Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815 °C

doi:10.1007/s40195-019-00986-0

Abstract

Abstract:

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

Key words: Selective laser melting, Low-cycle fatigue, Creep-fatigue interaction, Failure analysis

Xiao-An Hu, Gao-Le Zhao, Yun Jiang, Xian-Feng Ma, Fen-Cheng Liu, Jia Huang, Cheng-Li Dong. Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815 °C[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(4): 514-527.

Figures/Tables 20

Fig. 1 Powder size distribution

Fig. 2 Powder of Inconel 625: a shape of powder, b detail of granule shape

Table 1 Chemical compositions of Inconel 625 powder (wt%)

Fig. 3 Dimension of the tested samples (unit: mm)

Fig. 4 Microstructure of SLM Inconel 625 in three mutually perpendicular planes before testing

Table 2 LCF and CFI test matrix of SLM Inconel 625 at 815 °C

Fig. 5 Engineering stress and strain curves at 815 °C for SLM Inconel 625: a global view, b local view

Fig. 6 Comparison of elongation in forged [28] and SLM Inconel 625 after fracture at 815 °C

Fig. 7 Stress and strain relationship under LCF and CFI. a Pure LCF (specimen C2), b CFI (specimen D2)

Fig. 8 Fracture surfaces of specimen T1 at 815 °C

Fig. 9 Microstructures of the surface near the fracture section at 815 °C

Fig. 10 Microstructures of specimens after tensile testing at 815 °C

Fig. 11 Fracture surfaces of fatigue specimen A1: σmax = 200 MPa, Nf = 446,413

Fig. 12 Fracture surfaces of fatigue specimen A2: σmax = 200 MPa, Nf = 213,495

Fig. 13 Fracture surfaces of fatigue specimen A3: σmax = 200 MPa, Nf = 4471

Fig. 14 Fracture surfaces of D1: σmax = 200 MPa, Nf = 110

Fig. 15 a Yield, b ultimate stress, c elongation of annealed bar [28] and SLM Inconel 625 [32] at the fracture

Fig. 16 Three different damage mechanisms controlling LCF failure of SLM 625 under the maximum stress of 200 MPa at 815 °C

Fig. 17 Nondestructive X-ray testing results of specimens: a A1, b A2 , c A3

Fig. 18 SEM images of the longitudinal section of LCF and CFI specimens: a A1, b B1, c C1, d D1, e D2, f D3

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