High-speed Gas Metal Arc Welding Research Articles

Numerical analysis of molten metal behavior and undercut formation in high-speed GMAW

• A self-adaptive integrated arc heat-force distribution model is developed for GMAW. • Backward flow, gouging region and thin metal layer are processes initiating undercut. • Two dimensionless numbers are developed to predict the growth rate of undercut. A computational fluid dynamics model was established to investigate the undercut formation in high speed gas metal arc welding (GMAW). A double-ellipse integrated "arc current density-arc pressure-electromagnetic force-arc heat" distribution model, which could be self-adaptive to weld pool surface evolution, was developed. The multi-coupling transport phenomena in weld pool were simulated, and the effect of driving forces on the behaviors of molten metal and the formation of undercut was analyzed quantitatively. The results show that the high-velocity backward molten metal flow, large-size gouging region and prematurely solidified thin metal layer at weld toe were typical processes that initiated undercut. The evolution of undercut morphology was determined by the characteristics of inertia force, hydrostatic pressure and arc pressure in the different stages. Based on Buckingham- π theorem, the dimensionless growth rate of undercut area was a parabolic function of a dimensionless group including characteristics of stress state, molten metal flow and morphology of weld pool. This study clarified the physical origin of undercut defect in GMAW and might also provide some basic guidelines for its suppression.

Numerical simulation of coupled arc-droplet-weld pool behaviors during compound magnetic field assisted gas metal arc welding

A compound external magnetic field (EMF) can be used to prevent weld bead defects by controlling the metal transfer and arc behaviors during high-speed gas metal arc welding. A unified model containing the filler metal, arc, weld pool, and compound EMF was established to study the effect of the compound EMF on the welding process. The heat and mass transfer among the filler metal, arc, and weld pool under the action of the EMF was numerically analyzed. The simulation results showed that the high-temperature region in the molten pool was reduced and the temperature distribution was more uniform after the EMF was introduced. The speed of the forward liquid metal flow in the front of the molten pool increased due to the forward inclination of the filler metal and arc plasma, and a small metal bulge formed at the leading edge of the molten pool. Because of the periodic swing of the arc and the droplet, the transverse filling of the liquid metal strengthened. The generated additional electromagnetic force inside the weld pool was analyzed, and it had limited impact on the weld pool behavior.

Experimental measurement of fluid flow in high-speed GMAW assisted by transverse magnetic field

Abstract The visual measurement system was established to investigate the fluid flow during high-speed gas metal arc welding. The two-dimensional fluid flow velocity on weld pool surface was measured using the tracer particles. The characteristic parameters of the backward liquid metal flow were extracted, then the speed of the liquid metal flow was compared with different welding speeds. The results showed that the high-speed backward liquid metal flow and the pinching & earlier solidification of the backward liquid metal flow channel caused the humping bead formation. A transverse magnetic field was used to control the fluid flow behavior to prevent the humping bead. Due to the introduction of transverse magnetic field, the thickness of liquid metal layer underneath the arc was increased. The longitudinal velocity of the liquid metal flow was decreased while the transverse velocity was increased so the humping bead can be suppressed.

Study of high-speed GMAW assisted by compound external magnetic field

A magnetic generating device was proposed to regulate the transverse and backward flow of weld pool for high-speed gas metal arc welding (GMAW). Compound external magnetic field (EMF) was simulated to verify the feasibility of designed magnetic generating device. The highest welding speed with good weld bead appearance was 1.7 m/min with the wire feed speed of 9.0 m/min when compound EMF was applied. The effect of the compound EMF on the arc and droplet motion was studied by image acquisition system. The suppression mechanism of the compound EMF on the undercut defect of high-speed GMAW was analyzed. The mechanism of the influence of the applied compound EMF on different welding speeds and welding currents was studied by the application of the compound EMF-assisted welding process, and low-cost, high-quality, and high-speed welding technology was realized.

Hump weld bead monitoring based on transient temperature field of molten pool

The molten pool images of GMAW (Gas Metal-Arc Welding) was acquired during the base time period using a self-built system for measuring the transient temperature field of molten pool. The transient temperature field of the molten pool of GMAW at different welding speeds was calculated based on the principle of colorimetric temperature measurement. When analyzing the temperature distribution at the middle and tail region of the molten pool in the process of hump bead formation under high speed GMAW, it was found that the temperature distribution of the molten pool is uneven perpendicular to the welding direction, and this law does not accord with the time when the hump appears in the image, and the accuracy rate accorded with the law was also counted. Through this rule, the welding robot can adjust and control the process in time before the appearance of hump, and the prediction of hump before bead formation under GMAW is preliminarily realized.

Analysis of additional electromagnetic force for mitigating the humping bead in high-speed gas metal arc welding

To mitigate the occurrence of humping bead in high-speed gas metal arc welding, an external magnetic field is exerted into the weld pool to produce the forward electromagnetic force and to brake the backward flow molten jet. A thermal-magnetic coupling model is developed to analyze the distribution of the additional electromagnetic force in weld pool. The interaction of the external magnetic field in the arc region with the arc/liquid metal stream at the wire tip is taken into consideration, and the excitation current and the wire-magnet distance are optimized. The welding experiments on mild steel plates (Q235B) demonstrate that with help of the forward additional electromagnetic force, good weld bead quality without humping is obtained in high-speed gas metal arc welding.

