Study on interfacial morphologies of AA1060 and SS321 magnetic pulse welded joints

In this study, the application of magnetic pulse welding was employed for the production of tubular joints using 1060 aluminium and 321 stainless steel. Two crucial process parameters, the charging voltage, and the gap between the outer and inner tubes, were subjected to mechanical property tests and morphology analysis. The joints that exhibited favourable mechanical properties were obtained with a gap size of 1.25 mm and a charging voltage exceeding 3.5 kV. The presence of intermetallic compounds at the joint interface indicates the occurrence of fusion in the transition zone. The quantity of molten metal increased with an increase in the charging voltage. The aluminium content in the intermetallic compounds within the transition zone decreased as one moved from the 1060 aluminium side towards the 321 stainless steel side. The gap distance plays an important role in determining the efficiency of energy transformation at the joint interface during magnetic pulse welding (MPW). A smaller gap distance resulted in a substantial amount of energy being transformed into plastic deformation in the transition zone. Conversely, a greater gap distance led to a significant amount of energy from the electrical charging voltage being converted into thermal energy. The MPW joint consisted of bonding, mechanical interlock, and non-bonding regions. The mechanical properties of the joint were influenced by the composition of the intermetallic compound. Taking into account the impact of morphologies and elastic modulus of the transition zone, an increase in the melting of stainless steel during the welding process was found to be advantageous for the MPW of 1060 aluminium to 321 stainless steel.


Introduction
Aluminium has extensive and significant application in the tubular joints of high-speed trains and the aerospace industry, in contrast to stainless steel.The technique of multi-material joining plays a crucial role in the development of lightweight structures [1,2].Fusion-welding is unsuitable for welding aluminium and stainless steel together due to their distinct physical characteristics.The process often leads to the formation of brittle intermetallic compounds.A potential solution for joining dissimilar materials is magnetic pulse welding (MPW).This cold solid-state welding method employs electromagnetic forces to unite metal workpieces.MPW has been successfully used to weld various combinations of materials, including aluminium with copper, aluminium with magnesium, and aluminium with steel.
In figure 1, the schematic representation of the magnetic pulse welding system [2] is displayed.The capacitor bank is charged, leading to the initiation of a low inductance switch, and subsequently, a damped sinusoidal current is induced in the solenoid coil.Because of Faraday's law of electromagnetic induction, a secondary eddy current is generated in the flyer (outer workpiece) due to the alteration of magnetic flux.The repulsive Lorentz forces between the two conductors carrying current result in the motion of the flyer, leading to a high-speed impact (ranging from 200 m s −1 to 500 m s −1 ) with the target (inner workpiece).By satisfying the appropriate critical conditions of impact angle and velocity, a solid-state cold weld can potentially be formed [2].
The methodology of MPW welding exhibits similarities to that of explosive welding.MPW yields a metallurgical bond without any zones affected by heat, thus facilitating the superior preservation of the original heat treatment properties of joints [3,4].This technique boasts numerous advantages.Firstly, it obviates the need for direct contact between the tool and workpieces as well as the absence of a protective atmosphere, filler materials, or other auxiliary substances.Secondly, it eradicates the necessity for periodic adjustments, which is customary in conventional machining, and it avoids the creation of heat affected zones.Lastly, the duration of the MPW process does not exceed 100 μs; the only limitation is the loading and unloading process with the capacitor charge time [3][4][5][6].
MPW has been effectively utilised on several combinations of metals [7,8].Numerous investigations have been conducted on the phenomena occurring at the interface and the mechanisms of bonding in magnetic pulse welding.Raoelision et al [9][10][11] uncovered that in the magnetic pulse welded joint of Al Cu -1 , the formation of an interfacial intermetallic phase took place, whereas the bonding in the Al Al -1 joint was achieved through metal continuity at the interface.The intermetallic phases were found to be amorphous, nanoporous, and damaged by cracks in multiple directions.It has also been observed that significant plastic deformation occurred in both workpieces, and the distribution of intermetallic compound composition in both Al Cu -1 and Al Mg -1 joints was determined by the input energy [12][13][14].The composition of the intermetallic compound was determined by the amount of energy applied.Chen [13,14] discovered the existence of wave formation at the interface of Al Mg -1 MPW joints.The interfacial wave on the Mg layer was greater in comparison to the interfacial wave on the Al layer, and a fusion zone was observed at the interface [15][16][17].As the discharge voltage increased from 4 kV to 5 kV, the interfacial waves became more regular and significantly smoother.A gradual refinement of grain size adjacent to the weld interface has also been observed, which can be attributed to dynamic recrystallisation.
The Al Mg -1 and Al Cu -1 joints were deemed highly suitable for MPW due to their low-discrepancy (physical and chemical properties) and excellent inter-solubility [18].However, determining the Al Fe -1 MPW process parameters poses a significant challenge due to the formation of hard and brittle intermetallic compounds at the interface.In a study conducted by Lin et al [15], the influence of the critical thickness of the inner tube on the performance of 3A21 aluminium alloy and 1020 carbon steel MPW joints was investigated [19].The results indicated a linear relationship between the critical thickness of the inner tube and the discharge voltage, with an approximate increase of 0.5 mm for every 2 kV increase in discharge voltage.Furthermore, Yu [16] observed that at an 8 kV discharge voltage, the 3003 aluminium alloy and 20 carbon steel can produce metallurgical bonded joints, with mutual diffusion of Fe and Al elements occurring across the interface and within the transition zone [10,20].
The objective of this study was to examine the impact of the charging voltage, gap between workpieces, and interfacial morphology on the MPW joints of 1060 aluminium and 321 stainless steel.

