5.3 Dynamics of Metal Vapor/Plasma in Transient Keyhole
161
laser directly heats the surface of the workpiece. The laser spot is on local position of
the workpiece, and the metal vapor violently evaporates along the Z-axis direction to
produce a very large spewing out speed. After the keyhole is formed, the front wall
of the keyhole is directly heated by the laser energy, and the back wall has a local
high temperature region due to multiple reflections, and the evaporation is strong.
With periodic oscillation of the keyhole morphology, the degree of inclination of
the local evaporation position changes, the evaporating direction of the spewed out
metal also changes continuously, and the velocity of the spewed out metal vapor
in the Z-axis direction is reduced accordingly. This is consistent with the variation
pattern of average velocity of the vapor at the opening simulated vapor obtained by
simulation.
5.3.4.2 Metal Vapor Compressibility
As shown in Fig. 5.4, it is known that there is a high pressure inside the transient
keyhole much larger than 1 atm
1 and the density of the metal vapor is much smaller
than the air density. So, it is very possible that highly compressed low density vapor
exists in a millimeter keyhole. The compressibility is an important physical property
of the vapor plume generated in the deep penetration laser welding process. The
degree of compressibility of the metal vapor in the keyhole can, to some extent, reflect
the intensity of the metal vapor movement in the keyhole. Figure 5.12 illustrates the
peak Mach number evolutions with different welding speeds of 2, 3 and 4 m/min
and laser power at 1.5 kW. It can be easily found from Fig. 5.12 that the moment
Mach number can easily reach 0.8, and can even exceed 1.0, which indicates the
metal vapor may be violently compressed (Fig. 5.13).
5.4 Conclusion
(1) In consideration of complex phenomena such as heat and mass transfer and
phase transformation in the weld pool, with the Level Set method employed in
tracking the evolution process of keyhole’s free interface, and effects of recoil
pressure, environmental pressure, and the compressibility of vapor taken into
account, a mathematical model depicting dynamic behavior of metal vapor in
transient keyhole during deep penetration laser welding is established.
(2) A novel method for setting the boundary temperature of the wall of the keyhole
temperature-dependent compressible gas–liquid interface is proposed. The
evaporation boundary and reflection boundary are respectively set according
to evaporation and condensation caused by the temperature difference on the
wall surface of the keyhole. The temperature, pressure, density, and velocity
boundary conditions associated with vapor flow are derived.
1 1 atm = 101.325 kPa.
161
laser directly heats the surface of the workpiece. The laser spot is on local position of
the workpiece, and the metal vapor violently evaporates along the Z-axis direction to
produce a very large spewing out speed. After the keyhole is formed, the front wall
of the keyhole is directly heated by the laser energy, and the back wall has a local
high temperature region due to multiple reflections, and the evaporation is strong.
With periodic oscillation of the keyhole morphology, the degree of inclination of
the local evaporation position changes, the evaporating direction of the spewed out
metal also changes continuously, and the velocity of the spewed out metal vapor
in the Z-axis direction is reduced accordingly. This is consistent with the variation
pattern of average velocity of the vapor at the opening simulated vapor obtained by
simulation.
5.3.4.2 Metal Vapor Compressibility
As shown in Fig. 5.4, it is known that there is a high pressure inside the transient
keyhole much larger than 1 atm
1 and the density of the metal vapor is much smaller
than the air density. So, it is very possible that highly compressed low density vapor
exists in a millimeter keyhole. The compressibility is an important physical property
of the vapor plume generated in the deep penetration laser welding process. The
degree of compressibility of the metal vapor in the keyhole can, to some extent, reflect
the intensity of the metal vapor movement in the keyhole. Figure 5.12 illustrates the
peak Mach number evolutions with different welding speeds of 2, 3 and 4 m/min
and laser power at 1.5 kW. It can be easily found from Fig. 5.12 that the moment
Mach number can easily reach 0.8, and can even exceed 1.0, which indicates the
metal vapor may be violently compressed (Fig. 5.13).
5.4 Conclusion
(1) In consideration of complex phenomena such as heat and mass transfer and
phase transformation in the weld pool, with the Level Set method employed in
tracking the evolution process of keyhole’s free interface, and effects of recoil
pressure, environmental pressure, and the compressibility of vapor taken into
account, a mathematical model depicting dynamic behavior of metal vapor in
transient keyhole during deep penetration laser welding is established.
(2) A novel method for setting the boundary temperature of the wall of the keyhole
temperature-dependent compressible gas–liquid interface is proposed. The
evaporation boundary and reflection boundary are respectively set according
to evaporation and condensation caused by the temperature difference on the
wall surface of the keyhole. The temperature, pressure, density, and velocity
boundary conditions associated with vapor flow are derived.
1 1 atm = 101.325 kPa.