Effect of a Single-Sided Magnetic Field on Microstructure and Properties of Resistance Spot Weld Nuggets in H1000/DP590 Dissimilar Steels

Abstract

H1000 stainless steel is defined as a nickel-saving austenitic stainless steel, characterized by high strength and high elongation. DP590 steel is widely used in the manufacturing of vehicle bodies. DP590 dual-phase steel is classified as a high-strength low-alloy steel, known for its high strength and good formability. To address issues such as nugget deviation, inhomogeneous mixing of the internal nugget microstructure, and interfacial fracture during tensile-shear testing in resistance spot-welded joints of these dissimilar materials, a unilateral magnetic-controlled resistance spot-welding process was proposed. The influence of the external magnetic field on various properties of the joint was systematically investigated. The results indicate that the application of an external magnetic field significantly enhances the strength of H1000/DP590 dissimilar spot-welded joints, with joint strength increasing by approximately 14% and energy absorption capacity improving by about 30%. These improvements are attributed to the electromagnetic stirring effect induced by the magnetic field, through which the effective nugget diameter was enlarged, the microstructure was homogenized, and the macroscopic morphology of the nugget was modified. As a result, the bonding area between the nugget and the base metal is expanded, and the fracture mode of the joint is shifted from interfacial failure to partial button failure, thereby enhancing the mechanical properties of the joint.

1. Introduction

With the rapid development of the automotive industry, problems such as energy shortage and environmental pollution have emerged. Vehicle lightweight technology, as an important method for energy conservation and emission reduction, has attracted significant attention from governments and enterprises worldwide [1,2,3]. The application of dissimilar steel plates has become an important means of vehicle lightweighting, with such connections accounting for more than 60% of the spot welds in automotive body-in-white [4,5]. The extensive use of dissimilar steel plates in vehicle manufacturing has brought challenges to resistance spot welding, making the assurance of mechanical properties in dissimilar steel resistance spot-weld joints of great significance.

H1000 stainless steel, a third-generation ultra-high-strength steel produced by Outokumpu, combines high strength with high elongation, making it highly promising for applications in automotive structural components, such as body B-pillars. As a result, it has garnered significant attention from automobile manufacturers. High-strength dual-phase steels are widely used in structural parts, reinforcement components, and anti-collision parts in the automotive industry [6,7]. Due to varying strength requirements between the B-pillar and its adjacent components, different materials are often employed. Therefore, the application of H1000 stainless steel in B-pillars will inevitably raise connection challenges with dual-phase steels such as DP590.

Resistance spot welding remains the preferred joining process in vehicle manufacturing due to its simplicity, high automation, lack of required auxiliary materials, high welding quality, and low cost [8,9]. However, in dissimilar steel resistance spot welding, differences in plate thickness and material properties cause uneven heat generation and dissipation, leading to nugget offset and reduced effective nugget size. Additionally, material differences result in uneven mixing of the nugget microstructure, reducing joint mechanical properties and creating potential safety hazards [10].

Existing solutions for nugget offset include the use of asymmetric electrode caps [11,12] and DeltaSpot [13] technology, but these methods have limitations, including complex processes, difficulties in dressing asymmetric electrodes, and high costs. Few solutions exist for the problem of uneven microstructure mixing in dissimilar steel spot welds.

