2
1 Laser Welding Basics
Heat
conduction
Light absorption
and heating
Melting
Laser
beam
Liquid
interface
Vaporization
Plasma production
Fig. 1.1 Physical effect on the material surface under the laser action
material are mainly free electrons. Absorption in the metal is mainly completed by
the vibration of free electrons which can move freely under the electric field action
without the effect of the restoring force. The natural vibration frequency of the oscillators is ω 0 = 0, and the relationship between refractive index n, extinction coefficient
κ, and damping factor γ , plasma frequency ω p is as follows:
n =
⎧
⎪ ⎪ ⎨
⎪ ⎪ ⎩
1 −
ω 2
p
ω 2 +γ 2
2 + +
ω 2
p γ
ω 3 +γ 2 ω
2 +
1 −
ω
2
p
ω 2 +γ 2
2
⎫
⎪ ⎪ ⎬
⎪ ⎪ ⎭
1/2
(1.1)
κ =
⎧
⎪ ⎪ ⎨
⎪ ⎪ ⎩
1 −
ω 2
p
ω 2 +γ 2
2 + +
ω 2
p γ
ω 3 +γ 2 ω
2 −
1 −
ω
2
p
ω 2 +γ 2
2
⎫
⎪ ⎪ ⎬
⎪ ⎪ ⎭
1/2
(1.2)
where
ω p =
N e 2
mε 0
γ =
e
2
ω
2
0
6πε 0 c 3 m
The plasma frequency ω p is the strength parameter, which determines the relative
strength of the oscillators; the damping factor γ is the parameter of the band width.
As we can see from Eqs. (1.1) and (1.2), when the laser frequency is far less than the
plasma frequency, n and k increase rapidly, most of laser is reflected, and very little is
absorbed; when the laser frequency is close to the plasma frequency, n shows the local