8
1 Laser Welding Basics
where T m —Melting temperature of the material;
T v —Vaporization temperature of the material.
In terms of the material characteristics, the larger the ratio of the thermal conductivity coefficient k of the material to T v /T m is, the greater the melting depth Z max
will be. For the laser characteristics, to improve the melting depth, the relatively
low laser power density shall be adopted because it can take relatively long time to
heat the material surface to T v . Therefore, at the given laser pulse, the laser power
density shall be adjusted so that the material surface can just reach the vaporization
temperature at the end of laser pulse, in order to obtain the maximum melting depth.
Under normal circumstances, the maximum depth of the pure melting of the material
under laser irradiation is about several microns to several hundred microns.
What the shallow melting corresponds to is deep melting which is defined to be
situation where the melting depth is no less than the radius of the light spot. For
deep melting, only equilibrium vaporization occurs at the gas–liquid phase surface,
the vapor is transparent to the laser. The threshold value of the laser power density
causing deep melting of the material under continuous Gaussian beam irradiation is
about:
I md = 2kT m /(
√
πα A)
(1.15)
During deep melting, the diameter of the weld pool is larger than that of the light
spot. A stable vaporization wellblock with the diameter less than that of the light spot
appears in the center of the weld pool. The vapor in the wellblock has low density,
basically transparent to the laser, so that the laser can directly enter and irradiate on
the gas–liquid interface at the bottom of the wellblock, and then be absorbed. The
absorbed laser energy is used for heat dissipation of the side wall and vaporization
at the bottom. Assumed that the radius of both laser spot and vaporization wellblock
is R s , the depth of the deep melting can be expressed as:
Z v =
R
2
s AI 0
k q T v
1 − exp
−
k q T v
R 2
s L v ρ q
t
(1.16)
In the deep melting, the length-diameter ratio of the vaporization wellblock Z v /R s
cannot be too large (generally 0.5 ≤ Z v /R s ≤ 40), or unstable movement may occur
to vapor and solution.
If the laser intensity is relatively high, vaporization at the gas–liquid interface in
the wellblock will exacerbate, and the vapor pressure will rise. The vapor pressure and
vapor reaction can overcome the tension on the solution surface and the solution static
pressure, resulting in appearance of keyholes and transfer of liquid mass. The keyhole
is like black body, which is in favor of absorption of the beam energy, indicating the
“wall focusing effect". Since the laser beam after focusing is not parallel beam, it will
make certain angle of incidence with the keyhole wall. After radiating on the keyhole
wall, the laser beam reaches the bottom of the keyhole after multiple reflection, and
finally be completely absorbed.
1 Laser Welding Basics
where T m —Melting temperature of the material;
T v —Vaporization temperature of the material.
In terms of the material characteristics, the larger the ratio of the thermal conductivity coefficient k of the material to T v /T m is, the greater the melting depth Z max
will be. For the laser characteristics, to improve the melting depth, the relatively
low laser power density shall be adopted because it can take relatively long time to
heat the material surface to T v . Therefore, at the given laser pulse, the laser power
density shall be adjusted so that the material surface can just reach the vaporization
temperature at the end of laser pulse, in order to obtain the maximum melting depth.
Under normal circumstances, the maximum depth of the pure melting of the material
under laser irradiation is about several microns to several hundred microns.
What the shallow melting corresponds to is deep melting which is defined to be
situation where the melting depth is no less than the radius of the light spot. For
deep melting, only equilibrium vaporization occurs at the gas–liquid phase surface,
the vapor is transparent to the laser. The threshold value of the laser power density
causing deep melting of the material under continuous Gaussian beam irradiation is
about:
I md = 2kT m /(
√
πα A)
(1.15)
During deep melting, the diameter of the weld pool is larger than that of the light
spot. A stable vaporization wellblock with the diameter less than that of the light spot
appears in the center of the weld pool. The vapor in the wellblock has low density,
basically transparent to the laser, so that the laser can directly enter and irradiate on
the gas–liquid interface at the bottom of the wellblock, and then be absorbed. The
absorbed laser energy is used for heat dissipation of the side wall and vaporization
at the bottom. Assumed that the radius of both laser spot and vaporization wellblock
is R s , the depth of the deep melting can be expressed as:
Z v =
R
2
s AI 0
k q T v
1 − exp
−
k q T v
R 2
s L v ρ q
t
(1.16)
In the deep melting, the length-diameter ratio of the vaporization wellblock Z v /R s
cannot be too large (generally 0.5 ≤ Z v /R s ≤ 40), or unstable movement may occur
to vapor and solution.
If the laser intensity is relatively high, vaporization at the gas–liquid interface in
the wellblock will exacerbate, and the vapor pressure will rise. The vapor pressure and
vapor reaction can overcome the tension on the solution surface and the solution static
pressure, resulting in appearance of keyholes and transfer of liquid mass. The keyhole
is like black body, which is in favor of absorption of the beam energy, indicating the
“wall focusing effect". Since the laser beam after focusing is not parallel beam, it will
make certain angle of incidence with the keyhole wall. After radiating on the keyhole
wall, the laser beam reaches the bottom of the keyhole after multiple reflection, and
finally be completely absorbed.