Numerical Simulation and Industrial Application of High Speed Tandem TIG Welding

Self-developed high speed tandem TIG welding equipment were adopted to manufacture titanium welded tubes with high efficiency and high quality. The joint made by this high efficient welding process met Chinese standard requirements. A coupled electrode, arc and weld pool numerical model was also developed to investigate temperature and velocity distributions, and energy propagation of this welding process. The numerical results showed that the Marangoni stress was much higher than the arc shear stress, and was mainly positive after leading and trailing arcs in the x and y directions, so the molten metal flowed backward on the top weld pool surface. Previous studies proposed that a “pull-push” flow pattern defined as a backward molten metal flow after the leading arc and a forward molten metal flow before the trailing arc existed on the top weld pool surface in tandem arc welding processes, while it was not observed in this case. The calculated arc efficiency of the high speed tandem TIG welding was about 79.8%.


Introduction
Pure titanium and titanium alloy tubes possess advantages of high strength-to-weight ratios at room temperature and high temperature, high corrosion resistance and high temperature creep resistance [1,2], thus are widely utilized in electric power, marine engineering and petrochemical industries [3,4]. Compared with titanium seamless tubes, titanium welded tubes have similar or same chemical compositions, mechanical properties and process performances, while the production cost is much lower. Usually, tungsten inert gas (TIG) welding is adopted to manufacture titanium welded tubes, while the welding efficiency can't be further improved owing to the formation of weld defects, such as humping and burn through, in high current and high speed cases [5,6].
In order to improve the welding efficiency and quality, a high speed tandem TIG welding process was developed to successfully weld titanium plates in our previous study [7], and is adopted to manufacture titanium welded tubes in this study. In this high efficient welding process, a higher current leading arc aimed to obtain deep penetration, and a lower current trailing arc heated the trailing part of the weld pool, and suppressed the strong backward molten metal flow [8].
Energy propagation during a non-consumable gas shielded arc welding process plays an important role on the lifetime of a tungsten electrode [9] and final weld bead formation [10]. The main energy propagation mechanisms at the cathode surface are thermionic cooling, thermally conducted energy from the arc column, ion heating and radiation cooling. The main energy propagation mechanisms at the anode surface are electron condensation heating, thermally conducted energy from the arc column, and radiation cooling [11]. Previous studies showed that the maximum arc temperature [12,13] and arc pressure [14,15] in a twin TIG arc were lower in comparison with a single TIG arc with a same total current; therefore, the thermally conducted energy from a twin TIG arc to the cathodes and anode also decreased. As the size of a twin/tandem TIG arc increases, the radiation cooling of the arc column also becomes stronger. Qin et.al [8] and Jiang et.al [16] proposed that an obvious "pull-push" flow pattern defined as backward and forward molten metal flows on the top weld pool surface between two arcs existed in a high speed tandem TIG welding process. This "pull-push" flow pattern might cause more sufficient energy convection, such that the weld cooling rate in a high speed tandem TIG welding process was much higher than that in a TIG welding process. In summary, owing to the coupling effects of leading and trailing arcs, the energy propagation behaviors in a high speed tandem TIG welding process may be significantly different from these in a TIG welding process.
During an arc welding process, the shape of workpiece also affects the arc behaviors as well as energy propagation. Ogino et.al [17,18] investigated the arc behavior and heat input distribution of a TIG arc with a groove surface based on a 3D numerical model, and found that the maximum heat input was located at side surfaces of the groove, and energy efficiency was increased. Dong et.al [19] proposed that in a narrow gap TIG welding process, as the shape of the workpiece surface changed from concave to convex, the arc root shrank, and the energy efficiency was decreased. During the high speed tandem TIG welding of titanium tubes, the anode surface is convex upward, which may also has a great influence on the energy propagation.
In this study, high speed tandem TIG welding equipment were developed to manufacture titanium welded tubes. A coupled electrode, arc and weld pool numerical model was built to investigate temperature and velocity distributions, and energy propagation. Energy balance of the whole welding system was analyzed.
Weld microstructures and mechanical properties of the joint were also studied, showing that the high speed tandem TIG welding can improve the manufacturing efficiency and quality of titanium welded tubes.

