Enhanced strength-ductility synergy of bimetallic laminated steel structure of 304 stainless steel and low-carbon steel fabricated by wire and arc additive manufacturing

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Introduction
Bimetallic materials have attracted extensive attention in recent years because they break through the limitations of single-material properties and provide a more diverse combination of properties [1].Researchers have attempted to join different dissimilar metal structures, including aluminum, stainless steel, titanium alloys, magnesium alloys, copper alloys and nickel-based superalloys [2], which are widely needed in a variety of industries including aerospace, shipbuilding and automobiles.To date, several methods for fabricating bimetallic materials have been successfully demonstrated, such as arc welding [3], powder metallurgy [4], functionally graded coatings [5], chemical vapor deposition [6], and laser-based techniques [7].Compared with these methods, direct energy deposition (DED) directly manufactures large components at lower cost, which is widely applicable to the production of workpieces with higher flexibility and superior mechanical properties.
Wire and arc additive manufacturing (WAAM) is one of the DED methods, which uses the arc as the heat source to melt the welding wire from bottom to top, and deposits it layer by layer to achieve free forming [8,9].Due to its low cost and high productivity, WAAM has been widely used in aerospace, automobile, construction and other fields [10].Commonly used WAAM methods include gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) and plasma arc welding (PAW) processes [11].WAAM has been successfully used to manufacture alloy components such as aluminum, nickel, titanium and steel [12][13][14][15].Rodrigues et al. [16] investigated in-situ strengthening of high-strength low-alloy steel (HSLA) fabricated by WAAM, where SiC particles were added to the molten pool to refine grains for strengthening.Spalek et al. [17] studied the properties of WAAM fabricated laminated materials composites (LMC, alternately deposited ductile steel (SG2) and high-strength steel (X90)) under static tensile, impact and high cyclic fatigue loads.Under static tensile loading, the engineering yield strength and strain obeyed the mixing rule.WAAM fabricated LMC was resistant to the crack propagation under impact loading, especially at the interface between ductile and high-strength steel layers, where crack passivation, deviation and delamination were identified.Under high cyclic fatigue load, this LMC outperformed homogeneous steel produced by WAAM.Sun et al. [18] studied the microstructure and mechanical properties of low-carbon high-strength steel prepared by WAAM, and analyzed the difference in Taylor factor between the interlayer and the deposition region by electron backscatter diffraction (EBSD).It was found that nonuniform deformation and local stress concentration occurred in the interlayer area.
However, until now, the research on the fabrication of heterogeneous materials by WAAM is still limited.Ahsan et al. [19] studied the microstructure and mechanical behavior of austenitic stainless steel and Inconel 625 bimetallic structure, and found that WAAM has the potential to prepare bimetallic structure with controllable properties.Shen et al. [20] prepared a Ti6Al4V/Al6.21Cubimetallic structure with the addition of Nb interlayer by cold metal transition additive manufacturing, and found that adding Nb interlayer could significantly improve the mechanical properties of the Ti6Al4V/Al6.21Cubimetallic structure.Singh et al. [21] investigated the microstructure and mechanical properties of NiTi/stainless steel bimetallic structures built using WAAM.They found that there were no defects at the interface and the as-prepared bimetallic structures had good mechanical properties.Most of these studies are in the "A + B" mode and require materials or properties for specific occasions.There are few studies on two common materials, stainless steel (SS) and low carbon steel (LCS).Ahsan et al. [3,10] studied the effect of heat treatment on the microstructure and mechanical properties of LCS and 316L SS bimetallic materials manufactured by WAAM, but still in the "A + B" mode.
In this paper, it is proposed that WAAM can be used to alternately deposit LCS and 304 SS in the "LCS + 304 SS + LCS + 304 SS …" mode to fabricate bimetallic laminated steel structure (BLSS).The microstructure and mechanical properties of the BLSS were studied by SEM, EBSD, EDS and DIC.The synergistic effect of high strength and high ductility can be obtained for this BLSS, which provides a new idea for the preparation of laminated materials with excellent properties by WAAM.

