5.2 Dynamic Model of Metal Vapor/Plasma in Transient Keyhole
149
The outermost layer of the calculation area above the keyhole is set as the outlet,
and its pressure boundary is set to the atmospheric pressure by the first type boundary
condition.
5.3 Dynamics of Metal Vapor/Plasma in Transient Keyhole
5.3.1 Uneven Distribution and High Transient
5.3.1.1 Unevenness
Figure 5.3 illustrates the transient temperature distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,
5.308 ms, 15.277 ms and 20.844 ms) with laser power at 1.5 kW and welding speed
at 3 m/min (The black line represents the outline of the keyhole, the inside of the
keyhole indicates the temperature distribution of the metal vapor, and the outside
indicates the temperature distribution of the weld pool and the unmelted substrate).
The temperature distribution of the metal vapor is also uneven because the location
in the keyhole directly heated by the laser is near the fore edge of the keyhole and the
energy distribution on the wall of the keyhole is uneven due to multiple reflections and
Fresnel absorption. Figure 5.3c–f shows that there is a high temperature region in the
lower part of the keyhole. In particular, as shown in Fig. 5.3f, the lower temperature
of the metal vapor is about 3000 °C, while the temperature in higher temperature
region can reach above 4500 °C, and you can see the temperature difference in the
keyhole can reach 1000 °C.
Figure 5.4 illustrates the transient pressure distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,
5.308 ms, 15.277 ms and 20.844 ms) with laser power at 1.5 kW and welding speed
at 3 m/min (The black line represents the outline of the keyhole, the inside of the
keyhole indicates the pressure distribution of the metal vapor, and the outside indicates the pressure distribution of the weld pool and the unmelted substrate). At the
free interface of the keyhole under the action of a high power density laser beam,
the molten metal vapor forms a huge recoil pressure, which drives the flow of the
metal vapor in the keyhole. Because the gradient of the recoil pressure on the wall
of the keyhole is very large (up to several atmospheric pressure) and the keyhole
experiences complicated morphological changes, the vapor pressure distribution in
the keyhole is very uneven. From Fig. 5.4b–f, it can be seen that there is a negative
pressure (lower than the atmospheric pressure) inside the keyhole near the opening,
and from Fig. 5.4b–e, it can be seen that there is a high pressure region near the wall
of the keyhole where the pressure is higher than the atmospheric pressure by about
20 kPa.
Figure 5.5 illustrates the transient density distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,
149
The outermost layer of the calculation area above the keyhole is set as the outlet,
and its pressure boundary is set to the atmospheric pressure by the first type boundary
condition.
5.3 Dynamics of Metal Vapor/Plasma in Transient Keyhole
5.3.1 Uneven Distribution and High Transient
5.3.1.1 Unevenness
Figure 5.3 illustrates the transient temperature distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,
5.308 ms, 15.277 ms and 20.844 ms) with laser power at 1.5 kW and welding speed
at 3 m/min (The black line represents the outline of the keyhole, the inside of the
keyhole indicates the temperature distribution of the metal vapor, and the outside
indicates the temperature distribution of the weld pool and the unmelted substrate).
The temperature distribution of the metal vapor is also uneven because the location
in the keyhole directly heated by the laser is near the fore edge of the keyhole and the
energy distribution on the wall of the keyhole is uneven due to multiple reflections and
Fresnel absorption. Figure 5.3c–f shows that there is a high temperature region in the
lower part of the keyhole. In particular, as shown in Fig. 5.3f, the lower temperature
of the metal vapor is about 3000 °C, while the temperature in higher temperature
region can reach above 4500 °C, and you can see the temperature difference in the
keyhole can reach 1000 °C.
Figure 5.4 illustrates the transient pressure distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,
5.308 ms, 15.277 ms and 20.844 ms) with laser power at 1.5 kW and welding speed
at 3 m/min (The black line represents the outline of the keyhole, the inside of the
keyhole indicates the pressure distribution of the metal vapor, and the outside indicates the pressure distribution of the weld pool and the unmelted substrate). At the
free interface of the keyhole under the action of a high power density laser beam,
the molten metal vapor forms a huge recoil pressure, which drives the flow of the
metal vapor in the keyhole. Because the gradient of the recoil pressure on the wall
of the keyhole is very large (up to several atmospheric pressure) and the keyhole
experiences complicated morphological changes, the vapor pressure distribution in
the keyhole is very uneven. From Fig. 5.4b–f, it can be seen that there is a negative
pressure (lower than the atmospheric pressure) inside the keyhole near the opening,
and from Fig. 5.4b–e, it can be seen that there is a high pressure region near the wall
of the keyhole where the pressure is higher than the atmospheric pressure by about
20 kPa.
Figure 5.5 illustrates the transient density distribution of the metal vapor inside
the keyhole during laser welding at different times (0.350 ms, 1.841 ms, 2.990 ms,