Assumptions, governing equations and boundary conditions
Many general assumptions are employed to simplify the numerical model for the high speed tandem TIG welding process, such as local thermodynamic equilibrium (LTE) arc plasma [20], Newtonian fluids, turbulent flow of arc and molten metal. The buoyancy force in the molten pool is calculated by Boussinesq approximation and metal vapour effects are neglected [21]. Governing equations for arc and molten pool modeling include the mass, momentum, energy, current and turbulence kinetic energy conservation equations.
The assumptions and governing equations as well as some Maxwell's equations were discussed in our previous study of plasma arc welding [22], thus they are not presented here. Some special boundary conditions for the high speed tandem TIG welding of the titanium tube are shown.
The thermionic cooling, thermally conducted energy from the arc column, ion heating and radiation cooling at the cathode surfaces (leading and trailing electrode surfaces) are considered in the energy boundary condition [23].
where je is the electron current density, ji is the ion current density, jR is the Richardson current density, j is the total current density, ψc is the work function of the cathode, Vi is the ionization potential of argon, keff is the effective thermal conductivity, δ is the length of the sheath region, Tg and Tm are the arc temperature and metal temperature at the interface.εr is the surface radiation emissivity, α is the Stefan-Boltzmann constant, A is the Richardson constant, e is the elementary charge, ψKeff is the effective work function of the electrode, kB is the Boltzmann's constant.
The electron condensation heating, thermally conducted energy from the arc column, radiation cooling at the anode surface (top titanium tube surface) are considered in the energy boundary condition [24].
whereψa is the work function of the anode.
The bottom titanium tube surface can't get any energy from the arc column, so only convective energy transfer and radiation cooling are considered in the energy boundary condition.
where hcon is the convective heat transfer coefficient, T0 is the ambient temperature.
The Marangoni stress and arc shear stress are considered at the momentum boundary condition of the top weld pool surface in the tangential direction : Only Marangoni stress is considered at the momentum boundary condition of the bottom weld pool surface in the tangential direction: whereμis the fluid viscosity, vt is the tangential fluid velocity, n is the normal vector, S is the tangential vector, μp is the arc plasma viscosity, vp is the arc plasma velocity.
Top leading electrode surface (Wall) -300 −σ = 1 = where I1 and I2 are leading and trailing electrode currents, r1 and r2 are leading and trailing electrode radii, j1 and j2 are current densities at top leading and trailing electrode surfaces, ψ is the electric potential, A is the vector potential.

Equipment development and welding procedures
The titanium welded tube manufacturing equipment including a titanium strip feeding equipment, a titanium tube forming equipment, a titanium tube welding equipment, two titanium welded tube calibration equipment, a titanium welded tube annealing equipment and a titanium welded tube cutting equipment were developed by our team, as shown in Fig. 2. The titanium strips were exported through a skewed roller in the titanium strip feeding equipment, and were processed into tubes through various types of roll molds in the titanium tube forming equipment. These tubes were welded by the high speed tandem TIG welding, and the sizes were calibrated by a titanium welded tube calibration equipment. In order to eliminate the residual stress after the welding and calibration, the tubes were annealed in the titanium welded tube annealing equipment with annealing temperature of about 580℃; this equipment was filled with pure Ar to protect the tubes from being oxidized. The tubes were calibrated again by another titanium welded tube calibration equipment. The tubes were cut according to buyer's requirements. To ensure the quality of titanium welded tubes, microstructure analysis and mechanical tests were carried out following the GB/T3625-2007 standard. Page6

Fig. 2 Manufacturing flowchart of titanium welded tubes
During the welding, a tandem TIG welding power source (MPT-500D) was used. W-ThO2 electrodes was adopted. Pure titanium (TA2) strips with thickness of 0.7 mm were used. The density, thermal conductivity, electrical conductivity, specific heat and viscosity of TA2 were calculated by the JMatpro software, as shown in Fig. 3. Other thermo-physical material properties of TA2 can be seen in Table 2. After forming, the tube radius was about 9.55 mm. The leading and trailing TIG torches were fixed, while the titanium tube moved.  Detailed welding parameters can be seen in Table. 3. A sample of the titanium welded tube is shown in Fig.4. Silver-white weld without defects was obtained. The bead was cut, mounted, polished and etched using 5 ml HF + 10 ml HNO3 + 85 ml H2O solution with 30 s; the weld microstructure was observed using an optical microscope. The tensile test, flattening test and flaring test were carried out following the GB/T3625-2007 standard.

