**4. Interface behaviour and weld variances**

In a conventional welding process, a weld is basically defined by three distinct zones: the solidified molten zone at the joint, the surrounding heat affected zone, and the base metals whose properties remain the same. The weld is generally produced at large scale to ensure an efficient joint at the interface during a conventional welding process. But, in MPW, since the bonding principle is completely different, the notion of weld nature is particularly varied in term of size and morphology. High velocity impact welds are typically confined at the interface within a few micrometre thick zones. A permanent bond occurs immediately between the two components but the behaviour of the interface represents different weld variances. Investiga‐ tions of welded specimens from Al6060-T6/Al6060-T6 joints have allowed identifying the typical variances for a similar metal pair [32–35]. The major observations of such investigations are reported in the following sections, which also cover the effect of joining dissimilar combinations.

a shearing action at the interface. Therefore, as a result of the confined shearing, the nearby grains at the vicinity of the interface become flattened and elongated as shown in **Figure 4c**.

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The interfacial shearing causes a series of kinematic instabilities that changes the morphology of the interface whose behaviour is therefore similar to the Kelvin-Helmholtz instability that occurs at fluidic interfaces subjected to an interfacial shearing. The straight interface becomes wavy at the onset of instability. The development of wave at the interface is governed by two main phenomena: interferences of compressive shock waves due to the dynamic collision and a jetting phenomenon governed by an alternate inversion. As suggested by Ben-Artzy et al., the first phenomenon is a mechanism for the creation of periodic humps with regular shapes [36]. The wave height and periodicity depend on the size of the structure, the compressive stress intensity, and the interference of shock waves along the interface. Under these condi‐ tions, the initiation and development of hump can be considered as a consequence of defor‐ mation of the bonded and sheared interface by the reflection of mechanical waves. All the humps regardless of their amplitude reveal shear strain along the interface that would be

**Figure 5.** Typical magnetic pulse welded interfaces with wavy morphologies showing grains shearing along the wavy

The interfacial shearing can reach an instability threshold due to a high strain rate and severe shearing. In the literature of impact welding, a notion of shear instability is used to demonstrate this circumstance. The confined shearing remains at the interface, but becomes abruptly excessive so that a jetting phenomenon occurs. A tangential jet forms ahead of the collision point and the jet kinematics is controlled by the normal force component along the interface that develops from both shock waves and impact velocity. The jet may evolve following a series of upward and downward jetting to form a sequence of inverted curves along the interface. In that way, the interface develops by jetting phenomenon which is interpreted as both a weld indicator and a mechanism of interfacial humps formation. However, the kine‐ matics of the sheared interface is complex so that it is difficult to clearly state which mechanism really forms the wavy interface: the interference of compressive shock waves or the jetting phenomenon. These two factors can be concomitant or asynchronous or due to their conse‐ quences. Particularly, for regular shape humps, a simultaneous prominence may prevail as

**4.3. Jetting phenomenon and interfaces with irregular wavy shapes**

**4.2. Interfacial deformation and wavy nature of the bonded interfaces**

regularly shaped the wavy interface (**Figure 5**).

pattern [32, 33].

#### **4.1. Onset of weld without apparent interfacial deformation**

The interface experiences a progressive kinematic phenomenon that governs the generation of various interfacial morphologies when subjected to the high speed collision. In MPW, the weld natures are generally identifiable at the microscopic level, and the first case is an apparent bonding showing a metal continuity across a straight bonded interface (**Figure 4**). This corresponds to the onset of weld produced by a predominant high compressive stress which is a hydrostatic stress since the grains adjacent to the interface remain undeformed (**Figure 4**). The interfacial zone exhibits an equiaxed grain structure without any noticeable deformation supporting the bond formation due to hydrostatic stresses. Generally, the contact pressure is expected to be in the range of 1–20 GPa according to a simple assessment based on the expression of collision pressure *P* =*ρ*1*ρ*2*C*1*C*2*Vi* / (*ρ*1*C*<sup>1</sup> + *ρ*2*C*2), where *ρ*1 and *ρ*<sup>2</sup> are the material densities, *C*1 and *C*2 are the speed of longitudinal waves in those materials and *Vi* is the impact velocity. The welded joint can extensively be straight as long as the interface remains stable during the complete collision (**Figure 4b**). The hydrostatic bonding is uniform along the interface and the collision is essentially governed by a normal stress. However, due to the oblique collision, the tangential component of the impact velocity can be high enough to cause

