with a 0.5 mm thick Al 5754 flyer.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>Fig. 24. Post-mortem weld mark on the aluminum base plate. The welded line is circled in red, and the virtual wedge effect in green. An O-shape inductor was used.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>Fig. 25. Deformation of a flyer welded using MPW with an O-shape inductor. The welded spots are circled in red, and the virtual wedge effect in green.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Fig. 22. Post-mortem weld mark on MARS 380 base plate impacted by 0.5 mm thick Al flyer. The welding spots are circled in red and aluminum dust in blue. An O-shape inductor was used.</figcaption></figure>

approximately 2.75 mm, representing the midpoint of the weldable standoff distances. The relationship between weld strength and standoff distance follows for symmetrical pattern around this peak, indicating that optimal bond strength is achieved at this specific distance.

Figs. 31–33 present the results of single lap shear tests, displaying the tensile force-displacement curves for the Al5754 - MARS 380 combination and the Al5754-Al7075 pairs, using O-shaped and U-shaped inductors, respectively. The results focus on
<figure><img src="image_29.png" /><figcaption>**Fig. 26.** Welding patterns on the base plate using a U-shape inductor. The welded line is circled in green and the welding spots in red.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Fig. 27.** Deformation of a flyer welded using MPW with a U-shape inductor.</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>**Fig. 28.** Maximum force achieved during the single lap shear test as a function of the standoff distance *e* and the discharge energy *E* for the Al 5754-MARS 380 combination welded with an O-shaped inductor.</figcaption></figure>

configurations that withstood significant maximal forces during testing, above 2 kN.

The outcomes for the Al5754 - MARS 380 combination, as highlighted in Fig. 31, can be classified into five distinct weld strength levels based on the strength measured at the breaking point of the first weld. Welds exhibiting a strength below 1,000 N (shown in red in Fig. 34) are deemed insufficient for practical applications, while those around 1,600 N (displayed in orange) are considered weakly welded. Welds with strengths near 2,000 N (illustrated in yellow) are categorized as moderately welded. Additionally, welds with strengths between 2,000 and 3,500 N, which show flyer tearing near the weld spots (indicated in green), are identified as strong. Finally, welds exceeding 4,000 N

<figure><img src="image_32.png" /><figcaption>**Fig. 29.** Maximum force achieved during the single lap shear test as a function of the standoff distance *e* and the discharge energy *E* for the Al 5754-Al 7075 combination welded with an O-shaped inductor.</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Fig. 30.** Maximum force achieved during the single lap shear test as a function of the standoff distance *e* and the discharge energy *E* for the Al 5754-Al7075 combination welded with a U-shaped inductor.</figcaption></figure>

(represented in blue) not only surpass the strength of the flyer material but also serve as robust components of the assembly.

Similarly, the tensile test resulting from the Al5754 - Al7075 combinations, shown in Figs. 32 and 33, welded with either O-shaped or U-shaped inductors, can be divided into five categories, but with different thresholds compared to the Al5754 - MARS 380 combination. When using the O-shaped inductor, the strength categories are defined by levels of 2,500 N (blue), 2,000 N (green), 1,750 N (yellow), 1,200-1,500 N (orange), and less than 1,000 N (red), as shown in Fig. 35. For the U-shaped inductor, the categories are defined by strengths around 2,500 N (blue), 1,500-2,000 N (green), 1,500 N (yellow), 1,000 N (orange), and less than 800 N (red), shown in Fig. 36. In contrast to the Al5754 - MARS 380 combination, welds between Al5754 and Al7075 exhibit a sharp rupture pattern, likely due to differences in the welding geometry. Notably, flyer tearing does not occur at the welds in these cases.

