Weld Pool Dynamics Research Articles

Influence of welding speed on weld pool dynamics and welding quality in underwater wet FCAW

The effects of welding speed on the molten pool dynamics, the weld forming and the diffusible hydrogen content in deposited metal during underwater wet flux cored arc welding (FCAW) were studied. The molten pool was elongated with a higher solidification speed, as the welding speed was increased. The faster solidification improved the stability of the molten pool by reducing the bubbles and gas disturbance during FCAW, whereas it caused more hydrogen remaining in the deposited metal and increased the diffusible hydrogen content of the weld. According to the variation coefficient of weld reinforcement, the weld formation was improved at first and then deteriorated with the increasing welding speed. More bulges appeared in the weld at a low welding speed and pits were formed more easily with a high welding speed. The occurrence of the defects was closely related to the molten pool behaviors, which were determined by the pool angles during FACW.

Material flow analyses of high-efficiency joint process in VPPA keyhole flat welding by X-ray transmission system

The consideration of environment-friendly and low energy consumption tends to control and guide the progress of metal joint manufacturing processes. Variable polarity plasma arc (VPPA) welding is an environment-friendly, high quality and efficiency joint method for aluminum alloys due to its characteristics of such as little welding fume, and high energy density etc. However, the three-dimensional morphology and physical phenomena of penetration weld pool in VPPA welding are not clearly understood, which slows down its process development and optimization. This work aims to clarify the three-dimensional keyhole detouring flow of VPPA welding for aluminum alloy. The liquid metal distribution of penetration weld pool was generally obtained by metallographic observation. The measurement results show that the liquid metal layer attached to the three-dimensional keyhole boundary is thin. And the liquid metal mainly distributes in the bottom side of the keyhole. In order to understand the weld pool dynamics, the stereo synchronous imaging of tungsten tracer particles using two sets of X-ray transmission system was adopted, for the first time, to measure the 3D velocity field of convection flow inside weld pool of VPPA welding of aluminum alloy in real time. The flow on the weld pool surface at different locations of keyhole boundary was also measured by tracing zirconia and slag particles. The liquid metal flows simultaneously to the top and around the keyhole. The metal at the bottom of keyhole’s front wall, flows around the keyhole towards the backside, then separately flows to top side and bottom side, forming a vertical stationary point. The key to weld and avoid defect proofing are to ensure stable detouring flow on the keyhole bottom and proper stationary point position.

Sinusoidal oscillating laser welding of 7075 aluminum alloy: Hydrodynamics, porosity formation and optimization

This paper reports a mechanism understanding of how the sinusoidal beam oscillation reduces the porosity defects during laser welding of 7075 aluminum alloy. In this study, the coupled time dependent keyhole and weld pool dynamics as well as the porosity formation during the oscillating laser welding process were simulated. Also, the keyhole and weld pool behaviors were experimentally investigated using high speed imaging and the porosity rates of the welds with or without beam oscillations were characterized. The results indicated that the beam oscillation modulated the fluid flow in the weld pool into a swinging mode and stabilized it. In this mode, the average flow velocity exhibited periodical oscillation with the double frequency of that of the laser beam oscillation. The velocity oscillating amplitude decreased as the frequency of beam oscillation increased. When the oscillating frequency of the fluid flow increased to 100 Hz, the swinging flow with a small oscillation amplitude could be obtained. The results also indicated that with the beam oscillation the keyhole stability was improved and the porosities were reduced in the welds. This is mainly because the keyhole diameter was significantly increased and the surface tension effect, the major cause responsible for the keyhole oscillations, was weakened. However, in cases with beam oscillation, there were still some porosities in the welds. An optimization method to reduce the porosities by limiting the time of laser staying in unstable area was proposed. Porosity free welds were successfully made. This study could help improving the understanding of the weld pool dynamics in laser welding and guide the selection of the process parameters.

Investigation of heat transfer and fluid flow in high current GTA welding by a unified model

This research is aimed to understand the heat transfer and fluid flow of the arc plasma and weld pool with a significant deformation of the weld pool surface during high current gas tungsten arc welding (GTAW). A unified model including the tungsten electrode, arc plasma and the weld pool of low carbon steel is developed by presetting a free surface deformation from the experimental observations. Buoyance, Lorentz force, plasma drag force and Marangoni force are taken into account to investigate the weld pool dynamics with a great depressed free surface. Heat flux, current density, Marangoni force and plasma drag force at the weld pool surface are presented to understand the heat and momentum transfer from the arc plasma to the weld pool. It is found that the distributions of the heat flux and current density as well as temperature at the depressed weld pool surface are double-peak profiles; the molten metal flow of the weld pool with severe surface depression are determined by the plasma drag force rather than the Marangoni force. The effect of the Lorentz force is more important than that of the buoyance, but weaker than the Marangoni force. The results are consistent with the existing results of both the experiment and the theory.

