8.5 Dynamic Dilution Behavior of Moving Weld Pools …
243
Table 8.13 Technical parameters of laser welding
Laser power/kW Laser spot radius/mm Defocusing amount/mm Welding speed/ (m/min)
1.0
0.2
0
3
Table 8.14 Numerical simulation parameters
Grid quantity
Spatial step/m Initial time step/s Initial temperature
of parent metal/K
Ambient
temperature/K
100 × 60 × 80 3.0 × 10 –5
3.0 × 10 –6
300
300
numerical calculation parameters used are respectively set out in Tables 8.13, 8.14
and 8.15.
Figure 8.37 shows the simulated evolution process of the concentration of compositions inside the moving weld pool near the keyhole free surface and inside the
longitudinal section of the moving weld pool. The three-dimensional free interface
shape represents the keyhole morphology, and the black line represents the fusion
line. In the color scale, high concentration and low concentration are expressed in red
and blue respectively. Figure 8.37a–f respectively show the concentration of compositions near the free interface and inside the cross section of the weld pool after 0.12,
5.04, 9.96, 14.88, 19.8 and 24.72 ms. As can be seen from the figures, the keyhole and
weld pool gradually form with the movement of the laser head; in addition, the wire
composition coated on the workpiece surface is diluted by convection of the weld
pool, and presents a tendency to move to both sides of the weld pool and to the depth
direction at the same time. The weld pool dynamic mechanism of the laser welding
obtained through study in Chap. 3 indicates that a high-speed downward movement
exists near the front wall of the keyhole as a result of recoil pressure. Therefore, a
high concentration of wire composition can be found at the weld pool position near
the front wall of the keyhole. Besides, according to the conclusions in Chap. 3, there
are three typical vortex flows and small speeds at both sides of the upper part, the rear
part, and the position of the weld pool near the back bottom of the keyhole. Thus,
the concentration of wire composition at these positions is significantly higher than
at other positions from 9.96 ms, as shown in Fig. 8.37c–f.
Figure 8.38 shows the evolution process of wire composition at the solidification
front and melting front of the transient moving weld pool. According to the figure,
with the advancing of the welding process, the wire concentration gradually increases
at both sides of the moving weld pool, the bottom of the melting front and the bottom
of the solidification front. The foregoing simulation results suggest that after the wire
composition enters the moving weld pool from the upper part of the melting front, it
bypasses the keyhole to move backward due to the convection on the upper part of the
weld pool, and in addition, it moves towards the direction of penetration depth due
to the high-speed downward convection caused by the recoil pressure. Figure 8.39
shows the concentration evolution process of the wire composition in the longitudinal
section of the weld joint. The figure indicates that, as the welding process proceeds,
243
Table 8.13 Technical parameters of laser welding
Laser power/kW Laser spot radius/mm Defocusing amount/mm Welding speed/ (m/min)
1.0
0.2
0
3
Table 8.14 Numerical simulation parameters
Grid quantity
Spatial step/m Initial time step/s Initial temperature
of parent metal/K
Ambient
temperature/K
100 × 60 × 80 3.0 × 10 –5
3.0 × 10 –6
300
300
numerical calculation parameters used are respectively set out in Tables 8.13, 8.14
and 8.15.
Figure 8.37 shows the simulated evolution process of the concentration of compositions inside the moving weld pool near the keyhole free surface and inside the
longitudinal section of the moving weld pool. The three-dimensional free interface
shape represents the keyhole morphology, and the black line represents the fusion
line. In the color scale, high concentration and low concentration are expressed in red
and blue respectively. Figure 8.37a–f respectively show the concentration of compositions near the free interface and inside the cross section of the weld pool after 0.12,
5.04, 9.96, 14.88, 19.8 and 24.72 ms. As can be seen from the figures, the keyhole and
weld pool gradually form with the movement of the laser head; in addition, the wire
composition coated on the workpiece surface is diluted by convection of the weld
pool, and presents a tendency to move to both sides of the weld pool and to the depth
direction at the same time. The weld pool dynamic mechanism of the laser welding
obtained through study in Chap. 3 indicates that a high-speed downward movement
exists near the front wall of the keyhole as a result of recoil pressure. Therefore, a
high concentration of wire composition can be found at the weld pool position near
the front wall of the keyhole. Besides, according to the conclusions in Chap. 3, there
are three typical vortex flows and small speeds at both sides of the upper part, the rear
part, and the position of the weld pool near the back bottom of the keyhole. Thus,
the concentration of wire composition at these positions is significantly higher than
at other positions from 9.96 ms, as shown in Fig. 8.37c–f.
Figure 8.38 shows the evolution process of wire composition at the solidification
front and melting front of the transient moving weld pool. According to the figure,
with the advancing of the welding process, the wire concentration gradually increases
at both sides of the moving weld pool, the bottom of the melting front and the bottom
of the solidification front. The foregoing simulation results suggest that after the wire
composition enters the moving weld pool from the upper part of the melting front, it
bypasses the keyhole to move backward due to the convection on the upper part of the
weld pool, and in addition, it moves towards the direction of penetration depth due
to the high-speed downward convection caused by the recoil pressure. Figure 8.39
shows the concentration evolution process of the wire composition in the longitudinal
section of the weld joint. The figure indicates that, as the welding process proceeds,