Microstructures and properties of 7075 aluminum alloy and CP780 steel resistance spot welded joint assisted by magnetic field

Joining steel and aluminum is vital for lightweight automobile but still challenging due to their different physical properties. Herein, resistance spot welding tests were performed on CP780 high-strength steel (thickness 1 mm) and 7075 aluminum alloy (thickness 1.5 mm) dissimilar metals under steady-state magnetic field. The influences of magnetic field (B = 40 mT) on the structure of welded joints, the phase composition/content of intermetallic compounds, and tensile properties of welded joints were analyzed under different welding current conditions (I = 9 kA,10 kA, 11 kA, and 12 kA). At the same welding current, the Lorentz force generated by the additional magnetic field promoted the outward circumferential movement of the molten metal in the weld along the horizontal surface , as well as increased the diameter of the Fe/Al contact interface in the weld nugget along the horizontal direction, conducive to the effective utilization of heat of the resistance spot welding. Except under (11 kA-0 mT) and (11 kA-40 mT), welded joints under other welding parameters displayed a few welding defects, such as incomplete fusion and shrinkage cavity formed at the cross-section of the welded joints. Therefore, the synergism between the magnetic field and appropriate welding current held important roles in the formation of welded joints without obvious welding defects. The intermetallic compounds of all the welds were mainly composed of (Fe, Si)Al2 and (Fe, Si)Al5. Meanwhile, the thickness and content of the intermetallic compounds layer reduced under a magnetic field at the same welding current, significantly improving the tensile properties of the welded joints. The comprehensive properties of welded joints were the best under 11 kA-40 mT, with an average shear force increase from 3.02 kN to 3.49 kN (15.56%) and an average displacement increase from 1.01 mm to 1.22 mm (20.79%). Overall, the proposed dissimilar aluminum/steel resistance spot welded joint assisted by magnetic field looks promising for lightweight automobile use.


Introduction
The lightweight automobile is an effective way to deal with global energy and environmental problems, which can be achieved by combining lightweight materials with high-strength steel.Since the density of aluminum is about 1/3 that of steel, the use of aluminum alloy instead of steel could reduce the weight by about 50%.Therefore, the connection between steel and aluminum is vital for the construction of lightweight automobiles [1].In terms of physical characteristics, great differences exist between aluminum alloys and steel, such as melting point, electrical conductivity, thermal conductivity, and thermal expansion coefficient.Therefore, the formation of good welding between both materials is very difficult, resulting in brittle intermetallic compounds (IMCs) at the aluminum/steel interface during welding, adversely affecting the mechanical properties of aluminum/steel welded joints [2][3][4].
The resistance spot welding (RSW) method is advantageous in terms of high production efficiency, simple operation, and easy automation, thereby being widely utilized in automobile production [5].The welding temperature of RSW is lower than those of other fusion welding methods, which could effectively reduce the mutual reaction and diffusion between atoms at the interface, suppressing the generation of IMCs and facilitating the connection between aluminum/steel dissimilar metals [6].Therefore, studying the RSW technology of aluminum/steel dissimilar materials is of great significance to advanced lightweight automobiles.
Numerous studies have so far been conducted on the RSW technology of dissimilar metals of aluminum alloy and high-strength steel [7][8][9].Unlike arc welding and laser welding, the RSW does not require the use of filler wire.Besides, a short welding time of RSW results in rapid solidification of the molten metal with the easy occurrence of welding defects in the formed Fe/Al weld, deteriorating the mechanical properties of welded joints under inappropriately selected welding parameters [10].The mechanical properties of Fe/Al welded joints of RSW are mainly affected by two welding parameters (welding current and welding time).The increase in welding current and the extension of welding time would result in enhanced welded nugget diameter in the weld followed by stabilization of the trend [11].Besides, the thickness of IMCs layer in Fe/Al joint becomes thicker with the increase in welding current and welding time [12].Currently used methods are meant to reduce the thickness and content of IMCs in Fe/Al weld by optimizing welding parameters and adjusting the phase composition of the interlayer in the weld to yield welded joints with improved mechanical properties [13][14][15].
Some research studies have shown that the addition of a magnetic field during the welding process could effectively improve the mechanical properties of welded joints, making it a feasible auxiliary welding method.Despite the absence of an obvious arc and liquid molten pool in RSW, the application of a magnetic field could significantly affect the RSW process due to the existence of a current component during RSW process.Various findings have concluded that the application of a magnetic field during the RSW process of metals significantly improved the mechanical properties of the welded joint [16][17][18][19].
CP780 steel is an advanced automobile high-strength steel gradually replacing ordinary steel, especially in automobile body manufacturing [20].Meanwhile, 7075 belonging to the 7XXX series (Al-Mg-Si-Zn) alloy aluminum plate can be strengthened by heat treatment to yield high strength, good weldability, and corrosion resistance, useful for automobile manufacturing [21].Therefore, connecting CP 780 steel and 7075 aluminum is of great interest for lightweight automobiles but little has so far been performed in terms of studying weldability, microstructure, and mechanical properties of dissimilar metal RSW joints of both materials, as well as the changes in microstructures and properties of welded joints in the presence of a magnetic field.
Herein, CP780 high-strength steel and 7075 high-strength aluminum alloy dissimilar metals were subjected to RSW tests under steady-state magnetic field assistance.The influence of the steady-state magnetic field on the macro-morphology of welded joints, the microstructure, phase composition/content of IMCs, and the tensile properties of welded joints were explored to optimize the RSW process parameters of steel/aluminum dissimilar materials, broaden the application of magnetic field-assisted RSW technology, and provide the basic theoretical analysis and technical aspects of steel/aluminum RSW of dissimilar materials.

