9.3 Behaviors of Keyhole and Weld Pool in Vacuum Laser Welding
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Fig. 9.6 Comparison of keyhole depths in laser welding under vacuum and under atmospheric
pressure
(a) from 0 ms to 35 ms
(b) from 25 ms to 30 ms
Fig. 9.7 Evolution curves of keyhole depths in laser welding under vacuum and under atmospheric
pressure
9.3.2 Moving Weld Pool Flow Patterns
Evolutions of weld pool flow behaviors during vacuum laser welding are shown in
Figs. 9.8 and 9.9. The flow behaviors, which are quite similar to those during laser
welding under atmospheric pressure (see Figs. 9.10 and 9.11), are mainly as follows.
In areas near the keyhole opening on the weld pool surface, driven by recoil pressure
and Marangoni force, the fluid flows from the keyhole opening to the peripheral region
of weld pool. These flows can be always observed during the welding process. The
speed of these flows under vacuum is about 0.4 m/s, significantly smaller than the
speed (about 1.5 m/s) under atmospheric pressure. Besides, high-speed downward
flows driven by recoil pressure can be found near the keyhole wall. The magnitude