Effect of multi-walled structure on microstructure and mechanical properties of 1.25Cr-1.0Mo steel fabricated by GMAW-based WAAM using metal-cored wire

Wire-arc additive manufacturing (WAAM) offers multiple beneﬁts, such as high metal deposition,lowcapitalcost,suitablemechanicalproperties,andreasonablecosts.Inthepresentwork, Gas metal arc welding (GMAW) based WAAM was employed to manufacture a multi-walled component of 1.25Cr-0.5Mo at optimized parameters using metal-cored wire. The fabricated multi-walled structure was observed with seamless fusion and free from disbonding. The fabricated multi-walled component was studied through microstructure investigations, mechanical properties such as microhardness (MH), tensile test, and impact test at various positions (top side, middle side, and bottom side) of the built structure. Microstructure results have shown a tempered martensite structure in the bottom zone with coarse grains and ﬁner mi-crostructuresinthemiddleandtopzones.MHvaluesthroughoutthecomponentwereuniform and thus indicated a similar nature to the multi-walled component. A comparison of tensile propertieswas carried out among the results of metal-cored wires and multi-walled structures to check the internal eminence of the obtained component. For all sides of the multi-walled structure, all the tensile properties were found to be in the range values of 1.25Cr-0.5Mo metal-cored wire. The results of all three conditions for impact toughness showed far better strength than the requirement. Fracture surface morphologies of tensile and impact test parts showedthepresenceoflargedimpleswiththehomogenousdistribution.Thus,alltheobtained results have suggested the suitability of the GMAWAM process for the fabrication of a multi-walled structure of 1.25Cr-0.5Mo metal-cored wire for various industrial applications.