In recent years, numerous researchers have investigated magnetic-controlled resistance spot-welding technology, revealing that external magnetic fields can refine the internal grains of the weld nugget and improve joint quality. Shen Qi et al. [14,15] utilized a pair of axially magnetized annular NdFeB permanent magnets as an external magnetic field source to study magnetic-assisted resistance spot welding of dual-phase steel. The results indicated that the external magnetic field significantly altered the nugget morphology of the joint, resulting in a peanut-shell shape characterized by “thin in the middle and thick at both ends.” As the nugget diameter increased, the crystallization directionality within the nugget was weakened, the grain structure in the fusion zone was refined, and the hardenability of the microstructure decreased. Additionally, the strength, plasticity, and high-cycle fatigue performance of the joint were improved to some extent. Wang Yuhao et al. [16] investigated the improvement in weld quality in 2.5 mm AlSi7MnMg sheet joints by applying a multi-pulse magnetically assisted resistance spot-welding (MPMA-RSW) process with an external magnetic field. Results demonstrated that this MPMA-RSW technique interrupted the growth of the columnar grain zone (CGZ), shifting solidification from directional to nearly simultaneous mode. This refinement led to a 26.6% increase in nugget diameter, a 23.2% enhancement in peak lap-shear strength, and a 58.1% improvement in peak energy absorption. Luo Zhen et al. [17] reviewed the dissimilar welding of aluminum and steel, emphasizing the improving effects of external magnetic fields on conventional welding processes. They concluded that the external magnetic field, through the Lorentz force it generates, drives the movement of molten metal, adjusts the microstructure and composition in the weld zone, reduces porosity, and inhibits the formation of intermetallic compounds, thereby enhancing joint strength. According to the aforementioned studies, the auxiliary magnetic field resistance spot-welding method features simple equipment and process, low cost and maintenance expenses, and can also homogenize the nugget structure, refine grains, and enhance joint hardness, making it suitable for improving the quality of austenitic stainless steel spot-welding joints. However, all the above studies employed bilateral auxiliary magnetic fields. Since the nugget distribution in dissimilar steel joints is asymmetric, the internal metal flow is more complex. Compared to the double-sided magnetic field, the single-sided magnetic field exhibits lower intensity at the weld joint location. Under the double-sided magnetic field configuration, the molten metal in the weld pool scours the nugget edges under the influence of the Lorentz force. In contrast, although the Lorentz force is weaker in the single-sided magnetic field setup, the molten metal is subjected not only to the driving force that scours the nugget edges but also to a component that induces vertical agitation within the pool. This paper takes H1000 stainless steel and DP590 steel as the research objects, proposes a unilateral magnetic-assisted resistance spot-welding method, and investigates in detail the influence of an external magnetic field on the mechanical properties of the spot-welded joints between H1000 stainless steel and DP590 steel.

2. Materials and Methods

In this study, 1.2 mm thick H1000 stainless steel sheets and 1.6 mm thick DP590 steel sheets were used as base materials. The H1000 stainless steel is primarily composed of austenite, while the DP590 steel consists mainly of ferrite with martensite distributed in an island-like pattern. The chemical compositions and mechanical properties of the base materials are presented in

Table 1. Chemical compositions of H1000 and DP590 steels. Adapted from [18,19].

Element (wt%)	C	Si	Mn	p	S	Cr	Mo	Cu	Ni	Al	Fe
H1000	0.21	0.24	14.6	0.002	<0.010	13.9	0.13	0.44	0.34	−	Balance
DP590	0.09	0.35	0.99	0.012	<0.005	0.02	−	−	0.01	−	Balance

and

Table 2. Mechanical properties of H1000 and DP590 steels. Adapted from [18,19].

Experimental Materials	UTS (MPa)	EL (%)	ER (μΩ × cm)
H1000	1310–1325	23–25	78.0
DP590	590–700	19–27	15.7

[18,19].

The dimensions of the welded specimens for the tensile-shear test were 100 × 38 mm, and the specimens were welded in a parallel lap joint configuration. The overlapping area between the two steel plates was a 38 × 38 mm square, with the spot weld located at the center of the overlapping area. During the tensile test, spacers with the same thickness as the welded materials were added at both the upper and lower clamps. Considering that the Peltier effect [20,21] during resistance spot welding generates more heat on the sheet side in contact with the positive electrode, the H1000 stainless steel sheet with higher electrical resistivity was consistently placed in the lower position in this experiment, while the DP590 steel sheet was placed in the upper position.

In the present experiment, a resistance spot-welding machine (R2000iA/210F welding robot: Fanuc Corp., Japan; C-type servo welding gun: Obara Corp., Japan.) equipped with a servo gun was used, as shown in Figure 1a. Chromium-zirconium-copper electrode caps with an end face diameter of 6 mm were employed, with the main chemical composition being 0.7% chromium, 0.1% zirconium, and more than 98.5% copper. As shown in Figure 1a, a single annular neodymium-iron-boron permanent magnet was mounted on the lower electrode arm, with the N pole facing downward and the S pole facing upward. The permanent magnet had an inner diameter of 20 mm, an outer diameter of 40 mm, and a thickness of 20 mm, with its performance parameters presented in

Table 3. Performance parameters of the external magnetic field source. Adapted from [15].