Physical understanding of keyhole and weld pool dynamics in laser welding under different water pressures

Underwater laser welding, an efficient and labor-saving in-situ repair technology, is prospective in the field of underwater environment manufacturing. It simplifies the tedious process of disassembly and transportation when the underwater facilities are usually repaired ashore. However, the keyhole and weld pool dynamics in laser welding under different water pressure are little understood. In this study, we developed a mathematical model of underwater laser welding process with arbitrary ambient pressures for the first time. The underlying physics of the underwater laser welding process was explored, and the keyhole and weld pool dynamics between the underwater environment and the ground were compared. Results showed that a keyhole similar to that in the atmosphere is also formed in the water environment. And the keyhole was oscillated. The surface of the weld pool in contact with water cooled quickly, forming a special morphology of the weld pool. The flow in the weld pool was mainly driven by the recoil pressure. The temperature of the surface of the keyhole increased with the increase of water depth. That is because the laser beam energy density is large enough to heat the local metal to the boiling point which increases with the ambient pressure increase. It means that the weld pool was able to absorb more energy before evaporation. Then, less energy is applied to induce the recoil pressure to deepen the keyhole. As a result, a shallower but more stable keyhole is formed in underwater laser welding process under 5 MPa pressure. This study can provide better understanding of the heat and mass transfer behaviors in underwater laser welding.

A CFD-FEM Model of Residual Stress for Electron Beam Welding Including the Weld Imperfection Effect

Residual stress of electron beam welding (EBW) is associated with the heat transfer, fluid flow, and keyhole dynamics occurred in the process. In engineering, it was commonly simulated by pure heat conduction-based thermomechanical modeling with an experimentally calibrated heat source model. This method, however, is difficult to be used to consider the effect of weld imperfections such as the spiking and porosity defect. In this study, we proposed a novel combined computational fluid dynamics and finite element method (CFD-FEM) model to predicate the residual stresses of EBW. This model allows us to include the integral effect of heat transfer, fluid flow, and keyhole dynamics as well as process defects on residual stresses for the first time. A phenomenological heat source model dependent on the dynamic keyhole was considered in the CFD model part. A mapping algorithm was developed to build a new heat source model of FEM modeling part from the CFD simulations. Parametrical simulations and experiments of EBW of a 10-mm-thickness Ti-6-Al-4-V alloy plate were carried out and good agreements were obtained by the comparisons. It was shown that for deep penetration EBW the keyhole oscillates in radial and depth directions and the spiking defect occurs, particularly under large heat input parameters. Interestingly, the equivalent and principle residual stresses exhibits strong oscillations in the bottom part of welds along the welding direction, in which the amplitudes are around 9 pct (73.69 MPa) of the equivalent stress and 24 pct (121.84 MPa) of the principle stress in penetration direction, respectively, when the power of electron beam is 1.5 kW. As the power of electron beam increases, the effect of spiking on the residual stress enhances. When the welding power increases to 2.25 kW, the amplitudes of equivalent residual stress increase to 12 pct (119.2 MPa). Our results provide a quantitative method connecting the weld pool dynamics and process defects with the residual stress of EBW, which previously could be understood only in a qualitative way, and demonstrate the possibility of residual stress predications only from the information of materials and process parameters before welding, without the knowledge of post-weld profiles.

Effects of shielding gases on process stability of 10CrNi3MoV steel in hybrid laser-arc welding

Hybrid laser-arc welding (HLAW) of 10CrNi3MoV steel was carried out with different shielding gas mixtures including 100% Ar, 5% CO2 + Ar, 10% CO2 + Ar, 15% CO2 + Ar, 20% CO2 + Ar, 25% CO2 + Ar and 100% CO2. A CMOS high speed camera and real-time electric signal data were utilized to obtain the high-speed images (HSI) and electric signals of the droplet transfer and weld pool dynamics in real time during the HLAW process. The results showed that the increase of CO2 content increased the synergetic effect, especially the energy density of laser, and improved the penetration. The electric signal and HSI results revealed that, at low CO2 content (5% CO2 + Ar, 10% CO2 + Ar), the process was unstable with spatters and the metal transfer mode was globular mode. But at medium CO2 content (15% CO2 + Ar, 20% CO2 + Ar), the process was stable with less spatters and the metal transfer mode was spray mode. Stable process and accepted sound weld were obtained using 20% CO2 + Ar, which corresponded to the welding current 200 A and welding voltage 29 V. As CO2 content in the shielding gas increased, the increase in the weld current and welding voltage deteriorated the stability of the welding process, and the transfer mode shifted to repelled transfer, leading to the formation of large spatters.