Introduction
Additive manufacturing (AM) is a widely chosen method for developing intricate shape components by broadly intensifying manufacturing competence and flexibility [1].Recently, essential engineering materials like aluminum, titanium, and steel can be transformed into fully dense parts with the help of AM and obtain optimum properties [2].AM has recently become an excellent alternative for the layer-by-layer building of parts [3].AM techniques were divided into three major groups: sheet lamination, powder bed fusion, and direct energy deposition (DED) [4,5].The DED process used in most past studies contains wire-arc AM (WAAM) with wire feedstock as the energy source [6].There are several other energy sources of WAAM, such as Gas metal arc welding (GMAW), gas tungsten arc welding, and plasma arc welding [7].Regardless of the traditional GMAW technique, its AM-based technology is used for a more significant rate of depositions, suitable mechanical properties, and reasonable costs [8].WAAM has certain advantages over other AM technologies.WAAM is favorable in the case of batch productions because of its low capital cost.Components like welding torch, feeder wire system, and required programmed systems are available quickly, making WAAM a cost-efficient system.The metal wires required in WAAM are commercially accessible and simple in operation, unlike metal powder substitutes [9].GMAWAM process can provide a deposition rate of 8 kg/h [10].Even though the surface quality and dimensional accuracy are limited due to high metal deposition, it has the capacity to manufacture large-volume components [11].Postprocessing, like milling or grinding, is sometimes required in order to meet the necessary tolerances [12].Owing to the large rate of depositions, high temperature, excessive energy input, distortion, and stresses are generated, which is a challenge during such AM processes.Thus, in order to obtain favorable characteristics, a proper selection of parameters like travel speed (TS), feed rate, wire feed speed (WFS), arc voltage (V), and path planning is essential [4].
1.25Cre0.5Mosteels are employed in the production of equipment employed in processes that exhibits higher temperatures and pressures.A safe working environment is a priority that is fulfilled by the steel's metallurgical and mechanical properties [13e15].Despite having good weldability, CreMo steels undergo temper embrittlement due to exposure to the higher temperature range of 370e550 .Metal-cored and fluxcored are the two types of wires available [16].Flux cored wire increases the overall quality of welding as they have a filling of shielding gas in the core but reduces the deposition rate.On the other hand, metal-cored wires improve the current density and deposition rate as they have filler metal at the core [17,18].Metal cored wires have shown minimum angular distortion compared with the other wires [19].However, a more significant presence of sulphur in metal cored wires slightly reduces the toughness compared to flux cored wires.The filler passes deposited with the GMAW technique, and metal cored wire has provided favorable results according to a study performed by Das et al. [20].The above outline has proved that metal cored wires can be used for 1.25Cr-0.5Mosteels.
The effect of process parameters like current, V, and TS on microstructures of GTAWAM 308 L walls was investigated by Mai et al. [21].They observed that TS and V had a significant impact on grain sizes, grain refinements, and texture.Columnar austenite dendrites and residual ferrite phases were observed on the built microstructure.The walls of GMAWAM 308 L were found without defects and were constructed successfully.It was also observed that the bonding between the deposited layers was strong, and the height of the walls remained regular.Abioye et al. [22] used laser metal deposition for a multi-layered wall structure of 316LSi.The microhardness of the multi-layered wall at optimal parameters was found to be in the range of 167e194 HV.Obtained tensile strength results show that wall building direction plays a key role for multi-layered structures.Two different welding current techniques were employed for comparing the mechanical characteristics of GMAWAM-fabricated 30 layered structures on 316 L by Wu et al. [23].The evaluation of obtained results has shown coarser structures for higher welding current and also had its influence on the hardness of the built materials.A study by Xiong et al. [18] suggested a neural network process based on a closed-loop iteration to fabricate thin walled structures using WAAM.It was later found that the precision of the applied network was not satisfactory as changing parameters became more complex.
In the same way, properties of bead geometry were not concisely represented by the bead height and width.It was concluded that reverse forecasted parameters were inaccurate as different welding parameters could obtain the same bead geometries.For the investigations of mechanical and morphological characteristics of 304 L stainless steel, Laghi et al. [24] fabricated a steel structure using GMAW based WAAM process.The part's mechanical properties were anisotropic, which was concluded by the tensile test along the longitudinal, diagonal, and transverse directions.Elastic and plastic strength was found to be larger in a diagonal direction.The GMAW-based WAAM method has been utilized to investigate process variables on the surface quality of welded samples.From the study, it was observed that interlayer temperature has the highest effect on the surface quality of the part [25].It was suggested to use post-processing operations to acquire good surface quality with lower dimensional deviation [26].Optimizing process parameters might be beneficial to minimize issues directly related to the bead geometry.Williams et al. [27] studied GMAW-based WAAM and concluded that GMAW is a promising technique of WAAM due to the use of a wire spool as an electrode.Kumar et al. [28] used parametric optimization to build a multi-layered structure using GMAW-based WAAM.Their obtained microstructure results have shown ferrite and pearlite structures, coarser structures, and bainite structures in the bottom, middle, and top zones, respectively.Xiong et al. [29] investigated the potential impact WAAM technique on the fabricated multilayered inclined structure.The effect of welding torch, wire feed speed, and offset distance were studied on inclination angle.The results showed that larger inclination angles were obtained at higher travel speeds and lower wire feed speeds.The greatest inclination angle attained was above 45 with correct weld parameters.Another work by Li et al. [30] has shown extra material should be deposited at the edge of the parts with inclined features to improve bead geometry.Palani et al. [31] optimized the bead geometry using the response surface method to get the desired process parameters.However, the calculation process was complex, and the local solution was likely to be reached for the multi-response optimization but was not the optimum combination of variables.In another study, Kim et al. [32] employed the combined approach of genetic algorithm and RSM to optimize bead geometry (height and width) during the WAAM process.
Lu et al. [33] examined the microstructural and mechanical characteristics of a multi-walled structure fabricated on ASTM 104 by the GMAWAM process.The obtained microstructure was observed by granular structures comprising residual ferrite, residual austenite, granular ferrite, and perlite along the gain boundaries.Their results have shown enhanced mechanical properties for structures that are normal to building direction.Haden et al. [29], using the GMWAM technique, observed that the structure build's tensile strength was in the permissible range for both 304 steel.Chen et al. [34] studied GMAWAM 316 L walls in order to explore its microstructures.The study showed that the microstructure of 316 L walls had residual ferrite, sigma phases, and austenite dendrites inside the austenite matrix.
As per the published results, the appropriate selection of WAAM variables at individual levels plays a vital role in manufacturing multi-walled components with favorable mechanical properties.Very few studies were reported on manufacturing multi-walled components of 1.25Cr-0.5Moat optimized parameters and analysis of microstructure and mechanical characteristics through the GMAWAM process.Thus, optimal process variables of GMAWAM were employed to manufacture a multi-walled component.The current study investigated the process stability with microstructural properties of the built multi-walled component on 1.25Cr-0.5Mo.The study also investigated mechanical characterizations of microhardness, tensile test, impact test, and fracture surface morphologies of 1.25Cr-0.5Mo,whose results are considered important for future research and industrial applications.

