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Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy
Accepted Manuscript Title: Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy Author: Shengyong Pang Weidong Chen Jianxin Zhou Dunming Liao PII: DOI: Reference:
S0924-0136(14)00406-3 http://dx.doi.org/doi:10.1016/j.jmatprotec.2014.11.013 PROTEC 14177
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
10-2-2014 4-11-2014 5-11-2014
Please cite this article as: Pang, S., Chen, W., Zhou, J., Liao, D.,Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy, Journal of Materials Processing Technology (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy Shengyong Pang*, Weidong Chen, Jianxin Zhou, Dunming Liao
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State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University
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of Science and Technology (HUST ), Wuhan 430074, China
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ABSTRACT:
A three dimensional model incorporating heat transfer, fluid flow and the keyhole free surface,
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Marangoni forces, surface tension and recoil pressure is described. Possible multiple reflection
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Fresnel absorptions in complex keyhole profiles and laser plume interactions are also treated in the model. The results show that the keyhole is usually highly unstable and its depth undergoes severe
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oscillations in deep penetration tandem dual beam laser welding. The frequency of the depth oscillations is several kHz, which is comparable to that in single beam laser welding. Under the
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same heat input conditions, the amplitude of keyhole depth oscillations is smaller than that in
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single beam laser welding. Increasing the welding speed could improve the keyhole instability in dual beam laser welding. The mechanism of process stabilization of tandem dual beam laser welding over single beam laser welding is a joint effect of several beneficial physical factors, but not only the common known degassing effect. Good agreement is obtained between simulation
*
Corresponding author at: State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong
University of Science and Technology (HUST), Wuhan 430074, China. Tel.:+86 27 87541922; fax. : 86 27 87541922. E-mail address: [email protected] (Shengyong Pang). 1
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results and prior experiments.
KEYWORDS: Dual beam laser welding; keyhole instability; weld pool dynamics; numerical
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an
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simulation
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1. INTRODUTION Over the past decades, single beam laser welding has attracted more and more attentions owing to its remarkable advantages over other fusion welding processes. Many experimental and
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theoretical progresses have been made to understand its mechanisms (Dowden, 2009; Semak, 1997). However, high quality single beam laser welds are commonly obtained by trial and error
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experiments, especially during welding of materials with large heat conductivity or with volatile
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chemical elements, such as 5000 series or 7000 series aluminum alloys. For these alloys, good welding parameter window is usually narrow and difficult to be found from experiments, because
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the keyhole and weld pool may violently fluctuate in the welding process. To improve the quality
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of the welds, tandem dual beam laser welding has been proposed as a better alternative process for high performance joining of these alloys (Xie, 2002a, b). Nevertheless, the mechanisms of tandem
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dual beam laser welding are still not fully clear because the underlying physics is too complicated. In order to understand the mechanisms of dual beam laser welding, many experimental
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progresses have been made in past decades. Xie (2002a) suggested that a common keyhole or two
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separate keyholes can be produced in tandem dual beam laser welding of aluminum and steel alloys. Their study also suggested that when a common keyhole is created, welding defects such as porosity, surface holes, and undercut can be decreased, and the vapor plume fluctuation can be reduced in dual beam laser welding, as comparing to single beam laser welding. Iwase et al. (2000) studied the tandem dual beam Nd:YAG laser welding of Al-Mg aluminum alloys under different beam distances. It was suggested that the welding process becomes unstable when a single beam or a short beam distance is used, and the degassing effect of an enlarged keyhole aperture in dual
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beam laser welding is responsible for the improvement of process instability. Shibata et al. (2003) studied the pore characteristics during tandem dual beam Nd:YAG laser welding of automobile used aluminum alloys using a micro-focus in-situ X-Ray imaging apparatus. It was shown that the
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number of pores is closely related to the ratio of keyhole depth to the keyhole opening, and an enlarged keyhole opening and/or a shallow keyhole depth results in smaller pores. However, they
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did not give theoretical explanations of their results. Li et al. (2008) measured the plasma/vapor
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plume characteristics during single beam and dual beam CO2 laser welding processes, and found that the area variation of the plasma in single beam laser welding is 3-4 times larger than that of
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dual beam laser welding process, and the size of plasma/vapor plume is around 1.5-2 times larger.
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Punkari et al. (2003) investigated the effect of Mg content on dual beam YAG laser welding of thin thickness Al-Mg alloys, and found that weld penetration and the maximum welding speed
ed
allowing full penetration increase with Mg content. Narikiyo et al. (1999) found that when the incident angle of the laser beams are 30 or 45 degrees, transition from penetration with a keyhole
pt
bottom to penetration with two keyhole bottoms occurs in dual beam YAG laser welding. Deutsch
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et al. (2003) found that in dual beam YAG laser welding of Al-Mg alloys significant occluded porosity defects exist in partial penetration welds and suggested that the reason might result from violent keyhole instability. Despite aforementioned experimental progresses have been made to understand the mechanisms of tandem dual beam laser welding, currently mathematical modeling of tandem dual beam laser welding is very limited. Currently, mathematical modeling laser welding[0] considering the self-consistent coupling effect of keyhole instability and weld pool dynamics is challenging, and only few models have
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been presented for single beam laser welding (Cho et al., 2009; Dowden, 2009; Lee et al., 2002; Ki et al., 2002a, b; Pang et al., 2011; Zhou et al., 2006a, b; Zhou et al., 2007a, b; Zhou et al., 2008). Generally, it is more challenging to develop a model for the keyhole and weld pool behaviors in
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dual beam laser welding. Recently, several simplified heat transfer and fluid flow model of dual beam laser welding were reported. Chen and Kannatey-Asibu Jr (1996) developed a heat transfer
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and fluid flow model for conduction mode tandem dual beam laser welding and neglected the
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keyhole effect. Hou (1994) developed a finite element method based on heat transfer model for dual beam laser welding and also neglected the keyhole dynamics. Hu and Tsai (2003) proposed a
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three dimensional model of keyhole and weld pool dynamics during dual beam laser welding. The
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keyhole and weld pool evolutions were predicated. However, their model is only limited to stationary dual beam laser welding and the keyhole instability cannot be simulated. Zhou and Tsai
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(2010) later improved the model of Hu and Tsai (2003) by considering the effect of moving laser beam. However, the keyhole instability was still not simulated due to the capability limit of their
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model. To the author’s best knowledge, so far no theoretical modeling study has been reported to
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understand the coupling mechanisms of keyhole instability and weld pool dynamics in tandem dual beam laser welding.
In this paper, a comprehensive mathematical model is first developed to understand the
self-consistent transient keyhole instability and weld pool dynamics in tandem dual beam laser welding of aluminum alloys. Mechanisms of keyhole and weld pool behaviors under different process parameters are theoretically discussed and compared with experimental results.
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2. MATHEMATICAL MODELING The physics of laser welding involves very complex mesoscopic plume (or plasma plume)-liquid-solid multiphase transformation, free surface keyhole evolutions, and heat transfer
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and fluid flow in weld pool. It is imperative to make some simplifications or assumptions in order to simulate the laser welding process. Here, the hydrodynamic effect of vapor plume (or plasma
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plume) towards the keyhole and the effect of the amount of the shielding gas is neglected (Cho et
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al., 2009; Pang et al., 2011; Zhou et al., 2006a, b; Zhou et al., 2007a, b; Zhou et al., 2008). Moreover, the condensation of metallic vapor during laser welding is also neglected (Ki et al.,
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2002a, b; Pang et al., 2011).
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In the present model, the keyhole dynamics is mainly assumed to be governed by recoil pressure, surface tension, Marangoni shear stress and hydrodynamic pressure of molten liquid.
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Transient heat transfer, fluid flow and keyhole dynamics are considered. A level set method is developed to track the keyhole evolutions. For high power dual beam CO2 laser welding, energy
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dissipations of laser beam by scattering or Inverse Bremsstrahlung (IB) of plasma/vapor plume
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induced in laser welding might be significant. In the present study, these effects are approximated by artificially increasing the beam radius in numerical simulations. While for Nd:YAG laser welding, the IB effect is very small since ionization degree of vapor plume is very small, and only the scattering and refraction effect might be significant in case a very high power YAG laser was used (Dowden, 2009; Katayama, 2010). Here this effect is neglected in simulations since the power level in the present dual beam Nd:YAG laser welding processes, as discussed later, is relatively small. Besides, the latent heat of solidification and melting in laser welding process is
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treated by a commonly used temperature back compensation method (Pang et al., 2011). The heat dissipation during evaporation process of the keyhole wall is also included by considering the recession speed due to evaporation and the jump conditions across the Knudsen layer induced in
convection and radiation of workpiece are incorporated in the model.
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evaporation, following the theoretical model of Ki et al. (2002a). Also, heat dissipations due to
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The molten liquid in the weld pool during the welding process is assumed to be
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incompressible fluid flow, and the density is assumed to be const as the solid liquid phase transition occurs. Therefore, the mass, momentum and energy conservation equations can be
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expressed as (Pang et al., 2011):
U 0
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U C U U l U p l U U U g T Tref K K t
(1)
(2)
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T C p U T k T t
(3)
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where U is the velocity vector of the mixture flow, the density, p the pressure, g the
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gravitational vector, the thermal expansion coefficient, C p the thermal capacity, k the thermal conductivity, Tref the reference temperature, K the Carman-Kozeny coefficient. The Carman-Kozeny coefficient is related to the liquid fraction f l , which can be determined as (Zhou et al., 2006a):
fl 3d 2 K 180(1 fl ) 2
(4)
where d is proportional to the dendrite dimension, which is generally in the order of 10-2 cm. The inertial coefficient C , in equation (2), can be determined as (Rai et al., 2006): 7
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C 0.13 f l 3/ 2
(5)
The liquid fraction f l is assumed to be linearly varied with temperature, as follows:
T Tl Tl T Ts
(6)
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T Ts
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1 T Ts fl Tl Ts 0
Fig. 1. The schematic diagram of the ray tracing in tandem dual laser welding: (a) welding process;
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(b) multiple reflection absorption of laser beam in the keyhole The possible multiple reflections Fresnel absorptions on keyhole wall in tandem dual laser
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welding are described in Fig. 1, which can be treated with a robust ray tracing method (Pang et al.,
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2010). The detail calculation steps of this method are shown in Fig. 2, as follows: (a) the laser beam is firstly to be subdivided into small rays; (b) then the intersection points between each ray and the keyhole are calculated to determine the energy absorbing locations of the keyhole; (c) Further, the direction and energy of each ray after irradiation and the amount of the energy absorbed by the keyhole are calculated; repeat step (b) until each ray gets out of the keyhole (no intersection point between the ray and the keyhole) or the energy of the ray after irradiation is low enough. Since in tandem dual beam laser welding two laser spots are used in welding process, the
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Start
Subdivide the laser beam into small rays Calculate the intersection points between each ray and the keyhole
Y
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End
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Is the energy Of the ray is low enough ?
