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Article

Evaluation of Arc Signals, Microstructure and Mechanical Properties in Ultrasonic-Frequency Pulse Underwater Wet Welding Process with Q345 Steel

by
Shixiong Liu
1,
Hao Ji
1,
Wei Zhao
1,
Chengyu Hu
1,
Jibo Wang
2,*,
Hongliang Li
1,*,
Jianfeng Wang
3 and
Yucheng Lei
1
1
School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
2
Fujian Provincial Key Laboratory of Welding Quality Intelligent Evaluation, Longyan 364012, China
3
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China
*
Authors to whom correspondence should be addressed.
Metals 2022, 12(12), 2119; https://doi.org/10.3390/met12122119
Submission received: 7 November 2022 / Revised: 29 November 2022 / Accepted: 5 December 2022 / Published: 9 December 2022
(This article belongs to the Section Welding and Joining)
Browse Figures
Versions Notes

Abstract

:
The ultrasonic-frequency pulse underwater wet welding (UFP-UWW) process was achieved through a constant-voltage-mode power source connecting an ultrasonic-frequency pulse power source in parallel. The arc voltage and welding current waveforms, sound signal variations, microstructural characteristics and mechanical properties at different parameters were investigated. The results showed that the ultrasonic-frequency pulse voltage and current of the UFP-UWW process displayed a periodic high-frequency oscillation on the based values of the conventional UWW process. The arc stability of the UFP-UWW process improved owing to the fact that the proportions of the unstable arc burning region could be reduced to 1.56% after the introduction of the ultrasonic-frequency pulse current. No significant changes in weld width and penetration were observed while the weld dilution rate increased to 54.2% for the combination of 40 V–30 kHz, compared with the results of the conventional UWW process. The flux-cored arc (FCA) welding arc in the air had the same frequency response to the ultrasonic excitation signal, which verified the existence of the ultrasonic-frequency vibration induced by the periodic high-frequency electromagnetic forces. The application of the ultrasonic-frequency pulse produced finer columnar grains in the welds with an average length of 315 μm, although the amount of pro-eutectoid ferrite and acicular ferrite varied little. The mechanical properties of the welded joints were also noticeably enhanced with the application of different ultrasonic excitation frequencies. The optimum tensile strength and impact toughness of the welded joint were improved by 6.7% and 21.7% when the applied ultrasonic excitation voltage was 40 V for a pulsed frequency of 30 kHz. These results facilitate the application of ultrasonic arc welding technology in the marine field.

1. Introduction

Corrosion and fatigue damage in marine and offshore environments are major causes of primary steel strength degradation during service. It is important to research and develop underwater welding or repair technology for such structural components [1,2]. Underwater wet welding (UWW) is gaining more popularity in marine engineering for its high efficiency, comparatively low cost and simple operation. However, the UWW process is particularly complex, since arc burning, metal transfer and weld pool solidification are severely influenced by the water. On account of this fact, the fast cooling rate, high tendency to produce hydrogen-induced cracks and the poor process stability are the main issues with this welding process [3,4,5].
Domestic and foreign scholars have improved the quality of underwater wet welded joints from the directions of welding consumables, welding process stability and externally assisted techniques [6,7,8]. Currently, the main research focuses on suitable wet welding consumables for underwater welding are the electrodes and self-shielded flux-cored wires. Ferritic consumables with the comprehensive properties of low hydrogen content, good arc stability and satisfactory mechanical properties have been developed through optimizing the slag systems or the alloying system [9,10,11]. New types of austenitic stainless-steel-based and nickel-based filler metals have also been designed for the UWW process to enhance the properties of wet welded joints [12]. A detailed founding of the metal transfer process suggests that a large droplet repulsive transfer with a low frequency exists in the UWW process [13]. Excessive hydrogen and dynamic bubbles also cause poor arc stability in the UWW process. Based on the previous study, the optimization of welding consumables and process parameters alone cannot effectively reduce the number of defects and improve the quality of an underwater wet welded joint. Thus, it was essential to employ an aiding process.
New processes have been proposed to improve the underwater wet welding quality, such as tempered bead welding [14], real-time induction heating [15] and ultrasonic [16] and pulse wire feeding [17]. In particular, ultrasonic-assisted underwater welding can effectively improve the arc stability and welding quality, and it has been studied by many researchers. Some scholars have applied mechanical ultrasound at different locations of the welding process to enhance the properties of the weld metals by reducing the porosity, decreasing diffusible hydrogen content and crack sensitivity together with refining the grain [18,19,20].
Nevertheless, with the available aiding measures it is rarely possible to apply additional forces on the molten droplet caused by the obstruction of the water environment and arc bubbles. That is, the benefits of the mechanical ultrasound are counteracted because of this effect. Mechanical ultrasound has limitations in terms of the design cost and the flexibility in the welding process. An underwater pulse current FCAW with a pulse frequency of 20 Hz was proposed by Jia et al. to periodically regulate the forces applied on droplets, thereby improving the wet welding process stability [21,22].
In recent years, ultrasonic-frequency pulse arc welding (UFP-W) has been increasingly researched as an excellent welding technology by virtue of its high arc energy density, stable arc burning and high-quality welds. This technology has been successfully used in tungsten inert gas (TIG) welding [23], submerged arc welding [24], metal inert gas (MIG) welding [25] and shielded metal arc welding (SMAW) [26]. More importantly, UFP-W is efficient to reduce gas pores, improve fluidity in molten pool and promote the uniform distribution of alloy systems due to the drastic vibration of the molten pool caused by the electromagnetic force with ultrasonic frequency. Fang et al. found that the introduction of UFP-TIG with Q345 steel helped to refine the grains in the weld zone and increase the amount of acicular ferrite [27]. Chen et al. preliminarily investigated the feasibility of an ultrasonic-frequency pulse high current in the underwater wet welding process and pointed out that UFP-UWW helped to improve the welding process stability and increase the ductility of the welds [28].
Previous work by Lei et al., in our research group, introduced an ultrasonic arc in TIG and AC plasma welding, which had a current switch frequency of 10~100 kHz and a voltage adjustment range of 10–90 V (corresponding to a current adjustment range of approximately 30 A) [29]. These processes were successfully applied to the welding of aluminum alloy [30], aluminum matrix composite materials [31], oxide dispersion-strengthened alloys [32] and China low-activation martensitic steel [33]. The results showed that ultrasonic-frequency pulse arc welding could obtain high-quality welded joints with a uniform microstructure, finer grains, fewer pores and better mechanical properties. These benefits are meaningful for underwater wet welds to reduce the diffusible hydrogen level and mitigate the porosity problem at greater water depths. It should be noted that, although both Lei et al. and Chen at al. adopted a dual-power source to realize the ultrasonic coupling arc welding, the ultrasonic exciting current showed a large difference [28,29]. Chen et al. used a UFP current source with ultrasonic current up to 80 A, while the research of our group was conducted at a current of approximately 30 A. A high pulse current often means that a bulky UFP power supply and lengthy cables are needed, which may restrict the application of the UFP-UWW process. Therefore, it is necessary to develop a UFP-UWW process with a low ultrasonic exciting current.
The purpose of this paper was hence to combine ultrasonic-frequency arc welding technology with underwater wet welding. The influences of ultrasonic excitation voltage and frequency on the weld formation, microstructure and mechanical properties of underwater wet welded joints were studied. The main contribution of this paper is to provide a thorough experimental validation of the proposed UFP-UWW process with a low excitation current, which would improve the arc stability, microstructure and mechanical properties compared to the conventional UWW process. The advantage of applying the UFP-UWW process to overcome the adverse effects of extreme underwater environments has significant research and application prospects.