A Feasibility Study Comparing Two Commercial TIG Welding Machines for Deep Penetration

Developing a deep penetration TIG welding technique to produce welds of equal quality to the industrial standard practise of laser-based welding techniques has the potential to lower production complexity and cost. Higher currents levels are required to increase penetration depth in conventional TIG welding but this results in excessive weld bead width amongst other detrimental effects. However, through K-TIG and A-TIG techniques these detrimental effects can be circumnavigated. Prior experimental work on weld pool dynamics in conventional TIG welding in higher current regions has been sparse as TIG welding enhanced through novel techniques provides the best quality welds. This paper is an early feasibility study for novel deep penetration welding techniques motivated by observations made during research done at The University of Sheffield where unexpected activity in the weld pool was identified during TIG welding with a VBC IE500DHC between 300A – 1000A. This current range is labelled the ‘Red Region’. Understanding the fluid dynamics of the molten metal in the weld pool at the ‘ Red Region' current level will help in the creation of novel techniques for deep penetration TIG welding. Addressing this, this paper compares the quality of welds produced between 100A and 200A on 316 Stainless Steel by two industrially leading welding machines; the Miller Dynasty 350 and the VBCie 500DHC.

Localized boiling-induced spatters in the high-power laser welding of stainless steel: three-dimensional visualization and physical understanding

Serious spatter defects occur with the intensive interaction between a high-power density laser beam (> 107 W/cm2) and the material used in high-power (HP) laser welding. In this study, we developed a three-dimensional comprehensive model to physically simulate the keyhole and weld pool dynamics in HP laser welding. Our recent proposed adaptive mesh-refinement method was adopted to solve the model. An interesting phenomenon was directly observed: the intense localized boiling on the mesoscale keyhole wall induced many small spatters (LBiS), sized hundreds of microns, during 10 kW HP laser welding of 304 stainless steel. Experimental studies of parameters were also conducted to validate the model and characterize the spatter behaviors outside the keyhole wall during welding. The theoretical predictions of LBiS behaviors agreed well with the experimental observations. Moreover, the flying directions of LBiS generally approached vertical and were relatively insensitive to welding speed, even when the speed was increased to a very high value of 15 m/min. This finding shows that LBiS are very different from the commonly observed spatter defects produced by the intense friction effect of a vapor plume toward the keyhole, and this finding demonstrates the variety of the spatters. Furthermore, an energy conservation theory was proposed to characterize LBiS, and a novel dimensionless spatter number was formulated to calculate their formation threshold. The spatter number relates to the Reynold and Weber numbers of the molten flow of the weld pool and shows the relative importance of an inertia flow driven by the recoil pressure and the resistance from viscous dissipation, surface tension and gravity. The spatter number was less than 1 when LBiS was formed. The proposed theory was validated against the numerical predictions, and good agreements were obtained.

Effect of alloy element on weld pool dynamics in laser welding of aluminum alloys

In this manuscript, weld pool dynamics in laser welding of various series of aluminum alloys were investigated by the in situ X-ray phase contrast imaging system. The experimental results showed that metal irradiated by laser was evaporated immediately, which generated the keyhole. Then metal surrounding the keyhole was melted gradually with the heat from keyhole. The growth rate of keyhole depth had a positive linear correlation with the total content of low boiling temperature elements (TCE), so did the keyhole depth and diameter at the stable stage. Longitudinal view area of the molten pool had a negative linear correlation with the thermal conductivity of aluminum alloy. The measured laser absorption rate had the same variation trend with the ratio of keyhole depth to diameter, and the highest absorption rate of 58% appeared in laser welding of aluminum alloy with TCE equal to 2.1%. Violent fluctuation in keyhole shape was avoided in aluminum alloy with TCE lower than 2.1%, where the surface tension and recoil pressure of metal vapor were balanced. To sum up, the effect of alloy element on weld pool dynamics in laser welding of aluminum alloys was firstly quantified in this manuscript.