2.
Materials and methods

Experimental setup and plan
In the present work, metal-cored wire with a 1.2 mm diameter was utilized for deposition on a substrate plate of low alloy steel grade of 1.25Cr-0.5Mousing the Miller make PRO MIG-530 GMAW system.The filler material with metal-cored wire consisted of Metalloy 80B2 (Hobart Brothers: TRI-MARK), which is mainly suitable for welding Chromoly steels.A thin-walled component was made-up through the GMAWbased WAAM technique.Tables 1 and 2 displayed the chemical composition and mechanical characteristics of base metal and filler wire materials.Base metal was adequately prepared and polished beforehand of the deposition.Shielding gas consisting of Ar and O 2 was utilized with the composition of 98% and 2%, respectively, during the deposition process at a flow rate of 15 L/min.The overall entire organized experimental setup is shown in Fig. 1.While depositing the filler material on the base plate, a torch was used after the appropriate mounting of the base plate on the work table.12 mm of distance was kept between the workpiece and welding torch.
To reduce the interlayer temperature, a dwell time of 10 s was kept between successive layers' deposition.The optimized process variables were determined from our recently reported work by employing a Teaching-Learning based optimization algorithm: voltage of 18 V, wire-feed speed of 5.9 m/min, travel speed of 476 mm/min, and arc length of 3 mm [35].
After deposition, the wire-EDM process extracted a multiwalled structure from the base plate.For the extracted structure, heat treatment (HT) was performed at 670 C with a heating rate of 100 C/h and held for 6 h at 670 C. Cooling of the structure was performed up to 400 C for the duration of 4 h, and later on, it was permissible to cool naturally.

Testing and characterization
Fabricated multi-walled components were studied through investigations of microstructure, mechanical properties, and fractography morphologies in different positions.Fig. 2 shows a multi-walled component of 1.25Cr-0.5Mobuilt at optimal process parameters of the GMAWAM process, along with the different testing locations.In Fig. 2, MS refers to the microstructure while CS refers to the chemical composition of the test specimen.Initially, the side surfaces of multi-walled components were removed.Then, a wire-EDM setup was used to extract the required specimens.Three small samples were also extracted from each structure for the measurement of the chemical composition of the multi-walled structure through the thermo-scientific spectrometer.Extracted samples were initially cleaned and polished.Table 3 represents the chemical composition of the multi-walled component.The mentioned results are the average value of three test results for all elements.Comparing these results of the multi-walled component with wire elements (Table 1) has shown that the composition of all the critical elements fell within the prescribed range of filler wire material.Fig. 2 depicts the three metallographic locations of multiwalled components in the top, middle, and bottom zones.All three specimens were mounted and then polished using different abrasive papers and then polished through 0.5 mm Al 2 O 3 to acquire a mirror surface.Specimens were then etched in 2 Nital solutions for a suitable period to obtain a better microstructure.The microstructure of prepared multi-walled specimens was then explored by the optical microscope (RADICAL).
For better accuracy and reproducibility of the results, multiple specimens were tested for each condition, and their average value was considered for analysis.As per Fig. 2, mechanical properties such as microhardness, tensile test, impact test, and fractography were carried out on the built structure's top, middle, and bottom sides.Vickers microhardness tester (ESEWAY: EW-150) was employed for microhardness (MH) determination.A load of 500 gf and 10 s dwell time was applied during the testing.Microhardness was determined at seven positions on the cross-section of the center line.The MH value at each position was the average value of three indentations.AIT-300 Charpy impact setup was employed to perform the impact test at room temperature.Tensile testing was carried out as per ASTM E8 standard using an M À 100 universal testing setup.Fig. 2 displays the locations of samples for the impact test and tensile test.The fracture surface morphologies of tensile and impact specimens were assessed by Zeiss Ultra-55 SEM setup.

3.
Results and discussions

Multi-walled component
Multi-walled component fabricated on 1.25Cr-0.5Moat the optimized process variables of GMAWAM was displayed in Fig. 3. Uniform deposition can be observed for the multiwalled component.To reduce the interlayer temperature and transmission of accumulated heat with surrounding, a dwell time of 10 s was kept between the depositions of successive layers.Layers of structure can be seen with seamless fusion, and multi-walled component was also free from disbonding.Some of the metal lumps were observed on the side walls of the component.All of them were removed in the post-processing of the multi-walled component.Thus, the obtained optimal parametric settings of the GMAWAM technique successfully fabricated multi-walled components without any defect.This multi-walled component was studied through investigations of microstructure, mechanical properties, and fractography morphologies at various positions (top side, middle side, and bottom side) of the built structure.

