Effects of magnetic field and post-weld heat treatment on microstructure and properties of laser welded joints of 22MnB5-TRIP590 steel

Laser welding without and with an auxiliary magnetic field (B = 30 mT) is performed on 22MnB5-TRIP590 steel. Subsequently, post-weld heat treatment, involving quenching at 800 °C followed by tempering at 200 °C, is carried out on the welded joints prepared by these two welding processes (B = 0 mT, B = 30 mT). This study examines the improvement of microstructures and mechanical properties of the welded joints by applying the magnetic field (B = 30 mT). Furthermore, this research investigates whether these enhanced characteristics of the welded joints persist after post-weld heat treatment. When the magnetic field is applied, the overall width of the welded joint is significantly reduced, and the microstructure of the weld is changed mainly from proeutectoid ferrite, granular bainite, and upper bainite to lath martensite and lower bainite. Furthermore, the grains in the coarse grain zone are refined, and the plasticity and overall hardness of the welded joint are considerably improved. After post-weld heat treatment, the weld and coarse grain zone of both welded joints (B = 0 mT, B = 30 mT) are mainly tempered martensite, and the tensile strength and overall hardness are significantly improved compared to those without post-weld heat treatment. Although the plasticity of two welded joints is lower than that of two unheated-treated welded joints, the fracture positions are all at the base metal and are both ductile fractures. The improved characteristics, such as optimized macro-morphology, refined grain morphology, improved plasticity, welded joint efficiency, and weld hardness of magnetic field-assisted laser welded joint, are all retained after post-weld heat treatment. A thorough comparison reveals that the magnetic field-assisted laser welded joint with post-weld heat treatment has better comprehensive mechanical properties.


Introduction
Nowadays, advanced high-strength automotive steels such as transformation-induced plasticity (TRIP) steels, dual-phase (DP) steels, and hot-stamped high-strength boron steels are widely used in laser welding of automotive bodies, which significantly reduce the weight of automotive structures [1][2][3].
In particular, TRIP steel is widely used due to its transformation-induced plastic effect, excellent tensile strength, and high work-hardening index [4].The microstructure and properties of laser-welded TRIP steel joints have previously been studied [5][6][7][8][9].For instance, P. VEC [10] found that the weld zone structure of the laser-welded joint of TRIP690T steel is martensite, lower bainite, and a small amount of upper bainite.Furthermore, the fine grain zone near TRIP690T steel is martensite, lower bainite, and ferrite.In the critical temperature range, the proportion of ferrite increases as the distance from the weld center increases.Khot [11] discovered that laser power and welding speed had a significant impact on the strength of the TRIP steel welded joint, with the strength of the welded joint increasing by 11% when the laser power was kept at 1800 W, and the welding speed was varied from 25 to 30 mm s −1 .Xia [12] learned that in the quasi-static and dynamic tensile tests after welding, the strength-toughness balance in the weld zone of silicon-containing TRIP steel was significantly lower than that of aluminum-containing TRIP steel.
The most common hot stamping high-strength boron steel is 22MnB5 steel.The strength is increased from about 600 MPa to over 1200 MPa after hot stamping, and the hardening capacity is strong, so the base metal structure easily forms a large number of martensites [13].Qin and Zhao et al [14,15] carried out laser welding on 22MnB5 steel and found that the weld was composed of martensite and δ-ferrite.The fully quenched zone in the heat-affected zone (Hereinafter referred to as HAZ) is found to be martensite, the partially quenched zone is composed of martensite+ferrite, and the weld microstructure is predominantly martensite.The lowest HAZ hardness with tempered martensite is approximately 250 HV.At present, most research is focused on the welding technology, microstructure, and properties of 22MnB5 steel after hot forming and surface coating [16,17], but little is known about the weldability and post-weld heat treatment technology (PWHT) of 22MnB5 steel without hot forming.
At the same time, researchers found that the faster welding speed and high power lead to the instability of the laser welding process, which affects the weld formation and, thus, the microstructure and properties of the weld [18].Magnetic field, as an externally added nonpolluting and convenient energy field, can interact with charged particles in the arc and liquid metal in the molten pool during welding.This interaction generates Lorentz force, which helps stir the welding molten pool, improve laser energy utilization efficiency, refine microstructure, and improve defects and mechanical properties of joints [19][20][21].
Currently, most of the research is devoted to the high-power laser welding technology, microstructures, and properties of welded joints of advanced high-strength automotive steels such as TRIP steel and 22MnB5 steel.However, little is known about the microstructure and properties of laser-welded joints of dissimilar advanced high-strength automotive steels.Furthermore, there is only limited research on the influence of adding magnetic fields and PWHT on their microstructure and properties.
TRIP steel and 22MnB5 steel are both used in the manufacturing of automotive structures [1,2,13,14], so research on the joining of these two dissimilar sheets of steel is critical.At the same time, it is equally important to investigate the low-power laser welding process from the perspective of production cost.
In this paper, the effects of adding magnetic field and PWHT on the microstructure and mechanical properties of the welded joints of automotive 22MnB5 steel and TRIP590 steel are investigated.In particular, whether these improved characteristics of welded joints with adding magnetic field remain after PWHT are studied.The objective is to optimize the welding process parameters, improve safety, and expand the application range of magnetic field-assisted laser welding technology.This study will help provide theoretical and practical references for lightweight automotive research.