Materials and preparation
The bimetallic laminated steel structure (BLSS) was fabricated by WAAM using 304 stainless steel (304 SS) and ER70S-6 low carbon steel (LCS) welding consumable wires with diameters of 1.2 mm on the Q345 substrate plate (20 mm thick).The chemical composition of the base materials is listed in Table 1.
Before the deposition process, the substrate was cleaned with an angle grinder and steel brush to remove impurities from the surface, followed by acetone cleaning.During the WAAM process, 304 SS and LCS wires were alternately fed to deposit the BLSS with the bottom layer of LCS, as shown in Fig. 1a.The deposition was carried out in the opposite direction to the previous layer to ensure that the previously built wall did not collapse.The WAAM process was conducted by the Panasonic YA-1VAR61CJ0 industrial robot.The welding current of LCS and 304 SS is 100 A and 143 A, respectively.The voltage is automatically matched according to the current, and the welding speed is 5 mm/ min.The 304 SS layer was protected by carbon dioxide gas, and the LCS layer was shielded with pure argon gas with the flow rate of 20 L/min.The obtained thickness of each layer was about 2 mm.For comparison, both 304 SS and LCS wires were deposited individually to fabricate single-material thin-walled structures through WAAM.
As shown in Fig. 1b, it can be observed that the height of the deposited BLSS sample is uneven.Due to the manual arc starting and arc extinguishing operations during the welding process, the arc dwell time on both sides cannot be accurately controlled.The arc dwell time on the left was longer due to manual operation error, which caused the deposited part to melt again and collapse under the action of gravity, resulting in inconsistent heights on both sides of the prepared wall structure.Therefore, for the preparation of all subsequent microstructure and mechanical properties test specimens, the middle area of the deposited wall was selected to minimize the impact on the test results.

Microstructure characterization
The metallographic specimens were cut using wire electrical discharge machining (EDM) and ground with silicon carbide sandpaper from 120# to 2000#, followed by polishing with a polishing machine.Due to the different microstructure of the LCS and 304 SS layers, two different etchants were used to etch the corresponding layers.The LCS layers were first etched with 4% nitric acid alcohol for 3-5 s, and then the sample was rinsed with alcohol.Next, the 304 SS layers were etched using 50% aqua regia for 25-30 s until the surface was darkened.The metallographic structures were observed by an optical microscope (OM, Olympus GX51).Electron backscattered diffraction (EBSD, FEI QUANTA FEG450) analysis was performed to further characterize the microstructure of the samples after electrolytic polishing.Semi-quantitative analysis of the elemental distribution between the 304 SS and LCS layers was measured by energy-dispersive spectrum (EDS, Genesis XM2, EDAX Inc.) along the X-Z plane.

Mechanical testing
Microhardness tests were performed across the fusion line of the 304 SS and LCS layers using a Vickers hardness tester (HUAYIN 1000A) with a load of 1000 g and a dwell time of 15 s.The interval between two consecutive indentations was 0.5 mm.Tensile specimens along different loading directions presented in Fig. 1c were prepared by EDM, and the detailed dimensions are shown in Fig. 1d.Tensile tests were then carried out through an electronic universal testing machine (WDGDW-100, Changchun New Testing machine Co., LTD) with a loading rate of 0.5 mm/min at room temperature.For each condition, three samples were tested to guarantee the reliability of the data.During the tensile process, digital image correlation (DIC) assessment was also applied to capture images of the gauge length till the final fracture.The results were then analyzed by CSI-DIC software to obtain the strain distribution of the specimen during tensile deformation.The fracture morphology was observed by scanning electron microscope (SEM) to reveal the fracture mechanism.

