Ultrasonic vibration assisted gas tungsten arc welding of Inconel 690 alloy: Ultrasonic effect to refine grains and improve mechanical properties

Abstract

The Inconel 690 alloy is widely used in the manufacturing of nuclear equipment, such as pipe welding for steam generators (SG) and pressure vessels, due to its excellent high-temperature strength, corrosion resistance, and thermal stability. However, coarse grains have been observed in the welded joint of Inconel 690. Considering its crucial role as a structural material under high pressure, temperature, and corrosive conditions, improvements should be made to the microstructure of the welded joint. The ultrasonic-assisted gas tungsten arc welding (UA-GTAW) was used in Inconel 690 welding. The influence of ultrasonic vibration on the microstructure and mechanical properties of welded joints was studied. The results show that the ultrasonic refined the microstructure further to improve the mechanical properties. The UA-GTAW sample performed superiorities over regular GTAW joint in multi perspective including refined solidification grains, less element segregation, higher tensile strength and hardness. The Yield strength, ultimate tensile strength, and elongation increased from 320 MPa, 591 MPa, and 25.1 % to 387 MPa, 672 MPa, and 31.6 %, respectively.

1. Introduction

Since 1978, stress corrosion cracking (SSC) has been found in the Inconel 600 alloy welded joint of the safety terminal in the Duane Arnold Energy Center safety accident [1]. Inconel 690 alloy was developed by increasing the content of Cr to 30 %, effectively avoiding SCC to ensure the safe operation of nuclear power plants in the 1990s. It has excellent high-temperature, corrosion resistance, and thermal stability, which is suitable for application in nuclear equipment manufacturing [2]. However, the presence of coarsened solidification grains in the welded joint of Inconel 690 still poses a risk of inducing ductility-dip cracking (DDC), thereby compromising the service life and operational safety of nuclear power plants.

The Fukushima nuclear power plant accident has led to heightened global attention towards safety concerns pertaining to nuclear power plants, with particular emphasis on the microstructure and mechanical performance of welded joints made from Inconel 690 alloy. [3], [4]. The researchers found that the welds with DDC cracks are usually characterized by large grain size and high weld restraint. Nishimoto et al. conducted welding of Inconel 690 alloy under various restraint conditions and observed fractures, discovering that the presence of S and P elements can enhance ductile-to-brittle transition behavior. Furthermore, their study revealed that the DDC susceptibility increased with increasing S and P content [5], [6]. In addition to the segregation of S and P elements at grain boundaries, discontinuous Cr₂₃C₆ carbides at grain boundaries also lead to the generation of DDC. Young et al. maintain that the presence of cracks is attributed to the stress concentration induced by semi-continuous Cr₂₃C₆ precipitation on grain boundaries and the lattice mismatching between Cr₂₃C₆ and matrix interface [7], [8]. Min et al. designed the material composition of the nickel-based superalloy to reduce the content of Cr₂₃C₆ at the grain boundaries, successfully reducing the crack content in the microstructure [9]. Ti was incorporated into Inconel 690 alloys by MO et al. to form Ti (C, N) and mitigate the presence of Cr₂₃C₆ at grain boundaries [10].

Watanabe et al. found that cracks are often found at the trigeminal grain boundary, where a large stress concentration is formed, resulting in crack initiation [11], [12]. Chen et al. studied the DDC susceptibility based on the local strain concentration and compared the DDC distribution and orientation in different areas. They have found that Zigzag grain boundaries prevent crack growth, compared with straight grain boundaries [13], [14]. These works exhibit that the inadequate cracks and mechanical properties result from the coarse grains and the straight grain boundaries. However, there are migrations of solidification grain boundaries in Inconel 690 welded joints due to the thermal history of arc welding. Jang et al. observed the distribution of DDC and analyzed the relationship between DDC and migrated grain boundary (MGB). Their found that the formation of DDC is associated with the straight MGB morphology. There is no DDC in the tortuous MGB and creep-like morphology DDC at the tripe point of the MGB [15], [16]. So, controlling the weld composition, grain size and morphology are efficient ways to improve the microstructure and mechanical properties of Inconel 690 alloy. Simultaneously, the welding processing of nickel-based alloy exhibited notable sensitivity. Tushar Sonar er al. summarized the influence of welding process parameters on the microstructure and mechanical properties, highlighting that reducing heat input and increasing cooling rate can effectively mitigate Laves phase formation; however, this may lead to an increase in porosity and liquation cracking [17]. Sivaprasad et al. employed magnetic arc oscillation and current pulsing methods to enhance the refinement of Laves phases in the weld zone, thereby improving the microstructural characteristics [18]. Similarly, G.D. Janaki Ram et al. utilized the current pulsing method and observed a refined fusion zone microstructure with reduced presence of Laves phase due to low heat input [19]. Tushar Sonar et al. employed the interpulse magnetic arc constriction and high frequency pulsation technique to further enhance the distribution of Laves phase and observed that high-frequency pulse arc facilitates the diffusion of Nb element, leading to increased precipitation strengthening after heat treatment and subsequent enhancement in mechanical properties [20].

