In-situ observations of melt degassing and hydrogen removal enhanced by ultrasonics in underwater wet welding

Materials and Design 188 (2020) 108482

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In-situ observations of melt degassing and hydrogen removal enhanced by ultrasonics in underwater wet welding Hao Chen a,b, Ning Guo a,b,c,⁎, Kexin Xu b, Changsheng Xu a,b, Li Zhou a,b, Guodong Wang a,b a b c

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China Shandong Institute of Shipbuilding Technology, Weihai 264209, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Melt degassing phenomenon enhanced by ultrasound is studied by an in-situ imaging method. • Effects of ultrasonic power on weld porosity and diffusible hydrogen content are investigated • Appropriate ultrasonic power decreases the porosity and diffusible hydrogen content in the welds. • Mechanism of ultrasonic degassing and diffusible hydrogen decreasing are discussed.

a r t i c l e

i n f o

Article history: Received 11 October 2019 Received in revised form 27 December 2019 Accepted 6 January 2020 Available online 10 January 2020 Keywords: Underwater welding Ultrasonic melt processing Solidification Diffusible hydrogen In-situ X-ray imaging

a b s t r a c t In this study, the phenomenon of gas removal behaviors in the melt pool and the flow behaviors of melt pool were observed using an in-situ X-ray imaging method. Based on the ultrasonic effects, a novel processing method was developed to reduce porosity and decrease diffusible hydrogen content in the deposited metal. The effects of ultrasonic output power on the size and frequency of gas bubble collapsing were studied. Results showed that with the assistance of ultrasonic, the gas bubble size were smaller and the collapsing frequency was significantly increased. The porosity decreased from 1.4% to 0.5% and the hydrogen diffusible content decreased from 24.5 to 18.6 ml/100 g when the ultrasonic power increased to 720 W. The possible hydrogen removal mechanism was proposed by two aspects including the microcosmic and macrocosmic scale. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction As a method of high efficiency and extremely low cost, underwater wet welding is widely applied in offshore oil platforms, nuclear power plants and ship emergency repairs because it is no need of any complicated device [1,2]. Unfortunately, during the wet welding process, the welding consumables and melt pool are exposed directly in the water, which results in the dissociation of water [3]. The higher contents of ⁎ Corresponding author at: No.2 Wenhuaxi Road, Weihai 264209, China. E-mail address: [email protected] (N. Guo).

oxygen and hydrogen presenting in the melt pool will increase porosity and hydrogen-induced cracks in the wet welding joint [4]. The rapid solidification of deposited metal prevents the gas bubble escaping from melt pool in time owing to the efficient cooling effect of the surrounding water, which is also an important reason that resulted in the welding porosity [5]. In recent studies, a larger number of pores and high diffusible hydrogen level in the wet welding joint has been reported. Świerczyńska et al. studied the effect of underwater wet welding parameters and conditions on the diffusible hydrogen content in deposited metal [6]. Silva et al. analyzed pores and completed characterization of defects in underwater wet welds using lab scale X-

https://doi.org/10.1016/j.matdes.2020.108482 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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ray Micro CT [7]. The existing of surrounding water has unique effects on the melt pool behaviors, including fluid flow and gas bubbles escaping during the wet welding process. How to reduce the welding porosity and diffusible hydrogen content is a worthwhile project for improving the quality of wet joint and deposited metal. As a newly developed processing method, ultrasonic-assisted is introduced into the conventional welding field and becomes a research hotspot in recent years [8–10]. Depend on the ultrasonic cavitation and acoustic streaming, the ultrasonic-assisted method can promote wetting and spreading of molten metal, fragment grain, eliminate defects such as pores or lack fusion and enhance mechanical properties of deposited metal [11–14]. Yang et al. found that a short-time ultrasonic field evidently enhanced the wettability of Sn-37Pb/Cu compare to a common wetting field which had no introduced ultrasonic [11]. Yuan et al. and Liu et al. both produced effective grain refining by introducing the ultrasonic energy into molten metal, respectively [12,13]. Ji et al. studied the effect of ultrasonic transmission rate and obtained high strength joints in the ultrasonic-assisted brazing of Cu to alumina [14]. Because of the limitation of observing methods, the studies in underwater wet ultrasonic-assisted flux-cored arc welding (UAFCAW) manly focus on microstructure evolution and mechanical property of welding joints. But little research has been reported on the melt pool behavior in the field of UAFCAW and the only research is studied by numerical analysis method [15–17]. In order to directly observe the behaviors of melt pool, various techniques have been adopted, such as ultrasonic sensing, infrared sensing, imaged laser matrix dots and laser back-lighted shadow graphic method. Huang et al. studied the oscillation of stationary weld pool surface using the detecting and sensing method based on laser vision [18]. On that basis, Li et al. developed the laser-photoelectric-conversion method to accomplish the real-time measurement of weld pool oscillation frequency in GTAW-P process [19]. Liu et al. realized the visual sensing of the topside weld pool based on a low-cost CCD camera through image processing of the captured raw images [20]. Wang et al. observed the inner phenomenon of keyhole and molten pool through glass plate by high speed camera and spectrometer in high power density laser welding [21]. However, these methods cannot overcome the water perturbation surrounding the arc burning area. Rely on the short wavelength, high energy and strong penetrability, radiography has been well applied to in-situ study physical phenomena, such as pores formation, microstructure evolution, mass transfer and crack propagation, occurring inside of metallic materials [22–24]. The contrast of the image is obtained based on the X-ray energy absorbed by materials with different densities. It is feasible to observe melt pool behavior by using the in-situ X-ray imaging method, especially for the gas evolution behavior that is not possible to acquire using other techniques. For instance, Miyagi et al. observed the formation of porosity in the molten pool during solidification and cooling by X-ray imaging technology [25]. In this paper, with the help of X-ray imaging method, the effects of ultrasonic power on the gas evolution behaviors in melt pool are insitu observed. This research is aimed at revealing the influencing mechanism of ultrasonic on gas removal and diffusion hydrogen content in the UAFCAW process. 2. Experimental 2.1. Experimental platform design The schematic diagram of underwater wet UAFCAW system and insitu X-ray imaging system are shown in Fig. 1. The UAFCAW system contains the ultrasonic power, the ultrasonic transducer, the ultrasonic horn, the constant-voltage welding power source and the moveable platform. Welding with direct current electrode positive (DCEP) was conducted in a water tank placed on the moveable platform with a water depth of 0.5 m. The electrical energy was transformed into ultrasonic vibration by the ultrasonic transducer. The ultrasonic horn