Thermoelectric currents and thermoelectric-magnetic effects in full-penetration laser beam welding of aluminum alloy with magnetic field support

Thermoelectric currents (TECs) caused by Seebeck effect were usually neglected in welding simulation. This paper characterizes the TECs and thermoelectric-magnetic phenomena during the magnetically supported laser beam welding (MSLBW) of thick aluminum (Al) alloy. The simulation is based on a computational fluid dynamic (CFD) model developed in three dimensions. The model considers multi-physical mechanisms including thermal transfer, solid–liquid (S/L) transition, molten metal convection, magnetohydrodynamics (MHD) and Seebeck effect. The computed TEC distribution in laser welding pool highly depends on welding time and temperature gradient. The large current densities occur near the weld pool edge and the vectors are opposite at both side of S/L interface. The maximum TEC density obtained in the stabilized weld pool is 2.14 × 106 A/m2, leading to a self-induced magnetic field of 2.27 × 10−3 T and a Lorentz force of 2.62 × 103 N/m3. The influences of TECs on weld pool dynamics are quite limited in LBW but notable in MSLBW. The inversion of MF direction leads to different Lorentz force distributions, weld pool dimensions and seam profiles. The simulation data agrees well with the experimental results.

Correlation between pulse 300 series SS wire parameters

A centralized and extensive relationship linking arc characteristics is presented to explain the metal transfer phenomenon, electrode melting rate, metal transport and weld-solidification during the Pulse gas metal arc welding. The study details interpretation of numerous arc, heat and mass transfer correlations during molten stage of the electrode, initiation of droplet, growth, detachment, transfer and impingement into the weld-pool. An interactive coupling between pulse process parameters, temperature distribution and weld pool dynamics has been discussed. This review elucidates conditions of the pulse metal transfer in the arc column and into the weld pool on the basis of current waveform, magnitude of peak, base and pulse current for 300stainless steel wire.

Optimization possibility of beam scanning for electron beam welding: Physics understanding and parameters selection criteria

Scanning electron beam welding (SEBW) is a very important process for welding of thick plates in aerospace, aeronautical and power industries. However, since the invention of this process, the scanning parameters are typically selected through time consuming and costly trial and error. No existing mathematical criterion can be used to select the optimal parameters due to lack of physical understanding of the welding process. In this study, we developed a three-dimensional mathematical model of SEBW capable of modeling the coupled keyhole and weld pool dynamics for the first time, and used it to understand the underlying physics of the welding process and explore the process optimization criterion of circular mode beam scanning by combining welding experiments and modeling. We showed that beam scanning may not always stabilize the keyhole and weld pool, and may not improve the final weld quality of electron beam welding. However, beam scanning can modulate the movement of high temperature positions on keyhole wall, and contribute to a better uniformity of weld pool dynamics behind the keyhole. For circular beam scanning, we proved that low frequency scanning may lead to more welding defects such as porosity, spiking, and spatters because it increases the tendency of keyhole oscillations as compared to the no scanning case. High frequency scanning could stabilize the keyhole to a certain degree and modulate the fluid flow of the weld pool to make it more regular. Additionally, the scanning radius should be neither too small nor too large. Too small radius may lead to more defects, and too large radius can decrease the penetration depth significantly. A dual direction energy uniformity (DDEU) criterion was proposed to select the scanning parameters by considering the energy uniformity degree in the welding direction and the transverse direction. It was demonstrated that process parameters including beam scanning frequency and radius can be successfully optimized using the proposed criterion.

Numerical simulation of weld pool dynamics using a SPH approach

In this paper, we present a numerical model for the calculation of weld pool dynamics and the resulting weld seam structure with regard to the arc forces. The model uses an incompressible smoothed particle hydrodynamics method and is suitable for three-dimensional calculations with a high spatial resolution at low computational costs. The model is applied to a GTAW process, where the arc forces as well as the arc heat input is modelled on the basis of known values from measurement and literature. The validity of the model is shown for different arc currents by comparison of the calculated penetration profile with measurements. Finally, the model is used to perform parameter studies on the influence of the arc pressure and the arc shear on the penetration profile. The parameter studies show the strong influences of the arc forces on the penetration depth.

3D Multiphysical Modelling of Fluid Dynamics and Mass Transfer in Laser Welding of Dissimilar Materials

A three-dimensional multiphysical transient model was developed to investigate keyhole formation, weld pool dynamics, and mass transfer in laser welding of dissimilar materials. The coupling of heat transfer, fluid flow, keyhole free surface evolution, and solute diffusion between dissimilar metals was simulated. The adaptive heat source model was used to trace the change of keyhole shape, and the Rayleigh scattering of the laser beam was considered. The keyhole wall was calculated using the fluid volume equation, primarily considering the recoil pressure induced by metal evaporation, surface tension, and hydrostatic pressure. Fluid flow, diffusion, and keyhole formation were considered simultaneously in mass transport processes. Welding experiments of 304L stainless steel and industrial pure titanium TA2 were performed to verify the simulation results. It is shown that spatters are shaped during the welding process. The thickness of the intermetallic reaction layer between the two metals and the diffusion of elements in the weld are calculated, which are important criteria for welding quality. The simulation results correspond well with the experimental results.