Ultrasonic-assisted welding has emerged as an effective technique for enhancing the microstructure and mechanical properties of the welded joint. The propagation of ultrasonic waves within the medium yields a variety of effects, including cavitation, acoustic streaming, thermal vibrations, and mechanical vibrations [21], [22]. The researchers discovered that during the welding process, these ultrasonic effects alter the flow of the molten pool and the thermodynamic con0ditions of the solidification process, ultimately achieving the refinement of the welded joint microstructure and the uniform distribution of elements. The efficacy of ultrasonic-assisted welding has been corroborated in the welding of aluminum alloys, titanium alloys, stainless steels, and other metals [23], [24].

Hua et al. [25] used the 30 kHz ultrasonic to assist nickel-alloy welding for reducing the Laves phases. The average concentration gradient of Nb was decreased 98 % with the introduction of ultrasonic. Wang et al. welded Inconel718 and Inconel738 alloys using the ultrasonic assisted TIG welding method. The microstructure results of Inconel718 shows that the ultrasonic promotes the uniform distribution of elements, change the distribution of Laves phase in Inconel718 alloy welding joint, destroys the continuous distribution of Laves phase and reduces the size of Laves phase [26], [27]. Zhou et al. used the 20 kHz ultrasonic vibration assisted welding to weld Hastelloy C-276 and SUS304 [28]. The microstructure of the welded joint reveals that ultrasonic vibration promotes the reduction of secondary phases and element homogenization. Wang et al. enhanced the grain shape of the weld joint using the ultrasonic-assisted laser welding technique and analyzed the beneficial impact of equiaxial grains on the mechanical properties of Inconel 690 weld joints from a fracture perspective [29]. However, despite its conventionality, the ultrasonic-assisted GTA welding of Inconel 690 still requires further investigation.

In this work, the ultrasonic vibration was induced into the Inconel 690 GTAW processing simultaneously through an ultrasonic transducer following the GTA torch. The microstructure and mechanical properties of UA-GTA and regular GTA welded joints were focused. Furthermore, grain refinement and the molten pool shape evolution mechanism are clarified. The characterization of fracture samples showed the relationship between the refined microstructure and strengthening performance. The ultrasonic vibration significantly improved the microstructure and mechanical properties of the Inconel 690 welded joint.

2. Experimental apparatus, materials, and methods

The UA-GTAW system includes the ultrasonic vibration part: an ultrasonic transducer, an ultrasonic power supply; the GTAW part: a welding supply, an GTA torch; the control part: 3-axis welding motion platform and control center. The systems are shown in Fig. 1. The base metal used for welding was the Inconel 690 plates, with dimensions of 250 mm in length, 80 mm in width, and 3 mm in height. The composition details of the Inconel 690 alloy can be found in Table 1. Pure argon (99.9 %) with a flow rate of 18 L/min was used as shielding gas. The arc current was 150 A, with a welding speed of 100 mm/min. The arc length was set to 3 mm, and the arc voltage is 14.7 V. The diameter of tungsten was 3.2 mm, and its tip angle was 45°. The ultrasonic power was 800 W, which corresponding to an amplitude of 29 μm. The frequency of ultrasonic vibration was 35 kHz. The ultrasonic horn is positioned posterior to the welding torch. The distance between the torch and horn is set at 60 mm, in accordance with previous research [30]. During the welding process, after the initiation of arc by the torch, synchronous movement of the ultrasonic horn with the torch ensues, ultimately facilitating steadfast integration of ultrasonic assistance throughout the welding process.