Finally, Figs. 34–36 depict the reduced weldability window, categorizing configurations into three distinct zones based on weld strength for each material combination studied. The green zone represents robust welds with strong bond strength. The yellow zone includes configurations that, while achieving a weld, show weaker bond strength and may not be optimal for practical applications. The red zone represents configurations where the
<figure><img src="image_34.png" /><figcaption>**Fig. 31.** Single lap shear test curves showing tensile force (N) versus displacement (mm) for the Al 5754-MARS 380 combination welded with an O-shape inductor, sustaining more than 2,000 N before failure. (a) Welded bonds; (b) Strongly welded bonds.</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Fig. 32.** Single lap shear test curves showing tensile force (N) versus displacement (mm) for the Al 5754-Al 7075 combination welded with O-shaped inductor, handling more than 2,000 N before failure. (a) Welded bonds; (b) Strongly welded bonds.</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>**Fig. 33.** Tensile test curves showing tensile force (N) versus displacement (mm) for the Al 5754-Al 7075 combination welded with U-shaped inductor, handling more than 1,500 N before failure. (a) Welded bonds; (b) Strongly welded bonds.</figcaption></figure>

weld fails to provide adequate strength for use.

The weldability window for the Al5754/Al7075 combination is noticeably smaller than that of the Al5754/MARS 380 pair. However, weldable configurations for the Al5754/Al 7075 pair feature larger standoff distances, potentially allowing for the insertion of a thicker third layer between the flyer and base plate, which could be advantageous considering ballistic protection assemblies.

## 3.2. Flyer's acceleration

Figs. 37 and 39 illustrate the velocity profiles of the Al 5754 flyer when accelerated by O-shaped and U-shaped inductors, respectively. In both configurations, a velocity peak occurs around 20 µs. For the O-shaped inductor, this peak corresponds to flyer displacements of 3.05, 3.27, and 3.32 mm at energy inputs of 8, 10,
<figure><img src="image_37.png" /><figcaption>**Fig. 34.** Reduced weldability window for the Al5754 and MARS 380 pair welded with O-shaped inductor.</figcaption></figure>

<figure><img src="image_38.png" /><figcaption>**Fig. 35.** Reduced weldability window for the Al5754 and Al7075 pair welded with O-shaped inductor.</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>**Fig. 36.** Reduced weldability window for the Al5754 and Al7075 pair welded with U-shaped inductor.</figcaption></figure>

and 12 kJ, respectively, as shown in Fig. 38. By contrast, the U-shaped inductor results in slightly greater flyer displacements of 3.32, 3.33, and 3.38 mm for the same energy levels, as shown in Fig. 40. This suggests that, beyond the velocity peak, the flyer undergoes deceleration, a key consideration to evaluate the weldability window as discussed in section 3.1.

### 3.3. Ballistic campaign results

Figs. 41 and 42 present the results of the ballistics tests, featuring images from cameras 1 and 2 that capture the target before, during, and after the impact, offering a chronological view of the event. Camera 3, which focuses on the rear of the target, records images starting from the moment of impact, showing a clean break in the back plate without prior deformation. Additionally, a post-mortem image of the front plate highlights the impact effects of the bullet. In the relevant images, distinct features are marked for easier analysis: the bullet is circled in red, debris are marked in green. The images shown in Figs. 41 and 42 are labeled as Camera i, j, where 'i' refers to the camera number as indicated in Fig. 16, and 'j' corresponds to the time at which the image was captured.

For the tests using 7.62 mm × 51 mm NATO munition, several key observations can be made:

* The bullet successfully penetrates the structure, regardless of whether it strikes a welded point, the area between two welded points, or the isorel panel.

* When the bullet strikes a welded zone without isorel, a distinct petal pattern is created, this behavior is described in Ref. [24], whereas a clean perforation is observed when the bullet hits the isorel panel. The hole is likely due to the impedance mismatch between aluminum and isorel. Significant debris are seen both in front of and behind the target when the welded zone is hit, while less debris are visible at the front face when the isorel panel is impacted.

* The back plate is penetrated without any prior deformation, with debris found over 10 m away from the structure. The resulting perforation is shown in Fig. 43, where cracks associated with the failure are visible.

* If the bullet passes near a welded zone without directly hitting it, the weld remains intact, even when the bullet strikes very close to it, such as in the space between two welds.

