2.8 Summary
63
2.8 Summary
On the basis of mass conservation equation, momentum conservation equation and
energy conservation equation in fluid mechanics, the component conservation equation and the κ–ε equation are added according to different simulation objectives.
Given boundary conditions of deep penetration laser welding, the control volume
method is used to discretize nonlinear partial differential equations, and with the
CFD commercial software FLUENT solver and SIMPLE algorithm, the keyhole and
the flow velocity field of the weld pool, and the mass fraction of components under
the interaction of inert auxiliary airflow, the keyhole erupting airflow and the upper
part of the keyhole during the deep penetration laser welding are simulated. The
calculation results are compared with the changes in the shape and size of the weld
of titanium alloy laser welding, or the changes of the plasma smoke photo of highspeed photography. It is found that the simulation results are basically consistent
with the experimental results.
Through the above study, the following conclusions are drawn:
(1) It is suitable to employ a combined heat source model consisting of the rotating
Gauss heat source and double-ellipsoid heat source to simulate the formation
of the keyhole in deep penetration laser welding of titanium alloy and the flow
velocity field of the weld pool. The body heat source model can reflect the
basic physical process in deep penetration laser welding, and also embodies
the simulation characteristics of the control volume method.
(2) In the process the deep penetration laser welding with full penetration, the
diameter of the keyhole is not very sensitive to the laser power, but the inclined
angle of the keyhole changes obviously with the increase of the laser welding
speed. The surface tension pressure caused by the curvature of the free surface
of the keyhole has a strong relationship with the welding speed under the
condition of constant airflow pressure in the keyhole. With the increase of the
welding speed, the stability of the keyhole gradually decreases.
(3) The flow velocity above the surface of the weld pool for deep penetration
laser welding is the largest, and Marangoni force plays a leading role in the
convective heat transfer on the surface of the weld pool. Under the action of
moving heat source, the Marangoni flow evolves from a symmetrical shape
around the laser heating center to a tadpole shape in which the direction of the
long axis is parallel to the welding direction of the weld pool. In a velocity
“virtual” circle, on the surface of the weld pool, with half of the width of
the weld pool as the radius, the flow velocity of the weld pool is relatively
large, and outside the “virtual” circle, the flow velocity of the fluid in the weld
pool decreases gradually. Inside the weld pool, the flow velocity of the fluid is
significantly lower than that on the surface of the weld pool, but much larger
than the welding speed. The flow velocity value of fluid on the back side of the
weld pool is larger than the flow velocity value of metal inside the weld pool.
(4) The shape and size of the weld pool have a good correspondence with the size
and position of the flow velocity vortex in the weld pool. The vortex in the weld
63
2.8 Summary
On the basis of mass conservation equation, momentum conservation equation and
energy conservation equation in fluid mechanics, the component conservation equation and the κ–ε equation are added according to different simulation objectives.
Given boundary conditions of deep penetration laser welding, the control volume
method is used to discretize nonlinear partial differential equations, and with the
CFD commercial software FLUENT solver and SIMPLE algorithm, the keyhole and
the flow velocity field of the weld pool, and the mass fraction of components under
the interaction of inert auxiliary airflow, the keyhole erupting airflow and the upper
part of the keyhole during the deep penetration laser welding are simulated. The
calculation results are compared with the changes in the shape and size of the weld
of titanium alloy laser welding, or the changes of the plasma smoke photo of highspeed photography. It is found that the simulation results are basically consistent
with the experimental results.
Through the above study, the following conclusions are drawn:
(1) It is suitable to employ a combined heat source model consisting of the rotating
Gauss heat source and double-ellipsoid heat source to simulate the formation
of the keyhole in deep penetration laser welding of titanium alloy and the flow
velocity field of the weld pool. The body heat source model can reflect the
basic physical process in deep penetration laser welding, and also embodies
the simulation characteristics of the control volume method.
(2) In the process the deep penetration laser welding with full penetration, the
diameter of the keyhole is not very sensitive to the laser power, but the inclined
angle of the keyhole changes obviously with the increase of the laser welding
speed. The surface tension pressure caused by the curvature of the free surface
of the keyhole has a strong relationship with the welding speed under the
condition of constant airflow pressure in the keyhole. With the increase of the
welding speed, the stability of the keyhole gradually decreases.
(3) The flow velocity above the surface of the weld pool for deep penetration
laser welding is the largest, and Marangoni force plays a leading role in the
convective heat transfer on the surface of the weld pool. Under the action of
moving heat source, the Marangoni flow evolves from a symmetrical shape
around the laser heating center to a tadpole shape in which the direction of the
long axis is parallel to the welding direction of the weld pool. In a velocity
“virtual” circle, on the surface of the weld pool, with half of the width of
the weld pool as the radius, the flow velocity of the weld pool is relatively
large, and outside the “virtual” circle, the flow velocity of the fluid in the weld
pool decreases gradually. Inside the weld pool, the flow velocity of the fluid is
significantly lower than that on the surface of the weld pool, but much larger
than the welding speed. The flow velocity value of fluid on the back side of the
weld pool is larger than the flow velocity value of metal inside the weld pool.
(4) The shape and size of the weld pool have a good correspondence with the size
and position of the flow velocity vortex in the weld pool. The vortex in the weld