Effect of interlayer thickness and gap distance on vaporizing foil actuator welding of 5A06 aluminium alloy and 321 stainless steel

The composite structure of aluminium alloy and stainless steel provides a wide range of comprehensive advantages, encompassing properties such as lightweight, high strength, and corrosion resistance. These advantages make composite structure particularly suitable for various applications in industries such as transportation and chemicals. One innovative solid-phase welding technology that is well suited for joining dissimilar materials is vaporizing foil actuator welding. This technology allows for the welding of composite structures made of aluminium and stainless steel, despite the significant differences in physical and chemical properties. To enhance the vaporizing welding process, this paper proposes the introduction of an interlayer between the dissimilar materials. The interlayer consists of a third material that is added to bridge the gap between materials with differing hardness and plasticity. The main objective of introducing the interlayer is to minimise performance disparities and reduce the formation of intermetallic compounds at the interface. By examining the vaporizing foil actuator welding process of aluminium alloy and stainless steel with the interlayer, it aims to analyse the characteristics of the interface morphology. Additionally, this study investigates the energy conversion mechanism of the aluminium foil gasification process and explore the influence of the interlayer on the microstructure and mechanical properties of the interface between aluminium alloy and stainless-steel joints.


Introduction
The advancement and progress of industrial technologies in various sectors such as aviation, aerospace, transportation, and defence has brought about the utilisation of aluminium alloy as a replacement for steel materials [1], resulting in the achievement of structural lightweight properties [2].Therefore, there has been a gradual increase in the utilisation of welded components made from aluminium alloy and stainless steel.However, the conventional fusion welding processes employed in industries often give rise to a multitude of brittle intermetallic compounds at the welding interface.These compounds pose a significant challenge when it comes to establishing reliable joints between aluminium alloy and stainless steel due to the inherent disparities in their physical and chemical properties [3,4].Consequently, the welding of aluminium alloy and stainless steel becomes a complex task that demands careful consideration and innovative approaches to overcome the challenges [5,6].
High-velocity impact welding is a welding process that occurs in the solid-state, providing a viable solution for the joining of dissimilar materials [7][8][9][10].In recent times, the utilisation of the vaporizing Foil Actuator has showcased its versatility as a tool for various metalworking applications, with one such application being impact welding of dissimilar materials.The process of vaporizing Foil Actuator Welding (VFAW) is representative of a high-velocity impact technique [11,12].During this process, a flyer sheet is propelled towards the target sheet at impressive speeds ranging from 500 m s −1 to 800 m s −1 .These speeds are achieved through the rapid vaporization of a thin metallic conductor, which is driven by electrical pressure and occurs in a short period of approximately 10 μs [13].It is noteworthy that the VFAW process does not necessitate the use of external heat input, nor does it require the addition of filler metal or shielding gas [14].Instead, it highly relies on the occurrence of high-velocity inclined impacts between the workpieces.As a result of these impacts, the collision zone experiences elevated local temperatures, ultimately leading to partial melting and the initiation of metallurgical reactions [15].Reliable joints are created in certain regions as a result of interatomic diffusion that occurs when exposed to elevated temperatures, as explained in reference [16].The VFAW process offers a multitude of advantages, including its simplicity, high efficiency, the absence of heat-affected zones in the joints, and its ability to be easily automated.Consequently, this technique can be extensively utilised in the formation of lap joints involving nonferrous metals, such as Aluminium (Al), Magnesium (Mg), Titanium (Ti), Copper (Cu), as well as various steel materials, as stated in references [17,18].
Angshuman et al [19] reported in their study that when spot joints of aluminium alloy 6111-T4 and high strength low alloy 340 steel are welded using the vaporizing foil actuator welding process, the resulting joints exhibit high strengths as demonstrated through lap-shear and coach peel testing.Shuhai et al [17] devised a groove die with unique angles to facilitate the welding of 3003 aluminium alloy and pure titanium through the vaporizing foil actuator welding process.The joints created in tests dis-played an interfacial morphology characterised by regular wave bonding.In another investigation, Vivek [20] conducted welding experiments on the joints between a Zr-based Bulk Metallic Glass and copper on a small scale in the laboratory.Subsequently, the relationship between the interfacial structure and mechanical properties of these joints was thoroughly examined.Lastly, the vaporizing foil actuator welding technique was successfully utilised to achieve dissimilar joining of sheet aluminium AA6061-T4 to cast magnesium AM60B [21].The findings demonstrate the capability of VFAW to join dissimilar lightweight metals, specifically Al/Mg, which is expected to greatly facilitate the ongoing efforts to reduce vehicle weight.A modified version of vaporizing foil actuator welding, a solid-state impact welding technique, was employed to create similar JSC590R and 6061-T6 spot welded joints.These joints were then subjected to various tests, including lap-shear testing, microhardness measurements, optical imaging, and fatigue testing, to assess both mechanical and fatigue properties.The results were compared to those of resistance spot welding joints.The findings revealed that the VFAW joints exhibited improved strength and energy absorption compared to RSW joints for both material combinations [22].However, when the nugget size was considered, it was observed that the strength of JSC590R-JSC590R VFAW welds was lower than that of RSW welds.Impact welding is a material processing technology that allows for metallurgical bonding in the solid state through high-speed oblique collision.The effects of flier thickness and collision angle on the morphology of the weld interface were examined by applying vaporizing foil actuator welding to AA1100-O and AISI 1018 steel [23].A numerical simulation framework based on the finite element method with Eulerian formalism is developed to model the high-speed impact between metal plates.The model is thoroughly validated by comparing the wave characteristics from numerical simulations with the experimental results of evaporation foil actuator welding.Evaporation film actuator welding was used for the first time to produce high-strength welds between SiC-reinforced 8009 aluminium metal matrix composite and 7075 aluminium alloy.Mao et al investigated the high-strength impact welding of 7075 al with SiC-reinforced aluminium metal matrix composite [13].The VFAW process offers a solution for joining Al-MMC with different metals in the automotive and aerospace industries.
Preliminary investigations have shown that the VFAW process is not able to achieve a reliable weld for the composite structure made of aluminium alloy 5A06 and stainless steel 321.In this study, an optimisation approach for VFAW welding of dis-similar metals is used by introducing an interlayer.The interlayer consists of aluminium alloy 3003 and the aim is to investigate the feasibility of the VFAW process for aluminium alloy 5A06 and stainless steel 321.The study investigates the influence of energy input on the impact velocity, the interface morphology of the joints and the mechanical properties in the VFAW process for 5A06 aluminium alloy and 321 stainless steel with the interlayer, considering specific impact angles.As part of the research, the effects of the interlayer on the microstructure and mechanical properties of the welds were investigated in more detail.Both the mechanical strength and the microstructure were taken into account when evaluating the weld seams.Modern microscopy was used to analyse the characteristics of the mechanical interlock and the metallurgical joint areas.

