2.5 Effect of Welding Speed on Keyhole Profile
47
and pressure of the keyhole is just the opposite. The decrease of the inclination of the
front wall of the keyhole is beneficial to the absorption of laser beam by the keyhole
wall. In deep penetration laser welding, the stability of keyhole is the result of the
combined effect of fluid flow pressure in keyhole and surface tension pressure, static
pressure and dynamic pressure of weld pool fluid. On the premise that other pressures
remain unchanged, the increase of the surface tension pressure of the keyhole means
the decrease of the stability of the keyhole.
2.5.3 Effect of Laser Power on Keyhole Size
Deep penetration laser welding is divided into partial deep penetration welding and
full deep penetration welding. The former refers to the generation of laser keyholes
without penetrating the base metal, while the latter refers to full penetration of keyhole
through the plate. In partial deep penetration welding, it is easy to produce process
pore, or porosity. Such porosity can be caused by many factors; a major one is the
instability of the keyhole. To keep the keyhole wall open, two basic conditions must
be met: energy balance and the pressure balance. The following describes the impact
of laser power changes on keyhole size from the perspective of energy balance.
In laser welding, the existence of plasma considerably affects the efficiency of
energy coupling between laser and material. The periodic plasma flow from the
keyhole directly takes the energy of the air flow inside keyhole. Based on energy
balance, the stability of the keyhole wall depends on the heat absorption and dissipation. According to the available data, laser energy is absorbed by the air flow and
laser beam inside keyhole through the Fresnel absorption mechanism of the keyhole
wall and the reverse bremsstrahlung absorption mechanism. The heat absorbed is
used to overcome the resistance of forcing the keyhole closed, such as the surface
tension, the static pressure of the weld pool fluid and the dynamic pressure of the
flow.
Figure 2.17 shows the temperature field when the welding speed is 0.025 m/s. In
the picture, the temperature in the red area is 5000 K, and the temperature in the dark
yellow area is above 3590 K, which is also in the evaporation temperature range. This
temperature region is the equivalent pore. It can be seen that the keyhole is in the
shape of an upright wine cup, showing typical asymmetry in appearance. The double
horn shape of weld pool is also very obvious. The incident laser beam enters from
the upper part of the workpiece. If the plasma density in the keyhole is not high, part
of the laser energy passes through the vapor in the keyhole and interacts directly with
the welded base metal. According to the Fresnel absorption mechanism, the energy
of the laser beam is converted into the energy required for welding. In fact, the vapor
pressure in the keyhole changes in a quasi-periodic manner, so does the amount
of plasma. In this changing process, the Fresnel absorption mechanism has been
playing a role, but the absorption of laser beam energy by this mechanism has also
undergone a quasi-periodic change. Seto et al. from Osaka University observed the
quasi-periodic eruption process of keyhole plasma through high-speed photography.
47
and pressure of the keyhole is just the opposite. The decrease of the inclination of the
front wall of the keyhole is beneficial to the absorption of laser beam by the keyhole
wall. In deep penetration laser welding, the stability of keyhole is the result of the
combined effect of fluid flow pressure in keyhole and surface tension pressure, static
pressure and dynamic pressure of weld pool fluid. On the premise that other pressures
remain unchanged, the increase of the surface tension pressure of the keyhole means
the decrease of the stability of the keyhole.
2.5.3 Effect of Laser Power on Keyhole Size
Deep penetration laser welding is divided into partial deep penetration welding and
full deep penetration welding. The former refers to the generation of laser keyholes
without penetrating the base metal, while the latter refers to full penetration of keyhole
through the plate. In partial deep penetration welding, it is easy to produce process
pore, or porosity. Such porosity can be caused by many factors; a major one is the
instability of the keyhole. To keep the keyhole wall open, two basic conditions must
be met: energy balance and the pressure balance. The following describes the impact
of laser power changes on keyhole size from the perspective of energy balance.
In laser welding, the existence of plasma considerably affects the efficiency of
energy coupling between laser and material. The periodic plasma flow from the
keyhole directly takes the energy of the air flow inside keyhole. Based on energy
balance, the stability of the keyhole wall depends on the heat absorption and dissipation. According to the available data, laser energy is absorbed by the air flow and
laser beam inside keyhole through the Fresnel absorption mechanism of the keyhole
wall and the reverse bremsstrahlung absorption mechanism. The heat absorbed is
used to overcome the resistance of forcing the keyhole closed, such as the surface
tension, the static pressure of the weld pool fluid and the dynamic pressure of the
flow.
Figure 2.17 shows the temperature field when the welding speed is 0.025 m/s. In
the picture, the temperature in the red area is 5000 K, and the temperature in the dark
yellow area is above 3590 K, which is also in the evaporation temperature range. This
temperature region is the equivalent pore. It can be seen that the keyhole is in the
shape of an upright wine cup, showing typical asymmetry in appearance. The double
horn shape of weld pool is also very obvious. The incident laser beam enters from
the upper part of the workpiece. If the plasma density in the keyhole is not high, part
of the laser energy passes through the vapor in the keyhole and interacts directly with
the welded base metal. According to the Fresnel absorption mechanism, the energy
of the laser beam is converted into the energy required for welding. In fact, the vapor
pressure in the keyhole changes in a quasi-periodic manner, so does the amount
of plasma. In this changing process, the Fresnel absorption mechanism has been
playing a role, but the absorption of laser beam energy by this mechanism has also
undergone a quasi-periodic change. Seto et al. from Osaka University observed the
quasi-periodic eruption process of keyhole plasma through high-speed photography.