Microstructure and fatigue behavior of laser-powder bed fusion austenitic stainless steel

doi:10.1016/j.jmst.2019.08.047

Abstract

Abstract:

The microstructures and stress-controlled fatigue behavior of austenitic stainless steel (AISI 316 L stainless steel) fabricated via laser-powder bed fusion (L-PBF) technique were investigated. For L-PBF process, zigzag laser scanning strategy (scan rotation between successive layer was 0°, ZZ sample) and cross-hatching layer scanning strategy (scan rotation between successive layer was 67°, CH sample) were employed. By inducing different thermal history, it is found that the scan strategies of laser beam have a significant impact on grain size and morphology. Fatigue cracks generally initiated from persistent slip bands (PSBs) or grain boundaries (GBs). It is observed that PSBs could transfer the melt pool boundaries (MPBs) continuously. The MPBs have better strain compatibility compared with grain boundaries (GBs), thus MPBs would not be the initiation site of fatigue cracks. A higher fatigue limit strength could be achieved by employing a crosshatching scanning strategy. For the CH sample, fatigue cracks also initiated from GBs and PSBs. However, fatigue crack initiated from process-induced defects were observed in ZZ sample in high-cycle regions. Solidification microstructures and defects characteristics are important factors affecting the fatigue performance of L-PBF 316 L stainless. Process-induced defects originated from fluid instability can be effectively reduced by adjusting the laser scan strategy.

Key words: Laser-powder bed fusion, Austenitic stainless steel, Microstructures, Fatigue crack

Chenfan Yu, Peng Zhang, Zhefeng Zhang, Wei Liu. Microstructure and fatigue behavior of laser-powder bed fusion austenitic stainless steel[J]. J. Mater. Sci. Technol., 2020, 46: 191-200.

Figures/Tables 16

Fig. 1. Micromorphologies of 316 L stainless steel powders: (a) dispersive distributed powders; (b) cross-section of a single powder with a diameter of 30 μm; (c) schematic of the laser scan strategies: scanning direction rotated 67° between successive layer (cross-hatching sample) and laser scanning without rotation (zigzag sample); (d) illustration of sampling direction and build direction, outline dimensional drawing of specimens for mechanical tests.

Table 1 Chemical composition of raw 316 L stainless steel powder (wt%).

Cr	Mo	Ni	Mn	C	Si	P	N	Fe
17.49	2.36	12.84	0.47	0.01	0.41	0.013	0.09	Bal.

Fig. 2. Rapid solidification-resultant microstructures of laser-powder bed fusion 316 L stainless steel: (a) longitudinal cross-section of the CH sample; (b) melt pool boundary and cellular structures; (c) longitudinal cross-section of the ZZ sample; (d) columnar grain.

Fig. 3. Inverse pole figures (IPF) showing grain morphology and orientation of L-PBF 316 L stainless steel sample fabricated via different laser scanning strategy: (a) top side of the ZZ sample; (b) top side of the CH sample; (c) lateral side of the ZZ sample; (d) lateral side of the CH sample.

Fig. 4. EBSD images with low-angle grain boundaries (LAGBs, 5°-15°, red line) and high-angle grain boundaries (HAGBs, >15°, blue line) of L-PBF 316 L stainless steel fabricated with two laser scan strategy: (a) top side of the CH sample; (b) top side of the ZZ sample; misorientation distribution of (c) the CH sample and (d) the ZZ sample.

Fig. 5. Engineering stress-strain curves of CH and ZZ sample.

Table 2 Tensile properties of L-PBF 316 SS fabricated by different lasser scan strategies.

Sample	Ultimate tensile strength (MPa)	Elongation to failure (%)
CH	680.8	44.3
ZZ	705.5	46.3

Fig. 6. S-N curves of L-PBF 316 L stainless steel fabricated with different laser scan strategies.

Fig. 7. Comparison of fatigue performance between L-PBF 316 L SS and 316 L SS fabricated via other conventional methods [33,34].

Fig. 8. SEM images of fatigue damage morphologies on the surface of L-PBF 316 L SS: (a-c) ZZ sample (Δσ/2 = 230 MPa, Nf = 1,129,430.25 cycles); (d-f) CH sample (Δσ/2 = 400 MPa, Nf = 32,032.25 cycles).

Fig. 9. Typical interfaces of L-PBF 316 L SS and fatigue damage morphology (ZZ sample, Δσ/2 = 400 MPa, Nf = 66,503.25 cycles).

Fig. 10. Slip morphologies on the surface of fatigued CH sample (Δσ/2 = 300 MPa, Nf = 468,790 cycles) beside the (a) grain boundary and (b) melt pool boundary; (c) fatigue cracking along PSB; (d) fatigue cracking near the melt pool boundary. The black arrow indicates the PSB cracking.

Fig. 11. Illustration of the persistent slip bands (PSBs) morphology near the interface: (a) impingement and shear of PSBs on grain boundary (GB); (b) PSBs transfer through melt pool boundary (MPB).

Fig. 12. Fractographs of fatigued L-PBF 316 L stainless steel at constant stress amplitude of Δσ/2 = 230 MPa: (a) CH sample, 1,045,360 cycles; (b) magnification of fatigue initiation area in (a); (c) ZZ sample, 480,027 cycles; (d) magnification of fatigue crack initiation area in (c).

Fig. 13. Fatigue life of L-PBF 316 L stainless steel fabricated via different laser scan strategy under stress amplitude (Δσ/2) of 230 MPa.

Fig. 14. Typical defects morphologies on the surface of L-PBF 316 L SS fabricated using zigzag strategy (ZZ sample).

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