Materials and methods
The material of the flying workpiece used for the test is aluminium alloy 5A06-O, the material of the target workpiece is stainless steel 321 and the interlayer is aluminium alloy 3003.The material specifications are listed in table 1.In order to analyse the impact velocity of the different thickness of the interlayer on the bonding performance of 5A06 aluminium alloy and 321 stainless steel, the thickness of the interlayer was set to 0.51 mm, 0.64 mm, 0.81 mm, 1.02 mm and 1.19 mm.The distance between the interlayer and the flying workpiece is 3 mm, while the distance between the interlayer and the target workpiece is 1.5 mm.The factors influencing the energy input on the impact velocity, the mechanical properties of the joint and the interface morphology are analysed.
In the VFAW process, the impact force generated by the vaporization of the aluminium foil under the effect of the impulse current is the driving force that causes the flying workpiece to hit the target plate at high speed.The efficiency of the energy conversion is the prerequisite for the maximum acceleration of the flying workpiece.According to the relevant window function of explosion welding [15], the energy input for this investigation is set at 6 kJ, 8 kJ and 10 kJ.Since the thickness of the aluminium foil is closely related to the conversion of the evaporation energy, the results show that an aluminium foil with a thickness of 0.076 mm is used for an energy input of 6 kJ, while an aluminium foil with a thickness of 0.127 mm provides the best energy conversion for 8 kJ and 10 kJ.The material of the foil is an 1100 aluminium alloy.The schematic of the foil is shown in figure 1.The utilisation of a dog bone-shaped sample in figure 1 aligns with the ASTM standard for several reasons.Primarily, the choice of this shape facilitates the vaporization actuator welding process, where aluminium foil acts as the medium for energy transfer.The dog bone shape ensures a smooth transition in cross-sectional area, which is crucial for optimal vaporization in the designated local area.Extensive prior research has demonstrated the effectiveness of this specific shape of aluminium foil in analysing the feasibility of the welding process.The dog bone shape allows for precise control over the size of the welding area by adjusting the dimensions of the central region.This flexibility is essential for accommodating varying workpiece requirements.Therefore, the selection of the dog bone shape in figure 1 is based on its proven efficacy in facilitating the vaporization actuator welding process and its alignment with ASTM standards.The cutting of aluminium foils to the dimensions depicted in figure 1 was accomplished using a water jet cutting machine.
Figure 2 illustrates the schematic view elucidating the impact angle of the joints, specifically in relation to the interlayer configuration.Equation (1), provided herein, delineates the computational framework for ascertaining this angle.
θ--impact angle L--half width of the target workpiece s--width of subplate h--thickness of flyer and target workpiece t--thickness of interlayer d--vaporizing zone width In this investigation, the foundational assumption that the length of the vaporization zone of the aluminium foil spans 15 mm is made, while the width (L) of the workpiece measures 70 mm.Notably, the impact angle (θ) crucially hinges upon the distance (h) between the workpieces.For configurations reliant on the interlayer, it is   imperative to acknowledge that the thickness of both the flyer and target workpieces should equate to the cumulative thickness of the subplate and interlayer.The subplate, comprising glass fibre resin and subject to shearing processes, boasts a width (s) of 7 mm.Additionally, the width of the vaporization zone (d) ascertained from the mechanical test section approximates 7.5 mm.Within the purview of this study, we explore a range of subplate thicknesses, specifically 1.5 mm, 2.25 mm, and 3 mm, as outlined comprehensively in table 2.
The evolution of the vaporizing foil actuator welding process stems from advancements in magnetic pulse forming technology and harnesses the capabilities of a magnetic pulse forming power supply.Specifically, the power source employed in this investigation is the MAGNEFORM-16 magnetic pulse forming power supply, boasting a maximum charging energy of 16 kJ, a voltage rating of 8.16 kV, and a capacitance of 426 μF.The relationship between energy and charging voltage can be precisely determined through the application of equation (2).
The energy generated by the vaporizing foil actuator, leveraging the potential of aluminium foil, impels the impact of the airborne workpieces onto the target workpieces.The velocity of both the flyer and the interlayer during their impact on the target was meticulously documented through the application of a Photonic Doppler Velocimetry (PDV) system.The experimental setup, inclusive of the power supply and tooling, is visually represented in figure 3. Specific tools employed to measure the speed of the flying plate and the intermediary layer within the experimental framework are elucidated in figures 3(b) and (c), respectively.
The energy generated by the vaporizing foil actuator, leveraging the potential of aluminium foil, impels the impact of the airborne workpieces onto the target workpieces.The velocity of both the flyer and the interlayer during their impact on the target was meticulously documented through the application of a Photonic Doppler Velocimetry (PDV) system.The experimental setup, inclusive of the power supply and tooling, is visually represented in figure 3. Specific tools employed to measure the speed of the flying plate and the intermediary layer within the experimental framework are elucidated in figures 3(b) and (c), respectively.
In the examination of mechanical attributes, interface morphology, and impact velocity, three distinct joint configurations are employed, as illustrated in figure 4.Among these, figure 4(a) delineates the joint configuration tailored for metallographic analysis specimens, while figure 4(b) depicts the arrangement designed for measuring the velocity of the flying plate and interlayer.Figure 4(c), on the other hand, showcases the joint configuration optimised for assessing mechanical properties.Within the Vaporizing Foil Actuator Welding (VFAW) process utilising an interlayer, a dual-probe setup is deployed to illuminate both the interlayer and the flyer workpiece separately.A sophisticated two-channel self-focusing probe Photonic Doppler Velocimetry (PDV) system is employed to precisely gauge the velocity variations of the flyer workpiece and the interlayer.To facilitate direct illumination of the flyer workpiece, a 5 mm diameter hole is drilled into the middle layer, ensuring unimpeded access for the PDV probe.The separation distance between the two probes is maintained at 7 mm.Given that the narrowest section of the aluminium foil working area measures 12.7 mm, the PDV probe effectively illuminates both the flyer workpiece and the interlayer simultaneously.
PDV technology integrates the latest fibre laser component to gauge the frequency variance between the laser's initial frequency and the Doppler frequency shift [24,25].This method harnesses a high-speed oscilloscope for signal capture, followed by data analysis software for comprehensive analysis, culminating in the determination of the object's velocity.The PDV system is equipped with a laser boasting a wavelength of 1,550 nm and utilises a 1 × 4 splitter to facilitate 4-channel data acquisition.Following entry into the probe via the circulator, the laser illuminates the swiftly moving workpiece and is subsequently reflected back to the probe, maintaining coherence with the original laser signal within the circulator.Post mixing, frequency alterations are scrutinised using a comparator.Notably, the laser wavelength employed in this system remains fixed at 1,550 nm, enabling the collection of velocities of moving objects up to 1,500 m s −1 .
Figure 5 illustrates the underlying principle of the synchronous acquisition system for current, voltage, and the PDV system.Within this system, a Rogowski coil with a ratio of 50 kA:1 V was utilised to gather the changes in pulse current of the aluminium foil during the VFAW process.A Tektronix P6015A high voltage probe, with a ratio of 1,000 V:1 V, was employed to measure the pulse voltage change across the aluminium foil.This probe has the capability to measure pulse voltages up to 40 kV (peak value, with a pulse bandwidth of 100 μs).The LeCroy 620Zi oscilloscope, equipped with 4 channels, a bandwidth of 2 GHz, and a sampling rate of 20GS/s, was used to collect the voltage, current, and PDV signals.MATLAB software was then utilised to analyse the voltage and current of the aluminium foil, as well as the changes in velocity during the VFAW process.Furthermore, MATLAB was used to analyse the impact velocity of the flyer workpiece and the interlayer overtime during the VFAW process.
Lap-shear and peel tests were employed to examine the strength of the joint.A representation of the strength testing procedure is shown in figure 6.Three samples were subjected to testing for each input energy.In the peel testing, the flyer sheets were flexed at a 90°angle with respect to the interlayer and target, and the target sheets were affixed to a steel die using a bolt.In the mechanical properties testing of VFAW (Vaporization Flux-Aided Welding) welded joints, specific standards such as ASTM or DIN are not followed due to the unique  characteristics of the welding process and joint configuration.Unlike conventional lap shear test specimens, VFAW welded joints are utilised directly for testing without machining.This approach is necessitated by the variable nature of the welding area in VFAW joints, which exhibits an elliptical distribution along the centreline with a cross-sectional area significantly smaller than the width of the workpiece.The reliable welding joints are situated within this designated welding area of the aluminium foil vaporization.The mechanical testing of these VFAW welded joints was conducted using an MTS810 mechanical testing frame operating at a consistent extension rate of 1 mm/min.This testing methodology ensures accurate and reliable assessment of the mechanical properties of the welded joints, taking into account the unique characteristics and configuration of the VFAW process.
The specimens for the purpose of observing the microstructure were obtained from the work zone in the field.To attain a smooth and reflective surface, the central cross-section of the specimens underwent the following steps, adhering to standard protocols for cutting, grinding, and polishing.Grinding was carried out using progressively finer grades of silicon carbide grinding papers, specifically at 800 grits, 1200 grits, and 2000 grits.After grinding, an initial polishing step was executed for 10 s using diamond pastes with particle sizes of 5 μm, 2.5 μm, and 1 μm, until a satisfactory outcome was achieved.
The final polishing step lasted for 2 min and was conducted using a polishing cloth immersed in a solution consisting of a 0.05 μm colloidal silica suspension.Surface roughness measurement, microstructure observation, and element analysis distribution were performed utilising a Zeiss Ultra55 Scanning Electron Microscope (SEM) with EMAX energy dispersive x-ray spectroscopy (EDS) and an Olympus LEXT OLS4000.