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N
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N
Calculate the direction and energy of each ray after irradiation and the energy absorbed by the keyhole
Y
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Is there a intersection point ?
Fig. 2. The flow chart of the robust ray tracing method.
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absorbed energy due to multiple Fresnel absorptions are different from that in single beam
be expressed as:
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welding. In tandem dual beam laser welding, the absorbed energy density q on keyhole wall can
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N q I1 (r1 , z1 )( I1 n) Fr (1 ) I m (r1 , z1 ) Fr ( m ) m 1
N I 2 (r2 , z2 )( I 2 n) Fr ( 2 ) I k (r2 , z2 ) Fr ( k )
(7)
k 1
I (r , z )
3Q 3r 2 exp( ) R2 R
1 1 (1 cos )2 2 2 cos 2 cos 2 Fr ( ) 1 ( ) 2 1 (1 cos ) 2 2 2 cos 2 cos 2
(8)
(9)
where I1 (r1 , z1 ) , I 2 (r2 , z2 ) are the first and the second beam energy density distribution before irradiation, I m ( r1 , z1 ) is the first beam energy density of the m -th reflections, I k (r2 , z2 ) is 9
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the second beam energy density of the k -th reflections, is the angle between the incident beam and the normal of the point on keyhole wall,
Fr is the Fresnel absorption coefficient, N
is the incident times of beams after multiple reflections, I is the unit vector of incident beams,
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n is the unit normal vector, is a coefficient related to type of lasers and process parameters, R is the effective beam radius and Q is the power density.
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To make the model tractable, the friction effect of vapor plume ejections on the keyhole wall
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is neglected in the present model. The movement of the keyhole free surface boundary is supposed to be mainly affected by surface tension, Marangoni force and recoil pressure, shear stress of the
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fluid flow, hydrostatic pressure as well as hydrodynamic pressure of the fluid flow in the weld
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pool. Following the sharp interface boundary condition derived in the literature (Anisimov, 1968), the free surface boundary conditions can be derived and written as (Pang et al., 2011):
p f pr 2l n U n f
l n t1 t2 n 0 0
l n t1 t2 0 t1 l n t1 t2 n 0
T
2
T
T
T
1
T t1 t2 (11)
T
2
0 s t1 s t2 T 0 0 n t1 t2 t2 0 0 0 0
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n t1
(10)
U n 0 0 n t U 0 U 0 t t n t t
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U
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1
2
where p f is the pressure boundary value at the keyhole interface, pr the recoil pressure, surface tension coefficient,
U
f
the viscous stress boundary value at the keyhole interface,
l the viscosity for the fluid flow in the weld pool, and t1 , t2 are unit orthogonal tangent vectors perpendicular to the normal of the free surface. The recoil pressure model of this study is described as (Anisimov, 1968; Matsunawa et al., 1997; Semak et al., 1997):
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T Tv pr 0.54 p0 exp H v RTTv
(12)
where p0 is atmospheric pressure, R is universal gas constant, H v is evaporation latent heat and Tv is boiling temperature of Aluminum alloys.
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Some other important boundary conditions for energy conservation equation (3) should be
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given here. On the keyhole surface, due to the influence of the multiple reflections, thermal convection, radiation and evaporation, the heat fluxes boundary condition on the free surface is as
(13)
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T k q h(T T ) r (T 4 T4 ) VevpTv n
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follows:
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where q is the heat fluxes due to multiple reflections Fresnel absorption, which determined by equation (10) , h is the air convection coefficient,
r the black body radiation coefficient, s
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the Stephan-Boltzmann const and T the ambient temperature. qevp is the heat dissipation due
qevp Vevp Lv
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to evaporation, which can be formulated as:
(14)
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where is the Vevp free surface recession speed due to evaporation and Lv is the evaporation latent heat of the material. In this study, the recession speed is determined by considering the Knudsen jump conditions across the keyhole interface, following the studies of Ki et al. (2002a, b).
For other surface besides the keyhole free surface, only the effects of heat convection and radiation are considered. The heat fluxes boundary condition of these surfaces can be determined as:
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k
T h T T s r T 4 T4 n
(15)
3. EXPERIMENTAL AND SIMULATION PARAMETERS
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Fig. 3 shows the schematic of tandem dual beam laser welding. In Fig. 3, two beam spots with the same power density were approximate normally irradiated onto the workpiece. The
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material of workpiece was 5052 aluminum alloy. The chemical compositions of this alloy were
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shown in Table 1. D was the beam distance between the two spots and R was the effective radius of the laser spot. The temperature dependent thermal properties of 5052 aluminum alloy were
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computed using JMatPro (Sente Software, UK), then averaged as const according to temperature,
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and listed in Table 2. Other important parameters such as convection and radiation coefficients are
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pt
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given in Table 2.
Fig. 3. Schematic view of tandem dual beam laser welding process.
Table 1
5052 aluminum alloy chemical composition Element
Si
Fe
Mn
Mg
Ti
Else
Al
%
0.4
0.2
0.15
2.0
0.15
0.15
Balance
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Table 2
Symbol
Value
Density(kgm-3)
2600
Specific heat(Jkg-1K-1)
Cp
1000
Heat conductivity(Wm-1K-1)
k
90
Solidus temperature (K)
Ts
Liquidus temperature (K)
Tl
Boiling point (K)
Tv
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Property
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831
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Melting latent(Jkg-1)
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Tm
Evaporation latent(Jkg-1)
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Physical and thermal properties of 5052 aluminum alloy.
889
2790 5.03×105 1.07×107
Surface tension temperature coefficient(Nm-1K-1)
d / dT
-0.35×10-3
Kinematic viscosity (Nm-2s)
l
6.15×10-7
Black body radiation coefficient
r
0.3
Surface tension(Nm-1)
1.0
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Lv
The process parameters of dual beam laser welding experiments are listed in Table 3. The
welding experiments are used from Xie (2002b) and Shibata et al. (2003). The experimental process parameters are listed in Table 2. For tandem dual beam CO2 laser welding processes, the laser was split into two beam spots with 0.25mm radius. Helium gas was used as shielding gas for bead on plate welding experiments, and a flow rate of 20 L/min was used. For tandem dual beam Nd:YAG laser welding processes, the laser was split into two beam spots with 0.15 mm radius. 13
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The argon gas was used as shielding gas and the gas flow rate was 15 L/min. For all the process parameters, the workpiece were not penetrated. Table 3
Welding speed
Interbeam
Beam radius
Laser power
No.
(m/min)
distance (mm)
(mm)
(kW)
1
2.54
1.2
0.25
3.0+3.0
2
3.81
1.2
0.25
3
5.08
1.2
0.25
4
6.25
1.2
5
3.81
0
6
4.0
0.36
7
4.0
0.6
8
4.0
3.0+3.0
Xie, 2002b
0.25
3.0+3.0
Xie, 2002b
0.25
3.0+3.0
Xie, 2002b
0.15
2.0+2.0
Shibata et al., 2003
0.15
2.0+2.0
Shibata et al., 2003
0.15
2.0+2.0
Shibata et al., 2003
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Xie, 2002b
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Xie, 2002b
3.0+3.0
ed
pt 1.0
From reference
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Process
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Process parameters for dual beam laser welding of aluminum alloys.
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In order to treat the energy dissipations of laser beam by scattering or Inverse Bremsstrahlung
(IB) of plasma/vapor plume and the heat radiation effect of the high temperature plasma/vapor plume on the keyhole wall in high power CO2 laser welding, the beam radius used in the simulations was enlarged as 0.5 mm. This data was obtained from numerical experiences by comparing with experimental results. For Nd:YAG laser welding, it was shown that the plume scattering or refraction in Nd:YAG laser welding may be significant, when the laser power is very high (typically in the order of 10 kW) (Dowden, 2009; Katayama, 2010). However, in the present
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study the laser power of Nd:YAG laser (around 2 kW) was relatively small. Hence, the same beam radius (0.15 mm) was adopted in the simulations of dual beam Nd:YAG laser welding. To accurately simulate the mesoscopic keyhole and weld pool behaviors, the computational grid step
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was chosen to be 1/10 of the beam radius in all simulations.
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4. RESULTS
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4.1. Keyhole profile evolution
Keyhole profiles under different beam distances in tandem dual beam laser welding are
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investigated in Figure 4. It is observed that as the beam distance increases, the welding mechanism
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could be a common keyhole with a single tip (mechanism I), a common aperture with two keyhole tips (mechanism II) and two separated keyholes (mechanism III). When a common aperture with
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two keyhole tips forms, as shown in Fig. 4 (b), the depth of the keyhole tip of the lag beam (in the right) is obviously deeper than that of the lead one (in the left). Besides, the keyhole is far from
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stable and many humps occur on the free surface keyhole wall (Fig. 4).
(a)
(b)
(c)
Fig. 4. Longitudinal section views of the calculated keyhole profiles under different beam distance at 30.71 ms welding time (Process No. 6 ~ 8): (a) 0.36 mm, (b) 0.6 mm, (c) 1.0 mm. To provide insight into mechanism II, the three dimensional transient keyhole profiles during a typical dual beam laser welding process is further investigated in Figure 5. It is shown that after 15
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the very beginning of the welding process, the apertures of the two keyholes become to be merged together and a common keyhole with two tips forms (Fig. 5 (c) ~ (d)). At this time, the penetration speed of the keyhole tip of the lag beam becomes larger than the lead one. In addition, the keyhole
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profile is highly irregular, and many humps occur on the keyhole wall except the very beginning
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stage of keyhole formation.
(b)
(c)
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(a)
(d)
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Fig. 5. Longitudinal section views of the calculated transient keyhole profiles during dual beam
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CO2 laser welding (Process No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 54.002 ms.
