Hybrid post-processing effects of magnetic abrasive finishing and heat treatment on surface integrity and mechanical properties of additively manufactured Inconel 718 superalloys

doi:10.1016/j.jmst.2022.03.026

Abstract

Abstract:

Desired microstructure and surface integrity are critical to achieving the high performance of additively manufactured components. In the present work, the hybrid post-processes of magnetic abrasive finishing (MAF) and post-heat treatment (HT) were applied to the additively manufactured Inconel 718 superalloys. Their hybrid effects and influencing mechanism on the surface quality and mechanical properties of the additively manufactured samples have been studied comparatively. The results show that the MAF process effectively reduces the surface roughness by more than an order of magnitude due to the flexibility and geometric consistency of the magnetic particles and abrasives with the finished surfaces. The proper sequence of MAF and HT obtains enhanced mechanical properties for the homogenized-MAF-aged sample with the yield strength of 1147 MPa, the ultimate tensile strength of 1334 MPa, and the elongation of 22.9%, which exceeds the standard wrought material. The surface integrity, compressive residual stress field, and grain refinement induced by the MAF and subsequent aging heat treatment increase the cracking resistance and delay the fracture failure, which significantly benefits the mechanical properties. The MAF process combined with proper post-heat treatment provides an effective pathway to improve the mechanical properties of additively manufactured materials.

Key words: Magnetic abrasive finishing, Additive manufacturing, Inconel 718, Microstructure-property relationship, Heat treatment

Li Kun, Ma Ruijin, Zhang Ming, Chen Wen, Li Xiaobin, Z.Zhang David, Tang Qian, E.Murr Lawrence, Li Jinfeng, Cao Huajun. Hybrid post-processing effects of magnetic abrasive finishing and heat treatment on surface integrity and mechanical properties of additively manufactured Inconel 718 superalloys[J]. J. Mater. Sci. Technol., 2022, 128: 10-21.

Figures/Tables 18

Fig. 1. Building process of Inconel 718. (a) LPBF printing system, (b) dimensions of tensile specimen according to ASTM E8 standard, and (c) printed samples of cubes and tensile bars.

Fig. 2. Thermodynamic predictions for Inconel 718 using the Thermo-Calc software: (a) phase fraction vs temperature using the equilibrium model and (b) nonequilibrium simulation using the Scheil-Gulliver model.

Fig. 3. Thermal history of the post-heat treatment for the LPBF-produced 718 samples.

Fig. 4. Schematic of the experimental setup and MAF process for the LPBF-produced 718 samples.

Table 1. MAF experimental condition.

Parameter	Condition
Finished sample	Inconel 718 cubes and ASTM E8 tensile bars
Magnetic particle	G25 steel grit: 0.9 g
Magnetic abrasive	120 µm friable wheel grit with 0 wt.% Uncoated diamond abrasive: 0.4 g
Lubricant	Water-soluble barreling compound: 0.5 mL (0.025 mL/min)
Clearance	2 mm
Magnet	Nd-Fe-B magnet: Φ12.7 × 12.7 mm
Magnetic flux density	596 mT
Magnet feed	Feed rate: 1 mm/s, travel distance: 80 mm
Magnet revolution	600 r/min
Total finishing time	180 min

Fig. 5. Schematic diagram of the full experiments.

Fig. 6. Microstructure morphologies of LPBF-produced Inconel 718 with different post-heat treatments: (a) as-printed (AP) sample, (b) as-printed sample after homogenization (AP+H), (c) as-printed sample after homogenization and aging (AP+H+A), (c3) TEM morphology of precipitates in the AP+H+A sample, and (c4) selective area diffraction pattern of (c3) (a1-c1: EBSD IPF analysis, a2-c2: SEM observation).

Fig. 7. Surface morphologies of the samples with different post-processing conditions: (a) Sequence of H+A+MAF and (b) sequence of H+MAF+A (H—homogenization, A—aging, MAF—magnetic abrasive finishing).

Fig. 8. Roughness evolution of different surfaces as a function of MAF finishing time.

Table 2. Mechanical properties and printed-side surface roughness of the samples with different conditions.

Sample	YS (MPa)	UTS (MPa)	ε_t (%)	UOT (MJ/m)	Ra	Rz
AP	737 ± 5	1052 ± 8	27.6 ± 0.5	237.3	2.8 ± 0.3	11.4 ± 0.8
H+A	1140 ± 6	1306 ± 5	14.2 ± 0.7	175.9	2.7 ± 0.3	11.4 ± 0.9
H+A+MAF	1151 ± 5	1339 ± 6	19.5 ± 0.6	254.3	0.13 ± 0.08	1.1 ± 0.3
H+MAF+A	1147 ± 4	1334 ± 5	22.9 ± 0.8	290.4	0.18 ± 0.05	1.3 ± 0.4
Wrought Std. [47,48]	1034	1241	15	-	-	-

Fig. 9. Engineering stress-strain curves of the samples with different post-processing conditions.

Fig. 10. Roughness real-time profiles of the H+A+MAF samples with the finishing time: (a) printed surface and (b) EDM surface.

Fig. 11. Roughness real-time profiles of the H+MAF+A samples along with the finishing time: (a) printed surface and (b) EDM surface.

Fig. 12. Materials removal evolution of the samples under different post-processing conditions.

Fig. 13. Evolution of residual stress at the center surfaces of the tensile samples under different heat-treated and MAF conditions.

Fig. 14. Microstructure and hardness distribution from the top to the center along the thickness direction of the samples under different heat-treated and MAF conditions: (a-c) orientation maps from EBSD observations, (d-f) the corresponding GOS maps from EBSD observations, (g) hardness evolution from the sample surface.

Table 3. Grain characteristics extracted from the EBSD results.

Sample	Grain size (µm)	GB density (m^-1)	Peak GOS (°)	Average GOS (°)
H+A	85.4 ± 12.5	8.9 × 10⁴	6.07	0.25
H+A+MAF	64.3 ± 12.2	13.4 × 10⁴	8.66	2.6
H+MAF+A	32.6 ± 8.4	29.8 × 10⁴	9.2	3.8

Fig. 15. Fracture morphologies from the top to the center along the thickness direction of the samples under different heat-treated and MAF conditions: (a) H+A sample, (b) H+A+MAF sample, (c) H+MAF+A sample (0: overall morphology at low magnification, 1 and 2: facture morphology in the top zone, 3 and 4: facture morphology in the center zone).

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