amplified the vibration and transferred the ultrasonic energy to the workpiece. The frequency of ultrasonic vibration was 27 kHz and the maximum output power was 1200 W. During the welding process, the workpiece moved with the water tank by the driving of the step motor. And the ultrasonic horn kept a constant distance (30 mm) from the welding torch to ensure it didn't melt because of arc burning. 2.2. Materials and welding process The bead welding was performed on the surface of the E40 steel plates with dimensions of 120 mm × 40 mm × 10 mm. The filler material was a specially developed tubular self-shielded TiO2-Fe2O3 slag system flux-cored wire with a diameter of 1.6 mm. The sheath materials of flux-cored wire were low-carbon steel strip. Some metal powders such as Mn and Ni were added in the flux-core to improve the mechanical property of weld metal. The nominal chemical composition of base metal and filler metal (only steel sheath) are listed in Table 1. The optimized parameters in conventional wet welding are listed as follows: arc voltage of 28 V, wire feed speed of 3.5 m/min, workpiece traveling speed of 2 mm/s. The contact tip to work distance (CTWD) was 20 mm and the wire extension was 15 mm. In order to research the effects of ultrasonic vibration, different output powers of 240 W, 480 W, 720 W, 960 W and 1200 W, were used. By contrast, the welding experiment without ultrasonic was conducted with the same welding parameters. The X-ray imaging system consists of a micro-focused X-ray source, an image intensifier converting the X-ray transmitted image to the visible image, a high-speed camera, and a computer. The image intensifier converted the X-ray transmitted image to the visible image which is captured by the high-speed camera (Optronis CR series with maximum fps up to 5000 fps). In order to obtain a clear image of melt pool and gas bubble in the melt pool, the high-speed camera parameters with 256 × 256 pixels chip and frame rate of 1000 fps were used. To ensure X-ray radiolucency, the X-ray source parameters of 130 kV and 0.3 A were employed. The consecutive images in the movie of melt pool behaviors and gas evolution behaviors were extracted and analyzed in the underwater wet welding process. 2.3. Materials characterizations The porosity of deposited metal was quantitated and calculated by the Archimedean principle, as shown in Eq. (1). δ¼

  ρ 1− 1  100% ρ0

ð1Þ

where δ is the porosity, ρ0 is the actual density of deposited metal, ρ1 is the apparent density of the measured deposited metal. The diffusible hydrogen content in deposited metal was determined according to the gas chromatography (HD-6 provided by NCS Testing Technology CO., LTD), which could provide more accurate results compared with the glycerin method. Under each experimental parameter condition, at least four specimens were examined in the determination experiments of porosity and diffusible hydrogen content. The deposited metal and the heat-affected zone were etched with a 4% nitric acid solution. The microstructures were observed using an OLYMPUS GX51 optical digital microscope. 3. Results and discussion 3.1. Morphological evolution of melt pool obtained in the air and water As a universal phenomenon in the processing of liquid metals, pores exist widely in metal products such as castings and weldments [22]. The main reason is that the huge gap in gas solubility between the metal liquid and solid. Such as hydrogen dissolved in steel, the solubility in

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Fig. 1. Schematic diagram of the experimental configuration and an example X-ray image annotated to show the characteristic to help the observation during the experimental process.

