112
4 Simulation of Transient Keyhole and Weld Pool
Fig. 4.5 Internal flow trend
of weld pool with keyhole
stability obtained by X-ray
penetration test
4.2.3 Flow Characteristics of Weld Pool with Unstable
Keyhole
In the welding process, it is impossible to maintain a balance between the recoil
pressure on some keyhole walls and the surface tension and the impact force of
metal liquid, so the keyhole begins to oscillate. The oscillation is manifested in the
appearance of multiple convex platforms on the keyhole walls, as shown in Figs. 4.6
and 4.7. Once a convex platform appears, its upper surface absorbs more energy
because of direct exposure to laser, while its lower surface is blocked by the convex
platform and cannot receive direct laser exposure. As a result, there is a significant
temperature difference between the upper and lower parts of the convex platform,
as shown in Fig. 4.7. In the upper part of the convex platform, the temperature is
very high and may approach or exceed the boiling point of aluminum alloy so that
this upper part experiences intense evaporation, which produces recoil pressure. The
recoil pressure will drive the metal liquid in the weld pool near the platform to move
downwards at a high speed in an attempt to close the keyhole, as shown in Fig. 4.6b–h.
At the same time, because the curvature of the keyhole walls of the convex platform
is usually large, the surface tension on these walls is also large. In addition, because
of the small keyhole radius of the platform, the above-mentioned keyhole walls are
more sensitive to metal liquid impact of the same degree. As a result of the combined
action of the above reasons, the keyhole eventually collapses and becomes two pieces.
Figure 4.7d, f show two keyholes that are about to collapse, while Fig. 4.7g shows a
keyhole that has been broken into two parts. If air enters the keyhole before fracture,
the closed area of the keyhole at the bottom will contain a bubble, and if the bubble
is captured by the solidification front, a pore defect will occur. When the keyhole is
closed, the keyhole becomes obviously shallow. However, due to the effect of laser
irradiation, the recoil pressure forces the keyhole open and drive the free interface
4 Simulation of Transient Keyhole and Weld Pool
Fig. 4.5 Internal flow trend
of weld pool with keyhole
stability obtained by X-ray
penetration test
4.2.3 Flow Characteristics of Weld Pool with Unstable
Keyhole
In the welding process, it is impossible to maintain a balance between the recoil
pressure on some keyhole walls and the surface tension and the impact force of
metal liquid, so the keyhole begins to oscillate. The oscillation is manifested in the
appearance of multiple convex platforms on the keyhole walls, as shown in Figs. 4.6
and 4.7. Once a convex platform appears, its upper surface absorbs more energy
because of direct exposure to laser, while its lower surface is blocked by the convex
platform and cannot receive direct laser exposure. As a result, there is a significant
temperature difference between the upper and lower parts of the convex platform,
as shown in Fig. 4.7. In the upper part of the convex platform, the temperature is
very high and may approach or exceed the boiling point of aluminum alloy so that
this upper part experiences intense evaporation, which produces recoil pressure. The
recoil pressure will drive the metal liquid in the weld pool near the platform to move
downwards at a high speed in an attempt to close the keyhole, as shown in Fig. 4.6b–h.
At the same time, because the curvature of the keyhole walls of the convex platform
is usually large, the surface tension on these walls is also large. In addition, because
of the small keyhole radius of the platform, the above-mentioned keyhole walls are
more sensitive to metal liquid impact of the same degree. As a result of the combined
action of the above reasons, the keyhole eventually collapses and becomes two pieces.
Figure 4.7d, f show two keyholes that are about to collapse, while Fig. 4.7g shows a
keyhole that has been broken into two parts. If air enters the keyhole before fracture,
the closed area of the keyhole at the bottom will contain a bubble, and if the bubble
is captured by the solidification front, a pore defect will occur. When the keyhole is
closed, the keyhole becomes obviously shallow. However, due to the effect of laser
irradiation, the recoil pressure forces the keyhole open and drive the free interface