Materials and methods
The materials used in this test are 22MnB5 steel and TRIP590 steel, both produced by China Shanghai Baosteel Company, with a thickness of 1 mm and a cold rolled delivery status.The microstructure of 22MnB5 steel is shown in figure 1(a), with ferrite as the matrix, and pearlite and martensite are distributed on it.As shown in figure 1(b), TRIP590 steel has a multi-phase mixed structure that includes ferrite, pearlite, retained austenite, and martensite.The dimensions of both test materials are 55 mm × 60 mm × 1 mm.The main chemical components and mechanical properties of the 22MnB5 steel and TRIP590 steel are given in tables 1 and 2, respectively.
The heat source for this laser welding test was an IPG YLS-1000W low-power fiber laser, and the shielding gas was Ar.Before welding, the surface of the test plate was polished with sandpaper, then the parts to be welded were cleaned with acetone, and the two sides of the test plate were fixed to the stainless steel workbench with stainless steel clamps.
A strong NdFeB permanent magnet with a surface magnetic induction intensity of 100 mT and a size of 55 mm × 20 mm × 6 mm was placed horizontally under the stainless steel fixture, and its longitudinal axis coincides with the center of the joint gap and its longitudinal length is flush with the ends of the test plate in the welding direction.
The magnetic induction intensity on the surface of the weld was changed by adjusting the vertical spacing between the permanent magnet and the bottom of the test plate.The magnetic induction intensity measured with a Gaussmeter (model HT20) on the upper surface of the test plate to be welded was 30 mT.Laser dissimilar metal welding was carried out on 22MnB5-TRIP590 steel along the rolling direction to investigate the influence of magnetic field on the microstructure and properties of dissimilar metal welded joints.The test procedure is shown in figure 2.
The composition-temperature curves of two types of base metals are simulated and analyzed using Jmat-pro software (figure 3).It is confirmed that the lowest heating temperature is roughly 800 °C to ensure quenching after austenitizing.Considering the thin thickness of the test plate and the production efficiency, a short PWHT process for the welded joint is adopted.The heat treatment equipment adopts muffle furnace with model SX2-4-10.The relevant literature [6][7][8][9][13][14][15] is consulted to formulate the welding process and PWHT parameters, as shown in table 3 and figure 4.
After welding, the metallographic specimen of the welded joint was prepared by wire cutting, and the welded joint was etched for 10 s in a 4% Nital etchant Solution.The microstructure of the welded joint was analyzed   with the help of 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).
The hardness of the center position of each welded joint was then tested at 0.1 mm intervals along the horizontal direction.The test equipment was an HVS-10 automatic Vickers hardness tester, the loading pressure was 300 gf, and the time to maintain pressure was 10 s.For each parameter, three groups of tensile specimens were prepared in accordance with ASTM E8-2016, with a thickness of 1 mm and specimen size as shown in figure 5.After the weld surface was polished, the tensile strength of 12 groups of tensile specimens was tested by a CMT5305 electronic universal testing machine, and the average values of tensile strength, yield strength, and elongation of each group were recorded after the test.The tensile fracture morphology of the welded joints was observed by SEM.