Magnetic Flux Density Hcb (MA/m)	Remanence Br (T)	Maximum Magnetic Energy Product Bhmax (kJ/m3)
0.955	1.17~1.22	263~287

[15]. The enlarged view of the lower electrode in Figure 1a is shown in Figure 1b. The permanent magnet was secured using an insulating fixture made of nylon. The magnetic force generated by the unilateral magnetic field source varies according to the working distance. As shown in Figure 1b, the working distance (WD) of the magnetic field source was set to 3 mm. The schematic diagram of the magnetic field force generated by the unilateral magnetic source within the joint is shown in Figure 1c. According to the “Right-hand grip rule” and “Fleming’s left-hand rule,” the magnetic field distribution of the current can be determined, and the magnetic force distributions of both the current and the permanent magnet can be obtained, as shown in Figure 1c. Figure 2 shows the simulated magnetic field distribution of the external magnetic field.

According to the relevant welding standards and a comprehensive consideration of the property characteristics of various materials and the range of welding parameters [22], the welding parameters for magnetic-controlled resistance spot welding of dissimilar metals were set as follows: a welding current of 6.5 kA was applied, with the squeeze time, welding time, and hold time all set to 200 ms, and an electrode force of 4.0 kN was used. A schematic diagram of the welding sequence is shown in Figure 3.

The tensile-shear strength of the joint is an important indicator for evaluating the quality of resistance spot-weld joints. The tensile tests were conducted using a UTM5504 electronic universal testing machine (Jinan Hengsishengda Instrument Co., Ltd., Jinan, China) with a testing speed of 2 mm/min. In the present experiment, the nugget diameter was measured through the macro-morphology of metallographic specimens. Metallographic samples were prepared in strict accordance with standard metallographic specifications. A Leica stereomicroscope and a Leica CTR6 LED optical microscope (Leica Instruments Co., Ltd., Wetzlar, Germany) were used to observe the nugget morphology and microstructure. The surface hardness of the joint was measured using a Buehler Wilson VH1102 microhardness tester (Buehler Ltd., Lake Bluff, IL, USA). The microhardness tests were conducted at an ambient temperature of 25 °C with a load of 1.96 N, a point spacing of 0.3 mm, and a dwell time of 15 s, yielding the hardness distribution curve along the diagonal of the joint.

3. Results and Discussion

3.1. Macro and Micro Morphologies

Figure 4 presents the macro- and micro-morphologies of both conventional and magnetic-assisted resistance spot-welded joints of H1000 and DP590 steels. As shown in Figure 4a, the nugget of the conventional joint was characterized by an “inverted-bowl” structure, with a relatively flat and wide “bowl base.” When the magnetic-assisted joint in Figure 4b was observed, it was found that the nuggets on both the upper and lower sides tend to move away from the permanent magnet, forming a “small-boat” shaped nugget that is larger at the ends and thinner in the middle. Furthermore, the angle between the joint edge projection on the cross-section and the vertical plane was measured to be 30° for the magnetic-assisted joint, compared to 40° for the conventional joint, namely, the nugget edge on the DP590 side was steeper in the magnetic-assisted joint than that in the conventional one, as shown in Figure 4a,b. It is because under the stirring effect of the external magnetic field, the molten metal at the edge of the nugget was subjected to the circumferential magnetic force and undergoes centrifugal motion, thus concentrating toward the outside of the nugget and constantly eroding the outer wall of the nugget, allowing more base metal to integrate into the molten nugget, which also causes the nugget to increase in size.

As can be seen from Figure 4c,e,g, the nugget of the conventional joint was still largely composed of dendritic structures with a clear orientation. In contrast, the nugget center of the magnetic-assisted joint was mainly characterized by fine dendrites and equiaxed crystals, as shown in Figure 4d,f,h. This change is attributed to the electromagnetic stirring, by which the elongated dendrites typically found in conventional joints are disrupted. Consequently, the joint interior is dominated by smaller equiaxed crystals. Although some dendrites are still present, they are not consistently aligned. It occurs because the forced convection generated by electromagnetic stirring exerts substantial fluid shear stress on the elongated dendritic crystals. This shear force acts to “break” or “tear off” the fragile dendritic arms from their primary trunks. Furthermore, electromagnetic stirring creates a more uniform thermal and chemical environment. The combined action of these mechanisms significantly promotes the formation and growth of equiaxed crystals while suppressing the development of coarse columnar grains.