<figure><img src="image_40.png" /><figcaption>**Fig. 37.** Velocity profile of the Al 5754 flyer with O-shaped inductor.</figcaption></figure>
<figure><img src="image_41.png" /><figcaption>Fig. 38. Displacement profile of the Al 5754 flyer with O-shaped inductor.</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>Fig. 39. Velocity profile of the 0.5 mm thick Al 5754 flyer with U-shaped inductor.</figcaption></figure>

<figure><img src="image_43.png" /><figcaption>Fig. 40. Displacement profile of the 0.5 mm thick Al 5754 flyer with U-shaped inductor.</figcaption></figure>

In contrast, the tests using 9 mm × 19 mm parabellum munition yield different results:

* The bullet does not fully penetrate the structure, regardless of whether it strikes a welded zone, the area between two welds, or the isorel panel. The front plate and isorel are crossed, but the back plate successfully stops the bullet.

* A cleaner hole is produced in the isorel panel compared to the welding area, where the petal pattern is less distinct than with the 7.62 mm × 51 mm NATO bullet. As a result, fewer debris are observed on the front of the target.

* Since the bullet does not fully penetrate the structure, all of its kinetic energy is absorbed. This is evidenced by the propagation of Rayleigh waves on the front plate, highlighted in blue in Fig. 42. Additionally, the heat and pressure cause the bullet to melt, forming a lead jet, shown in Fig. 44, which shears nearby welded points. This combination of mechanical waves and molten lead results in the rupture of welds when the bullet directly impacts a welded zone. Some lead fragments could even be found in the transparent panel that protected the camera and the projectors.

* When the bullet hits the isorel panel, a similar molten lead jet is observed (see Fig. 45), though the nearby welds remain intact despite the impact.

* The camera system failed to capture the bullet's impact on the isorel panel due to a triggering malfunction.

This experimental campaign aims to evaluate the performance of welds created by magnetic pulse welding when subjected to bullet impacts. The results for 7.62 mm × 51 mm NATO impact show that the welded zones remain intact if the bullet does not directly strike them, even when it passes through the structure. However, with 9 mm × 19 mm parabellum munitions, the bullet fail to fully penetrate resulting in molten lead formation. When the bullet strikes a welded zone, the combination of released kinetic energy, molten lead, and Rayleigh waves, in addition the lead jet, are sufficient to shear nearby weld interfaces. This highlights a potential vulnerability in the welding process when subjected to ballistic impacts.

## 4. Conclusions

This study explored the application of magnetic pulse welding (MPW) for joining dissimilar materials commonly used in ballistic protection for lightweight armored vehicles. Two material combinations were selected for investigation: Al5754-Al7075 and Al5754-MARS380, both widely used in shielding applications.

The purpose of the study was to propose a method to explore the weldability of selected materials. Initial experiments focused on small specimens to assess various MPW parameters. The study began with the Al5754-Al7075 combination using an O-shaped inductor. Subsequently, modifications were introduced by replacing the base material with steel (MARS 380) or switching to a U-shaped inductor. A key parameter in the MPW process, the standoff distance between the joining surfaces was controlled using spacers. In practical applications, these spacers could be substituted with ballistic materials such as Tensylon or Kevlar. Notably, the material of the spacer did not affect the welding itself and was therefore not a primary focus of this research. The performance of the welded specimens was, then, evaluated through a series of single lap shear tests.

