Narrow-gap Gas Metal Arc Welding Research Articles

Investigation of Microstructure and Mechanical Properties of High-Depth-to-Width-Ratio Horizontal NG-GMAW Joint for S500Q Steel.

A novel high depth-to-width ratio of 15:1 narrow-gap gas metal arc welding technique was developed for the welding of S500Q steel in a horizontal butt joint. The bead arrangement of the I groove was optimized to produce a high-quality connection with the upper sidewall of the joint. The microstructure and mechanical properties were observed and evaluated by optical microscopy, scanning electron microscopy, tensile testing, and micro-hardness and impact toughness testing at 1/5, 2/5, 3/5, and 4/5 thickness of the joint. The 3/5 T position exhibited the highest strength, which was attributed to the presence of finer carbide precipitates. The highest micro-hardness appeared at 4/5 T. The highest impact toughness appeared at 3/5 T. The formation of coarse granular bainite was the major reason for the decrease in impact toughness in other regions. A microscopic fracture at 1/5 T and 3/5 T was further analyzed. It was observed that the width of the fibrous zone at 3/5 T was significantly larger than that at 1/5 T. The radial zones at 1/5 T were observed to exhibit cleavage, with secondary cracks on the fracture surface.

Implementation of a two-stage algorithm for NG-GMAW seam tracking and oscillation width adaptation in pipeline welding

Seam tracking and oscillation width adaptation could effectively reduce the sidewall fusion defects in automatic narrow-gap gas metal arc welding (NG-GMAW). This study proposes a two-stage algorithm to realise seam tracking and oscillation width adaptation in NG-GMAW pipeline welding. The first stage involves using the principal component analysis (PCA) eigenvectors to adjust the oscillating centre, enabling seam tracking and oscillation width adaptation for the hot and fill layer. In the second stage, dataset was prepared based on the tracking data from the first stage, which can guide the welding torch trajectory for the cap layer. Experimental results show that the proposed scheme achieves seam tracking and oscillation width adaptation with an accuracy of 0.1 mm.

Real-time seam tracking during narrow gap GMAW process based on the wide dynamic vision sensing method

Welding torch horizontal seam tracking is a crucial condition to realize automatic welding of narrow gap gas metal arc welding (NG-GMAW) robots. In the present study, a real-time welding seam tracking technology based on the passive vision sensor is proposed. To this end, a wide dynamic range (WDR) vision sensor is used to capture real-time welding images. In order to realize real-time identification of the welding torch center deviation in the process of swinging arc in a welding bead, a welding seam tracking algorithm based on the window information fusion (WIF) is proposed. In this regard, the local feature moving window recognition (LMWR) algorithm is applied to extract the groove center. Based on the improved welding image processing program, the weld pool center and welding wire center are prepared for seam tracking. In this regard, considering the trifling variation of groove and weld pool center, the welding horizontal deviation is obtained by combining them with welding wire center calculation. Aiming at establishing a correlation between consecutive images, the Kalman filter is used to estimate the deviation optimally. The tracking algorithm is successfully verified in the weld bead with a horizontal deviation, and the tracking accuracy of ±0.47 mm is achieved. The performed analyses demonstrate that the developed algorithm meets the requirements of real-time seam tracking in the GMAW process of the welding robot.

Applying Statistical Models to Optimize the Weld Bead Geometry in the Vertical Oscillation Arc Narrow Gap All-Position GMAW

Vertical oscillation arc welding for narrow gap gas metal arc welding (NG-GMAW) has a relatively simple structure, and it is widely used in all-position pipeline field welding. However, it has some shortcomings, such as incomplete fusion defects on the sidewall and interlayer. Aiming at resolving these shortcomings, a mathematical model is proposed to obtain appropriate welding parameters in different positions. In this model, the response surface methodology (RSM) based on the central composite design (CCD) was developed to study the interactions between welding parameters and the weld bead geometry. Then the analysis of variance (ANOVA) was used to evaluate the accuracy and significance of the proposed model. Finally, experiments were carried out in flat, vertical, and overhead positions to obtain the optimal parameters. The macroscopic metallography of the transversal section of the weld bead under the optimizing welding parameters showed that the weld beads were free of defects in the sidewall and interlayers.