**Figure 4.** Typical interfacial features of an onset of bonding [32, 33]: (a) magnified view of a straight bonded interface, (b) typical large bonded zone and (c) typical bonded interface with onset of interfacial shearing.

a shearing action at the interface. Therefore, as a result of the confined shearing, the nearby grains at the vicinity of the interface become flattened and elongated as shown in **Figure 4c**.

#### **4.2. Interfacial deformation and wavy nature of the bonded interfaces**

**4. Interface behaviour and weld variances**

**4.1. Onset of weld without apparent interfacial deformation**

combinations.

250 Joining Technologies

In a conventional welding process, a weld is basically defined by three distinct zones: the solidified molten zone at the joint, the surrounding heat affected zone, and the base metals whose properties remain the same. The weld is generally produced at large scale to ensure an efficient joint at the interface during a conventional welding process. But, in MPW, since the bonding principle is completely different, the notion of weld nature is particularly varied in term of size and morphology. High velocity impact welds are typically confined at the interface within a few micrometre thick zones. A permanent bond occurs immediately between the two components but the behaviour of the interface represents different weld variances. Investiga‐ tions of welded specimens from Al6060-T6/Al6060-T6 joints have allowed identifying the typical variances for a similar metal pair [32–35]. The major observations of such investigations are reported in the following sections, which also cover the effect of joining dissimilar

The interface experiences a progressive kinematic phenomenon that governs the generation of various interfacial morphologies when subjected to the high speed collision. In MPW, the weld natures are generally identifiable at the microscopic level, and the first case is an apparent bonding showing a metal continuity across a straight bonded interface (**Figure 4**). This corresponds to the onset of weld produced by a predominant high compressive stress which is a hydrostatic stress since the grains adjacent to the interface remain undeformed (**Figure 4**). The interfacial zone exhibits an equiaxed grain structure without any noticeable deformation supporting the bond formation due to hydrostatic stresses. Generally, the contact pressure is expected to be in the range of 1–20 GPa according to a simple assessment based on the expression of collision pressure *P* =*ρ*1*ρ*2*C*1*C*2*Vi* / (*ρ*1*C*<sup>1</sup> + *ρ*2*C*2), where *ρ*1 and *ρ*<sup>2</sup> are the material densities, *C*1 and *C*2 are the speed of longitudinal waves in those materials and *Vi* is the impact velocity. The welded joint can extensively be straight as long as the interface remains stable during the complete collision (**Figure 4b**). The hydrostatic bonding is uniform along the interface and the collision is essentially governed by a normal stress. However, due to the oblique collision, the tangential component of the impact velocity can be high enough to cause

**Figure 4.** Typical interfacial features of an onset of bonding [32, 33]: (a) magnified view of a straight bonded interface,

(b) typical large bonded zone and (c) typical bonded interface with onset of interfacial shearing.

The interfacial shearing causes a series of kinematic instabilities that changes the morphology of the interface whose behaviour is therefore similar to the Kelvin-Helmholtz instability that occurs at fluidic interfaces subjected to an interfacial shearing. The straight interface becomes wavy at the onset of instability. The development of wave at the interface is governed by two main phenomena: interferences of compressive shock waves due to the dynamic collision and a jetting phenomenon governed by an alternate inversion. As suggested by Ben-Artzy et al., the first phenomenon is a mechanism for the creation of periodic humps with regular shapes [36]. The wave height and periodicity depend on the size of the structure, the compressive stress intensity, and the interference of shock waves along the interface. Under these condi‐ tions, the initiation and development of hump can be considered as a consequence of defor‐ mation of the bonded and sheared interface by the reflection of mechanical waves. All the humps regardless of their amplitude reveal shear strain along the interface that would be regularly shaped the wavy interface (**Figure 5**).