2. Materials and Experimental Methods

Underwater wet welding experiments were conducted at a depth of 0.5 m in fresh water. The base metal selected was low-alloy Q345 steel with the dimensions of 220 mm × 60 mm × 6 mm, produced by China Baowu Steel Group Co., Ltd. (Shanghai, China). Figure 1 exhibits the microstructure of the base metal, which consisted of white-banded ferrite and a small amount of black pearlite. The consumable with a diameter of 1.6 mm was a TiO2–Fe2O3 slag system self-shielded flux-cored wire. The chemical composition of the filler metal and the base metal used in this study are provided in Table 1. The chemical compositions of the Q345 steel were provided by the manufacturer. The compositions of the filler metal were analyzed by optical emission spectrometry (Hitachi High-Tech Analytical Science, Shanghai, China). The parameters for the ultrasonic-frequency pulse underwater wet welding are illustrated in Table 2. The conventional underwater wet welding parameters were an arc voltage of 27 V, wire feed speed of 3.6 m/min and a welding speed 0.003 m/s.
Figure 2 depicts a schematic diagram of the ultrasonic-frequency pulse underwater wet welding system, including the welding power source, ultrasonic-frequency pulse power source, welding motion system and current–voltage acquisition system. A pulse MIG-350 (Aotai Electric Co., Ltd., Ji’nan, China) welding power source was utilized in the underwater wet flux-cored arc welding process with the direct current electrode positive (DCEP) mode. The ultrasonic-frequency pulse power source was coupled with the conventional MIG-350 welding power source (Aotai Electric Co., Ltd., Ji’nan, China), which produced a high amplitude voltage (100 V) and ultrasonic frequencies (20 kHz~100 kHz) in pulsed form under 50% duty cycle. A robot (KUKA Robotics China Co., Ltd., Shanghai, China) was implemented as the welding motion system.
The electrical signal acquisition system was composed of a Hall current sensor (SW3t200c100v6, Beijing STM Measurement & Control Technology Co., Ltd., Beijing, China), a Hall voltage sensor (SA1t50v5v6, Beijing STM Measurement & Control Technology Co., Ltd., Beijing, China) and an NI data acquisition card (USB6251, National Instruments Corporation, Austin, America). After the coupling, the welding current was measured using a current sensor, and the arc voltage between the tip of the welding torch and the workpiece was obtained by a voltage divider resistor. Subsequently, the NI data acquisition card along with the PC was used to collect and store the welding current and arc voltage simultaneously. Eventually, an off-line analysis of the welding current and arc voltage was performed according to the application designed in the LabVIEW software (2021 SP1, National Instruments Corporation, Austin, TX, USA).
The collection of arc sound pressure was conducted using a 4939 microphone (Brüel & Kjær, Nærum, Denmark) with a 4 mV/Pa sensitivity and in a 4 Hz~100 kHz measurement frequency range. The mechanical energy of the sound waves was converted into electrical signals, and the signal collected by the microphone was amplified using a 2670 microphone preamplifier (Brüel & Kjær, Nærum, Denmark). The sound pressure signal was then measured by a NEXU2690 modulation amplifier with a 316 mv/Pa scale conversion (Brüel & Kjær, Nærum, Denmark).
After welding, the transverse sections were extracted to analyze the microstructures of the welded joints. As described in GB/T13298-2015, the metallographic samples were ground to a 1500-grit finish, polished with 1 μm diamond solutions and then etched with 4% nitric acid alcohol to reveal the weld metal microstructure using an optical microscope. The ISO 4136:2001 standard was used to conduct a tensile test with a DDL-type electronic universal testing machine at a speed of 5 mm/min. Following the ISO 9016:2001 standard, a Charpy impact test was carried out to measure the impact toughness values at room temperature. Each group of tensile and Charpy impact tests was repeated three times at least. Finally, scanning electron microscopy was used to observe and analyze the tensile fracture and impact fracture.

3. Results and Discussion

3.1. Oscillograms of the Coupling Welding Current and Arc Voltage

The oscillograms of the arc voltage and welding current of the conventional UWW are given in Figure 3. Obviously, the measured values of the voltage and current fluctuated, and the variation of the current was sharper than that of the arc voltage because a constant-voltage power source was used in the welding process. Dong et al. suggested that the severe fluctuations of the electrical signal were mainly attributed to the frequent floating and bursting of bubbles in arc combustion [34].
When welding with the UFP-UWW process, the electrical signal significantly changed, as displayed in Figure 4a. A high-frequency current was superimposed on a conventional current waveform with an average value of 190 A, generating the final arc voltage and coupling current shown Figure 4b. The arc voltage had periodic variations after the excitation of the ultrasonic-frequency pulse current. The employed frequency in the current was measured to be approximately 30 kHz, and the amplitude was approximately 30 A. The capacitive and inductive components in the isolated-coupling device affected the non-stationary response process of the circuit, leading to triangular cyclical characteristics of the excitation current, which is shown in Figure 4c.
The formation mechanism of the ultrasonic-frequency pulse current could be explained as follows [35]. Through the parallel coupling of the ultrasonic-frequency excitation signals with the conventional welding arc, the arc plasma experienced periodic contraction and expansion to excite the ultrasonic wave. Then, the arc voltage and welding current also presented a high-frequency oscillation on the base value. The previous study also indicated that the welding arc had a good dynamic response characteristic without the restriction of ultrasonic-frequency ranges, and the arc also oscillated with the ultrasonic-frequency pulse [36].
Figure 5 shows the arc voltage and welding current waveforms at different ultrasonic excitation voltages at a constant frequency of 30 kHz. It can be seen that the HF pulse current was successfully superimposed on the welding current of the conventional UWW process. The measured welding current and arc voltage pulsated with the same frequency of 30 kHz. The amplitude of ultrasonic-frequency voltage increased from approximately 7.5 to 15 V when the excitation voltage ranged from 20 to 40 V, as illustrated in Figure 5b,c. When the excitation voltage was higher than 40 V, the amplitude of arc voltage basically tended to stable at a value of 15 V.
Figure 6 shows the arc voltage and welding current waveforms at different ultrasonic frequencies at an ultrasonic excitation voltage of 40 V. Increasing the pulse frequency from 20 to 60 kHz, the arc voltage and welding current in the UFP-UWW process also exhibited similar features with the same frequency. The amplitude of the ultrasonic-frequency pulse voltage was measured to approximately 15 V and maintained constant within the investigated frequencies. Meanwhile, the fluctuations of ultrasonic-frequency voltage were more pronounced at high frequencies, which may deteriorate the arc stability of the underwater wet welding process.
From Figure 3 and Figure 4, it can be seen that the welding arc for the conventional UWW and the UFP-UWW process had many arc extinction and short-circuit regions. The short-circuit region (arc voltage < 10 V) and arc extinction region (arc voltage > 55 V) are two parameters that help to analyze the welding stability between constant-voltage and pulse-current wet FCAW. In order to examine the UFP-UWW process’s stability, the proportions of low (<10 V) or high (>55 V) arc voltage with different welding parameters were computed, as shown in Table 3. The proportions of the short-circuit regions (arc voltage < 10 V) and arc extinction regions (arc voltage > 55 V) for the different excitation voltages were 1.56% at 20 V, 2.46% at 40 V and 2.8% at 60 V. This meant that the proportions of low (<10 V) or high (>55 V) arc voltage areas were apparently lower than that the constant-voltage mode (2.70). This implied a steadier welding process with this welding parameter.
On the other hand, when increasing the pulse frequency from 20 to 60 kHz, the proportions of short-circuit regions and arc extinction regions rose from 3.28% to 14.64%. This result indicated that a number of short-circuit regions and arc extinction regions existed in the UFP-UWW process. It was indicated that a too high pulse frequency (>50 kHz in this study) was not conducive to improving the stability of the welding process.