4.2. Keyhole depth evolution The keyhole instability (Figure 5) could lead to the keyhole depth fluctuations during laser
welding. Fig. 6 shows the calculated maximum keyhole depth evolutions during tandem dual beam laser welding under different welding speed conditions. It could be found that the maximum keyhole depth significantly fluctuates during welding processes, except at the very beginning stage. As the welding speed increases from 2.54m/min to 6.25m/min, the average amplitude of
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depth fluctuations gradually becomes smaller. This suggests that increasing weld speed may
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improve keyhole instability in dual beam laser welding.
(b)
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an
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(a)
(d)
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(c)
Fig. 6. The calculated maximum keyhole depth evolutions during dual beam CO2 laser welding
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under difference welding speeds (Process No. 1 ~ 4): (a) 2.54 m/min, (b) 3.81 m/min, (c) 5.08 m/min, (d) 6.25 m/min.
To better understand the keyhole dynamics, the keyhole instability is quantitatively
characterized using its depth fluctuations. The frequencies of keyhole depth fluctuations of typical dual beam laser welding processes are counted from Fig. 7, which are around 1.4 kHz. By comparing with single beam laser welding, it can be found that dual beam laser welding can significantly decrease the amplitude of keyhole depth fluctuations under the same heat input condition (Figure 6 (b) and 8 (a)). Moreover, the frequencies in single beam laser welding are
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cr
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around 1.7 kHz (Figure 8 (b)), which are slightly larger than those in dual beam laser welding.
(a)
(b)
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Fig. 7. The calculated maximum keyhole depth evolutions during dual beam CO2 laser welding
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(a)
ed
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from 45 ms welding time to 55 ms welding time: (a) Process No.1, (b) Process No.2.
(b)
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Fig. 8. The calculated maximum keyhole depth evolutions during single beam CO2 laser welding (Process No. 5): (a) from 0 ms to 35 ms, (b) from 25 ms to 35 ms.
The keyhole depth evolutions of the two keyhole tips during dual beam laser welding
(mechanism II) are further quantitatively studied (Fig. 9). As seen from Fig. 9, the amplitude of depth fluctuations of the keyhole tip of the lag beam is obviously larger than that of the lead beam. For the tip of the lag beam, the frequencies are in the order of 1.4 kHz, while for the tip of the lead beam, the frequencies are approximately 2~3 kHz.
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ip t
(b)
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(a)
Fig. 9. The calculated maximum depth evolutions of separated keyhole tips during dual beam CO2
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laser welding (Process No. 2): (a) from 0 ms to 55 ms, (b) from 45 ms to 55 ms.
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4.3. Transient velocity evolution
Fig. 10 and 11 show the top and longitudinal section views of transient velocity field during
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dual beam laser welding. It can be found that, when the two keyholes merge to one (Mechanism
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II), the surface velocity near the keyhole aperture is more complex and violent than flows on the other parts of the weld pool surface (Fig. 10 (c) ~ (d)). Near the front keyhole wall, there are some
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downward violent fluid flows (Fig. 11). Moreover, there are some characteristic flows behind the
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blind rear wall of the keyhole of the lag beam (Fig. 11 (c) ~ (d)). First, along the top part of rear keyhole wall, there are upward fluid flows. Second, behind the blind rear wall of the keyhole tip of the lag beam, there are downward fluid flows, which can produce a vortex flow behind the blind rear of the keyhole tip.
(a)
(b)
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(c)
(d)
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Fig. 10. Top views of the calculated velocity field during dual beam CO2 laser welding (Process
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us
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No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 46.117 ms.
(b)
(d)
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(c)
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(a)
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Fig. 11. Longitudinal section views of the calculated velocity field during dual beam CO2 laser welding (Process No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 46.117 ms.
By comparing transient velocity evolutions with those in single beam laser welding (Fig. 12
~ 13), it is obvious that there are significant different fluid flow patterns of the weld pool under the same heat input conditions. For example, there are more violent fluid flows in the weld pool during single beam laser welding than dual beam laser welding. Also the maximum magnitude of downward fluid flow near the keyhole wall in single beam laser welding is obviously larger than that of dual beam laser welding. Nevertheless, some similar fluid flow patterns of weld pool, for 20
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instance, the downward motion and the vortex fluid flow behind the rear wall of the keyhole, exist
(b)
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(a)
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in both dual beam and single beam laser welding.
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(c)
(d)
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Fig. 12. Top views of the calculated velocity field during single beam CO2 laser welding (Process
pt
ed
No. 5). (a) 2.415 ms, (b) 7.922 ms, (c) 30.196 ms, (d) 36.447 ms.
(b)
(c)
(d)
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(a)
Fig. 13. Longitudinal section views of the calculated velocity field during single beam CO2 laser welding (Process No. 5): (a) 2.415 ms, (b) 7.922 ms, (c) 30.196 ms, (d) 36.447 ms.
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5. DISCUSSION 5.1. Dynamic keyhole behavior The presented results in Fig. 5 indicate that many dynamic humps occur on keyhole wall in
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tandem dual beam laser welding, which is similar to single beam laser welding (Pang et al., 2011). These humps are mainly induced by the force imbalance between surface tension, recoil pressure,
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hydrodynamic and hydrostatic pressure of fluid flow (Pang et al., 2011). These phenomena
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suggest that the keyhole is still unstable in deep penetration tandem dual beam laser welding. Fig. 14 shows some snapshots of bubble formation during the dual beam laser welding process, which
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demonstrates that the keyhole is highly unstable and the instability may lead to porosity defects in
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welds. This unstable nature of keyhole shown here is consistent with previous experimental investigations. For example, Xie (2002a), Iwase et al. (2000), Shibata et al. (2003) and Deutsch et
ed
al. (2003) found small irregular pores induced by keyhole dynamics are existed in tandem dual beam laser welds. Li et al. (2008) proved that the plasma plume is also existed in dynamic
Ac ce
pt
oscillating during tandem dual beam laser welding.
(a)
(b)
Fig. 14. Bubble formations due to depth instability of the keyhole tip of the lag beam during dual beam CO2 laser welding (Process No. 2): (a) 27.91 ms, (b) 48.50 ms. The quantitative characterization of dynamic keyhole depths (Section 4.2) reveals that
22
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welding process might become more stable with higher welding speeds during tandem dual beam laser welding (Fig. 6). This can be simply explained as follows. By increase of the welding speed, the keyhole depth becomes smaller, the keyhole shape becomes shallower. Hence the surface
ip t
tension force on the dynamic keyhole wall becomes smaller, which means the keyhole becomes more stable in the keyhole welding mode. This theoretical finding is also in good agreement with
cr
the X-Ray imaging experimental results of Shibata et al. (2003). In their study, fewer pores are
us
found in high welding speed conditions during dual beam laser welding, which indeed suggests that the keyhole is more stable. Interestingly, the role of welding speed on dynamic keyhole in
an
dual beam laser welding is similar to that in single beam laser welding. Recently, both theoretical
M
and experimental studies have shown that increasing the welding speed could produce a more stable welding process during single beam laser welding of aluminum and other alloys (Dowden,
ed
2009; Pang et al., 2011). The present theoretical finding about the effect of welding speed can provide a theoretical basis for process optimizations in tandem dual beam laser welding.
pt
Different frequencies of keyhole depth oscillations are found in tandem dual beam laser
Ac ce
welding and single beam laser welding (Fig. 7 and 8). In the current dual beam laser welding, since the laser is evenly subdivided into two beam spots, the power density in dual beam laser welding is smaller than that in single beam laser welding. Hence, a large recoil pressure can be expected in single beam laser welding. The downward movement of keyhole humps can be faster and a larger frequency (1.7 kHz) is theoretically observed in single beam laser welding than in dual beam laser welding. Generally, it is believed that keyhole instability is closely associated with vapor plume oscillations. Previous experimental studies of vapor plume comparisons between dual
23
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beam and single beam laser welding indicated that the vapor plume is more stable in dual beam laser welding as comparing to single beam laser welding (Li et al., 2008; Xie, 2002a, b), which seems to be in good agreement with the present theoretical results of keyhole instability. Moreover,
ip t
different frequencies of two keyhole tips are also theoretically found in dual beam laser welding (Fig. 9, Mechanism II). This difference is mainly attributed to the depth variations of two keyhole
cr
tips observed in Fig. 4(b). Note that the power density is the same for the two spots in the current
us
investigations, therefore, the dynamic humps on the keyhole of the lead beam can move to the keyhole bottom earlier. Hence, a smaller period of keyhole depth variations can be observed. It
an
should be pointed out that aforementioned two different plume oscillations have been
M
experimentally found by Xie (2002a) in dual beam laser welding, however, no theoretical explanations were given. Here, a clear explanation of this phenomenon is proposed by using the
ed
proposed comprehensive model. In a word, the present quantitative characterization of dynamic keyhole provides new insights
pt
into the mechanisms of keyhole instability in dual beam laser welding. These results can serve as a
Ac ce
theoretical basis for on-contact monitoring the dynamic keyhole behavior in dual beam laser welding.
5.2. Mechanisms of weld pool dynamics Complex weld pool dynamics are predicated during tandem dual beam laser welding (Fig. 10 and 11). Generally, the fluid flows of weld pool are closely associated with hydrodynamic factors on the keyhole wall and welding parameters. In the present model, the recoil pressure and
24
Page 24 of 35
Marangoni shear stress are considered as two main driving forces. Some fluid flows, such as the surface fluid flows around the keyhole aperture (Fig. 10 (c) and 10 (d)), upward fluid flows along the top part of rear keyhole wall (Fig. 11 (c) and 11 (d)), are mainly driven by the recoil pressure
ip t
and possibly the Marangoni shear stress. While the others, such as high speed downward fluid flows near the front keyhole wall surface (Fig. 11), downward fluid flows behind the blind rear
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wall of the keyhole tip of the lag beam (Fig. 11 (c) and 11 (d)), are mainly driven by the recoil
us
pressure.