Microstructure
Optical micrographs of the as-deposited BLSS and single-material 304 SS and LCS structures are presented in Fig. 2. For the single-304 SS region in Fig. 2a, vermicular ferrite is distributed in the austenite matrix, and the columnar grains grow continuously along the building direction due to the maximum temperature gradient.This grain growth mode has also been observed in other AM processes of titanium alloy [22].In contrast, the microstructure of the 304 SS layer in the BLSS (Fig. 2b) shows many lamellar clusters with different orientations of flake grains, suggesting the formation of a martensitic structure.The formation of martensite in the 304 SS layer of the BLSS is likely due to the interaction of the two raw materials.To be specific, dilution occurred during the WAAM process, and both the equivalent Ni and Cr contents decreased in the 304 SS layer due to the corresponding low contents in the LCS material, therefore, resulting in the formation of martensite according to the Shaeffler diagram [23].Similar   microstructures have also been observed in laser autogenous welded LCS and austenitic stainless steel dissimilar joint [24].The single-material LCS structure (Fig. 2c) shows an obvious blocky ferritic structure.In the LCS layer of the BLSS, the blocky ferritic structure still exists, and an uneven distribution of fine grains can also be observed, as shown in Fig. 2d.As a result, the overall average grain size of the LCS layer is smaller than that of the single-material LCS structure.
EBSD analysis was performed to further study the microstructure of the additive manufactured BLSS.The results of the LCS layer, 304 SS layer and LCS/304 SS interface are shown in Figs.3-5, respectively.As shown in Fig. 3a, the phase map of the LCS layer shows a single BCC structure, confirming the formation of ferrite during cooling.This result is consistent with the optical micrograph.The inverse pole figure (IPF) in Fig. 3b also verifies that in the LCS layer, there are fine grains distributed between the coarse grains, mainly at the coarse grain boundaries [25].Grain orientations are randomly distributed, with no obvious preferred orientation or strong texture.The Kernel Average Misorientation (KAM) shown in Fig. 3c quantifies the average misorientation around a kernel point with respect to the next-nearest neighbors [26], which reflects the degree of lattice mismatches and the density of dislocations [27].The degree of local misorientation in the additive manufactured LCS layer is unevenly distributed, and the green regions with higher lattice mismatch and dislocation density are mainly distributed at low-angle grain boundaries (LAGB, less than 10 • ).Fig. 3d shows the statistical misorientation angle distribution in this region, demonstrating the existence of a large number of LAGB, especially those with a misorientation angle less than 5 • (~27%).
For the 304 SS layer of the BLSS, the phase structure is also pure BCC, as shown in Fig. 4a, which is confirmed to be the martensite structure.A large number of flake clusters with different orientations are observed from the IPF in Fig. 4b, consistent with the optical micrograph.A higher and more uniform KAM distribution is observed in the 304 SS layer, as shown in Fig. 4c, corresponding to a higher lattice mismatch and dislocation density, which is beneficial for enhanced hardness and strength.Fig. 4d shows the misorientation angle distribution in this region, with the fraction of LAGB reaching ~50%, which is consistent with the increased dislocation density.
EBSD results of the LCS/304 SS interface of the BLSS are presented in Fig. 5.As expected, a single BCC structure is obtained (Fig. 5a).Compared to the LCS and 304 SS layers, the IPF in Fig. 5b shows a much finer grain distribution.This may be because, on the one hand, during the WAAM process, when two different materials are successively deposited, the continuous growth of grains is inhibited to a certain extent due to the differences in the physicochemical properties of the materials themselves.On the other hand, due to the time interval between the two consecutive layers, the temperature gradient between the newly deposited layer and the previous layer is large, which increases the cooling rate and suppresses the growth of grains in turn.The distribution of KAM is between the LCS layer and the 304 SS layer (Fig. 5c), but higher high-angle grain boundaries (HAGB, higher than 10 • ) are detected in this region, as shown in Fig. 5d.This is due to the presence of more fine grains leading to the increase in grain boundaries, which to some extent increases the dislocation density and results in an increased KAM compared to the LCS layer.
This work achieves alternate deposition of 304 SS and LCS by WAAM, which differs from the previously reported bimetallic structure with only one bimetallic interface [3].Since each two consecutive deposited layers undergo different thermal cycles and corresponding metal dilution and diffusion processes, the obtained microstructures are also different.For example, the microstructure of the 304 SS layer in the current BLSS is martensite rather than δ-ferrite embedded in the austenite matrix observed in the bimetallic structure [3].Compared with the low-carbon high-strength steel fabricated by WAAM [18], where the microstructure consisted of equiaxed, columnar, and inter-layer zones, the current BLSS shows uniform microstructure distribution within each single layer with no significant differences in phase structure.

