Numerical Modeling Of GMAW ARC

A comprehensive mathematical model and the associated numerical technique have been developed to simulate the coupled, interactive transport phenomena between the electrode (droplets), the arc plasma, and the workpiece (weld pool) during a stationary axisymmetric gas metal arc welding process. The simulation involves arc plasma generation, electrode melting, droplet formation, detachment, transfer, and impingement onto the workpiece, and weld pool dynamics. During transfer from the tip of the electrode to the workpiece, the droplet subjects to gravity, electromagnetic force, surface tension, and arc plasma drag force. Transient temperature and velocity distributions of the arc plasma, shapes of the electrode, droplet, and weld pool, and heat transfer and fluid flow in the weld pool are all calculated in a single, unified model. The predicted solidified weld bead shape compares favourably with the experimental result.

A three dimensional transient model for heat transfer and fluid flow of weld pool during electron beam freeform fabrication of Ti-6-Al-4-V alloy

Heat transfer and fluid flow and their effects on the solidification microstructure in full-penetration laser welding of aluminum sheet

In this study, the accuracy of the model developed for oscillating laser-GMA hybrid welding has been improved by integrating more experimentally measured parameters, such as the droplet velocity, transient arc power, and fusion area on sheets at the transverse cross-section. The effects of lateral distance of hybrid heat source relative to the top sheet edge on heat transfer and fluid flow in weld pool are numerically simulated. It is found that with more offset of the hybrid heat source to the top sheet, a larger depression and stronger lateral flow occur in weld pool, which leads to a delayed and weak backwards flow. These characteristics of heat transfer and fluid flow play a great role in the formation of weld bead defects of lap joint configuration, such as irregularity, discontinuity, lack of fusion at the top sheet. Compared to the experimental investigation, the numerical simulation provides not only quantitative description of the occurrence of those weld bead defects but also a deep physical insight into the whole process. Thus, it is useful for the parameter optimizations and further applications of this hybrid welding process.

Numerical simulation provides a way to improve our understandings of the heat transfer and fluid flow behaviors of the weld pool during laser keyhole welding. However, current numerical studies are only limited to serial simulations which running on a single CPU. In this study, a parallel numerical study of the heat transfer and fluid flow of the weld pool is presented. A mathematical model considering the effect of Marangoni force, buoyancy force, friction force of the mushy zone region and the effect of keyhole is presented. A combined keyhole volume and surface heat source model is also developed. The coupled transient heat transfer and Navier-stokes equations are solved with a high order accuracy parallel projection method. The simulation code is parallelized with the OpenMP language. It is shown that 200% speedup can be achieved on a shared memory quad-core CPU using the presented parallel simulation system. The simulation results agree well with the in-situ high speed CCD video imaging experiments and the literature results.

A three-dimensional transient model is developed to solve for heat transfer, fluid flow, and species distribution during a continuous gas metal arc welding (GMAW) process for joining dissimilar aluminum alloys. The phase-change process during melting and solidification is modeled using a fixed-grid enthalpy-porositytechnique, and Scheil's model is used to determine coupling among composition, temperature, and the liquid fraction. The effect of molten droplet addition to the weld pool is simulated using a “cavity” model, in which the droplet heat and species addition to the molten pool are considered as volumetric heat and species sources, respectively, distributed in an imaginary cylindrical cavity within the molten pool. To establish the model for joining dissimilar alloys, results for joining two pieces of a similar alloy are also presented. The dissimilar welding model is demonstrated using a case study in which a plate of wrought aluminum alloy (with approximately 0.5 wt% Si) is butt-welded to an aluminum cast alloy plate (with approximately 10 wt% Si) of equal thickness using a GMAW process. Macrosegregation, along with the associated heat transfer and fluid flow phenomena and their role in the weld pool development, are discussed. The model is able to capture some of the key features of the process, such as differential heating of the two alloys, asymmetric weld pool development, mixing of the molten alloys, and the final composition after solidification.

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

This article analyzes the transient complex heat transfer and fluid flow in molten metal and arc plasma during the gas metal arc welding process. The model predicts the formation, growth, detachment, and transfer of droplets from the tip of a continuously fed electrode under the influences of several competing forces including gravity, electromagnetic force, arc pressure, plasma shear stress, and surface tension. Simulations were conducted for five different current levels to study the effects of current on the distributions of temperature, velocity, pressure, and current density in the droplet and/or the arc plasma. Agreement between the simulated results and published experimental data was obtained.

