5.3 Dynamics of Metal Vapor/Plasma in Transient Keyhole
153
pressure distribution (Fig. 5.2) and temperature distribution (Fig. 5.1), the density
distribution of vapor in the keyhole is also uneven. There exists a low density region
in the center of the keyhole where the density is lower than 0.2 kg/m
3 , far lower
than the ambient gas (air and argon) density. The low density might be caused by
the high-velocity flow which carries away excessive metal vapor from the region.
The density at the opening of the keyhole is higher, which might be the result of the
action of ambient gases (protective gas and air) and compression of the metal vapor.
The density of the metal vapor is higher at the bottom of the keyhole, which might
the result of compression of the metal vapor.
Figure 5.6 illustrates the transient Mach number 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 Mach number distribution of the metal vapor,
and the outside indicates the Mach number distribution of the weld pool and the
unmelted substrate). As shown in Fig. 5.6, the Mach number distribution of vapor in
the keyhole is also uneven. The Mach number of vapor at the bottom of the keyhole
is smaller and can be as small as 0.01, but due to the presence of high temperature on
the wall of the local keyhole, the molten metal vaporizes and spews out metal vapor
at a very high speed, resulting in a relatively large Mach number distribution, and
the largest Mach number can be larger than 0.8. Meanwhile, even the keyhole depth
reaches a quasi-steady state (Fig. 5.6c, d), the Mach number still varies violently due
the drastic changing of the profile of the keyhole.
The distributions of temperature, pressure, density and Mach number of the vapor
in the keyhole and its evolution with time shown in Figs. 5.3, 5.4, 5.5 and 5.6 indicates
that the metal vapor plume in the transient keyhole features nonuniformity during
deep penetration laser welding.
5.3.1.2 High Transient Property of Metal Vapor
Figure 5.7 shows the evolution of velocity of vapor in the keyhole during the 1.256
us transient period from 11.717949 to 11.719205 ms with laser power at 1.5 kW and
welding speed at 3 m/min. In Fig. 5.7a, there is a small hump on the right lower
part of the wall of the keyhole, and the temperature of the hump is rather high,
about 3100 °C. As shown in Fig. 5.7a–f, violent evaporation occurs at that region,
and strong metal vapor are generated there. The time that the velocity experiences
significant changes is only 396 ns, as shown in Fig. 5.7a, b. With the progress of the
welding process, at 11.718604 ms, the velocity field inside the keyhole approaches
to be a quasi-steady state except that the metal vapor at the hump continue to increase
slightly.
Figure 5.8 and Fig. 5.9 respectively show the evolution of pressure and Mach
Number of vapor in the keyhole during the 1.256 us transient period from
11.717949 ms to 11.719205 ms with laser power at 1.5 kW and welding speed
153
pressure distribution (Fig. 5.2) and temperature distribution (Fig. 5.1), the density
distribution of vapor in the keyhole is also uneven. There exists a low density region
in the center of the keyhole where the density is lower than 0.2 kg/m
3 , far lower
than the ambient gas (air and argon) density. The low density might be caused by
the high-velocity flow which carries away excessive metal vapor from the region.
The density at the opening of the keyhole is higher, which might be the result of the
action of ambient gases (protective gas and air) and compression of the metal vapor.
The density of the metal vapor is higher at the bottom of the keyhole, which might
the result of compression of the metal vapor.
Figure 5.6 illustrates the transient Mach number 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 Mach number distribution of the metal vapor,
and the outside indicates the Mach number distribution of the weld pool and the
unmelted substrate). As shown in Fig. 5.6, the Mach number distribution of vapor in
the keyhole is also uneven. The Mach number of vapor at the bottom of the keyhole
is smaller and can be as small as 0.01, but due to the presence of high temperature on
the wall of the local keyhole, the molten metal vaporizes and spews out metal vapor
at a very high speed, resulting in a relatively large Mach number distribution, and
the largest Mach number can be larger than 0.8. Meanwhile, even the keyhole depth
reaches a quasi-steady state (Fig. 5.6c, d), the Mach number still varies violently due
the drastic changing of the profile of the keyhole.
The distributions of temperature, pressure, density and Mach number of the vapor
in the keyhole and its evolution with time shown in Figs. 5.3, 5.4, 5.5 and 5.6 indicates
that the metal vapor plume in the transient keyhole features nonuniformity during
deep penetration laser welding.
5.3.1.2 High Transient Property of Metal Vapor
Figure 5.7 shows the evolution of velocity of vapor in the keyhole during the 1.256
us transient period from 11.717949 to 11.719205 ms with laser power at 1.5 kW and
welding speed at 3 m/min. In Fig. 5.7a, there is a small hump on the right lower
part of the wall of the keyhole, and the temperature of the hump is rather high,
about 3100 °C. As shown in Fig. 5.7a–f, violent evaporation occurs at that region,
and strong metal vapor are generated there. The time that the velocity experiences
significant changes is only 396 ns, as shown in Fig. 5.7a, b. With the progress of the
welding process, at 11.718604 ms, the velocity field inside the keyhole approaches
to be a quasi-steady state except that the metal vapor at the hump continue to increase
slightly.
Figure 5.8 and Fig. 5.9 respectively show the evolution of pressure and Mach
Number of vapor in the keyhole during the 1.256 us transient period from
11.717949 ms to 11.719205 ms with laser power at 1.5 kW and welding speed