Materials and methods
MPW connections were carried out on samples of 1060 aluminium and 321 stainless steel alloys using a Pulsar 20-9 machine.The experimental conditions can be found in table 1.The configurations of the inductive coil and field shaper are illustrated in figure 2. The inductive coil consists of 4.5 turns, while the field shaper is responsible for focusing the magnetic field, has a work zone with a length of 10 mm and a radial gap of 1.5 mm.The energy input and the gap between the workpieces are important parameters in impact welding.These parameters determine whether bonding and IMP will occur.To assess the influence on mechanical properties and interface morphology, three different gaps of 1 mm, 1.25 mm, and 1.5 mm were set for the workpieces.The dimensions of the specimens are shown in figure 3. The outer tube is made of annealed 1060 aluminium and has a diameter of 16 mm, a thickness of 1 mm, and a length of 95 mm.The inner tube is made of 321 stainless steel and has an outer diameter (d) of either 12 mm, 11.5 mm, or 11 mm, depending on the desired gap.The length of the inner or outer tube is 65 mm, while the thickness is 3 mm.The selected rods and their sizing were determined based on their intended use as a sealing joint within a laser apparatus, showcasing promising applications within the realm of laser technology.Prior to MPW, the contact surface of the specimen needs to be polished with 400-grit sandpaper, followed by cleaning with acetone in an ultrasonic cleaning machine for 10 seconds.A total of eight aluminium-stainless steel welds were produced in discharge voltages ranging from 3.5 kV to 7 kV with an incremental increase of 0.5 kV.
The MTS C45-105 machine was employed to determine the tensile strength.The specimens for observing the microstructure were collected from the work zone in the field.To achieve a polished and mirror-like finish,   the central cross-section of the specimens underwent the following steps.Standard procedures were followed to prepare the specimens for cutting, grinding, and polishing.Grinding was carried out using progressively finer grades of silicon carbide grinding papers, specifically at 800 grits, 1200 grit, and 2000 grit.After grinding, an initial polishing step was performed for 10 s using diamond pastes with particle sizes of 5 μm, 2.5 μm, and 1 μm, until a satisfactory result was obtained.The final polishing step lasted for 2 min and was conducted using a polishing cloth soaked in a solution of a 0.05 μm colloidal silica suspension [15,16].Surface roughness measurement, microstructure observation, and element analysis distribution were tested using a Hitachi SU-4300 Scanning Electron Microscope with EMAX EDS and an Olympus LEXT OLS4000.The elastic modulus of the interfaces was determined using the Agilent Nano indentation apparatus G200.

