Formation and Evolution of Surface Morphology in Overhang Structure of IN718 Superalloy Fabricated by Laser Powder Bed Fusion

doi:10.1007/s40195-023-01546-3

Abstract

Abstract:

Controlling the overhang surface quality is still a formidable challenge in manufacturing the components with complex structures during laser powder bed fusion (LPBF). This study systematically uncovers the effects of the volume energy density (VED) and overhang angle on the evolution of surface morphology and corresponding surface roughness (Ra) of top and down-skin surfaces of IN718 superalloy samples. The results show that balling, Plateau-Rayleigh instability, open pore and humping caused by the material stacking are the main factors contributing to the apparent deterioration of top surface quality. When the VED is 80-100 J/mm³, the high down-skin surface roughness is attributed to the serious dross caused by recoil pressure and sinking of the melt pool. Using insufficient VED (15-50 J/mm³) can easily lead to poor metallurgical bonding and material spalling on the down-skin surface. In addition, overhang angle also significantly affects down-skin surface roughness due to the stair effect and the adhered unmelted powders. An improvement in the surface quality of down-skin surface is observed when the overhang angle increases. Based on the finding of this investigation, an optical VED (59.5 J/mm³) significantly improves the top and down-skin surface quality and porosity of overhang samples. This study provides an insight into synergy ascension of the top and down-skin surface quality in the overhang structure.

Key words: Laser powder bed fusion, IN718 superalloy, Surface roughness, Overhang structure

Haotian Zhou, Haijun Su, Yinuo Guo, Yuan Liu, Di Zhao, Peixin Yang, Zhonglin Shen, Le Xia, Min Guo. Formation and Evolution of Surface Morphology in Overhang Structure of IN718 Superalloy Fabricated by Laser Powder Bed Fusion[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(9): 1433-1453.

Figures/Tables 27

Fig. 1 a SEM morphology of the IN718 powder; b the powder size distribution of the IN718 powder

Fig. 2 a Front view of the samples at the overhang angle 45°; b side view of the samples at the overhang angle 45°; c schematic of the scanning strategy and the surface morphology; d the actual printed overhang sample diagram

Table 1 Process parameters for fabricating IN718 by LPBF process

Sample no.	Laser power (W)	Scanning speed (mm/s)	Hatch space (μm)	Volume energy density (J/mm³)
1	150	600	60	83.3
2	150	1000	75	40.0
3	150	1400	90	23.8
4	150	1800	105	15.9
5	200	600	75	88.9
6	200	1000	60	66.7
7	200	1400	105	27.2
8	200	1800	90	24.7
9	250	600	90	92.6
10	250	1000	105	47.6
11	250	1400	60	59.5
12	250	1800	75	37.0
13	300	600	105	95.2
14	300	1000	90	66.7
15	300	1400	75	57.1
16	300	1800	60	55.6

Fig. 3 Surface roughness of the top and down-skin surface at different overhang angles: a 30°; b 45°; c 60°

Fig. 4 Schematic diagram of staircase effect

Fig. 5 Main effects plot of the top surface roughness at 45°

Table 2 ANOVA for top surface roughness. Overhang angle = 45°. R2 = 87.61%

Parameter	DF	Seq. SS	Adj. MS	F	p	Significance
Laser power	3	1359.56	453.19	8.44	0.014	Highly significant
Scanning speed	3	827.92	275.97	5.14	0.043	Significant
Hatch space	3	91.24	30.41	0.57	0.657	Not significant
Error	6	322.13	53.69
Total	15	2554.5

Fig. 6 Effect of VED on the top surface roughness at 45°

Fig. 7 SEM images of top surface under low VED at 45°: a 23.8 J/mm3; b 24.7 J/mm3; c 27.2 J/mm3; d 37.0 J/mm3. 3D surface morphology under low VED: e 23.8 J/mm3; f 24.7 J/mm3; g 27.2 J/mm3; h 37.0 J/mm3

Fig. 8 Schematic diagram of the Rayleigh-Plateau instability: a necking-down stage; b melt pool breaks into droplets. SEM image: c irregular melt track with necking-down; d irregular melt track with breakup

Fig. 9 Top surface morphologies under medium VED at 45°: a SEM image at 59.5 J/mm3; b SEM image of sample 14 at 66.7 J/mm3; c 3D surface morphology at 59.5 J/mm3; d 3D surface morphology of sample 14 at 66.7 J/mm3

Fig. 10 Schematic diagram of the melt pool flow between the adjacent melt track: a flat surface; b humping caused by low hatch spacing

