Impact of electromagnetic stirring on the gas metal arc welding of an MAR-M247 superalloy

In this paper, the impact of electromagnetic stirring (EMS) on the gas metal arc welding (GTAW) of an MAR-M247 superalloy was investigated. Results revealed that, without electromagnetic stirring, it was easy for carbides in the heat-affected zone (HAZ) of the weld bead to liquefy during welding, leading to weld bead cracks. Electromagnetic stirring refined the grains in the HAZ and the weld bead, leading to grain strengthening and subsequently resulting in the effective improvement in the hardness of the weld bead. In addition, electromagnetic stirring significantly facilitated the formation of the weld bead by the removal of large inclusions which in turn effectively improved crack resistance of the joint. It also accelerated the floating up of gas holes thereby reducing the generation of porosity.


Introduction
MAR-M247 is a typical cast polycrystalline nickel-based superalloy, which was developed by Martin Marietta in the late 1970s [1]. It exhibits excellent castability at high temperatures, as well as satisfactory creep resistance and hot corrosion resistance [2][3][4][5][6][7][8][9]; it is the main material used for the fabrication of gas turbine blades in the aviation and energy industries [8,10]. The MAR-M247 superalloy exhibits an extremely complex composition, composed of several solid-solution strengthening elements and precipitationhardening elements [7,[11][12][13]. Its microstructure mainly comprises austenite γ-phase base, γ′ strengthening phase, γ-γ′ eutectic structure [14][15][16][17], and various borides and carbides crystallized by solidification [14]. The Ni base exhibits limited solubility to these trace elements. The trace elements interact with Ni in the base during welding, to form a lowmelting-point eutectic phase, and segregate at the grain boundaries, leading to the precipitation of brittle carbides [18][19][20] during welding, resulting in clear hot cracks on the weld bead. This is also one of the main reasons that MAR-M247 is not applicable for repair welding.
Generally, the Ni-base superalloys (e.g., Hastelloy x) can be welded by fusion welding, such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), shielded metal arc welding (SMAW), and submerged arc welding (SAW) [21]. However, owing to the poor thermal conductivity, strong viscosity of the liquid metal, and low oxidation resistance of the alloy elements [18], the molten pool of the MAR-M247 superalloy cannot be wetted and developed as easily as the molten pool of steel. Therefore, the formation of the weld bead is poor, its penetration is shallow, and fluidity cannot be improved even with an increase in the current, whereas the hot crack sensitivity of the weld bead can be accelerated by an increase in the current. GTAW is typically used for the welding of Ni-based superalloys [22][23][24] exhibiting a clean surface after welding. However, if the welding heat input is high during the implementation of multi-pass welding, the eutectic structure of the low-melting-point weld head at the grain boundaries of the HAZ is prone to liquefaction under welding stress, leading to defects in the HAZ, which can include liquefaction defects, weld bead defects, humped weld bead defects, weld erosion, and gas holes. Therefore, it is crucial to employ a modified welding method to solve this processing problem.
Laser welding [25][26][27], electron-beam welding [28][29][30], friction stir welding [31][32][33][34][35][36], and other advanced welding methods generate less welding stress, thereby helping to reduce welding hot cracks; however, these welding methods are not yet practically applicable in repair welding. Recently, electromagnetic stirring (EMS) during welding has come to be recognized as an effective method for facilitating bead formation, as well as enhancing the microstructure and mechanical properties of the weld [37][38][39][40][41][42][43]. EMS can effectively decrease welding defects, such as weld erosion, hump beads, gas holes, and cracks during fusion welding. In 1971, Tseng and Savage [44] reported that electromagnetic stirring can improve the shape coefficient of the GTAW weld bead and reduce its sensitivity to hot cracking, consequently improving crack resistance. Under the application of an external magnetic field, the proportion of coarse grains in the weld bead decreased, and fine equiaxed grains were formed. The average grain size decreased, thereby enlarging the grain boundary area and enhancing the strengthening effect of the grain boundaries. Consequently, joint strength was improved. The addition of a magnetic field could facilitate optimization of the distribution of inclusions in the weld bead. Inclusions are dispersed in the metal of the weld bead as second-phase particles, which serve as the base for the nucleation of weld bead grains. Magnetic stirring on the other hand can alleviate the segregation of alloying elements in the molten pool, thereby considerably reducing the tendency of hot cracking. In addition, it can change the mass transfer and heat transfer processes of the molten pool [37] and the orientation of grain growth. Large angle grain orientations can increase the energy required for the development of hot cracks, thereby effectively hindering their expansion. Currently, ESM magnetic field control technology has been developed for metal welding, including stainless steel [45][46][47][48][49], aluminum alloys [42,[50][51][52][53], and magnesium alloys [54][55][56][57], but it has not been employed for welding of the MAR-M247 superalloy. Therefore, in this study, the effects of GTAW with an EMS device on the mechanical properties and microstructure of the MAR-M247 superalloy are explored. Table 1 summarizes the composition of the MAR-M247 superalloy used in this experiment, and Fig. 1 shows the equipment comprising the GTAW device along with the EMS device. The EMS metal is fixed on a welding gun, and EMS is powered by the magnetic-field power-supply equipment, which can adjust the magnitude of the current and the frequency of the external magnetic field. The GTAW welder is an automatic welding machine, which utilizes an automatic wire feeding device and can automatically digitalize and adjust the wire feeding speed, movement speed, arc height, current, and voltage, to stabilize the welding parameters for the welding process.