Charging voltage influence
To investigate the impact of the charging voltage on the quality of MPW joints made from 1060 aluminium and 321 stainless steel, a total of eight specimens were fabricated with a gap size of 1.25 mm.The charging voltage was steadily varied between 3.5 kV and 7 kV with an incremental increase of 0.5 kV.The visual appearance of the joints exhibited a gradual enhancement as the charging voltage was raised within the range of 3.5 kV to 6 kV, as illustrated in figure 4. Specifically, the outer tube displayed wrinkles at charging voltages of 3.5 kV and 4 kV.However, a satisfactory surface appearance was achieved when the charging voltage was set between 4.5 kV and 6 kV.It is worth to note that certain small wrinkles and cracks emerged on the surface as the charging voltage was further increased.These cracks became more pronounced at the end of the outer tube when a charging voltage of 6.5 kV was applied with a gap of 1.25 mm.The primary factor contributing to the damage of the outer tube was identified as the high strain rate during the magnetic pulse welding process.At a tensile force of approximately 5 kN, the joints that had a charging voltage exceeding 3.5 kV experienced fracture, specifically on the base made of 1060 aluminium.This occurrence demonstrates that the welded area undergoes work hardening and stress concentration during the magnetic pulse welding process.In figure 5, the fracture of the outer tube was found in various regions.As the charging voltage increased, the fracture location became further from the joint.It was primarily due to the gradual increase in the deformation area of the joints as the charging voltage increased.Adjacent to the field shaper work zones, work hardening effects can be seen.When the charging voltage was below 4.5 kV, the fracture regions appeared at a distance of 20 mm from the end  of the outer tube.However, as the charging voltage increased to 5 kV and 6.5 kV, the distance of the fracture regions increased to 22 mm and 27 mm, respectively.
The morphology analysis of the longitudinal sections of MPW joints presented that 1060 aluminium and 321 stainless steel joints had an apparent transition zone at the interfaces and the morphology of transition zones differed as the charging voltages increased form 3.5 kV to 7 kV.Three regions of the MPW joint interfaces were severed for analysis.The joint interfaces constituted of transition (bonding), mechanical-clinch and nonbonding zones from the middle to the boundaries of both sides, as shown in figure 6.In figure 7 the morphology of non-bonding zones at charging voltages between 4 kV and 6 kV are presented.
The morphology analysis of the longitudinal sections of MPW joints revealed that there was a distinct region of change at the interfaces between the 1060 aluminium and 321 stainless steel joints.Furthermore, the morphology of the transition zones varied depending on the charging voltages, which ranged from 3.5 kV to 7 kV.In this analysis, three specific regions of the MPW joint interfaces were used.These interfaces were composed of transition (bonding), mechanical-clinch, and non-bonding zones, which extended from the middle to the boundaries of both sides, as shown in figure 6. Figure 7 displays the morphology of the nonbonding zones when the charging voltages ranged from 4 kV to 6 kV.These regions were primarily composed of pure metal particles and intermetallic compounds with varying elemental compositions.Table 2 presents the elemental content of (1-8) points within this region at different charging voltages.The size of the particles increased with the increase in charging voltage.These particles were formed at the bonding zones and accumulated in this area due to the impact of the outer tube jet on the inner tube.The particle sizes were significantly small, and the main constituent was aluminium at 4 kV and 4.5 kV.It can be explained by the fact that the charging voltage primarily converted into kinetic energy at low charging voltages.At the charging voltages of 5 kV and 5.5 kV, a larger number of particles aggregated in this region resulting in a doubling of the particle size.The particles were compressed together at 6 kV.This because a significant amount of energy being converted into thermal energy at high charging voltages, causing the aluminium and stainless-steel interfaces on both sides to fuse.
The phenomenon of non-bonding zones was found in the element content of points 1-8.Table 3 indicates that aluminium was the dominant element component at both 4 kV and 4.5 kV.The content of Fe exhibited a significant increase when the charging voltages surpassed 4.5 kV.This exemplified that the melting of the stainless-steel side was more pronounced at higher charging voltages compared to the application of lower charging voltages.
A region characterised by mechanical clinching can be seen between the transition and non-bonding regions, and its physical appearance is illustrated in figure 8.The length of the mechanical clinching process remained consistently at approximately 260 μm when a charging voltage of 4 kV was used but decreased in magnitude as the charging voltage increased.Conversely, the presence of intermetallic compounds in this specific area was negligible when a charging voltage of 5.5 kV or higher was employed.Moreover, the intermetallic compounds within this particular region were securely bonded to both the aluminium and stainless-steel components, exhibiting no evidence of any imperfections such as cracks or shrinkage cavities.While the temperature in this region did not surpass the fusion point of stainless-steel, it is reasonable to assert that the elevated temperature of the aluminium component facilitated the diffusion of stainless steel, culminating in the formation of intermetallic compounds.
The morphologies of the bonding zone are shown in figure 9.At a charging voltage of 4 kV, the thickness of the intermetallic compounds was approximately 16 μm.The thickness increased to 52 μm at 4.5 kV, 48 μm at 5 kV, 42 μm at 5.5 kV, and 9 μm at 6 kV.It can be noted that higher intermetallic compound thicknesses were more susceptible to the formation of defects, such as cracks.It reveals that the rapid solidification at the transition zones and the inherent brittleness of the intermetallic compounds.In the case of charging voltages of 4 kV and 4.5 kV, the irregular ripples observed in this area were a result of the plastic deformation of stainless steel under the influence of an impact force.However, as the charging voltages increased to 5 kV and 5.5 kV, the deformation of the transition zones became more uniform.Interestingly, regular ripples reappeared in this area when the charging voltage reached 6 kV.The increase in intermetallic thickness to 52 μm at 5 kV leads to the increase in energy input.Conversely, the intermetallic thickness decreased to 9 μm when the energy input was increased to 6 kV, because of a high impact velocity at the interface.The primary morphological anomalies found in the region of adhesion were shrinkage cavities, as shown in figure 10.The characteristics of the MPW joint are governed by the composition of the interface.Commencing from the stainless-steel side, 25 locations were selected at intervals of 20 μm along lines 1 and 2 for the purpose of assessing elastic modulus.
The elastic modulus measurements of both lines 1 and 2 in figure 10 are detailed in figure 11.At a charging voltage of 4.5 kV, the maximum elastic modulus was observed to be 4.76 GPa, while at 5.5 kV, it was found to be  3.02 GPa for stainless steel.This increase in elastic modulus near the interfaces can be explained by the fact that the work-hardening effect caused by the impact of the outer tube.Moreover, it was observed that the elastic modulus of stainless steel was lower at a charging voltage of 5.5 kV compared to 4.5 kV, which can be attributed to the higher stainless-steel fusion at 5.5 kV.As the charging voltage increased, the elastic modulus at the interface decreased towards aluminium, along with a decrease in the elastic modulus of the intermetallic compounds.The highest temperature and Fe content compared to other regions can be seen in the bonding zones.Furthermore, the temperature at the interfaces increased as the charging voltages increased, leading to rapid interfacial heating and solidification.The intermetallic compounds present in the Al-Fe system consist of stable phases (FeAl 3 , Fe 2 Al 5 , FeAl, Fe 3 Al) and metastable phases (FeAl 2 , Fe 2 Al 9 , FeAl 6 ) [17,18].It was observed that the Al-rich phases exhibited high elastic modulus but low tensile strength, while the Fe-rich phases displayed the opposite behaviour.The formation of Fe 2 Al 9 and FeAl 6 phases was found to be more favourable at the Al-rich interfaces.It is advantageous to have a comprehensive understanding of the interface morphology and its influence on elastic modulus to better control the stainless-steel fusion in the 1060 aluminium and 321 stainless steel MPW process.
In figure 12, it presents the line scan of the element content that was performed perpendicular to the bonding zones at charging voltages of 4.5 kV and 5.5 kV.The analysis of the morphology in this specific area and the  results obtained from the element content line scan revealed that the inter-metallic compounds present in the transition zone exhibited heterogeneity.The rapid cooling and solidification process resulted in an insufficient metallurgical reaction of the inter-metallic compounds during the MPW.The low iron (Fe) content within these compounds caused the adjacent stainless-steel side to display continuous cracks at the charging voltage of 4.5 kV.On the other hand, the aluminium content gradually decreased from the aluminium side to the stainlesssteel side and was effectively separated by the charging voltage of 5.5 kV.It is worth noting that the distribution of the inter-metallic compounds in the transition zones proved to be advantageous for the MPW joints.

