188
7 Keyhole and Weld Pool Dynamics in Dual-Beam Laser Welding
p r = 0.54 AB 0 (T )
−1/2 exp(−
U
kT
)
(7.13)
where, A—constant related to material;
B 0 —constant related to material;
U—latent heat of evaporation of each atom;
T —surface temperature of the keyhole;
k—Boltzmann’s constant.
On the free interface of the keyhole, due to Fresnel absorption, thermal convection,
radiation and evaporation, there exist the following temperature boundary conditions
k
∂ T
∂
n
= q − h(T − T ∞ ) − ε r σ
T
4
− T
4
∞
− ρV evp T v
(7.14)
where, q—laser energy density absorbed by Fresnel effect, determined by Formula
(7.8);
T v —evaporating temperature;
V evp —receding speed of the keyhole interface caused by evaporation, determined
in this study by the formula proposed by Ki, et al.
On other boundaries of the calculated region, there exist the following temperature
boundary conditions
k
∂ T
∂
n
= −h(T − T ∞ ) − ε r σ
T
4
− T
4
∞
(7.15)
where, h—convection coefficient;
ε r —black body radiation coefficient;
σ —Boltzmann constant.
7.3 Coupling Behavior of Keyhole and Weld Pool
in Dual-Beam Welding
7.3.1 Evolution Behavior of Dynamic Keyhole in Welding
Figures 7.2 and 7.3 show the temperature field distribution and morphological evolution of the keyhole in typical parallel dual beam laser welding respectively. In the
study, the material is 304 stainless steel, the welding speed is 2 m/min, the power
of each beam is 2 kW, the beam spot spacing is 0.5 mm, and the laser spot radius is
0.25 mm. As shown in Figs. 7.2a, b and 7.3a, b, when the welding starts, the workpiece
absorbs laser energy, with the temperature rising rapidly, and starts to evaporate. The
recoil pressure generated by metal evaporation causes the metal liquid surface to dip,
forming two relatively independent keyholes. At 0.83 ms, two separate keyholes are
connected to form a single keyhole, and the center of the keyhole opening is concave,
7 Keyhole and Weld Pool Dynamics in Dual-Beam Laser Welding
p r = 0.54 AB 0 (T )
−1/2 exp(−
U
kT
)
(7.13)
where, A—constant related to material;
B 0 —constant related to material;
U—latent heat of evaporation of each atom;
T —surface temperature of the keyhole;
k—Boltzmann’s constant.
On the free interface of the keyhole, due to Fresnel absorption, thermal convection,
radiation and evaporation, there exist the following temperature boundary conditions
k
∂ T
∂
n
= q − h(T − T ∞ ) − ε r σ
T
4
− T
4
∞
− ρV evp T v
(7.14)
where, q—laser energy density absorbed by Fresnel effect, determined by Formula
(7.8);
T v —evaporating temperature;
V evp —receding speed of the keyhole interface caused by evaporation, determined
in this study by the formula proposed by Ki, et al.
On other boundaries of the calculated region, there exist the following temperature
boundary conditions
k
∂ T
∂
n
= −h(T − T ∞ ) − ε r σ
T
4
− T
4
∞
(7.15)
where, h—convection coefficient;
ε r —black body radiation coefficient;
σ —Boltzmann constant.
7.3 Coupling Behavior of Keyhole and Weld Pool
in Dual-Beam Welding
7.3.1 Evolution Behavior of Dynamic Keyhole in Welding
Figures 7.2 and 7.3 show the temperature field distribution and morphological evolution of the keyhole in typical parallel dual beam laser welding respectively. In the
study, the material is 304 stainless steel, the welding speed is 2 m/min, the power
of each beam is 2 kW, the beam spot spacing is 0.5 mm, and the laser spot radius is
0.25 mm. As shown in Figs. 7.2a, b and 7.3a, b, when the welding starts, the workpiece
absorbs laser energy, with the temperature rising rapidly, and starts to evaporate. The
recoil pressure generated by metal evaporation causes the metal liquid surface to dip,
forming two relatively independent keyholes. At 0.83 ms, two separate keyholes are
connected to form a single keyhole, and the center of the keyhole opening is concave,