Microstructure and properties of magnetic field assisted laser wire-filled welded 22MnB5 steel joints

The magnetic field-assisted laser wire-filled welding test of 1.5 mm automotive 22MnB5 steel is performed to investigate the influence of magnetic field on the microstructure and properties of the welded joints. When no magnetic field is applied, and the laser heat input is 190 J mm−1, the welded joint width and the grain size of the coarse grain region are large. Also, there is an obvious hump defect at the bottom of the weld. Under the same heat input conditions, when a 5 mT and 15 mT steady magnetic field is applied, the thermoelectric magnetic force generated by the magnetic field promoted the flow of molten pool and concentrated laser energy. It is found that the hump defect is eliminated, the width of the welded joint is reduced, the grain size of the coarse grain region is significantly reduced, and the overall hardness of the welded joint is improved. However, different magnetic induction intensities have different effects on the solid phase transformation of the weld. When no magnetic field is added, the weld center is mainly composed of granular bainite and polygonal ferrite due to the slow cooling rate of the molten pool. When the applied magnetic field is 5 mT, the center of the weld is mainly composed of brittle and hard upper bainite because the thermoelectric magnetic force stirs the molten pool and accelerates the cooling rate of the molten pool but the overall mechanical properties of the welded joint were relatively poor. At 15 mT, lath martensite and lower bainite predominate in the weld center due to the increased cooling rate of the molten pool, thereby increasing the overall mechanical properties of the welded joint. Therefore, choosing the appropriate magnetic induction intensity is critical for improving the microstructure and properties of welded joints.


Introduction
Lightweight materials have emerged as a prominent area of research and technological innovation in the automotive field due to their significant advantages in energy conservation, emission reduction, and environmental protection [1]. In the field of lightweight materials and structures, advanced automotive highstrength steels such as high-strength boron steel have found widespread application as a substitute for ordinary automotive steel [2]. The most common high-strength boron steel is 22MnB5 steel, which has a lot of ferrite and pearlite in the matrix structure. After hot stamping, the strength of 22MnB5 steel is found to increase from about 600 MPa to more than 1200 MPa, and a large amount of martensite is formed in the matrix structure [3,4].
Laser welding has the advantages of high energy density, good weld quality, and small deformation for advanced high-strength steels, and has gradually become an important joining technology and process in automobile manufacturing [5,6]. Among many laser welding technologies, laser filler wire welding not only greatly improves the assembly gap tolerance between automotive body parts to be welded, but also solves the problem that laser deep penetration welding requires an excessively large groove gap. It can also improve the microstructure distribution of the weld area by using welding wires of different compositions, thereby regulating the performance of laser filler wire welding, which is widely used in automobile manufacturing [7].At the same Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. time, some researchers combined artificial intelligence predictions with welding experiments and confirmed that the increased strength and toughness of high-strength steel are closely related to the chemical composition and microstructure of the weld and heat-affected zone (HAZ) [8][9][10]. Therefore, it is particularly important to develop appropriate welding materials and welding process parameters.
Only a few scholars have studied the laser filler wire welding of 22MnB5 steel, and their work primarily focuses on the welding materials and process parameters and their influence on the surface coating, microstructure, and properties of the joint [11]. For example, Lin et al [12] performed a comparative analysis of laser filler wire welding and laser welding to weld 22MnB5 steel with 1.5 mm thick Al-Si coating on its surface. They investigated the effect of wire filling on the microstructure and mechanical properties of the weld zone and demonstrated that the joint strength of laser welding is 1210 MPa and the elongation is 1.1%. When laser filler wire welding is used, the joint strength is increased to 1550 MPa and the elongation is greater than 3%. At the same time, it was found that filler wire welding diluted the content of Al-Si coating on the surface of 22MnB5 steel, and the δ-ferrite phase also decreased after Al reduction and homogenization. Xu et al [13] used highstrength steel wire for laser filler welding of 1.5 mm Al-Si coated steel and discovered that wire filler welding prevents the formation of δ-ferrite, resulting in high-quality laser welding of Al-Si coated 22MnB5. The tensile strength and elongation of the wire-filled weld joints are increased by 7.5% and 183.9%, respectively, when compared to those without filler wire. Qin and Zhao et al [14,15] used laser wire penetration welding on a 22MnB5 steel lap joint. The effects of laser beam offset welding and laser wire filler welding on the microstructure and mechanical properties of the joint were investigated. The weld zone consists of martensite and δ-ferrite. When all other welding parameters are held constant, the ferrite content with filler wire is lower than that without filler wire. There are three different zones in the HAZ and their microstructures are martensite (inner HAZ), mixed alloy of martensite and ferrite (intermediate HAZ), and tempered martensite (outer HAZ). The HAZ with tempered martensite has the lowest hardness (about 250 HV). It is found that increasing the laser beam offset or adding filler wire helps in forming a good weld appearance. The δ-ferrite has a band-like structure with low strength and ductility, which appears along the fusion boundary and develops into a region with poor mechanical properties.
At the same time, researchers found that because of the high welding speed, the heat input used in laser wire welding is large, which not only affects the weld formation but also increases the area of the HAZ of the joint, affecting the microstructure and properties of the joint [12][13][14][15][16][17]. The magnetic field, as an additional nonpolluting and convenient energy field, interacts with the liquid metal in the molten pool during welding and generates a thermoelectric magnetic force [18,19].
The existing research [20][21][22] has shown that, under certain conditions, the presence of thermoelectric magnetic force in the weld pool can promote local flow and increase crystallization speed. For example, the selection of appropriate magnetic field parameters can optimize the weld structure, reduce welding defects, and improve the mechanical properties of the joint during laser wire-filled welding of stainless steel and other metal materials [23,24]. However, limited research has been conducted on the structure and properties of laser wire-filled welded joints of advanced high-strength steel for automobiles with the assistance of a magnetic field.
This paper examines the influence of different magnetic induction intensities on the macro-morphology, microstructure, alloy element transition, micro-hardness, and tensile properties of laser wire-filled 22MnB5 automotive steel welded joints, and then optimizes the welding process parameters. The research reported in this paper will provide valuable assistance in improving the safety performance and expanding the application scope of high-strength steel. At the same time, it will serve as a theoretical and practical reference for automotive lightweight material research.