Results and discussion
During the process of vaporizing aluminium foil, the rise time of the pulse current is very brief, ranging from 8 μs to 12 μs.The energy primarily acts on the aluminium foil and is subsequently released as the foil expands after vaporization, exerting force on the adjacent surface.
As a result, the flyer workpiece experiences a greater acceleration.Figure 7 illustrates the change in voltage and current at both ends of a 0.076 mm thick aluminium foil over time, as well as the velocity of the flyer workpiece during the VFAW process with an energy input of 6 kJ.It can be observed from figure 7 that the pulse current reaches a peak value of 130 kA after 10 μs and then gradually weakens.The voltage on both sides of the aluminium foil jumps once the current reaches its peak value and subsequently decreases.It indicates that the aluminium foil, also referred to as the working area, has vaporized.The time from power on to the occurrence of vaporization is known as the aluminium foil's vaporizing burst time.By analysing the flight speed curve of the flyer workpiece, it can be noted that during the rising stage of the current acting on the aluminium foil, the flyer workpiece generates an eddy current due to the magnetic field resulting from the current passing through the foil.The induced eddy current, mainly present near the surface on one side of the magnetic field due to the skin effect, undergoes plastic deformation caused by the electromagnetic force in the magnetic field.Consequently, the flyer workpiece acquires a velocity of approximately 50 m s −1 .As the pulse current continues to increase, the aluminium foil vaporizes, and the flyer workpiece experiences a significant acceleration.The velocity increases from 50 m s −1 to about 250 m s −1 , then further rises to approximately 620 m s −1 after a sudden increase, and finally impacts the target workpiece.

