1 School of Mechanical Engineering, Guizhou University, Guiyang 550025, China 2 College of Mechanical and Vehicle Engineering, Hunan University, Changsha 410082, China
Cite this article:
REN Song, WU Jiazhu, ZHANG Yi, ZHANG Dabin, CAO Yang, YIN Cunhong. Numerical Simulation on Effects of Spatial Laser Beam Profiles on Heat Transport During Laser Directed Energy Deposition of 316L Stainless Steel. Acta Metall Sin, 2024, 60(12): 1678-1690.
The distribution characteristics and magnitude of energy density on the cross section of a laser beam are determined by its spatial profile, which directly impacts heat transport during laser material processing. Hence, it is essential to understand the influence of spatial profiles on heat transport during laser directed energy deposition with synchronous material delivery. Herein, a three-dimensional heat transport model that takes into account important physical events such as laser-powder-pool coupling, thermal-fluid coupling, solid-liquid phase change, and multiple heat transfer was established. The model was validated using single-track single-layer deposition experiments. The effects of four spatial laser beam profiles, including Gaussian (GP), super-Gaussian (SGP1 and SGP2), and pure flat-topped (FTP) profiles, on the heat transport and fluid flow within the molten pool were investigated. Simulated results show that peak temperatures of the molten pool decrease sequentially under GP, SGP1, SGP2 and FTP, and the temperature gradients on the solidification interface increase gradually from the top to the bottom of the molten pool. Temperature gradients on the solidification interface positively correlate with the angle between the normal direction of the solidification interface and the laser scanning direction, and negatively correlate with the distances from the beam center on the molten pool surface. Under all four spatial laser beam profiles, temperature gradients at the same positions on the solidification interface near the rear of the molten pool increase, while those at the bottom of the molten pool decrease. The molten pool exhibits an outward annular flow pattern under all four spatial laser beam profiles with fluid flows mainly driven by Marangoni shear stress. Heat transfer within the molten pool is dominated by Marangoni convection and heat conduction. Average fluid velocities within the molten pool decrease successively according to the following order: Gaussian, super-Gaussian, and pure flat-topped profiles.
Fund: National Natural Science Foundation of China(51975205);Guizhou Provincial Science and Technology Projects([2021]265);Guizhou Provincial Science and Technology Projects([2023]017);Natural Science Foundation of Guizhou University((2021)15);Guizhou University Graduate Innovative Talent Program Project(202203)
Corresponding Authors:
WU Jiazhu, associate professor, Tel: (0851)83627516, E-mail: wujz_pillar@163.com
Fig.1 Principle schematic of laser directed energy deposition
Fig.2 Schematics of the spatial density distributions of GP (a), SGP1 (b), SGP2 (c), and FTP (d) respectively when the laser power P = 700 W and the beam radius rb = 1.2 mm (GP—Gaussian profile, SGP—super-Gaussian profile, FTP—flat-topped profile)
Fig.3 Schematic of the coordinate systems { O }, { L }, and {iPS} of the coaxial powder stream (The injection angle φ refers to the angle between the axis ziPS of the powder stream component and the laser axis zL; the divergence angle θ is the angle between the axis ziPS and boundary of the powder stream component; Rin is the distance from the intersection point OS of the extended powder stream component to the laser axis zL)
Fig.4 Meshing of 3D deposition model (unit: mm)
Physical parameter
Value
Unit
Ref.
Solidus temperature Tsol
1648
K
Liquidus temperature Tliq
1673
K
Solid specific heat csol
604
J·kg-1·K-1
Liquid specific heat cliq
824
J·kg-1·K-1
Solid thermal conductivity ksol
25
W·m-1·K-1
Liquid thermal conductivity kliq
36
W·m-1·K-1
Room temperature Tref
293.15
K
Solid density ρsol
8000
kg·m-3
Liquid density ρliq
6893
kg·m-3
Emissivity
0.7
[26]
Laser absorptivity
0.38
Latent heat of fusion L
2.5 × 105
J·kg-1
[29]
Convective heat transfer coefficient hcon
80
W·m-2·K-1
[30]
Thermal expansion coefficients αexp
5.85 × 10-5
K-1
[31]
Dynamic viscosity μ
6 × 10-3
kg·m-1·s-1
[32]
Permittivity of vacuum σ'
5.67 × 10-8
W·m-2·K-4
Table 1 Parameter values of the thermal transfer model
Table 3 Process parameters of the deposition experiment
Fig.6 Experimental and simulated results at laser powers of 600 W (a), 700 W (b), and 800 W(c) under the action of SGP1
Laser power / W
Result
Deposition track width / mm
Deposition
depth / mm
Deposition
height / mm
600
Experiment
2129.08
281.86
194.90
Numerical simulation
2102.37
256.37
173.56
Relative error
1.25%
9.04%
10.95%
700
Experiment
2365.13
356.83
218.89
Numerical simulation
2239.64
330.61
210.52
Relative error
5.33%
7.35%
3.82%
800
Experiment
2509.46
428.59
248.83
Numerical simulation
2544.80
382.74
234.36
Relative error
1.39%
10.70%
5.82%
Table 4 Characteristic parameters of molten pool at laser powers of 600, 700, and 800 W under the action of SGP1
Fig.7 Temperature (T) fields of molten pool under actions of GP (a), SGP1 (b), SGP2 (c), and FTP (d)
Fig.8 Temperature distributions of trajectory 1 in Fig.4 under the four spatial laser beam profiles
Fig.9 Temperature gradient of longitudinal slice of molten pool under GP
Fig.10 Temperature gradient distributions on two longitudinal sections α (a) and β (b) under GP
Fig.11 Temperature gradients at the observation points on the solidification boundary of the α-plane (a) and β-plane (b) under the four spatial laser beam profiles
Fig.12 Distances from the observation points on the solidification boundary of α-plane (a) and β-plane (b) to the beam center of the molten pool surface under the four spatial laser beam profiles
Fig.13 Angles between the laser scanning direction and the normal direction at the observation points on the solidification boundary of α-plane (a) and β-plane (b) under the four spatial laser beam profiles
Fig.14 Velocity fields of molten pool under actions of GP (a), SGP1 (b), SGP2 (c), and FTP (d)
Fig.15 Velocity distributions of trajectory 1 in Fig.4 under the four spatial laser beam profiles
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