Effect of external magnetic field on weld pool flow conditions in high-speed gas metal arc welding

In high-speed gas metal arc welding process, weld defects like humping bead and undercutting appear and the weld bead quality worsens when welding speed exceeds a critical value. The previous investigations have proved that the backward flowing molten jet with high momentum in the weld pool is responsible for the formation of humping bead in high-speed gas metal arc welding. In this study, an external electromagnetic field generator is developed to change and control the flow condition in the weld pool and suppress the occurrence tendency of the humping bead. Bead-on-plate welding experiments are performed on mild steel Q235 test-pieces, and the influences of the exerted magnetic strength on the arc behavior, the fluid flow inside the weld pool, the weld bead shape and the weld penetration and width are investigated. The reason why the external electromagnetic field can suppress the humping bead is explained. The results show that external magnetic field can remarkably adjust the weld pool fluid flow field and decrease the momentum of the backward flowing molten jet. Therefore, weld defects like humping bead and undercutting can be effectively suppressed, the quality of weld bead is remarkably improved, and the critical welding speed is notably increased.

Effect of droplet heat content distribution on humping formation in high speed GMAW

1) MOE Key Laboratory for Liquid-Solid Structure Evolution and Materials Processing, Instituteof Materials Joining, Shandong University, Jinan 250061, China2) State Key Laboratory for Advanced Welding & Joining, Harbin Institute of Technology, Harbin 150001,China3) BAM Federal Institute for Materials Research and Testing, Division V.5 Safety of Joined Components,Berlin 12205, GermanyManuscript received 22 March 2011; in revised form 18 June 2011The momentum of strong backward °owing melt jet and the thermal action fromtransferred droplets are two dominating factors afiecting the formation of humpingbead in high speed gas metal arc welding (GMAW). Appropriate describing the in°u-ence of the distribution mode of droplet heat content in the weld pool is essential tounderstand the physical mechanism of humping bead formation. Based on the exper-imental results, four kinds of droplet heat content distribution modes are proposedand employed to calculate the transient evolution of the temperature fleld and weldpool during high speed GMAW process. Through making comparison of predictedand measured weld bead dimensions, a suitable and adaptive distribution mode ofdroplet heat content is found,

Numerical simulation of humping phenomenon in high speed gas metal arc welding

It is of great significance to obtain a thorough understanding of the physical mechanisms responsible for humping bead phenomenon in high speed gas metal arc welding (GMAW) in order to raise welding efficiency. Experiments were conducted to observe the weld pool behaviors in high speed GMAW, and it was found that both the severely deformed weld pool surface and strong backward flowing play a dominant role in humping bead formation. In this study, a mathematical model is developed to quantitatively analyze the forming mechanism of humping beads for high speed GMAW through considering both the momentum and heat content distribution of the backward flowing molten metal inside the weld pool. The transient development of temperature profiles in the weld pool with severe deformation demonstrates the humping bead forming process under some welding conditions. The predicted and measured humping bead dimensions are in agreement.

Numerical Analysis of Forming Mechanism of Hump Bead in High Speed GMAW

A thorough understanding of the physical mechanisms responsible for humping bead formation in high speed GMAW (gas metal arc welding) is essential to improve the welding productivity in today’s competitive manufacturing industries. Based on the experimental observing results, a mathematical model is developed to quantitatively analyze the forming mechanism of humping bead for high speed GMAW, considering both the momentum and heat content of the backward flowing molten metal in weld pools. One term related to the momentum of backward flowing molten metal is added to the equation of the weld pool surface deformation, and the heat content of overheated droplets is distributed within the layer covering the whole pool. The humping bead forming process and its dimension and 3-D geometry are numerically simulated under some welding conditions. It is found that the model can describe and characterize the humping formation in high speed GMAW quantitatively.

Visualization of hump formation in high-speed gas metal arc welding

The hump bead is a typical weld defect observed in high-speed welding. Its occurrence limits the improvement of welding productivity. Visualization of hump formation during high-speed gas metal arc welding (GMAW) is helpful in the better understanding of the humping phenomena so that effective measures can be taken to suppress or decrease the tendency of hump formation and achieve higher productivity welding. In this study, an experimental system was developed to implement vision-based observation of the weld pool behavior during high-speed GMAW. Considering the weld pool characteristics in high-speed welding, a narrow band-pass and neutral density filter was equipped for the CCD camera, the suitable exposure time was selected and side view orientation of the CCD camera was employed. The events that took place at the rear portion of the weld pools were imaged during the welding processes with and without hump bead formation, respectively. It was found that the variation of the weld pool surface height and the solid–liquid interface at the pool trailing with time shows some useful information to judge whether the humping phenomenon occurs or not.

Predicting of bead undercut defects in high-speed gas metal arc welding (GMAW)

In the gas metal arc welding (GMAW) process, when the welding speed reaches a certain threshold, there will be an onset of weld bead undercut defects which limit the further increase of the welding speed. Establishing a mathematical model for high-speedGMAWto predict the tendency of bead undercuts is of great significance to prevent such defects. Under the action of various forces, the transferred metal from filler wire to the weld pool, and the geometry and dimension of the pool itself decide if the bead undercut occurs or not. The previous model simplified the pool shape too much. In this paper, based on the actual weld pool geometry and dimension calculated from a numerical model, a hydrostatic model for liquid metal surface is used to study the onset of bead undercut defects in the high-speed welding process and the effects of different welding parameters on the bead undercut tendency.