Elements distribution
The distribution of alloying elements in the WAAM built BLSS is not uniform, especially near the LCS/304 SS interface.EDS analysis across the interface was conducted, and the results are shown in Fig. 6.Fig. 6a shows the SEM image of the interface with the marked location for EDS line scan (marked with red line), and the line scan result is presented in Fig. 6b.Significant changes in elemental composition can be observed on both sides of the interface.That is, from the LCS side to the 304 SS side, Fe shows a gradually decreasing trend, indicating that Fe diffused from the LCS layer to the 304 SS layer, while Cr and Ni show a gradually increasing trend, indicating that both diffused from the 304 SS layer to the LCS layer, which could compensate for the softening effect caused by decarburization in the LCS layer [28], and thereby enhance the interface strength.The lower ratio of Cr and Ni in the 304 SS layer would cause the CCT curve to shift to the right, leading to the decreased martensite transition line and free energy of the system, thus providing sufficient driving force for the martensite phase transformation [23].The Mn content across the interface is basically unchanged, which may be due to the relatively low Mn contents in both raw materials.The width of the diffusion layer at the interface is about 30 μm, and the distribution of the diffusion layer is smooth, indicating that there is no macroscopic  intermetallic phase, which is related to the rapid cooling rate of the WAAM process.Elemental mapping across the interface for different elements is presented in Fig. 6c, demonstrating good agreement with the line scan results.
It is mentioned in the microstructure section that there are fine grains distributed unevenly in the LCS layer and interface (Figs. 3b and 5b), which may also be associated with the elemental diffusion and distribution.According to the molten metal solidification theory [29], the nucleation rate of metal solidification increases when the relative content of alloying elements fluctuates.As shown in Fig. 6, the relative contents of alloying elements in the BLSS fluctuate greatly, especially in the region close to the LCS/304 SS interface.The nucleation rate of the molten metal near this region during cooling is therefore greater than that at other regions.That is, as the nucleation begins, there will be more nucleation sites in the interface.When the grains start to grow, the adjacent grains quickly grow to contact the grain boundaries, after which the grain growth process stops, resulting in the formation of a large number of fine grains in these regions.Fig. 5b shows that a large number of fine grains are distributed at the BLSS interface, and a high HAGB fraction is observed in Fig. 5d, confirming the formation of more nucleation sites during solidification.Even within the same layer, the distribution of alloying elements is not completely uniform, such as the presence of an inhomogeneous distribution of fine grains near the grain boundaries in the LCS layer (Fig. 3b), which is closely related to the elemental diffusion [25].

