170
6 Behaviors of Keyhole and Weld Pool Under the Effect …
6.3 Behaviors of Keyhole and Weld Pool Under the Effect
of Side-Blown Gas
6.3.1 Deformation of Weld Pool Surface Under the Effect
of Side-Blown Gas
In the actual welding process, the surface of the weld pool was deformed under the
combined effects of surface tension, high-temperature metallic vapors and auxiliary
gas flow. In order to distinguish the influence of auxiliary gas flow on the weld pool
from the influence of high-temperature metallic vapors on the surface of the weld
pool, the calculations are simplified by ignoring the effects of temperature on the
metal surface tension of the weld pool and the deformation of the keyhole and the
lower surface of the weld pool.
From the results in Fig. 6.6, it is clear that after the calculation starts, the gas–
liquid interface presents the following changes: (1) the junction point of gas, liquid
and solid near the welding line moves to the center of the weld pool; (2) the area
around the center of the upper surface of the weld pool bulges, exceeding the height
of the upper workpiece surface. After that, under the effects of side-blown auxiliary
gas flow, the surrounding liquid metal on the surface of weld pool transfers to the rear
of the weld pool. Finally, the gas–liquid interface around the melt front at the front
of the weld pool is lower than the workpiece surface, while the gas–liquid interface
near the solidification front at the rear of the weld pool is higher than the workpiece
surface.
Figure 6.7 shows the surface shapes of the weld pool at 7 ms at different observation angles. Figure 6.7c presents the weld reinforcement and undercut on the surface
of the weld pool. According to the calculation results, the weld undercut defects
occur mainly because the gas–liquid-solid phase boundary point near the welding
line moves toward the center of the weld pool. Since for the convenience of calculations, many complicated factors are neglected, it is easy to see that it is the surface
tension that drives the three-phase junction to move towards the center of the weld
pool. Figure 6.8 shows the velocity vector diagram of the surface of the weld pool
at 0.1 ms. The calculation results in Fig. 6.8 demonstrate that the liquid metal near
the solid–liquid interface on the upper surface of the weld pool flows to the center
of the weld pool at 0.1 ms. As displayed in Fig. 6.7c, weld reinforcement mainly
results from the fact that the liquid metal in the weld pool moves towards the rear
of welds, and then accumulates and solidifies. Figure 6.9 shows the velocity field on
the weld pool surface at t = 7 ms. It is apparent that the metal liquid on the weld pool
surface flows towards the rear part of the weld pool. Since the calculation neglects
the surface tension gradient and rapidly moving heat source, it is the side-blown flow
that forces the metal liquid to move towards the rear of the weld pool.
Figure 6.10 shows the pressure distribution of the gas phase on symmetry plane
at 7 ms. When auxiliary gas is applied to the workpiece, a local high-pressure region
appears near the gas–liquid surface which is locally concave at the front of the weld
6 Behaviors of Keyhole and Weld Pool Under the Effect …
6.3 Behaviors of Keyhole and Weld Pool Under the Effect
of Side-Blown Gas
6.3.1 Deformation of Weld Pool Surface Under the Effect
of Side-Blown Gas
In the actual welding process, the surface of the weld pool was deformed under the
combined effects of surface tension, high-temperature metallic vapors and auxiliary
gas flow. In order to distinguish the influence of auxiliary gas flow on the weld pool
from the influence of high-temperature metallic vapors on the surface of the weld
pool, the calculations are simplified by ignoring the effects of temperature on the
metal surface tension of the weld pool and the deformation of the keyhole and the
lower surface of the weld pool.
From the results in Fig. 6.6, it is clear that after the calculation starts, the gas–
liquid interface presents the following changes: (1) the junction point of gas, liquid
and solid near the welding line moves to the center of the weld pool; (2) the area
around the center of the upper surface of the weld pool bulges, exceeding the height
of the upper workpiece surface. After that, under the effects of side-blown auxiliary
gas flow, the surrounding liquid metal on the surface of weld pool transfers to the rear
of the weld pool. Finally, the gas–liquid interface around the melt front at the front
of the weld pool is lower than the workpiece surface, while the gas–liquid interface
near the solidification front at the rear of the weld pool is higher than the workpiece
surface.
Figure 6.7 shows the surface shapes of the weld pool at 7 ms at different observation angles. Figure 6.7c presents the weld reinforcement and undercut on the surface
of the weld pool. According to the calculation results, the weld undercut defects
occur mainly because the gas–liquid-solid phase boundary point near the welding
line moves toward the center of the weld pool. Since for the convenience of calculations, many complicated factors are neglected, it is easy to see that it is the surface
tension that drives the three-phase junction to move towards the center of the weld
pool. Figure 6.8 shows the velocity vector diagram of the surface of the weld pool
at 0.1 ms. The calculation results in Fig. 6.8 demonstrate that the liquid metal near
the solid–liquid interface on the upper surface of the weld pool flows to the center
of the weld pool at 0.1 ms. As displayed in Fig. 6.7c, weld reinforcement mainly
results from the fact that the liquid metal in the weld pool moves towards the rear
of welds, and then accumulates and solidifies. Figure 6.9 shows the velocity field on
the weld pool surface at t = 7 ms. It is apparent that the metal liquid on the weld pool
surface flows towards the rear part of the weld pool. Since the calculation neglects
the surface tension gradient and rapidly moving heat source, it is the side-blown flow
that forces the metal liquid to move towards the rear of the weld pool.
Figure 6.10 shows the pressure distribution of the gas phase on symmetry plane
at 7 ms. When auxiliary gas is applied to the workpiece, a local high-pressure region
appears near the gas–liquid surface which is locally concave at the front of the weld