Fig. 1.

The schematic of the UA-GTA system.

Table 1.

Chemical compositions of the welding plates (mass fraction%).

Alloys	Cr	Fe	Mn	C	Cu	Si	Ti	Ni
Range	27–31	7–11	<0.5	<0.05	<0.5	<0.5	<0.15	Balance
Measured value	28.1	7.86	0.26	0.002	0.19	0.36	0.07	Balance

The weld joints were precisely cut into various sizes in accordance with specific requirements using wire-cut electrical discharge machining (WEDM). The samples used for characterization were sequentially polished and polished with 200 #, 800 #, and 2000 # sandpaper. Subsequently, electrolytic polishing is conducted using a mixture of perchloric acid and acetic acid under a direct current of 30 V for a duration of 30 s. Optical microscopy (OM) was employed to observe the penetration. The OM samples were etched using a mixture of Cr₂O₃ and H₂O under an applied voltage of 15 V for 15 s. Electron backscatter diffraction (EBSD; ZESS Gemini560) was utilized to investigate the morphology and orientation for grains in weld zone. Scanning electron microscopy (SEM; ZESS SIGMA 300) was conducted to characterize the distribution and morphology of the secondary phases. Electron probe microanalysis (EPMA; JEOL-8230) was performed to examine the elemental distribution. Transmission electron microscopy (TEM; FEI Talos F200X) was employed for phase analyze of the secondary phase. Samples for TEM analysis were prepared by slicing into dimensions of 0.5 mm × 5 mm × 5 mm, followed by ion thinning using a precision ion polishing machine (Gatan 695). The TEM data were processed using the Gatan Digital Micrograph software. Tensile strength of welds was tested with a tensile testing machine (Instron 5569). The hardness of weld zone was measured with a sclerometer (HVS-1000A).

3. Results and discussion

3.1. The effect of ultrasonic on weld zone morphology

Fig. 3 shows the weld zone shapes of the UA-GTAW and GTAW Inconel 690 joints. The action of ultrasonic vibration results in a significant alteration of the shape of the molten pool, and the depth and width of molten pool increase from 1.57 mm and 10.2 mm to 2.1 mm and 11 mm respectively. Concurrently, the morphology of the molten pool also has a transformation. In the regular GTA welded joint, the bottom of the molten pool is flat; however, upon the assisted of ultrasonic vibration, it assumes a W-shape which the penetration depth on both sides of the molten pool is greater than that in the center of the molten pool.

Fig. 3.

The macrophotograph of two kinds of Inconel 690 welded joints.

The morphology alteration of the molten pool is primarily attributed to the ultrasonic-induced convection of the liquid metal and the advancement of ultrasonic effects on the solid–liquid interface, as shown in Fig. 4. With the assistance of ultrasonic vibration, the high-frequency vibration of mechanical waves can cause intense molecular vibrations at the microscopic scale, increasing friction and internal energy dissipation between molecules, thereby converting into additional thermal energy. This additional thermal energy can supplement the heat generated during the arc welding process [31]. Especially in the solid–liquid interface, the minute vibrational stimulus provokes the local movement of atoms, prompting the transition of atoms at the interface from long-range order to long-range disorder (the arrangement of atoms in the liquid metal), thereby enhancing penetration [32]. Concurrently, the morphology alteration is mainly associated with the liquid metal convection caused by ultrasonic vibration. In regular GTAW, the flow of the molten pool is mainly determined by the arc pressure and surface tension. However, when the ultrasonic wave propagated into the weld molten pool, the ultrasonic waves caused periodic vibration of metal atoms, resulting in periodic changes in local density in the molten pool, thus forming alternating high-density areas and low-density areas. When metal atoms move toward the direction of vibration, a high-density area is formed, forming a positive pressure; while when atoms move away from the direction of vibration, a low-density area is formed, forming a negative pressure [33]. This local pressure difference drives the flow of liquid metal, further promoting convection phenomena, promoting excavation behavior on both sides of the melt pool, and then changing the morphology of the melt pool, as shown in Fig. 4.