Effects of arc interaction on lack-of-fusion and porosity in tandem NG-GMAW of 5083 Al-Mg alloy

The tandem narrow-gap gas metal arc welding (NG-GMAW) is an advanced high efficient technology for joining thick section metals. However, the common defects such as lack-of-fusion (LOF) at groove sidewalls, porosity in weld bead are still hard to be avoided, especially for joining thick section of aluminum alloy. In this study, a pulsed tandem NG-GMAW process was applied to join thick 5083 Al-Mg alloy plates. The LOF and its formation mechanism at varied arc length were investigated by observing the tandem arc interaction behaviors with high-speed photography and by synchronized recording the corresponding welding signals. The results indicate, the arc heat distribution, which is likely influenced by the tandem arc arrangement, is a critical factor to the formation of LOF. However, a properly adjusted arc length can improve the arc interaction and the weld formation. Moreover, a proper arc interaction behavior can change the direction of arc force, which makes contribution to the stirring effect on molten pool. It is beneficial to reduce porosity. The oxygen enriched pores (pore-II) caused by short-circuiting transfer were found when arc length correction coefficient (ALCC) is −15, which needs to be avoided during the welding process.

Influence of Interwire Distance and Arc Length on Welding Process and Defect Formation Mechanism in Double-Wire Pulsed Narrow-Gap Gas Metal Arc Welding

Influence of Interwire Distance and Arc Length on Welding Process and Defect Formation Mechanism in Double-Wire Pulsed Narrow-Gap Gas Metal Arc Welding

Effects of Active Gases on Droplet Transfer and Weld Morphology in Pulsed-Current NG-GMAW of Mild Steel

The current research of narrow-gap gas metal arc welding (NG-GMAW) primarily focuses on improving the sidewall fusion and avoiding the lack-of-fusion defect. However, the high cost and operation difficulty of the methods limit the industrial application. In this study, small amount of active gases CO2 and O2 were added into pure argon inert shielding gas to improve the weld formation of pulsed-current narrow-gap gas metal arc welding (NG-GMAW) of mild steel. Their effects on droplet transfer and arc behavior were investigated. A high-speed visual sensing system was utilized to observe the metal transfer process and arc morphology. When the proportion of CO2, being added into the pure argon shielding gas, changes from 5% to 25%, the metal transfer mode changes from pulsed spray streaming transfer to pulsed projected spray transfer, while it remains the pulsed spray streaming transfer when 2% to 10% O2 is added. Both CO2 and O2 are favorable to stabilizing arc and welding process. O2 is even more effective than CO2. However, O2 is more likely to cause slags on the weld surface, while CO2 can improve the weld appearance in some sense. The weld surface concavity in NG-GMAW is greatly influenced by the addition of active gas, but the weld width and weld penetration almost keep constant. This study proposes a new method which is beneficial to improving the weld bead formation and welding process stability.

Influence of interwire angle on undercutting formation and arc behavior in pulsed tandem narrow-gap GMAW

The tandem narrow-gap gas metal arc welding was adopted in this study. The effects of interwire angle on the welding process stability and arc behavior were investigated. With the interwire angle increasing, the welding process stability gets improved. The arc interruptions which frequently occur in the leading arc are eliminated when the interwire angle is >0°. The mutual electromagnetic attraction between the two arcs decreases with the interwire angle increasing due to the increased arc distance. The two arcs move from the groove bottom to the sidewalls, which transfer more heat to the sidewalls resulting in increased weld width. This leads to serious undercutting defects especially when the interwire angle is 20°. The molten metal under the arcs flows to the rear of welding pool under the action of plasma drag force, arc pressure and droplet impingement. As a result, there is no adequate molten metal to fill the sidewalls, giving rise to undercutting defects. The undercutting at the groove sidewall close to the trailing arc is smaller. This is because the liquid metal under the trailing arc is more massive, which means more molten metal flows to the sidewall, alleviating the undercutting defect.

Heat Source Characteristics of Ternary-Gas-Shielded Tandem Narrow-Gap GMAW.

The characteristics of the welding heat source for tandem narrow-gap gas metal arc welding are examined for different ternary shielding gas (Ar-CO2-He) compositions. Results of previous calculations of arc properties for bead-on-plate geometry are adapted to the narrow-gap geometry to predict these characteristics. The heat source concentration factor decreases and the maximum heat flux density increases as the helium content increases, which leads to an increased welding heat efficiency. Addition of CO2 up to around 10% also increases the heat efficiency. When the CO2 content exceeds 10%, the heat source concentration factor increases significantly and the heat efficiency decreases. The shielding gas composition also affects the heat source distribution. The heat source characteristics are applied to a computational fluid dynamic model of the weld pool to predict the weld shape, and the predictions are verified by experiment. The results indicate that the appropriate addition of helium to the shielding gas can increase the heat transferred to the peripheral regions of the arc and increase the sidewall penetration.

