激光熔覆过程质量传输的边界耦合双方程数值模拟

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"这篇文章是关于激光熔覆过程中质量转移的边界耦合双方程数值模拟研究，由华南理工大学机械工程学院的研究团队进行。研究基于连续体模型，对流体流动、热传递和质量传递进行了耦合数值模拟。在激光熔池中的质量传递模拟中，分别计算了包覆层与基材界面两侧的浓度分布，并在共边界处进行耦合。通过索伯列夫模型，根据平衡溶质分配系数计算得到非平衡溶质分配系数。研究团队利用基于商业软件PHOENICS 1.4开发的自定义软件，对实验中的钢铁材料激光熔覆时铁元素在熔池中的分布进行了分析。" 本文的重点知识包括： 1. 激光熔覆：这是一种表面改性技术，通过高能激光束将合金粉末熔化并沉积在基材表面，形成具有优异性能的包覆层。在这个过程中，涉及流体动力学、热传递和化学反应等多个物理过程。 2. 数值模拟：为了理解和预测激光熔覆过程中的复杂现象，研究者采用数值模拟方法，这可以更精确地模拟实际操作中的流体流动、热传递和质量转移等现象，而无需进行昂贵且可能破坏性的实验。 3. 连续体模型：这是一种在流体力学和热力学中广泛使用的理论框架，它假设物质是连续分布的，允许用偏微分方程来描述物质的运动和变化。 4. 边界耦合：在模拟中，边界条件是关键因素。在激光熔覆中，包覆层和基材的界面是关键的边界，其两侧的浓度分布被单独计算并在此边界上耦合，确保了模拟的准确性。 5. 溶质非平衡分配系数：在熔融金属冷却凝固时，溶质元素的分布可能偏离平衡状态。非平衡溶质分配系数是描述这种现象的关键参数，可以通过索伯列夫模型计算得到。 6. 索伯列夫模型：这是一种描述熔池中溶质分配的理论模型，用于计算在快速凝固过程中非平衡状态下溶质的分布。 7. 商业软件PHOENICS：这是一款强大的计算流体动力学（CFD）软件，被用来进行热传递和流体流动的模拟。研究团队基于此软件开发了自定义工具，以适应激光熔覆的具体需求。 8. 钢铁材料的激光熔覆：实验案例中，研究者关注的是在钢铁材料表面进行激光熔覆时，铁元素在熔池中的分布，这对于理解和优化熔覆过程以及最终涂层的质量至关重要。通过这些知识点，研究者可以深入理解激光熔覆过程中的物理和化学行为，为改进工艺参数、提高涂层质量提供理论支持。

356 CHINESE OPTICS LETTERS / Vol. 6, No. 5 / May 10, 2008

Boundary coupled dual-equation numerical simulation on

mass transfer in the process of laser cladding

Yanlu Huang (

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), Yongqiang Yang (

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), Guoqiang Wei (

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Wenqing Shi (

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

), and Yibin Li (

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)

School of Mechanical Engineering, South China University of Technology, Guangzhou 510641

Received October 23, 2007

The coupled numerical simulation on fluid flow, heat transfer, and mass transfer in the process of laser

cladding is undertaken on the basis of the continuum model. In the simulation of mass transfer in the

laser molten pool, the concentration distribution in the regions on different sides of the interface between

cladding layer and substrate is calculated separately and coupled at the co-boundary. The non-equilibrium

solute partition coefficient is obtained from equilibrium solute partition coefficient according to the Sobolev

model. By using the developed software which is based on the commercial software PHOENICS 1.4, the

distribution of Fe in laser molten pool in an experiment of cladding Stellite 6 on 12CrMoV is calculated.

The obtained results well coincide with the experimental ones.

OCIS codes: 140.0140, 140.3390, 000.3860.

Laser cladding is generally used to improve hardness,

wear and corrosion resistance, and some other surface

characteristics of individual products

[1−4]

. It is known

that the mechanical prop e rties for metal parts strongly

depend on the parameters of crysta lline structure, which

depends on the alloy type, the cooling rate, and the alloy

composition. For be st performance , a low dilution for the

cladding layer is expected. Genera lly, alloying should be

uniform over the laser melted zone, while a grated distri-

bution is demanded in some cases. Uniform alloying will

be achieved, if the alloying additions are melted quickly

as introduction into the pool and the combined convec-

tion diffusion of matter in the melt pool redistributes the

solute effectively throughout the molten pool. Though

insulation of solute would be expected to occur at the

solidifying interface, high cooling rates, in combination

with essentially uniform liquid compo sition at the solid-

liquid interface, could greatly reduce the scale of local

heterogeneity in co mpo sition.

Laser cladding is essentially a fusion and solidification

process, which involves complicated interactions among

the laser beam, metal powders, the substrate material,

and processing gases. There are over ten variables which

strongly influence the characteristics of the clad part

[5]

Process optimization requires both theoretical and ex-

perimental understanding of the associated physical phe-

nomena. Numerical modelling offers a cost-efficient way

to better under stand the related complex physics in a

laser cladding process. It helps to reveal the effects and

significance of each processing parameters on the desired

characteristics of clad par ts. Successful theoretical guid-

ance enables intelligent closed-lo op control in a high qual-

ity and stability process, and s aves time and cost in trans-

ferring the laser cladding technology from one system to

another. In the previous works, a model for the simula-

tion of fluid flow, heat transfer, formation of the cladding

layer, and interaction be tween laser beam and powder

stream during the process of laser cladding with powder

feeding has been presented, and it successfully predicted

the characteristics of the laser cladding processing

[5,6]

It has been pointed out that convection plays a ma-

jor role in arc, laser, and electron beam weld pools

[6−10]

Convection is the most importa nt factor influencing the

geometry of the pool including pool shape, undercut, and

ripples, and c an re sult in defects such as variable pene-

tration, poros ity, and lack of fusion. Convection is also

primarily respo ns ible for mixing and therefore affects the

composition of the weld pool. Hence, some k nowledge o f

the mechanisms and characteris tics of material redistri-

bution within the molten pool is desired to better under-

stand the process of alloy production in laser cladding.

The objectives of this work are to formulate a model for

mass transport during laser cladding and e valuate the

model predictions for different situatio ns.

The mathematical model in this study is based on the

following assumptions. 1) Laser beam profile is assumed

to be Gaussian and constant along the z direction, i.e.,

the depth-of-focus effect is ignored. 2) The liquid metal

is considered as an incompressible Newtonian fluid. Flow

in the mo lten pool is considered to be laminar . 3) The

fluid flow is driven by the buoyancy force in the molten

pool and surface tension gradients on the molten pool

surface. 4) The solid and liquid phases are considered as

a continuum medium. The velocity of solid phase is zero.

5) There is no diffusion transpo rt in solid phase.

The continuum model for binary solid-liquid phase

change system

[11]

was used, and a fixed-grid and mov-

ing coordinate sys tem was adopted to deal with fluid

flow and heat tr ansfer pr oblem involving moving heat

source. The governing equations in the moving frame

can be written as follows.

The first one is the continuity equation

∂(ρ)

∂t

∂(ρu

)

∂x

= 0. (1)

The second one is the momentum c onservation equa-

tion

∂(ρu

)

∂t

∂

∂x

(ρu

) =

∂

∂x

(µ

∂u

∂x

) −

∂p

∂x

+ S

, (2)

1671-7694/2008/050356-05

 2008 Chinese Optics Letters

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