Microstructure of multi-walled structure in different zones
The microstructural studies were carried out to understand the underlying changes in phases and grain structure at three locations (i.e., Top, middle, and bottom), as shown in Fig. 2. The images were captured using optical microscopy, as shown in Figs.
4e6.It has to be noted that the analyses were carried out after subjecting the multi-walled component to post-weld heat treatment (PWHT).The effective grain structure of the component consists of a dendritic structure at all three locations.Fig. 3 e Multi-walled component on 1.25Cr-0.5Mousing metal cored wire.However, as shown in Fig. 4a, the bottom layer microstructure shows the grains having a tempering effect which the coarser granular structure can confirm.This was due to the deposition of the subsequent filler metal layers on it.Essentially, as the bottom layer was the first to be deposited, it has experienced extreme cooling rates due to the cold plate and atmosphere.Hence, the bottom layer revealed a tempered martensite structure.In addition, an interconnected layer of carbides was also noticed in the micro-image (Fig. 4b).This was because, as the component was subjected to PWHT, any untampered martensite formed during solidification gets a suitable time for carbide precipitations.These carbides were rounded in shape and were randomly scattered, as seen in the microstructure, and possibly rich in chromium as it is the main alloying element.
The microstructures of the middle zone, as shown in Fig. 5a  and b, have shown somewhat similar structures as that of the bottom zone.However, the amount of carbides was a little lesser, along with a slightly finer microstructure.This was because the amount of layers subsequently deposited on the middle layer was less as compared to the bottom layer, and this deposition acted as heat input to the already present layer.This can be seen in Fig. 5a and b.
The microstructure of the top layer, as shown in Fig. 6, also depicted a similar phenomenon wherein negligible heat was subjected to the top few layers due to deposition.The carbide amount here would be even lesser than in the middle section because this formation would be possible during the PWHT.Thus a mix of untampered martensite (which doesn't convert to carbide) and colonies of tempered martensite can be seen in Fig. 6a and b.

Mechanical characteristics
Investigations of mechanical properties such as microhardness, tensile test, impact test, and fractography were carried out on the built structure's top, middle, and bottom sides.For better accuracy and reproducibility of the results, multiple specimens were tested for each condition, and their average value was considered for analysis.

Microhardness testing
Microhardness (MH) was measured in three different zones (top, middle, and bottom) of the built multi-walled structure as per the locations mentioned in Fig. 2. Ten samples were tested in each zone, and their average reading was taken for analysis.Fig. 7 depicts a graph for microhardness readings.Average MH values of 184.75 ± 4.38 HV, 189.31 ± 5.06 HV, and 192.28 ± 5.89 HV were recorded in the top, middle, and bottom zones, respectively.MH in the top zone depicted a slightly lower value, while the bottom zone gave a marginally higher value of MH.The primary reason behind this variation is that the top layer experienced a lower cooling rate owing to frequent thermal cycles and the bottom layer experienced a slightly higher cooling rate.The results were in line with the reasonable agreement reported by Das et al. [20] and Gao et al. [36].However, the deviations were minimal.They can be treated as uniform MH across the built structure.Moreover, these uniform values of MH in all their sections indicated a similar nature of multi-walled components and recommended that the specimen will not have brittle failure.