Gap influence
For a tube MPW joint, the thickness of flyer is assumed to be constant to analyse the flyer deformation rate.The deformation rate σ can be expressed as follows: Where D out is the outer diameter of flyer, D in is the inner diameter of flyer, d in is the inner diameter of flyer after MPW.As the distances between gaps increased, there was a gradual increase in the rates of deformation during the MPW process.Specifically, the rates of deformation were found to be 13.3%, 16.6%, and 20% for gaps measuring 1 mm, 1.25 mm, and 1.5 mm, respectively.The regions that underwent deformation has not shown any wrinkles when the energy inputs were suitable.However, when significantly higher energy inputs were used, non-bonding and cracks can be seen.Figure 13 illustrates the joint surfaces with gaps measuring 1.5 mm, 1.25 mm, and 1 mm at a voltage of 7 kV.As the gaps increased, the tendency for the joint surfaces to be damaged also increased.At a gap of 1.5 mm, the joint surfaces appeared wrinkled, while at the end of the outer tube with an initial gap of 1.25 mm, cracks were observed.When the gaps were reduced to 1 mm, the tube end exhibited a flaky type of damage.The conversion of low input energy into kinetic energy and deformation.As a result, more energy acted on the surface of the outer tube, making the joints of the outer tube surfaces with smaller gaps more susceptible to flaky types of damage.
Figures 14 and 15 show the morphologies of bonding and non-bonding zones with gaps measuring 1 mm and 1.5 mm at a potential difference of 5.5 kV, respectively.In contrast, figures 7 and 9 show the morphologies of bonding and non-bonding zones created with a gap of 1.25 mm in a previous study.It was found that the size of particles in this area was greater than 1 mm when a gap of 1.25 mm and 1.5 mm was utilised.The intermetallic compounds observed in the bonding zone were found to be heterogeneous and discontinuous, with gaps measuring 1 mm.The thickness of these intermetallic compounds was determined to be 23 μm with gaps measuring 1.5 mm and 62 μm with gaps measuring 1.25 mm.The selection of points 1-3 (with a gap of 1.5 mm, in figures 3(a), 4 and 5 (with a gap of 1 mm, in figures 3(b), 7 and 8 (with a gap of 1.25 mm, in figure 7) for elemental content analysis is listed in table 3. The analysis of the elemental content revealed the presence of stainless steel and various intermetallic compounds at the interface with different gap distances at a charging voltage of 5.5 kV.The temperature at the interfaces was identified as the key factor influencing this phenomenon.The heat generated at the interfaces was a result of the conversion of energy from the outer tube.The impact speeds were responsible for determining the content and quantity of intermetallic compounds.The acceleration of the outer tube's impact on the inner tube gradually increased until the quarter of the discharge cycle.The gap distance, in turn, determined the impact speeds and energy conversion.The high impact speeds significantly increased the fusion of aluminium and stainless steel in bonding zones, but also led to an increase in pressure in this area.As a result, more intermetallic compounds and metal particles were expelled to both sides.Additionally, a 1 mm gap leading to a low impact velocity higher-pressure effect on the transition zone and intermetallic compounds were dispersed in this region.When comparing the elemental content of transition

