2.7 Numerical Simulation of Fluid Flow in Weld Pool …
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affected by the size of the eddies. The flow rate of weld pool fluid decreases rapidly
with the increase of the distance from the free surface. In terms of the driving force
of laser welding pool flow, there are Marangoni force, recoil force, buoyancy force,
gravity force and friction force of irregular eruption of keyhole gas flow. Because
this section focuses on the flow of laser welding pool in the case of full penetration, deep penetration laser welding is different from laser spot welding or partial
deep penetration laser welding whose weld pool flow is driven by the recoil force
generated by vapor evaporation. In the weld pool, if there is no auxiliary effect of
eddy flow, the flow driving force is only the buoyancy of the weld pool, so the flow
velocity is much lower than that of the weld pool surface. On the back of the weld
pool, the weld pool fluid is still the working surface, but because of the difference of
the temperature gradient between the positive and negative sides of the workpiece,
the surface tension gradient of the weld pool is smaller on the back side than on the
front side, and the size of the weld pool is also significantly smaller than that on the
front side. Therefore, the maximum flow velocity on the surface of the weld pool is
much lower than that on the front side, due to the Marangoni force.
Fig. 2.31 Effect of laser power on sizes of the cross section of TC4 titanium alloy CO 2 laser weld
joint
59
affected by the size of the eddies. The flow rate of weld pool fluid decreases rapidly
with the increase of the distance from the free surface. In terms of the driving force
of laser welding pool flow, there are Marangoni force, recoil force, buoyancy force,
gravity force and friction force of irregular eruption of keyhole gas flow. Because
this section focuses on the flow of laser welding pool in the case of full penetration, deep penetration laser welding is different from laser spot welding or partial
deep penetration laser welding whose weld pool flow is driven by the recoil force
generated by vapor evaporation. In the weld pool, if there is no auxiliary effect of
eddy flow, the flow driving force is only the buoyancy of the weld pool, so the flow
velocity is much lower than that of the weld pool surface. On the back of the weld
pool, the weld pool fluid is still the working surface, but because of the difference of
the temperature gradient between the positive and negative sides of the workpiece,
the surface tension gradient of the weld pool is smaller on the back side than on the
front side, and the size of the weld pool is also significantly smaller than that on the
front side. Therefore, the maximum flow velocity on the surface of the weld pool is
much lower than that on the front side, due to the Marangoni force.
Fig. 2.31 Effect of laser power on sizes of the cross section of TC4 titanium alloy CO 2 laser weld
joint