Materials
The base metal used in this experiment was 22MnB5 high-strength boron steel produced by Baosteel company in China, and the supply state was cold rolled. The dimensions of all specimens were 65 mm × 60 mm × 1.5 mm. The welding wire (CARBOFIL 1 A ER70S-6) was supplied by OERLIKON company, which is a 1 mm low-alloy high-strength steel solid welding wire. The chemical composition and mechanical properties of the two materials are shown in tables 1 and 2, respectively.

2.2.
Magnetic field-assisted laser wire filler welding test Before welding, the surface of the test board is ground with sandpaper, the parts to be welded are cleaned with acetone, and both sides of the test board are fixed to the 304 stainless steel workbench with stainless steel clamps. Although most researchers used high power in laser welding experiments [6][7][8][11][12][13][14][15], it is equally important to investigate the laser welding process at low power to reduce the manufacturing cost. In this laser welding experiment, a YLS-1000W low-power fiber laser supplied by IPG company is used as a welding heat source, Ar is used as a shielding gas, and a YJ-105 wire feeder produced by Panasonic company is used to supply filler welding wire.
To explore the influence of magnetic induction intensity on the microstructure and properties of laser wirefilled 22MnB5 welded joints, a horizontally positioned strong NdFeB permanent magnet was used. The magnet has a surface magnetic field of 100 mT and dimensions of 65 mm × 20 mm × 6 mm. It was securely fixed under the same fixture, with its longitudinal axis aligned with the center of the joint gap. In addition, the longitudinal length of the permanent magnet is flush with both ends of the test board in the welding direction. As shown in figure 1, the magnetic induction intensity on the weld surface is varied by adjusting the vertical height distance between the permanent magnet and the bottom of the test board. The magnetic induction intensity of the upper surface of the test board to be welded is measured using a Gauss meter (model HT20). Then, while maintaining the same laser welding parameters, two different vertical magnetic induction intensities were introduced based on literature [12,13,25,26] and our previous research work and three separate laser wire-filled welding tests were carried out (see table 3).    Equation (1) was used to calculate the heat input: where P (W) is the laser power and V (mm/s) is the welding speed.

Metallography analysis
After welding, the metallographic specimens of the welded joints were prepared by wire cutting, which were then etched for 12 seconds in a 4% nitric acid + alcohol (nital). The microstructure of the welded joints of three groups of specimens was examined and compared. The examination was conducted using the Zeiss Axio Vert. A1 inverted metallographic Optical Microscope (OM), Zeiss SIGMA 300 field emission Scanning Electron Microscope (SEM), and FEI Tecnai G2 T20 Transmission Electron Microscope (TEM).

