12
1 Laser Welding Basics
in its absorptivity; conversely, as the absorptivity weakens, the laser power density
reaching the surface of the material increases, and therefore the evaporation effect is
enhanced, resulting in an increase in the density and temperature of the plasma, as
well as in its absorptivity.
The absorption of the laser by the plasma prevents the laser from reaching the
surface of the material, cutting off the energy coupling between the laser and the
material. This effect is called plasma shielding. The ratio of energy absorbed by
plasma to the energy of the incident laser is called plasma shielding coefficient. The
plasma shielding coefficient is related to the laser wavelength. The plasma shielding
effect of a long wavelength laser is stronger than that of a short wavelength laser, and
appears earlier. In laser welding technology, the absorption and scattering of plasma
affect the transmission efficiency of laser and reduce the laser energy reaching the
work piece. Meanwhile, the negative lens effect (refraction) of plasma expands the
area of action of laser energy on the work piece, thus reducing welding quality.
1.2 Principles and Characteristics of Laser Welding
Laser welding is a special fusion welding method in which a focused laser beam with
high energy density (10
6 –10
12 W/cm
2 ) is used as a heat source to heat and melt the
work piece. It is a fusion welding based on the above-mentioned photothermal effect
of the interaction between the laser and the material. Its premise is that the laser is
absorbed by the material and converted into the thermal energy required for welding.
In general, the physical phenomena resulting from laser action on the material surface
vary, including the increase of surface temperature, melting, evaporation, formation
of keyhole, and generation of laser induced plasma (see Fig. 1.2).
These physical phenomena determine the thermal action mechanism of the
welding process, which leads to two laser welding modes: heat conduction welding
and deep penetration welding. The transition of the two modes depends primarily
Solid state heating
Surface remelting
Keyhole effect
Plasma shielding
Fig. 1.2 Physical processes of laser of different intensities acting on metal surface
1 Laser Welding Basics
in its absorptivity; conversely, as the absorptivity weakens, the laser power density
reaching the surface of the material increases, and therefore the evaporation effect is
enhanced, resulting in an increase in the density and temperature of the plasma, as
well as in its absorptivity.
The absorption of the laser by the plasma prevents the laser from reaching the
surface of the material, cutting off the energy coupling between the laser and the
material. This effect is called plasma shielding. The ratio of energy absorbed by
plasma to the energy of the incident laser is called plasma shielding coefficient. The
plasma shielding coefficient is related to the laser wavelength. The plasma shielding
effect of a long wavelength laser is stronger than that of a short wavelength laser, and
appears earlier. In laser welding technology, the absorption and scattering of plasma
affect the transmission efficiency of laser and reduce the laser energy reaching the
work piece. Meanwhile, the negative lens effect (refraction) of plasma expands the
area of action of laser energy on the work piece, thus reducing welding quality.
1.2 Principles and Characteristics of Laser Welding
Laser welding is a special fusion welding method in which a focused laser beam with
high energy density (10
6 –10
12 W/cm
2 ) is used as a heat source to heat and melt the
work piece. It is a fusion welding based on the above-mentioned photothermal effect
of the interaction between the laser and the material. Its premise is that the laser is
absorbed by the material and converted into the thermal energy required for welding.
In general, the physical phenomena resulting from laser action on the material surface
vary, including the increase of surface temperature, melting, evaporation, formation
of keyhole, and generation of laser induced plasma (see Fig. 1.2).
These physical phenomena determine the thermal action mechanism of the
welding process, which leads to two laser welding modes: heat conduction welding
and deep penetration welding. The transition of the two modes depends primarily
Solid state heating
Surface remelting
Keyhole effect
Plasma shielding
Fig. 1.2 Physical processes of laser of different intensities acting on metal surface