10
1 Laser Welding Basics
equilibrium evaporation models, the vapor pressure is considered to be proportional
to the laser power density.
When the laser power density is high, the material evaporation rate increases,
and the vapor pressure increases. When it is significantly higher than the ambient
pressure, the number of particles in the vapor that return to the solution decreases, and
the velocity distribution will deviate from the balanced Maxwell Distribution. The
vapor particles leaving the liquid surface must pass a distance and collide with each
other to reestablish a translational equilibrium. The thin layer above the liquid surface
where the vapor is in the transition from a non-equilibrium state to an equilibrium
state is called the Knudsen layer. In the extreme of strong evaporation, the vapor on
the outer surface of the Knudsen layer flows at the sound velocity, and the vapor
flowing outward is noticeably colder and thinner than the saturated vapor on the
surface of the solution.
Under continuous laser irradiation, the velocity of evaporation front (namely the
receding velocity) rises rapidly from zero to an approximately constant value, and
the evaporation enters a steady state. The establishment time t sv of a steady state is
related to the steady receding velocity U v . Usually t sv is dozens of times greater than
t v . For example, when AI 0 is 10
7 W/cm
2 , the receding speed of aluminum is about
2.34 m/s, and the corresponding velocity t sv ≈ 15 μs, while t v is less than 0.3 μs.
Due to the action of the vapor pressure, t v in Eq. (1.18) should be greater than the
normal evaporating temperature under standard atmospheric pressure. When the laser
power density is between 10
6 and 10
7 W/cm
2 , the temperature of the evaporation
front of the metal is slightly greater than the normal evaporating temperature. When
the laser power density is between 10
6 and 10
10 W/cm
2 , the temperature of the
former evaporation front is several times to dozens of times that of the latter. Since
T v increases with the laser power density, it leads to the receding velocity U v in the
range above 10
8 W/cm
2 decreasing with the increase of the laser power density, and
the decrease is more obvious in the range above 10
0 W/cm
2 .
The gaseous mass transfer rate ˙
m v depends on the receding velocity, ˙
m v = ρU v ,
and the total gaseous transfer mass is ρ
t
t v
U v dt. When the laser power density is
low, the heat lost by thermal diffusion has an obvious influence on U v , and the rate
of gaseous mass transfer is also low. The calculated values of the receding velocity
and gaseous mass transfer rate are usually noticeably different from the experimental
results, which is caused by two reasons. For one thing, it is because the reflectivity,
absorptivity and thermal physical characteristics are all related to temperature and
the actual laser waveform is highly irregular. For the other, it is because that mass
transfer is actually a comprehensive result of various mechanisms, in which the main
part of the transferred mass is composed of the splashed droplets by vapor pressure.
1.1.5 Laser-Induced Plasma and Its Effects
When laser acts on the surface of the material, vapor is induced. The vapor continues
to absorb laser energy, causing an increase in the temperature and eventually forming
1 Laser Welding Basics
equilibrium evaporation models, the vapor pressure is considered to be proportional
to the laser power density.
When the laser power density is high, the material evaporation rate increases,
and the vapor pressure increases. When it is significantly higher than the ambient
pressure, the number of particles in the vapor that return to the solution decreases, and
the velocity distribution will deviate from the balanced Maxwell Distribution. The
vapor particles leaving the liquid surface must pass a distance and collide with each
other to reestablish a translational equilibrium. The thin layer above the liquid surface
where the vapor is in the transition from a non-equilibrium state to an equilibrium
state is called the Knudsen layer. In the extreme of strong evaporation, the vapor on
the outer surface of the Knudsen layer flows at the sound velocity, and the vapor
flowing outward is noticeably colder and thinner than the saturated vapor on the
surface of the solution.
Under continuous laser irradiation, the velocity of evaporation front (namely the
receding velocity) rises rapidly from zero to an approximately constant value, and
the evaporation enters a steady state. The establishment time t sv of a steady state is
related to the steady receding velocity U v . Usually t sv is dozens of times greater than
t v . For example, when AI 0 is 10
7 W/cm
2 , the receding speed of aluminum is about
2.34 m/s, and the corresponding velocity t sv ≈ 15 μs, while t v is less than 0.3 μs.
Due to the action of the vapor pressure, t v in Eq. (1.18) should be greater than the
normal evaporating temperature under standard atmospheric pressure. When the laser
power density is between 10
6 and 10
7 W/cm
2 , the temperature of the evaporation
front of the metal is slightly greater than the normal evaporating temperature. When
the laser power density is between 10
6 and 10
10 W/cm
2 , the temperature of the
former evaporation front is several times to dozens of times that of the latter. Since
T v increases with the laser power density, it leads to the receding velocity U v in the
range above 10
8 W/cm
2 decreasing with the increase of the laser power density, and
the decrease is more obvious in the range above 10
0 W/cm
2 .
The gaseous mass transfer rate ˙
m v depends on the receding velocity, ˙
m v = ρU v ,
and the total gaseous transfer mass is ρ
t
t v
U v dt. When the laser power density is
low, the heat lost by thermal diffusion has an obvious influence on U v , and the rate
of gaseous mass transfer is also low. The calculated values of the receding velocity
and gaseous mass transfer rate are usually noticeably different from the experimental
results, which is caused by two reasons. For one thing, it is because the reflectivity,
absorptivity and thermal physical characteristics are all related to temperature and
the actual laser waveform is highly irregular. For the other, it is because that mass
transfer is actually a comprehensive result of various mechanisms, in which the main
part of the transferred mass is composed of the splashed droplets by vapor pressure.
1.1.5 Laser-Induced Plasma and Its Effects
When laser acts on the surface of the material, vapor is induced. The vapor continues
to absorb laser energy, causing an increase in the temperature and eventually forming