184
7 Keyhole and Weld Pool Dynamics in Dual-Beam Laser Welding
small residual stress deformation and good joint quality. Moreover, it can solve the
problems such as poor gap adaptability of conventional laser welding, large porosity
tendency and serious burning loss of alloying elements. According to the arrangement
of the beam, the dual beam laser welding can be divided into parallel and tandem
arrangement. If the lining of the focus between two beams is parallel to the welding
direction, it is called tandem dual beam welding. If the lining of the focus between
two beams is perpendicular to the welding direction, it is called parallel dual beam
welding.
Taking dual beam welding as the research object, this chapter studies the establishment of transient coupling model of keyhole and weld pool in dual beam laser
welding, focusing on the dynamics behavior of the keyhole and weld pool in dual
beam welding, and exploring the stability mechanism in dual beam laser welding.
7.2 Transient Coupling Model of Keyhole and Weld Pool
in Dual-Beam Welding
7.2.1 Control Equation of Transient Coupling Model
7.2.1.1 Basic Assumption
The dual beam laser welding process includes the very complex polyphase transition of plasma—gas—liquid—solid, free interface evolution and heat transfer flow
coupling in the weld pool. Therefore, the current numerical calculation needs to be
simplified. The metal vapor plume and fluid mechanics effect of protective gas on the
keyhole are ignored, and the condensation process of metal vapor is not considered.
For high power CO 2 laser welding, laser beam scattering and inverse bremsstrahlung
absorption of plasma matter a lot. In numerical simulations, these effects are characterized by artificially increasing the radius of the laser beam. However, in Nd:
YAG laser welding, due to the low ionization of metal vapor, inverse bremsstrahlung
absorption has little effect, thus scattering and refraction effect is simply considered. But for general low power dual beam laser welding process, the effect can be
neglected. What’s more, according to the method proposed by Ki et al., heat loss
due to evaporation near the keyhole wall is addressed. The model also considers the
thermal convection and thermal radiation of workpiece surface.
7.2.1.2 Control Equation
Assume that the metal liquid in the welding weld pool is incompressible fluid, the
density of liquid metal does not change during the solid-liquid phase transition.
Therefore, the conservation equation of mass, momentum and energy is expressed
as follows
7 Keyhole and Weld Pool Dynamics in Dual-Beam Laser Welding
small residual stress deformation and good joint quality. Moreover, it can solve the
problems such as poor gap adaptability of conventional laser welding, large porosity
tendency and serious burning loss of alloying elements. According to the arrangement
of the beam, the dual beam laser welding can be divided into parallel and tandem
arrangement. If the lining of the focus between two beams is parallel to the welding
direction, it is called tandem dual beam welding. If the lining of the focus between
two beams is perpendicular to the welding direction, it is called parallel dual beam
welding.
Taking dual beam welding as the research object, this chapter studies the establishment of transient coupling model of keyhole and weld pool in dual beam laser
welding, focusing on the dynamics behavior of the keyhole and weld pool in dual
beam welding, and exploring the stability mechanism in dual beam laser welding.
7.2 Transient Coupling Model of Keyhole and Weld Pool
in Dual-Beam Welding
7.2.1 Control Equation of Transient Coupling Model
7.2.1.1 Basic Assumption
The dual beam laser welding process includes the very complex polyphase transition of plasma—gas—liquid—solid, free interface evolution and heat transfer flow
coupling in the weld pool. Therefore, the current numerical calculation needs to be
simplified. The metal vapor plume and fluid mechanics effect of protective gas on the
keyhole are ignored, and the condensation process of metal vapor is not considered.
For high power CO 2 laser welding, laser beam scattering and inverse bremsstrahlung
absorption of plasma matter a lot. In numerical simulations, these effects are characterized by artificially increasing the radius of the laser beam. However, in Nd:
YAG laser welding, due to the low ionization of metal vapor, inverse bremsstrahlung
absorption has little effect, thus scattering and refraction effect is simply considered. But for general low power dual beam laser welding process, the effect can be
neglected. What’s more, according to the method proposed by Ki et al., heat loss
due to evaporation near the keyhole wall is addressed. The model also considers the
thermal convection and thermal radiation of workpiece surface.
7.2.1.2 Control Equation
Assume that the metal liquid in the welding weld pool is incompressible fluid, the
density of liquid metal does not change during the solid-liquid phase transition.
Therefore, the conservation equation of mass, momentum and energy is expressed
as follows