等离子体辐射

685
T Mos´cicki et al
∂ (ρv) + ∇ · (ρvv) = −∇p + ∇ · (τ ) + ρg + F ,
(3)
∂t
∂ ∂t
(ρYi)
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+
∇
·
(ρvYi)
=
∇
·
ρ Di,m ∇ Yi ,
(4)
where τ is the viscous tensor
τ = µ (∇v + ∇vT ) − 2 ∇ · vI .
2. Theoretical model
The model was solved in axial symmetry with the use of the
commercially available program Fluent 6.1 [7]. The set of
equations used by Fluent consists of equation of conservation
gas mixture, cp—specific heat at constant pressure, v—
the velocity vector, k—the thermal conductivity, T —the
temperature, κ—the absorption coefficient, IL—the laser
assumed that the plasma flowing from the keyhole was the
ionized iron vapour while the shielding gas was either argon
switched off in Fluent solver.
The source terms including the laser intensity and
radiation losses were inserted into the program. The
computations were made for stationary solutions. It was
1. Introduction
During laser welding, the interaction of intense laser radiation with a workpiece leads to the formation of a long, thin, cylindrical cavity in a metal, called a keyhole. Generation of a keyhole enables the laser beam to penetrate the workpiece and is essential for deep welding. The keyhole contains ionized metal vapour and is surrounded by molten material called the weld pool. The metal vapour which flows from the keyhole mixes with the shielding gas flowing from the opposite direction and forms a plasma plume over the keyhole mouth. The plasma plume has considerable influence on the processing conditions. Plasma strongly absorbs laser radiation and significantly changes energy transfer from the laser beam to a material. Although there is comprehensive literature on this topic [1–4] only few papers treat the problem theoretically [5, 6].
intensity, R—the radiation loss function, g—the gravity, F —
the external force, µ—the dynamic viscosity, Di,m—the mass diffusion coefficient, I —the unit tensor. All the material
INSTITUTE OF PHYSICS PUBLISHING J. Phys. D: Appl. Phys. 39 (2006) 685–692
JOURNAL OF PHYSICS D: APPLIED PHYSICS doi:10.1088/0022-3727/39/4/014
Modelling of plasma plume induced during laser welding
increases with the distance from the surface of the welded material.
This paper also deals with the theoretical modelling of the plasma plume induced during welding with a CO2 laser. However, our results are considerably different since they reveal the fact that in the case when argon is the shielding gas there are actually two plasmas—argon plasma and metal plasma. Additionally, the case when helium is the shielding gas is also computed and both cases, with argon and helium, are compared.
functions depend on the temperature and mass fraction only.
The thermal conductivity used in our paper contains
a reactive conductivity component and hence the energy
of mass, energy, momentum and the diffusion equation in the
form
∂ρ + ∇ · (ρv) = 0,
(1)
∂t
∂ (ρE)+∇ ∂t
·
(v(ρE+p))
=
∇
· (k∇T
+
τ eff
· v) + κIL
−
R,
(2)
0022-3727/06/040685+08$30.00 © 2006 IOP Publishing Ltd Printed in the UK
transport term associated with the species diffusion fluxes was
omitted. The pressure work term and the viscous dissipation
term are small and can be neglected but these terms cannot be
Received 17 October 2005, in final form 22 December 2005 Published 3 February 2006 Online at stacks.iop.org/JPhysD/39/685
Abstract A theoretical modelling of the plasma plume induced during welding of iron sheets with CO2 laser is presented. The set of equations consists of the equations of conservation of mass, energy, momentum and the diffusion equation and is solved with the use of the commercially available program Fluent 6.1. The computations are made for a laser power of 1700 W and for two shielding gases—argon and helium. The results show a significant difference between these two cases. When helium is used as the shielding gas, the plasma is much smaller and burns only where the metal vapour is slightly diluted by helium. In the case when argon is the shielding gas, there are actually two plasmas: argon plasma and metal plasma. The flowfield shows that the velocity increases in the hot region but only part of the mass flux enters the plasma core. In the case when argon is used as the shielding gas, the total absorption of the laser radiation amounts to 18–33% of the laser power depending on argon and iron vapour velocities. In the case of helium the total absorption is much lower and amounts to ∼5% of the laser power.
Numerical simulations of Chen and Wang [5,6] confirmed the assumption that the plasma plume induced during laser welding is a mixture of metal vapour and the shielding gas. Their calculations for the case when argon is used as the shielding gas showed that the stationary plasma plume is a mixture of metal vapour and argon and the fraction of argon
(5)
3
E is energy E = h − p/ρ + 0.5v2, h enthalpy h =
j Yj hj ,
hj =
T Tref
cp,j
dT
,
ρ
is
the
mass
density,
p—
the pressure, Yi—the mass fraction of iron vapour in the
T Mos´cicki, J Hoffman and Z Szyman´ ski
Institute of Fundamental Technological Research, S´ wietokrzyska 21, 00-049 Warsaw, Poland
E-mail: tmosc@ippt.gov.pl, jhoffman@ippt.gov.pl and zszym@ippt.gov.pl