Fig. 4.

Schematic diagram of the effect of ultrasonic vibration on the shape of the molten pool.

3.2. The effect of ultrasonic on microstructure

The EBSD results of the Inconel 690 weld zone are shown in Fig. 5. Fig. 5(a) and (c) show the grain morphology and orientation of regular GTA and UA-GTA welded joints. In both weld zones, the morphology of solidified grains is columnar grain growing along the opposite direction of the temperature gradient and there were no obvious orientations. The grain of the regular GTA weld zone has the preferred orientation in the (1 0 0) direction, but the pole figure of the (1 0 0) direction (Fig. 5(b)) shows that its maximum polar density is 1.65, which indicates that its grain orientation is anisotropy in the cross-section perpendicular to the welding direction. The maximum polar density of the grain of the ultrasonic-assisted weld zone is also small at 2.01. The ultrasonic vibration has little effect on the orientation of the grain of the weld zone.

Fig. 5.

IPF maps and pole figure of the welded joints (a-b) GTAW Inconel 690 welded joint; (c-d) UA-GTAW Inconel 690 welded joint.

However, there are significant alterations in the morphology and size of grains under the assistance of ultrasonic. Fig. 6 shows the alterations of grain morphology in different weld zones accompanied by quantitative statistical analysis results. The aspect ratio, defined as the ratio of the long axis to the short axis of the ellipse, which is fitted by grain, serves as the characterization index for grain morphology. The white lines within the grains in Fig. 6(a) and (d) represent the aspect ratio. Solidified grains with an aspect ratio exceeding 2 are categorized as columnar grains, while those below 2 are classified as equiaxed grains. Fig. 6(a) shows a higher number of yellow grains with a large aspect ratio in the regular GTA weld zone. Notably, columnar grains constitute 57.9 % of all grains and 94.21 % of the total grain area, as shown in Fig. 6(c). In contrast, the microstructure of the UA-GTA weld zone exhibits a significant alteration in grain morphology. Specifically, yellow grains with a large aspect ratio were markedly reduced, while cyan grains with a smaller aspect ratio experienced a substantial increase. Columnar grains with an aspect ratio greater than 2 constitute only 54.4 % of the total count, and their area proportion diminishes to 84.3 %, as shown in Fig. 6(f).

Fig. 6.

Grain morphology and Aspect ratio distribution of weld zones: (a) grain morphology of GTAW Inconel 690 welded joint; (b) statistics of grain aspect ratios; (c) distribution of the aspect ratio for different grains;(d) grain morphology of UA-GTAW Inconel 690 welded joint; (e) statistics of grain aspect ratios; (f) distribution of the aspect ratio for different grains.

Fig. 7 shows the grain boundaries misorientation angle and grain size distribution of the GTA and UA-GTA Inconel 690 welded joints. The total length of grain boundaries increased from 6.67 cm to 8.31 cm due to the grain refinement of the weld zone. The length of grain boundaries with the rotation misorientation between 5° and 15° also increased, from 2.41 mm to 6.2 mm, an increase of 157 %, as shown in Fig. 7(a) and (b). The increase in grain boundary length enhances the ability of the microstructure of welded joints to hinder crack propagation. The longer low-angle grain boundaries reduce stress concentration and improves compatible deformation capability. At the same time, the arrangement of atoms in proximity to low-angle grain boundaries is more orderly and acts as a potent barrier to hinder the movement of dislocations compared to the disordered arrangement of atoms at high-angle grain boundaries, improving the strength and hardness of the welded joint. Fig. 7(c) shows the grain size distribution of the GTA and UA-GTA welded joints. The percentage of grain size within the range of 100 to 200 μm in GTA welded joints is approximately 78 %. And the average equivalent diameter of grains is 145.6 μm. In the UA-GTA welded joint, the equivalent diameter is mostly concentrated between 30 and 110 μm (the percentage is 69 %) and the average equivalent diameter is 60.4 μm. The diminution of grain size not only exerts a substantial influence on enhancing the compatibility of deformation capacity among grains but more significantly, it diminishes the abundance of coarse columnar grains and concurrently lessens the straight grain boundaries that facilitate crack propagation. Fig. 8 shows the distribution of the grain boundary curvature in the GTAW and UA-GTAW Inconel 690 weld zone. The grain boundaries with larger curvature are colored in red, while the grain boundaries with smaller curvature show as green boundaries. In Fig. 8(a), the grain boundary between two grains tends to be tortuous, which is due to that the solidified grain boundaries in nickel-based alloy are formed by the intersection of multiple clusters of subgrains, and each cluster of subgrains has different growth direction and length, so its solidified grain boundary presents a more tortuous zigzag shape. But the grain boundary still has the tendency to reduce its grain boundary energy, especially in Inconel 690 alloys. The zigzag solidified grain boundary diminishes its inherent energy through the process of straightening. This leads to boundary migration, ultimately resulting in the formation of a straight migration grain boundary. As shown in Fig. 8(c), grain refinement results in the concentration of grain boundaries with larger curvature at the junctions of multiple grains in UA-GTA welding. Even if there is a migration of these grain boundaries, they continue to exhibit high curvature. Furthermore, due to an increase in the total length of the grain boundary, those with large curvature tend to be longer. This characteristic effectively inhibits crack propagation and encourages coordinated grain deformation.