molten metal increases by a factor of 5 or more. This results in gas enrichment and formation of bubbles ahead of the solidification front. Once the bubbles cannot escape from the molten metal in time, which will be finally trapped into pores [26,27]. In order to capture the gas evolution behaviors, the in-situ X-ray imaging of melt pool during the FCAW process was performed, as shown in Fig. 1. Fig. 2a shows the typical gas bubble formation and escaping behavior in melt pool during the FCAW process in the air. It can be observed that the melt pool is long and relatively flat. Some bubbles exist in the melt pool with smaller sizes (about 0.5–1.0 mm in diameter) as shown in Supplementary movie 1. With the cooling of the melt pool, as the result of reduction in gas solubility, most of the gases dissolved in the molten metal are degassed from pool in the term of bubbles. At the time of t0 + 8 ms, the figure shows the generation process of the gas bubble. Then, at the time of t0 + 44 ms, more bubbles are produced and become the bubble clusters. From t0 + 52 to t0 + 72 ms, these bubbles move toward the end of the melt pool because of Marangoni-driven flow [28,29]. From t0 + 100 to t0 + 224 ms, they are expanding and floating up to the upper surface of the melt pool. At this stage, the small bubbles coalesce into several larger bubbles (about 1.0–1.5 mm in diameter). In the end, as shown at t0 + 292 ms, these bubbles completely escape from the melt pool and collapse at the interface between vapor and melting metal liquid. This process of gas discharge is over and the next cycle is beginning. About the source of gas existing in the melt pool, in addition to the above-mentioned reason, the gases also can be generated by the decomposition of core powders in the welding wire and the mixed slag and liquid metal [30,31]. Fig. 2b shows the typical bubble behavior in the melt pool during the conventional underwater wet FCAW process. The completely differences in bubble formation, movement and escaping behavior are Table 1 Chemical composition of the base metal and filler metal (only steel sheath) (wt%). Material

C

Mn

Ni

Cr

Si

P

S

Fe

E40 Filler metal

0.17 0.10

1.36 0.41

0.05 -

0.02 -

0.45 0.05

0.006 0.035

0.3 0.025

Bal. Bal.

shown under two different welding environments. Compared with welding in the air, the images and movies reveal a shorter melt pool with more violent oscillation. From t1 to t1 + 8 ms, several bubbles (about 1.5 mm in diameter) generate due to the gas evolution in the cooling molten metal. However, the surrounding water accelerates the cooling rate of melt pool. So, the bubble coalesces and grows rapidly caused by a lot of gas degassed from melt pool. Meanwhile, because of the hydraulic pressure, the gas bubbles must gather together until getting enough diameter (about more than 4.0 mm) to escape from the melt pool, as shown from t1 + 17 to t1 + 36. One phenomenon worth noting is that the large bubbles tend to escape at the end of melt pool, which indicates that the process of bubble coalescence needs enough time. Fig. 2b also uncovers the phenomenon that gas escapes from the melt pool and melt pool fluctuations (see details in Supplementary movie 2). With the expanding of the gas bubble, Once the gas pressure exceeds the surface tension of the bubble, the bubble collapses (t1 + 24 ms and t1 + 106 ms) and ejects a cloud of gas (t1 + 36 ms and t1 + 134 ms). Some views suggested that most of gas in the bubble came from the electrolysis and evaporation of water surrounding the arc burning zone. The spectrum of arc plasma proved that H atoms became involved due to the decomposition of water [32]. It also can be evidenced by the high content of diffusible hydrogen in deposited metal [33–35]. 3.2. Effects of ultrasonic on melt pool dynamics Based on the cavitation and acoustic stream, ultrasonic treatment can be used to promote the flow of melt pool and remove hydrogen in the molten metal [36,37]. Using the in-situ X-ray imaging method, a series of images of melt pool dynamics are captured. In order to compare the melt pool dynamic behaviors more accurately, the first images of all series (from t2 to t6) are captured at the welding time of 20 s. Because it is a relatively stable stage during the whole manufacturing processing. Fig. 3a shows gas bubble behaviors and melt pool dynamics when the ultrasonic output power is 240 W. The first bubble forms (t2 ms), expands (t2 + 5 ms) and then collapses (t2 + 9 ms) (see details in Supplementary movie 3). Compared with the conventional wet welding

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Fig. 2. Time-series radiographs showing dynamic evolution of the melt pool morphology and the gas bubble in the melt pool obtained using the same experimental parameters without ultrasonic-assisted: melt pool created in (a) air environment and (b) water environment. Red dotted circles and arrows indicate the process of bubble generating, expanding and collapsing. Blue dotted circles and arrows highlight the location of the molten droplets. Purple arrows show the gas escaping from the collapsed bubble.

process (Fig. 1b), this whole process takes a shorter time. The next two new bubbles collapse quickly in the process of moving to the end of melt pool. In addition, some gas also exists in the inside of droplet and the enters melt pool along with the droplet transfer process (t2 + 106 ms). And then, the gas bubble coalesces with other new bubbles and moves to the end of the pool. At t2 + 186 ms, the expanding bubble collapses and ejects the gas. An obvious change is that the volume of the gas bubble has a significant decrease because of the introduction of ultrasonic wave. More turbulent flowing in the melt pool is also obtained (t2 + 142 ms). Fig. 3b–e illustrate the melt pool dynamics during ultrasonicassisted underwater wet welding with an ultrasonic output power of 480 W, 720 W, 960 W and 1200 W, respectively (see details in Supplementary movie 4–7). In addition to the above significant changes, it is clearer to observe that more tiny bubbles coalesce and become a large bubble. More bubbles collapse at the front of melt pool instead of the end of that, which is shown in Fig. 3d (t5 + 69 ms). With the increase of ultrasonic wave energy, the images reveal more violent fluctuations of liquid metal in the melt pool. The bubbles move toward to the end of melt pool and collapse within several milliseconds, as shown in Fig. 3b and e. Moreover, many melt pool shock spatters can be observed, as marked by the dark yellow dotted circles. In general, three typical spatter modes exist in the wet welding process, including “droplet repelled spatter”, “explosive spatter” and “melt pool shock spatter” [38]. Jia et al. studied two spatter generation mechanisms, i.e. “droplet repelled spatter” and “explosive spatter” [39]. However, it was difficult to recognize the “melt pool shock spatter” by common visualization method in their researches. The stronger acoustic streaming effect induced by more ultrasonic energy accelerates the fluid flow and oscillation of the melt pool. Under the influence of ultrasonic vibration, the flow velocity of melt pool can reach 1.0 m/s or more. The rapidly relative motion between the melt pool and water suggests the presence of the Kelvin Helmholtz instability at the interface, which leads