3.2. Arc Sound Signal Analysis

The welding process itself is a heating process, which contains the vibrating plasma atmosphere and the vibration inside the liquid pool. During the welding process, the pulsation of plasma flow and the intrinsic vibration frequency of molten pool will be diffused out in the form of sound waves. The plasma flow inside the arc itself is the basic sound source to generate vibration. When the motion of the vibration source causes the particles in the surrounding arc atmosphere to deviate from their normal position and spread from near to far, the arc sound signal waveform is finally formed [37]. In this section, the sound signal analysis of the FCA welding arc in the air was carried out to verify the possible existence of ultrasonic-frequency vibration in the arc plasma. Once it was confirmed that there was ultrasonic-frequency sound in the arc, the violent stirring of weld pool caused by cavitation and acoustic streaming effects could be used to explain the effects of the UFP current on weld quality. Therefore, the sound pressure at different excitation parameters was preliminarily identified. The time-domain feature of the arc sound signal describes the welding process’s variability with time as the independent variable [38]. The time-domain features could reveal the partial variance in the welding process, but additional features need to be further extracted to identify the weld quality. The frequency-domain feature was used to analyze the arc sound signal based on the fast Fourier transform (FFT).
Figure 7 describes the time-domain and frequency-domain images of the sound frequency of the conventional FCAW arc in the air. The frequency band of the conventional FCA welding arc was mainly concentrated at 0~20 kHz. Figure 8 illustrates the frequency-domain images of the arc sound signal in UFP-FCAW in the air. Obviously, after the introduction of the ultrasonic-frequency current, the sound pressure amplitude of the FCAW arc changed. In addition to being concentrated in 0~20 kHz, the frequency-domain image also had corresponding high-frequency components in the ultrasonic-frequency band. When the excitation frequency was 30 kHz, the arc sound frequency had an ultrasonic band near 29.97 kHz, which showed that the FCA welding arc had the same frequency response to the ultrasonic excitation signal. Meanwhile, the amplitude value of the FCA arc sound pressure appeared to increase and then decrease with increasing exciting voltage, as shown in Figure 8a–c.
Meanwhile, the amplitude value of the FCA arc sound pressure decreased with the increasing excitation pulse frequency. The above results verified the existence of the ultrasonic-frequency vibration induced by the periodic high-frequency electromagnetic forces. Meanwhile, the amplitude of sound pressure induced by the arc excitation represented the intensity of the ultrasonic-frequency vibration in the arc, which could be used to improve the welding quality including promoting the arc stability, refining the microstructure and enhancing the mechanical performance [39].

3.3. Weld Appearances

The weld morphology and cross-section at different process parameters are displayed in Figure 9. It can be seen that the weld surfaces with the conventional UWW process had some defects, such as weld pits, spatters or serpentine weld. After the introduction of the UFP, the weld appearances were improved with few defects. Compared to the conventional UWW process, the unstable arc burning region of the UFP-UWW process in cases 1–4 became narrow, which meant that there were few weld pits and spatters. Meanwhile, the ultrasonic-frequency pulse current could increase the arc pressure and arc stiffness in a certain frequency range, making the arc more directional and improving the quality of the weld appearance. When the pulse frequency was more than 50 kHz in cases 5–6, the weld appearances became poor, as shown in Figure 7d–h. This may be attributed to the too high pulse frequency, leading to the excessive rise of a Lorentz force and the radial electromagnetic force in the arc, which would hinder the droplet transfer and increase the number of spatters. Thus, the weld appearance, with pits and spatters, was not continuous and smooth.
Figure 10 illustrates cross-sections under different ultrasonic excitation parameters. The geometry was similar in all cases in terms of the shape. The corresponding values of weld width, weld penetration and weld dilution rate were shown in Figure 11. Figure 11a shows the measured width, penetration and dilution rate as a function of excitation voltage for a pulsed frequency of 30 kHz. With increasing the excitation voltage from 20 to 60 V, the average values of the weld width were firstly increasing then decreasing slightly. The weld penetration for tests 2 and 4 approximately equaled to that acquired without ultrasonic excitation. When the excitation parameter was 40 V–30 kHz in test 3, the maximum weld width of 11.9 mm was obtained while the weld penetration was only 3.7 mm. For the same parent and welding method, the weld geometry mainly depended on the welding heat input. Table 4 lists the calculated welding heat input of every test. It was obvious that the welding heat input at test 3 was minimal. The introduction of ultrasonic current should increase the heat input of the welding process, but the calculated results did not support this conclusion. This was because the ultrasonic frequency electromagnetic field accelerated the thermal convention and part of the heat input transferred to mechanical energy [28]. Meanwhile, a proportion of energy from the ultrasonic current was consumed in the coupling circuit. Cunha [40] also found a lack of a quantitative relation between the weld geometry and the ultrasonic excitation parameters. Voigt [24] also observed this phenomenon when applying the UFP current to the submerged arc welding process. The layer of flux enveloping the welding arc may be a reason why the arc was not free to oscillate. The arc burning process in underwater surroundings was inevitably influenced by the disturbance of bubbles and the water flow, which may decrease the arc stiffness exerted by the application of an ultrasonic-frequency pulse.
Regarding the weld geometry as a function of the pulsed frequency for an excitation voltage of 40 V, some different results were obtained, as depicted in Figure 11b. The average values of the weld width were all less than that acquired without ultrasonic excitation, which could be expected to decrease with the arc constriction caused by the ultrasonic-frequency current. The weld penetration was still maintained at approximately 4 mm, except for that at the pulsed frequency of 60 kHz, the value of which was only 3.6 mm. The weld penetration was probably associated with the melting pool agitation promoted by the current pulsation at high-frequency levels. Chen [28] also confirmed that the UFP energy can be converted to welding heat input and forces (i.e., agitation) with a high frequency. In view of this aspect, the arc sound pressure amplitude was less than 1.56 Pa, meaning that the ultrasonic vibration effect induced by the UFP current on the molten pool at a pulsed frequency of 60 kHz was weak. Thus, a low weld penetration was obtained. Interestingly, greater values of the weld dilution rate were obtained when compared with the results of the conventional underwater wet welding process, although a direct relation between the dilution rates and the exciting parameters could not be determined. This fact indicated that the ultrasonic vibration and electromagnetic field stirring improved the convective flows of the molten pool. When excited by the ultrasonic-frequency pulse current, the arc could emit ultrasonic energy. Then, the welding arc was not only a heat source but also a force source. The forced vibration of the molten pool generated by the change of the arc pressure and plasma flow force was transferred to the molten pool and affected the weld dilution rate [36]. It was acknowledged that the UFP-TIG process had features of shrinking arc, increasing arc pressure and improving weld penetration. However, the positive effect of the ultrasonic frequency current in the underwater wet welding process was not as pronounced as in the TIG process. The arc burning and metal transfer in the underwater FCAW process were inevitably influenced by the disturbance of dynamic bubbles and the water flow, which may mitigate the advantages of the ultrasonic-frequency current. The combined effects of the welding heat input and ultrasonic-frequency agitation may explain the irregular changes of weld geometry.