Under the same heat input conditions, less turbulent fluid flows are observed in dual beam
an
laser welding than in single beam laser welding (Fig. 10~13). This qualitatively agrees well with
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previous experimental studies that a more stable weld pool can be obtained during dual beam laser welding (Shibata et al., 2003; Xie, 2002a, b). Previously, Relative to single beam laser welding, a
ed
major improving factor of fluid flow of dual beam laser welding was the difference of vapor plume dynamics between the two processes (Shibata et al., 2003; Xie, 2002a, b). Nevertheless, the
pt
friction effect of vapor plume on the keyhole wall is neglected in present model. Hence, other
Ac ce
mechanisms should be also helpful for the stabilization of weld pool in dual beam laser welding. This study proposes some other possible factors. First, the tandem arrangement of two spots leads to a smaller power density, even if the heat input is the same. Hence, a smaller recoil pressure could be expected during laser material interactions. Second, a larger keyhole aperture in dual beam laser welding (mechanism II) leads to a smaller surface tension force. Therefore, the closure possibility of keyhole aperture is lower, and the movement possibility of the keyhole wall near the aperture towards the beam center is lower. This effect could also contribute to the stabilization of
25
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surface fluid flow of weld pool. Third, the tandem arrangement of two beam spots will produce a wider weld pool in dual beam laser welding as comparing to single beam laser welding. It will also be helpful for the stabilization of weld pool in dual beam laser welding.
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In the present study, the mechanisms of transient fluid flows in weld pool during dual beam laser welding are investigated and the differences of fluid flows between dual beam and single
cr
beam laser welding are analyzed. It is shown that besides the dragging effect of vapor plume, the
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lower power density, the smaller surface tension force and the larger surface area of weld pool also
M
5.3. Mechanisms of process stabilization
an
play important roles in the stabilization of weld pool in dual beam laser welding.
Previous experimental results indicated that tandem dual beam laser welding can
ed
significantly improve the process stability of single beam laser welding (Xie, 2002a, b). The mechanism of process stabilization was considered to be the degassing effect of the enlarged
pt
keyhole aperture in dual beam laser welding (Iwase et al., 2000).
Ac ce
In present study, the effect of vapor plume frictions has been neglected in the theoretical
model. Nevertheless, a more stable welding process has also been obtained from the present simulation results. The present results indicate that dual beam technique can obviously decrease the keyhole depth. A shallower keyhole is usually less vulnerable to the pinching effect of surface tension and hydrodynamic pressure of molten liquid, which could lead to a more stable process. Moreover, the fluid flow of weld pool could become more stable in dual beam laser welding, since the tandem arrangement of two spots can lower the power density, reduce the surface tension force
26
Page 26 of 35
and increase the surface area of weld pool. All these physical factors are beneficial for the process stabilization. Hence, the mechanism of process stabilization of dual beam laser welding over single beam laser welding was not solely caused by the degassing effect. Rather, it is produced by
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the joint effect of several beneficial physical factors.
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6. MODEL VALIDATION
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To validate the simulated weld pool dimensions, Fig. 15 compares the predicated penetration depth between simulation and experimental results (Shibata et al., 2003) under different beam
an
distance conditions (Process No. 6, 7 and 8 in Table 2). It could be seen that a reasonable
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agreement is obtained. As shown in Fig. 16, the comparisons of the length of the keyhole aperture between experiments (Shibata et al., 2003) and simulations are further made (Process No. 6, 7 and
ed
8). A good agreement is obtained. Besides, a comparison on the penetration depth under different welding speed conditions is shown in Fig. 17 (Process No. 1, 2, 3 and 4) (Xie, 2002b), a direct
pt
comparison of cross sectional shape of weld bead and keyhole aperture between simulation and
Ac ce
experimental results under a typical process (Process No. 2) is shown in Fig. 18 (Xie, 2002b). Both good agreements are obtained. To further validate the mesh size, a comparison on calculating weld profiles between different mesh sizes are also made (Fig. 19, Process No. 2), consistency results are obtained, which prove that the choice of mesh size in the present research is reasonable.
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Page 27 of 35
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Fig. 15. Penetration depth comparisons of dual beam YAG laser welding.
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an
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(Process No. 6, 7 and 8).
ed
Fig. 16. Comparisons of length of keyhole aperture in dual beam YAG laser welding.
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(Process No. 6, 7 and 8).
Fig. 17. Penetration depth comparisons of dual beam CO2 laser welding. (Process No. 1, 2, 3 and 4).
28
Page 28 of 35
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Fig. 18. Comparison of cross sectional shape of weld bead and keyhole aperture between
M
an
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simulation and experimental results (Process No. 2).
ed
Fig. 19. Comparison on weld profiles between different mesh size. (Process No. 2) To validate the proposed welding mechanisms and keyhole profiles, a comparison between
pt
simulation (Fig. 4) and experimental keyhole shapes (Fig. 18) is further performed. It could be
Ac ce
found that a reasonable agreement is obtained. Note also that the lead laser beam has a preheating effect on the lag one. Due to this effect, the depth of the keyhole tip of the lag beam is obviously larger in mechanism II. Due to the resolutions of experimental images, it is difficult to distinguish the two keyhole tips. Nevertheless, the predicated keyhole dimensions and shapes are still reasonable as comparing to Fig. 18. The preheating effect observed in present theoretical study has also been experimentally found in other high density beam welding processes. In tandem dual beam electron beam welding, Arata and Nabegata (1978) observed the two distinct keyhole tips
29
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with a common keyhole aperture using high speed X-Ray imaging and clearly found that the tip of
(a)
cr
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the lag beam is deeper than that of the lead one (shown in Fig. 20).
(b)
(c)
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0.6mm, (c) 1.0mm.
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Fig. 20. Experimental results of welding mechanisms (Shibata et al., 2003): (a) 0.36 mm, (b)
Based on the above comparisons, it is concluded that the proposed model can reasonably
7. CONCLUSIONS
ed
M
predicate the keyhole and weld pool behaviors in tandem dual beam laser welding.
pt
1. A comprehensive three dimensional model is first developed for understanding the transient
Ac ce
keyhole instability and weld pool dynamics during tandem dual beam laser welding of 5052 aluminum alloys. The modeling results of the proposed model reasonably agree with the experiments.
2. As the beam distances increases, the welding mechanism could be a common keyhole with a single tip, a common aperture with two keyhole tips and two separated keyholes. When a common keyhole with two tips is produced under certain process conditions, the depth of the keyhole tip of the lag beam is larger than that of the lead one due to the preheating effect. 3. The frequency of keyhole depth oscillations in tandem dual beam laser welding is in the order 30
Page 30 of 35
of several kHz, which is comparable to that of single beam laser welding. The amplitude of keyhole depth oscillations is smaller than that of single beam laser welding. The keyhole depth instability can be also improved by increasing the welding speed.
ip t
4. Under the same heat input conditions, there are less turbulent fluid flows in the weld pool in tandem dual beam laser welding than in single beam laser welding. The lower power density, the
us
contributes to the stabilization of weld pool in the welding process.
cr
smaller surface tension force and the larger surface area of weld pool in dual beam laser welding
5. The mechanism of process stabilization of dual beam laser welding over single beam laser
an
welding was not solely caused by the degassing effect of larger keyhole but also by a combination
M
of several beneficial physical factors.
ed
ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China
pt
(No. 51105153), the Talent Recruitment Foundation of Huazhong University of Science and
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Technology (No. 0124110034) and the National Basic Research Program of Chine (973 Program, No. 2014CB046703). The authors are grateful to Wang Wen for helping to improve the manuscript.
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Theoretical Physics 27, 182-183.
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Arata, Y., Nabegata, E., 1978. Tandem Electron Beam Welding (Report-I). Transactions of JWRI 7, 101-109.
Chen, T., Kannatey-Asibu Jr, E., 1996. Convection pattern and weld pool shape during conduction-mode dual
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beam laser welding, PhD thesis, University of Michigan, Ann Arbor, USA, pp.259-264.
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Cho, J., Na, S., 2006. Implementation of real-time multiple reflection and Fresnel absorption of laser beam in
keyhole. Journal of Physics D: Applied Physics 39, 5372.
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Cho, J., Na, S., 2009. Three-dimensional analysis of molten pool in GMA-laser hybrid welding. Welding Journal
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88, 35-43.
Deutsch, M., Punkari, A., Weckman, D., Kerr, H., 2003. Weldability of 1.6 mm thick aluminium alloy 5182 sheet
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by single and dual beam Nd: YAG laser welding. Science and Technology of Welding & Joining 8, 246-256.
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Dowden, J., 2009. The Theory of Laser Materials Processing: Heat and Mass Transfer in Modern Technology.
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Hou, C., 1994. Finite element simulation of dual-beam laser welding using ANSYS, PhD thesis, Pittsburgh, USA,
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Hu, J., Tsai, H., 2003. Fluid flow and weld pool dynamics in dual-beam laser keyhole welding, in:
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Iwase, T., Sakamoto, H., Shibata, K., Hohenberger, B., Dausinger, F., 2000. Dual-focus technique for high-power
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Kaplan, A., 1994. A model of deep penetration laser welding based on calculation of the keyhole profile. Journal of
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Katayama, S., Kawahito, Y., Mizutani, M., 2010. Elucidation of laser welding phenomena and factors affecting
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Ki, H., Mazumder, J., Mohanty, P.S., 2002a. Modeling of laser keyhole welding: Part I. Mathematical modeling,
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numerical methodology, role of recoil pressure, multiple reflections, and free surface evolution. Metallurgical and
Materials Transactions A 33, 1817-1830.
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Ki, H., Mazumder, J., Mohanty, P.S., 2002b. Modeling of laser keyhole welding: Part II. Simulation of keyhole
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evolution, velocity, temperature profile, and experimental verification. Metallurgical and Materials Transactions A
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Lee, J.Y., Ko, S.H., Farson, D.F., Yoo, C.D., 2002. Mechanism of keyhole formation and stability in stationary
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Li L., Chen Y., Tao W., 2008. Research on dual-beam welding characteristics of aluminum alloy. Chinese Journal
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Narikiyo, T., Miura, H., Fujinaga, S., Takenaka, H., Ohmori, A., Inoue, K., 1999. Keyhole bottom separation in
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Pang, S., Chen, L., Chen, T., Yin, Y., Hu, L., Liu, J., 2010. A calculation method of laser energy density of arbitrary
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Punkari, A., Weckman, D., Kerr, H., 2003. Effects of magnesium content on dual beam Nd: YAG laser welding of
ip t
Al–Mg alloys. Science and Technology of Welding & Joining 8, 269-281.