Results and discussion
3.1.Cross-sectional morphology Figure 6 shows cross-sections of four groups of welded joints and their macro-morphology.There are no obvious defects such as air holes, weld inclusions, and incomplete fusion in the four groups of welded joints, and the welds are symmetrical and well formed.
When B = 0 mT, the width of the upper and lower ends of the weld is approximately equal to 2.5 mm, and the width of the HAZ on both sides is in the range of 1.1-1.3mm.
When B = 30 mT, a thermal current is generated in the molten pool that flows from the front of the liquid phase zone to the solid-state zone.The magnetic field influences the thermal current during movement, resulting in thermoelectric magnetic force [22][23][24].This force drives the local flow of the molten pool, resulting in a noticeable shift in the flow direction at the center of the molten pool.As a result, energy contraction occurs in the lower part of the molten pool [25,26], making the width of the upper end and lower end of the weld about 1.7 mm and 0.8 mm, thus reducing them by 32.0% and 68.0%, respectively.Because of the thermoelectric magnetic stirring welding pool, the laser energy is more concentrated [26].The width of HAZ is reduced to 0.7-0.9mm, a reduction of 30.77%-36.36%.
Figure 6 shows the cross-sectional morphology of the two groups of welded joints after PWHT.It can be seen that the overall outline of the welds is nearly identical to that before PWHT, but the overall outline of the HAZs has disappeared, and their width cannot be seen clearly.

Microscopic morphology 3.2.1. Weld center
When B = 0 mT, the weld stays in the high-temperature zone for a long time due to the larger laser heat input.Moreover, ferrite and bainite transformations mainly occur.According to OM and SEM observations (figures 7(a) and (b)), the weld contains a large number of proeutectoid ferrite and some feathery upper bainite, with massive ferrite serving as the matrix and fine carbon-rich grains distributed on the matrix, forming a typical island-like granular bainite structure [27][28][29].These microstructures were also confirmed by TEM analysis, as shown in figures 7(c) and (d).Although there is a small amount of upper bainite structure, the presence of a large number of proeutectoid ferrite and granular bainite ensures the good plasticity of the weld.
When B = 30 mT, the molten pool is fully affected by electromagnetic stirring, which accelerates the cooling rate of the weld and promotes the bainite and martensite transformations of the weld.Figures 8(a       After tempering at 200 °C, the lath bundle of martensite has not changed significantly, and the lath martensite structure is still visible.This is because when low-carbon martensite is tempered at about 200 °C, the energy state of C atom segregation is lower than that of precipitated carbide, and C atoms are still concentrated near the crystal boundary and dislocation line [30], resulting in a weld of good mechanical properties.According to TEM observations and related literature [30,31], the microstructure of the two groups of welds mainly comprises tempered martensite, as shown in figures 9(c) and 10(c).
Comparing the two tempered martensite microstructures(figures 9 and 10) reveals differences in the aggregation state of carbon atoms due to the magnetic field.
When B = 0 mT, the high temperature of the welding pool exists for a long time.Hence, the grain morphology of the weld is mainly massive bulk grains, with relatively few grain boundaries and dislocation lines [27,29,30] after PWHT (figures 9(a) and (b)) , which is not conducive to the diffusion of carbon atoms in the weld.Therefore, the carbon atoms in tempered martensite of the weld (B = 0mT + PWHT) are concentrated near the relatively few crystal boundaries and dislocation lines, as shown in figure 9(c).
Contrary to the above analysis, the welding pool is stirred by thermoelectric magnetic force when B = 30 mT, shortening its high-temperature existence time.This shortened time and the electromagnetic stirring refine the weld grain [19][20][21], making its grain shape distribution relatively irregular (figures 10(a) and (b)) after PWHT, increasing the number of grain boundaries and dislocation lines and benefiting the dispersed distribution of carbon atoms in the weld.Therefore, the distribution of carbon atom aggregation in tempered martensite of the weld (B = 30mT + PWHT) is relatively dispersed, as shown in figure 10(c).Figures 7-10 reveal that the addition of a magnetic field will produce the stirring process of electromagnetic force in the molten pool, promote the flow of the molten pool, accelerate the cooling rate and solid phase transformation of the weld, and affect the microstructure of the weld.Although the PWHT ensures that the microstructure of the two welds under different magnetic induction intensities is consistent, the different grain morphology and distribution characteristics may affect the mechanical properties of the weld.