**Figure 5.** Typical magnetic pulse welded interfaces with wavy morphologies showing grains shearing along the wavy pattern [32, 33].

#### **4.3. Jetting phenomenon and interfaces with irregular wavy shapes**

The interfacial shearing can reach an instability threshold due to a high strain rate and severe shearing. In the literature of impact welding, a notion of shear instability is used to demonstrate this circumstance. The confined shearing remains at the interface, but becomes abruptly excessive so that a jetting phenomenon occurs. A tangential jet forms ahead of the collision point and the jet kinematics is controlled by the normal force component along the interface that develops from both shock waves and impact velocity. The jet may evolve following a series of upward and downward jetting to form a sequence of inverted curves along the interface. In that way, the interface develops by jetting phenomenon which is interpreted as both a weld indicator and a mechanism of interfacial humps formation. However, the kine‐ matics of the sheared interface is complex so that it is difficult to clearly state which mechanism really forms the wavy interface: the interference of compressive shock waves or the jetting phenomenon. These two factors can be concomitant or asynchronous or due to their conse‐ quences. Particularly, for regular shape humps, a simultaneous prominence may prevail as observed in **Figure 6**. The deformation of the interface creates a hump with nearly symmetric shape (regular shape) while the grains adjacent to the bonded interface are strongly sheared with an upward kinematics at the left front of the hump and a downward kinematics at the right front (**Figure 6a**). If the jetting phenomenon becomes predominant, irregular shapes arise from the progression of humps. A jetting aspect and its orientation can be clearly observed that depend on the governing stresses. On the typical case evidenced in **Figure 6b**, a downward jet occurs. This could initially be an upward jet that is inverted by an interference of shock waves at the interface, or a pure downward jet guided by the local stress evolution, or kinematic instability governed by an opposed shear alone similar to that of Kelvin-Helmholtz instability occurs at fluidic interfaces subjected to a shearing. Hence, the mechanism of wavy interface formation is to be fully explored despite the experimental identification of particular mor‐ phologies.

in size at the vortex affected zone (VAZ), and their enlargement depends on the local kine‐ matics and stress distribution. Note that the size of voids can reach up to several hundred μm, in the same range of the height of the humps, i.e. approximately equal to the size of the weld width. The mechanism of void formation could be due to an ejection molten fluid, a solidifi‐ cation shrinkage or a local fragmentation combined with particulates jetting governed by shear stresses [32]. In any event, the welded interface becomes defective due to the formation of

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The MPW of similar metal pairs generally reveal the weld natures among those aforemen‐ tioned variances. This large depiction is useful to identify the weldability. It is clear that the single case of wavy morphology is not sufficient to substantiate the weld formation as commonly suggested. Although this weld indicator is valuable, the bonding is able to occur independently of the interface shape thus the development of humps can evolve a defective joint. In addition, the relevance of wavy morphology is discussed for dissimilar metal combi‐ nations. However, this situation is identified as no longer a reliable consideration of a bond formation. Albeit the literature of impact welding includes several cases of regular or irregular wavy shaped dissimilar joints, there exist some cases where the formation of humps is conducive to brittle joints. This scenario was found when replacing the previous Al6060-T6/ Al6060-T6 pair by a dissimilar Al6060-T6/Cu combination, the copper being the inner rod in order to ensure the same in-flight behaviour for the flyer prior to the collision. Further investigations for in-flight behaviour of similar and dissimilar cases are also explored using

In term of material dissimilarity, the copper is softer, having a higher melting temperature and dissipating the interfacial heating more quickly than those of the aluminium alloy. The combination of these properties produces a different response at the Al/Cu interface during the collision. In contrast to the previous case (Al/Al), the major noticeable changes are associated with the interface nature and particularly prominent with the vortex development. When the vortex instability develops, the copper forms a solid spiral due to its higher malle‐ ability and higher melting temperature than those of the flyer part. The VAZ consists of a rollup so that the copper and the aluminium are locally intermixed (**Figure 8a**). The kinematics of the vortex contributes to the interface bonding by an interlocking mechanism governed by the

discontinuous voids (**Figure 7**).