Materials and methods
The 7075 high-strength aluminum alloy and CP780 high-strength steel with supply status of respectively T6 and cold rolled were used as test materials.The specimen sizes of both materials measured 80 mm × 20 mm × 1.5 mm and 80 mm × 20 mm × 1 mm, respectively.Their chemical compositions and mechanical properties are summarized in tables 1 and 2, respectively.
According to the lap form of welded joints in literature [3,17], 7075 aluminum alloy was on the top and CP780 steel was set on the bottom, overlapped up and down in the horizontal direction with an overlapping area of 20 mm × 20 mm.The size of the welding sample is shown in the figure 1(a).At the center of the lap welded joint, a Panasonic YR-350S single-phase AC resistance welder was utilized for RSW.The electrode material consisted of Cr-Zr-Cu, and the diameter of the electrode end face was 5 mm.
The influence of the magnetic field on the microstructure and mechanical properties of the RSW joint was studied through magnetic field-assisted resistance spot welding testing (hereinafter referred to as MA-RSW).According to the literature [16][17][18][19], using the same pre-welding preparation method, a total of eight NdFeB strong permanent magnets (6 mm in diameter) with 40 mT surface magnetic induction intensity were horizontally fixed below the electrode.As shown in the arrangement of magnets (figure 1(b)), the magnetic induction lines generated by permanent magnets passed through the welding nugget to generate Lorentz force.
According to the literature [3,[10][11][12] , and under 2.0 kN electrode pressure and 0.04 s welding time, eight groups of RSW tests were carried out by changing the welding current.As shown in table 3, a total of six welding tests were performed for each group of parameters.
After welding, the metallographic specimen of the joint was prepared by wire cutting.The samples were then made by grinding and polishing an MP-2B metallographic specimen with a machine equipped with double discs and a step-less speed change feature.Finally, the samples were etched with a solution composed of 5 ml nitric acid, 1 ml HF, and 44 ml H 2 O.The cross-sectional macroscopic morphologies of the joints were observed by

Group number
Welding current (kA) B (mT) Zeiss Stemi508 stereomicroscope, and the Fe/Al contact interface diameter was recorded.The phase composition of the weld center was analyzed by x-ray diffraction (XRD, Rigaku D/Max-2500V-PC).The IMCs microstructure, energy spectrum analysis (line and point scanning), and the thickness of intermediate transition layer between aluminum and steel were viewed by field emission scanning electron microscopy (FESEM, Zeiss SIGMA 300).Eight groups of joints with different welding parameters were subjected to a CMT5305 electronic universal testing machine.For each group, three specimens were prepared and the shear force-displacement curve of each group of parameters was recorded after testing.
To ensure uniform load during tensile testing, steel plates with sizes of 1 mm × 20 mm × 20 mm and 1.5 mm × 20 mm × 20 mm) were placed on the two end surfaces of 7075 aluminum alloy and CP780 steel specimens during tensile testing, respectively.After tensile tests, the fracture morphology of welded joint was observed by Zeiss Stemi508 stereomicroscope and FESEM (Zeiss SIGMA 300).
The above-used experimental setup is shown in table 4.