to some melts stripped from melt pool. In addition, the hot liquid metal keeps vaporizing the surrounding water and produced a vapor film around the melt pool due to the Leidenfrost Phenomenon [40]. The vapor film eventually evolves into a bubble, and the rising of the bubble due to the buoyancy effect carries these melts as spatters away (see Fig. 3e and Supplementary movie 7). This demonstrates a possible generation mechanism of the “melt pool shock spatter”. The average diameter and the frequency of bubble collapsing are counted and recorded in Fig. 4a. In order to eliminate the influence of process stability and heat input changes, all the data is collected over the same period of welding time. In the conventional wet FCAW without ultrasonic, the average diameter of bubble collapsing is 4.21 mm with the frequency of 10.21 Hz. A relatively slow gas evolution process reveals that more time is consumed to coalesce a large enough bubble. When an ultrasonic vibration with 240 W output power is used, the average diameter of bubble collapsing significantly decreases to 2.01 mm and the frequency increases to 19.69 Hz. This result shows that the introducing of ultrasonic wave inhibits the production of larger gas bubbles while enhances the efficiency of gas escaping from the melt pool. In recent study, we found that the transient cavitation will produce a large impact on the bubble, which promotes the large bubbles to break down into the smaller ones [41]. With further increasing ultrasonic power to 480 W, the volume of bubble shows a sharp increase, from 2.01 mm to 3.01 mm. The bubble collapsing frequency reaches a peak value of 37.51 Hz, which means that more bubbles generate and collapse during the identical time. In other words, increasing the output power of ultrasonic also promotes the gas releasing and escaping from the melt pool. As the ultrasonic output power increases to 720 W, the diameter keeps going up to the maximum value of 3.30 mm. However, the frequency of bubble collapsing shows inflection point and decreases to 28.12 Hz. As the ultrasonic power increases up to 1200 W, the bubble collapsing diameter also

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Fig. 3. Time-series radiographs of dynamic evolution of the melt pool morphology acquired during underwater wet welding under the same welding parameters but different ultrasonic output power: (a)–(e) Ultrasonic output power are 240 W, 480 W, 720 W, 960 W and 1200 W, respectively. Red arrows indicate the process of bubble generating, expanding and collapsing. Yellow dotted lines highlight that gas exists in the inside of droplet and enters melt pool. Blue arrows show the moving direction of bubble in the melt pool. Tawny dotted circles highlight the “melt pool shock spatter”.

reveals a decrease from 3.30 mm to 2.61 mm. The bubble collapsing frequency also has a decreasing trend when the ultrasonic output power is more than 480 W, while the frequency value is still much bigger than that of with lower ultrasonic power (240 W) or without ultrasonic. It is worth noting that the bubble collapsing diameter increases firstly and decreases later as ultrasonic energy further increases. The reason for this phenomenon is related to the combined action of cavitation and acoustic stream induced by ultrasonic. The

previous studies have observed the generation of cavitation bubble by a synchrotron X-radiography method [42]. The researchers have confirmed that the acoustic cavitation and streaming flow play a crucial role in the fragmentation of the intermetallic dendrites [43]. In this study, the ultrasonic induces the formation of cavitation bubble in the melt pool during the welding process. These smaller bubbles rapidly move and coalesce with the larger gas bubble driven by the acoustic streaming effect. So, the bubble collapsing diameter

6 H. Chen et al. / Materials and Design 188 (2020) 108482 Fig. 4. Statistic analysis and contrast of gas bubble characteristics in melt pool under different ultrasonic output power: (a) The changes of bubble collapsing size and frequency with the increase of ultrasonic output power and (b) The moments of bubble collapsing in the same amount of time (1200 ms) under conditions without ultrasonic-assisted and with 480 W ultrasonic output power.