Mechanical properties
Microhardness tests were carried out on the single-material LCS/304 SS structure and BLSS prepared by WAAM.Fig. 7a shows the test area for hardness, here the BLSS is taken as an example, where the dark layer is 304 SS and the bright layer is LCS.The hardness test line is spread across several dark and bright layers to ensure the accuracy of the hardness data.Fig. 7b shows the hardness values in the XOZ plane for the three samples.For the single-material 304 SS and LCS structure, the hardness value fluctuates less between the interface of different layers, which is related to the relatively uniform grain structure and distribution.In contrast, the hardness of the BLSS material varies greatly between different layers due to different microstructures.As shown in Fig. 7b, the short black vertical line represents the interface between the LCS/304 SS layer.It can be seen that the hardness of the 304 SS layer is significantly higher than that of the LCS layer.Although the lattice types of both regions are BCC structures, the martensitic hardness in the 304 SS layer is higher than that of the ferrite in the LCS layer.In addition, the hardness of the BLSS material is much higher than that of the singlematerial structure, which is due to the interdiffusion of alloying elements in the LCS and 304 SS layers during the WAAM process, resulting in the changes of the microstructure.Among them, the 304 SS layer evolves from austenite and ferrite structure to martensite with higher  hardness, and the ferrite grains in the LCS layer are smaller than that in the single-material LCS structure.According to the Hall-Petch relationship, the hardness of ferrite increases with decreasing grain size.Therefore, the different layers of the BLSS all show significantly enhanced hardness.
Compared to the bimetallic structure fabricated from similar raw materials (SS and LCS) [3], the average hardness of the current BLSS is higher, with a minimum value above 300 HV.In Ahsan et al.' work [3], the authors also studied the effect of different heat treatment conditions on the hardness of SS316L + LCS bimetallic structures.The results show that the as-deposited and 800 • C treated samples have their highest hardness right at the interface, which is consistent with the current study.They pointed out that in addition the diffusion of carbon element, lattice mismatch at the interface could be another reason for the higher hardness.Compared with the high-strength steels manufactured by the same WAAM process [16,18], the hardness is similar and both are significantly higher than the two raw materials used in this study.Therefore, it can be concluded that the BLSS obtained in this experiment is comparable to the high-strength steel in terms of hardness.
Tensile tests of the BLSS and single-LCS/304 SS along two directions, namely the transversal direction (Z) and the longitudinal direction (Y), were performed to study the mechanical properties.The engineering stress-strain curves and corresponding mechanical properties are plotted in Fig. 8. Tensile properties vary greatly due to different microstructures.For the WAAM deposited single-304 SS samples, the tensile strength in the Y direction (534.2MPa) is close to that of conventionally fabricated 304 SS (520 MPa) [30], and the elongation (34.6%) decreases slightly.While the Z-direction properties show a slight decrease in both strength (455.8MPa) and ductility (32.6%).In contrast, the mechanical properties of the single-LCS samples along different loading directions show larger differences.The tensile strength in the Y direction (457.2MPa) is close to that of the raw LCS wire (ER70S-6, 450 MPa, provided by the wire company), while the performance in the Z direction is much lower.It can be noticed that the tensile strength and elongation along the Z direction of these two single-material structures are lower than those of the base metals, which may be due to the presence of undetected microscopic welding defects between consecutive layers, which affects the performance of the additive manufactured materials.
For the BLSS, it can be seen from Fig. 8 that the tensile properties are significantly improved compared to the single-material structures.The tensile strength in the Y direction (999.8MPa) is 173% of 304 SS and 184% of LCS, and the elongation (28.2%) is between two single-material structures.The tensile strength in the Z direction (905.9MPa) is slightly reduced compared to that in the Y direction, which is 154% of 304 SS and 163% of LCS, respectively, with the elongation (18.9%) in between.As mentioned above, the metal solidification nucleation rate increases due to the elemental fluctuation between different layers, and the formation of fine grains at both the LCS layer and the interface plays an important role in improving the material strength.Meanwhile, the martensite formed at the 304 SS layer also significantly enhances the mechanical properties.To sum up, the tensile properties of the BLSS material are much greater than those of the single-material structures.Compared with additively manufactured duplex stainless steels [31], this BLSS has similar tensile strength but much higher elongation.It also has higher strength than additively manufactured high-strength steels, despite a slight decrease in elongation [16].Compared with the single-interface bimetallic structure of LCS and SS3016L prepared by WAAM [3], the strength of the current BLSS material is about three times that of its as-deposited sample, and even higher than that of the samples after different heat treatment conditions.This is closely related to the good metallurgical bonding and elemental diffusion between the two dissimilar materials.The bimetallic laminated structure prepared in this study enables the interaction of the two materials at the interface to form multiple interfaces with fine-grained structure, which lays the foundation for high yield strength and high fracture strength.
During the tensile test, in order to better observe the deformation process of the BLSS specimens, DIC equipment was used to record the tensile process, and then the local strain was calculated.The results along the Z and Y directions are shown in Fig. 9a and b, respectively.The gauge length between the two black lines was measured as 10 mm before the tensile test.P1~P8 were taken from different stages of the tensile process, as shown in Fig. 8. From Fig. 9, it can be observed that the maximum local strain is 20.3% in the Z direction and 56.5% in the Y direction, respectively.Compared with the Y direction, the strain concentration is more obvious in the Z direction.In Fig. 9a, the region of maximum strain transitions from P4 to P8, where the stress reaches its maximum value.As shown in Fig. 9b, in the Y direction, points P1~P3 are in the elastic deformation stage, and the strain distribution is uniform.After entering the plastic deformation stage, as indicated by points P4 and P5, the strain increases gradually, but the overall distribution is relatively uniform.As the stress further increases, the strain concentration gradually becomes apparent, and there is a pronounced necking before the final fracture, as shown by point P8.
In the BLSS material, martensite with high strength and poor toughness is formed in the 304 SS layer, and ferrite with finer grains is formed in the LCS layer.EBSD and SEM observations show that there are no obvious defects or intermetallic compounds at the interface.The combined effect of these three aspects makes the strength of the BLSS material much higher than that of the two single-material structures.Considering the different corrosion resistance of LCS and 304 SS, the surface of the LCS layer of the tensile fracture sample was etched with 4% nitric acid alcohol for 3-5 s to determine the fracture location.According to the macroscopic morphology of the fracture shown in Fig. 10, it is found that the length of the Y-direction fracture along the longitudinal direction is greater than 2 mm, and the fracture spans two different layers, resulting in excellent elongation.While the fracture in the Z direction is smooth and located in the LCS layer where the fine grains are not uniformly distributed, the elongation is therefore between the two single-material structures.
In order to further understand the fracture mechanism of the BLSS materials along different loading directions, the fracture morphology of the tensile specimens was observed by SEM, as shown in Fig. 11.A large number of dimples are observed on the fracture surfaces of both singlematerial 304 SS and LCS structures (Fig. 11a-d), indicating that both materials undergo plastic fracture in the Y and Z tensile directions.For the BLSS materials, rock candy-like fractures and some shallow dimples are observed on the fracture surfaces in both directions, revealing a mixed mode of ductile and brittle fracture.It is also observed that the grain size at the fracture in the Y direction is smaller than that in the Z direction, which could also explain the higher strength in the Y