Rai, R., DebRoy, T., 2006. Tailoring weld geometry during keyhole mode laser welding using a genetic algorithm
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Semak, V., Matsunawa, A., 1997. The role of recoil pressure in energy balance during laser materials processing.
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Shibata, K., Iwase, T., Sakamoto, H., Dausinger, F.H., Hohenberger, B., Mueller, M., Matsunawa, A., Seto, N.,
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Xie, J., 2002a. Dual beam laser welding. Welding Journal 81, 223-230.
Xie, J., 2002b. Weld morphology and thermal modeling in dual-beam laser welding. Welding Journal 81, 283-290.
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Zhou, J., Tsai, H. L., Lehnhoff, T., 2006a. Investigation of transport phenomena and defect formation in pulsed
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laser keyhole welding of zinc-coated steels. Journal of Physics D: Applied Physics 39, 5338-5355.
Zhou, J., Tsai, H. L., Wang, P. C., 2006b. Transport Phenomena and Keyhole Dynamics during Pulsed Laser
Welding. Journal of Heat Transfer 128, 680-690.
Zhou, J., Tsai, H. L., 2007a. Effects of electromagnetic force on melt flow and porosity prevention in pulsed laser
keyhole welding. International Journal of Heat and Mass Transfer 50, 2217-2235.
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129, 1014-1024.
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Zhou, J., Tsai, H. L., 2010. Investigation of Fluid Flow and Heat Transfer in 3D Dual-Beam Laser Keyhole
Welding, in: Proc.Int.MSEC '10, The ASME Manufacturing Engineering Division, Pennsylvania, USA, vol. 2, pp.
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S0924-0136(14)00406-3 http://dx.doi.org/doi:10.1016/j.jmatprotec.2014.11.013 PROTEC 14177
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
10-2-2014 4-11-2014 5-11-2014
Please cite this article as: Pang, S., Chen, W., Zhou, J., Liao, D.,Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy, Journal of Materials Processing Technology (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Self-consistent modeling of keyhole and weld pool dynamics in tandem dual beam laser welding of aluminum alloy Shengyong Pang*, Weidong Chen, Jianxin Zhou, Dunming Liao
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State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University
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of Science and Technology (HUST ), Wuhan 430074, China
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ABSTRACT:
A three dimensional model incorporating heat transfer, fluid flow and the keyhole free surface,
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Marangoni forces, surface tension and recoil pressure is described. Possible multiple reflection
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Fresnel absorptions in complex keyhole profiles and laser plume interactions are also treated in the model. The results show that the keyhole is usually highly unstable and its depth undergoes severe
ed
oscillations in deep penetration tandem dual beam laser welding. The frequency of the depth oscillations is several kHz, which is comparable to that in single beam laser welding. Under the
pt
same heat input conditions, the amplitude of keyhole depth oscillations is smaller than that in
Ac ce
single beam laser welding. Increasing the welding speed could improve the keyhole instability in dual beam laser welding. The mechanism of process stabilization of tandem dual beam laser welding over single beam laser welding is a joint effect of several beneficial physical factors, but not only the common known degassing effect. Good agreement is obtained between simulation
*
Corresponding author at: State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong
University of Science and Technology (HUST), Wuhan 430074, China. Tel.:+86 27 87541922; fax. : 86 27 87541922. E-mail address: [email protected] (Shengyong Pang). 1
Page 1 of 35
results and prior experiments.
KEYWORDS: Dual beam laser welding; keyhole instability; weld pool dynamics; numerical
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pt
ed
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an
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simulation
2
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1. INTRODUTION Over the past decades, single beam laser welding has attracted more and more attentions owing to its remarkable advantages over other fusion welding processes. Many experimental and
ip t
theoretical progresses have been made to understand its mechanisms (Dowden, 2009; Semak, 1997). However, high quality single beam laser welds are commonly obtained by trial and error
cr
experiments, especially during welding of materials with large heat conductivity or with volatile
us
chemical elements, such as 5000 series or 7000 series aluminum alloys. For these alloys, good welding parameter window is usually narrow and difficult to be found from experiments, because
an
the keyhole and weld pool may violently fluctuate in the welding process. To improve the quality
M
of the welds, tandem dual beam laser welding has been proposed as a better alternative process for high performance joining of these alloys (Xie, 2002a, b). Nevertheless, the mechanisms of tandem
ed
dual beam laser welding are still not fully clear because the underlying physics is too complicated. In order to understand the mechanisms of dual beam laser welding, many experimental
pt
progresses have been made in past decades. Xie (2002a) suggested that a common keyhole or two
Ac ce
separate keyholes can be produced in tandem dual beam laser welding of aluminum and steel alloys. Their study also suggested that when a common keyhole is created, welding defects such as porosity, surface holes, and undercut can be decreased, and the vapor plume fluctuation can be reduced in dual beam laser welding, as comparing to single beam laser welding. Iwase et al. (2000) studied the tandem dual beam Nd:YAG laser welding of Al-Mg aluminum alloys under different beam distances. It was suggested that the welding process becomes unstable when a single beam or a short beam distance is used, and the degassing effect of an enlarged keyhole aperture in dual
3
Page 3 of 35
beam laser welding is responsible for the improvement of process instability. Shibata et al. (2003) studied the pore characteristics during tandem dual beam Nd:YAG laser welding of automobile used aluminum alloys using a micro-focus in-situ X-Ray imaging apparatus. It was shown that the
ip t
number of pores is closely related to the ratio of keyhole depth to the keyhole opening, and an enlarged keyhole opening and/or a shallow keyhole depth results in smaller pores. However, they
cr
did not give theoretical explanations of their results. Li et al. (2008) measured the plasma/vapor
us
plume characteristics during single beam and dual beam CO2 laser welding processes, and found that the area variation of the plasma in single beam laser welding is 3-4 times larger than that of
an
dual beam laser welding process, and the size of plasma/vapor plume is around 1.5-2 times larger.
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Punkari et al. (2003) investigated the effect of Mg content on dual beam YAG laser welding of thin thickness Al-Mg alloys, and found that weld penetration and the maximum welding speed
ed
allowing full penetration increase with Mg content. Narikiyo et al. (1999) found that when the incident angle of the laser beams are 30 or 45 degrees, transition from penetration with a keyhole
pt
bottom to penetration with two keyhole bottoms occurs in dual beam YAG laser welding. Deutsch
Ac ce
et al. (2003) found that in dual beam YAG laser welding of Al-Mg alloys significant occluded porosity defects exist in partial penetration welds and suggested that the reason might result from violent keyhole instability. Despite aforementioned experimental progresses have been made to understand the mechanisms of tandem dual beam laser welding, currently mathematical modeling of tandem dual beam laser welding is very limited. Currently, mathematical modeling laser welding[0] considering the self-consistent coupling effect of keyhole instability and weld pool dynamics is challenging, and only few models have
4
Page 4 of 35
been presented for single beam laser welding (Cho et al., 2009; Dowden, 2009; Lee et al., 2002; Ki et al., 2002a, b; Pang et al., 2011; Zhou et al., 2006a, b; Zhou et al., 2007a, b; Zhou et al., 2008). Generally, it is more challenging to develop a model for the keyhole and weld pool behaviors in
ip t
dual beam laser welding. Recently, several simplified heat transfer and fluid flow model of dual beam laser welding were reported. Chen and Kannatey-Asibu Jr (1996) developed a heat transfer
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and fluid flow model for conduction mode tandem dual beam laser welding and neglected the
us
keyhole effect. Hou (1994) developed a finite element method based on heat transfer model for dual beam laser welding and also neglected the keyhole dynamics. Hu and Tsai (2003) proposed a
an
three dimensional model of keyhole and weld pool dynamics during dual beam laser welding. The
M
keyhole and weld pool evolutions were predicated. However, their model is only limited to stationary dual beam laser welding and the keyhole instability cannot be simulated. Zhou and Tsai
ed
(2010) later improved the model of Hu and Tsai (2003) by considering the effect of moving laser beam. However, the keyhole instability was still not simulated due to the capability limit of their
pt
model. To the author’s best knowledge, so far no theoretical modeling study has been reported to
Ac ce
understand the coupling mechanisms of keyhole instability and weld pool dynamics in tandem dual beam laser welding.
In this paper, a comprehensive mathematical model is first developed to understand the
self-consistent transient keyhole instability and weld pool dynamics in tandem dual beam laser welding of aluminum alloys. Mechanisms of keyhole and weld pool behaviors under different process parameters are theoretically discussed and compared with experimental results.
5
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2. MATHEMATICAL MODELING The physics of laser welding involves very complex mesoscopic plume (or plasma plume)-liquid-solid multiphase transformation, free surface keyhole evolutions, and heat transfer
ip t
and fluid flow in weld pool. It is imperative to make some simplifications or assumptions in order to simulate the laser welding process. Here, the hydrodynamic effect of vapor plume (or plasma
cr
plume) towards the keyhole and the effect of the amount of the shielding gas is neglected (Cho et
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al., 2009; Pang et al., 2011; Zhou et al., 2006a, b; Zhou et al., 2007a, b; Zhou et al., 2008). Moreover, the condensation of metallic vapor during laser welding is also neglected (Ki et al.,
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2002a, b; Pang et al., 2011).
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In the present model, the keyhole dynamics is mainly assumed to be governed by recoil pressure, surface tension, Marangoni shear stress and hydrodynamic pressure of molten liquid.
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Transient heat transfer, fluid flow and keyhole dynamics are considered. A level set method is developed to track the keyhole evolutions. For high power dual beam CO2 laser welding, energy
pt
dissipations of laser beam by scattering or Inverse Bremsstrahlung (IB) of plasma/vapor plume
Ac ce
induced in laser welding might be significant. In the present study, these effects are approximated by artificially increasing the beam radius in numerical simulations. While for Nd:YAG laser welding, the IB effect is very small since ionization degree of vapor plume is very small, and only the scattering and refraction effect might be significant in case a very high power YAG laser was used (Dowden, 2009; Katayama, 2010). Here this effect is neglected in simulations since the power level in the present dual beam Nd:YAG laser welding processes, as discussed later, is relatively small. Besides, the latent heat of solidification and melting in laser welding process is
6
Page 6 of 35
treated by a commonly used temperature back compensation method (Pang et al., 2011). The heat dissipation during evaporation process of the keyhole wall is also included by considering the recession speed due to evaporation and the jump conditions across the Knudsen layer induced in
convection and radiation of workpiece are incorporated in the model.