Macro-morphology
The obvious difference in melting points between aluminum alloy (640 °C) and high-strength steel (1514 °C) resulted in the partial melting of aluminum alloy while high-strength steel remained solid during the RSW process.The aluminum/steel RSW joint showed fusion-brazing joint characteristics, with the mixing of the two metals occurring at the Fe/Al contact interface [13,14].The micro-morphologies of welded joint structures of RSW (0 mT) and MA-RSW (40 mT) are depicted in figure 2. Obviously, both structures contained two welded nuggets.Since most heat for welded nugget formation on the steel side was provided by the internal resistance on the steel side [18], one of the welded nuggets started forming at the central position inside the CP780 steel plate to grow outward elliptically.The higher thermal conductivity of aluminum alloy (2-3 times that of steel), resulting in the spread of some heat formed by contact resistance to the steel side [19].Consequently, the other welded nugget on the aluminum side started to form at the welded joint contact interface of 7075 aluminum alloy and CP780 steel to grow toward the 7075 aluminum alloy side semi-elliptically.
The measurements revealed obvious differences in the Fe/Al contact interface diameter of the cross sections of eight groups of welded joints.The numerical changes in the diameter of the welded joint Fe/Al contact interface of each group of parameters are displayed in figure 3.
As shown in figures 2 and 3, the addition of the magnetic field increased the diameter of the Fe/Al contact interface, which can be explained by the interaction between the external magnetic field and welding current generating a circumferential Lorentz force suitable for promoting the planar circumferential movement of the molten metal to the outside of the welded nugget.Under the combined action of the induced magnetic field and the external magnetic field, the Lorentz force monotonically increased from the center of the welded nugget to its edge, conducive to pushing the molten metal from the welding center to the periphery.The working principle can be described by the research reported by Li et al [13,16,17,22,23], as shown in figure 4. Such effect promoted the outward extension of the molten metal of upper part of the 7075 aluminum alloy welded nugget at the Fe/Al contact interface, thereby enhancing the diameter of the Fe/Al contact interface and the effective utilization of the heat of RSW, consistent with Shen et al research [18,19].
In the absence of a magnetic field, the longitudinal comparison (figure 2) of the contact interface of Fe/Al indicated an obvious incomplete fusion at the contact interface of Fe/Al under low welding current (I = 9 kA) due to insufficient resistance heat.At I = 12 kA, the resistance heat was high, and the molten metal stayed at high temperatures for a long time, resulting in facilitated splashing and loss of the internal volume of the welded nugget, facilitating the formation of a shrinkage cavity at the Fe/Al contact interface [12,13].Consequently, the Fe/Al contact interface with welding defects significantly reduced the mechanical properties of welded joints.At  welding currents of 10 kA and 11 kA, no obvious welding defects were noticed on the Fe/Al contact interface of the welded joint.
At B = 40 mT, very small welding currents(I = 9 kA, 10 kA) resulted in shorter high-temperature residence time at the contact interface and welding nugget, along with a shorter action time of Lorentz force, unable to fully expand in all directions in a short time.This, in turn, unevenly distributed heat at the joint contact interface, resulting in welding defects, such as incomplete fusion at the internal Fe/Al contact interface, as shown in the morphologies of 9 kA-40 mT and 10 kA-40 mT welded joints in figure 2. Under larger welding currents(12 kA), the molten metal in the weld can easily be splashed due to the action of Lorentz force for a long time, inducing a shrinkage cavity defect at the Fe/Al contact interface, as shown in the morphology of the 12 kA-40 mT welded joint in figure 2.
Therefore, the MA-RSW process required synergy from the magnetic field and welding current, with the best lifting effect obtained by selecting the appropriate welding current.The comparison in figure 2 suggested that welded joints without welding defects can be obtained by adding a magnetic field at a welding current of 11 kA.Under these conditions, the diameter of the Fe/Al contact interface increased, conducive to enhancing the Fe/Al contact area of the welded joint with a potential role in promoting the mechanical properties of the welded joint.