Fig. 7.

Grain-boundary misorientation angle and grain size distribution of the welded joints: (a-b) grain boundary distribution of GTAW and UA-GTAW Inconel 690 welded joint; (c) grain size distribution of GTAW and UA-GTAW Inconel 690 welded joint.

Fig. 8.

Grain boundary curvature distribution: (a) the distribution and (b) the statistics of grain boundary curvature of GTAW Inconel 690 weld zone; (c) the distribution and (d) the statistics of grain boundary curvature of UA-GTAW Inconel 690 weld zone.

The alteration of grain morphology, grain size and grain boundary demonstrate a significant enhancement on the microstructure of the weld zone in Inconel 690 alloy due to ultrasonic assistance. In the area of the white dotted line in Fig. 5(c), many fine grains can be observed. According to previous work and grain morphology and orientation, these fine grains may be caused by ultrasonic action during the growth of dendrite. The influence of ultrasonic on the solidification microstructure of weld zone primarily manifests through the two processes of increasing the nucleation rate and hindering grain growth. From the perspective of changing the nucleation rate, the ultrasonic vibration may result in the cavitation effect and convection within the molten pool and influence the solidification process during the weld solidification stage. The temperature gradient (G) and solidification rate (R) are thereby subject to alterations in the ultrasonic effect. The process of cavitation collapse is characterized by a significant energy release, which mitigates the temperature gradient and cooling rate of the molten pool and supplies the nucleation energy. The formula for nucleation rate can be expressed as follows [34]:

$$N=C\times e^{(-\mathrm{\Delta }{G}^{\ast }/kT)}\times e^{(-Q_d/kT)}\ast$$ (1)

Here C is a constant, $\mathrm{\Delta }{G}^{\ast }$ indicates the critical nucleation-free energy, k is also a constant, T is the absolute temperature, and $Q_d$ is a temperature-independent parameter. Since the cavitation effect supplements the energy required for nucleation, with the decrease of $\mathrm{\Delta }{G}^{\ast }$, the nucleation rate will increase to a certain extent, so it can promote the grain refinement. The simultaneous occurrence of cavitation and acoustic streaming effects during grain growth also impedes dendrite growth by fragmenting the dendrite, as shown in Fig. 9.

Fig. 9.

Schematic diagram of UA-GTAW of Inconel 690 alloy.

The expression of cavitation threshold, which refers to the minimum sound pressure required to induce the cavitation effect in a molten pool, can be estimated using the following equation [35]:

$$P_B=P_0-P_V+\frac{2}{3\sqrt{3}}{[\frac{{(\frac{2\sigma }{R_0})}^3}{P_0-P_V+\frac{2\sigma }{R_0}}]}^{\frac{1}{2}}$$ (2)

where σ (N/m) is the surface tension coefficient of Inconel 690 alloy (approximately 1.83 N/m [36]), $P_0$ is 1 atm (0.1013 MPa), $P_V$ (MPa) represents the saturation vapor pressure (2000 Pa), R₀ is the nuclei radius of the dissolved air in the melt (approximately 10 μm). The calculation result of cavitation threshold in liquid Inconel 690 alloy is 0.2259 MPa.

