1.1 Interaction of Lasers and Materials
9
The high-pressure steam in the keyhole pushes the melt to splash out along the edge
of the weld pool or the wall of the vapor well, causing liquid mass transfer. Liquid
mass transfer of this kind will greatly improve the efficiency of some laser processing,
such as laser drilling or laser cutting. The squeezing effect of material vapor is
an important cause of liquid mass transfer. Assuming that the vapor is produced
from equilibrium vaporization at the gas–liquid section, ignoring the momentum
and energy of the vapor, and only taking into consideration the vapor pressure P v ,
when the vapor pushes the nonviscous incompressible solution layer of a certain
thickness, the work of the vapor will be completely converted into the kinetic energy
of the solution, and the mass of the liquid material transferred from a unit area of
laser beam spot within a unit time at the ambient pressure of P 0 is the liquid mass
transfer rate ˙
m q of liquid mass, which is
˙
m q =
2λ q
R s
ln
T v
T m
1/2
ρ
3/4
q [2(P v − P 0 )]
1/4
(1.17)
1.1.4 Evaporation of Materials Under Laser Action
It can be seen from the melting of material under laser action, the melting process is
usually accompanied by evaporation of the material. The evaporation mechanism is
closely related to laser power density.
The time from the start of laser irradiation to the material surface reaching the
evaporating temperature T v is called the evaporation starting time t v , which can be
estimated as
t v =
π
4α
kT v
AI 0
2
(1.18)
For the metal with a low evaporating temperature, when AI 0 is 10
4 ~10
6 W/cm
2 ,
t v is about a few milliseconds to microseconds; for the metal with a high evaporating temperature, when AI 0 is 10
5 ~10
7 W/cm
2 , t v is about a few milliseconds to
several hundred nanoseconds; If t v is much less than the width of the laser pulse, the
evaporation of the material surface is considered to start immediately once the laser
irradiates.
When the laser power density is not very high, the evaporation of material is
not drastic, the saturated vapor pressure is balanced with the ambient pressure, and
the velocity distribution of the vapor particle is isotropic, in the Maxwell Distribution of translational equilibrium. Myriad laser equilibrium evaporation models have
been established by scholars across the world. At present, the theoretical model of
laser evaporation has only qualitative significance, and the quantitative understanding
mainly relies on experiments. In all evaporation pressure formulas obtained with laser
9
The high-pressure steam in the keyhole pushes the melt to splash out along the edge
of the weld pool or the wall of the vapor well, causing liquid mass transfer. Liquid
mass transfer of this kind will greatly improve the efficiency of some laser processing,
such as laser drilling or laser cutting. The squeezing effect of material vapor is
an important cause of liquid mass transfer. Assuming that the vapor is produced
from equilibrium vaporization at the gas–liquid section, ignoring the momentum
and energy of the vapor, and only taking into consideration the vapor pressure P v ,
when the vapor pushes the nonviscous incompressible solution layer of a certain
thickness, the work of the vapor will be completely converted into the kinetic energy
of the solution, and the mass of the liquid material transferred from a unit area of
laser beam spot within a unit time at the ambient pressure of P 0 is the liquid mass
transfer rate ˙
m q of liquid mass, which is
˙
m q =
2λ q
R s
ln
T v
T m
1/2
ρ
3/4
q [2(P v − P 0 )]
1/4
(1.17)
1.1.4 Evaporation of Materials Under Laser Action
It can be seen from the melting of material under laser action, the melting process is
usually accompanied by evaporation of the material. The evaporation mechanism is
closely related to laser power density.
The time from the start of laser irradiation to the material surface reaching the
evaporating temperature T v is called the evaporation starting time t v , which can be
estimated as
t v =
π
4α
kT v
AI 0
2
(1.18)
For the metal with a low evaporating temperature, when AI 0 is 10
4 ~10
6 W/cm
2 ,
t v is about a few milliseconds to microseconds; for the metal with a high evaporating temperature, when AI 0 is 10
5 ~10
7 W/cm
2 , t v is about a few milliseconds to
several hundred nanoseconds; If t v is much less than the width of the laser pulse, the
evaporation of the material surface is considered to start immediately once the laser
irradiates.
When the laser power density is not very high, the evaporation of material is
not drastic, the saturated vapor pressure is balanced with the ambient pressure, and
the velocity distribution of the vapor particle is isotropic, in the Maxwell Distribution of translational equilibrium. Myriad laser equilibrium evaporation models have
been established by scholars across the world. At present, the theoretical model of
laser evaporation has only qualitative significance, and the quantitative understanding
mainly relies on experiments. In all evaporation pressure formulas obtained with laser