Molten pool behaviors and their influences on welding defects in narrow gap GMAW of 5083 Al-alloy

The welding defects such as porosity and lack-of-fusion is more likely to occur in the narrow gap gas metal arc welding (NG-GMAW) of aluminum alloy. In this study, a three-dimensional transient numerical simulation was carried out to investigate the molten pool behaviors as well as their influences on welding defects. A pulsed arc welding mode was employed, which is accompanied with elliptically symmetric arc models. Three different weld bead surface shapes i.e. the convex, flat and concave surface were achieved both by experiment and simulation with varied welding current. The flow pattern varied significantly due to the different thermal and force conditions. The formation process of lack-of-fusion defect was analyzed and the effect of flow pattern on porosity in all cases were discussed. The results indicate that the upward fluid flow near the sidewall border of molten pool facilitates the formation of lack-of-fusion defect and the downward and backward fluid flow near the sidewalls may induce the porosity.

Optimization of shielding gas composition in narrow gap GMA welding based on response surface methodology

In gas metal arc (GMA) welding, metal melting behaviors and the weld geometry are affected by shielding gas composition. Helium has special properties, and it can provide a wider weld profile which is benefit for increasing sidewall penetration in narrow gap GMA welding. In this paper, helium was added to shielding gas for narrow gap GMA welding to ensure the sidewall penetration. The shielding gas includes argon, carbon dioxide, and helium. The statistical models for narrow gap GMAW weld bead sidewall penetration were developed using response surface methodology (RSM) based on central composite design (CCD). The developed models were checked for their adequacy and significance by analysis of variance (ANOVA), and the effects of shielding gas composition on sidewall penetration were studied. Finally, the optimal ternary shielding gas for tandem narrow gap GMA welding were obtained by numerical optimization using RSM. The largest sidewall penetration can be obtained under 79%Ar–10%CO2–11%He.

Study on insufficient fusion of NG-GMAW for 5083 Al alloy

The lack of fusion at sidewall is one of the most frequently occurred problems in narrow-gap gas metal arc welding for joining thick plates of aluminum alloys. For better understanding the sidewall fusion mechanism and eventually solving such a problem, a series of welding experiments were carried out to investigate the sidewall penetration characteristics for 5083 Al alloys under various technological conditions. A Taguchi method was adopted to study the effect of welding current, arc voltage, and welding speed on the sidewall penetration and surface concavity, which is related to the quality of welding joint. The result shows that higher welding current or lower welding speed will contribute to a better fusion between weld bead and sidewalls; thus, optimized welding parameters can be obtained. The analysis of formation mechanism for lack of fusion defect shows that the molten pool surface tension, which is greatly influenced by temperature and oxide, is a main reason. Therefore, increasing the heat input to sidewalls by arc-swing, improving the gas protection of molten pool against oxidization, or reducing the heat dispersion of molten pool by preheating, are all effective measures for eliminating the lack of sidewall fusion defect.

Study on arc characteristics and their influences on weld bead geometry in narrow gap GMAW of 5083 Al-alloy

As an efficient process, narrow gap gas metal arc welding (NG-GMAW) was employed to join thick plates of 5083 Al-alloy. In this paper, arc characteristics in NG-GMAW of 5083 Al-alloy and their influence on the weld bead geometry were studied with synchronous acquisition system for various groove types, welding modes, welding current levels, and swing frequencies. The arc profiles were measured and appear to be entirely different compared with that in bead-on-plate GMAW process, meanwhile the arc size changes with varied welding mode, welding current, and arc swing frequency. Arc characteristics highly affect the weld bead geometrical parameters such as bead width, welding penetration, sidewall penetration, and weld surface concavity. Linear regressions were made between the arc characteristics and the weld bead geometrical parameters both in swing and non-swing arc process. It showed that a negative linear correlation exists between welding penetration and arc length, sidewall penetration, and arc warpage, while a positive linear correlation exists between weld surface concavity and arc warpage in non-swing arc process. In contrast, a non-linear correlation exists among them and the variation tendency remains the same in swing arc process.

Characteristics and formation mechanism of sidewall pores in NG-GMAW of 5083 Al-alloy

Narrow-gap gas metal arc welding was carried out on thick plate of 5083 Al-alloy, and the characteristics of welding pores were analyzed in terms of their size and distribution. The large pores were found to occur frequently in the weld zone adjacent to the sidewalls of narrow gap groove. Their amount and size increased along with increasing welding current and/or decreasing travel speed, which was totally different from conventional GMAW of aluminum alloy. The hydrogen was found to be the main cause to the pores by using SEM and EDS analysis. The formation of sidewall pores in narrow-gap weld beads were greatly related to the groove type, the shape of molten pool and its solidification behavior, and it was verified by polarization observation and EBSD analysis. The optimized parameters for avoiding sidewall pores and lack of sidewall fusion were 175A and 5mm/s in the narrow-gap welding of given aluminum alloy.

