180
6 Behaviors of Keyhole and Weld Pool Under the Effect …
applied side-blown gas pushes the liquid metal on the upper surface of the weld pool
to flow backwards to the rear part of the weld pool. This enables a concave shape to
form on the liquid surface near the keyhole opening in the front part of the weld pool,
and results in the reduction of the melt around the keyhole opening. The formation of
the concave shape and reduction of the melt greatly help the keyhole keep open and
the keyhole and weld pool stay stable in laser welding. Consequently, as the laser
action lasts, the peak average pressure of the keyhole decrease step by step under the
effect of side-blown gas, as shown in the result of the period from 1.5 to 2.8 ms in
Fig. 6.17. When the side-blown gas is not used, the melt does not flow backwards to
the rear part of the weld pool. Under the effect of surface tension, a large amount of
melt surrounding the upper keyhole opening tends to close the opening, which gives
rise to an increase in the average pressure within the keyhole. As a result, the keyhole
opening becomes wide again under the strong impact of high-pressure metallic vapor.
During an actual laser welding process, this phenomenon might periodically occur so
that the weld pool would be disturbed violently. It is should be noted that despite the
lack of side-blown gas, the impact of high-pressure metallic vapor could be strong
enough to cause a hump to emerge on local weld pool surface near the upper keyhole
opening, as shown in Fig. 6.16 at t 0 + 2.3 ms. In this case, however, because of
the absence of side-blown gas, the hump does not move towards the rear part of the
weld pool; instead, under the combined effects of surface tension and gravity, the
hump reserves energy enough to allow it to flow backwards the keyhole opening,
marking the start of the process of closing the keyhole. Therefore, as the laser welding
continues, the peak value of the average pressure increases stepwise, and the keyhole
and weld pool become increasingly unstable as shown in Figs. 6.16 and 6.17.
6.5 Conclusions
Below are the findings of the research on the influences of side-blown gas on
deformation of the weld pool surface.
(1) Under the effect of surface tension, the junction point of gas, liquid and solid
phases near the welding line moves to the center of the weld pool and the area
near the center of upper surface of the weld pool bulges, with the hump higher
than the upper surface of workpiece. Then, under the effect of side-blown
auxiliary gas flow, the liquid metal near the surface of the weld pool transfers
to the rear of the weld pool. Finally, the gas–liquid interface around the melt
front in the front of the weld pool is lower than the workpiece surface, while
that near the solidification front in the rear of the weld pool is higher than the
workpiece surface.
(2) The main reason for the appearance of weld undercut defects is that the junction
point of gas, liquid and solid phases near the welding line moves to the center
of the weld pool driven by surface tension. The weld reinforcement is mainly
caused by the movement of liquid metal in the weld pool to the rear of welds,
6 Behaviors of Keyhole and Weld Pool Under the Effect …
applied side-blown gas pushes the liquid metal on the upper surface of the weld pool
to flow backwards to the rear part of the weld pool. This enables a concave shape to
form on the liquid surface near the keyhole opening in the front part of the weld pool,
and results in the reduction of the melt around the keyhole opening. The formation of
the concave shape and reduction of the melt greatly help the keyhole keep open and
the keyhole and weld pool stay stable in laser welding. Consequently, as the laser
action lasts, the peak average pressure of the keyhole decrease step by step under the
effect of side-blown gas, as shown in the result of the period from 1.5 to 2.8 ms in
Fig. 6.17. When the side-blown gas is not used, the melt does not flow backwards to
the rear part of the weld pool. Under the effect of surface tension, a large amount of
melt surrounding the upper keyhole opening tends to close the opening, which gives
rise to an increase in the average pressure within the keyhole. As a result, the keyhole
opening becomes wide again under the strong impact of high-pressure metallic vapor.
During an actual laser welding process, this phenomenon might periodically occur so
that the weld pool would be disturbed violently. It is should be noted that despite the
lack of side-blown gas, the impact of high-pressure metallic vapor could be strong
enough to cause a hump to emerge on local weld pool surface near the upper keyhole
opening, as shown in Fig. 6.16 at t 0 + 2.3 ms. In this case, however, because of
the absence of side-blown gas, the hump does not move towards the rear part of the
weld pool; instead, under the combined effects of surface tension and gravity, the
hump reserves energy enough to allow it to flow backwards the keyhole opening,
marking the start of the process of closing the keyhole. Therefore, as the laser welding
continues, the peak value of the average pressure increases stepwise, and the keyhole
and weld pool become increasingly unstable as shown in Figs. 6.16 and 6.17.
6.5 Conclusions
Below are the findings of the research on the influences of side-blown gas on
deformation of the weld pool surface.
(1) Under the effect of surface tension, the junction point of gas, liquid and solid
phases near the welding line moves to the center of the weld pool and the area
near the center of upper surface of the weld pool bulges, with the hump higher
than the upper surface of workpiece. Then, under the effect of side-blown
auxiliary gas flow, the liquid metal near the surface of the weld pool transfers
to the rear of the weld pool. Finally, the gas–liquid interface around the melt
front in the front of the weld pool is lower than the workpiece surface, while
that near the solidification front in the rear of the weld pool is higher than the
workpiece surface.
(2) The main reason for the appearance of weld undercut defects is that the junction
point of gas, liquid and solid phases near the welding line moves to the center
of the weld pool driven by surface tension. The weld reinforcement is mainly
caused by the movement of liquid metal in the weld pool to the rear of welds,