Effect of interlayer on impact velocity in VFAW process
The selection of the interlayer in the VFAW process has an effect on the velocity of impact during the welding process of the 5A06 aluminium alloy and 321 stainless steel.
Figure 8 illustrates the changes in velocity of the flyer workpiece and the interlayer over time with an energy input of 6 kJ.At approximately 14 μs after the discharge of the equipment, the aluminium foil vaporizes under the influence of a pulse current with a peak value of 160 kA, and leads to a voltage jump and a subsequent decrease in current.The flight speed curve of the flyer workpiece reveals that 5 μs after the discharge, the flyer workpiece undergoes a minor acceleration due to the electromagnetic force.After 10 μs, the acceleration experiences an instantaneous increase as a result of the substantial impact from the vaporization energy of the aluminium foil.The flyer workpiece then collides with the interlayer at a velocity of 611 m s −1 and subsequently strikes the target workpiece at a velocity of approximately 685 m s −1 under the continuous vaporization energy of the aluminium foil.
The velocity curves of the flyer workpiece and the interlayer with energy inputs of 8 kJ and 10 kJ are displayed in figure 9.It can be observed that once the flyer workpiece collides with the interlayer, it continues to accelerate.The acceleration decreases and eventually impacts the target workpiece at a velocity of 685 m s −1 with an energy input of 6 kJ.The acceleration remains constant and the final collision velocities are 796 m s −1 and 933 m s −1 respectively with energy inputs of 8 kJ and 10 kJ.It can be noted from the flight curve of the flyer workpiece that when the energy input is 6 kJ, the energy density is lower compared to that of 8 kJ and 10 kJ.The total mass of the flyer workpiece increases after colliding with the interlayer, leading to a decrease in acceleration.With an   increase in energy input, the time required for the vaporization of the aluminium foil increases from 9.8 μs to 12.4 μs, the final collision velocity increases from 685 m s −1 to 925 m s −1 , and the instantaneous gasification current increases from 130 kA to 190 kA.The energy input increases by 80%, the instantaneous gasification current increases by approximately 47%, and the final impact velocity increases by 37%.