Experiments
The welding methods ( Fig. 2) selected herein are (1) bead-on-plate (BOP) and (2) V-shaped butt welding. In the BOP welding test, the alloy specimen is wire cut into 100 mm × 60 mm × 6 mm plates (Fig. 2a), and welding is executed both with and without a Turbaloy@625 solder for analysis of the differences. Three-pass overlay welding is carried out using the Turbaloy@625 solder and another plate. In the v-butt welding test, the MAR-M247 superalloy ingot is cut into V-shaped joints. The joints are grooved by wire cutting, with a side angle of 45° and a plate spacing of 5 mm (Fig. 2b). Before welding, the joints are ground down with sandpaper to remove any oxide film from the surface and in the groove. After grinding, the joints are cleaned with acetone, and v-butt GTAW and GTAW + EMS are conducted. Before v-butt welding, the v-butt plate was subjected to a vacuum solution treatment at 1185 °C for 2 h, followed by cooling to room temperature in an argon gas atmosphere, to eliminate residual stress from the specimen. After welding, the v-butt welding specimens were subjected to a vacuum solid-solution treatment at 1185 °C for 2 h, followed by cooling to room temperature with argon. Subsequently, after 20 h of artificial aging in a vacuum at 871 ℃, the material is allowed to cool to room temperature in the furnace. The heat input temperatures of GTAW and GTAW + EMS during welding are measured using an infrared thermometer. The alloy microstructure is observed using an optical microscope. The metallographic specimens are treated by coarse grinding, fine grinding, and polishing. After polishing, the microstructure of the alloy is observed under different magnifications. Scanning electron microscopy (SEM, acceleration voltage set at 15 kV) is employed to observe the morphology of the weld bead and fracture surface. The observations are utilized to understand the microstructure and fracture mechanism of the weld bead. Energy-dispersive spectrometry (EDS) is employed for semi-quantitative analysis of intermetallic compounds of the alloy, and X-ray analysis is employed for the detection and location of welding defects. Wire cutting and slow cutting machines are used to sample the defects. After sampling, metallographic defects are observed and analyzed by SEM, and EDS and X-ray analysis are employed to analyze the composition surrounding the defects to understand their causes.
The Vicker's hardness test is employed to measure the hardness (load of 500 gf and a loading time of 10 s) and analyze changes in the mechanical properties of the materials after welding. The 10-ton MTS universal testing machine is utilized for tensile testing, which is conducted in the stroke control mode. Throughout the entire process, the tensile test speed rate is divided into two sections: initially, it is 0.2 mm/ min, and when the strain is equal to 1%, it is switched to 2 mm/min. Figure 3 shows the microstructure of the as-cast and solution-quenched state of the MAR-M247 nickel-based superalloy. From the as-cast state (Fig. 3a), the microstructure of MAR-M247 is mainly composed of the austenite γ phase and a number of Chinese script-shaped carbides. However, no significant difference between the as-cast state and solutionquenched state (Fig. 3b) was observed, indicating that the solution-quenched heat treatment does not produce significant changes in the morphology of Chinese script-shaped carbides and that the carbides can reach up to 50 to 100 μm in size (Fig. 3c). Figure 4 shows the SEM microstructure of the solutionquenched state MAR-M247 nickel-based superalloy. The microstructure of the solution-quenched state MAR-M247 austenite γ phase is clearly observed in the SEM images (as shown in Fig. 4a). In addition to the large number of Chinese script-shaped carbides, the fine γ′ strengthening phase, rose-shaped γ-γ′ eutectic structure, and thick-plated γ′ phase also are found (as shown in Fig. 4b, c, d). Figure 5 shows the results of electron probe micro analyzer (EPMA) analysis of the Chinese script-shaped carbides which are mainly composed of Ti, Ta, and Hf. Figure 6 shows the arc shapes of GTAW and electromagnetic stirring assisted GTAW. The GTAW carried out without an external electromagnetic field exhibits a bell-shaped arc (Fig. 6a), while GTAW + EMS exhibits  (Fig. 6b). Figure 7 shows weld bead structure of the solution-quenched state MAR-M247 superalloy welded by bead-on-plane (BOP) welding. There are notably clear waves in the GTAW weld bead, while the GTAW + EMS weld bead is gentler and smoother, indicating that the quality of the GTAW weld bead can be significantly improved by the application of an electromagnetic field.