H. Chen et al. / Materials and Design 188 (2020) 108482

increases firstly, so do the collapsing frequency when ultrasonic power is 480 W. The stronger ultrasonic vibration results in stronger acoustic streaming, which will inhabit the expanding and coalescence of gas bubble once more. Meanwhile, the presence of acoustic streaming leads to that large gas bubble cannot exist in a stable state. Some small bubbles have escaped from melt pool before they coalesced and expanded to a large bubble, as shown in Fig. 3d and e. These bubbles with small size also decrease the average collapsing diameter. Fig. 4b shows the moments of bubble collapsing in the same amount of time (1200 ms) under conditions without ultrasonicassisted and with 480 W ultrasonic power, respectively. The red line on the chart shows that the periodicity of bubble collapsing during ultrasonic-assisted welding is more obvious than that of without ultrasonic. Besides, it also reveals that the frequency of bubble collapsing has a significant increase. According to Figs. 3 and 4, it can be concluded that the gas evolution behavior has obtained the maximum enhancement when the ultrasonic output power is in the range between 480 W and 720 W. The effect of degassing doesn't appear to be further increased at a higher output power of ultrasonic. This result is consistent with the previous research by Chen et al. [44]. By further reducing the thickness of the workpiece, we obtained clearer image and movie of melt pool. Fig. 5 reveals the contrast of bubble formation in the melt pool between without and with ultrasonic-assisted (480 W) during underwater wet welding (see Supplementary movies 8 and 9). Fig. 5b shows that the larger ultrasonic power induces stronger cavitation effect, which produces more tiny cavitation bubbles and prevents the generation of large gas bubbles. However, too many cavitation bubbles perhaps remain

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in the deposited metal and then become the micro-pores because they cannot escape from melt pool in time. 3.3. Effects of ultrasonic on weld appearance Fig. 6 shows the weld appearances and X-ray nondestructive testing images without ultrasonic-assisted and with different ultrasonic output power. Fig. 6a exhibits the morphology features of weld bead obtained by the conventional wet welding method without ultrasonic-assisted. The weld surface appearance is generally acceptable, but some invisible imperfections inside the weld also are revealed by X-ray detection image. The irregular shadows existing on the X-ray image indicate the pits located on the surface of weld bead. The circular shadows marked by the red dotted line reveal the locations of pores existing inside of deposited metal. It is observed that the pores often generate at the starting point and ending point of the weld. Obviously, this result is due to the welding heat input reduction at these locations. For the gas bubble, it is too late to escape from melt pool because of the rapid cooling of deposited metal. Fig. 6b–d show the weld bead characteristics welded by ultrasonic-assisted wet welding method with ultrasonic output power of 240 W, 480 W, and 720 W, respectively. The assistance of ultrasonic significantly reduces the number of pores and decreases the size of pores in weld bead. The obvious pores located at the weld starting point and ending point have disappeared or became less noticeable. As shown in Fig. 6c, the red dotted line marks some very small pores (compared with Fig. 6a) which locate at the ending point of weld. These fine pores perhaps are the tiny cavitation bubbles remaining in the deposited metal due to the rapid solidification of melt. This result has confirmed the cavitation bubble formation (Fig. 6b). However,

Fig. 5. The contrast of bubble formation in the melt pool during the underwater wet welding process: (a) without ultrasonic-assisted and (b) with ultrasonic-assisted (480 W).

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Fig. 6. The weld appearance, X-ray nondestructive testing images without ultrasonic-assisted and under different ultrasonic output power: (a) under condition without ultrasonic; (b)– (f) ultrasonic output power are 240 W, 480 W, 720 W, 960 W and 1200 W, respectively.

when the output power increases to 960 W even 1200 W, the number and distribution of bubbles are increased (Fig. 6e and f). The cause of this result has been explained in the previous paragraph. In addition to the changing of bubble evolution behaviors, higher ultrasonic power also induces the formation of welding spatters and the distortion of weld shape. As shown in Fig. 6d, the white dotted line shows the trace of spatters on both sides of weld. The increase of ultrasonic power notably increases the number of welding spatters. The violent vibration of ultrasonic horn sharpening the cavitation of surrounding water, which induces the outer water drastic fluctuation. Many studies have point out that the droplet was surrounded by the arc bubble which was generated by the burning of powder inside the flux-core wire and the evaporation of water [45–47]. The water flow also influences the arc bubble and drives the gas flow. According to the previous research, when arc bubble detaches and rises in the water, a gas flow drag force will be produced and hinders the droplet to transfer into the melt pool. This result is like those in the previous study that we found that it was easier for droplet to become “droplet repelled spatter” in a water flow-field [48].

Fig. 5f displays that some droplet repelled spatters which are firmly welded on the substrate surface and cannot be removed. The blue dotted line shows the distortion changing of weld shape. Too strong ultrasonic vibration aggravates the irregular melt flow in the melt pool. The weld shape shows a severely distortion after the rapid solidification of melt pool, which means that the assistance of ultrasonic with too high output power is not conducive to obtain a good weld shape in the underwater wet welding process. 3.4. Characteristics of porosity and diffusible hydrogen According to quantitatively study the effect of ultrasonic-assisted on gas removal, the porosity of deposited metal is measured by Archimedes method, as shown in Fig. 7. It could be found that porosity shows a clear reduction from 1.4% to 0.5% when the ultrasonic output power increases from 0 to 720 W. As the power continues to increase, the porosity of deposited metal increases to a small extent instead. The porosities of weld with ultrasonic power of 960 W and 1200 W are