Conclusions
A bimetallic laminated steel structure of 304 stainless steel and lowcarbon steel was successfully fabricated using wire and arc additive manufacturing.Microstructural characterization and mechanical testing results indicate good metallurgical bonding and enhanced mechanical properties.The main conclusions are summarized as follows: (1) Both the 304 SS layer and the LCS layer in the BLSS material exhibit microstructures different from that of the single-material structure, forming martensite and ferrite with finer grain size, respectively.(2) The fine grains at the LCS/304 SS interface are more obvious, and the grain orientations are randomly distributed, which directly affects the mechanical properties of the interface.Z-direction fracture occurs in the LCS layer, demonstrating good metallurgical bonding at the interface and its superior mechanical properties.

Fig. 1 .
Fig. 1.(a) Schematic diagram of the WAAM experimental setup; (b) As-deposited BLSS of 304 SS and LCS; (c) Visualization of the extraction locations of metallographic and mechanical test specimens; (d) Dimensions of tensile specimens.
Y. Chen et al.

Fig. 6 .
Fig. 6.(a) SEM image of the LCS/304 SS interface with the marked location for EDS line scan and (b) EDS line scan results, (c) elemental mapping across the interface.

Fig. 11 .
Fig. 11.SEM images of fractures: (a) Y direction and (b) Z direction of single-304 SS structure; (c) Y direction and (d) Z direction of single-LCS structure; (e) Y direction and (f) Z direction of the BLSS material.
Y. Chen et al.