Tensile testing
Investigations of tensile test properties have been carried out at the top, middle, and bottom sides of the built structure.Tensile testing was carried out as per ASTM E8 standard using an M À 100 universal testing setup.Table 4 depicted the summary of the results of the tensile test, while Fig. 8 represented a graph comparing the results of the multi-walled structure and base metal.As per Table 4, YS, UTS, and percentage elongation of metal-cored wire were reported as 540 MPa, 640 MPa, and 22%, respectively.Mechanical properties were compared among the tensile test results of metal-cored wires and multi-walled structures to check the internal eminence of the obtained component.For the top side of the multi-walled structure, tensile properties have shown YS of 579 MPa, UTS of 658 MPa, and EL of 24.21%.These recorded results were the average values obtained from repeated trials.YS, UTS, and EL of 571 MPa, 652 MPa, and 24.03% were achieved in the middle side, while 568 MPa, 647 MPa, and 23.79% were attained on the bottom side of the structure.Thus, the multi-walled structure has shown average YS, UTS, and EL values of 572.66 ± 6.35 MPa, 652.33 ± 5.67 MPa, and 24.01 ± 0.22%, respectively.All the obtained results fall in the range values of 1.25Cr-0.5Mometal-cored wire.The obtained results were in line with the reasonable agreement of Das et al. [20].Therefore, results of tensile properties endorse that the accepted component from GMAWAM of 1.25Cr-0.5Mometalcored wire is suitable for industrial applications.Another observation from the results of the tensile characteristics of Table 4 shows the negligible deviation among the properties of the multi-walled structure's top, middle and bottom sides.This uniform behavior of tensile properties depicted adequate supremacy of deposited components for various applications.YS on the bottom side depicted a slightly higher value, while the top side showed a marginally lower value of YS of multi-walled structure.This variation was found as per the Hall-Petch relationship [37].This analysis has demonstrated that the YS of parts will give larger values in zones if more refined grains are obtained in the microstructure [36,37].Thus, these obtained results are in good agreement with the results of the microstructure presented in the 3.2 section.Another reason behind this variation was the higher cooling rate experienced by bottom side layers and lower cooling rate experienced by top side layers owing to frequent thermal cycles.The obtained results were in line with the reasonable agreement reported by Lin et al. [38] and Vora et al. [39].However, all these deviations in tensile properties were shown a negligible deviation among the top,  middle and bottom sides of the multi-walled structure.This uniform behavior of tensile properties depicted adequate supremacy of deposited components for various applications.
SEM for the fractography of the tensile test part of the bottom side has been depicted as per Fig. 9.A large number of dimples with homogenous distribution were observed on the fracture surface of the specimen.This implies a superior toughness of the fabricated parts for GMAWAM [40].The presence of a higher amount of dimples adjacent to each other confirmed the excellent ductility of the multi-walled component.Top side and bottom side components have also reported similar results.As seen in Fig. 9, carbide entrapped within the dimpled structure can be seen, which is in line with microstructural analysis and resultant mechanical properties.This shows the GMAWAM process's suitability for fabricating a multi-walled structure of 1.25Cr-0.5Mometal-cored wire.

Impact testing
ASTM E23 standard was followed to prepare the specimen for impact testing.After etching the specimen, a Charpy impact test was performed by preparing a V-notch.It has been carried out on the built structure's top, middle, and bottom sides at room temperature.Average values of 239 ± 4.72 J, 243 ± 3.42 J, and 249.47 ± 5.95 J were recorded in the top, middle, and bottom zones, respectively, as shown in Fig. 10.However, as per the standard condition (API-1104), 58 J is the least requisite for the toughness of weld specimens.The obtained results for all three conditions showed far better strength than the requirement.Also, uniform behavior in toughness results in all three zones depicting adequate supremacy of deposited components through GMAWAM for various applications.
Fig. 11 depicts the SEM fractography of the fractured impact part on the bottom side of the structure.A large number of dimples with homogenous distribution were observed on the fracture surface of the specimen.This revealed ductile behavior.Additionally, the results of impact toughness showing a higher strength suggest that multiwalled component has outstanding ductility.The absence of micro-pores and micro-cracks ensured that the built structure had good impact characteristics.Top side and bottom side components have also reported the same results.This shows the GMAWAM process's suitability for fabricating a multi-walled structure of 1.25Cr-0.5Mometalcored wire.

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Fig. 5 e
Fig. 5 e (a) Optical microscopy, (b) SEM image for the middle zone.

Fig. 6 e
Fig. 6 e (a) Optical microscopy, (b) SEM image for top zone.

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Fig. 7 e
Fig. 7 e Microhardness of multi-walled structures in different zones.
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Fig. 8 eFig. 9 e
Fig. 8 e Comparison of tensile properties between the metal-cored wire and multi-walled structure for YS, UTS, and EL.
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Fig. 10 eFig. 11 e
Fig. 10 e Impact test results of multi-walled structures in different zones.
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Table 1 e
Chemical composition of substrate plate and filler wire.

Table 2 e
Mechanical characteristics of substrate plate and filler wire.

Table 3 e
Chemical composition of multi-walled component.

Table 4 e
Result summary for tensile properties.