Underwater Flux-cored Arc Welding Research Articles

Modeling the coupled bubble-arc-droplet evolution in underwater flux-cored arc welding

For the numerical simulation of underwater wet flux-cored arc welding, the most crucial issue is to investigate the intense interactions between the underwater bubble, arc plasma, and molten metal. However, it is a great challenge to couple them into a single numerical model. In this study, a 3D three-phase flow model is originally established, which successfully couples the dynamic bubble, arc, and droplet. A modified Lee model is employed to realize spontaneous phase transition from liquid water to bubble gas. According to the simulation results, the bubble evolution is divided into four main stages, and two bubble separation modes are recognized. It is found that, both the changing pressure field around the bubble and the varying flow direction of surrounding water contribute to the unique bubble evolution patterns. As for the droplet, its violent up-and-down oscillation at the wire tip is mainly caused by the opposite gas drag forces, which are produced by the turbulent gas flow inside the bubbles. The upward gas drag force can even make the neck of the droplet disappear. With different droplet detaching angles, two predominant droplet transfer modes are numerically produced; when the angle reaches 147°, the droplet is pushed away and becomes a spatter. Furthermore, the underwater arc is found to be subjected to compressions from both the radial direction due to bubble necking and the axial direction due to droplet growth. The arc temperature and velocity vary significantly not only during the whole droplet transfer period, but also within each bubble evolution cycle. To verify the reliability of the model, underwater welding experiments and visual sensing are conducted. The simulated results match well with the experimental ones, with an 8% error in the droplet transfer period and only a 1.4% error in the bubble evolution cycle.

A novel strategy to improve melting efficiency and arc stability in underwater FCAW via contact tip air chamber

The conventional method of underwater flux-cored arc welding (UWW-FCAW) is characterized by water around the welding torch's electric parts, the tubular wire, the molten pool and the workpiece. In the present work, underwater FCAW was performed by keeping the contact tip dry, inside a specially developed torch, and as in conventional underwater welding. The welds were carried out in a flat position at simulated depths of 0.3 m, 10 m and 30 m using a hyperbaric chamber. Welding electrical signals of voltage and current and contact tip temperature were acquired and processed to determine process behavior. As a result, the welding current was reduced when the contact tip was kept dry inside the torch because of the higher temperature achieved by resistance heating when it was insulated from the water. The ambient surrounding the contact tip interfered with the coefficient of heat transfer and, consequently, with its temperature. Welds performed with the dry contact tip also presented slight variations in bead shape parameters in different water depths. Higher arc stability was achieved by welding with the contact tip inside the air chamber, as minor variations of electric signals and fewer arc extinction events were observed compared to conventional underwater welds. The combination of improved arc stability with higher electrode temperature may also have contributed to minor porosity and smaller variation of the reinforcement along the weld bead.

Numerical modeling of coupled arc plasma, metal transfer and molten pool evolution for underwater flux-cored arc welding

Numerical modeling of coupled arc plasma, metal transfer and molten pool evolution for underwater flux-cored arc welding

Numerical Analysis of the Metal Transfer and Welding Arc Behaviors in Underwater Flux-cored Arc Welding

Based on the VOF method, a three-dimensional numerical analysis model coupling both droplets and arc behaviors is developed to simulate the metal transfer and arc plasma flow processes in underwater wet flux-cored arc welding. The relationships between several physical quantities such as droplet shape and temperature, electromagnetic force and current density of droplets and arc at different periods are analyzed. As it turns out, the special arc plasma component is the main reason why the arc concentrates on the bottom of the droplet, rather than being wrapped. As the volume of the droplet increases, the arc temperature decreases. The constriction of arc shape leads to the result that the welding current flow goes through from the droplet bottom–a small area to subjacent arc. Therefore, at the anode spot an intense electromagnetic spot force is formed. The droplets are also affected by the gas flow resistance inside the bubble. Under the combinative actions of the spot force and the gas flow resistance, the main mode of droplet transfer is large droplet repelled transfer. For verifying the reliability of the model, the real-time metal transfer process is captured by a high-speed camera, the experimental values and the simulated values are found to be in good agreement.

Investigation on the mass transfer control, process stability and welding quality during underwater pulse current FCAW for Q235

Pulse current technology is employed to adjust the mass transfer and improve the welding quality, and effects of the pulse frequency (f), load duty cycle (D) and peak current (Ip) on the droplet were analyzed based on Taguchi method during underwater flux-cored arc welding (FCAW). In underwater wet pulse FCAW (FCAW-P), the mass transfer process is observed and the interaction mechanism of the pulse current on the droplet is investigated using in-situ X-ray imaging method. Within one pulse period, the decreasing repulsive force and the increasing gravitational assist the droplets detach from the wire or touch with the welding pool at the end of the peak pulse duration and reduce the droplet detachment angle, improving the mass transfer stability and thus increasing the arc stability and ameliorating the bead appearance. The stable mass transfer process and welding arc increase the effective heat input, and thus decreases the acicular ferrite and increases the pro-eutectoid ferrite in the bead. The hardness distribution and the tensile strength of the beads deposited in conventional and pulse current underwater wet FCAW are identical basically, demonstrating the application of pulse current technology does not deteriorate the mechanical properties.

