158
5 Dynamic Behaviors of Metal Vapor/Plasma Plume …
at 3 m/min. As shown in Figs. 5.8 and 5.9a, b, the significant changes of the pressure and Mach number of metal vapor in the keyhole experience are also due to the
violent evaporation of metal vapor at the hump of the back wall of the keyhole. It is
also shown from Figs. 5.8 and 5.9c–f that the Mach number at the local position of
the back hump keeps increasing, and that the nearby vapor pressure state is changed
simultaneously. Therefore, the variation of metal vapor velocity, pressure and Mach
number in a short period of time shows that the state of metal vapor in the keyhole
changes obviously with time and has a high transient property.
5.3.2 Multi-directional Flow Behavior in the Keyhole
From the simulation results in Fig. 5.10, a high-pressure region will be formed in
the middle lower bottom part of the keyholes. If the keyhole depth is large, the
pressure inside the keyhole, especially at the middle lower part, cannot be easily
released, and a high-pressure region can be easily formed. The main component of
the gaseous phase in the bubble caused by the keyhole’s instability is proved to be
ambient gas. It is known that recondensation due to rapid cooling is due to the flow
of protective gas into the keyhole during the pulsed laser of liquid zinc. However,
how environmental gases enter the bubble is still not well understood, especially in
continuous laser welding processes. This problem can be explained by our current
theoretical results. As can be seen from the figures, during the laser welding process,
a negative pressure zone is formed at the edges of the keyhole opening. When vapor
is spewed out of the keyhole at a very high speed, it may cause strong vortex in
those places. Under the influence of this flow, some atmospheres may be quickly
absorbed into the keyhole, resulting in the metal vapor inside the keyhole possibly
flowing toward the bottom while flowing toward the opening. These gases entering
the keyhole can reach the bottom of the keyhole, and if the keyhole is split into two
parts at this time, the separate part at the bottom of the keyhole will be filled with
ambient gases, eventually resulting in cavities.
5.3.3 Violent Oscillation and Swinging Behavior
Figure 5.11 shows the movement of the metal vapor above the opening when the metal
vapor is ejected. It can be clearly seen that under the current process conditions, the
metal vapor is not spewed out in a fixed direction but oscillates back and forth, and this
is consistent with the simulated swing angle of the metal vapor at the opening. The
oscillation of the metal vapor means that the direction of the component of the metal
vapor velocity parallel to the workpiece is changing. Due to constant oscillation of
the wall surface of the keyhole during the laser welding process, the local evaporation
on the wall surface of the keyhole is dramatic, and changes with position and time.
The recoil pressure generated during evaporation causes the metal vapor to move
5 Dynamic Behaviors of Metal Vapor/Plasma Plume …
at 3 m/min. As shown in Figs. 5.8 and 5.9a, b, the significant changes of the pressure and Mach number of metal vapor in the keyhole experience are also due to the
violent evaporation of metal vapor at the hump of the back wall of the keyhole. It is
also shown from Figs. 5.8 and 5.9c–f that the Mach number at the local position of
the back hump keeps increasing, and that the nearby vapor pressure state is changed
simultaneously. Therefore, the variation of metal vapor velocity, pressure and Mach
number in a short period of time shows that the state of metal vapor in the keyhole
changes obviously with time and has a high transient property.
5.3.2 Multi-directional Flow Behavior in the Keyhole
From the simulation results in Fig. 5.10, a high-pressure region will be formed in
the middle lower bottom part of the keyholes. If the keyhole depth is large, the
pressure inside the keyhole, especially at the middle lower part, cannot be easily
released, and a high-pressure region can be easily formed. The main component of
the gaseous phase in the bubble caused by the keyhole’s instability is proved to be
ambient gas. It is known that recondensation due to rapid cooling is due to the flow
of protective gas into the keyhole during the pulsed laser of liquid zinc. However,
how environmental gases enter the bubble is still not well understood, especially in
continuous laser welding processes. This problem can be explained by our current
theoretical results. As can be seen from the figures, during the laser welding process,
a negative pressure zone is formed at the edges of the keyhole opening. When vapor
is spewed out of the keyhole at a very high speed, it may cause strong vortex in
those places. Under the influence of this flow, some atmospheres may be quickly
absorbed into the keyhole, resulting in the metal vapor inside the keyhole possibly
flowing toward the bottom while flowing toward the opening. These gases entering
the keyhole can reach the bottom of the keyhole, and if the keyhole is split into two
parts at this time, the separate part at the bottom of the keyhole will be filled with
ambient gases, eventually resulting in cavities.
5.3.3 Violent Oscillation and Swinging Behavior
Figure 5.11 shows the movement of the metal vapor above the opening when the metal
vapor is ejected. It can be clearly seen that under the current process conditions, the
metal vapor is not spewed out in a fixed direction but oscillates back and forth, and this
is consistent with the simulated swing angle of the metal vapor at the opening. The
oscillation of the metal vapor means that the direction of the component of the metal
vapor velocity parallel to the workpiece is changing. Due to constant oscillation of
the wall surface of the keyhole during the laser welding process, the local evaporation
on the wall surface of the keyhole is dramatic, and changes with position and time.
The recoil pressure generated during evaporation causes the metal vapor to move