3.4. Microstructural Analysis

The bottom zone of the welds adjacent to the fusion boundary in the conventional UWW and the UFP-UWW process are shown in Figure 12. Table 5 statistically gives the measured dimension of columnar grain in the bottom part of the welds with different excitation parameters. The welds close to the fusion boundary were composed of coarse columnar grains for the conventional UWW weld, as displayed in Figure 12a. A large amount of pro-eutectoid ferrite (PF) with the light white color was distributed at the prior austenite grain boundaries. These coarse granular PF with the width of 10–30 μm were distributed on the boundary of columnar grain. Under the action of ultrasound in the UFP-UWW process, the length and width of the columnar grains decreased evidently compared to that of the conventional UWW process. With increasing the excitation voltage from 20 V to 60 V, the average width and length of the columnar grains was firstly decreasing and then increasing slightly. When the excitation voltage was 40 V and the excitation frequency was 30 kHz, the columnar grains in Figure 12c had the lowest value of 86 μm in the width, which was caused by the minimum heat input and the highest sound pressure at this parameter. As the excitation voltage reached to 60 V, larger columnar grains were again acquired due to the weak electromagnetic stirring and the slight rise in welding heat input.
Figure 12e–h shows the columnar grains zone adjacent to the fusion boundary. The excitation frequency also had a pronounced effect on the size of columnar grains. The calculated heat inputs for the UFP-UWW process at different excitation frequencies were higher than that of the conventional UWW process. This result indicated that the pronounced decrease in the size of columnar grain was attributed to the continuous oscillation in the weld pool induced by the ultrasonic-frequency electromagnetic stirring [41]. Particularly, for the parameter combination 40 V–30 kHz, the columnar grains were only 92 μm in width. When the pulse frequency was 30 kHz, the average width and length of columnar grains were 92 µm and 328 µm, respectively. Then, the pulse frequency further increased to 40 kHz and 50 kHz, and the average width and length of columnar grains became 112 µm and 273 µm and 102 µm and 301 µm. When the pulse frequency reached up to 60 kHz, the average width and length of columnar grains reduced to 105 µm and 320 µm. Therefore, with the increase of the pulse frequency, the size of columnar grains became small. This variation was in accordance with the change of sound pressure amplitude, suggesting that ultrasonic current had an important role in refining grains. Overall, the average width and length of the columnar grains was only 106 μm and 316 μm. This represented a 39% decrease in width and a 36% decrease in length of the columnar grains close to the fusion boundary. Chen also found that the use of ultrasonics could refine grains by enhancing dendritic fragmentation [42]. In addition, He et al. proposed that the grain refinement was because of the ultrasonic wave induced by the UFP current in the weld pool [43].
The effect of the ultrasonic excitation voltage on the microstructure of the welds is shown in Figure 13. The typical microstructure of the welds in a conventional UWW process was composed of pro-eutectoid ferrite (PF), side plate ferrite (FSP) and acicular ferrite (AF). During the cooling of the weld metal, the PF and BF first formed at the grain boundary of the prior austenite. When the temperature continued to decrease, the FSP was grown along grain boundaries in a slatted pattern into the grain. At lower temperatures, the AF was formed. When the UFP-UWW process was applied, the columnar PF grain became intermittent, and the length of pro-eutectoid ferrite was small due to the ultrasonic-frequency variation of the electromagnetic force in the UFP-UWW process. High fracture toughness was associated with fine acicular ferrites while the PF and side-plate ferrite morphologies were not as desirable as acicular ferrite in terms of weld toughness. Table 6 lists the measured values of ferrite content in the welds during the UFP-UWW process.
The microstructures in two welding processes were the same, including PF, FSP and AF, but the proportions of ferrite with different morphologies changed under the action of ultrasonic current. As shown in Figure 12a, the amount of AF was 13% for the conventional UWW process. When the excitation voltages were 20 V, 40 V and 60 V, the proportions of AF were 13%, 17% and 14%, respectively. This indicated that the amount of AF firstly increased and then decreased slightly. It should be noted that the ferrite type in low-carbon welds was mainly dependent on the weld metal’s composition, the prior austenite’s grain size and the cooling rate [44]. The alloying elements in the underwater wet welds were also at a similar level due to the same type of flux-cored wires used. Although the welding heat input was approximately the same in all of the tests, the ultrasonic-frequency electromagnetic field accelerated the thermal convection circulation and the stirring flow of liquid metal particles in the molten pool, thereby breaking coarse dendrites, increasing the undercooling degree and forming more nucleation particles [23]. As discussed in Section 3.2, the sound pressure amplitude firstly increased and then decreased slightly when increasing the excitation voltage from 20 V to 60 V. The proportion of AF may change with the variation of the ultrasonic-frequency electromagnetic field induced by the change of sound pressure amplitude. At the excitation voltage of 40 V, a maximum sound pressure of 14.88 Pa could be obtained. Then, the function of thermal convection circulation and the stirring flow of liquid metal particles in the molten pool at this parameter were most pronounced. Therefore, a high proportion of AF was acquired at this parameter. Chen et al. also reported that the introduction of UFP increased the amount of AF and decreased the amount of PF near the fusion boundary [28]. Another interesting result was that the introduction of the UFP current helped to refine the size of inclusions in the underwater wet welds. As displayed in Figure 13a, the black inclusions with a diameter of 10.6 µm could be easily found. When the excitation voltage ranged from 20 V to 60 V, the average diameter of the inclusions reduced to less than 4.3 µm, and the inclusions with large size were rarely observed, which was very beneficial for the enhancement of mechanical properties of underwater wet welds.
Figure 14 displays the microstructure of the welds as a function of the pulsed frequency for an excitation voltage of 40 V. According to the statistical results shown in Table 6, similar results were obtained for the measured values of the ferrite content in the welds compared to the results obtained under different excitation voltages. When the pulse frequency was 30 kHz, the measured value of AF was 15% while the amount of PF + FSP was 85%. Then, the pulse frequency further increased to 40 kHz and 50 pulse frequency, and the proportions of AF became 16% and 13%, respectively. When the pulse frequency reached up to 60 kHz, the proportion of AF reduced to 12%. The welding heat input and alloying element content in the underwater wet welds were also at a similar level. Therefore, the positive change of the microstructure was induced by the introduction of the UFP current. Considering that the arc sound pressure decreased with increasing excitation pulse frequency, it was inferred that the function of thermal convection circulation and the stirring flow of liquid metal particles in the molten pool were relieved at high frequencies. Therefore, the amount of AF tended to decrease with the increasing of pulse frequency.
As discussed, the key role of the ultrasonic-frequency current on the microstructure was to refine the grains and break the growth of coarsened columnar grains in the welds. Under the exertion of the UFP, the heat transfer process of the molten pool was greatly enhanced, and the residence time of the molten pool at a high temperature was reduced, which restricted the growth of lengthy PF. The ultrasonic-frequency stirring of the electromagnetic force in the UFC-UWW process was also conducive to the floating and distribution of nonmetallic inclusions in the molten pool.