Effect of the gap distance of workpiece
The utilisation of the interlayer can enhance the welding adaptability of dissimilar metals in VFAW and has minimal influence on the final impact velocity when the energy input is sufficient.The achievement of reliable welding between the interlayer, target workpiece, and flyer workpiece is a crucial factor in attaining joints with exceptional performance.The impact velocity and impact angle are vital parameters in the VFAW process.
Figure 10 illustrates that with the same total gap distance, the gap distance between the flyer workpiece and interlayer is larger than the gap distance between the interlayer and the target workpiece.This is advantageous as it allows the flyer workpiece to achieve a greater impact velocity under the energy of the aluminium foil vaporization.With an increase in energy input, the energy density increases and the impact of gap distance distribution on the final impact velocity diminishes.Therefore, in order to achieve a greater final impact velocity within the permissible range of process parameters, it is necessary for the gap distance between the interlayer and the flyer workpiece to be larger than the gap distance between the interlayer and the target workpiece.This condition is favourable for obtaining a higher impact velocity.
The thickness of the interlayer between the flyer workpiece and the target workpiece determines the impact angle and final impact velocity.The investigation was carried out to examine the impact of energy inputs at 6 kJ, 8 kJ, and 10 kJ.The thickness of the middle layer is set at 1.02 mm.The gap thickness of the interlayer between the flyer workpiece and the target workpiece is presented in table 2. Figure 11 provides a list of the final collision velocities of the interlayer impacting the target plate under different gap conditions.
When the energy input is set at 6 kJ, the impact velocity shows an increase within the range of the total gap distance, which varies from 3 mm to 4.5 mm.Furthermore, the impact velocity continues to rise as the total gap distance increases, provided that the total gap distance remains between 4.5 mm and 6 mm.On the other hand, when the energy input is set at 8 kJ, the impact velocity decreases with an increase in the total gap distance; however, the amplitude of this decrease is smaller compared to the impact velocity at an energy input of 6 kJ.It is worth noting that the impact velocity gradually increases as the total gap distance increases at an energy input of 10 kJ.The growth rate of the impact velocity tends to level off when the gap reaches 5.25 mm which is caused by two main reasons.Firstly, the increase in energy input enhances the energy density generated by the vaporization of the aluminium foil due to the vaporization process.Consequently, this leads to a stronger ability to drive the flyer workpiece, resulting in continuous acceleration.Secondly, as the total distance increases, the deformation resistance of the flyer workpiece and the interlayer also increase.

Effect of interlayer thickness on impact velocity
The VFAW joint of aluminium alloy and stainless steel with an interlayer is achieved through the connection of the interlayer to the flyer workpiece and the target workpiece, respectively.The welding strength of the interlayer with both the flyer workpiece and the target workpiece, as well as the strength of the interlayer itself, are crucial factors that significantly affect the performance of the joint.In order to enhance the strength of the joint, it is possible to increase the thickness of the interlayer.To examine the impact of interlayer thickness on the impact velocity, experiments were conducted using energy inputs of 6 kJ, 8 kJ, and 10 kJ.The gap distance between the interlayer and the flyer workpiece was set at 3 mm, while the gap distance between the interlayer and the target workpiece was set at 1.5 mm.The influence of interlayer thickness on the final impact velocity was analysed for thicknesses of 0.51 mm, 0.64 mm, 0.81 mm, 1.02 mm, and 1.19 mm.
The final velocity of impact with varying interlayer thicknesses is presented in figure 11.The finding indicates that the impact velocity falls within the range of energy input from 6 kJ to 10 kJ, with the final impact velocity ranging from 587 m s −1 to 1028 m s −1 .The final impact velocity decreases as the interlayer thickness increases.This decreasing trend of the final impact velocity becomes more pronounced when the energy input is 6 kJ.The velocity decreases from 741 m s −1 to 587 m s −1 as the interlayer thickness increases from 0.51 mm to 1.19 mm.The final impact velocity decreases to a lesser extent as the interlayer thickness increases at energy inputs of 8 kJ and 10 kJ.As the interlayer thickness increases, the resistance of the undeformed region to the accelerated flight of the intermediate deformed region gradually increases causing a decrease in the final impact velocity.