**Figure 7.** Typical magnetic pulse welds with defects at the VAZ [33, 34].

multi-physics simulations in Section 6.

**4.5. Weld with an interfacial mixing or intermetallic compounds**

**Figure 6.** Wavy morphology with strong confined shearing, jetting, irregularly shaped interface and defect onset [32, 33]. (a) Wave formation with upward kinematics, (b) downward kinematics and (c) wavy interface with onset of de‐ fects.

#### **4.4. Vortex development and formation of defective welds**

Generally, the wavy shape is identified as a particular feature of a high speed welded joint and it has also been suggested as a weld indicator. Nevertheless, distinction between regular and irregular development of wavy interfaces could be useful. Formations of irregular humps due to the predominance of the jetting phenomenon indicate a severe shearing that can promote a confined heating. The plastic work can also be high enough to cause a strong thermomechan‐ ical softening, even a melting, and thus the strong shear stress would be unfavourable in the weld formation. Potential matters from ejection also lead to the formation of cavities within the welded interface. The interfacial observation in **Figure 7** represents a typical onset of defects appearing within the irregular wavy interface produced by the jetting phenomenon. Forma‐ tion of holes is evidenced at the jet tip and at the vicinity of the strongly sheared interfaces (**Figure 7**). Moreover the defects are confined at the bonded joint. The defective weld can be also caused by the kinematic progression of the interface. The jet flow may evolve towards complex kinematics such as a swirling flow. Zones of jetting occurrence become potential sites of vortex development similar to the Kelvin-Helmholtz instability in fluids. Experimentally, it was evident that the jetting affected zone (JAZ) containing prominent voids with circular morphology supports the suggestion of vortex development (**Figure 7**). These voids increase

in size at the vortex affected zone (VAZ), and their enlargement depends on the local kine‐ matics and stress distribution. Note that the size of voids can reach up to several hundred μm, in the same range of the height of the humps, i.e. approximately equal to the size of the weld width. The mechanism of void formation could be due to an ejection molten fluid, a solidifi‐ cation shrinkage or a local fragmentation combined with particulates jetting governed by shear stresses [32]. In any event, the welded interface becomes defective due to the formation of discontinuous voids (**Figure 7**).

**Figure 7.** Typical magnetic pulse welds with defects at the VAZ [33, 34].

observed in **Figure 6**. The deformation of the interface creates a hump with nearly symmetric shape (regular shape) while the grains adjacent to the bonded interface are strongly sheared with an upward kinematics at the left front of the hump and a downward kinematics at the right front (**Figure 6a**). If the jetting phenomenon becomes predominant, irregular shapes arise from the progression of humps. A jetting aspect and its orientation can be clearly observed that depend on the governing stresses. On the typical case evidenced in **Figure 6b**, a downward jet occurs. This could initially be an upward jet that is inverted by an interference of shock waves at the interface, or a pure downward jet guided by the local stress evolution, or kinematic instability governed by an opposed shear alone similar to that of Kelvin-Helmholtz instability occurs at fluidic interfaces subjected to a shearing. Hence, the mechanism of wavy interface formation is to be fully explored despite the experimental identification of particular mor‐

**Figure 6.** Wavy morphology with strong confined shearing, jetting, irregularly shaped interface and defect onset [32, 33]. (a) Wave formation with upward kinematics, (b) downward kinematics and (c) wavy interface with onset of de‐