Because of its high gas velocity and heat input,plasma arc welding(PAW) can penetrate thicker workpieces with a single pass because PAW can operate in the keyhole mode. Compared with electron beam and laser beam welding,keyhole PAW is more cost effective and more tolerant of joint preparation,so that it is widely used in manufacturing structures with medium thickness. However,the keyhole establishment and sustainment during the initial stage of PAW process, i.e.,the keyholing process,has a critical effect on the process stability and the weld quality.Thus, modelling and simulating of the keyholing process and its influence on fluid flow and heat transfer in keyhole PAW process is of great significance to completely understand the process mechanism.With considering the interaction between weld pool and keyhole,a three dimensional transient model of fluid flow and heat transfer in weld pool is developed for numerical analysis of keyholing process in PAW.The volume of fluid method(VOF) is used to track the keyhole shape and size.The latent heat and momentum sink due to solidifying and melting are dealt with by enthalpy-porosity technique. Considering the larger ratio of PAW weld depth to width,a combined volumetric heat source model is established,and one of its distribution parameters is adjusted dynamically with the variation of keyhole depth.The evolution of fluid flow and thermal field in weld pool,and the keyholing process are quantitatively analyzed on the stainless steel plates of thickness 8 mm.The feature of fluid flow in weld pool is revealed.The predicted keyhole size at bottom side of workpiece and fusion line at transverse cross-section of welds agree with the experimentally measured results.

Investigation of heat transfer and fluid flow in activating TIG welding by numerical modeling

A mathematical model of heat transfer and fluid flow in the gas metal arc welding process

Several uncertain parameters affect the reliability of heat transfer and fluid flow calculations during conduction mode laser spot welding because their values cannot be prescribed from fundamental principles. These parameters include absorptivity of the laser beam, effective thermal conductivity and effective viscosity of liquid metal in the weld pool. Values of these parameters are usually adjusted by trial and error so that the computed results agree with the corresponding experimental values. Here it is shown that by integrating multivariable constrained optimisation with convective heat transfer and fluid flow calculations, the values of the uncertain parameters can be obtained from a limited volume of experimental data. The optimisation technique requires numerically calculated sensitivity values of weld dimensions with respect to absorptivity, effective thermal conductivity and effective viscosity and minimises the discrepancy between the predicted and the measured weld dimensions. The numerical heat transfer and fluid flow model embodying the optimised values of the uncertain parameters could accurately compute values of weld dimensions for new welding conditions. Reliability of heat transfer and fluid flow calculations can be significantly enhanced by determining the values of uncertain parameters from a limited volume of experimental data using a multivariable optimisation technique with a numerical heat transfer and fluid flow model.

A numerical study of three-dimensional heat transfer and fluid flow in a moving gas metal arc welding (GMAW) process is performed by considering various driving forces of fluid flow such as buoyancy, Lorentz force, and surface tension. The computation of the current density distribution and the resulting Lorentz force field is performed by solving the Maxwell equations numerically in the domain of the workpiece. The phase change process during melting and solidification is modeled using the enthalpy-porosity technique. Mass and energy transports by droplet transfer are also considered through a thermal analysis of the electrode. The droplet heat addition to the molten pool is considered to be a volumetric heat source distributed in an imaginary cylindrical cavity within the weld pool ("cavity" model). This nature of the heat source distributed due to the falling droplets takes into account the momentum and thermal energy of the droplets. The numerical model is able to capture the well-known "finger" penetration commonly observed in the GMAW process. Numerical prediction regarding the weld pool shape and size is compared with the corresponding experimental results, showing good qualitative agreement between the two. The weld pool geometry is also found to be dependant on some key parameters of welding, such as the torch speed and power input to the workpiece.

In comparison with conventional single-sided arc welding, double-sided arc welding powered by a single power supply has remarkable advantages in enhancing penetration, minimizing distortion and improving welding production. In this paper, a three-dimensional steady numerical model is developed for the heat transfer and fluid flow in a fusion type plasma arc (PA)–gas tungsten arc (GTA) double-sided welding process. Based on the numerical model, the distributions of the fluid flow and temperature field are calculated. Numerical results show that the peak temperature and temperature gradient in the weld pool in the PA side are higher than the values in the GTA side. Within the weld pool, the electromagnetic force drives the melted metal to move from two sides to the central part of the weld pool, and this effect is positive to penetrating the workpiece. The fluid flow of the melted metal in the free surface of the weld pool is fiercer than the flow within the weld pool, and the biggest flow velocity of the melted metal occurs in the free surface in the PA side. A comparison of the cross section of the weld bead with the experimental result shows that the numerical model's accuracy is reasonable.

Gas Metal Arc Welding (GMAW) is a welding process where an electrode wire is continuously fed from an automatic wire feeder through a conduit and welding gun to the base metal, where a weld pool is created. The formation of droplet and transfer of droplet are governed by the conservation equations. This study on GMAW aims to simulate transient behavior of welding arc and shielding gas flow. Computational Fluid Dynamics (CFD) is used as a tool to understand multifaceted physics involved in GMAW process. A two dimensional axisymmetric model is prepared to reduce computational time. The heat transfer and fluid flow in the arc column were studied based on the transient distributions of velocity, turbulence, voltage, current density, and temperature. An interactive coupling between welding arc, plasma, current and temperature were considered. The assumed steady state and laminar gas flow in traditional models studied so for does not reflect the real distributions in the welding process. Hence influence of the welding arc on the shielding gas flow and vice versa was taken up for study. From the study it is found that as the arc is struck, the shielding gas is accelerated towards axis. When the plasma reaches towards workpiece, axial momentum of gases is changed to radial momentum and flows away from the workpiece. The shielding gas also carries current from electrode to workpiece which helps in reducing spatter of the arc and hence concentrated arc is obtained