Effect of interlayer thickness on mechanical properties
The tensile strength of the VFAW joint is evaluated using the method displayed on the left side of figure 6.The tensile strength of the VFAW joint is shown in figure 12 at energy inputs of 6 kJ, 8 kJ, and 10 kJ, with a gap distance of 3 mm between the interlayer and the flyer workpiece, and a gap distance of 1.5 mm between the interlayer and the target workpiece.Figure 12 illustrates that the tensile strength of the joints gradually increases with the thickness of the interlayer when the interlayer thickness is less than 1.02 mm.Tensile strength also increases with the energy input.As the interlayer thickness increases from 1.02 mm to 1.2 mm, the maximum tensile force of the joint decreases from 44 kN (at an energy input of 10 kJ) to 39.1 kN, 35 kN (at an energy input of 8 kJ) to 33.7 kN, and 29.9 kN (at an energy input of 6 kJ) to 28.2 kN.
The results of the tensile strength test reveal two types of joint failure locations: one at the intersection between the interlayer and the flyer workpiece, and the other at a shear fracture of the interlayer along the junction of the target workpiece.The morphology of the failure section of the joint exhibits a noticeable unwelded area at the intersection between the interlayer and the target workpiece.The failure region is mainly located on the flyer workpiece and the interlayer, as depicted in figure 13, while the unwelded area in the center is illustrated in figure 14.
The failure region of the tensile strength test of the joint is situated at the junction where the flyer workpiece and the interlayer meet, while the thickness of the middle layer exceeds 0.841 mm.A small portion of the interlayer exhibits shear fracture at an energy input of 6 kJ, as shown on the left side of figure 10.It reveals that the low energy input and the insufficient impact velocity of the flyer workpiece and the target workpiece after colliding with the interlayer.As a result, the bonding strength between the flyer workpiece and the interlayer is lower than the bonding strength between the interlayer and the target workpiece.For interlayer thicknesses of 0.51 mm and 0.64 mm, the fracture area of the joint in the tensile strength test primarily follows the shear fracture of the bonding area between the interlayer and the target workpiece.The diameter of the fracture zone gradually increases as the energy input increases to 8 kJ and 10 kJ.The maximum force of tensile strength in the joint occurs when the energy input is 10 kJ and the interlayer thickness is 1.02 mm, resulting in a maximum tensile strength of 44 kN.
The lap-shear strength of the VFAW joint is evaluated using the method shown on the right side of figure 6.At energy inputs of 6 kJ, 8 kJ, and 10 kJ, with an interlayer thickness of 1.02 mm and a gap distance of 3 mm between the interlayer and the flyer workpiece, as well as a gap distance of 1.5 mm between the interlayer and the target workpiece, the tensile strength of the VFAW joint is illustrated in figure 15.
The failure section of the lap-shear strength, stroke of the joint, and the interface indicate that as the energy input increases, the maximum lap-shear strength of the joint gradually increases.At an input energy of 10 kJ, the joint demonstrates the highest shear resistance, reaching a maximum shear resistance of approximately 2.1 kN and a stroke of about 3 mm.For energy inputs of 6 kJ and 8 kJ, the maximum shear resistance is around 1.6 kN, both occurring within the stroke, and failure occurs at approximately 4 mm.The results of the tensile and lap-shear strength tests reveal that as the energy input rises, the tensile properties of the joints experience a significant increase.There is minimal disparity in shear resistance when the energy input is 6 kJ and 8 kJ.A notable increase as well as two abrupt drops in shear resistance occur as the energy input reaches 10 kJ.The reason for the occurrence is that the area of the joint's connection region expands with the augmentation of energy input, and concurrently, the rise in impact velocity causes the interlayer to partially overlap with the welding area of the flyer workpiece and the target workpiece.
In the central region of the joint, there exists an unwelded region with a diameter of approximately 10 mm at the interface between the interlayer corresponding to the area of aluminium foil vaporization and the target workpiece.Due to the collision angle being 0 in this region, the energy of the aluminium foil vaporization has an impact.As the energy increases, the area of instantaneous vaporization of the aluminium foil expands gradually from the centre to both sides, causing the region to change from a circular shape to an elliptical shape, thereby making it impossible to form a reliable weld.With the increase in energy input, the pulse current at both ends of the aluminium foil also gradually increases, but it does not continue to increase once the long axis of the unwelded elliptical region in the middle reaches 15 mm.By analysing the impact velocity, mechanical properties of the joint, and the appearance of tensile failure in the joint, it can be observed that the bonding interface strength between the aluminium alloy and stainless steel VFAW joint with an interlayer is uneven.The bonding region between the interlayer and the flyer workpiece is closer to the centre side, while the bonding region between the interlayer and the target workpiece is larger.The bonding strength in the central region is weaker than that in the outer region, resulting in an interlaced distribution of bonding strength in the interface region between the intermediate layer and the flyer and target workpieces.Through a comparison of the diameter of the welding region, it is found that the tensile strength of the joint is similar to the tensile strength of the 3003 aluminium alloy in the middle layer.
3.5.Interface morphology of joints with the best mechanical properties 3.5.1.Interfacial morphology of 5A06 and 3003 At energy inputs of 10 kJ, with an interlayer thickness of 1.02 mm and a gap distance of 3 mm between the interlayer and the flyer workpiece, as well as a gap distance of 1.5 mm between the interlayer and the target workpiece, the interface morphology of 5A06 and 3003 is depicted in figure 16.
The leftmost portion of the figure 16 illustrates the central region of the joint.Figure 16 reveals that it is evident that the central area of the interface between the 3003 aluminium alloy and the 5A06 aluminium alloy forms a straight bond, while the outer edges exhibit regular undulating bonds.The morphology of the bonding area mainly consists of undulating bonds, with a length of approximately 6 mm and a distance of about 7 mm from the central area.There are significant variations in the height of the wave crest and the distance between the two waves.The height of the wave crest and the distance between the two waves gradually increase from the centre to both sides.
Taking into consideration the results of the mechanical property tests, fracture morphology, and the interface morphology between 5A06 and 3003, the width and spacing of the wave peaks in the corrugated bonding area at the interface are critical factors that determine the bonding strength between the flyer workpiece and the interlayer in the VFAW process with an interlayer.