The ideal sound pressure can be estimated by the following formula, assuming that the temperature of the molten pool is uniform and there is no convection:

$$P=\lambda \ast \mathrm{s}\mathrm{i}\mathrm{n}(2\pi ft)$$ (3)

Here λ is the ultrasonic amplitude of molten pool (approximately 4 μm), f is the ultrasonic frequency, t is the time. The results of ideal sound pressure calculation were below the cavitation threshold. However, due to non-uniform distribution of sound pressure within the molten pool, distinct regions with varying pressure densities exist [35], thereby facilitating cavitation occurrence in the higher-pressure area. The formation and subsequent rupture of cavities engender localized high temperatures and pressures, thereby giving rise to micro vortices. Concurrently, the augmentation of these micro vortices is facilitated by ultrasonic vibration. As a result, convection and agitation of the fluid enveloping the dendrite transpire, leading to a modification in the surface tension of the liquid metal. This results in stress concentrations on the dendrite surface, yielding localized high stresses [37], [38]. Simultaneously, the propagation of mechanical waves generated by ultrasonic vibrations in the liquid metal may give rise to surface waves and elastic waves. These waves distort at the interface between the dendrite surface and the liquid metal, causing local stress non-uniformity. The ultimate presence of strong local stresses on the dendrite surface induces stress and generates dislocations within the dendrite. These local stresses and dislocations facilitate deformation and cracks in the dendrite, leading to local dendrite fractures [39]. This localized fracture process facilitates the development of the refinement of the dendrites, which in turn promotes a finer microstructure of the welded joint.

Fig. 10 shows the distribution of the element within the Inconel 690 weld zone but Ni and highlights the disparity in Fe, Cr, and C content between GTA and UA-GTA welded joints. In regular GTA welded joints, the formation of the Cr₂₃C₆ compound between the dendrites leads to a reduction in Cr content between the dendrites, increasing the likelihood of corrosion failure in the service environment of nuclear plants. As illustrated in Fig. 10(a), the Cr content between the dendrites exhibits a relatively low concentration, in contrast to the high Cr content within the dendrite cores. In the UA-GTA welded joint, the distribution of the Cr element under ultrasonic vibration is obviously more uniform, as shown in Fig. 9(c). The distribution concentration of Cr between the dendrites and in the core of the dendrite is almost the same. In addition, the distribution of Fe and C elements is also affected by ultrasonic vibration, resulting in a more uniform distribution in the UA-GTA welded joint. These findings suggest that the incorporation of ultrasonic in the welding process of Inconel 690 alloy not only enhances grain morphology and size but also promotes the homogeneous distribution of elements, resulting in a more consistent performance of the welded joints. However, ultrasonic vibration appears to have minimal impact on the second phase present on the solidified grain boundary. By employing secondary electron microscopy to examine the distribution of the second phase, it was ascertained that this phase is present within and between the dendrites. Notably, more the second phases were observed at the interface of the dendrites, while it rarely presents at the core. And Fig. 11(a) and (c) show the effect of ultrasonic on the distribution and morphology of secondary phase is limit. Fig. 12(a) shows the bright field image of the secondary phase at the interface of dendrites. The HR-TEM images and the corresponding FFT images show that the crystal structure of the secondary phase is face-centered cubic, similar to that of the matrix (γ (Ni, Cr, Fe)), as shown in Fig. 12 (b) and (c). And the secondary phase is incoherent with the matrix. The super EDS results presented in Fig. 12(e) indicated that the secondary phase has been identified as MC carbide. The segregation of Cr element is avoided under the influence of ultrasonic; however, the distribution of the second phase remains unaltered primarily due to distinct elemental segregation behaviors in the molten state. The element redistribution during solidification of Inconel 690 alloy can be estimated with the help of Brody–Fremings formula as follows [40].

$$C_S=k{C}_0{[1-\frac{f_s}{1-\alpha k}]}^{k-1}$$ (4)

Fig. 10.