3.5. Mechanical Properties

Figure 15 exhibits the average tensile strength and impact toughness of the weld metal with different ultrasonic excitation parameters. At a constant ultrasonic exciting frequency of 30 kHz (Figure 11a), the average tensile strength of the weld metals obtained for different ultrasonic excitation voltages was higher than that of the conventional UWW process (484 MPa). When the excitation voltage was 20 V, the average tensile strength and impact toughness of the weld metal were 511 MPa and 61.7 kJ/cm, respectively. At an ultrasonic exciting voltage of 40 V, the maximum tensile strength of 516 MPa and the highest impact toughness of 65.7 kJ/cm were attained. When the excitation voltage increased to 60 V, the tensile strength and impact toughness of the welds decreased slightly. Generally, the tensile strength and impact toughness of the welds for the UFP-UWW process could be improved to 6.7% and 21.7% of that for the conventional UWW process. It was obvious that the impact toughness increased significantly while the tensile strength increased slightly. The coarse PF on the grain boundary of the conventional UWW welds would degrade the toughness of weld metal and provide a path for crack growth. Therefore, after the introduction of ultrasonic pulse current, the small columnar grains, the increased content of AF and uniform distribution of non-metallic inclusions in the UFP-UWW welds were the main reasons for the improvement of mechanical performance, as discussed in Section 3.4. Moreover, the fracture location of welded joints obtained from underwater welding normally occurs in the weld metal.
Figure 15b displays the tensile strength and impact toughness of the welded joints obtained with different pulsed frequencies at a constant excitation voltage (40 V). As the ultrasonic pulse frequency increased from 20 kHz to 60 kHz, the tensile strength and impact toughness maintained at a high level, although they showed a slight decrease. The result showed that the ultrasonically excited arc produced an electromagnetically stirred molten pool, thereby improving the microstructure and mechanical properties of underwater wet welds. The enhancement of mechanical properties of the welds was mainly attributed to the refined columnar grains, the increased content of AF and the uniform distribution of non-metallic inclusions in the welds. It should be noted that the columnar grains became large with increasing the pulse frequency from 20 kHz to 60 kHz, but the columnar grains were still small compared to that of the conventional UWW process, as shown in Table 4. This fact was caused by the low amplitude of the sound pressure at high frequencies, which represented that the intensity of the ultrasonic-frequency vibration in the arc was weak. As a result, the positive effect of ultrasonic frequency effect was not obvious at very high frequencies. Generally, the tensile strength and impact toughness of the weld metal for the UFP-UWW process at different frequencies were higher than that for the conventional UWW process. At an ultrasonic exciting voltage of 20 V, the maximum tensile strength of 517 MPa and the highest impact toughness of 65.7 kJ/cm were obtained.
The effect of excitation voltage on fracture morphology of UFP-UWW impact specimens is illustrated in Figure 16. All the fractures exhibited a similar ductile fracture feature with a number of dimples and spherical second phase inclusions distributed in the dimples. When the ultrasonic-frequency pulse current was introduced into the UWW process, the diameter of the dimples in the impact fracture became smaller and the depth became larger, as shown in Figure 8a–c. Meanwhile, more spherical second-phase inclusions were observed in the center part of the dimple, which would also be beneficial for the impact toughness. However, at the ultrasonic exciting parameters of 60 V–30 kHz, the features of dimples with large diameter and small depth were similar to that of the impact fracture in conventional underwater welds. This was in accordance with the low impact toughness at these parameters. The smaller amplitude of arc sound pressure (weak ultrasonic frequency stirring) at these parameters may be responsible for the low impact toughness.
Figure 17 shows the variation of the impact fracture morphology as a function of the pulse frequency at 40 V. Massive dimples with nonmetallic inclusions in the central zone were observed in the fractured surfaces. This indicated that a ductile mode was attained in the impact specimens of the UFP-UWW process. As the pulse frequency increased from 20 to 60 kHz, the features of impact fracture changed little, which is in agreement with the similar impact toughness of these specimens.
According to the above analysis, the following advantages of the ultrasonic-frequency current could be obtained: Compared with the conventional UWW process, the introduction of the ultrasonic-frequency current can increase the Lorentz force and the radial electromagnetic force in the arc to some extent, which changes the behavior of the arc burning and droplet transfer. A satisfactory arc stability for the UFP-UWW process could be acquired within certain excitation parameters. Secondly, the ultrasonic-frequency electromagnetic field accelerates the thermal convection circulation and the stirring flow of liquid metal particles in the molten pool, thereby breaking coarse dendrites, increasing the undercooling degree and forming more nucleation particles. Therefore, the ultrasonic-frequency current can break the growth of coarsened columnar grains and refine the grains in the welds. Meanwhile, the introduction of the UFP current can increase the proportion of acicular ferrite to some extent, which depended on the excitation parameters. Therefore, the tensile strength and the impact toughness of the welds increased. Future studies on the UFP-UWW process should focus on the weld process stability and joint’s quality at larger depths, the functioning mechanism of the UFP current on the underwater welding process and the susceptibility to hydrogen-induced cracking of underwater wet welded joints.

4. Conclusions

The weld appearance, sound and electrical signals, microstructure and mechanical properties of the underwater wet welding of Q345 steel under different ultrasonic excitation parameters were studied and compared, and the following conclusions were obtained:
  • The ultrasonic-frequency pulse voltage and current of the UFP-UWW process were successfully excited with a low ultrasonic-frequency current (high voltage). The addition of the ultrasonic-frequency pulse within a certain range provided a stabilization effect on the welding process. A too high pulse frequency (>50 kHz in this study) is not conducive to improving the arc stability of the UFP-UWFCAW process.
  • The sound signals during the FCA welding indicated the presence of an ultrasonic field when introducing the ultrasonic-frequency pulse excitation. The amplitude value of the FCA arc sound pressure responded differently with the excitation voltage and pulse frequency.
  • The UFP-UWW process could obtain continuous and smooth welds with no visible defects. Greater values of the weld dilution rate were obtained when compared with the results of the conventional UWW process, although a direct correlation between the dilution rates and the pulse parameters could not be acquired. The ultrasonic-frequency vibration induced by the periodic high-frequency electromagnetic forces refined the coarsened columnar grains in the welds while the amount of pro-eutectoid ferrite and acicular ferrite varied little.
  • There was an optimum excitation parameter at which the positive effect of the UFP was maximum. The optimum excitation parameters in this study were 40 V–30 kHz. The ultimate tensile strength of the joint at the parameter was 517 MPa, and the impact toughness of the weld metal was 65.7 J/cm2.
  • The UFP-UWW process could surely improve the arc stability, microstructure and mechanical performance of the welds. However, the systematic studies on the UFP-UWFCAW process were still inadequate, particularly in the quantitative relationship between the benefits of the UFP and the excitation parameters, susceptibility to hydrogen-induced cracking and weld quality at greater depths.

Author Contributions

S.L.: formal analysis, writing—original draft and funding acquisition; H.J.: data curation and writing—original draft; C.H. and W.Z.: data curation; J.W. (Jibo Wang): formal analysis; H.L.: conceptualization, funding acquisition, and supervision; J.W. (Jianfeng Wang): formal analysis; Y.L.: resources. All authors have read and agreed to the published version of the manuscript.

Funding

This research and APC were funded by the National Natural Science Foundation of China, grant number 51675310, the Natural Science Foundation of Fujian Province, grant number 2020J05196 and the State Key Laboratory of Advanced Welding and Joining, grant number AWJ-21M09 and AWJ-23M01, and the Postgraduate Research and Practice Innovation Program of Jiangsu Province (grant nos. KYCX22_3637).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data generated during the present study are available from the corresponding author on reasonable request.

Conflicts of 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.