Mechanical properties
The hardness of the central horizontal line along the vertical welded joint thickness direction was tested at 0.1 mm intervals along the horizontal direction. The hardness test was conducted using an HVS-10 automatic Vickers hardness measuring instrument, with a loading pressure of 300 gf, and a holding time of 10 s. The tensile strength of the specimens was measured using a CMT5305 electronic universal testing machine. The thickness of the specimen was 1.5 mm, and the tensile specimen is prepared in accordance with ASTM E8-2016 after the weld surface is polished. The specimen size and sampling positions are shown in figure 2. The average values of tensile strength, yield strength, and elongation of each group are determined. The fracture morphology was observed by Zeiss SIGMA 300 field emission scanning electron microscope.

Results and discussion
3.1. Cross-sectional morphology Figure 3 shows the overall micro-morphology of the welded joints under different magnetic induction intensities. When no magnetic field is applied, the weld metal area is large, resembling an oval shape, the bottom of the weld has an obvious root hump defect, and the width of the HAZ is in the range of 1.4-1.6 mm. The weld metal area is quite small and resembles a conical shape when the magnetic induction intensity is 5 mT or 15 mT under identical heat input conditions (190 J mm −1 ). At the same time, the weld root hump defect disappears, the bottom shrinks, and the width of the HAZ decreases to about 0.7-1.4 mm and 0.5-0.8 mm, respectively. According to the literature [18,22], the current in the molten pool is mainly thermal current caused by temperature gradient. When the magnetic field acts on the thermal current in the molten pool, it produces thermoelectric magnetic force [23], driving the local flow of the molten pool and reducing the energy at the bottom of the molten pool [26,27]. In the equation (2), Thermoelectric magnetic force (F L ) satisfies the following conditions in a specific temperature and magnetic induction intensity range: where F L is the thermoelectric magnetic force, σ is the electrical conductivity of the molten pool, S is the thermoelectric power, DT is the temperature gradient, and B is the magnetic induction intensity [28,29]. When the welding heat input is constant, the stirring effect of the thermoelectric magnetic force F L gradually increases with B. When B is gradually increased, the cooling rate of the weld pool gradually increases, resulting in a greater degree of energy contraction at the center and bottom of the weld pool. Consequently, the metal area of the weld decreases further, causing a gradual change in the shape of the weld from 'oval' to 'conical'. This modification effectively eliminates the root hump defect at the bottom of the weld. Since the flow of the weld pool is improved and the laser energy is more concentrated after the magnetic field is applied [20], the cooling speed of the weld pool is accelerated, resulting in a significant reduction in the width of the HAZ of the welded joint. These findings are consistent with the results shown in figure 3. At the same time, the remarkable changes in macroscopic morphology directly reflect the changes in the solid phase transformation process of the weld metal.

Micro-morphology 3.2.1. Weld center
When the magnetic field is not applied, the required laser heat input (190 J mm −1 ) is large to ensure the complete melting of the filler material. This leads to a prolonged residence time of the weld center in the hightemperature zone, mainly facilitating ferrite and bainite transformations. According to OM and SEM observations, the weld center contains a large number of crisscrossed and dispersed polygonal ferrite [30,31], which serves as the matrix, and fine carbon-rich grains are distributed throughout the matrix. As shown in figure 4, fine carbon-rich grains are dispersed both at the grain boundaries of polygonal ferrite and in polygonal ferrite grains, forming a typical island-like granular bainite structure. These microstructures were also confirmed by TEM observation (figure 4(c)). The presence of these microstructures will significantly improve the plasticity of the weld. According to equation (2), when the magnetic induction strength is 5 mT and the heat input is the same (190 J mm −1 ), the thermoelectric magnetic force is relatively small and the electromagnetic stirring has a minimal impact on the center of the weld pool. As a result, the cooling rate of the weld center is slightly increased, the ferrite transformation in the high-temperature range is suppressed [32], and the solid phase transformation in the weld center is mainly bainite transformation. The carbon atoms in the strip ferrite diffuse to the phase interfaces on both sides [31] and precipitate between the ferrite strips, forming a mixture of a large number of strip ferrite and intermittent strip cementite sandwiched between them. OM, SEM, and TEM observations confirm the presence of a large amount of brittle and hard upper bainite, which exists in the typical 'feather' shape in the weld center [33,34], as shown in figure 5. The presence of a large number of upper bainite structures is undoubtedly detrimental to weld plasticity.
When the magnetic induction intensity is increased to 15 mT, the buildup of the magnetic field on the weld center is enhanced, allowing the weld pool to be fully subjected to electromagnetic stirring, the cooling rate of the weld center is accelerated , and the bainite and martensite transformations of the weld center are promoted. The microstructure of the weld center changed from 'feathery' to a large number of parallel lath bundles and block structures as observed by OM and SEM (figures 6(a) and (b)). The presence of a large number of martensite and There is a clear relationship between the magnetic induction intensity and the electromagnetic stirring force of the molten pool. The increased stirring force promotes flow within the molten pool, resulting in a faster solid phase transformation in the weld center. These factors can influence the microstructure and properties of laser wire-filled 22MnB5 welded joints.