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evaporation, following the theoretical model of Ki et al. (2002a). Also, heat dissipations due to
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The molten liquid in the weld pool during the welding process is assumed to be
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incompressible fluid flow, and the density is assumed to be const as the solid liquid phase transition occurs. Therefore, the mass, momentum and energy conservation equations can be
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expressed as (Pang et al., 2011):
U 0
M
U C U U l U p l U U U g T Tref K K t
(1)
(2)
ed
T C p U T k T t
(3)
pt
where U is the velocity vector of the mixture flow, the density, p the pressure, g the
Ac ce
gravitational vector, the thermal expansion coefficient, C p the thermal capacity, k the thermal conductivity, Tref the reference temperature, K the Carman-Kozeny coefficient. The Carman-Kozeny coefficient is related to the liquid fraction f l , which can be determined as (Zhou et al., 2006a):
fl 3d 2 K 180(1 fl ) 2
(4)
where d is proportional to the dendrite dimension, which is generally in the order of 10-2 cm. The inertial coefficient C , in equation (2), can be determined as (Rai et al., 2006): 7
Page 7 of 35
C 0.13 f l 3/ 2
(5)
The liquid fraction f l is assumed to be linearly varied with temperature, as follows:
T Tl Tl T Ts
(6)
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T Ts
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us
cr
1 T Ts fl Tl Ts 0
Fig. 1. The schematic diagram of the ray tracing in tandem dual laser welding: (a) welding process;
ed
(b) multiple reflection absorption of laser beam in the keyhole The possible multiple reflections Fresnel absorptions on keyhole wall in tandem dual laser
pt
welding are described in Fig. 1, which can be treated with a robust ray tracing method (Pang et al.,
Ac ce
2010). The detail calculation steps of this method are shown in Fig. 2, as follows: (a) the laser beam is firstly to be subdivided into small rays; (b) then the intersection points between each ray and the keyhole are calculated to determine the energy absorbing locations of the keyhole; (c) Further, the direction and energy of each ray after irradiation and the amount of the energy absorbed by the keyhole are calculated; repeat step (b) until each ray gets out of the keyhole (no intersection point between the ray and the keyhole) or the energy of the ray after irradiation is low enough. Since in tandem dual beam laser welding two laser spots are used in welding process, the
8
Page 8 of 35
Start
Subdivide the laser beam into small rays Calculate the intersection points between each ray and the keyhole
Y
M
End
an
Is the energy Of the ray is low enough ?
us
N
cr
N
Calculate the direction and energy of each ray after irradiation and the energy absorbed by the keyhole
Y
ip t
Is there a intersection point ?
Fig. 2. The flow chart of the robust ray tracing method.
ed
absorbed energy due to multiple Fresnel absorptions are different from that in single beam
be expressed as:
pt
welding. In tandem dual beam laser welding, the absorbed energy density q on keyhole wall can
Ac ce
N q I1 (r1 , z1 )( I1 n) Fr (1 ) I m (r1 , z1 ) Fr ( m ) m 1
N I 2 (r2 , z2 )( I 2 n) Fr ( 2 ) I k (r2 , z2 ) Fr ( k )
(7)
k 1
I (r , z )
3Q 3r 2 exp( ) R2 R
1 1 (1 cos )2 2 2 cos 2 cos 2 Fr ( ) 1 ( ) 2 1 (1 cos ) 2 2 2 cos 2 cos 2
(8)
(9)
where I1 (r1 , z1 ) , I 2 (r2 , z2 ) are the first and the second beam energy density distribution before irradiation, I m ( r1 , z1 ) is the first beam energy density of the m -th reflections, I k (r2 , z2 ) is 9
Page 9 of 35
the second beam energy density of the k -th reflections, is the angle between the incident beam and the normal of the point on keyhole wall,
Fr is the Fresnel absorption coefficient, N
is the incident times of beams after multiple reflections, I is the unit vector of incident beams,
ip t
n is the unit normal vector, is a coefficient related to type of lasers and process parameters, R is the effective beam radius and Q is the power density.
cr
To make the model tractable, the friction effect of vapor plume ejections on the keyhole wall
us
is neglected in the present model. The movement of the keyhole free surface boundary is supposed to be mainly affected by surface tension, Marangoni force and recoil pressure, shear stress of the
an
fluid flow, hydrostatic pressure as well as hydrodynamic pressure of the fluid flow in the weld
M
pool. Following the sharp interface boundary condition derived in the literature (Anisimov, 1968), the free surface boundary conditions can be derived and written as (Pang et al., 2011):
p f pr 2l n U n f
l n t1 t2 n 0 0
l n t1 t2 0 t1 l n t1 t2 n 0
T
2
T
T
T
1
T t1 t2 (11)
T
2
0 s t1 s t2 T 0 0 n t1 t2 t2 0 0 0 0
Ac ce
n t1
(10)
U n 0 0 n t U 0 U 0 t t n t t
pt
U
ed
1
2
where p f is the pressure boundary value at the keyhole interface, pr the recoil pressure, surface tension coefficient,
U
f
the viscous stress boundary value at the keyhole interface,
l the viscosity for the fluid flow in the weld pool, and t1 , t2 are unit orthogonal tangent vectors perpendicular to the normal of the free surface. The recoil pressure model of this study is described as (Anisimov, 1968; Matsunawa et al., 1997; Semak et al., 1997):
10
Page 10 of 35
T Tv pr 0.54 p0 exp H v RTTv
(12)
where p0 is atmospheric pressure, R is universal gas constant, H v is evaporation latent heat and Tv is boiling temperature of Aluminum alloys.
ip t
Some other important boundary conditions for energy conservation equation (3) should be
cr
given here. On the keyhole surface, due to the influence of the multiple reflections, thermal convection, radiation and evaporation, the heat fluxes boundary condition on the free surface is as
(13)
an
T k q h(T T ) r (T 4 T4 ) VevpTv n
us
follows:
M
where q is the heat fluxes due to multiple reflections Fresnel absorption, which determined by equation (10) , h is the air convection coefficient,
r the black body radiation coefficient, s
ed
the Stephan-Boltzmann const and T the ambient temperature. qevp is the heat dissipation due
qevp Vevp Lv
pt
to evaporation, which can be formulated as:
(14)
Ac ce
where is the Vevp free surface recession speed due to evaporation and Lv is the evaporation latent heat of the material. In this study, the recession speed is determined by considering the Knudsen jump conditions across the keyhole interface, following the studies of Ki et al. (2002a, b).
For other surface besides the keyhole free surface, only the effects of heat convection and radiation are considered. The heat fluxes boundary condition of these surfaces can be determined as:
11
Page 11 of 35
k
T h T T s r T 4 T4 n
(15)
3. EXPERIMENTAL AND SIMULATION PARAMETERS
ip t
Fig. 3 shows the schematic of tandem dual beam laser welding. In Fig. 3, two beam spots with the same power density were approximate normally irradiated onto the workpiece. The
cr
material of workpiece was 5052 aluminum alloy. The chemical compositions of this alloy were
us
shown in Table 1. D was the beam distance between the two spots and R was the effective radius of the laser spot. The temperature dependent thermal properties of 5052 aluminum alloy were
an
computed using JMatPro (Sente Software, UK), then averaged as const according to temperature,
M
and listed in Table 2. Other important parameters such as convection and radiation coefficients are
Ac ce
pt
ed
given in Table 2.
Fig. 3. Schematic view of tandem dual beam laser welding process.
Table 1
5052 aluminum alloy chemical composition Element
Si
Fe
Mn
Mg
Ti
Else
Al
%
0.4
0.2
0.15
2.0
0.15
0.15
Balance
12
Page 12 of 35
Table 2
Symbol
Value
Density(kgm-3)
2600
Specific heat(Jkg-1K-1)
Cp
1000
Heat conductivity(Wm-1K-1)
k
90
Solidus temperature (K)
Ts
Liquidus temperature (K)
Tl
Boiling point (K)
Tv
cr
Property
us
831
an
Melting latent(Jkg-1)
M
Tm
Evaporation latent(Jkg-1)
ip t
Physical and thermal properties of 5052 aluminum alloy.
889
2790 5.03×105 1.07×107
Surface tension temperature coefficient(Nm-1K-1)
d / dT
-0.35×10-3
Kinematic viscosity (Nm-2s)
l
6.15×10-7
Black body radiation coefficient
r
0.3
Surface tension(Nm-1)
1.0
Ac ce
pt
ed
Lv
The process parameters of dual beam laser welding experiments are listed in Table 3. The
welding experiments are used from Xie (2002b) and Shibata et al. (2003). The experimental process parameters are listed in Table 2. For tandem dual beam CO2 laser welding processes, the laser was split into two beam spots with 0.25mm radius. Helium gas was used as shielding gas for bead on plate welding experiments, and a flow rate of 20 L/min was used. For tandem dual beam Nd:YAG laser welding processes, the laser was split into two beam spots with 0.15 mm radius. 13
Page 13 of 35
The argon gas was used as shielding gas and the gas flow rate was 15 L/min. For all the process parameters, the workpiece were not penetrated. Table 3
Welding speed
Interbeam
Beam radius
Laser power
No.
(m/min)
distance (mm)
(mm)
(kW)
1
2.54
1.2
0.25
3.0+3.0
2
3.81
1.2
0.25
3
5.08
1.2
0.25
4
6.25
1.2
5
3.81
0
6
4.0
0.36
7
4.0
0.6
8
4.0
3.0+3.0
Xie, 2002b
0.25
3.0+3.0
Xie, 2002b
0.25
3.0+3.0
Xie, 2002b
0.15
2.0+2.0
Shibata et al., 2003
0.15
2.0+2.0
Shibata et al., 2003
0.15
2.0+2.0
Shibata et al., 2003
an
Xie, 2002b
M
us
Xie, 2002b
3.0+3.0
ed
pt 1.0
From reference
cr
Process
ip t
Process parameters for dual beam laser welding of aluminum alloys.