Coarse grain zone
Since the contents of alloying elements in 22MnB5 steel and TRIP590 steel are similar, their HAZ microstructure is also similar.
When there is no PWHT, the peak temperature in the HAZ of 22MnB5 steel and TRIP590 steel increased above the critical transformation temperature of Ac3, and the grains grew rapidly to form a coarse grain zone structure near the weld fusion boundary after complete austenitisation [8,9,15,32].
According to the Jmat-pro Simulation (figure 3) and [6-9, 13-15], the hardening tendency of 22MnB5 Steel is relatively high, while the hardening tendency of TRIP590 steel is relatively small.Hence, when B = 0 mT, massive martensite with relatively coarse grains is formed in the coarse grain zone due to the greater hardening tendency of 22MnB5 steel, and the martensite grains of TRIP590 steel formed in the coarse grain zone are relatively small.
As shown in figures 11(a) and (b), the bulk grain sizes in coarse grain zones on both sides of 22MnB5 steel and TRIP590 steel are in the range of 25-60 μm and 20-50 μm, respectively.
When B = 30 mT, the laser energy is more concentrated, which accelerates the cooling rate of the coarse grain zone and refines the grains.As shown in figures 11(c) and (d), the grain sizes in the coarse grain zones of 22MnB5 steel and TRIP590 steel are in the range of 15-45 μm and 15-40 μm, respectively.
When the two groups of welded joints are subjected to PWHT, massive tempered martensite occurs on both sides of the welded joints [30,31,33].When B = 0 mT, the coarse grain size of 22MnB5 steel and TRIP590 steel is in the range of 20-30 μm and 20-40 μm, respectively, as shown in figures 11(e) and (f).When B = 30 mT, the Although the PWHT causes the microstructure of the coarse grain zone of the two welded joints to be uniform, the characteristics of relatively refined grains in the coarse grain zone of the welded joint (B = 30 mT) retain, which improves the mechanical properties of the welded joint.

Microhardness
Figure 12 shows the hardness distribution of four groups of welded joints.When B = 0 mT, the hardness is low because the weld is mainly composed of a large number of ferrite and granular bainite structures, and the average hardness of the weld is 239 HV.According to the analysis in section 3.2.2, the highest HAZ value of 22MnB5 steel is 399 HV due to its high hardening tendency.In contrast, the highest HAZ value of TRIP590 steel is 374HV due to the low hardening tendency.The hardness values of both sides gradually decreased towards the base metal, and the softening phenomenon was not observed.
When B = 30 mT, the interior of the molten pool is subjected to electromagnetic stirring, and the area where bainite and martensite structures are formed in the weld gradually increases, resulting in an average hardness of 279 HV, which is 16.74% higher than that of the weld with B = 0 mT.The highest hardness of 22MnB5 steel and TRIP590 steel in HAZ increased to 422 HV and 399 HV, respectively.Because the laser energy is more concentrated after the magnetic field is applied, the width of the HAZ and degree of grain coarsening are reduced, and the overall hardness of the welded joint is increased.The softening phenomenon is not observed on either side of the welded joint.
After PWHT, the HAZ microstructure of the welded joints with B = 0 mT and B = 30 mT is mainly tempered martensite.Therefore, the tendency of hardening of HAZ on both sides of welded joints is alleviated, and the overall hardness distributions are relatively stable.According to the simulation results of Jmat-pro (figure 3) of 22MnB5 and TRIP590 and references [6-9, 13-15], the 22MnB5 base metal is mainly tempered martensite after PWHT (relatively high hardness and austenitization degree), while the TRIP590 base metal is mainly tempered martensite and ferrite (relatively low hardness and austenitization degree).Hence, the hardness distribution gradually decreases from the 22MnB5 steel side to the TRIP590 steel side.
According to the analysis in section 3.2.1, the weld is mainly composed of tempered martensite when B = 0 mT + PWHT; the average hardness value in the weld is increased to 303 HV.When B = 30 mT + PWHT, the grain morphology in the weld is refined and staggered due to the relative concentration of laser energy caused by the addition of a magnetic field, making the distribution of carbide in tempered martensite in the weld relatively dispersed (figure 10).Hence, the average hardness is increased to 313 HV.According to the analysis in section 3.2.2, the hardness of the coarse grain zone on both sides of the welded joint is relatively higher when B = 30mT + PWHT due to the smaller grain size.
Therefore, the overall hardness of the welded joint is slightly higher than the welded joint (B = 0 mT + PWHT).Hardness measurements and analysis show that the widths of the weld and HAZ are consistent with those shown in figure 6.
According to figure 12, adding a magnetic field can improve the overall hardness of a welded joint.At the same time, PWHT resulted in an overall increase in the hardness of both welded joints, contributing to a more stable hardness distribution.Notably, the characteristics of increased weld hardness of the welded joint with B = 30 mT + PWHT still retain, which is crucial for improving the mechanical properties of the welded joint.