Generally, the wavy shape is identified as a particular feature of a high speed welded joint and it has also been suggested as a weld indicator. Nevertheless, distinction between regular and irregular development of wavy interfaces could be useful. Formations of irregular humps due to the predominance of the jetting phenomenon indicate a severe shearing that can promote a confined heating. The plastic work can also be high enough to cause a strong thermomechan‐ ical softening, even a melting, and thus the strong shear stress would be unfavourable in the weld formation. Potential matters from ejection also lead to the formation of cavities within the welded interface. The interfacial observation in **Figure 7** represents a typical onset of defects appearing within the irregular wavy interface produced by the jetting phenomenon. Forma‐ tion of holes is evidenced at the jet tip and at the vicinity of the strongly sheared interfaces (**Figure 7**). Moreover the defects are confined at the bonded joint. The defective weld can be also caused by the kinematic progression of the interface. The jet flow may evolve towards complex kinematics such as a swirling flow. Zones of jetting occurrence become potential sites of vortex development similar to the Kelvin-Helmholtz instability in fluids. Experimentally, it was evident that the jetting affected zone (JAZ) containing prominent voids with circular morphology supports the suggestion of vortex development (**Figure 7**). These voids increase

**4.4. Vortex development and formation of defective welds**

phologies.

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fects.

The MPW of similar metal pairs generally reveal the weld natures among those aforemen‐ tioned variances. This large depiction is useful to identify the weldability. It is clear that the single case of wavy morphology is not sufficient to substantiate the weld formation as commonly suggested. Although this weld indicator is valuable, the bonding is able to occur independently of the interface shape thus the development of humps can evolve a defective joint. In addition, the relevance of wavy morphology is discussed for dissimilar metal combi‐ nations. However, this situation is identified as no longer a reliable consideration of a bond formation. Albeit the literature of impact welding includes several cases of regular or irregular wavy shaped dissimilar joints, there exist some cases where the formation of humps is conducive to brittle joints. This scenario was found when replacing the previous Al6060-T6/ Al6060-T6 pair by a dissimilar Al6060-T6/Cu combination, the copper being the inner rod in order to ensure the same in-flight behaviour for the flyer prior to the collision. Further investigations for in-flight behaviour of similar and dissimilar cases are also explored using multi-physics simulations in Section 6.

#### **4.5. Weld with an interfacial mixing or intermetallic compounds**

In term of material dissimilarity, the copper is softer, having a higher melting temperature and dissipating the interfacial heating more quickly than those of the aluminium alloy. The combination of these properties produces a different response at the Al/Cu interface during the collision. In contrast to the previous case (Al/Al), the major noticeable changes are associated with the interface nature and particularly prominent with the vortex development. When the vortex instability develops, the copper forms a solid spiral due to its higher malle‐ ability and higher melting temperature than those of the flyer part. The VAZ consists of a rollup so that the copper and the aluminium are locally intermixed (**Figure 8a**). The kinematics of the vortex contributes to the interface bonding by an interlocking mechanism governed by the intermixing phenomena. Note that the vortex instability evolves at high strain rate and excessive deformation so that the plastic work heating enables to melt the aluminium during the swirling phenomenon. The molten phase is quickly solidified due to confinement of the heating and the rapid heat dissipation is facilitated by the good thermal conductivity of both copper and aluminium. These two factors promote a high cooling rate in the range of 104-6 K/s that prevents the atomic structural changes such as crystallisation during slow thermal kinetic. The hyperquenching freezes the random allocation of atoms within the initial molten phase and produces an intermetallic phase which was proven to be amorphous [37]. Hence, the dissimilar Al/Cu combination is conducive to intermetallic formation, within the VAZ and along the interface that alters the physical nature of the weld. The intermetallic phases may appear within discontinuous pockets (**Figure 8b**) or as a continuous layer with a non-uniform thickness (**Figure 8c**). In any case, the presence of the intermediate intermetallic media introduces a new weld variance that is particularly suggested for dissimilar metal joints produced in MPW.