Temperature and velocity distributions of electrode, arc and weld pool
The temperature distribution of electrode, arc and weld pool in the symmetry plane is shown in Fig. 5. In high speed tandem TIG welding, the polarities of leading and trailing electrodes are same, so arcs attract each other under the influence of the electromagnetic force [25]. The maximum arc temperature (17535 K) locates at the region below the leading electrode. The maximum leading electrode temperature and maximum weld pool temperature located at the regions near the leading arc are 3743 K and 3149 K, respectively. Two high temperature regions can be seen in the weld pool.   tandem TIG weld pool [8,16]. This flow pattern can also be observed in a tandem/twin MIG weld pool [27,28], a TIG-MIG weld pool [29] and a hybrid plasma-MIG pool [30]. However, in our study, the molten metal Page9 mainly flows backward at the top weld pool surface. Fig.7 Molten metal temperature and flow on the top weld pool surface As shown in Fig. 8, owing to the outward flow of leading arc plasma, in the x direction, the arc shear stress acting on the top weld pool surface is negative before the leading arc, and positive after the leading arc.
Owing to the arc attraction, the trailing arc plasma mainly flows forward, so in the x direction, the arc shear stress acting on the top weld pool surface near the trailing arc is negative. In the y direction, the arc shear stress is mainly positive near both leading and trailing arcs.
In the x direction, the Marangoni stress acting on the top weld pool surface is negative before the leading arc, and mainly positive after leading and trailing arcs. In the y direction, the Marangoni stress acting on the top weld pool surface is mainly positive near both leading and trailing arcs. As the Marangoni stress is much higher than the arc shear stress, it can be concluded that under the influence of the Marangoni stress, the molten metal on the top weld pool surface flows backward.

Arc energy balance and efficiency of the welding system
Energy balance of the whole welding system is shown in Fig. 9 [32] measured the TIG arc efficiency, and found that the arc efficiency in electrode negative polarity (about 80%) is higher than that in electrode positive polarity (about 60%). In summary, the calculated arc efficiency of a tandem TIG arc in this study is reasonable. Fig.9 Energy balance and efficiency of the whole welding system

Weld microstructures and mechanical properties
The weld microstructures in the high speed tandem TIG welding are shown in Fig. 10. Fine equiaxed α grains exist in the base metal. Jagged grains form in the heat affected zone (HAZ) close to the base metal (region 1 and region2). As shown in region 3, coarser jagged grains form in the HAZ close to the fusion zone (FZ). In the FZ, there are many coarse equiaxed dendrite grains, and a few fine equiaxed dendrite grains.
Page11 Fig. 10 Weld microstructures in the high speed tandem TIG welding Both in the numerical simulation and experiment, no weld defect forms in this welding process. As shown in Fig. 11 In summary, the calculated arc efficiency, maximum electrode and arc temperatures, and weld geometry are acceptable.

Conclusions
In this study, titanium tubes were successfully welded by self-developed high speed tandem TIG welding equipment. A coupled electrode, arc and weld pool numerical model was built to investigate temperature and velocity distributions, and energy propagation of this welding process. The following conclusions can be drawn: (1) The trailing arc current is much lower, under the influence of the electromagnetic force, arc plasma under the trailing electrode firstly flows downward and toward the leading electrode, but then flows upward.
(2) Two high temperature regions locate at the top weld surface, and the "pull-push" flow pattern proposed in previous studies is not formed in this case. The Marangoni stress on the top weld pool surface is mainly positive after leading and trailing arcs in the x and y directions, so the molten metal on the top weld pool surface flows backward.
(3) The calculated arc efficiency of the high speed tandem TIG welding is about 79.8%.
(4) The joint made by the high speed tandem TIG welding meets the requirements of GB/T3625-2007 standard.