Multiscale and multiphysics explorations of the transient deposition processes and additive characteristics during laser 3D printing

doi:10.1016/j.jmst.2020.11.032

Abstract

Abstract:

Laser directed energy deposition (DED) involves complex physical processes, and the trial and error examinations are time consuming and cost expensive. The research paradigm can be reshaped using advanced phenomenological models via computing the spatiotemporal variations of the build features. In this work, multi-layer and multi-track laser DED of Ti-6Al-4V were systematically explored on multiple scales including the 1D track, the 2D layer and the 3D full build considering the complex transport of energy, mass, and momentum in the moving freeform molten pool. The results showed that convex, near-flat, and wavy builds were generated using gradually larger hatch spacings. The profiles of individual tracks and layers were extracted through the unique advantages of the model. The individual tracks exhibited various patterns and rotated with specific inclinations to form distinct layer profiles. The net increments of the deposit generated upon the printing of a new track during the continuous deposition process showed that the smaller hatch spacing caused higher overlap rate of horizontally adjacent tracks but lower remelting rate of vertically adjacent tracks in neighboring layers. The 3D numerical model was validated with corresponding experiments for various process conditions. The scientific findings can provide useful insights for further researches of DED.

Key words: Additive manufacturing, Multi-layer and multi-track, Numerical model, Build features, Molten pool

H.L. Wei, F.Q. Liu, L. Wei, T.T. Liu, W.H. Liao. Multiscale and multiphysics explorations of the transient deposition processes and additive characteristics during laser 3D printing[J]. J. Mater. Sci. Technol., 2021, 77: 196-208.

Figures/Tables 18

Fig. 1. Schematic illustration of the dynamic mesh distribution during multi-layer and multi-track laser DED. (a) Scanning strategy for the five-layer and six-track deposition process. (b) the 1 st track of the 1 st layer, (c) the 6th track of the 1 st layer, (d) the 6th track of the 5th layer.

Table 1. Process parameters of the laser DED cases.

Case No.	Laser Power (W)	Scanning Speed (mm/s)	Powder Feed Rate (g/min)	Hatch Spacing (mm)	Quantity of Layers	Quantity of Tracks
1	1800	5	20	2	5	30
2	1800	5	20	3	5	30
3	1800	5	20	4.2	5	30
4	1800	5	20	2	20	120
5	1800	5	20	3	20	120
6	1800	5	20	4.2	20	120

Table 2. Thermophysical properties of the substrate and metal powders [14,38].

Name	Ti-6Al-4V	Ti
Density (kg m^-3)	4000	-
Solidus temperature (K)	1878	-
Liquidus temperature (K)	1928	-
Latent heat of fusion (m²s^-2)	2.85×10⁵	-
Thermal conductivity of solid (W m^-1K^-1)	1.57 + 2.9 × 10^-2T - 7 × 10^-6T²	25
Thermal conductivity of liquid (W m^-1K^-1)	33	31
Specific heat of solid (J kg^-1K^-1)	512.4 + 0.15 T - 1 × 10^-6T²	760
Specific heat of liquid (J kg^-1K^-1)	825	783
Viscosity (kg m^-1s^-1)	4×10^-3	4×10^-3
dσ/dT (N m^-1K^-1)	-0.26×10^-3	-

Fig. 2. Computed temperature fields and build profiles for the five-layer and six-track case using hatch spacing of 2 mm.

Fig. 3. Computed temperature fields and build profiles for the five-layer and six-track case using hatch spacing of 3 mm.

Fig. 4. Computed temperature fields and build profiles for the five-layer and six-track case using hatch spacing of 4.2 mm.

Fig. 5. Transverse sections of the five-layer and six-track builds at the mid-length of the build depicting the net increments at the bottom of each track: (a) hatch spacing of 2 mm, (b) hatch spacing of 3 mm, (c) hatch spacing of 4.2 mm.

Fig. 6. Transverse sections of the five-layer and six-track builds at the mid-length of the build depicting the net increments on the top of each track: (a) hatch spacing of 2 mm, (b) hatch spacing of 3 mm, (c) hatch spacing of 4.2 mm.

Fig. 7. Transverse sections of individual layers showing the progressive deposition of multiple tracks using hatch spacing of 2 mm.

Fig. 8. Transverse sections of individual layers showing the progressive deposition of multiple tracks using hatch spacing of 3 mm.

Fig. 9. Transverse sections of individual layers showing the progressive deposition of multiple tracks using hatch spacing of 4.2 mm.

Fig. 10. Variations of height/width ratios of individual layers for cases using different hatch spacings.

Fig. 11. Transverse sections of individual tracks showing the progressive deposition of multiple layers with hatch spacing of 2 mm. (a)-(c) net increments on the top of each track, (d)-(f) progressive updates at the bottom of each track.

Fig. 12. Transverse sections of individual tracks showing the progressive deposition of multiple layers with hatch spacing of 3 mm. (a)-(c) net increments on the top of each track, (d)-(f) progressive updates at the bottom of each track.

Fig. 13. Transverse sections of individual tracks showing the progressive deposition of multiple layers with hatch spacing of 4.2 mm. (a)-(c) net increments on the top of each track, (d)-(f) progressive updates at the bottom of each track.

Fig. 14. Comparison of the experimental and computed results for five-layer and six-track laser DED of Ti-6Al-4 V with different hatch spacings on the top surface. Laser power 1800 W, laser scanning speed 10 mm/s, powder feed rate 20 g/min. (a) and (d) hatch spacing 2 mm, (b) and (e) hatch spacing 3 mm, (c) and (f) hatch spacing 4.2 mm. (a)-(c) experimental results, (d)-(f) modeling results.

Fig. 15. Comparison of the experimental transverse sections for five-layer and six-track laser DED of Ti-6Al-4 V with different hatch spacings. The dashed lines delineate the profiles of the computational results. (a) hatch spacing of 2 mm, (b) hatch spacing of 3 mm, (c) hatch spacing of 4.2 mm.

Fig. 16. Profiles of the twenty-layer and six-track builds using various hatch spacings. (a) and (b) hatch spacing of 2 mm, (c) and (d) hatch spacing of 3 mm, (e) and (f) hatch spacing of 4.2 mm. (b), (d) and (f) show the dynamic mesh structure in the transverse sections at the mid-length of the builds.

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