ejection of intermetallic phases from the interface [33–35]. Finally, the interface debonding could be also resulted from a separation of molten IMCs from the aluminium solid wall while opening stresses could act during the confined melting. Such hypothesis was supported by experimental observations revealing typical features of a liquid that freely spreads prior to the solidification [33–35]. Indeed, the top surface of an identified intermetallic region exhibits a smooth appearance without fracture pattern due to debonding, and the heterogeneous nature of this surface evidences a free fluid flow [33]. These entire phenomena associated with intermetallic formation adversely affect the welding efficiency of dissimilar metal combina‐ tions. The sole alternative is to minimize the thickness of inter metallic layer using an identified lowest critical impact energy. Hence, from similar to dissimilar metal combination, the notion of bonding and weldability is phenomenologically different so that their identification should be accurately established. The next section is numerical simulation to reproduce the interfacial morphologies, which could be used to identify the potential weld variances including the

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**Figure 9.** Typical defective dissimilar magnetic pulse welds with (a) onset of cracks within the intermetallic phases, (b)

This section begins with a literature survey regarding the numerical simulation of the interface behaviour during impact welding, prior to the suggestion of a suitable method, namely Eulerian simulation to compute the collision and weld generation. This method is applied for both similar and dissimilar metal combinations (Al/Al and Al/Cu) to show the convincing predictions of typical interfacial features including the wavy morphology and formation of

The numerical simulations of the weld generation during impact welding processes can be classified into five distinct methods known as Lagrangian, Adaptive Lagrangian-Eulerian (ALE), Eulerian, smooth particle hydrodynamic (SPH) and molecular dynamics (MD). Generally, Lagrangian computation fails during the development of excessive interfacial

weldability window for similar and dissimilar assemblies.

**5. Numerical simulation of the interface behaviour**

**5.1. A brief literature review of impact welding simulations**

fragmentation, and (c) catastrophic failure [33].

defects.

**Figure 8.** Typical dissimilar magnetic pulse welds with (a) vortex development, (b) intermetallic pockets and (c) con‐ tinuous intermetallic layer at the interface [33].

#### **4.6. Fracture within intermetallic phases and detrimental welds**

The formation of a permanent bonding becomes difficult with the accumulation of intermedi‐ ate intermetallic compounds (IMCs). The fast shrinkage during the solidification stage involves cold cracking phenomenon governed by the heterogeneous heat conduction com‐ bined with the incompatibilities of the thermal expansion coefficient. For the case of thin IMC layer, light microscope observations at low magnification reveal transverse cracks across the thickness without further propagations outside the intermetallic zone (**Figure 9a**). These microcracks randomly coalesce and formarbitrary multidirectional crack propagation. The intermetallic media facilitates the cracks to fragment which lead to a catastrophic failure of the joint (**Figure 9b**). It was evidenced that the thickening of the intermetallic phase is favourable to form numerous cracks so that its occurrence can strongly impair the weld integrity. Eventually, the interface can completely break due to a propagation of a macro crack within the intermetallic zone along the interface. According to experimental analyses, this major fracture is attributable to the development of predominant shrinkage stresses during the intermetallic solidification or to a detrimental shearing stress arises from the liquid or solid ejection of intermetallic phases from the interface [33–35]. Finally, the interface debonding could be also resulted from a separation of molten IMCs from the aluminium solid wall while opening stresses could act during the confined melting. Such hypothesis was supported by experimental observations revealing typical features of a liquid that freely spreads prior to the solidification [33–35]. Indeed, the top surface of an identified intermetallic region exhibits a smooth appearance without fracture pattern due to debonding, and the heterogeneous nature of this surface evidences a free fluid flow [33]. These entire phenomena associated with intermetallic formation adversely affect the welding efficiency of dissimilar metal combina‐ tions. The sole alternative is to minimize the thickness of inter metallic layer using an identified lowest critical impact energy. Hence, from similar to dissimilar metal combination, the notion of bonding and weldability is phenomenologically different so that their identification should be accurately established. The next section is numerical simulation to reproduce the interfacial morphologies, which could be used to identify the potential weld variances including the weldability window for similar and dissimilar assemblies.

**Figure 9.** Typical defective dissimilar magnetic pulse welds with (a) onset of cracks within the intermetallic phases, (b) fragmentation, and (c) catastrophic failure [33].