The distribution of elements on the welded joint (a-b) GTAW Inconel 690 welded joint; (c-d) UA-GTAW Inconel 690 welded joint.

Fig. 11.

The distribution of secondary phases in the welded joint (a-b) GTAW Inconel 690 welded joint; (c-d) UA-GTAW Inconel 690 welded joint.

Fig. 12.

The TEM result: (a) bright-field image of secondary phase; (b-c) HRTEM micrograph and its Fast Fourier Transform image of secondary phase; (d) Inverse Fast Fourier Transform (IFFT) image; (e) super EDS images.

Here, C_s is the component concentration at the solid–liquid interface, C₀ is the alloy composition concentration, f_s is the solid phase fraction, k is the equilibrium distribution coefficient, and α is the dimensionless coefficient indicating the degree of solid phase diffusion. The k values of Ni, Cr and Fe are greater than 1, however the k value of Ti is less than 1. Consequently, Ti atoms readily induce component segregation at the solid–liquid interface Due to its low diffusion activation energy (only 135 kJ/mol), which is approximately half that of other alloying elements (Cr, Fe), C tends to accumulate in dendrite gaps during solidification and form carbides with other alloying elements. The segregation tendency of interdendritic carbide is challenging to be mitigated by the application of ultrasonic treatment. Consequently, the potential impact of ultrasonic on modifying the distribution pattern of MC carbides remains limited [41].

3.3. The effect of ultrasonic on mechanical properties

The mechanical properties of Inconel 690 alloy were evaluated through tensile and hardness tests conducted on the GTA and UA-GTA welded joints. Tensile samples were oriented orthogonally to the welding direction, as shown in Fig. 2, to assess the performance of the welded joints. Fig. 13 shows the tensile strength and corresponding curves for both GTA and UA-GTA welded joints. Ultrasonic vibration-induced microstructural optimization significantly enhances the tensile strength of UA-GTA welded joints, surpassing that of GTA welded joints. The mean yield strength (YS), ultimate tensile strength (UTS), and elongation values for regular GTA welded joints were recorded as 320 MPa, 591 MPa, and 25.1 %, respectively. Conversely, these values increase to 387 MPa, 672 MPa, and 31.6 % for UA-GTA welded joints. Compared to UA-GTA welded joints, notable enhancements of approximately 20.9 %, 13.7 %, and 25.8 % were observed in yield strength (YS), ultimate tensile strength (UTS), and elongation for GTA welded joints. Fig. 14 shows the tensile fractures of GTA and UA-GTA welded joints. Both samples exhibit ductile fractures characterized by distinct dimples on the fracture surface. The tensile fracture of the GTA and UA-GTA welded joint demonstrates a combination of small and large dimples. However, the fracture of UA-GTAW displays slightly larger dimples than the regular GTAW. Lower magnification fractography reveals that both GTAW and UA-GTAW exhibit similar diameters for the dimples, while in UA-GTA, there appears to be an increased depth of dimples. Deeper dimples in UA-GTAW indicate prolonged plastic deformation stages, indicating enhanced plasticity, as shown in Fig. 13(b).

Fig. 2.

Schematic of position and dimensions of samples cutting (unit: mm).

Fig. 13.

Tensile properties and tensile curve of GTAW and UA-GTAW Inconel 690 welded joints.

Fig. 14.

The fracture of samples after the tensile test: (a), (b), (c) and (d): GTA welded joint; (e), (f), (g) and (h) UA-GTA welded joint.

The synergistic enhancement of strength and toughness of the UA-GTA samples is mainly attributed to the changes in grain slip systems and the coordinated deformation of polycrystals. Fig. 15(a) and (c) show the Schmid factor mappings in the grains of regular GTA and UA-GTA weld zone, respectively. The color of grains represents the maximum Schmid factor of each grain under horizontal loading conditions in mappings. The slip direction and plane of the slip system corresponding to the max Schmidt factor in the grain were marked with red arrows and blue lines in the mappings. The Schmidt factors of the whole grains in two kinds of weld zone were greater than 0.3. In the GTA and UA-GTA welds, 66.9 % and 77.9 % of the grains exhibit Schmidt factors greater than 0.45, indicating a pronounced propensity for slip under load. The grains of GTA and UA-GTA weld zone were both no oriented, as shown in Fig. 5. But the grains in the UA-GTA weld zone exhibited the better deformability, with approximately 80 % of the grain area capable of undergoing slip under load perpendicular to weld, representing a 27 % increase compared to that observed in the GTA weld zone. At the same time, the grain size in the UA-GTA weld is smaller, and the deformation coordination ability is better under load, so it shows more plastic deformation ability, as shown in the Fig. 13(b).

