Effects of circular beam oscillation technique on formability and solidification behaviour of selective laser melted Inconel 718: From single tracks to cuboid samples

doi:10.1016/j.jmst.2019.09.044

Abstract

Abstract:

The inherent drawbacks of selective laser melting technique including serious micro-pore and element microsegregation problems destroy the mechanical property of the component. To overcome this problem, a new approach, circular beam oscillation, was successfully applied in the SLMed Inconel 718 samples including single tracks, thin walls and cuboid samples. On one hand, circular beam oscillation reduces the micro-pores in molten pools and cuboid samples, increasing the relative density of the cuboid sample to 99.95 %. On the other hand, circular beam oscillation suppresses the element microsegregation, reducing the formation of Laves phases in SLMed Inconel 718 samples. Moreover, circular beam oscillation enhances the <001> texture of thin walls and the <101> texture of cuboid samples. The improvement of formability and microstructure of the SLMed samples with oscillation is closely related to cooling rate, thermal gradient and stirring effect during the solidification process. Therefore, circular beam oscillation shows the possibility to overcome the key bottlenecks of the traditional SLM technology and to realize a further industrial application of SLM technology.

Key words: Circular beam oscillation, Selective laser melting, Inconel 718, Solidification behaviour

Huihui Yang, Guanyi Jing, Piao Gao, Zemin Wang, Xiangyou Li. Effects of circular beam oscillation technique on formability and solidification behaviour of selective laser melted Inconel 718: From single tracks to cuboid samples[J]. J. Mater. Sci. Technol., 2020, 51: 137-150.

Figures/Tables 21

Fig. 1. Schematic diagram of circular oscillating scanning.

Table 1 Design of experiments for SLMed Inconel 718 single-track, multi-layer and cuboid samples.

Group	Single-track	Multi-layer	Cuboid sample
Laser power, P (W)	300	300	300
The speed of single molten pool formation, v (mm/s)	10	10	10
Scanning distance, S (mm)	-	-	0.3-1
Layer thickness, T (μm)	40	40	80
Amplitude, A (mm)	0-1.1	0, 0.5	0, 0.5
Oscillating frequency, F (Hz)	0-800	0, 300	0-500

Fig. 2. Top surface morphologies of SLMed circular beam non-oscillating (a) and oscillating (b-l) Inconel 718 molten pools.

Fig. 3. Cross-section morphologies of SLMed circular beam non-oscillating (a) and oscillating (b-k) Inconel 718 molten pools.

Fig. 4. (a) Chart to describe the depth, width and height of rectangle-shape molten pool; Measured sizes of rectangle-shape molten pools under various oscillating frequencies (b) and amplitudes (c).

Fig. 5. Microstructures in SLMed Inconel 718 molten pools without (a) or with (b-j) oscillation.

Fig. 6. Effects of oscillating frequency (a) and amplitude (b) on PDAS and Laf in SLMed Inconel 718 molten pools.

Fig. 7. Effects of oscillating frequency (a) and amplitude (b) on microhardness in SLMed Inconel 718 molten pools.

Fig. 8. Time series snapshots of Inconel 718 molten pool evolution during different SLM processes including non-oscillation (a1-d1) and oscillation (a2-d4).

Table 2 Cooling rates measured from video acquired during formation processes of molten pools with and without oscillation.

Laser power, P (W)	Scanning speed, v (mm/s)	Amplitude, A (mm)	Oscillating frequency, F (Hz)	Δt (s)	Cooling rate, T˙ (K/s)
300	10	0	0	69.05	2.4 × 10⁴
		0.5	50	5.96	2.78 × 10⁵
		0.5	100	40.93	4.06 × 10⁴
		0.5	500	30.05	5.53 × 10⁴

Fig. 9. Optical and SEM images of dendrites at different parts of SLMed non-oscillating (a) and oscillating (b)Inconel 718 thin walls.

Fig. 10. Cooling rate and area fraction of Laves phases in SLMed oscillating and non-oscillating Inconel 718 thin walls.

Fig. 11. Optical images (a, a1) and EBSD images at different zones of oscillating and non-oscillating thin walls: inverse pole maps (b, c and b1, c1) and pole maps (d, e and d1, e1).

Fig. 12. Microstructural evolution mechanism diagram of oscillating (a1-d1) and non-oscillating (a-d) Inconel 718 thin walls during SLM process.

Fig. 13. Cross sections of SLMed Inconel 718 cuboid samples under various oscillating parameters.

Fig. 14. XRD patterns of SLMed oscillating and non-oscillating Inconel 718 cuboid samples.

Fig. 15. SEM figures of SLMed cuboid samples with and without oscillation (a, c); (b, d) are inverse pole figures.

Fig. 16. Enlarged SEM figures of SLMed oscillating and non-oscillating (a, c) Inconel 718 cuboid samples; (b, d) are grain boundaries map with various angles in a bigger area.

Fig. 17. Microstructures on cross section of the non-oscillating SLMed Inconel 718 cuboid sample.

Fig. 18. Microstructures on cross section of oscillating SLMed Inconel 718 cuboid sample.

Fig. 19. Formation mechanisms diagram of different textures of SLMed Inconel 718 cuboid samples with and without circular beam oscillation.

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