Tensile properties
The tensile properties of welded joints were tested, and the results are shown in figure 13.When B = 0 mT, the weld contains a large number of proeutectoid ferrite and granular bainite, both of which have good plasticity.The fracture positions of the welded joints are all at the base metal (22MnB5 steel), and the elongation is high, with an average value of 18.13%.
When B = 30 mT, the thermoelectric magnetic force that fully stirs the molten pool helps the laser energy concentration, which accelerates the bainite and martensite transformation.As a result, the weld exhibits a large number of martensite and lower bainite structures, and the grains in the coarse grain zone are obviously refined.This enhancement improves the plasticity of the welded joint, resulting in the fracture position at the base metal of 22MnB5 steel, and the average elongation of the welded joint reaches 26.08%, showing an improvement of 43.85%.However, the tensile strength has not improved significantly, and its average value is 523.67 MPa and 525.33 MPa for B = 0 mT and B = 30 mT, respectively.
The residual stress caused by quenching was eliminated by tempering, and the microstructure uniformity was improved.There were a lot of tempered martensite microstructures in the welded joints, which significantly improved their tensile strength, with average values of 707.33 MPa and 728.33 MPa for B = 0 mT and B = 30 mT, respectively.The fracture positions of both welded joints were at the base metal (TRIP590 steel), but their average elongation was decreased, as shown in figures 13(a) and (c).
The abovementioned phenomenon can be attributed to the microstructure of the base metal of the welded joint after PWHT.According to the analysis in sections 3.2 and 3.3, compared with the welded joint without PWHT, the microstructure of the TRIP590 side of the base metal has many tempering martensite + ferrite grains, which not only improves the strength of the welded joint, but also reduces the plasticity, and their strength was lower than the 22MnB5 base metal which only has tempering martensite grains.Therefore, the fracture positions are all on one side of the TRIP590 base metal.
Hence, for B = 0 mT + PWHT, the average elongation of the welded joint is 6.95%.In contrast, when B = 30 mT+ PWHT, the analysis in section 3.2 reveals that a higher average elongation of 10.99% is achieved due to the grain structure of the coarse grain zone being refined and the dispersed distribution of carbon atoms in welds.
Figure 13(c) also compares the change in welded joint efficiency after adding the magnetic field.The welded joint efficiency is very important to reflect the degree of strength weakening of the welded joint, it is also a comprehensive reflection of the mechanical properties of welded joints.The higher the efficiency of the welded joint, the better its comprehensive mechanical properties.
Equation (1) is used to express the calculation of the welded joint efficiency: where σ W is the tensile strength of the welded joint, and σ M is the tensile strength of the base metal.
Before PWHT, the fracture position is always at the base metal of 22MnB5.Hence, σ M is consistent regardless of the magnetic field.Therefore, a comparison of the average value σ W of strength obtained from its tensile test reveals that Φ (B = 0 mT) < Φ (B = 30 mT).When PWHT, σ M is consistent because the fracture position is all at the base metal of TRIP590 regardless of the magnetic field.Therefore, a comparison of their average value of strength σ W confirms that Φ (B = 0 mT + PWHT) < Φ (B = 30 mT + PWHT).
The comparison revealed that the tensile strength of welded joints with PWHT is significantly improved compared to those without PWHT.Meanwhile, the characteristic of improved elongation and welded joint efficiency of welded joints assisted by the magnetic field is also retained after PWHT.Therefore, for B = 30 mT + PWHT, and the comprehensive mechanical properties of a welded joint are improved.

