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4 Simulation of Transient Keyhole and Weld Pool
4.4 Summary
(1) For the first time, the numerical simulation visually reveals the unsteady threedimensional transient keyhole behavior and the flow field inside the weld pool
that are in line with the X-ray penetration test result, and it is found that the
keyhole instability is closely related to the periodic boss on the pore wall. The
boss is caused by the imbalance of the recoil pressure, and surface tension of
the wall surface of the keyhole and the impact force of the fluid. The instability
of the keyhole leads to periodic closing of the keyhole, forming bubbles at the
bottom or middle of the keyhole, thus causing high-frequency oscillation of
the keyhole depth.
(2) By means of visual numerical simulation, stable keyhole shape and corresponding fluid dynamics behavior in the weld pool that are in line with the
X-ray are penetration test result obtained for the first time. When the keyhole
is stable, the flow velocity near the keyhole wall is almost parallel to the hole
wall and moves upward.
(3) Recoil pressure is the key factor to form keyhole and obtain deep penetration
laser welding. Intense and persistent evaporation is the main factor to distinguish conduction welding from deep-penetration welding. With less surface
tension, the oscillation frequency of keyhole depth is low, but the amplitude
of keyhole depth oscillation is higher. The thermal capillary force has great
influence on the speed of weld pool surface. The larger the thermal capillary
force, the more intense the weld pool surface movement, but the penetration depth is smaller. Although thermal capillary force has great influence on
keyhole and weld pool coupling, it is not the decisive factor for the formation of the weld pool. In general, the energy absorbed by the keyhole wall is
mainly determined by the initial Fresnel absorption and the first and second
multiple reflection Fresnel absorptions. With the same other parameters, the
alloy welding process with low thermal conductivity is more stable, but there
is no simple monotonic function relationship between the dynamic viscosity
and keyhole stability. Under certain conditions, appropriate improvement of
welding speed, reduction of welding power, and an increase of spot radius, i.e.,
reduction of line energy and power density can effectively reduce the amplitude
of keyhole depth oscillation, to obtain deep penetration laser welding without
keyhole oscillation. However, appropriately reducing line energy and power
density has a slight influence on the oscillation period of keyhole depth.
4 Simulation of Transient Keyhole and Weld Pool
4.4 Summary
(1) For the first time, the numerical simulation visually reveals the unsteady threedimensional transient keyhole behavior and the flow field inside the weld pool
that are in line with the X-ray penetration test result, and it is found that the
keyhole instability is closely related to the periodic boss on the pore wall. The
boss is caused by the imbalance of the recoil pressure, and surface tension of
the wall surface of the keyhole and the impact force of the fluid. The instability
of the keyhole leads to periodic closing of the keyhole, forming bubbles at the
bottom or middle of the keyhole, thus causing high-frequency oscillation of
the keyhole depth.
(2) By means of visual numerical simulation, stable keyhole shape and corresponding fluid dynamics behavior in the weld pool that are in line with the
X-ray are penetration test result obtained for the first time. When the keyhole
is stable, the flow velocity near the keyhole wall is almost parallel to the hole
wall and moves upward.
(3) Recoil pressure is the key factor to form keyhole and obtain deep penetration
laser welding. Intense and persistent evaporation is the main factor to distinguish conduction welding from deep-penetration welding. With less surface
tension, the oscillation frequency of keyhole depth is low, but the amplitude
of keyhole depth oscillation is higher. The thermal capillary force has great
influence on the speed of weld pool surface. The larger the thermal capillary
force, the more intense the weld pool surface movement, but the penetration depth is smaller. Although thermal capillary force has great influence on
keyhole and weld pool coupling, it is not the decisive factor for the formation of the weld pool. In general, the energy absorbed by the keyhole wall is
mainly determined by the initial Fresnel absorption and the first and second
multiple reflection Fresnel absorptions. With the same other parameters, the
alloy welding process with low thermal conductivity is more stable, but there
is no simple monotonic function relationship between the dynamic viscosity
and keyhole stability. Under certain conditions, appropriate improvement of
welding speed, reduction of welding power, and an increase of spot radius, i.e.,
reduction of line energy and power density can effectively reduce the amplitude
of keyhole depth oscillation, to obtain deep penetration laser welding without
keyhole oscillation. However, appropriately reducing line energy and power
density has a slight influence on the oscillation period of keyhole depth.