272
9 Dynamical Behaviors of Keyhole and Weld Pool …
If this condition is verified, there should be a shock wave at the exit of the keyhole
and the vapor flow is then choked. One can see that this condition is fulfilled when the
ambient pressure is lower than the critical one (Eq. (9.17)), i.e., when the penetration
depth saturation previously discussed occurs. To sum up, for industrial applications of
laser welding under vacuum, it is not necessary to have a very good level of vacuum,
as in the case of usual electron-beam welding. The critical ambient pressure theory
should be useful in the design and application of proper industrial vacuum chambers.
9.5 Summary
(1) The recently proposed surface pressure model which can be used to describe
the forces on the evaporation surface during laser welding is introduced. This
model has seriously considered the important effect of ambient pressure on the
evaporation surface, based on the recoil pressure model of Anismov. Therefore,
the surface pressure model can be used to calculate the surface pressure on the
evaporation surface in laser welding under various ambient pressures.
(2) The keyhole wall temperature in vacuum laser welding generally ranges from
2300 to 2500 K. The highest temperature (2900 K), 200 K lower than the
boiling point (3100 K) of the material under atmospheric conditions, usually
occurs at the top part of the humped keyhole wall irradiated directly by the laser
beam. Under atmospheric pressure, the average keyhole wall temperature can
reach more than 2900 K. The highest temperature is also located at the top
part of the humped keyhole wall directly exposed to laser beam. The value is
around the boiling point of the material under atmospheric pressure, or can
reach even more than 3400 K.
(3) The feature of transient keyhole oscillation in vacuum laser welding is much
similar to that in laser welding under atmospheric pressure. The keyhole oscillation is also caused by the failure of the recoil pressure acting on the keyhole
wall to offset the joint pressure of the surface tension, the impact force of
fluid flow and hydrostatic pressure. Additionally, the keyhole under vacuum
is much deeper than that under atmospheric pressure, causing larger absolute
oscillation amplitude. However, the keyhole is more stable because the ratio
of the oscillation amplitude to the keyhole depth under vacuum is smaller.
(4) The weld pool flow pattern in vacuum laser welding is also similar to that in
laser welding under atmospheric pressure. Specifically, the fluid flows from the
keyhole opening to the periphery of the weld pool driven by the recoil pressure
and Marangoni force, and flows rapidly downward near the keyhole wall driven
by the recoil pressure. However, the fluid flow in weld pool in vacuum laser
welding is more moderate than that under atmospheric pressure. The speed
of flow in surface weld pool under vacuum is only about 0.5 m/s, compared
to 1.5 m/s under atmospheric pressure. This could be the important reason
that the spatter defect is reduced in laser welding under vacuum. Additionally,
compared with the violent vortex at the keyhole bottom in the rear of weld pool
9 Dynamical Behaviors of Keyhole and Weld Pool …
If this condition is verified, there should be a shock wave at the exit of the keyhole
and the vapor flow is then choked. One can see that this condition is fulfilled when the
ambient pressure is lower than the critical one (Eq. (9.17)), i.e., when the penetration
depth saturation previously discussed occurs. To sum up, for industrial applications of
laser welding under vacuum, it is not necessary to have a very good level of vacuum,
as in the case of usual electron-beam welding. The critical ambient pressure theory
should be useful in the design and application of proper industrial vacuum chambers.
9.5 Summary
(1) The recently proposed surface pressure model which can be used to describe
the forces on the evaporation surface during laser welding is introduced. This
model has seriously considered the important effect of ambient pressure on the
evaporation surface, based on the recoil pressure model of Anismov. Therefore,
the surface pressure model can be used to calculate the surface pressure on the
evaporation surface in laser welding under various ambient pressures.
(2) The keyhole wall temperature in vacuum laser welding generally ranges from
2300 to 2500 K. The highest temperature (2900 K), 200 K lower than the
boiling point (3100 K) of the material under atmospheric conditions, usually
occurs at the top part of the humped keyhole wall irradiated directly by the laser
beam. Under atmospheric pressure, the average keyhole wall temperature can
reach more than 2900 K. The highest temperature is also located at the top
part of the humped keyhole wall directly exposed to laser beam. The value is
around the boiling point of the material under atmospheric pressure, or can
reach even more than 3400 K.
(3) The feature of transient keyhole oscillation in vacuum laser welding is much
similar to that in laser welding under atmospheric pressure. The keyhole oscillation is also caused by the failure of the recoil pressure acting on the keyhole
wall to offset the joint pressure of the surface tension, the impact force of
fluid flow and hydrostatic pressure. Additionally, the keyhole under vacuum
is much deeper than that under atmospheric pressure, causing larger absolute
oscillation amplitude. However, the keyhole is more stable because the ratio
of the oscillation amplitude to the keyhole depth under vacuum is smaller.
(4) The weld pool flow pattern in vacuum laser welding is also similar to that in
laser welding under atmospheric pressure. Specifically, the fluid flows from the
keyhole opening to the periphery of the weld pool driven by the recoil pressure
and Marangoni force, and flows rapidly downward near the keyhole wall driven
by the recoil pressure. However, the fluid flow in weld pool in vacuum laser
welding is more moderate than that under atmospheric pressure. The speed
of flow in surface weld pool under vacuum is only about 0.5 m/s, compared
to 1.5 m/s under atmospheric pressure. This could be the important reason
that the spatter defect is reduced in laser welding under vacuum. Additionally,
compared with the violent vortex at the keyhole bottom in the rear of weld pool