The maximum achievable spacer thickness, and thus the maximum thickness of integrated shielding materials, appeared to
<table><caption>**Fig. 41.** Results of ballistic test for 7.62 mm × 51 mm NATO munition.</caption><thead><tr><th>Munition</th><th colspan="3">7.62×51 NATO</th></tr><tr><th>Target</th><th>Isorel</th><th>Between welding points</th><th>On welding point</th></tr></thead><tbody><tr><td>Camera 1_1</td><td></td><td></td><td></td></tr><tr><td>Camera 1_2</td><td></td><td></td><td></td></tr><tr><td>Camera 1_3</td><td></td><td></td><td></td></tr><tr><td>Camera 2_1</td><td></td><td></td><td></td></tr><tr><td>Camera 2_2</td><td></td><td></td><td></td></tr><tr><td>Camera 2_3</td><td></td><td></td><td></td></tr><tr><td>Camera 3_1</td><td></td><td></td><td></td></tr><tr><td>Camera 3_2</td><td></td><td></td><td></td></tr><tr><td>Post-mortem</td><td></td><td></td><td></td></tr></tbody></table>

be between 3 and 3.5 mm for the Al5754-Al7075 combination welded with the O-shaped inductor. This configuration was subsequently used to construct substructure panels for ballistic performance testing. It is important to note that the limitations of the pulse generator used in the experiments do not allow to fully explore the theoretical weldability window.

Ballistics tests were then conducted using 7.62 mm × 51 mm NATO and 9 mm × 19 mm parabellum munitions, with shots fired at three specific locations: directly on a weld point, between two

weld points, and near the welded zone. The 7.62 mm × 51 mm NATO bullets passed through the structure without damaging the adjacent welding points, while the 9 mm × 19 mm parabellum ones were stopped by the structure but caused debonding at the impact zone, likely due to a molten lead jet effect and the action of Rayleigh waves.

Overall, this study successfully demonstrated the feasibility of using MPW to join dissimilar materials for ballistic protection of armoured vehicles. The outcomes highlight MPW's potential for
<table><caption>**Fig. 42.** Results of ballistic test for 9 mm × 19 mm parabellum munition.</caption><thead><tr><th>Munition</th><th colspan="3">9×19 parabellum</th></tr><tr><th>Target</th><th>Isorel</th><th>Between welding points</th><th>On welding point</th></tr></thead><tbody><tr><td>Camera 1_1</td><td>No trigger</td><td></td><td></td></tr><tr><td>Camera 1_2</td><td>No trigger</td><td></td><td></td></tr><tr><td>Camera 1_3</td><td>No trigger</td><td></td><td></td></tr><tr><td>Camera 2_1</td><td>No trigger</td><td></td><td></td></tr><tr><td>Camera 2_2</td><td>No trigger</td><td></td><td></td></tr><tr><td>Camera 2_3</td><td>/</td><td>/</td><td>/</td></tr><tr><td>Camera 3_1</td><td>Doesn't pass</td><td>Doesn't pass</td><td>Doesn't pass</td></tr><tr><td>Camera 3_2</td><td>Doesn't pass</td><td>Doesn't pass</td><td>Doesn't pass</td></tr><tr><td>Post-mortem</td><td></td><td></td><td></td></tr></tbody></table>

<figure><img src="image_44.png" /><figcaption>**Fig. 43.** Bullet exit hole at the back face of the target.</figcaption></figure>

assembling these material combinations, enabling the development of lighter and potentially more effective shielding solutions. The established weldability windows and ballistic performance data of the substructures offer a valuable foundation for future research and development in the fields of manufacturing and materials science. In particular, correlating the results of quasi-static mechanical tests with the ballistic performance of assemblies would be a highly relevant next step and represent one of the main perspectives for future investigations.

<figure><img src="image_45.png" /><figcaption>**Fig. 44.** Molten lead from the bullet.</figcaption></figure>

CRediT authorship contribution statement

**Benoit Lagain:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Thomas Heuzé:**
<figure><img src="image_46.png" /><figcaption>Fig. 45. Molten lead from the bullet on isorel.</figcaption></figure>

Writing – review & editing, Supervision, Project administration. Methodology, Funding acquisition, Conceptualization. **Guillaume Racineux:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **Michel Arrigoni:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Benoit Lagain is funded on the one hand by Agence de l'Innovation de Défense (AID) grant reference number 2021 65 0044, and on the other hand by Ecole Centrale de Nantes. The authors also thanks Douillet Armoury in Plumeliau, France and OLEA Technology for their support during ballistic test campaign.

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