References

  1. Barnabas, S.G.; Rajakarunakaran, S.; Pandian, G.S.; Buhari, A.M.I.; Muralidharan, V. Review on enhancement techniques necessary for the improvement of underwater welding. Mater. Today Proc. 2021, 45, 1191–1195. [Google Scholar] [CrossRef]
  2. Yang, Z.X.; Kan, B.; Li, J.X.; Su, Y.J.; Qiao, L.J. Hydrostatic pressure effects on stress corrosion cracking of X70 pipeline steel in a simulated deep-sea environment. Int. J. Hydrogen Energy 2017, 42, 27446–27457. [Google Scholar] [CrossRef]
  3. Tomków, J.; Landowski, M.; Rogalski, G. Application Possibilities of the S960 Steel in Underwater Welded Structures. Facta Univ. Ser. Mech. Eng. 2022, 20, 199–209. [Google Scholar] [CrossRef]
  4. Vashishtha, P.; Wattal, R.; Pandey, S.; Bhadauria, N. Problems encountered in underwater welding and remedies- a review. Mater. Today Proc. 2022, 64, 1433–1439. [Google Scholar] [CrossRef]
  5. Fu, Y.L.; Guo, N.; Du, Y.P.; Chen, H.; Xu, C.S.; Feng, J.C. Effect of metal transfer mode on spatter and arc stability in underwater flux-cored wire wet welding. J. Manuf. Process. 2018, 35, 161–168. [Google Scholar] [CrossRef]
  6. Santos, V.R.; Bracarense, A.Q.; Pessoa, E.C.P.; Marinho, R.R.; Rizzo, F.C.; Junior, R.C.; Monteiro, M.J. Development of oxyrutile low alloy ferritic electrode for wet welding. J. Mater. Res. Technol. 2022, 21, 1223–1247. [Google Scholar] [CrossRef]
  7. Costa, P.S.; Altamirano-Guerrero, G.; Ochoa-Palacios, R.M.; Reséndiz-Flores, E.O.; Guía-Hernández, L.A.; Ramírez-Luna, L.E. Optimization of welding parameters in underwater wet FCAW on a structural steel using support vector regression and sequential quadratic programming. Int. J. Adv. Manuf. Technol. 2022, 121, 4225–4236. [Google Scholar] [CrossRef]
  8. Sun, Q.J.; Cheng, W.Q.; Liu, Y.B.; Wang, J.F.; Cai, C.W.; Feng, J.C. Microstructure and mechanical properties of ultrasonic assisted underwater wet welding joints. Mater. Des. 2016, 103, 63–70. [Google Scholar] [CrossRef]
  9. Li, H.L.; Liu, S.X.; Ma, Q.; Wang, P.F.; Liu, D.; Zhu, Q. Investigation of process stability and weld quality of underwater wet flux-cored arc welding of low-alloy high-strength steel with oxy-rutile wire. Pol. Marit. Res. 2021, 111, 100–109. [Google Scholar] [CrossRef]
  10. Zhang, X.; Guo, N.; Xu, C.S.; Du, Y.P.; Chen, B.; Feng, J.C. Influence of CaF2 on microstructural characteristics and mechanical properties of 304 stainless steel underwater wet welding using flux-cored wire. J. Manuf. Process. 2019, 45, 138–146. [Google Scholar] [CrossRef]
  11. Amaral, E.C.; Moreno-Uribe, A.M.; Bracarense, A.Q. Effects of PTFE on operational characteristics and diffusible H and O contents of weld metal in underwater wet welding. J. Manuf. Process. 2021, 61, 270–279. [Google Scholar] [CrossRef]
  12. Sun, K.; Zeng, M.; Shi, Y.H.; Hu, Y.; Shen, X.Q. Microstructure and corrosion behavior of S32101 stainless steel underwater dry and wet welded joints. J. Mater. Process. Technol. 2018, 256, 190–201. [Google Scholar] [CrossRef]
  13. Li, H.L.; Liu, D.; Guo, N.; Chen, H.; Du, Y.P.; Feng, J.C. The effect of alumino-thermic addition on underwater wet welding process stability. J. Mater. Process. Technol. 2017, 245, 149–156. [Google Scholar] [CrossRef]
  14. Tomków, J.; Fydrych, D.; Rogalski, G. Role of bead sequence in underwater welding. Materials 2019, 12, 3372. [Google Scholar] [CrossRef] [Green Version]
  15. Brätz, O.; Klett, J.; Wolf, T.; Henkel, K.-M.; Maier, H.J.; Hassel, T. Induction Heating in Underwater Wet Welding—Thermal Input, Microstructure and Diffusible Hydrogen Content. Materials 2022, 15, 1417. [Google Scholar] [CrossRef]
  16. Wang, J.F.; Sun, Q.J.; Hou, S.J.; Zhang, T.; Jin, P.; Feng, J.C. Dynamic control of current and voltage waveforms and droplet transfer for ultrasonic-wave-assisted underwater wet welding. Mater. Des. 2019, 181, 108051. [Google Scholar] [CrossRef]
  17. Guo, N.; Du, Y.P.; Maksimov, S.; Feng, J.C.; Yin, Z.Q.; Krazhanovskyi, D.; Fu, Y.L. Study of metal transfer control in underwater wet FCAW using pulsed wire feed method. Weld. World 2018, 62, 87–94. [Google Scholar] [CrossRef]
  18. Wang, J.F.; Sun, Q.J.; Ma, J.K.; Teng, J.B.; Jin, P.; Feng, J.C. Investigation of acoustic radiator affecting bubble-acoustic interaction in ultrasonic wave-assisted UWW at shallow water. J. Manuf. Process. 2019, 37, 563–577. [Google Scholar] [CrossRef]
  19. Chen, H.; Guo, N.; Xu, K.; Liu, C.; Wang, G.D. Investigating the Advantages of Ultrasonic-assisted Welding Technique Applied in Underwater Wet Welding by in-situ X-ray Imaging Method. Materials 2020, 13, 563–577. [Google Scholar] [CrossRef] [Green Version]
  20. Chen, H.; Guo, N.; Zhang, Z.H.; Liu, C.; Zhou, L.; Wang, G.D. A novel strategy for metal transfer controlling in underwater wet welding using ultrasonic-assisted method. Mater. Lett. 2020, 270, 127692. [Google Scholar] [CrossRef]
  21. Jia, C.B.; Wu, J.F.; Han, Y.F.; Zhang, Y.; Yang, Q.Y.; Wu, C.S. Underwater pulse-current FCAW-part 1, waveform and process features. Weld. J. 2020, 99, 135–145. [Google Scholar] [CrossRef]
  22. Wu, J.F.; Han, Y.F.; Jia, C.B.; Yang, Q.Y.; Wu, C.S. Underwater pulse-current FCAW–part 2, Bubble behaviors and waveform optimization. Weld. J. 2020, 99, 303–311. [Google Scholar] [CrossRef]
  23. Wang, Y.J.; Chen, M.A.; Wu, C.S. HF pulse effect on microstructure and properties of AC TIG butt-welded joint of 6061Al alloy. J. Manuf. Process. 2020, 56, 878–886. [Google Scholar] [CrossRef]
  24. Voigt, A.L.; Da Cunha, T.V. Effects of ultrasonic pulsation of the welding current on the geometry of the fusion zone and heat-affected zone in submerged arc welding. Weld. Int. 2017, 32, 172–178. [Google Scholar] [CrossRef]
  25. Kuang, X.C.; Qi, B.J.; Zheng, H. Effect of pulse mode and frequency on microstructure and properties of 2219 aluminum alloy by ultrahigh-frequency pulse Metal-Inert Gas Welding. J. Mater. Res. Technol. 2022, 20, 3391–3407. [Google Scholar] [CrossRef]
  26. He, L.B.; Hao, H.W.; Li, L.M.; Wen, X.W.; Wu, M.S. Effect of arc-ultrasonic on welding structure and property of Q235A steel with SMAW. Hot Work. Technol. 2006, 1, 3. [Google Scholar]
  27. Fang, Y.X.; Liu, Z.M.; Cui, S.L.; Zhang, Y.; Qiu, J.Y.; Luo, Z. Improving Q345 weld microstructure and mechanical properties with high frequency current arc in keyhole mode TIG welding. J. Mater. Process. Technol. 2017, 250, 280–288. [Google Scholar] [CrossRef]
  28. Chen, J.; Wen, Z.; Jia, C.B.; Zhao, B.; Wu, C.S. The mechanisms of underwater wet flux-cored arc welding assisted by ultrasonic frequency pulse high-current. J. Mater. Process. Technol. 2022, 304, 117567. [Google Scholar] [CrossRef]