IMCs microstructural characteristics
The FESEM microstructures of the transition zone at the aluminum/steel contact interface in the weld center (11 kA-0 mT, 11 kA-40 mT) are displayed in figures 5(a) and (b).The data corroborated previous literature [24][25][26][27], suggesting the formation of a transition zone at the steel/aluminum alloy contact interface with a typical layered structure in the center of the weld.The black area represented the intermediate transition layer between steel and aluminum plates at the contact interface.At the RSW(11 kA-0 mT) and MA-RSW(11 kA-40 mT) welding center, both sides of the steel/aluminum contact interface showed flat interfaces, and the morphological characteristics of IMCs on both sides were not obvious.
According to the literature [28,29], the chemical reaction between Al atom and Fe atom occurred first during the formation of the transition layer.Then the Fe/Al IMCs phase (such as FeAl 2 , and FeAl 5 et al ) with relatively stable thermodynamic properties were generated in the transition layer which can also be described as IMCs layer [28,29].
The energy spectrum analysis of point scanning of the IMCs layer by FESEM in figures 5a1 and 5b1 showed aluminum and steel experiencing mutual diffusion, as shown in table 5.The XRD results in figure 6, table 5, and related literature [26][27][28][29] all confirmed solid solutions of FeAl 2 and FeAl 5 as the main phase components of IMCs layer at the Fe/Al contact interface of the two welded joints(11 kA-0 mT, 11 kA-40 mT).Since the phase components contained small amounts of Si that cannot be ignored, the phase compositions can be expressed as (Fe, Si)Al 2 (hereinafter referred to as FeAl 2 ) and (Fe, Si) Al 5 (hereinafter referred to as FeAl 5 ).
The line scanning analysis results perpendicular to the aluminum/steel contact interface by FESEM in figures 5a2 and 5b2 indicated quantifiable values of thickness of the intermediate transition layer of the two welded joints.The calculations revealed IMCs layer thickness at the weld center of the RSW of 3.0 μm, and IMCs layer thickness at the weld center of the MA-RSW of 2.2 μm, equivalent to a thickness reduction of 26.67%.These data corroborated the thickness measurements of the black intermediate transition layer in figures 5a1 and 5b1.
According to the reaction-diffusion principle, the thickness X of the interfacial reaction layer can be quantified by equation (1) [26]: where k 0 represents the growth constant of the reaction layer, Q is the activation energy of the growth of the reaction layer, R refers to the gas constant, T is the heating temperature, and t denotes the reaction time.
According to equation (1), X was related to T. Consequently, higher heating temperatures should induce longer reaction times and thicker reaction layers.In the MA-RSW process, the existence of Lorentz force promoted the circular motion of molten metal, resulting in a relatively low maximum temperature in the central area of the joint Fe/Al contact interface with rapid heat dissipation.This, in turn, reduced the reaction time to a certain extent, preventing the nucleation of FeAl   As thickness increased, the crack sensitivity rose while the welded joint ductility decreased [27].As a result, MA-RSW (11 kA-40 mT) joint with thin IMCs layers should have better mechanical properties.

Tensile properties
The shear force-displacement curves and average values of shear force and displacement obtained by all eight groups of welded joints with different parameters are compared in figures 8 and 9.
Note that the fracture position of all welded joints was taken at the weld.In the absence of a magnetic field, the average values of shear force and displacement first rose with the increase in welding current and then decreased until reaching a peak at I = 11 kA.The average shear force increased from 3.02 kN (I = 11 kA , 0mT) to 3.49 kN (I = 11 kA , 40 mT), equivalent to an increase of 15.56%.The average displacement rose from 1.01 mm (I = 11 kA , 0mT) to 1.22 mm (I = 11 kA , 40 mT) , with an increase of 20.79% .
This can be explained by enhancement in the diameter of the Fe/Al contact interface caused by Lorentz force after adding magnetic field.In addition, the thickness of the interfacial IMCs layer was significantly reduced, conducive to improving tensile properties [26,27].The above results were consistent with the analysis results of sections 3.1 and 3.2.
The fracture images of RSW (11 kA-0 mT) and MA-RSW (11 kA-40 mT) joints on aluminum plates by FESEM are displayed in figure 10.The stereomicroscope observations of the fracture surfaces of two kinds of welded joints (figures 9(a) and (c) suggested a relatively small fracture surface of the RSW joint and a relatively large fracture surface of the MA-RSW joint.The FESEM images of fracture surfaces in figures 10(b) and (d) indicated mainly brittle fracture surfaces of both kinds of welded joints.Also, both contained small numbers of ductile fractures.The dimples in the fracture surface of RSW joint looked relatively shallow due to the higher IMCs content in the weld, as well as the relatively thick IMCs layer, prone to the occurrence of brittle fracture and significantly reduced mechanical properties of the joint (figure 10(b)).However, the fracture dimples of the joint in MA-RSW appeared relatively deep due to the significantly reduced thickness and content of IMCs layer.Although the fracture was still brittle, the plasticity of the welded joint was relatively improved (figure 10(d)).