Research on narrow-gap GMAW with swing arc system in horizontal position

In this paper, a novel swing arc welding system developed for horizontal narrow gap gas metal arc welding (NG-GMAW) which was used in welding and joining heavy plate structures was designed and attempted. Using this welding system, two sound heavy plate beads, i.e., “multi-layer single-pass” welding and “multi-layer double-pass” welding structures, were obtained through adjusting swing amplitude and frequency. Two different welding structures are both without sagged molten pool and interlayer slags, especially obtaining the complete sidewall penetration. All these welding features ensure the bead with higher tensile properties and hardness than the base metal, respectively. This welding system displays considerable flexibility and convenience and possesses a tremendous advantage in the auto-welding industry.

Review on Design and Development of Narrow Gap Welding Torches

Narrow gap welding is an effective process for joining thick plates (2 to 12) of carbon steel, low alloys steel and high strength low alloy steels. But it requires some development in torch design, wire feeding mechanism and welding procedure for preventing all sorts of weld defects before practical application. Narrow gap welding technology is associated with various conventional arc welding technology like GMAW, GTAW, SAW, FCAW&SMAW. Several fabricators world over, especially in Japan have developed their own equipment working on different principles [1] Standard torches for NGW are not available routinely in the market. Torches for NGW are specially designed (custom made) for various NGW applications. Design of torch will differ from processes to process and application to application. Paper highlights the different torch designs used by researcher for different narrow gap welding technologies like I) Narrow Gap-Gas Metal Arc Welding (NG-GMAW) Torch Design II) Narrow Gap-Gas Tungsten Arc Welding (NG-GTAW) Torch Design III) Narrow Gap- Submerged Arc Welding (NG-SAW) Torch Design.

Stability of Double Wires Narrow-gap Gas Metal Arc Welding

Some special parameters of double wires narrow gap GMAW (NG-GMAW) welding can be adjusted individually, such as the angle and distance between the two wires along the welding direction and the distance between the wire and the sidewall of the groove. These parameters have great influence on the welding stability. Experiments results show that when the distance between the two wires is less than 15 mm or greater than 50 mm, the two welding arcs are in one molten pool or two independent molten pools, and the welding process is very stable. When the distance is 20～30 mm, the molten pool is transforming from single one to two independent one. Under the action of unbalanced electromagnetic force, the arc is compelled to break frequently and the stable welding process is destroyed. When the two wires are parallel along the welding direction, the welding process is stable; if the front wire is inclined to the rear wire, the wandering of the front arc is aggravated. The distance between the wire and the groove sidewall has great influence on the arc modality. When the distance is 2.5 mm, the arc burns stably between the wire and the sidewall as well as the bottom of the groove. But if the distance is less than 1.5 mm, the arc burns between the sidewall or bottom randomly, and the welding stability deteriorates.

Assessment of Properties and Microstructure of X100 Pipeline Girth Welds

Over the last few years, interest in the development and construction of large diameter, high pressure pipelines from the Mackenzie Delta in northern Canada and Alaska’s North Slope has steadily increased as a result of the projected decline in recoverable natural gas from existing reserves in western Canada. The success of such enormous pipeline projects, which will require utilisation of a strain-based design approach, is largely based on successful application of high strength line pipe steels, such as X80 and X100. To fully realise the economic benefits of high strength steels, welding processes and procedures must be developed to ensure that the requirements for high strength and good low temperature toughness are confidently obtainable in field girth welds used in the construction of such pipelines. This study aims to provide a better understanding of the factors that control weld metal strength and toughness for a series of mechanised X100 girth welds produced using a range of narrow-gap gas-metal arc welding procedures. The attainment of high yield strength (≥ 810 MPa) is considered to be an important target to ensure overmatching strength relative to the longitudinal property distribution of X100 line pipe steel. Some factors that have been considered in this evaluation include influences of gas-metal arc welding process variants (single and multi-wire) and changes in welding procedure specifications, including joint preparation. Key components of this work relate to the reliable measurement of weld metal strength variation through-thickness and as a function of position around the circumference of a given girth weld.

Improving Electron Beam Weldability of Heavy Steel Plates for PWR-Steam Generator.

Installation and replacement of many PWR-steam generators are planned inside and outside Japan. The steel plates for steam generators are heavy in thickness, and increase the number of welding passes and prolong the welding time. Electron beam welding (EBW) can greatly reduce the welding period compared with conventional welding methods (narrow-gap gas metal arc welding (GMAW) and submerged arc welding (SAW)). The problems in applying EBW are to prevent weld defects and to improve the toughness of the weld metal. Defect-free welding procedures were successfully established even in thick steel plates. The factors that deteriorate weld-metal (WM) toughness of EBW were investigated. The manufacturing process, which utilizes a new secondary refining process at steelmaking and a high-torque mill at plate mill in actual mass-production, were established. EBW base metal and WM have better properties including fracture toughness than those of conventional welding processes. As a result, an application of EBW to t...