BOP welding analysis
The microstructure of the weld bead and HAZ of the solution-quenched state MAR-M247 superalloy welded without solder is also observed (the image of marked 1 and 2 in Fig. 7). From the image of marked 1 in Fig. 7,  it can be seen that the Chinese script-shaped carbides in the weld bead molten zone of the GTAW specimen are up to 50 μm in size, while those in the weld bead zone of the GTAW + EMS specimen are significantly down to 10 − 15 μm (as shown in Fig. 7 within marked 2). This result indicates that the application of an external magnetic field can help to refine the Chinese script-shaped carbides in the molten zone of the weld bead. In addition, the composition of the Chinese script-shaped carbides in the weld bead is analyzed by EPMA, and the results revealed them to be mainly composed of Ti, Ta, and Hf (Fig. 8).   Figure 9 shows a cross section of the solution-quenched state MAR-M247 superalloy welded with BOP welding. As can be observed in the figure, the weld bead height of the GAWT and GTAW + EMS specimens is 1.48 mm, but the weld bead widths of the GAWT and GTAW + EMS specimens are 6.91 mm and 5.61 mm, respectively, indicating that GAWT + EMS can effectively narrow the weld bead width and subsequently increase the weld bead formation   Figure 10 shows a cross section of the microstructure of the GTAW and GTAW + EMS bead-on-plate weld bead and HAZ of the solution-quenched MAR-M247 superalloy welded with 625 solder. The width of the columnar grains of GTAW is 25 μm, while that of GTAW + EMS is only 15 μm. Notably, coarse grains ~ 200 μm in size are observed in the HAZ of the GTAW specimen, but not in that of the GTAW + EMS specimen, indicating that the application of magnetic field can lead to an effective reduction of the grain size in the molten zone and HAZ.  Figure 11 shows the appearance and X-ray inspection analysis chart of the GTAW and GTAW + EMS specimens after three-pass overlay welding. As can be observed from the figure, the width of the weld bead with a coverage of 50% after three-pass overlay welding is 9 mm, while that for GTAW + EMS is clearly narrower, 7.7 mm. X-ray analysis reveals the presence of clear cracks in the GTAW, but not in the GTAW + EMS specimens, indicating that GTAW + EMS can effectively reduce the generation of welding hot cracks in the MAR-M247 superalloy.