Conclusion
Steady-state magnetic field-assisted resistance spot welding tests of CP780 high-strength steel and 7075 aluminum alloy dissimilar metals were tested under various welding currents (I = 9 kA, 10 kA, 11 kA, and 12 kA).Characteristics related to macroscopic morphology, IMCs microstructural characteristics, and tensile properties of RSW joint and MA-RSW joint were examined and the results were compared.The following conclusions can be drawn: (1) The welded joint structure of the two welding methods presented two welded nuggets (one in the aluminum plate and the other in the steel plate).Under the same welding current parameters, the diameter of the Fe/Al contact interface was increased under a magnetic field with B = 40 mT, significantly improving the heat utilization efficiency of RSW.At I = 11 kA, welded joints with no obvious welding defects were obtained in the presence of a magnetic field.Under other welding parameters, a few welding defects, such as incomplete fusion and shrinkage cavities formed at the cross-section of the welded joint.Thus, synergy from the magnetic field and the appropriate welding current could form welded joints without defects.
(2) (Fe, Si)Al 2 and (Fe, Si)Al 5 were confirmed as the main phase components in the intermediate IMCs transition layers of the two joints.IMCs layer was thicker in the central area of welded joint, and the thickness of IMCs layer decreased as distance from the center increased.In the presence of the magnetic field, the induced Lorentz force promoted the circular motion of molten metal, resulting in a relatively low maximum temperature in the area of the Fe/Al contact interface of the welded joint combined with rapid heat dissipation.These features were conducive to reducing the reaction time to a certain extent, as well as declining the thickness and content of the IMCs layer in formed interface.At I = 11 kA and in the presence of a magnetic field, the peak thickness of IMCs layer decreased by 26.67%.equivalent to an increase of 20.79%.The comprehensive comparison showed I = 11 kA and B = 40 mT as the optimized parameters to obtain welded joints with the best mechanical properties.In sum, the proposed dissimilar aluminum/steel resistance spot welded joint assisted by magnetic field looks promising for lightweight automobile parts manufacturing.

Figure 1 .
Figure 1.MA-RSW welding experiment (a) Size of welding sample; (b) Schematic diagram of MA-RSW experimental setup.

Figure 3 .
Figure 3. Diameter of Fe/Al contact interface.

Figure 4 .
Figure 4.The working principle of the MA-RSW process.

Figure 5 .
Figure 5.The IMCs Characteristics obtained by FESEM: (a) The microstructural characteristics of transition zone in weld center by RSW (11 kA-0 mT).(b) The microstructural characteristics of transition zone in weld center by MA-RSW (11 kA-40 mT).
2 and FeAl 5 , and declined the thickness and content of the IMCs layer.Comparison with figure 6(a) revealed a relative reduction in the composition of FeAl 2 and FeAl 5 in figure 6(b), confirming the analysis from figure 5 and equation (1).In particular, the reaction layer looked thicker in the central area and decreased as a function of the increase in distance from the center [26, 27].The IMCs layer thickness distributions of two welded joints (11 kA-0 mT, 11 kA-40 mT) are summarized in figure 7. The thicknesses of the evenly distributed 11 regions of the two welded joints were examined by FESEM.The average thickness of IMCs layer was calculated as the area surrounded by aluminum/IMCs and IMCs/steel boundary line exceeding the width of the area fixed at 250 μm.The maximum IMCs layer thickness in the RSW (11 kA-0 mT) joint was determined as 3.0 μm, which was near the weld center.The minimum IMCs layer thickness at the outermost position was 1.1 μm.The maximum thickness of IMCs layer in the MA-RSW(11 kA-

( 3 )
At I = 11 kA and under the action of the external magnetic field, the fracture area of the MA-RSW welded joint looked relatively large and the plasticity of the welded joint was relatively high due to the increased Fe/ Al contact interface and relatively decreased IMCs layer thickness and content.The average shear force increased from 3.02 kN (I = 11 kA , 0 mT) to 3.49 kN (I = 11 kA , 40 mT), equivalent to an increase of 15.56%.The average displacement rose from 1.01 mm (I = 11 kA , 0 mT) to 1.22 mm (I = 11 kA , 40 mT),

Figure 9 .
Figure 9.The average values of shear force and displacement of welded joints.

Table 1 .
Maximal concentrations of elements in 7075 aluminum alloy and CP780 steel (wt%).

Table 4 .
The summary sheet of the experimental setup.