Study on the Structure Deformation in the Process of Gas Metal Arc Welding (GMAW)

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Figure 2. Goldak’s model [12,13]

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Table 1. Parameters of Goldak’s double model

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Table 2. Chemical low carbon steel [12]

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Table 3. Material properties of welding plates [12]

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Figure 3 shows the mesh of the welded plate used in the established 3D solid model. Fine mesh is used around weld path, but coarse mesh is used away from the weld. The initial condition involves preheating temperature, and all nodes are loaded except for fillers. Figure 4 shows the welding model with the original coordinate. The Y-axis is the plate width direction, the X-axis is the welding direction, and the Z-axis is the plate thickness direction. The units in weld zone are activated one by one during the welding process. The welding processes are shown as in Figure 5.

Figure 3. Mesh model for simulation

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Figure 4. Location for observing the welding deformation

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In this study, linear elastic shrinkage method using relatively new FEM software, SIMUFACT WELDING, developed by Simufact Engineering GmbH, is employed to simulate the welding process and to predict the welding distortion in a butt joint of thickness 3 mm. For verification, a series of experiments were also performed using a fully automated welding system with GMAW power source. The shielding gas for the welding process was carbon dioxide (CO2). To measure the initial and final dimensions of the specimen, a coordinate measuring machine was used. The welding parameters that have been used during the experiments are shown in Table 4. For observing the influence of welding process on the deformation of structure, three processes were designed for simulation and experiment. The description of welding process is shown in Figure 5. First, two plates will be welded from point P1 to P3. Second, two plates will be welded from the outside to the center (point P2) of the weld path. Finally, two plates will be welded from the center (point P2) to the outside of the weld path.

Table 4. Welding parameters used for simulation and experimental method

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3. Results and Discussions

Plate’s deformation at the end of the welding cycle is shown in Figure 6a and Figure 6b. With different processes, as shown in Figure 4, this result shows that the deformation of two plates is almost the same with different processes. The fixed plate has the smaller deformation than the free plate. The maximum deformation is located at line T–T (Figure 5). This result is because the clamping fore position and deformation of plate are strongly limited. Therefore, in the welding cycle, the thermal deformation decreased clearly. On the contrary, the free plate was deformed by the thermal stress. Figure 6a and Figure 6b also show that the free plate was bent to the welding side as shown in the 3D view, because of stress distribution. Because the weld only appears at the top surface, the stress distribution at the top surface is greater than those on the bottom surface because of the thermal imbalance of the two sides ^{[14, 15]}.

Figure 5. Welding processes

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Figure 6. Simulation results at the end of the welding cycle

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By comparing the deformation value between three processes, the results show that the maximum deformation was observed with process 1, which has the distortion ranging from −0.3 mm to 3.15 mm. This range decreased when process 2 and 3 was used. The reduction could be explained by thermal stress. In process 1, the thermal energy appeared at the welding line from point 1 to point 3. That is, with the same welding time, the plates in process 1 received continuously thermal energy, and the result was the heat concentrate appeared more clearly at the welding position, as shown in Figure 6c. Hence, the thermal stress is also greater than that of the other process, especially with the position of line T–T, as shown in Figure 6d. With process 2 and 3, although the welding time is the same with process 1, however, in these processes, the welding cycle includes two steps. Therefore, the heat concentration reduced clearly, and the residual stress decreased.

The plate deformation on line T–T is also studied. The results were collected and compared in Figure 7. Based on this result, it is clear that process 2 and 3 have greatly improved plate deformation. In addition, the deformation distribution on line T–T shows that the maximum distortion points are affected by the welding process.

Figure 7. Simulation result of plate deformation on line T–T

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In process 1, when the plates were welded continuously from point 1 to point 3, the larger deformation is located at the side of point 3. This result could be explained by thermal deformation. At the beginning of the welding cycle, both plates were cool. Therefore, the thermal deformation did not occur. However, when the welding process started, plates were heated, and the thermal deformation occurred. The longer the time for welding, the more the deformation. The temperature distribution can clearly be observed in Figure 6c. By simulation, the maximum distortion occurred at point 114.2 mm on X-axis.

In process 2 and 3, with two welding steps, the total deformation was reduced clearly because the thermal energy has more time to transfer to the other location in the plates. By comparing the deformation distribution, the maximum distortion in process 2 and 3 are located at the center (X = 102.6 mm) and to the right (X = 158.7 mm), respectively. The location of maximum deformation can be explained by the temperature distribution as well as residual stress. With process 2, the highest temperature is located between point 2 and 3 (Figure 6c), whereas with process 3, the highest temperature is located close to point 3 (Figure 6c). This difference in maximum temperature leads to the change in residual stress, as shown in Figure 6d. Therefore, the maximum deformation points of processes 2 and 3 appear at different positions.

Figure 8. Plate deformation on line T–T with different processes

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