Heat transfer and fluid flow and their effects on the solidification microstructure in full-penetration laser welding of aluminum sheet

doi:10.1016/j.jmst.2019.10.027

Abstract

Abstract:

Understanding the behaviors of heat transfer and fluid flow in weld pool and their effects on the solidification microstructure are significant for performance improvement of laser welds. This paper develops a three-dimensional numerical model to understand the multi-physical processes such as heat transfer, melt convection and solidification behavior in full-penetration laser welding of thin 5083 aluminum sheet. Solidification parameters including temperature gradient G and solidification rate R, and their combined forms are evaluated to interpret solidification microstructure. The predicted weld dimensions and the microstructure morphology and scale agree well with experiments. Results indicate that heat conduction is the dominant mechanism of heat transfer in weld pool, and melt convection plays a critical role in microstructure scale. The mushy zone shape/size and solidification parameters can be modulated by changing process parameters. Dendritic structures form because of the low G/R value. The scale of dendritic structures can be reduced by increasing GR via decreasing heat input. The columnar to equiaxed transition is predicted quantitatively via the process related G³/R. These findings illustrate how heat transfer and fluid flow affect the solidification parameters and hence the microstructure, and show how to improve microstructure by optimizing the process.

Key words: Laser welding, Heat transfer, Fluid flow, Solidification microstructure, Aluminum

Shaoning Geng, Ping Jiang, Xinyu Shao, Lingyu Guo, Xuesong Gao. Heat transfer and fluid flow and their effects on the solidification microstructure in full-penetration laser welding of aluminum sheet[J]. J. Mater. Sci. Technol., 2020, 46: 50-63.

Figures/Tables 17

Fig. 1. Laser welding system.

Fig. 2. Schematic of the full-penetration laser welding of aluminum sheet.

Fig. 3. Schematic of the computational domain.

Table 1 Thermo-physical properties of 5083 aluminum alloy used in this work [36,37].

Physical properties	Value
Density of liquid (D_l)	2380 kg/m³
Density of solid (D_s)	2660 kg/m³
Viscosity (μ)	1.2 × 10^-3 kg/m s
Specific heat of liquid (C_p,l)	1198 J/kg K
Specific heat of solid (C_p,s)	1150 J/kg K
Latent heat of fusion (L_f)	3.87 × 10⁵ J/kg
Thermal conductivity of liquid (λ_l)	90 W/m K
Thermal conductivity of solid (λ_s)	120 W/m K
Liquidus temperature (T_l)	911 K
Solidus temperature (T_s)	864 K
Heat transfer coefficient (h_conv)	20 W/m² K
radiation emissivity	0.3
Surface tension (γ₀)	0.871 N/m
Surface tension gradient (dγ/dT)	-0.155 × 10^-3 N/m K

Table 2 Parameters of heat source.

Heat source model parameters	Value
Effective absorption coefficient (η_l)	0.60 [38,39]
Effective radius of top surface (r_e)	0.25 mm
Effective radius of bottom surface (r_i)	0.125 mm
Proportion factor (χ)	1.4

Fig. 4. Temperature variation with time at one point (0.5 mm away from the centerline) at the mid-depth horizontal plane using different grid sizes and time steps.

Fig. 5. Comparison of the experimental and predicted weld cross-section profile: (a) PL = 2500 W, V = 80 mm/s; (b) PL = 3000 W, V = 120 mm/s; (c) PL = 3500 W, V = 180 mm/s.

Fig. 6. Comparison of the top surface morphologies of weld pool between the experimental high-speed camera images and numerical simulations. (a) Experimental: PL = 2500 W, V = 80 mm/s; (b) Simulated: PL = 2500 W, V = 80 mm/s; (c) Experimental: PL = 3000 W, V = 120 mm/s; (d) Simulated: PL = 3000 W, V = 120 mm/s; (e) Experimental: PL = 3500 W, V = 180 mm/s; (f) Simulated: PL = 3500 W, V = 180 mm/s. Note that the weld pool profile in high-speed camera images can be recognized by the fluctuation of molten metal.

Fig. 7. Calculated temperature and velocity fields for Case I (PL = 2500 W and V = 80 mm/s) observed from different views: (a) 3D view, (b) top view, (c) bottom view, (d) front view, and (e) right view of section A in a.

Fig. 8. Temperature distribution along weld centerline for Case I (PL = 2500 W and V = 80 mm/s).

Fig. 9. Calculated temperature and velocity fields for different welding parameters: (a) PL = 2500 W, V = 80 mm/s; (b) PL = 3000 W, V = 120 mm/s; (c) PL = 3500 W, V = 180 mm/s.

Fig. 10. Schematic diagram illustrating the effect of temperature gradient G and growth rate R on the morphology and size of solidification microstructure [15].

Fig. 11. The spatial variation of the temperature gradient G and solidification rate R along the representative isothermal (TL + TS)/2 in the mushy zone as a function of weld width at the mid-depth horizontal plane. The welding parameters are PL = 2500 W and V = 80 mm/s.

Fig. 12. Comparison of the solidification parameters along the representative isothermal (887.5 K) in mushy zone for different welding cases: (a) temperature gradient G, (b) solidification rate R, (c) morphology factor G/R, (d) cooling rate GR. The welding parameters are: Case I - PL = 2500 W, V = 80 mm/s; Case II - PL = 3000 W, V = 120 mm/s; Case III - PL = 3500 W, V = 180 mm/s.

Fig. 13. The dendritic structures and their microstructure scale under various cooling rate in three welding cases: (a) (c) Case I - PL = 2500 W, V = 80 mm/s; (b) (e) Case II - PL = 3000 W, V = 120 mm/s; (c) (f) Case III - PL = 3500 W, V = 180 mm/s. (a), (b) and (c) are experimental results tested by SEM; (d) (e) and (f) are phase-field simulated results colored by the Mg content.

Fig. 14. (a) SDAS obtained from experiments and phase-field simulations under different cooling rates, and the corresponding fitted curves based on Eq. (23); (b) comparison of the fitted a and n among the experimental results, the phase-field simulated results, and Easton et al.’s experimental results [45].

Fig. 15. Spatial variation of the CET parameter, G3/R, along the representative isothermal (887.5 K) in mushy zone and the corresponding experimental grain structures tested by EBSD for different cases: (a)(b) Case I - PL = 2500 W, V = 80 mm/s; (c)(d) Case II - PL = 3000 W, V = 120 mm/s; (e)(f) Case III - PL = 3500 W, V = 180 mm/s.

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