  29. Lei, Y.C.; Luo, Y.; Gong, C.C.; Xiao, B. Effect of traces of CO2 on arc-ultrasonic TIG-welded MGH956 alloy. Mater. Manuf. Process. 2014, 29, 1004–1010. [Google Scholar] [CrossRef]
  30. Lei, Y.C.; Xue, H.L.; Hu, W.X.; Liu, Z.Z.; Yan, J.C. Effect of arc ultrasonic vibration on microstructure of joint of plasma arc ‘in situ’ welding of SiCp/6061Al. Sci. Technol. Weld. Join. 2013, 16, 575–580. [Google Scholar] [CrossRef]
  31. Lei, Y.C.; Xue, H.L.; Hu, W.X.; Liu, Z.Z.; Yan, J.C. Effects of ac arc ultrasonic on plasma arc in situ welding of SiCp/6061Al MMCs. Sci. Technol. Weld. Join. 2013, 16, 561–566. [Google Scholar] [CrossRef]
  32. Zhu, Q.; Lei, Y.C.; Wang, Y.L.; Huang, W.; Xiao, B.; Ye, Y.M. Effects of arc-ultrasonic on pores distribution and tensile property in TIG welding joints of MGH956 alloy. Fusion Eng. Des. 2014, 89, 2964–2970. [Google Scholar] [CrossRef]
  33. Chen, X.Z.; Shen, Z.; Wang, J.J.; Chen, J.; Lei, Y.C.; Huang, Q.Y. Effects of an ultrasonically excited TIG arc on CLAM steel weld joints. Int. J. Adv. Manuf. Technol. 2011, 60, 537–544. [Google Scholar] [CrossRef]
  34. Dong, S.F.; Han, Y.F.; Jia, C.B.; Wu, C.S.; Zhang, M.F.; Yang, Q.Y.; Yang, J. Organic adhesive assisted underwater submerged-arc welding. J. Mater. Process. Technol. 2020, 284, 116739. [Google Scholar] [CrossRef]
  35. He, L.B.; Wu, M.S.; Li, L.M.; Hao, H.W. Ultrasonic generation by exciting electric arc: A tool for grain refinement in welding process. Appl. Phys. Lett. 2006, 89, 131504. [Google Scholar] [CrossRef]
  36. Wang, Y.J.; Yu, C.; Lu, H.; Chen, J.M. Research status and future perspectives on ultrasonic arc welding technique. J. Manuf. Process. 2020, 58, 936–954. [Google Scholar] [CrossRef]
  37. Huang, Y.M.; Chen, S.B. Key Technologies of Intelligentized Welding Manufacturing: The Spectral Diagnosis Technology for Pulsed Gas Tungsten Arc Welding of Aluminum Alloys; Springer: Berlin/Heidelberg, Germany, 2019. [Google Scholar]
  38. Lv, N.; Xu, Y.L.; Li, S.C.; Yu, X.W.; Chen, S.B. Automated control of welding penetration based on audio sensing technology. J. Mater. Process. Technol. 2017, 250, 81–98. [Google Scholar] [CrossRef]
  39. He, L.B.; Li, L.M.; Hao, H.W.; Wu, M.S.; Zhou, R.L. Grain refinement and high performance of titanium alloy joint using arc-ultrasonic gas tungsten arc welding. Sci. Technol. Weld. Join. 2013, 11, 72–74. [Google Scholar] [CrossRef]
  40. Cunha, T.V.; Bohórquez, C.E.N. Ultrasound in arc welding: A review. Ultrasonics 2015, 56, 201–209. [Google Scholar] [CrossRef]
  41. Wang, Z.M.; Jiang, D.H.; Wu, J.W.; Xu, M.J. A review on high-frequency pulsed arc welding. J. Manuf. Process. 2020, 60, 503–519. [Google Scholar] [CrossRef]
  42. Chen, C.; Fan, C.L.; Cai, X.Y.; Lin, S.B.; Yang, C.L.; Zhuo, Y.M. Microstructure and mechanical properties of Q235 steel welded joint in pulsed and un-pulsed ultrasonic assisted gas tungsten arc welding. J. Mater. Process. Technol. 2020, 275, 116335. [Google Scholar] [CrossRef]
  43. He, L.B.; Li, L.M.; Wu, M.S. The acoustic characteristics of high frequency modulated arc and its application in welding structure refinement. Appl. Acoust. 2010, 29, 222–226. [Google Scholar]
  44. Chaveriat, P.F.; Kim, G.S.; Shah, S.; Indacochea, J.E. Low Carbon Steel Weld Metal Microstructures the Role of Oxygen and Manganese. J. Mater. Eng. Vol. 1987, 9, 253–267. [Google Scholar] [CrossRef]
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Figure 1. Microstructure of the Q345 base metal.
Figure 1. Microstructure of the Q345 base metal.
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Figure 2. Schematic diagram of the UFP-UWFCAW process.
Figure 2. Schematic diagram of the UFP-UWFCAW process.
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Figure 3. Welding current and arc voltage waveforms of the conventional UWW process.
Figure 3. Welding current and arc voltage waveforms of the conventional UWW process.
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Figure 4. Welding current and arc voltage waveforms of the UFP-UWW process at the excitation parameters of 20 V–30 kHz: (a) the overall welding current (blue) and arc voltage (red) waveforms of the UFP-UWW process; (b) detailed arc voltage waveform at the time interval between 435.9 and 436.6 ms (red) and detailed welding current waveform at the time interval between 435.9 and 436.6 ms (blue).
Figure 4. Welding current and arc voltage waveforms of the UFP-UWW process at the excitation parameters of 20 V–30 kHz: (a) the overall welding current (blue) and arc voltage (red) waveforms of the UFP-UWW process; (b) detailed arc voltage waveform at the time interval between 435.9 and 436.6 ms (red) and detailed welding current waveform at the time interval between 435.9 and 436.6 ms (blue).
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Figure 5. Arc voltage and welding current waveforms at different ultrasonic excitation voltages at an ultrasonic frequency of 30 kHz: (a) 0; (b) 20 V; (c) 40 V; (d) 60 V.
Figure 5. Arc voltage and welding current waveforms at different ultrasonic excitation voltages at an ultrasonic frequency of 30 kHz: (a) 0; (b) 20 V; (c) 40 V; (d) 60 V.
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Figure 6. Arc voltage and coupling current waveforms at different ultrasonic frequencies with an ultrasonic excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
Figure 6. Arc voltage and coupling current waveforms at different ultrasonic frequencies with an ultrasonic excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
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Figure 7. The sound pressure in conventional FCA welding arc in the air. (a) the time-domain image; (b) the frequency-domain image.
Figure 7. The sound pressure in conventional FCA welding arc in the air. (a) the time-domain image; (b) the frequency-domain image.
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Figure 8. The frequency-domain images of the sound frequency of the FCA welding arc in the air under different excitation conditions: (a) 20 V–30 kHz; (b) 40 V–30 kHz; (c) 60 V–30 kHz; (d) 40 V–20 kHz; (e) 40 V–40 kHz; (f) 40 V–50 kHz.
Figure 8. The frequency-domain images of the sound frequency of the FCA welding arc in the air under different excitation conditions: (a) 20 V–30 kHz; (b) 40 V–30 kHz; (c) 60 V–30 kHz; (d) 40 V–20 kHz; (e) 40 V–40 kHz; (f) 40 V–50 kHz.
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Figure 9. Weld appearances at different excitation voltages and frequencies: (a) conventional UWW; (b) 20 V–30 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz (scale 1 mm).
