8.4 Instability of Keyhole and Weld Pool in Welding with Filler Wires
227
chapter; in addition, under the same power and welding speed conditions, there are
much more large pores inside the cross section of the weld bead than that during the
single-beam laser welding process (see Fig. 8.24 (1) and (2)), which strongly proves
that the keyhole may be more unstable in the laser welding with filler wires process
than in the single-beam laser welding process when the wire is located in front of
the keyhole. In addition, even if the stability of the moving weld pool is improved
by increasing the wire feed rate, there are still many large pores on the weld bead,
which further indicates that the stability of the keyhole in the laser welding with
filler wires process is very poor. According to the mechanism of single-beam laser
welding process confirmed in Chapter 3, under general conditions, the shallower the
penetration, that is, the shorter the keyhole, the higher the stability of the keyhole.
However, the results of Fig. 8.24 (1) and (2) confirm that a keyhole with shallower
penetration is more unstable. Obviously, the only possible explanation is that the
addition of the wire increases the instability of the keyhole in the laser welding with
filler wires process.
In view of the above phenomenon, according to the test results, Dr. Yu Yangchun
speculated that the wire may form periodic unstable molten drops in the laser welding
with filler wires process, and during the transition into the weld pool, some of the
molten drops enter the keyhole, thus increasing the instability of the keyhole. In
addition, the jitter of the wire feeder may also increase the instability of the keyhole.
In this study, through theoretical simulation, the process of the wire entering the
keyhole and increasing the instability of the front wall of the keyhole is reproduced
visually, which, in theory, directly confirms that Dr. Yu Yangchun’s speculation that
the molten drops entering the keyhole may increase the instability of the keyhole is
reasonable. In addition, the theoretical study in this chapter also confirms that even
if periodic molten drops can exist steadily in the process of welding with filler wire
and enter the weld pool under ideal conditions without unstable wire feed structure,
it is still likely to increase the instability of the keyhole, and even if the wire enters
the weld pool in an ideal and uniform contact transition (liquid bridge transition)
way, it is still likely to increase the instability of the keyhole.
8.5 Dynamic Dilution Behavior of Moving Weld Pools
in Welding with Filler Wires
8.5.1 Model of Dilution in Weld Pools
In laser welding with filler wires, the chemical compositions of the wire convectively
move along with the velocity field of the moving weld pool after entering the moving
weld pool. At the same time, the chemical compositions can diffuse inside the weld
pool. The calculation by using the thermophysical property calculation software
JMatPro shows that the diffusion coefficients of general wire metal elements in a
liquid weld pool are small, about 10
−8 m
2 /s orders of magnitude. Despite varying
227
chapter; in addition, under the same power and welding speed conditions, there are
much more large pores inside the cross section of the weld bead than that during the
single-beam laser welding process (see Fig. 8.24 (1) and (2)), which strongly proves
that the keyhole may be more unstable in the laser welding with filler wires process
than in the single-beam laser welding process when the wire is located in front of
the keyhole. In addition, even if the stability of the moving weld pool is improved
by increasing the wire feed rate, there are still many large pores on the weld bead,
which further indicates that the stability of the keyhole in the laser welding with
filler wires process is very poor. According to the mechanism of single-beam laser
welding process confirmed in Chapter 3, under general conditions, the shallower the
penetration, that is, the shorter the keyhole, the higher the stability of the keyhole.
However, the results of Fig. 8.24 (1) and (2) confirm that a keyhole with shallower
penetration is more unstable. Obviously, the only possible explanation is that the
addition of the wire increases the instability of the keyhole in the laser welding with
filler wires process.
In view of the above phenomenon, according to the test results, Dr. Yu Yangchun
speculated that the wire may form periodic unstable molten drops in the laser welding
with filler wires process, and during the transition into the weld pool, some of the
molten drops enter the keyhole, thus increasing the instability of the keyhole. In
addition, the jitter of the wire feeder may also increase the instability of the keyhole.
In this study, through theoretical simulation, the process of the wire entering the
keyhole and increasing the instability of the front wall of the keyhole is reproduced
visually, which, in theory, directly confirms that Dr. Yu Yangchun’s speculation that
the molten drops entering the keyhole may increase the instability of the keyhole is
reasonable. In addition, the theoretical study in this chapter also confirms that even
if periodic molten drops can exist steadily in the process of welding with filler wire
and enter the weld pool under ideal conditions without unstable wire feed structure,
it is still likely to increase the instability of the keyhole, and even if the wire enters
the weld pool in an ideal and uniform contact transition (liquid bridge transition)
way, it is still likely to increase the instability of the keyhole.
8.5 Dynamic Dilution Behavior of Moving Weld Pools
in Welding with Filler Wires
8.5.1 Model of Dilution in Weld Pools
In laser welding with filler wires, the chemical compositions of the wire convectively
move along with the velocity field of the moving weld pool after entering the moving
weld pool. At the same time, the chemical compositions can diffuse inside the weld
pool. The calculation by using the thermophysical property calculation software
JMatPro shows that the diffusion coefficients of general wire metal elements in a
liquid weld pool are small, about 10
−8 m
2 /s orders of magnitude. Despite varying