Fig. 11 SEM images of the top surface under different VED: a sample 6 with 66.7 J/mm3; b 83.3 J/m m3; c 88.9 J/mm3; d 92.6 J/mm3. 3D surface morphology images under different VED: e sample 6 with 66.7 J/mm3; f 83.3 J/mm3; g 88.9 J/mm3; h 92.6 J/mm3

Fig. 12 Top surface with VED of 95.2 J/mm3: a SEM image; b 3D surface morphology

Fig. 13 Main effects plot of the down-skin surface roughness at different overhang angles

Table 3 ANOVA for down-skin surface roughness. Overhang angle = 30°. R2 = 88.32%

Parameter	DF	Seq. SS	Adj. MS	F	p	Significance
Laser power	3	2288.9	763.0	6.49	0.026	Significant
Scanning speed	3	2630.9	877.0	7.45	0.019	Highly significant
Hatch space	3	418.4	139.5	1.19	0.391	Not significant
Error	6	705.9	117.6
Total	15	6044.1

Table 4 ANOVA for down-skin surface roughness. Overhang angle = 45°. R2 = 88.72%

Parameter	DF	Seq. SS	Adj. MS	F	p	Significance
Laser power	3	420.2	140.06	4.46	0.057	Not significant
Scanning speed	3	563.2	187.73	5.98	0.031	Significant
Hatch space	3	498.2	166.06	5.29	0.040	Significant
Error	6	188.4	31.40
Total	15	1669.9

Table 5 ANOVA for down-skin surface roughness. Overhang angle = 60°. R2 = 88.08%

Parameter	DF	Seq. SS	Adj. MS	F	p	Significance
Laser power	3	247.39	82.46	6.18	0.029	Significant
Scanning speed	3	134.05	44.68	3.35	0.097	Not significant
Hatch space	3	209.65	69.88	5.24	0.041	Significant
Error	6	80	13.33
Total	15	671.10

Fig. 14 SEM images of down-skin surface under different overhang angles and process parameters: a 30° at 83.3 J/mm3; b 30° at 66.7 J/mm3; c 30° at 40.0 J/mm3; d 45° at 83.3 J/mm3; e 45° at 66.7 J/mm3; f 45° at 40.0 J/mm3; g 60° at 83.3 J/mm3; h 45° at 66.7 J/mm3; i 60° at 40.0 J/mm3

Fig. 15 Effect of VED on the down-skin surface roughness under different overhang angles

Fig. 16 SEM images of the down-skin surface under high VED: a 88.9 J/mm3 at 30°; b 92.6 J/mm3 at 30°; c 95.2 J/mm3 at 30°; d 88.9 J/mm3 at 60°; e 92.6 J/mm3 at 60°; f 95.2 J/mm3 at 60°

Fig. 17 3D surface morphology images of the down-skin surface under high VED: a 88.9 J/mm3 at 30°; b 92.6 J/mm3 at 30°; c 95.2 J/mm3 at 30°; d 88.9 J/mm3 at 60°; e 92.6 J/mm3 at 60°; f 95.2 J/mm3 at 60°

Fig. 18 SEM images of the down-skin surface under low VED: a 23.8 J/mm3 at 30°; b 24.7 J/mm3 at 30°; c 40.0 J/mm3 at 30°; d 23.8 J/mm3 at 60°; e 24.7 J/mm3 at 60°; f 40.0 J/mm3 at 60°

Fig. 19 3D surface morphology images of the down-skin surface under low VED: a 23.8 J/mm3 at 30°; b 24.7 J/mm3 at 30°; c 40.0 J/mm3 at 30°; d 23.8 J/mm3 at 60°; e 24.7 J/mm3 at 60°; f 40.0 J/mm3 at 60°

Fig. 20 Top surface of the sample 11 with 59.52 J/mm3: a SEM image at 30°; b 3D surface morphology at 30°; c SEM image at 60°; d 3D surface morphology at 60°

Fig. 21 Effect of contour scan on the down-skin surface morphology: a 30° without contour scan; b 30° with contour scan; c 60° without contour scan; d 60° with contour scan

Fig. 22 Evolution of pore under different VEDs and overhang angles: a 47.62 J/mm3 at 30°; b 47.62 J/mm3 at 60°; c 59.52 J/mm3 at 30°; d 59.52 J/mm3 at 60°; e 95.24 J/mm3 at 30°; f 95.24 J/mm3 at 60°

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