Fracture morphology
When B = 0 mT, the SEM observations show that there are a large number of dimples at the fracture surface, indicating that the fracture is a micropore aggregation fracture.The larger the average diameter of the dimples and the greater the depth, the better the plasticity of the steel [30,31].It can be concluded that the welded joint has good plasticity due to many lager dimples.
By comparing the dimple characteristics at the fracture surface shown in figures 14(a) and (b), it is clear that the average diameter and depth of the dimple of the welded joint with B = 30 mT are relatively large, and thus, the plasticity is relatively higher, which is consistent with the numerical calculation of the elongation shown in figure 13.A comparison of fracture morphology further verifies that the plasticity of welded joints is improved after adding the magnetic field and obtaining better comprehensive mechanical properties.
After PWHT, i.e., when B = 0 mT and B = 30 mT, the microstructure of the base metal of TRIP590 steel is tempered martensite and ferrite, decreasing the average diameter and depth of the dimples compared to the tensile fracture of the welded joint without PWHT.However, the fracture morphology shows the characteristics of plastic fracture due to ferrite structures, as shown in figures 14(c) and (d).
Figures 14(c) and (d) show that the average diameter and depth of dimples (B = 30 mT + PWHT) in the fracture are relatively higher than the welded joint (B = 0 mT + PWHT) due to the grain structure of the welded joint being refined and the dispersed distribution of carbon atoms in welds.A comparison of fracture morphology reveals that the plasticity of the welded joint (B = 30 mT + PWHT) is relatively higher, which is also consistent with the elongation analyses elongation in figure 13.
The above analysis shows that the phenomenon of plastic improvement of welded joints (B = 30 mT + PWHT) remains, further confirming the previous analysis.

Conclusion
Four groups of laser welding process (B = 0 mT, B = 30 mT, B = 0 mT + PWHT, and B = 30 mT+ PWHT) tests were conducted on a 22MnB5-TRIP590 steel joint, and the following major conclusions can be drawn: (1) When B = 0 mT, the width of the laser welded 22MnB5-TRIP590 steel joint was relatively wide.The solid phase transformation process in the weld center was mainly ferrite transformation and bainite transformation due to the long high-temperature time of the welding pool, leading to lower hardness, higher elongation, and better plasticity of welded joints.The grain size of the coarse grain zone on both sides was relatively large.(2) When B = 30 mT, the continuous stirring of thermoelectric magnetic force helps concentrate the laser energy, accelerating the cooling rate of the molten pool.Hence, the width of the weld and the HAZ, as well as the grain size of the coarse grain zone on both sides, are significantly reduced.The solid phase transformation process in the weld center is mainly bainite and martensite transformation.The average hardness, plasticity, elongation and efficiency of the welded joints is remarkably increased.The comprehensive mechanical properties are relatively good.
(3) After the same PWHT (quenching at 800 °C+tempering at 200 °C), the microstructure of welded joints from two welding processes (B = 0 mT and B = 30 mT) was mainly tempered martensite.Although the average elongation of welded joints is reduced, both welded joints have ductile fracture morphologies.The hardness and tensile strength of welded joints are significantly improved and the grain size of the coarse grain zone is reduced compared with those without PWHT.
(4) The improved properties of optimized macro-morphology, refined grain morphology, improved plasticity, welded joint efficiency, and hardness of magnetic field-assisted laser welded joint (B = 30mT) are all retained after PWHT.Comparing the mechanical properties of four groups of welded joints revealed that the 30 mT + PWHT welded joint has the best comprehensive mechanical properties.
) and (b) show
a large number of parallel lath bundles in the microstructure of the weld center as observed using OM and SEM.As shown in figures 8(c) and (d), TEM observation confirmed that there are a large number of lower bainite and lath martensite structures in the weld[27,29], respectively.Figures9 and 10show the micro-morphology of welded joints of two groups quenched with the same process parameters and tempered at low temperatures.The microstructure of the weld center of the two groups

Figure 5 .
Figure 5. Schematic diagram of the tensile specimen.

Table 3 .
Welding parameters of the welded joints.