Conclusions
This study examined the impact of charging voltages and gaps on MPW of 1060 aluminium outer tubes and 321 stainless steel inner tubes.The findings are subsequently presented in the following: (1) The 1060 aluminium and 321 stainless steel MPW joints shows satisfactory mechanical properties and a visually pleasing appearance when subjected to charging voltages within the ranges of 3.5 kV and 6 kV, with gap distances of 1 mm, 1.25 mm, and 1.5 mm.It was observed that lower charging voltages, specifically those with a gap distance of 1.5 mm, were susceptible to the occurrence of wrinkling, while higher charging voltages, specifically those with a gap distance of 1 mm, resulted in the formation of flaky damages.
(2) Fusion occurred at the interfaces of the joints.The fusion of the 321 stainless steel side increased proportionally to the charging voltages, and the intermetallic compounds in the transition zones exhibited an augmentation in Fe, which was advantageous for the 1060 aluminium and stainless steel MPW joints.The distribution of the aluminium element decreased at the transition zones, from the 1060 aluminium to the 321 stainless steel, resulting in the optimal combination of intermetallic compounds.
A comprehensive examination of the morphology and properties of intermetallic compounds was carried out, while the most favourable experimental parameters consisted of varying charging voltages ranging from 4.5kV to 5.5kV, with gaps of 1.25 mm, for the 1060 aluminium and 321 stainless steel tube MPW of diameter 16 mm on the Pulsar 20-9 device.

Figure 4 .
Figure 4.The surface appearance of joints.

Figure 5 .
Figure 5. Tensile test breaks at various charging voltages.

Figure 6 .
Figure 6.Three region of cross-section.

Figure 14 .
Figure 14.The morphologies of non-bonding area at 5.5 kV.

Figure 16 .
Figure 16.Transition zone content distribution with a 1.5 mm gap.