3.2. Microhardness Profiles

The microhardness distribution across the cross-sections of conventional and magnetic-assisted RSW joints of H1000 and DP590 steels is plotted in Figure 5. The red square-dotted line in the figure represented the microhardness distribution of the conventional spot-welding joint, while the blue circular-dotted line represented that of the magnetic-assisted spot-welding joint. As shown in the figure, the various regions of the spot-welding joint were divided into base material (BM), heat-affected zone (HAZ), and fusion zone (FZ) based on the thermal cycles they experience. The width of the heat-affected zone (HAZ) is defined as the maximum perpendicular distance from the weld fusion line to the boundary of the unaffected base metal. The heat-affected zone on the left was on H1000 steel, and the one on the right was on DP590 steel. The heat-affected zone on the H1000 steel side was narrower than that on the DP590 steel side. On the H1000 stainless steel side, the hardness of the heat-affected zone decreased from the region adjacent to the base material toward the fusion zone. In contrast, on the DP590 steel side, the hardness of the heat-affected zone initially increases with distance from the nugget center, then stabilizes in a plateau-like trend, and finally decreases as it moves farther away from the nugget center, eventually leveling off to the base material hardness of approximately 225 HV.

Based on the information presented in Figure 5, the average hardness in the nugget zone of the conventional joint was measured at 354.3 HV, with a standard deviation of 18.6, while the average hardness in the nugget zone of the magnetic-assisted joint was recorded as 379.7 HV, with a standard deviation of 15.9. Although the average nugget hardness was increased in the magnetic-assisted joint compared to the conventional joint, it can be observed from the figure that this overall improvement in hardness was primarily attributed to the presence of shrinkage pores and voids in the conventional joint, which cause the hardness values at certain locations to be reduced. From the standard deviation values, it is concluded that the fluctuation range of hardness in the nugget zone was reduced for the magnetic-assisted joint. It is because the shrinkage pores and voids within the joint are reduced by the electromagnetic stirring effect, and the internal microstructure of the nugget is made more uniform, with the grains being refined, as shown in Figure 4. As a result, the dispersion of hardness in the nugget zone is significantly reduced. It also leads to the average nugget hardness of the magnetic-assisted joint being increased compared to that of the conventional joint.

3.3. Mechanical Properties

Figure 6 shows the mechanical property test results for both conventional and magnetic-assisted resistance spot-welded joints of H1000 stainless steel and DP590 steel. Traditional resistance spot-welding and magnetic-assisted resistance spot welding are divided into two groups, with each group conducting five spot-weld tensile tests. As can be seen from Figure 6a, the tensile strength of the magnetic-assisted joint was significantly increased by approximately 14% compared to that of the conventional joint. This enhancement in strength was attributed to two main factors. Firstly, the nugget diameter is enlarged as the molten metal is promoted by the external magnetic force to scour the nugget boundary. Secondly, the edge of the magnetic-assisted spot-welded joint is more vertical, as shown in Figure 4b. Consequently, the resistance to crack propagation from the nugget edge is increased during tensile-shear testing, leading to improved joint strength.

The energy absorbed by a material during the deformation process until failure is numerically equal to the work done by the force along the displacement direction. This corresponds to the area enclosed by the force-displacement curve in Figure 6a, a vertical line drawn from the displacement axis, and the displacement axis itself. As shown in Figure 6b, the energy absorption until failure for the magnetic-assisted joint was significantly improved by approximately 30% compared to the conventional joint. It is attributed to its larger nugget diameter and greater penetration depth on the DP590 side. Meanwhile, as shown in Figure 4, the internal microstructure of the magnetic-assisted joint is more uniform, with fewer defects such as shrinkage cavities, which increases both the peak load and the elongation, thereby enhancing the energy absorption capacity of the joint.

3.4. Fracture Morphology

Figure 7 shows the typical fracture morphologies of conventional and magnetic-assisted resistance spot-welded joints for H1000 and DP590 steels. As shown in Figure 7a,b, interfacial fracture was exhibited in the conventional joint during tensile testing. However, the fracture path was not aligned with the nugget boundary on the DP590 side; instead, the fracture occurred through the center of the nugget. Since traditional optical microscopy cannot adequately represent the three-dimensional morphology of the specimen, a Keyence VK-X200 laser confocal microscope (Keyence Corporation, Osaka, Japan) was used to observe joint morphology in order to better display the spatial variations of the conventional fracture surface. The 3D morphology of the conventional joint is presented in Figure 7c,d, where the fracture surface can be observed to be uneven and irregular, exhibiting a peak-like appearance. It is closely related to the small effective nugget diameter, coarse nugget grains, and non-uniform microstructure distribution. In contrast, the fracture mode of the magnetic-assisted joint is altered to a partial button fracture, with most of the nugget being pulled out, as shown in Figure 7e,f. This change is caused by the electromagnetic stirring, through which the effective nugget diameter is increased, the grain size is refined, and the microstructure is made more homogeneous. Furthermore, defects such as shrinkage pores are reduced, leading to the mechanical properties of the joint being improved and consequently the fracture mode being changed.