Interfacial morphology of 3003 and 321 stainless steel
In the joint VFAW, which is formed by combining the 5A06 aluminium alloy and the 321 stainless steel with an interlayer made of 3003 aluminium alloy, it has been observed that the failure areas occur at the bonding interface between the 3003 aluminium alloy and the 321 stainless steel when the tensile force exceeds 25 kN.In the case of dissimilar metal materials that exhibit significant differences in performance, the bonding area primarily consists of intermetallic compounds during high-speed impacts at the interface.The nature and distribution of the products at the interface between the 3003 aluminium alloy and the 321 stainless steel play a critical role in determining the performance of the VFAW joint, which involves the 5A06 aluminium alloy and the 321 stainless steel with an interlayer.The performance of intermetallic compounds at the interface of dissimilar metal joints directly influences the overall interface performance.
By examining figure 17, it can be observed that the interface morphology of the 3003 aluminium alloy and the 321 stainless steel, with an energy input of 10 kJ, a 3 mm gap between the interlayer and the flyer workpieces, and a 1.5 mm gap between the interlayer and the target workpiece, can be illustrated from the centre to the outside.The result reveals that the interface bonding area between the 3003 aluminium alloy and the 321 stainless steel mainly consists of three parts: the unwelded area, the intermetallic compound bonding area, and the mechanical-clinch area.However, due to the shape of the aluminium foil, a reliable bond cannot be formed in the central region of the interface because the impact angle between the interlayer and the flyer workpiece is zero.The impact then expands from the central area to the outside.Under the given collision angle, and influenced by the heat generated during the collision, an intermetallic compound bonding area with a length of approximately 2 mm and an uneven thickness is formed at the interface.The thickest part of this area measures around 28 μm, and the interface exhibits irregular waviness.
Upon examination of the failure area in the samples following lap-shear and tensile strength tests, it is evident that the bonding area at the interface between 3003 aluminium alloy and 321 stainless steel is significantly larger compared to the interface between 3003 aluminium alloy and 5A06 aluminium alloy.The outer strength of the bonding area at the interface between 3003 aluminium alloy and 321 stainless steel is higher  than that of the centre side.All joint failure areas are located in the central region of 3003 aluminium alloy and 321 stainless steel.In the VFAW process with an interlayer, the impact angle increases as the distance from the centre increases.The impact angle near the central side of the corrugated bonding area is smaller than that on the outside, resulting in a weaker connection effect in this area compared to the outer side.The impact angle in the middle area is zero, leading to a poor connection effect in this region.Driven by the vaporization energy of the aluminium foil, the flyer workpiece 5A06 aluminium alloy first impacts with the interlayer, and then both impact with the target workpiece.After the welding of the interlayer 3003 aluminium alloy and the target workpiece 321 stainless steel is completed, the interlayer 3003 aluminium alloy and flyer workpiece 5A06 aluminium alloy have a certain relative impact speed, thereby enlarging the welding area of 3003 aluminium alloy and 321 stainless steel compared to that of 3003 aluminium alloy and 5A06 aluminium alloy.An SEM with EDS was used for quantitative analysis of the composition and element diffusion at the interface.Characteristic points were selected at the interface for element content analysis.The element contents of points 1-3 in figure 17 are presented in table 3, and the results of the EDS line 1 scan are illustrated in figure 18.
According to the Al-Fe binary phase diagram, a series of intermetallic compounds such as AlFe, Al 2 Fe, Al 3 Fe and AlFe 3 can be generated by the solid-liquid reaction of Al-Fe.Kahn [26] and Meng [27] results shows that there are a lot of AlFe(Cr,Ni,Ti) IMCs in the interface of 6061 aluminium alloy and 316 stainless steel joints by VFAW process.The results of element content distribution show that the chemical composition of the intermetallic compound at the interface remains relatively constant across different areas.Based on the ratio of aluminium elements to iron elements, it can be estimated that there are AlFe-dominated intermetallic compounds at the interface of well-welded joints.The presence of intermetallic compounds indicates that the metals on both sides of the interface undergo melting and are accompanied by metallurgical reactions.Analysing the element line scan results of the interface, it can be observed that for the mechanical-clinch zone of the interface, the content curves of aluminium and iron elements at the 5A06 and 321 interfaces do not show sharp vertical changes but have a slope.This phenomenon indicates that element diffusion occurs at the interface.
In the region of the welding interface between 3003 aluminium alloy and 321 stainless steel, situated centrally, the velocity of impact between the interlayer and the target workpiece is higher compared to the outer region.The primary bonding mechanism between the two materials involves the formation of intermetallic compounds.Near the joint's outer area, where the interface between 3003 aluminium alloy and 321 stainless steel is located, the two metals combine closely, resulting in element diffusion and a significant increase in strength compared to the central area.Based on the results of tensile and lap-shear strength tests, it can be  concluded that the morphology of the interface and the resulting products in the welding area between 3003 aluminium alloy and 321 stainless steel play a crucial role in the overall mechanical performance of the joint.

