Abstract
K-TIG is a welding process based on traditional TIG welding, which increases the welding current to 300 A or even higher value and forms the“keyhole”effect by the feat of tungsten electrode cooling system to achieve the ultimate deep penetration welding. The welding width of K-TIG welding is wider than that of plasma and laser weld, and the weld pool is larger. The traditional heat source model is not suitable for the characteristics of heat source distribution in K-TIG weld. Based on the SYSWELD simulation platform and the experimental results of K-TIG welding for titanium alloy, a combined heat source model for numerical simulation of K-TIG deep penetration welding of titanium alloy was developed. The results show that when the distribution coefficient of double ellipsoidal heat source is 0.75 and the acting depth is 4 mm, the simulation weld pool is consistent with the actual joint cross section, and the front weld width is 12 mm and the back weld width is 5 mm. The finite element simulation results of temperature loop curve and residual stress are basically consistent with the experimental results, verifying the accuracy of the established K-TIG heat source model.
As an indispensable material, medium-thick plate has been widely used in manufacturing industries such as pressure vessels, rail transportation and shipbuilding. Welding effi-ciency directly affects the construction cycle and cost of the product, and there is an urgent need to promote the application of efficient, high-quality, low-cost medium-thick plate welding methods. The Australian government's Federal Scientific and Industrial Research Organizatio
The above research is mainly from the experimental point of view for K-TIG welding seam forming laws and welding “keyhole” mechanism, but numerical simulation for K-TIG welding is seldom reported. With the development of computer technology, numerical simulation is increasingly used in welding-related research to avoid the cost of multiple experiments, and there is a variety of analytical tasks to complete the experimen
The test material was TC4 titanium alloy with the dimensions of 350 mm×150 mm×8 mm, and Ao Tai ATIG-1000 welding power was used for welding experiments. K-TIG priming welding had no wire filling, no opening of the breach and no gap. Welding process parameters are shown in
Welding current, I/A | Welding voltage, U/V | Welding speed/mm·mi | Protective gas | Protective gas flow/L·mi |
---|---|---|---|---|
450 | 17 | 380 | 99.99% Ar | 30 |
When the welding test was conducted, a K-type thermocouple was buried on the upper surface of the test plate, and the HP-8125 dynamic data acquisition system was connected to the thermocouple to collect the thermal cycling curve of the thermocouple position during the welding process. At the same time, the Sigmar-ASM blind hole method of stress measurement instrument was used to measure the residual stress at some key points on the upper surface of the welded specimen, including transverse residual stress (residual stress perpendicular to the welding direction) and longitudinal residual stress (residual stress parallel to the welding direction). The principles of the blind hole method used to measure weld residual stresses were described in Ref.[

Fig.1 Test point location diagram

Fig.2 Titanium alloy welded plate

Fig.3 Macroscopic topography of the joint
welding, plasma welding and other small-hole type welding methods, as a one-time penetration technology, and the melt pool presents a wide “funnel-shaped”. As the K-TIG welding arc is not compressed, the front of the weld is wider. K-TIG welding process is not filled with wire because of the base material self-melting. The role of surface tension leads to the welding in the middle of high and low sides, and the biting defect appears.
According to the size of the welded test plate and the macroscopic metallographic dimensions of the joint in

Fig.4 Finite element model of plate butt joint
Considering the nonlinear characteristics of the material properties in the welding process, JMatPro was used to cal-culate the thermophysical and thermomechanical parameters of TC4, as shown in

Fig.5 Thermal properties (a) and mechanical properties (b) of TC4 material
K-TIG welding is a typical small-hole welding process. Among the existing small-hole welding methods, plasma arc welding is also developed based on traditional TIG welding by contracting the arc and increasing the energy density of the arc, which is highly similar to K-TIG welding. Hu et a