Fig. 15.

The slip transmission analysis within the grains at the weld zone of regular GTA and UA-GTA: (a) Schmid factor mapping in the grains of regular GTA weld zone; (b) distribution of slip directions; (c) Schmid factor mapping in the grains of UA-GTA weld zone.

Fig. 16 shows the hardness test results. The shade of color in the figure signifies the hardness value of each location within the welded joints. In regular GTA welded joints, a significant disparity is observed between the hardness of the weld zone and that of the base metal. The hardness at the central weld zone is marginally higher, while the hardness at both sides of the weld zone is lower. The average value of the overall hardness is 232 HV_0.2. In the UA-GTA welded joint, although a certain discrepancy persists between the weld zone hardness and the base metal, the overall hardness is markedly higher than that of the regular GTA weld joint. It also shows the characteristic of higher hardness at the center of the weld zone and lower hardness at the sides. The average overall hardness reaches 261 HV_0.2. The application of ultrasonic vibration significantly enhances the weld joint hardness. Fig. 17 shows the impact of microstructure resulting from Vicker-pyramid indenting. Minute cracks are visible in the indentation of the GTA weld zone. These cracks, measuring in the micron scale, may propagate along the dendrite. However, the short length of the crack indicates that its further extension is limited by the zigzag dendrite boundaries. No crack was observed in the indentation of the UA-GTA weld zone. The microstructure of hardness indentations further corroborates the role of ultrasonic in optimizing microstructure and mechanical properties.

Fig. 16.

Hardness of GTAW and UA-GTAW Inconel 690 welded joints.

Fig. 17.

The indentation after hardness test (a): GTA; (b): UA-GTA.

4. Conclusions

The study proposes a strategy of utilizing ultrasonic vibration to assist in GTA welding of Inconel 690 alloy. By microstructure characteristics and mechanical properties tests, the influence of ultrasonic on melt pool morphology, grain morphology, element distribution, and macro–micro mechanical properties was investigated. Furthermore, a correlation between process-structure–property was established. The following conclusions are drawn:

• (1)The stable assistance of ultrasonic during the welding process of Inconel 690 alloy was achieved by synchronizing the movement of the ultrasonic horn and GTA torch. The assistance of ultrasonic enhances penetration and optimizes weldability in Inconel 690 alloy by inducing acoustic softening and leveraging the acoustic streaming effect.
• (2)The grain in the weld zone undergoes significant refinement, with the average grain diameter decreasing from 145.6 to 60.4. Simultaneously, ultrasonic vibration optimizes the grain morphology by transforming larger aspect ratio columnar grains into smaller aspect ratio ones. This change in both size and morphology leads to a modification of grain boundary shape and type, effectively reducing crack-sensitivity in Inconel 690 alloy welds.
• (3)The application of ultrasonic assistance modifies the flow of the molten pool, thereby mitigating elements segregation in the weld zone and further enhancing its consistency. However, it has limited impact on the distribution of MC carbide due to the low diffusion barrier of carbon element.
• (4)As a result of the refinement of microstructure and optimization of grain orientation, the yield strength of the weld joint increased by 20.9 %, while the elongation rate experienced a significant improvement of 25.8 %. Furthermore, there was an enhancement in overall hardness within the weld zone, with an increase from 232 HV_0.2 to 261 HV_0.2.

CRediT authorship contribution statement

Yunhao Xia: Writing – original draft, Visualization, Investigation. Bolun Dong: Writing – review & editing, Methodology. Xiaoyu Cai: Supervision, Funding acquisition. Sanbao Lin: Supervision, Resources, Methodology, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.