Ac ce
In order to treat the energy dissipations of laser beam by scattering or Inverse Bremsstrahlung
(IB) of plasma/vapor plume and the heat radiation effect of the high temperature plasma/vapor plume on the keyhole wall in high power CO2 laser welding, the beam radius used in the simulations was enlarged as 0.5 mm. This data was obtained from numerical experiences by comparing with experimental results. For Nd:YAG laser welding, it was shown that the plume scattering or refraction in Nd:YAG laser welding may be significant, when the laser power is very high (typically in the order of 10 kW) (Dowden, 2009; Katayama, 2010). However, in the present
14
Page 14 of 35
study the laser power of Nd:YAG laser (around 2 kW) was relatively small. Hence, the same beam radius (0.15 mm) was adopted in the simulations of dual beam Nd:YAG laser welding. To accurately simulate the mesoscopic keyhole and weld pool behaviors, the computational grid step
ip t
was chosen to be 1/10 of the beam radius in all simulations.
cr
4. RESULTS
us
4.1. Keyhole profile evolution
Keyhole profiles under different beam distances in tandem dual beam laser welding are
an
investigated in Figure 4. It is observed that as the beam distance increases, the welding mechanism
M
could be a common keyhole with a single tip (mechanism I), a common aperture with two keyhole tips (mechanism II) and two separated keyholes (mechanism III). When a common aperture with
ed
two keyhole tips forms, as shown in Fig. 4 (b), the depth of the keyhole tip of the lag beam (in the right) is obviously deeper than that of the lead one (in the left). Besides, the keyhole is far from
Ac ce
pt
stable and many humps occur on the free surface keyhole wall (Fig. 4).
(a)
(b)
(c)
Fig. 4. Longitudinal section views of the calculated keyhole profiles under different beam distance at 30.71 ms welding time (Process No. 6 ~ 8): (a) 0.36 mm, (b) 0.6 mm, (c) 1.0 mm. To provide insight into mechanism II, the three dimensional transient keyhole profiles during a typical dual beam laser welding process is further investigated in Figure 5. It is shown that after 15
Page 15 of 35
the very beginning of the welding process, the apertures of the two keyholes become to be merged together and a common keyhole with two tips forms (Fig. 5 (c) ~ (d)). At this time, the penetration speed of the keyhole tip of the lag beam becomes larger than the lead one. In addition, the keyhole
ip t
profile is highly irregular, and many humps occur on the keyhole wall except the very beginning
us
cr
stage of keyhole formation.
(b)
(c)
ed
M
an
(a)
(d)
pt
Fig. 5. Longitudinal section views of the calculated transient keyhole profiles during dual beam
Ac ce
CO2 laser welding (Process No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 54.002 ms.
4.2. Keyhole depth evolution The keyhole instability (Figure 5) could lead to the keyhole depth fluctuations during laser
welding. Fig. 6 shows the calculated maximum keyhole depth evolutions during tandem dual beam laser welding under different welding speed conditions. It could be found that the maximum keyhole depth significantly fluctuates during welding processes, except at the very beginning stage. As the welding speed increases from 2.54m/min to 6.25m/min, the average amplitude of
16
Page 16 of 35
depth fluctuations gradually becomes smaller. This suggests that increasing weld speed may
cr
ip t
improve keyhole instability in dual beam laser welding.
(b)
M
an
us
(a)
(d)
ed
(c)
Fig. 6. The calculated maximum keyhole depth evolutions during dual beam CO2 laser welding
Ac ce
pt
under difference welding speeds (Process No. 1 ~ 4): (a) 2.54 m/min, (b) 3.81 m/min, (c) 5.08 m/min, (d) 6.25 m/min.
To better understand the keyhole dynamics, the keyhole instability is quantitatively
characterized using its depth fluctuations. The frequencies of keyhole depth fluctuations of typical dual beam laser welding processes are counted from Fig. 7, which are around 1.4 kHz. By comparing with single beam laser welding, it can be found that dual beam laser welding can significantly decrease the amplitude of keyhole depth fluctuations under the same heat input condition (Figure 6 (b) and 8 (a)). Moreover, the frequencies in single beam laser welding are
17
Page 17 of 35
cr
ip t
around 1.7 kHz (Figure 8 (b)), which are slightly larger than those in dual beam laser welding.
(a)
(b)
us
Fig. 7. The calculated maximum keyhole depth evolutions during dual beam CO2 laser welding
pt
(a)
ed
M
an
from 45 ms welding time to 55 ms welding time: (a) Process No.1, (b) Process No.2.
(b)
Ac ce
Fig. 8. The calculated maximum keyhole depth evolutions during single beam CO2 laser welding (Process No. 5): (a) from 0 ms to 35 ms, (b) from 25 ms to 35 ms.
The keyhole depth evolutions of the two keyhole tips during dual beam laser welding
(mechanism II) are further quantitatively studied (Fig. 9). As seen from Fig. 9, the amplitude of depth fluctuations of the keyhole tip of the lag beam is obviously larger than that of the lead beam. For the tip of the lag beam, the frequencies are in the order of 1.4 kHz, while for the tip of the lead beam, the frequencies are approximately 2~3 kHz.
18
Page 18 of 35
ip t
(b)
cr
(a)
Fig. 9. The calculated maximum depth evolutions of separated keyhole tips during dual beam CO2
us
laser welding (Process No. 2): (a) from 0 ms to 55 ms, (b) from 45 ms to 55 ms.
an
4.3. Transient velocity evolution
Fig. 10 and 11 show the top and longitudinal section views of transient velocity field during
M
dual beam laser welding. It can be found that, when the two keyholes merge to one (Mechanism
ed
II), the surface velocity near the keyhole aperture is more complex and violent than flows on the other parts of the weld pool surface (Fig. 10 (c) ~ (d)). Near the front keyhole wall, there are some
pt
downward violent fluid flows (Fig. 11). Moreover, there are some characteristic flows behind the
Ac ce
blind rear wall of the keyhole of the lag beam (Fig. 11 (c) ~ (d)). First, along the top part of rear keyhole wall, there are upward fluid flows. Second, behind the blind rear wall of the keyhole tip of the lag beam, there are downward fluid flows, which can produce a vortex flow behind the blind rear of the keyhole tip.
(a)
(b)
19
Page 19 of 35
(c)
(d)
ip t
Fig. 10. Top views of the calculated velocity field during dual beam CO2 laser welding (Process
an
us
cr
No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 46.117 ms.
(b)
(d)
pt
(c)
ed
M
(a)
Ac ce
Fig. 11. Longitudinal section views of the calculated velocity field during dual beam CO2 laser welding (Process No. 2): (a) 0.150 ms, (b) 3.282 ms, (c) 37.907 ms, (d) 46.117 ms.
By comparing transient velocity evolutions with those in single beam laser welding (Fig. 12
~ 13), it is obvious that there are significant different fluid flow patterns of the weld pool under the same heat input conditions. For example, there are more violent fluid flows in the weld pool during single beam laser welding than dual beam laser welding. Also the maximum magnitude of downward fluid flow near the keyhole wall in single beam laser welding is obviously larger than that of dual beam laser welding. Nevertheless, some similar fluid flow patterns of weld pool, for 20
Page 20 of 35
instance, the downward motion and the vortex fluid flow behind the rear wall of the keyhole, exist
(b)
us
cr
(a)
ip t
in both dual beam and single beam laser welding.
an
(c)
(d)
M
Fig. 12. Top views of the calculated velocity field during single beam CO2 laser welding (Process
pt
ed
No. 5). (a) 2.415 ms, (b) 7.922 ms, (c) 30.196 ms, (d) 36.447 ms.
(b)
(c)
(d)
Ac ce
(a)
Fig. 13. Longitudinal section views of the calculated velocity field during single beam CO2 laser welding (Process No. 5): (a) 2.415 ms, (b) 7.922 ms, (c) 30.196 ms, (d) 36.447 ms.
21
Page 21 of 35
5. DISCUSSION 5.1. Dynamic keyhole behavior The presented results in Fig. 5 indicate that many dynamic humps occur on keyhole wall in
ip t
tandem dual beam laser welding, which is similar to single beam laser welding (Pang et al., 2011). These humps are mainly induced by the force imbalance between surface tension, recoil pressure,
cr
hydrodynamic and hydrostatic pressure of fluid flow (Pang et al., 2011). These phenomena
us
suggest that the keyhole is still unstable in deep penetration tandem dual beam laser welding. Fig. 14 shows some snapshots of bubble formation during the dual beam laser welding process, which
an
demonstrates that the keyhole is highly unstable and the instability may lead to porosity defects in
M
welds. This unstable nature of keyhole shown here is consistent with previous experimental investigations. For example, Xie (2002a), Iwase et al. (2000), Shibata et al. (2003) and Deutsch et
ed
al. (2003) found small irregular pores induced by keyhole dynamics are existed in tandem dual beam laser welds. Li et al. (2008) proved that the plasma plume is also existed in dynamic
Ac ce
pt
oscillating during tandem dual beam laser welding.