Coarse grain region
During welding, the HAZ experiences rapid thermal cycles with high peak temperatures that give rise to austenite grain growth, especially in the coarse grain region near the fusion zone [35]. When the magnetic induction intensity is 0 mT, the coarse-grained region on one side of 22MnB5 steel exhibits an increased hardening tendency due to the severe grain growth and a high degree of austenite homogenization, resulting in the formation of martensite. The total grain size in this region becomes coarse, ranging from approximately 10 mm to 40 mm ( figure 7(a)), thereby contributing to greater hardness and strength. The addition of a magnetic field reduces the width of the HAZ of the welded joint due to the concentration of laser energy and the acceleration of the solid phase transformation. Through SEM observation and comparison, it has been established that the grain size of the coarse-grained region of the welded joints decreases significantly with the increasing magnetic induction intensity. When B = 5 mT, the grain size ranges from 8 mm to 18 mm ( figure 7(b)). However, when B = 15 mT, the grain size ranges from 5 mm to 15 mm ( figure 7(c)). This reduction in grain size further improves the mechanical properties of the coarse-grained region of the welded joints. Figure 8 shows the hardness distribution of three groups of welded joints. There are many ferrite and granular bainite structures in the weld center when the magnetic field is not applied, resulting in a low overall hardness in the welded joint. The average hardness of the weld metal is 231.37 HV. Because 22MnB5 steel can be easily hardened, it has a high hardening tendency in the HAZ and is easy to form a martensite structure. The hardness value is relatively high, with a maximum value of 353.11 HV, and the hardness value gradually decreases towards the base metal, with no softening phenomenon in this area.

Microhardness
When the magnetic fields increased to 5 mT and 15 mT, the area of electromagnetic stirring in the molten pool increased, and the area where bainite and martensite structures are formed in the weld center also enlarged.  The average hardness of the welds is gradually increased. In particular, when compared to the 0 mT condition, the average hardness in the weld is increased by 7.0% and 46.7% for the magnetic field of 5 mT and 15 mT, respectively. As the laser energy is more concentrated after adding the magnetic field, the width of the HAZ is shortened, the degree of grain coarsening is reduced, and the overall hardness value of this zone is increased, with the highest hardness values of the HAZ being 412.12 HV and 479.34 HV, respectively. In addition, the softening phenomenon was not observed. Hardness measurement and analysis show that the widths of weld and HAZ under different magnetic induction intensities are consistent with those measured in figure 3. This shows that the magnetic field can improve the overall hardness of the welded joints, which is critical for improving the comprehensive mechanical properties. Figure 9(a) shows the fracture position of three groups of welded joints and base metal after tensile testing. When the magnetic field is not applied, there are a large number of granular bainite and polygonal ferrite in the weld, resulting in good plasticity. The fracture position of the welded joint is at the base metal, and the elongation is high (32.4%), which is close to the elongation of the base metal (32.9%), as shown in figures 9 and (b).

