Modeling of 3D temperature field in butt welded joint of 6060 alloy sheets using the ANSYS program

In work, the modeling of a three-dimensional temperature field in a butt weld connection of two 6060 aluminum alloy sheets using Finite Element Method is presented. The calculations were performed for two welding methods: TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas). The Goldak's double ellipsoidal heat source model has been used in modeling. The thermal-mechanical properties of the material were assumed to depend on the temperature. The Workbench, DesignModeler, Mechanical, Fluent and CFD-Post modules of the ANSYS program were used for numerical simulations. In the description of the geometry of joints, cube type elements were used, with density of grid in the heat affected zone. The parabolic shapes of face and root were assumed based on the literature and results of the experiment. The temperature distributions in cross-sections of welded joints as well as welding thermal cycles at selected points were analyzed. The results of numerical simulations were verified experimentally. Comparison of calculated and obtained in the experiment the characteristic limits of heat affected zones showed satisfactory compatibility. The directions of heat propagation determined by vectors of cooling rates coincide with the longitudinal axis of dendritic grains determined on the basis of metallographic tests.


Introduction
Thermal phenomena are important in the technological processes of metals and their alloys. In many processes, such as casting [1, 2], heat and laser treatment [3,4], welding [5,6], welding coatings [7,8], they are the essence of the technological process, i.e. calling the right temperature is to achieve the desired state of the material leading to a change in the state of focus or its properties. In other processes, the increase of the temperature of the processed material is aimed at determining such properties that enable the final technological method, eg. FSW (Friction Stir Welding) [9], coating [10]. In some processes, thermal effects are a secondary effect, such as in machining [11]. Depending on the material, thermo-mechanical properties change as the temperature changes [12], and in the case of steel (cast steel, cast iron), there are structural changes (phase transformations in solid state).
In modelling the temperature field of welding processes, the selection of the heat source model and the calculation method is very important. The analysis of models of heat sources used in the description of thermal phenomena in welding processes is presented, among others in the works [13,14]. The first proposals were point heat source models [15,16]. Because the point models did not allow for describing the temperature field near the weld axis, Eagar [17] proposed a Gaussian distributed surface source. The breakthrough moment was the double ellipsoid model of the heat source proposed by Goldak [18]. Goldak's proposal was the first model taking into account the volumetric nature of the welding source. In work [19] a volumetric model with Gaussian power distribution in the horizontal plane and parabolic change in the vertical direction was proposed.  [20,21] and to analyze the influence of this inclination on the temperature distribution in welded joints [22,23].
Because single-distributed models of heat sources only took into account the heat of the welding arc, disregarding the heat transferred to the weld pool through the molten additional material (eg electrodes), in works [24-27] two-distributed models were proposed. It allowed, among others, to obtain an irregular line of fusion (often observed in welding practice), impossible to reproduce with the use of single-modal source.
Despite many models and numerous publications on temperature field modelling, intensive search for models and algorithms is still in progress to describe the temperature field enabling to obtain temperature distributions as close as possible to real values [28][29][30][31][32][33].
In the descriptions of the temperature field during welding, analytical [34,35], semi-analytical [36] and numerical [33, 37-39] solutions for the differential heat conduction equation are used.
This paper presents modelling of 3D temperature field in butt weld joint of 6060 alloy sheets using the ANSYS program. Numerical simulations have been carried out for two welding methods (GTA and GMA) verified experimentally.

Experimental work
Welding tests of single pass butt welded joint of 6060 aluminum alloy sheets were carried out in the Welding Laboratory of Czestochowa University of Technology. Welded joints were made using two methods (in the argon shield): -GTA method (141) -PN-EN ISO 4063:2011 [55], -GMA method (131) -PN-EN ISO 4063:2011 [55].
The scheme of single-pass butt welding of aluminum alloy sheets is presented in Figure 1. The chemical composition and thermomechanical properties of has been summarized in Tables 1 and 2.   The welding process involved the manufacture of a butt joint of two 6060 alloy sheets with dimensions of 200x60x5 mm using the GMA (131) method. The welding process was carried out on the Synermig 400 welding device (OZAS, Opole, Poland) presented in Figure 3. The device is intended for joining construction steels, high-alloy steels of type 18 -8 and alloys (AlMg5 and AlSi5) [56]. We can weld with direct current and alternating current. The welding parameters using GTA method is setting in Table 4. Composition of additional material AlMg5 Wire feeding speed 8.5 (m/min) Shielding gas Ar 4.5 Intensity of shielding gas 15 (l/min)

Examples of numerical simulations
The problem of modeling the temperature field in welding processes using the finite element method was made using Ansys packages (4 programs from Ansys software): -Ansys DesignModeler used to make a solid geometry, -Ansys Meshing used to divide the solid into finite elements, -Ansys Fluent used to define the model and calculations, -Ansys CFD-Post for the development and analysis of results, Ansys Fluent uses the following equation to solve heat transfer problems for a solid: where: ρ -density, h -enthalpy, k -conductivity, T -temperature, Q -volumetric heat source, ⃗speed field. When modeling the three-dimensional temperature field in welding processes, the dual ellipsoidal moving heat source proposed by Goldak presented in the figure 4 was used. The Goldak's model consists of two semi-ellipsoidal volumes that create a heat flux. For points (x, y, z) belonging to the semi-ellipse located in the front part of the source, the heat flux is described by the equation: However, for points (x, y, z) belonging to the back of the source:     The cross-section shows the temperature distribution at time t = 69 s from the beginning of welding. The graph shows the temperature change with the change of distance from the heat source. It may observe a sharp drop in temperature in the initial phase of the graph, which presents densely arranged isotherms near the source. Figure 7 shows two points for which a diagram of thermal cycles was performed during the time of connecting the two butt-welded alloy plates using the GTA method. The results obtained give satisfactory results. The red color on the left is the area in which we obtained a temperature above solidus where the material melts. The dimensions of the remelting zone obtained during modeling in relation to experimental studies show divergences less than 5%.

Modeling of the temperature field during butt welding of 6060 aluminum alloy sheets using the GMA method (131)
During the modelling of the temperature field during the butt welding using the GMA method, two sheets of 6060 alloy, the solid was divided into 444609 cubic elements and 487760 nodes with a densegrid in the joint area [57].     The result of numerical simulations gives satisfactory results. The red color on the left shows the area in which we reached the temperature above the solidus where the material melts. The difference in dimensions obtained in the simulation with respect to experimental tests is below 5%.

Conclusion
Numerical simulations of the temperature field in welding processes for sheets made of aluminum alloys: -butt welded joint made with the GTA method (using a infusible (tungsten) electrode in the Argon shield with the addition of a deposited metal in the form of a wire), -butt welded joint made with the GMA method (using a fusible electrode in the Argon shield), allowed to determine the fusion zone of welded sheets in the mentioned processes. The numerical simulation results were verified experimentally by comparing the shapes and dimensions of remelting zones obtained theoretically (computationally) and experimentally (on the basis of metallographic specimen).
The obtained results are the origin point for the calculation of strain and stress states in the welding processes considered in the article.