4. Comprehensive Analysis

Based on the above analysis of the resistance spot-welding performance between H1000 stainless steel and DP590 steel, the influence of an external magnetic field on the performance of dissimilar-metal resistance spot-welded joints of H1000 stainless steel and DP590 steel can be determined. Overall, the unilateral application of an external magnetic field can significantly improve the quality of the H1000/DP590 dissimilar joints. The specific mechanisms can be summarized as follows:

(1)

Influence on Nugget Morphology and Dimensions

Figure 8 illustrates the improvement of an external magnetic field on the nugget morphology of the dissimilar metals between H1000 stainless steel and DP590 steel. As shown in Figure 8a, since the H1000 steel plate is positioned closer to the permanent magnet, the magnetic field intensity on the H1000 side was higher than on the DP590 side, resulting in varying degrees of electromagnetic stirring induced within the nugget. The influence of the auxiliary magnetic field on the nugget morphology can be explained by means of Figure 8b. During the traditional RSW process, the molten metal gathers toward the center of the nugget owing to the driving effect of the induced magnetic force. Therefore, a regular and flat nugget is more conducive to formation, as shown in Figure 1c and Figure 8b. However, the circumferential external magnetic force changes the flow path in the nugget. The molten metal, driven by the resultant magnetic force, flows away from the nugget center and scours the nugget edge, finally forming a nugget that is enlarged at both ends and thinner in the middle. Compared with conventional spot-welded joints, the applied magnetic field increases the effective nugget diameter of dissimilar joints. It has significantly increased the effective thickness of the nugget. In addition, the angle between the nugget periphery and the vertical plane has been reduced, allowing more DP590 material to be incorporated into the nugget. Therefore, more molten metal was driven to scour the nugget edge, which further strengthens the effect of the auxiliary magnetic field and leads to a more significant improvement in the quality of the nugget.

(2)

Influence on the Microstructure of the Joint

As shown in Figure 4, a more uniform mixing within the nugget of the H1000/DP590 dissimilar joint is achieved by the unilateral auxiliary magnetic field. To further verify this conclusion, energy-dispersive spectroscopy (EDS) was performed on the nugget. The results, presented in Figure 9 and

Table 4. Chromium content (wt.%) in conventional/magnetic-assisted spot-welding joints of H1000 stainless steel.

Element	RSW		SMA-RSW
Cr	DP590 side	H1000 side	DP590 side	H1000 side
	7.18	9.31	8.96	8.15

, confirm this observation. At ten different locations, each on the DP590 side and the H1000 side of the nugget, the chromium content was analyzed. The average values were listed in Table 4. It can be seen that in the conventional spot-welded joint, the chromium content in the nugget differs significantly between the two sides, whereas in the magnetic-controlled spot-welded joint, the chromium content on both sides is more comparable. It indicates that the mixing and flow of the molten metal within the nugget are enhanced by the electromagnetic stirring effect of the applied magnetic field.

5. Conclusions

In this study, a resistance spot-welding method with a unilateral external magnetic field is proposed to address the issues of a small effective nugget size and microstructural inhomogeneity in dissimilar metal joints between H1000 stainless steel and DP590 steel. The weld quality of the H1000/DP590 dissimilar steel joint is effectively enhanced. The main conclusions are drawn as follows:

(1)

The nugget deviation phenomenon, which is typically observed in conventional resistance spot welding, was mitigated by the unilateral external magnetic field, and the effective nugget diameter of the H1000/DP590 dissimilar joint was increased.

(2)

The elongated dendrites in the nugget center were disrupted by the external magnetic field, resulting in a joint interior that is composed of fine equiaxed crystals and dendrites. Compared to conventional resistance spot welding, the microstructure within the nugget was more uniformly mixed, and the fluctuation in nugget hardness was reduced by the application of the external magnetic field.

(3)

The strength of the magnetically-assisted spot weld joint increased by approximately 14%, and the failure energy increased by approximately 30%. The joint exhibited a partial button fracture mode.