Arc stability indexes evaluation of ultrasonic wave-assisted underwater FCAW using electrical signal analysis

Ultrasonic wave-assisted underwater flux-cored arc welding (FCAW) has the potential to control the dynamic bubble and then improve arc stability. In this study, the four indexes evaluation methods to determine arc stability, i.e., oscillograms, cyclograms, probability distribution, variation coefficients of the welding current and arc voltage, were conducted to study the effect of ultrasonic wave on the arc stability. Results show that it is reasonable to evaluate the variation in arc stability with the welding parameters including wire feed speed and arc voltage, or without/with an ultrasonic wave by using the four indexes evaluation methods. Further, the control of the dynamic bubble induced by the exertion of ultrasonic wave can be used to explain why the arc stability is improved. The key points to excellent arc stability depend on both the designation of the welding parameters and reasonable control of the bubble with structural integrity. Therefore, through the novel method which combines optimized welding parameters with a stable bubble environment, the achieved welding process can effectively reduce the bubble disturbance and enhance the arc stability. Results presented in the paper aim to understand the combined influence mechanism of welding parameters and ultrasonic wave on the arc stability in underwater wet welding.

Investigation on the bubble dynamic behaviors and corresponding regulation method in underwater flux-cored arc welding

Observed two bubble growth modes indicated that the stability of arc burning and bubble growth relied on each other. Under selected welding parameters, the maximum diameters of generated bubbles complied with approximately normal distribution. Higher preset welding parameters generated larger bubbles with gradually receding influences. Vaporization, condensation, and reactions happened rapidly inside the bubbles even after detachment and when coming to water surface. Gas-assisted underwater flux-cored arc welding was proposed to improve the slag coverage. This method was also effective in achieving better weld appearance.

Effect of ultrasonic vibration on microstructural evolution and mechanical properties of underwater wet welding joint

The propagation of high-intensity ultrasonic vibration into weld pool can influence on microstructure and mechanical properties of welded joints. This research introduced a hybrid method for improving weld quality by using superimposed ultrasonic vibration on workpiece directly in underwater flux cored arc welding (FCAW). With the purpose of revealing how macrostructure, microstructure and mechanical properties of welded joints are affected by ultrasonic vibration within the weld metal, the present study has been carried out. The results indicated that proeutectoid ferrite, ferrite side plate and acicular ferrite constitute the microstructures of weld metal. Ultrasonic vibration can affect the size and morphology of prior austenite grain during weld pool solidification and eventually accelerate the formation of fine ferrite microstructure of weld metal in high proportion in the subsequently solid-state phase transformation. Excellent balance of high tensile strength and impact toughness is realized as more fine columnar microstructures in weld metal with using ultrasonic vibration. These findings suggest that the addition of ultrasonic vibration is an effective way to improve the tensile strength and impact toughness of weld metal during underwater wet welding.

Microstructure and mechanical properties of ultrasonic assisted underwater wet welding joints

A new weld method, ultrasonic assisted underwater wet welding process (U-FCAW), was explored in order to achieve high performance welding joints. The addition of ultrasonic can form an acoustic field between the workpiece and the ultrasonic radiator. The joints were welded by ultrasonic assisted underwater wet welding process (U-FCAW) and underwater flux cored arc welding (FCAW), respectively. The effect of ultrasonic on the arc stability, microstructure and mechanical properties, such as tensile, bending and hardness distribution, was investigated. The results indicated that arc stability improved when ultrasonic was applied. The amount of martensite (M) and upper bainite (BU) was decreased, while the granular bainite (BG) and acicular ferrite (AF) increased, when ultrasonic was applied during welding. The tensile strength and the bending properties were substantially enhanced. The fracture occurrence of the welded joints during tensile testing was transferred from the joint to base metal, compared to FCAW. A 46% and 48% increase was found in the tensile strength of the upper and lower layers, respectively. The maximum angle during bending test was increased from 21° to 84°.

Vision-based seam tracking system of the underwater flux-cored arc welding

To track seam automatically during underwater flux-cored arc welding(UFCAW),a vision-based seam tracking system has been studied.This seam tracking system consists of a vision sensing system,an image processing and seam recog- nizing system,a control system and executive unit.The edge of the weld line is precisely detected by a wavelet transform method.The seam position error is calculated by a seam posi- tion recognition algorithm based on the binary image.Based on the theoretic analyze of the structure model of the vision-based seam tracking system,a control rule self-adjust fuzzy controller is designed.The factors of decreasing the precision of the seam tracking system are analysised,and some measures are adopted to improve the tracking precision.The tracking experiments results of the UFCAW confirm that the control rule self-adjust fuzzy controller meet the demand of the tracking precision.In the tracking trials of sidelong,reflex and curve weld line,the system can all meet the demand of the tracking precision.With the improved mini drain water cap,the tracking experiments of the underwater local dry welding are carried.The results indi- cate that this process can not only meet the demand of improve welding quality but also satisfy the tracking results.

Vision based seam tracking system for underwater flux cored arc welding

Manual underwater welding is usually time consuming, expensive and hard to perform because of the rigorous underwater environment. Automatic underwater welding can be performed faster at less cost and with higher quality. An automatic seam tracking system for underwater flux cored arc welding has been developed. It consists of a vision sensing module, a seam recognition module, a fuzzy controller (FC) and an X–Y travel platform. The vision sensing module can capture clear seam images during welding, successfully resolving the problem of welding arc interference during underwater flux cored arc welding. The seam recognition module filters, enhances and thresholds the seam images and then recognises the seam deviation angle with a three layer back propagation neural network. The FC outputs the control parameters according to the seam deviation angle and then controls the X–Y platform to drive the torch to the centre of the seam. In this study, three different welds were considered: a straight line, a kinked straight line and an S curved line. These welds were tracked real timely during underwater flux cored arc welding. The results show that this seam tracking system can meet the requirements of the automatic underwater flux cored arc welding.