BOP three-pass overlay welding
The pattern of cracks produced by GTAW three-pass overlay welding (Fig. 12) is observed using metallographic microscopy. It can be seen that Chinese script-shaped carbides liquefy around the cracks during welding solidification, supposedly one of the main causes of welding cracks. The SEM images with EDS analysis (as shown Fig. 13) also show the cracks to be filled with Chinese script-shaped carbides. Figure 14 shows the instantaneous temperature of the HAZ during the GTAW and GTAW + EMS welding process measured using an infrared thermometer. As can be observed from the figure, the effect of the arc on the instantaneous temperature of the HAZ of the MAR-M247 specimen during the welding process of GTAW is observed at ~ 1600 °C, while the instantaneous temperature during GTAW + EMS can drop to ~ 1113 °C, indicating that the effect of the heat input source of the arc on the HAZ can be considerably reduced by the application of the external electromagnetic field speeding up the cooling rate of the weld pool. It is speculated that this is the main reason for the refinement of the carbide size and grain size in the HAZ.

Hardness
The hardness of v-butt welded specimens is measured along the center line of the welding cross section of the MAR-M247 superalloy welded with GTAW and GTAW + EMS. Figure 15 shows the results obtained from hardness measurements. As shown in Fig. 16, the GTAW and GTAW + EMS welded joint can be divided into three zones: the weld zone, the heat affected zone, and the base metal. The hardness of the weld zone of the GTAW and GTAW + EMS specimen exhibits the same trend. The hardness is lowest in the center of the weld zone center and increases with the distance from the center. High hardness is still exhibited in the base metal zone because it is far away from the weld zone and not much affected by the welding heat input source. Note that the hardness values of the HAZ and weld zone center of the GTAW + EMS specimen are significantly greater than those produced by GTAW. In addition, the results revealed that the GTAW + EMS hardness is better than that of GTAW because GTAW + EMS can effectively refine the grains in the HAZ and the weld zone, reflecting the grain strengthening effect. Figure 16 shows the tensile mechanical properties of the GTAW and GTAW + EMS welded specimens of the     Figure 17 shows the tensile fracture surfaces of the GTAW and GTAW + EMS-welded specimens of the MAR-M247 superalloy. As can be seen in the figure, there is a clearly observable cleavage surface on the fracture surface of the GTAW-welded specimen, while the fracture surface of the GTAW + EMS specimen is mainly a dimpled structure, indicating that the ductility of the GTAW + EMS-welded specimen is better than that of the GTAW-welded specimen, which is consistent with the results observed from the tensile property analysis.

Conclusion
1. The as-cast state MAR-M247 matrix is mainly composed of the austenite γ phase, and the γ′ phase, Chinese script-shaped carbides, γ-γ′ eutectic structures, and thick-plate γ′. These carbides are mainly composed of Ti, Ta, and Hf, which are widely and evenly distributed in the MAR-M247 matrix. Furthermore, the solidsolution heat treatment does not lead to any significant change in the morphology of the Chinese script-shaped carbides in the MAR-M247 alloy. 2. The results revealed that without the application of the electromagnetic field, a bell-shaped GTAW arc is observed, and carbides in the HAZ of the GTAW weld bead are easily liquefied due to the input of considerable heat during welding, thereby leading to welding cracks. Results also revealed that GTAW + EMS exhibited a meteor-shaped arc, which can considerably decrease the weld bead temperature, improve the WRFF coefficient, and reduce the generation of hot cracks. 3. GTAW + EMS can dramatically reduce the effect of the arc heat input source on the welded workpieces and can further refine the size of carbides in the weld bead as well as the size of grains in the HAZ, effectively improving the mechanical properties.
Author contribution TYC: conceptualization, methodology, investigation, supervision, funding acquisition, validation, writing-review and editing. LCY: conceptualization, investigation, and validation. All authors equally contributed the overall manuscript. CRY : conceptualization, investigation, methodology, writing-original draft, writing-review and editing.
Availability of data and material Not applicable.