Figure 9. Weld appearances at different excitation voltages and frequencies: (a) conventional UWW; (b) 20 V–30 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz (scale 1 mm).
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Figure 10. Macrographs of the cross-section of the weld beads at different ultrasonic-frequency pulse conditions: (a) conventional UWW; (b) 20 V–30 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz (scale 1 mm).
Figure 10. Macrographs of the cross-section of the weld beads at different ultrasonic-frequency pulse conditions: (a) conventional UWW; (b) 20 V–30 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz (scale 1 mm).
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Figure 11. Average value of the width, penetration and dilution rate of the welds: (a) different ultrasonic excitation voltages with a constant frequency of 30 kHz; (b) different pulse frequencies with a constant voltage of 40 V.
Figure 11. Average value of the width, penetration and dilution rate of the welds: (a) different ultrasonic excitation voltages with a constant frequency of 30 kHz; (b) different pulse frequencies with a constant voltage of 40 V.
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Figure 12. Microstructure of fusion line under different ultrasonic excitation parameters: (a) conventional UWW; (b) 20 V–2 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz.
Figure 12. Microstructure of fusion line under different ultrasonic excitation parameters: (a) conventional UWW; (b) 20 V–2 kHz; (c) 40 V–30 kHz; (d) 60 V–30 kHz; (e) 40 V–20 kHz; (f) 40 V–40 kHz; (g) 40 V–50 kHz; (h) 40 V–60 kHz.
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Figure 13. Microstructure of weld metal under different excitation voltages at an ultrasonic pulse frequency of 30 kHz: (a) 0 V; (b) 20 V; (c) 40 V; (d) 60 V.
Figure 13. Microstructure of weld metal under different excitation voltages at an ultrasonic pulse frequency of 30 kHz: (a) 0 V; (b) 20 V; (c) 40 V; (d) 60 V.
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Figure 14. Microstructure of the weld metal under different pulse frequencies at an ultrasonic excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
Figure 14. Microstructure of the weld metal under different pulse frequencies at an ultrasonic excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
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Figure 15. The average tensile strength and impact toughness of welded joints: (a) different ultrasonic excitation voltages at 30 kHz; (b) different ultrasonic excitation pulse frequencies at 40 V.
Figure 15. The average tensile strength and impact toughness of welded joints: (a) different ultrasonic excitation voltages at 30 kHz; (b) different ultrasonic excitation pulse frequencies at 40 V.
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Figure 16. Impact fracture surface with different excitation voltage at a constant pulse frequency of 30 kHz: (a) 0 V; (b) 20 V; (c) 40 V; (d) 60 V.
Figure 16. Impact fracture surface with different excitation voltage at a constant pulse frequency of 30 kHz: (a) 0 V; (b) 20 V; (c) 40 V; (d) 60 V.
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Figure 17. Impact fracture surface with different pulse frequencies at a constant excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
Figure 17. Impact fracture surface with different pulse frequencies at a constant excitation voltage of 40 V: (a) 20 kHz; (b) 40 kHz; (c) 50 kHz; (d) 60 kHz.
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Table
Table 1. Chemical composition of the base material and filler metal (wt.%).
Table 1. Chemical composition of the base material and filler metal (wt.%).
MaterialsCPMnCrNiSSiCuFe
Base metal0.20.0141.120.030.010.0030.180.04Bal.
Filler metal0.1-0.41-0.8----
Table
Table 2. Parameters of ultrasonic-frequency pulse underwater wet welding.
Table 2. Parameters of ultrasonic-frequency pulse underwater wet welding.
RunUltrasonic Voltage
(V)		Ultrasonic Frequency
(kHz)		Wire Feed
Speed (m/min)		Arc Voltage
(V)		Welding Speed
(mm/s)
1003.6273
22030
34030
46030
54020
64040
74050
84060
Table
Table 3. The proportions of low (<10 V) or high (<55 V) arc voltage at different welding parameters (%).
Table 3. The proportions of low (<10 V) or high (<55 V) arc voltage at different welding parameters (%).
TestShort-Circuit Region
(Arc Voltage < 10 V)		Arc Extinction Region
(Arc Voltage > 55 V)		The Total Value of
Unstable Arc Burning
Regions
Conventional UWW2.440.352.79
20 V–30 k1.120.441.56
40 V–30 k1.50.962.46
60 V–30 k1.51.32.8
40 V–20 k0.752.533.28
40 V–40 k2.332.444.77
40 V–50 k7.417.2314.64
40 V–60 k6.326.9113.23
Table
Table 4. Measured values of the welding parameters during the UFP-UWW process.
Table 4. Measured values of the welding parameters during the UFP-UWW process.
TestAverage Measured Arc Voltage
(V)		Average Measured Welding Current
(A)		Calculated Welding Heat Input
(kJ/cm)
126.3191.416.18
226.1188.816.17
326.9185.916.11
427.3188.316.21
528.3191.216.30
627.8192.316.27
728.9186.116.31
827.5190.616.22
Table
Table 5. Measured dimension of columnar grain in the bottom part of the welded joints (μm).
Table 5. Measured dimension of columnar grain in the bottom part of the welded joints (μm).
TestAverage Width of the
Columnar Grains		Average Length of the Columnar Grains
1175491
299301
386342
4143345
592328
6112273
7102301
8105320
Table
Table 6. Measured values of the ferrite content in the welds during the UFP-UWW process.
Table 6. Measured values of the ferrite content in the welds during the UFP-UWW process.
TestPFAFFSP
1461341
2491338
3431740
4481438
5471538
6421642
7441343
8481240
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Liu, S.; Ji, H.; Zhao, W.; Hu, C.; Wang, J.; Li, H.; Wang, J.; Lei, Y. Evaluation of Arc Signals, Microstructure and Mechanical Properties in Ultrasonic-Frequency Pulse Underwater Wet Welding Process with Q345 Steel. Metals 2022, 12, 2119. https://doi.org/10.3390/met12122119

AMA Style

Liu S, Ji H, Zhao W, Hu C, Wang J, Li H, Wang J, Lei Y. Evaluation of Arc Signals, Microstructure and Mechanical Properties in Ultrasonic-Frequency Pulse Underwater Wet Welding Process with Q345 Steel. Metals. 2022; 12(12):2119. https://doi.org/10.3390/met12122119

Chicago/Turabian Style

Liu, Shixiong, Hao Ji, Wei Zhao, Chengyu Hu, Jibo Wang, Hongliang Li, Jianfeng Wang, and Yucheng Lei. 2022. "Evaluation of Arc Signals, Microstructure and Mechanical Properties in Ultrasonic-Frequency Pulse Underwater Wet Welding Process with Q345 Steel" Metals 12, no. 12: 2119. https://doi.org/10.3390/met12122119

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