Real-time porosity reduction during metal directed energy deposition using a pulse laser

doi:10.1016/j.jmst.2021.12.013

Abstract

Abstract:

Porosity is a challenging issue in additive manufacturing and is detrimental to the quality of the additively manufactured products. In this study, a real-time porosity reduction technique was developed by incorporating a pulse laser into a laser metal powder directed energy deposition (DED) system. The incorporated pulse laser can accelerate fluid flow within the melt pool and facilitate the escape of pores before complete solidification. It achieves this real-time porosity reduction by inducing accelerated and turbulent Marangoni flow, ultrasonic waves, and shock waves into the melt pool. The uniqueness and advantages of the proposed technique include the following: (1) For a laser metal powder DED process, this study proposed a noncontact, nondestructive, and real-time porosity reduction technique at the melt pool level. (2) It was experimentally and numerically validated that the developed technique did not alter the geometry and the grain structure of the manufactured Ti-6Al-4V samples. (3) Because the porosity reduction is accomplished at the melt pool level, its application is not limited by the size, shape, or complexity of the printing target. (4) The developed technique can be readily incorporated into the existing DED systems without any modification of the original tool-path design. The experimental results showed that the pore volume fraction decreased from 0.132% to 0.005%, no pores larger than 6 × 104 µm3 were observed, and a 91% reduction in the total pore number was achieved when the pulse laser energy reached 11.5 mJ.

Key words: Real-time porosity reduction, Pulse laser, Melt pool, Directed energy deposition, Ti-6Al-4V

Hoon Sohn, Peipei Liu, Hansol Yoon, Kiyoon Yi, Liu Yang, Sangjun Kim. Real-time porosity reduction during metal directed energy deposition using a pulse laser[J]. J. Mater. Sci. Technol., 2022, 116: 214-223.

Figures/Tables 17

Fig. 1. Schematic configuration of a DED system with an incorporated PR laser, (a) coaxial design, (b) off-axial design, and (c) the hardware realization of the off-axial design.

Table 1. Parameters of DED and PR lasers.

Empty Cell	Parameters (unit)
Empty Cell	Laser type	Wavelength (nm)	Pulse duration (ns)	Max. pulse energy (mj)	Max. peak power (MW)	Repetition rate (Hz)	Max. averaged power (W)
DED laser	Yb-fiber cw laser	1070	N/A	N/A	N/A	N/A	1000
PR laser	Nd:YAG pulse laser	532	10	11.5	1.15	100	1.15

Fig. 2. Overview of real-time porosity reduction technique using PR laser.

Table 2. Physical properties of Ti-6Al-4V used for sample manufacturing [33].

Property (unit)	Liquidus temperature (K)	Solidus temperature (K)	Evaporation temperature (K)	Solid density (kgm-3)	Liquid density (kgm-3)	Thermal conductivity (Wm-1K-1)	Heat capacity at a constant pressure (Jkg-1K-1)	Surface tension (Nm-1)	Laser absorption coefficient
Value	1923	1877	3533	4420-0.154(T-298K)	3920-0.680(T-1923K)	1.260+0.016T (T ≤ 1268K) 3.513+0.013T (1268K< T ≤ 1923K) -12.752+0.024T (T > 1923 K)	500	1.53-0.28 × 10^-3(T-1941K)	0.4

Table 3. Summary of the Ti-6Al-4V samples and corresponding PR laser energy levels.

Empty Cell	PR laser energy (mJ)
Printing scheme	Track_A	Track_B	Track_C	Track_D
80 mm-long single track	0.0	7.5	9.7	11.5
Empty Cell	Cuboid_A	Cuboid_B	Cuboid_C	Cuboid_D
3-layer cuboid	0.0	7.5	9.7	11.5

Fig. 3. Description of the Ti-6Al-4V samples and corresponding tool-paths.

Fig. 4. Effect of PR laser energy on porosity of cuboid samples (measured by X-ray microscopy); pores are highlighted in different colors according to the pore size.

Fig. 5. Relationships of (a) pore number, (b) pore volume fraction, and (c) averaged pore size with respect to the PR laser energy (based on X-ray microscopy of cuboid samples).

Fig. 6. Effect of PR laser energy on pore size distribution (based on X-ray microscopy of cuboid samples).

Fig. 7. (a) Optical microscopy (OM) images and (b) the corresponding cross-sectioned profiles (including heat affected zones) obtained from single-track samples manufactured under different PR laser energy levels.

Fig. 8. Comparison of geometries of cuboid samples manufactured under different PR laser energy levels (measured by laser line scanner): (a) 3D geometry of Cuboid_A, and (b) the representative cross-sectioned profiles along the XZ plane.

Fig. 9. Effect of PR laser energy level on dimensions of cuboid samples (based on measurements with laser line scanner); the standard deviation values are shown in parentheses.

Fig. 10. Scanning electron microscopy (SEM) images of cuboid samples manufactured under different PR laser energy levels.

Fig. 11. Effect of PR laser energy level on length and thickness of α lath obtained from different cuboid samples; the standard deviation values are shown in parentheses.

Fig. 12. Schematic illustration of the effects of a PR laser on melt pool: (a) accelerated and turbulent Marangoni flow, (b) ultrasonic waves, and (c) shock waves.

Fig. 13. (a) Model description (A 10 mJ PR laser was applied at 0.4 s, and the concurrent temperature distribution of the model at 0.4 s is shown), (b) temperature distribution of the melt pool at t = 0.4 s + 1 μs with a PR laser, (c) temperature distribution of the melt pool at t = 0.4 s + 1 μs without a PR laser, and (d) temperature difference between (b) and (c).

Fig. 14. Effect of PR laser on time-variation of the top surface temperature of the melt pool at t = 0.4 s + 1 μs (a) with and (b) without a PR laser (based on Flow-3D simulation).

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