Tensile properties
When B = 5 mT, the bainite transformation in the weld center is dominant due to the accelerated cooling rate, and there are a large number of brittle and hard upper bainite structures in the weld, which is not conducive to improving the plasticity of the weld, so the fracture positions are all at the weld center position, and the elongation is low (12.1%), as shown in figures 9(a) and (b). The yield strength of the welded joint is 409 MPa, but the tensile strength (451 MPa) of the welded joint is decreased, as shown in figure 9(c).
When B = 15 mT, the thermoelectric magnetic force can fully stir the molten pool and accelerate the martensite transformation due to the increased cooling rate, resulting in a large number of martensite and lower bainite structures in the weld center, which improves the mechanical properties of the weld and makes the fracture position at the base metal, as shown in figure 9(a). As shown in figures 9(b) and (c), the elongation reaches 30.0%, which is less than the elongation of the base metal (32.9%), and the yield strength (452 MPa) of the welded joint is significantly improved. Therefore, the addition of different magnetic induction intensities has a significant effect on the strength and elongation of the welded joints.

Fracture morphology
The fracture morphology of the tensile specimens is shown in figure 10. It can be observed that the fracture modes of the base metal and the 0 mT tensile specimen are plastic fractures, and there are a large number of deep dimples in the fracture, as shown in figures 10(a) and (b).
When B = 5 mT, the cooling rate of the weld is accelerated due to the existence of thermoelectric magnetic force, resulting in a high concentration of brittle and hard upper bainite in the weld center, reducing the plasticity of the weld. As shown in figure 10(c), the fracture positions of all specimens are at the center of the weld, dimples are shallow and their number in the fracture surface is reduced, with a small number of cleavage fracture morphology and relatively poor plasticity.
When B = 15 mT, martensite, and lower bainite structures are present in the center of the weld due to the relatively fast cooling rate, which improves the plasticity of the weld. As shown in figure 10(d), the fracture positions of all specimens are at the base metal, the number of deep dimples in the fracture surface is large, and there is no cleavage fracture morphology, indicating that the plasticity is relatively good. Hence, to effectively improve the comprehensive mechanical properties of the welded joints, it is necessary to select a suitable magnetic induction intensity, accelerate the martensite transformation process, and inhibit the formation of upper bainite in the weld.

Conclusion
The following major conclusions can be drawn from this research: (1) Without the application of a magnetic field, the shape of the weld is approximately 'oval', and the root hump defect of the weld bottom appears. The width of HAZ is in the range of 1.4-1.6 mm. There is a lot of granular bainite and polygonal ferrite in the weld center. The average hardness of the weld is 231.37 HV. The grain size of the coarse grain region is relatively large, ranging from approximately 10 mm to 40 mm. (2) When the magnetic field of 5 mT and 15 mT is applied under the same heat input conditions (190 J mm −1 ), the weld shape is approximately conical. The presence of thermoelectric magnetic force will drive the convection motion of the molten pool, shrink the energy at the bottom of the molten pool and increase the rate of the solid phase transformation in the weld. Hence, the width of the HAZ decreases to about 0.7-1.4 mm and 0.5-0.8 mm, respectively. Moreover, the root hump defect at the bottom of the weld disappears. When B = 5 mT, the solid phase transformation in the weld center is dominated by bainite transformation, resulting in a large number of brittle and hard upper bainite. The average hardness of the weld is increased by 7.0%, and the coarse grain size ranges from approximately 8 mm to 18 mm.
When B = 15 mT, martensite transformation, and bainite transformation predominate in the solid phase transformation in the weld center, and a large number of martensite and lower bainite with good mechanical properties are formed, resulting in the highest overall hardness in the welded joint. The average hardness of the weld is increased by 46.7%, and the coarse grain size ranges from approximately 5 mm to 15 mm.
(3) When the magnetic field is not applied, the tensile fracture position of the welded joint is at the base metal, which belongs to plastic fracture, the yield strength is 372 MPa, and the elongation is 32.4%. When B = 5 mT, the fracture position of the welded joint is at the weld center, and the yield strength is 409 MPa, but the elongation (12.1%), tensile strength (451 MPa), and plasticity are reduced when compared to those without magnetic field. When B = 15 mT, the fracture position of the welded joint is at the base metal, and the elongation (30.0%), yield strength (452 MPa) and plasticity are relatively increased.
(4) Various magnetic field parameters have distinct effects on the microstructure and properties of the welded joints. Comparatively speaking, under the same heat input condition, the addition of a 15 mT magnetic field can improve the comprehensive mechanical properties of the 22MnB5 steel laser wire-filled welded joint.
The parameters, technical terms, and abbreviations used in this paper are summarized in table A1. Scanning electron microscope SEM 10 Transmission electron microscope TEM 11 Heat affected zone HAZ 12 Weld metal WM 13 Base metal BM