4.2 Transient Coupling Dynamics of Keyholes and Weld Pool
115
to move in the direction of the depth of penetration, thus increasing the depth of
the keyhole again. On the other hand, the convex platform will appear again on the
keyhole walls, which facilitates the next keyhole oscillation. From Figs. 4.1 and 4.2,
it can be seen that in deep penetration laser welding, the above oscillation process
of the keyhole is repeated periodically, and because of keyhole oscillation, bubbles
are easy to form at the bottom or the middle of the keyhole.
In the past numerical simulation of the coupling behavior between the transient
keyhole and moving weld pool, foreign scholars have done some excellent research
work. Semak et al. are the first to confirm the phenomenon of convex platforms
on a keyhole wall through numerical simulation. However, their numerical model
is one-dimensional and considers only the phenomenon of convex platform on the
front keyhole wall. Lee et al. have theoretically proved that the interaction between
laser and convex platform may lead to keyhole closure and the generation of bubbles
through numerical simulation. However, this model is two-dimensional and only
suitable for stationary laser welding. Ki et al. carried out a theoretical study on the
coupling behavior between three-dimensional transient keyholes and moving weld
pools by numerical simulation for the first time. In this study, however, they failed to
visualize the process of pore generation. By comparing the present simulation results
and findings of previous researches, it is proved that the discontinuous mathematical
model proposed in this paper can be used to simulate keyhole instability and bubble
formation in deep penetration laser welding.
Figure 4.7 also shows the flow field distribution on the longitudinal section of the
weld pool during welding. The flow field in the weld pool is very complex when
the keyhole oscillates. The following analyzes and discusses several typical flow
mechanisms in the moving weld pool. First, there is a periodic high-speed downward
flow field driven by recoil pressure in the weld pool near the keyhole. Under the
current technological conditions, the maximum velocity of the characteristic fluid
dynamics can reach above 10 m/s. That characteristic flow pattern exists not only on
the front wall and both sides of the keyhole, but also on its rear wall. From Fig. 4.7c,
f, g, h, it can be seen that the high velocity hydrodynamics results in a strong vortex
flow state in the weld pool near the back edge at the bottom of the keyhole. The
mechanism of the vortex flow is caused by the interaction between the high-speed
downward flow and the boundary of the weld pool. In addition, generally speaking,
the vortex flow is not conducive to bubble floating, so controlling the characteristic
flow pattern may help to reduce the pore defects in the weld.
Secondly, the recoil pressure on the walls of the keyhole will cause some liquid to
flow laterally around the keyhole. This characteristic flow is distributed on both sides
of the weld pool, as shown in Fig. 4.9. Due to the existence of temperature gradient,
some liquid flows under the effect of thermal capillary force near the keyhole, as
shown in Fig. 4.8. In addition, in the vicinity of the keyhole, some liquid flows
upwards along the back wall of the keyhole due to the driving effect of the recoil
pressure, as shown in Figs. 4.8 and 4.9.
There is a strong vortex flow on both sides of the weld pool due to the interaction
of the boundary of the weld pool with transverse flow, thermal capillary flow and
upward flow driven by recoil pressure, as shown in Fig. 4.8. Finally, because the
115
to move in the direction of the depth of penetration, thus increasing the depth of
the keyhole again. On the other hand, the convex platform will appear again on the
keyhole walls, which facilitates the next keyhole oscillation. From Figs. 4.1 and 4.2,
it can be seen that in deep penetration laser welding, the above oscillation process
of the keyhole is repeated periodically, and because of keyhole oscillation, bubbles
are easy to form at the bottom or the middle of the keyhole.
In the past numerical simulation of the coupling behavior between the transient
keyhole and moving weld pool, foreign scholars have done some excellent research
work. Semak et al. are the first to confirm the phenomenon of convex platforms
on a keyhole wall through numerical simulation. However, their numerical model
is one-dimensional and considers only the phenomenon of convex platform on the
front keyhole wall. Lee et al. have theoretically proved that the interaction between
laser and convex platform may lead to keyhole closure and the generation of bubbles
through numerical simulation. However, this model is two-dimensional and only
suitable for stationary laser welding. Ki et al. carried out a theoretical study on the
coupling behavior between three-dimensional transient keyholes and moving weld
pools by numerical simulation for the first time. In this study, however, they failed to
visualize the process of pore generation. By comparing the present simulation results
and findings of previous researches, it is proved that the discontinuous mathematical
model proposed in this paper can be used to simulate keyhole instability and bubble
formation in deep penetration laser welding.
Figure 4.7 also shows the flow field distribution on the longitudinal section of the
weld pool during welding. The flow field in the weld pool is very complex when
the keyhole oscillates. The following analyzes and discusses several typical flow
mechanisms in the moving weld pool. First, there is a periodic high-speed downward
flow field driven by recoil pressure in the weld pool near the keyhole. Under the
current technological conditions, the maximum velocity of the characteristic fluid
dynamics can reach above 10 m/s. That characteristic flow pattern exists not only on
the front wall and both sides of the keyhole, but also on its rear wall. From Fig. 4.7c,
f, g, h, it can be seen that the high velocity hydrodynamics results in a strong vortex
flow state in the weld pool near the back edge at the bottom of the keyhole. The
mechanism of the vortex flow is caused by the interaction between the high-speed
downward flow and the boundary of the weld pool. In addition, generally speaking,
the vortex flow is not conducive to bubble floating, so controlling the characteristic
flow pattern may help to reduce the pore defects in the weld.
Secondly, the recoil pressure on the walls of the keyhole will cause some liquid to
flow laterally around the keyhole. This characteristic flow is distributed on both sides
of the weld pool, as shown in Fig. 4.9. Due to the existence of temperature gradient,
some liquid flows under the effect of thermal capillary force near the keyhole, as
shown in Fig. 4.8. In addition, in the vicinity of the keyhole, some liquid flows
upwards along the back wall of the keyhole due to the driving effect of the recoil
pressure, as shown in Figs. 4.8 and 4.9.
There is a strong vortex flow on both sides of the weld pool due to the interaction
of the boundary of the weld pool with transverse flow, thermal capillary flow and
upward flow driven by recoil pressure, as shown in Fig. 4.8. Finally, because the