Influence of magnetic field orientation on molten pool dynamics during magnet-assisted laser butt welding of thick aluminum alloy plates

An externally applied magnetic field has been proved effective in controlling the molten flow behavior during the laser beam welding. This paper reveals how a magnetic field applied in different orientations influences the weld pool dynamics during the full-penetration laser butt welding of thick aluminum alloy plates. A multi-physics coupling model was developed and numerically solved by considering the electromagnetic interaction between static magnetic field and magnetofluid. The dynamic and electromagnetic information within the weld pool were predicted and comparatively analyzed. The results show that Lorentz force and Hartmann effect induced in magnet-assisted weld pool are quite sensitive to magnetic field direction. The horizontally directed magnetic field leads to limited Hartmann effect by inducing additional molten vortexes, while the field aligned vertically to aluminum surface can obviously reduce the molten flow velocity and weaken the Marangoni convection. The work can provide a reference in optimizing the operating parameters when an electromagnetic system is employed to improve the welding quality.

Numerical simulation of the weld pool dynamics during pulsed laser welding using adapted heat source models

A transient simulation including the impact of the laser energy, the melting of the metal and the development of the weld pool was conducted to observe the evolution of the vapor capillary and the solidification of the melt in pulsed laser beam welding of AISI 304 steel. The phase field method was implemented to investigate the evolution and behavior of the liquid-gas interface during welding and to describe the condensed and vapor phases. The effects of phase transition, recoil pressure, thermo-capillary and natural convection, vaporization and temperature dependent material properties were taken into account. A Gaussian-like heat source under consideration of the Fresnel absorption model was used to model the energy input of the laser beam. The heat source model was extended by a newly developed empirical approach of describing multiple beam reflections in the keyhole. To validate this new model, the numerical results were compared to experimental data and good agreement regarding the size and shape of the weld pool was observed.

Melt flow and thermal transfer during magnetically supported laser beam welding of thick aluminum alloy plates

A multi-field coupled numerical model was proposed to calculate the weld pool dynamics during the full-penetration laser beam welding of 12mm thick aluminum alloy plates under the function of a longitudinal magnetic field. The model dealt with the melt flow, heat transfer, solid-liquid transition and magnetic diffusion phenomena in welding process. The Lorentz force directing from the keyhole to the liquid-solid interface was induced in magnetic field supported cases. The Marangoni convection near the weld pool surfaces was obviously weakened due to the electromagnetic braking effect, which was featured with decreased velocity and compressed vortex. The heat transfer condition was largely changed, resulting in declined weld pool dimensions. When the magnetic flux density applied was larger than 0.5T, the convective heat exchange at the middle height of weld pool became notable again due to the reversed melt flow direction. The liquid-solid lines with lower curvature were achieved. The reinforcement of Hartmann effect became limited. The model and the simulated weld pool morphologies were verified in experiment.

Understanding and overcoming of abnormity at start and end of the weld bead in additive manufacturing with GMAW

Weld bead geometry at start and end of the bead is often abnormal compared with the middle region, which will greatly affect the forming in gas metal arc welding (GMAW) additive manufacturing. The study’s aim is to investigate the causes and the optimization strategy of the weld bead abnormity at the unstable region. The weld pool dynamics, convection, and the extension process were analyzed through a three-dimensional transient fluid model and the finite element analysis of thermal behavior. The results showed that the abnormal bead geometry can be attributed to the backward fluid flow and the metal swelling in the weld pool, and the length of the initial bulky region is positively correlated with the inclined shape at the end, as well as the length of the weld pool. Some strategies to control the bead abnormity through adjusting the welding parameters, the crater filling options, and the path planning patterns were proposed. These methods contributed to the continuous and smooth deposition surface and laid the foundation of GMAW-based additive manufacturing process.

Numerical simulation of TIG weld pool dynamics using smoothed particle hydrodynamics

A three-dimensional computational weld pool model for TIG welding is described. The model is based on the smoothed particle hydrodynamics method and takes various forces in the weld pool into account including shear stress and arc pressure, surface tension, marangoni effect, gravity and the welding speed. The numerical results are compared to cross sections from experiments for different welding currents and show a very good agreement. The model is then used to describe the weld pool dynamics and the formation process of the weld penetration as well as the temperature distribution in the weld pool.