Fig.6 Schematic diagrams of heat source model: (a) double ellipsoidal heat source and (b) cylinder heat source
As for double ellipsoidal heat source functio
(1) |
The posterior semi-ellipsoid can be expressed as:
(2) |
Qf (x, y, z) is the heat flow distribution in the volume of the front hemisphere; Qr (x, y, z) is the heat flow distribution in the volume of the posterior hemisphere; a1, a2, b and c are the shape parameters of the double ellipsoid model, as shown in
The cylindrical heat source function is follows:
(3) |
Gaussian column heat source model sets the radial heat flow of the column heat source as Gaussian distribution, while uniform distribution in the depth direction of the column, and the heat source is an embedded cylinder. h is the welding melt depth and R is the radius of action of the heat source, as shown in
In the combined heat sources for welding simulation, the double ellipsoidal heat source load action height is defined as H, Gaussian cylinder heat source load action height is defined as h, single-pass welding melt depth is defined as Z, and Z=H+h. The proportion coefficient of each combined part of the heat source power accounts for the total heat source power, known as the power distribution coefficient. Double ellipsoidal heat source distribution coefficient is Xd, and Gaussian cylinder heat source distribution coefficient is Xc, satisfying Xd+Xc=l and Qd+Qc=Qeffective. K-TIG deep fusion welding total power Qeffective=ηUI, where η is the effective coefficient. Since larger part of the arc heat production in the K-TIG welding process is used to vaporize the metal to produce small holes, the energy utilization rate is relatively low, the effective coefficient η in the simulation is taken as 0.5
According to macroscopic metallographic dimensions of welded joints in
Heat source | af /mm | ar /mm | b/mm | c/mm | R/mm |
---|---|---|---|---|---|
Double ellipsoidal+cylindrical | 3.4 | 6.6 | 12 | 2 | 2.5 |
When using the combination of heat source simulation, the depth of action of the heat source and the heat source distribution coefficient have a significant impact on the simulation results, and unreasonable values will make the role of one of the heat source to be masked by the other heat source. According to the welding simulation pool boundary guidelines, the depth of action of the heat source and the energy distribution coefficient are obtained through the simulation and experimental cross-checking, and the calibration scheme is shown in
Scheme | Upper double ellipsoidal heat source | Lower Gaussian cylinder heat source | |||
---|---|---|---|---|---|
Xd | H/mm | Xc | h/mm | ||
1 | 0.65 | 2 | 0.35 | 6 | |
2 | 0.65 | 4 | 0.35 | 4 | |
3 | 0.65 | 6 | 0.35 | 2 | |
4 | 0.75 | 2 | 0.25 | 6 | |
5 | 0.75 | 4 | 0.25 | 4 | |
6 | 0.75 | 6 | 0.25 | 2 | |
7 | 0.85 | 2 | 0.15 | 6 | |
8 | 0.85 | 4 | 0.15 | 4 | |
9 | 0.85 | 6 | 0.15 | 2 |
The controlling equation for the heat transfer of the arc heat in the workpiece during welding is as follows.
(4) |
where T is the temperature; λ is the heat conduction coeffi-cient; qv is the heat generating power of the internal heat source; ρ is the density; c is the specific heat capacity; t is the heat transfer time; x, y and z are the coordinates in the plenary coordinate system (x, y, z).
The workpiece not only absorbs heat from the arc and molten metal droplets during the welding process, but also dissipates heat to the surrounding environment, so the heat exchange between the workpiece and the external environment is considered in the finite element model by defining the heat dissipation surface. This study considers two forms of heat loss, i.e., convection and radiation heat loss, through Newton's law and Stefan-Boltzmann's la
Based on the thermal-elastic-plastic finite element method, the simulation calculation results of temperature field which is the same as that of the actual welding process are obtained. Using the welding simulation melt pool boundary guidelines for heat source calibration, simulated melt pool size and the actual welded joint cross-section melt pool size are compared, and if the simulated melt pool area boundary and the actual welding melt pool size match, it can be considered that the selected heat source model is reasonabl

Fig.7 Comparison of heat source parameters and melt pool size under different conditions
According to the above nine groups of tests, it is known that a combined heat source model with a double ellipsoidal heat source distribution coefficient of 0.75 and a double ellipsoidal heat source action depth of 4 mm should be selected for the numerical simulation of K-TIG deep fusion welding of 8 mm titanium alloy. With these parameter, the dimensions of the simulated weld zone and heat affected zone match well with the actual joint, as shown in

Fig.8 Temperature field distribution of upper (a, c, e) and lower (b, d, f) surfaces of workpiece in K-TIG welding at different time: (a‒b) 5 s, (c‒d) 25 s, and (e‒f) 50 s
By solving the established model, the transient temperature field distribution of the test plate in welding process is shown in
In the experiments, the thermal cycle curves at three locations (8, 10 and 15 mm from the weld toe) are recorded by K-type thermocouples. For comparison, the corresponding thermal cycle data from the numerical model of these three locations (point A, point B and point C in

Fig.9 Comparison between simulation results and experimental results of welding thermal cycle
the welding cooling rate are within an acceptable rang
The results of the temperature field calculation as a thermal load are loaded into the thermal-elastic-plastic finite element module for welding stress calculations.

Fig.10 Nephogram of transverse (a) and longitudinal (b) residual stress distributions on the surface of the test plate
In the central cross-sectional position perpendicular to the welding direction, set path 1 (

Fig.11 Transverse (a) and longitudinal (b) residual stress distributions at the center section in the welding direction

Fig.12 Transverse (a) and longitudinal (b) residual stress distributions in path 1
1) Based on the temperature field simulation of the fusion line criterion, in the numerical simulation of K-TIG deep fusion welding for 8 mm titanium alloy, a combination of heat sources is used, with double ellipsoidal heat source distribution coefficient of 0.75 and the depth of action taken as 4 mm. The simulation of the melt pool and the actual cross-section of the joint match well, with the frontal fusion width of 12 mm and back fusion width of 5 mm.
2) Comparison of temperature thermal cycling curve results shows that the finite element model established in this study and the use of heat source model can accurately restore the actual heat input during the welding process.
3) The residual stresses obtained by thermal-elastic-
plastic finite element calculations and test results by blind hole method are basically the same, indicating that the finite element model established in this study can effectively and accurately assess the deep-fusion welding stress distribution.
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