H. Chen et al. / Materials and Design 188 (2020) 108482

approximately 1.2% and 0.95%, respectively. The quantitative conclusions are consistent with the observation results of the visual detection image of deposited metal above. The enhanced effect of ultrasonic on degassing or porosity decreasing is not always proportional to ultrasonic output power. Because the production of many cavitation bubbles cannot be ignored when excess ultrasonic wave power is introduced into melt pool. The minimal size of the cavitation bubbles which can escape from the weld pool before solidification can be calculated according to the formula of Stokes, as shown in Eq. (2). ve ¼

2ðρL−ρGÞgR2 9η

ð2Þ

where ve is the velocity of escaping from the weld pool, ρL and ρG are the density of the molten steel (7.86 × 103 kg m−3) and hydrogen in bubbles (0.0899 kg m−3), respectively, g is the gravity constant, η is the viscosity of the melt (5.88 × 10−3 N s m−2) and R the radius of the cavitation bubble. In the previous study, the depth (D) of weld pool is 2.36 mm and the solidification time (t) is 6.7 s under the same experimental parameters [41]. Assuming the cavitation bubble moving from bottom of weld pool to the surface, the minimum value of ve is approximately 0.35 mm/s (ve = D/t). The minimal pore diameter that can escape from the weld pool is approximately 22 μm according to Eq. (2). In theory, when the diameter of the cavitation bubble is smaller than the critical value of 22 μm, these cavitation bubbles cannot escape from weld pool and become pores existed in the solidified weld metal. However, observations show that most of the bubbles will move farther due to the flow of molten metal. So, the minimal pore diameter is more than 22 μm. Combining the production of a large number of cavitation bubble, it could be explained why the porosity increases again with the further increase of ultrasonic output power. The changes of diffusible hydrogen content in deposited metal under different ultrasonic power also are shown in Fig. 7. It can be clearly observed that underwater wet weld shows quite high content of diffusible hydrogen and the value can reach to 24.5 ml/100 g. The diffusible hydrogen of weld obtained in air atmosphere (marked by blue dotted line) is about 14.24 ml/100 g. Compared with this result, the rapid cooling of melt inhibits the escape of free hydrogen atoms due to the water environment. With the assistance of ultrasonic wave, the content of diffusible hydrogen decreases to 18.6 ml/ 100 g as the ultrasonic power increases to 720 W. Compared to the diffusible hydrogen content of that weld without ultrasonic, this is a relatively lower value and close to the value obtained in the air

atmosphere. The previous researches indicate that the hydrogenating of deposited metal is a major cause leading to the formation of quenched structures and cold cracks, especially in the heat-affected zone (HAZ) [49,50]. Fig. 8a–c shows the optical microscope photographs of deposited metal and the magnified image of HAZ obtained without ultrasonic and with the assistance of ultrasonic (480 W and 720 W), respectively. Fig. 8d shows an obvious hydrogen induced crack in coarse grain heat-affected zone (CGHAZ) where the coarsest martensite forms. This crack tends to propagate perpendicular to the direction of the longitudinal tensile residual stress. It is well known that the increase of diffusible hydrogen will promote the susceptibility of both the deposited metal and HAZ to hydrogen-induced crack. However, in the ultrasonic-assisted wet weld bead, the microcrack sizes show a significant decrease when the ultrasonic power is 480 W (Fig. 8e). Fig. 8f shows that there is no visible crack occurring in the HAZ of wet weld bead with ultrasonic-assisted of 720 W. This result reveals that ultrasonic vibration can effectively reduce the diffusible hydrogen content in deposited metal. In addition, it is important to note that when the ultrasonic power continues to increase, the graph shows an increase, rather than a decrease in the content of diffusible hydrogen. This law of changing is consistent with the change of porosity in the deposited metal, as shown in Fig. 7. Hence, the abnormal increase of diffusible hydrogen content is likely to be linked to the strong cavitation effect. 3.5. Mechanism analysis of ultrasonic vibration Some studies pointed out that the melt pool is supersaturated with hydrogen, rising from the vapor bubble surrounding the burning arc [32,51]. In this case, the ratios of the growth rates of the gas bubble Vb and the extension of the crystallization front V c reflect the probability of the pore formation. If Vb b Vc, the gas bubbles cannot separate from the growing crystallites and float up in time, which forms several pores in the deposited metal. Obviously, contacting directly with water environment causes a shortening of the cooling time in the method of wet welding. Fig. 9a and b reveal the schematic illustration of pores formation in the slow and rapid crystallization process, respectively. The gas bubbles floating slower than the crystallization rate will be trapped in the solidified deposited metal and become pores. Zhang et al. employed workpiece vibration to break the dendrite arms by introducing the bending stress on them and the remaining dendrites are shorter [52]. By introducing the ultrasonic vibration into the melt pool, more violent melt flow will be induced by cavitation and acoustic streaming. There have been many reports that oscillating ultrasonic bubbles can effectively fragment not only secondary dendrite arms but also the primary dendrite ones under high ultrasound power [53,54]. So, ultrasonic has been applied widely as an efficient solution to provide structure refinement in melt processing [55,56]. In this study, the flow of molten metal around the weld pool can be compared with a classical fluid mechanics problem that of flow past a circular cylinder [57]. The melt flows are the turbulences with rapid heat transfer, and it is easier to break the dendrites for the stronger turbulence [52]. The turbulence intensity increases with the increases of Reynolds number (Re) and the Re is defined by Eq. (3). Re ¼ ρvL=μ

Fig. 7. The porosity and diffusible hydrogen content of deposited metal during the underwater experiment at different ultrasonic power. (The error bars are the standard deviation of statistics data.)

