1.2 Principles and Characteristics of Laser Welding
13
on the power density of the laser spot acting on the material. For some particular
materials, there is a specific threshold of power density. When the power density of
the laser acting on the material is lower than this threshold value, the laser energy is
absorbed by the surface of the material and transferred to the interior of the material
quickly, forming a heat conduction weld with a relatively large width and depth.
However, when the power density of the laser acting on the material is higher than
this threshold value, the laser energy makes the material surface heat up, melt and
evaporate rapidly before the work piece surface can transfer the heat into the material,
and with the continuous input of laser energy, a keyhole in the direction of penetrating the thickness is formed. The keyhole, surrounded by a weld pool of liquid
metal, is filled with high-temperature metal vapor and plasma. The expansion force
of the high-temperature metal vapor and plasma acts together with the gravity and
surface tension of the liquid metal around the keyhole to maintain a stable existence
of the keyhole (see Fig. 1.3).
The keyhole moves along the welding direction, and the weld pool behind it
quickly cools and solidifies, forming a deep penetration weld with a large width and
depth, as shown in Fig. 1.4. Therefore, the laser welding mode is closely related to the
laser power density and welding heat input which determine the thermal mechanism.
Laser welding is an extremely complex physical and chemical process, containing
phenomena such as rapid heating, melting, evaporation, ionization, rapid cooling
and non-equilibrium solidification of the material, and involving the formation and
of different states of matter, such as solid, liquid, gaseous and plasma forms and the
complex interaction between them. The welding area is in a dynamic supernormal
thermophysical state, in which the behavior of heat and mass transfer and the process
of non-equilibrium solidification and structure evolution are more complicated than
that of conventional heat source welding. The dynamic behavior of these complex
phenomena has an important influence on the welding quality and the mechanical properties of the joint. Furthermore, the key components to be welded often
have complex three-dimensional shapes, and dynamic working conditions, such as
Fig. 1.3 Diagram of keyhole
formation in laser welding
Laser
beam
Plasma
Keyhole
Welding direction
Crystallized
weld
Work piece
Metal of weld
pool
13
on the power density of the laser spot acting on the material. For some particular
materials, there is a specific threshold of power density. When the power density of
the laser acting on the material is lower than this threshold value, the laser energy is
absorbed by the surface of the material and transferred to the interior of the material
quickly, forming a heat conduction weld with a relatively large width and depth.
However, when the power density of the laser acting on the material is higher than
this threshold value, the laser energy makes the material surface heat up, melt and
evaporate rapidly before the work piece surface can transfer the heat into the material,
and with the continuous input of laser energy, a keyhole in the direction of penetrating the thickness is formed. The keyhole, surrounded by a weld pool of liquid
metal, is filled with high-temperature metal vapor and plasma. The expansion force
of the high-temperature metal vapor and plasma acts together with the gravity and
surface tension of the liquid metal around the keyhole to maintain a stable existence
of the keyhole (see Fig. 1.3).
The keyhole moves along the welding direction, and the weld pool behind it
quickly cools and solidifies, forming a deep penetration weld with a large width and
depth, as shown in Fig. 1.4. Therefore, the laser welding mode is closely related to the
laser power density and welding heat input which determine the thermal mechanism.
Laser welding is an extremely complex physical and chemical process, containing
phenomena such as rapid heating, melting, evaporation, ionization, rapid cooling
and non-equilibrium solidification of the material, and involving the formation and
of different states of matter, such as solid, liquid, gaseous and plasma forms and the
complex interaction between them. The welding area is in a dynamic supernormal
thermophysical state, in which the behavior of heat and mass transfer and the process
of non-equilibrium solidification and structure evolution are more complicated than
that of conventional heat source welding. The dynamic behavior of these complex
phenomena has an important influence on the welding quality and the mechanical properties of the joint. Furthermore, the key components to be welded often
have complex three-dimensional shapes, and dynamic working conditions, such as
Fig. 1.3 Diagram of keyhole
formation in laser welding
Laser
beam
Plasma
Keyhole
Welding direction
Crystallized
weld
Work piece
Metal of weld
pool