Conclusions
The present study shows the utilisation of vaporizing foil actuator welding for joining 5A06 aluminium alloy and 321 stainless steel, with a 3003 aluminium alloy interlayer.The resulting joints exhibit favourable mechanical properties.
(1) During the VFAW process, when using the 5A06 aluminium alloy and 321 stain-less steel interlayer, a higher energy input leads to a reduction in the vaporizing burst time of the aluminium foil.The result in an increased velocity of impact between the flyer workpiece, interlayer, and the target workpiece.As a result, both the interlayer and the welding area between the flyer workpiece and the target workpiece undergo expansion.
(2) The tensile strength of the VFAW joint, which is dependent on the interlayer, is mainly influenced by the distribution and bonding surface area of the intermediate layer with the flyer and target workpieces.On the other hand, the lap-shear strength is deter-mined by the properties of intermetallic compounds formed in the welding area between the interlayer and the target workpiece.The VFAW joints of 5A06 aluminium alloy and 321 stainless steel, with 3003 aluminium alloy as the interlayer, exhibit commendable tensile and lap-shear strength.Failure occurs at the interface between 3003 aluminium alloy and 321 stainless steel.The maximum tensile strength is 44 kN, and the maximum lap-shear strength is 2.1 kN at an energy input of 10 kJ.
(3) The examination of the interfacial morphology is carried out using optical microscopy (OM) and scanning electron microscopy (SEM).Additionally, energy dispersive x-ray spectroscopy (EDS) is employed to analyse the intermetallic compounds present at the interface.The results reveal the presence of a wavy interface area between the interlay-er and the flyer workpiece.Notably, the height of the wave crests and the distance between the waves progressively increase from the central area towards the outer region.The bonding between the interlayer and the target workpiece is achieved through a combination of element diffusion and the formation of intermetallic compounds.
Furthermore, to validate the formation of the IMCs identified through the EDS analysis and to gain deeper insights into their composition and structure, future research could involve the XRD analysis.This additional characterization technique would provide a more thorough understanding of the weld interface and contribute to elucidating the role of the IMCs in determining the mechanical properties of the weld joints.Moreover, investigating the effects of different welding parameters and alloy compositions on the IMC formation and subsequent mechanical behaviour could offer valuable insights for optimizing the welding process and improving the overall performance of welded joints.

Figure 5 .
Figure 5.The schematic and equipment of voltage current detection and impact velocity acquisition system.

Figure 7 .
Figure 7.The current, voltage and flyer velocity of 0.072 mm foil with energy input 6 kJ.

Figure 8 .
Figure 8. Velocity of flyer and interlayer with 6 kJ energy input.

Figure 9 .
Figure 9. Velocity of flyer and interlayer with 8 kJ and 10 kJ energy input.

Figure 10 .
Figure 10.Velocity of flyer and interlayer with different total gap thickness.

Figure 11 .
Figure 11.Final impact velocity with different interlayer thickness.

Figure 12 .
Figure 12.Tensile strength of VFAW joints with different interlayer thickness.

Figure 13 .
Figure 13.Fracture appearance of VFAW joints after tensile strength test.

Figure 15 .
Figure 15.Lap-shear test with different energy input.

Table 3 .
Element content of the interface of joints.