9

ð3Þ

where, ρ is the density of molten metal, v is the mean velocity of molten metal, L is the characteristic length, and μ is the dynamic viscosity of molten metal. The density of molten steel is 7.86 × 103 kg m−3 and the dynamic viscosity of steel is 5.88 × 10−3 N s m−2 above 2000 K [58]. The characteristic length is approximately the width of weld, of about 12 mm (measured from Fig. 6). By measuring the movement distance and time of molten metal in the high-speed X-ray images, the mean velocity of molten metal can be obtained. Results show velocities

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Fig. 8. Optical microscope photographs of deposited metal and the magnified images of the heat-affected zone (HAZ). (a)–(c) Microstructure images of deposited metal obtained underwater without ultrasonic, with ultrasonic-assisted of 480 W, and with ultrasonic-assisted of 480 W. (d)–(f) Magnified images of HAZ highlighted by red boxes in (a)–(c).

of molten metal are 0.35 m/s in conventional wet welding without ultrasonic and 0.71 m/s in ultrasonic-assisted welding with 720 W output power, respectively. This means that the calculated value of Re is doubled, increasing from 5614 to 11,389. Ultrasonic vibration accelerates the flow of weld pool and breaks the stable status of grain growth. In this case, the gas bubbles already trapped by crystallized dendrites are released back into molten metal because of the breaking of dendritic crystals, as shown in Fig. 9c. The gas bubbles escaping from solidified metal continue to float up in the melt. So, compared with the conventional weld, the pores in the weld with ultrasonic-assisted are more likely to exist on the weld surface rather than inside the weld. This conclusion is consistent with the observed micro-pores on the surface of weld in Fig. 6c.

The solubility of hydrogen in iron as a function of temperature is shown in Fig. 10 [59]. With the decrease of temperature, the solubility of hydrogen shows a gradual decrease when the temperature is below 2600 °C. Then, the solubility of hydrogen decreases sharply when the temperature decreases to the melting point of iron, about 1538 °C. In addition, because the crystal structure transforms from δ-Fe to γ-Fe and αFe, the solubility of hydrogen will continue to fall sharply at the temperature point of phase transition. Some researches pointed out that the arc atmosphere was mainly H2 in the underwater wet welding [32,41]. So, there is supersaturated hydrogen dissolved in the melt pool under the arc zone because of high temperature and high partial pressure of hydrogen. Before the solidification of molten metal, the supersaturated

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Fig. 9. The schematic illustration of pores formation in the melt pool under different crystallization conditions: (a) in the slow crystallization rate; (b) in the rapid crystallization rate and (c) under the condition of ultrasonic vibration.

hydrogen forms hydrogen bubbles and escapes from the liquid metal in order to make the hydrogen concentration below the solubility limit. Ultrasonic waves generate large alternating stresses within a liquid by creating regions of positive pressure and negative pressure. The cavitation bubble can form and grow during the stage of negative pressure. The negative pressure in the expanding bubble promotes part of free hydrogen into the bubble [36,60]. Then the hydrogen is

transported out of the melt pool with the floating up and breaking of bubble and this process is illustrated in Fig. 11a. However, the abundance of hydrogen remains in the deposited metal due to the sudden fall in hydrogen solubility when the liquid metal is solidified, followed by two falls of hydrogen solubility because of phase transition. It is unlikely that the supersaturated hydrogen precipitates as the form of bubble in the solidified metal. Instead, hydrogen atoms

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The localized high temperature and pressure can accelerate the precipitated hydrogen to diffuse into the interstitial positions of lattice, forming more interstitial solid solutions. Ultrasonic effects also enhance the chemical reaction between hydrogen and metallic element involved in flux-cored wire such as lithium and nickel, which leads to the generation of more unstable metal hydrides. Fig. 11b shows the enhancement mechanism of atomic diffusion that explains the phenomenon that diffusible hydrogen content increases at a higher ultrasonic output power (more than 960 W). 4. Conclusions The key effects of ultrasonic vibration on the dynamic behavior of melt pool for the wet welding are in-situ observed by the X-ray transmission method. The evolutions of gas-bubble in the melt pool under different ultrasonic output power acquire comparison and analysis. The conclusions are as follows: Fig. 10. The solubility of hydrogen in iron as a function of temperature [59].

are precipitated from melt in three forms: interstitial solid solutions, unstable metal hydrides and hydrogen traps. In general, the source of diffusible hydrogen is mainly the diffusion motion of hydrogen existing in the interstitial solid solutions and unstable metal hydrides. Acoustic streaming can accelerate the dissolution rate of particles by promoting elemental diffusion [61]. The previous study pointed out that the rapid implosive collapse of the cavitation bubble can induce tiny hot spots with the localized extreme temperature and pressure estimated to be 5000 K and 0.1 GPa, respectively [62].

