8.3 Keyhole and Weld Pool Dynamics in Welding with Filler Wires
213
weld pool and keyhole. At the interval of 7–11 ms, it can be found that when the
wire feed rate is low, the time reaching the peak of maximum speed is slightly later
than the process at the high wire feed rate, mainly because when the wire feed rate
is relatively low, the time affecting the weld pool is slower than that when the wire
feed rate is high. At the interval of 11–15 ms, when the wire feed rate is low, the
change of maximum speed of the weld pool is slightly greater than that when the
wire feed rate is 0.1 m/s, so it can be assumed that the welding pool mobility is more
unstable. It is suggested through analysis that, the reason for the above result is that
when the wire feed rate is high, there are already two droplets entering into the weld
pool within the time under study, so that the volume is larger than that when the wire
feed rate is low. Therefore, at the same laser power, change of the maximum speeds
of the weld pool is gentler than that when the wire feed rate is low.
It shows from the above simulation results that, when the wire enters into the weld
pool in the free transition mode, increasing the wire feed rate may cause instability
of the keyhole. But under certain condition, reasonably raising the wire feed speed
can increase the volume of the weld pool, hence to improve the stability of the weld
pool to certain extent.
8.3.3 Influences of Wire Diameter on Transient Keyhole
and Weld Pool Behaviors in Free Transition
Unless otherwise noted, the numerical simulation parameters mentioned in the
section are consistent with the corresponding parameters in Sect. 8.3.1.
Figure 8.8 shows the comparison of the simulation results at the transient welding
moment of 13.59 ms under different wire diameters in free transition mode. The
figures (a) through (c) show the keyhole cross-section morphology, weld pool
morphology and free interface morphology under the wire diameter of 1.2 mm.
The figures (d) through (f) show the corresponding results under the wire diameter
of 0.9 mm.
The larger the molten droplet diameter, the more intense the protrusion of the
front wall of the keyhole, as shown in the figure. It can be speculated that excessive
large diameter of wire will bring adverse effects on the stability of the keyhole. In
addition, in case of fixed wire feed rate, the momentum of the metal-droplet transition
to the weld pool increases due to the increased radius, so the addition of wire has
an intensified disturbance effect on the weld pool. To further illustrate this issue,
the fluctuation of maximum speed absolute value of the weld pool under the two
conditions is analyzed.
Figure 8.9 shows the comparison of maximum speed absolute values of the weld
pool under different wire diameters in free transition mode. Where, the red line is
the variation curve of the maximum speed absolute value of the weld pool at a wire
diameter of 1.2 mm as a function of time, and the green line is the variation curve of
the maximum speed absolute value of the weld pool at a wire diameter of 0.9 mm
213
weld pool and keyhole. At the interval of 7–11 ms, it can be found that when the
wire feed rate is low, the time reaching the peak of maximum speed is slightly later
than the process at the high wire feed rate, mainly because when the wire feed rate
is relatively low, the time affecting the weld pool is slower than that when the wire
feed rate is high. At the interval of 11–15 ms, when the wire feed rate is low, the
change of maximum speed of the weld pool is slightly greater than that when the
wire feed rate is 0.1 m/s, so it can be assumed that the welding pool mobility is more
unstable. It is suggested through analysis that, the reason for the above result is that
when the wire feed rate is high, there are already two droplets entering into the weld
pool within the time under study, so that the volume is larger than that when the wire
feed rate is low. Therefore, at the same laser power, change of the maximum speeds
of the weld pool is gentler than that when the wire feed rate is low.
It shows from the above simulation results that, when the wire enters into the weld
pool in the free transition mode, increasing the wire feed rate may cause instability
of the keyhole. But under certain condition, reasonably raising the wire feed speed
can increase the volume of the weld pool, hence to improve the stability of the weld
pool to certain extent.
8.3.3 Influences of Wire Diameter on Transient Keyhole
and Weld Pool Behaviors in Free Transition
Unless otherwise noted, the numerical simulation parameters mentioned in the
section are consistent with the corresponding parameters in Sect. 8.3.1.
Figure 8.8 shows the comparison of the simulation results at the transient welding
moment of 13.59 ms under different wire diameters in free transition mode. The
figures (a) through (c) show the keyhole cross-section morphology, weld pool
morphology and free interface morphology under the wire diameter of 1.2 mm.
The figures (d) through (f) show the corresponding results under the wire diameter
of 0.9 mm.
The larger the molten droplet diameter, the more intense the protrusion of the
front wall of the keyhole, as shown in the figure. It can be speculated that excessive
large diameter of wire will bring adverse effects on the stability of the keyhole. In
addition, in case of fixed wire feed rate, the momentum of the metal-droplet transition
to the weld pool increases due to the increased radius, so the addition of wire has
an intensified disturbance effect on the weld pool. To further illustrate this issue,
the fluctuation of maximum speed absolute value of the weld pool under the two
conditions is analyzed.
Figure 8.9 shows the comparison of maximum speed absolute values of the weld
pool under different wire diameters in free transition mode. Where, the red line is
the variation curve of the maximum speed absolute value of the weld pool at a wire
diameter of 1.2 mm as a function of time, and the green line is the variation curve of
the maximum speed absolute value of the weld pool at a wire diameter of 0.9 mm