(a)
(b)
Fig. 14. Bubble formations due to depth instability of the keyhole tip of the lag beam during dual beam CO2 laser welding (Process No. 2): (a) 27.91 ms, (b) 48.50 ms. The quantitative characterization of dynamic keyhole depths (Section 4.2) reveals that
22
Page 22 of 35
welding process might become more stable with higher welding speeds during tandem dual beam laser welding (Fig. 6). This can be simply explained as follows. By increase of the welding speed, the keyhole depth becomes smaller, the keyhole shape becomes shallower. Hence the surface
ip t
tension force on the dynamic keyhole wall becomes smaller, which means the keyhole becomes more stable in the keyhole welding mode. This theoretical finding is also in good agreement with
cr
the X-Ray imaging experimental results of Shibata et al. (2003). In their study, fewer pores are
us
found in high welding speed conditions during dual beam laser welding, which indeed suggests that the keyhole is more stable. Interestingly, the role of welding speed on dynamic keyhole in
an
dual beam laser welding is similar to that in single beam laser welding. Recently, both theoretical
M
and experimental studies have shown that increasing the welding speed could produce a more stable welding process during single beam laser welding of aluminum and other alloys (Dowden,
ed
2009; Pang et al., 2011). The present theoretical finding about the effect of welding speed can provide a theoretical basis for process optimizations in tandem dual beam laser welding.
pt
Different frequencies of keyhole depth oscillations are found in tandem dual beam laser
Ac ce
welding and single beam laser welding (Fig. 7 and 8). In the current dual beam laser welding, since the laser is evenly subdivided into two beam spots, the power density in dual beam laser welding is smaller than that in single beam laser welding. Hence, a large recoil pressure can be expected in single beam laser welding. The downward movement of keyhole humps can be faster and a larger frequency (1.7 kHz) is theoretically observed in single beam laser welding than in dual beam laser welding. Generally, it is believed that keyhole instability is closely associated with vapor plume oscillations. Previous experimental studies of vapor plume comparisons between dual
23
Page 23 of 35
beam and single beam laser welding indicated that the vapor plume is more stable in dual beam laser welding as comparing to single beam laser welding (Li et al., 2008; Xie, 2002a, b), which seems to be in good agreement with the present theoretical results of keyhole instability. Moreover,
ip t
different frequencies of two keyhole tips are also theoretically found in dual beam laser welding (Fig. 9, Mechanism II). This difference is mainly attributed to the depth variations of two keyhole
cr
tips observed in Fig. 4(b). Note that the power density is the same for the two spots in the current
us
investigations, therefore, the dynamic humps on the keyhole of the lead beam can move to the keyhole bottom earlier. Hence, a smaller period of keyhole depth variations can be observed. It
an
should be pointed out that aforementioned two different plume oscillations have been
M
experimentally found by Xie (2002a) in dual beam laser welding, however, no theoretical explanations were given. Here, a clear explanation of this phenomenon is proposed by using the
ed
proposed comprehensive model. In a word, the present quantitative characterization of dynamic keyhole provides new insights
pt
into the mechanisms of keyhole instability in dual beam laser welding. These results can serve as a
Ac ce
theoretical basis for on-contact monitoring the dynamic keyhole behavior in dual beam laser welding.
5.2. Mechanisms of weld pool dynamics Complex weld pool dynamics are predicated during tandem dual beam laser welding (Fig. 10 and 11). Generally, the fluid flows of weld pool are closely associated with hydrodynamic factors on the keyhole wall and welding parameters. In the present model, the recoil pressure and
24
Page 24 of 35
Marangoni shear stress are considered as two main driving forces. Some fluid flows, such as the surface fluid flows around the keyhole aperture (Fig. 10 (c) and 10 (d)), upward fluid flows along the top part of rear keyhole wall (Fig. 11 (c) and 11 (d)), are mainly driven by the recoil pressure
ip t
and possibly the Marangoni shear stress. While the others, such as high speed downward fluid flows near the front keyhole wall surface (Fig. 11), downward fluid flows behind the blind rear
cr
wall of the keyhole tip of the lag beam (Fig. 11 (c) and 11 (d)), are mainly driven by the recoil
us
pressure.
Under the same heat input conditions, less turbulent fluid flows are observed in dual beam
an
laser welding than in single beam laser welding (Fig. 10~13). This qualitatively agrees well with
M
previous experimental studies that a more stable weld pool can be obtained during dual beam laser welding (Shibata et al., 2003; Xie, 2002a, b). Previously, Relative to single beam laser welding, a
ed
major improving factor of fluid flow of dual beam laser welding was the difference of vapor plume dynamics between the two processes (Shibata et al., 2003; Xie, 2002a, b). Nevertheless, the
pt
friction effect of vapor plume on the keyhole wall is neglected in present model. Hence, other
Ac ce
mechanisms should be also helpful for the stabilization of weld pool in dual beam laser welding. This study proposes some other possible factors. First, the tandem arrangement of two spots leads to a smaller power density, even if the heat input is the same. Hence, a smaller recoil pressure could be expected during laser material interactions. Second, a larger keyhole aperture in dual beam laser welding (mechanism II) leads to a smaller surface tension force. Therefore, the closure possibility of keyhole aperture is lower, and the movement possibility of the keyhole wall near the aperture towards the beam center is lower. This effect could also contribute to the stabilization of
25
Page 25 of 35
surface fluid flow of weld pool. Third, the tandem arrangement of two beam spots will produce a wider weld pool in dual beam laser welding as comparing to single beam laser welding. It will also be helpful for the stabilization of weld pool in dual beam laser welding.
ip t
In the present study, the mechanisms of transient fluid flows in weld pool during dual beam laser welding are investigated and the differences of fluid flows between dual beam and single
cr
beam laser welding are analyzed. It is shown that besides the dragging effect of vapor plume, the
us
lower power density, the smaller surface tension force and the larger surface area of weld pool also
M
5.3. Mechanisms of process stabilization
an
play important roles in the stabilization of weld pool in dual beam laser welding.
Previous experimental results indicated that tandem dual beam laser welding can
ed
significantly improve the process stability of single beam laser welding (Xie, 2002a, b). The mechanism of process stabilization was considered to be the degassing effect of the enlarged
pt
keyhole aperture in dual beam laser welding (Iwase et al., 2000).
Ac ce
In present study, the effect of vapor plume frictions has been neglected in the theoretical
model. Nevertheless, a more stable welding process has also been obtained from the present simulation results. The present results indicate that dual beam technique can obviously decrease the keyhole depth. A shallower keyhole is usually less vulnerable to the pinching effect of surface tension and hydrodynamic pressure of molten liquid, which could lead to a more stable process. Moreover, the fluid flow of weld pool could become more stable in dual beam laser welding, since the tandem arrangement of two spots can lower the power density, reduce the surface tension force
26
Page 26 of 35
and increase the surface area of weld pool. All these physical factors are beneficial for the process stabilization. Hence, the mechanism of process stabilization of dual beam laser welding over single beam laser welding was not solely caused by the degassing effect. Rather, it is produced by
ip t
the joint effect of several beneficial physical factors.
cr
6. MODEL VALIDATION
us
To validate the simulated weld pool dimensions, Fig. 15 compares the predicated penetration depth between simulation and experimental results (Shibata et al., 2003) under different beam
an
distance conditions (Process No. 6, 7 and 8 in Table 2). It could be seen that a reasonable
M
agreement is obtained. As shown in Fig. 16, the comparisons of the length of the keyhole aperture between experiments (Shibata et al., 2003) and simulations are further made (Process No. 6, 7 and
ed
8). A good agreement is obtained. Besides, a comparison on the penetration depth under different welding speed conditions is shown in Fig. 17 (Process No. 1, 2, 3 and 4) (Xie, 2002b), a direct
pt
comparison of cross sectional shape of weld bead and keyhole aperture between simulation and
Ac ce
experimental results under a typical process (Process No. 2) is shown in Fig. 18 (Xie, 2002b). Both good agreements are obtained. To further validate the mesh size, a comparison on calculating weld profiles between different mesh sizes are also made (Fig. 19, Process No. 2), consistency results are obtained, which prove that the choice of mesh size in the present research is reasonable.
27
Page 27 of 35
ip t
cr
Fig. 15. Penetration depth comparisons of dual beam YAG laser welding.
M
an
us
(Process No. 6, 7 and 8).
ed
Fig. 16. Comparisons of length of keyhole aperture in dual beam YAG laser welding.
Ac ce
pt
(Process No. 6, 7 and 8).
Fig. 17. Penetration depth comparisons of dual beam CO2 laser welding. (Process No. 1, 2, 3 and 4).
28
Page 28 of 35
ip t
cr
Fig. 18. Comparison of cross sectional shape of weld bead and keyhole aperture between
M
an
us
simulation and experimental results (Process No. 2).
ed
Fig. 19. Comparison on weld profiles between different mesh size. (Process No. 2) To validate the proposed welding mechanisms and keyhole profiles, a comparison between
pt
simulation (Fig. 4) and experimental keyhole shapes (Fig. 18) is further performed. It could be
Ac ce
found that a reasonable agreement is obtained. Note also that the lead laser beam has a preheating effect on the lag one. Due to this effect, the depth of the keyhole tip of the lag beam is obviously larger in mechanism II. Due to the resolutions of experimental images, it is difficult to distinguish the two keyhole tips. Nevertheless, the predicated keyhole dimensions and shapes are still reasonable as comparing to Fig. 18. The preheating effect observed in present theoretical study has also been experimentally found in other high density beam welding processes. In tandem dual beam electron beam welding, Arata and Nabegata (1978) observed the two distinct keyhole tips
29
Page 29 of 35
with a common keyhole aperture using high speed X-Ray imaging and clearly found that the tip of
(a)
cr
ip t
the lag beam is deeper than that of the lead one (shown in Fig. 20).
(b)
(c)
an
0.6mm, (c) 1.0mm.
us
Fig. 20. Experimental results of welding mechanisms (Shibata et al., 2003): (a) 0.36 mm, (b)
Based on the above comparisons, it is concluded that the proposed model can reasonably
7. CONCLUSIONS
ed
M
predicate the keyhole and weld pool behaviors in tandem dual beam laser welding.
pt
1. A comprehensive three dimensional model is first developed for understanding the transient
Ac ce
keyhole instability and weld pool dynamics during tandem dual beam laser welding of 5052 aluminum alloys. The modeling results of the proposed model reasonably agree with the experiments.
2. As the beam distances increases, the welding mechanism could be a common keyhole with a single tip, a common aperture with two keyhole tips and two separated keyholes. When a common keyhole with two tips is produced under certain process conditions, the depth of the keyhole tip of the lag beam is larger than that of the lead one due to the preheating effect. 3. The frequency of keyhole depth oscillations in tandem dual beam laser welding is in the order 30
Page 30 of 35
of several kHz, which is comparable to that of single beam laser welding. The amplitude of keyhole depth oscillations is smaller than that of single beam laser welding. The keyhole depth instability can be also improved by increasing the welding speed.
ip t
4. Under the same heat input conditions, there are less turbulent fluid flows in the weld pool in tandem dual beam laser welding than in single beam laser welding. The lower power density, the
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contributes to the stabilization of weld pool in the welding process.
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smaller surface tension force and the larger surface area of weld pool in dual beam laser welding
5. The mechanism of process stabilization of dual beam laser welding over single beam laser
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welding was not solely caused by the degassing effect of larger keyhole but also by a combination
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of several beneficial physical factors.
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ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China
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(No. 51105153), the Talent Recruitment Foundation of Huazhong University of Science and
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Technology (No. 0124110034) and the National Basic Research Program of Chine (973 Program, No. 2014CB046703). The authors are grateful to Wang Wen for helping to improve the manuscript.
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