(1) Ultrasonic energy breaks the large gas bubble and produces tiny cavitation bubbles, which decreases the average size of bubbles in the melt pool. Ultrasonic cavitation and acoustic streaming overwhelmingly promote the bubbles escaping from the melt pool. (2) The porosity decreases from 1.4% to 0.5% and the hydrogen diffusible content decreased from 24.5 to 18.6 ml/100 g when the ultrasonic power increased to 720 W. The porosity and diffusible hydrogen content decreases at first and then increases with an increase in the ultrasonic power. (3) Over-strong ultrasonic effects induce excessive cavitation bubbles, increasing the porosity in deposited metal. The amount of

Fig. 11. Schematics of the ultrasonic effects on the diffusible hydrogen during solidification of melt pool. (a) Effect of cavitation bubble on hydrogen removal under a proper ultrasonic output power. (b) Enhancement mechanism of H atomic diffusion at a higher ultrasonic output power.

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interstitial solid solutions and unstable metal hydrides are increased by cavitation and acoustic stream, which increase the diffusible hydrogen content in the weld.

Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2020.108482. Author contribution statement Hao Chen: Conceptualization, Software, Formal analysis, Writing original draft. Ning Guo: Writing - review & editing, Validation, Supervision, Resources. Kexin Xu: Investigation, Software, Visualization. Changsheng Xu: Investigation, Visualization. Li Zhou: Formal analysis, Data curation. Guodong Wang: Conceptualization, Supervision, Project administration. 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. Acknowledgements The authors are grateful for the financial support for this study from the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.201602, HIT.NSRIF.201704, HIT.MKSTISP.201617), the Shan dong Provincial Key Research and Development Plan (Grant No.2016ZDJS05A07, 2017CXGC0922, 2018GGX103003) and Natural Science Foundation of Shandong Province (Grant No. ZR2017QEE005, ZR2017PEE010). References [1] H. Li, D. Liu, Y. Yan, N. Guo, Y. Liu, J. Feng, Effects of heat input on arc stability and weld quality in underwater wet flux-cored arc welding of E40 steel, J. Manuf. Process. 31 (2018) 833–843, https://doi.org/10.1016/j.jmapro.2018.01.013. [2] J. Wang, Q. Sun, S. Hou, T. Zhang, P. Jin, J. Feng, Dynamic control of current and voltage waveforms and droplet transfer for ultrasonic-wave-assisted underwater wet welding, Mater. Design 181 (2019), 108051. https://doi.org/10.1016/j.matdes. 2019.108051. [3] N. Guo, Y. Fu, Y. Wang, Y. Du, J. Feng, Z. Deng, Effects of welding velocity on metal transfer mode and weld morphology in underwater flux-cored wire welding, J. Mater. Process. Tech. 239 (2017) 103–112, https://doi.org/10.1016/j.jmatprotec. 2016.08.019. [4] E.C.P. Pessoa, A.Q. Bracarense, E.M. Zica, S. Liu, F. Perez-Guerrero, Porosity variation along multipass underwater wet welds and its influence on mechanical properties, J. Mater. Process. Tech. 179 (2006) 239–243, https://doi.org/10.1016/j.jmatprotec. 2006.03.071. [5] E. Padilla, N. Chawla, L.F. Silva, V.R. Dos Santos, S. Paciornik, Image analysis of cracks in the weld metal of a wet welded steel joint by three-dimensional (3D) X-ray microtomography, Mater. Charact. 83 (2013) 139–144, https://doi.org/10.1016/j. matchar.2013.06.016. [6] A. Świerczyńska, D. Fydrych, G. Rogalski, Diffusible hydrogen management in underwater wet self-shielded flux cored arc welding, Int. J. Hydrogen Energ. 42 (2017) 24532–24540, https://doi.org/10.1016/j.ijhydene.2017.07.225. [7] L.F. Silva, V.R. Dos Santos, S. Paciornik, J.C.E. Mertens, N. Chawla, Multiscale 3D characterization of discontinuities in underwater wet welds, Mater. Charact. 107 (2015) 358–366, https://doi.org/10.1016/j.matchar.2015.07.030. [8] H. Ji, H. Chen, M. Li, Overwhelming reaction enhanced by ultrasonics during brazing of alumina to copper in air by Zn-14Al hypereutectic filler, Ultrason. Sonochem. 35 (2017) 61–71, https://doi.org/10.1016/j.ultsonch.2016.09.003. [9] C. Chen, C. Fan, X. Cai, S. Lin, C. Yang, Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method, J. Mater. Process. Tech. 271 (2019) 144–151, https://doi.org/10.1016/j. jmatprotec.2019.03.030. [10] Q.J. Sun, W.Q. Cheng, Y.B. Liu, J.F. Wang, C.W. Cai, J.C. Feng, Microstructure and mechanical properties of ultrasonic assisted underwater wet welding joints, Mater. Design 103 (2016) 63–70, https://doi.org/10.1016/j.matdes.2016.04.019. [11] Y. Yang, S. Li, Y. Liang, B. Li, The wetting phenomenon and precursor film characteristics of Sn-37Pb/Cu under ultrasonic fields, Mater. Lett. 234 (2019) 92–95, https:// doi.org/10.1016/j.matlet.2018.09.005. [12] T. Yuan, S. Kou, Z. Luo, Grain refining by ultrasonic stirring of the weld pool, Acta Mater. 106 (2016) 144–154, https://doi.org/10.1016/j.actamat.2016.01.016.

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