On the origin of the high tensile strength and ductility of additively manufactured 316L stainless steel: Multiscale investigation

doi:10.1016/j.jmst.2019.09.017

Abstract

Abstract:

We report that 316L austenitic stainless steel fabricated by direct laser deposition (DLD), an additive manufacturing (AM) process, have a higher yield strength than that of conventional 316L while keeping high ductility. More interestingly, no clear anisotropy in tensile properties was observed between the building and the scanning direction of the 3D printed steel. Metallographic examination of the as-built parts shows a heterogeneous solidification cellular microstructure. Transmission electron microscopy observations coupled with Energy Dispersive X-ray Spectrometry (EDS) reveal the presence of chemical micro-segregation correlated with high dislocation density at cell boundaries as well as the in-situ formation of well-dispersed oxides and transition-metal-rich precipitates. The hierarchical heterogeneous microstructure in the AM parts induces excellent strength of the 316L stainless steel while the low staking fault energy of the as-built 316L promotes the occurrence of abundant deformation twinning, in the origin of the high ductility of the AM steel. Without additional post-process treatments, the AM 316L proves that it can be used as a structural material or component for repair in mechanical construction.

Key words: Additive manufacturing, Austenitic stainless steel, Dislocation structure, Twinning, Mechanical behavior, Anisotropy

Bassem Barkia, Pascal Aubry, Paul Haghi-Ashtiani, Thierry Auger, Lionel Gosmain, Frédéric Schuster, Hicham Maskrot. On the origin of the high tensile strength and ductility of additively manufactured 316L stainless steel: Multiscale investigation[J]. J. Mater. Sci. Technol., 2020, 41: 209-218.

Figures/Tables 15

Table 1 Chemical compositions of the as-received 316 L powder compared to the ASTM specifications for 316L (wt.%).

Element	C	Cr	Co	Mn	P	Ni	Si	S	Mo	V	Cu	Fe
Powder	0.016	17.7	0.15	0.29	0.010	12.6	0.58	0.026	2.33	0.026	0.024	Bal.
ASTM	0.03_Max	16-18	—	2.0_Max	0.045_Max	10-14	0.75_Max	0.03_Max	2-3	—	—	Bal.

Fig. 1. Schematic of the horizontally and vertically built parts showing the positions from which tensile specimens were extracted and the areas on which microstructural analyzes were performed.

Table 2 Processing parameters used in directed laser deposition of the two additively manufactured parts.

Processing parameters	Laser power (W)	Travel speed (mm/s)	Layer thickness (mm)	Hatch spacing (mm)	Powder flow rate (g/min)
Vertical & horizontal bars	600	6.35	0.40	0.60	6

Fig. 2. The XRD patterns taken on the powder, the lateral and the top surface of the as-built 316L.

Fig. 3. 3D view of the as-built microstructure (a) optical micrograph; (b) Image quality (IQ) + color grain map.

Fig. 4. SEM images illustrating the typical cellular microstructure at (a) low and (b-d) high magnification corresponding to the yellow areas indicated in Fig. 4(a); (b) equiaxed cells; (c,d) evidence of elongated cells at the melt pool boundaries.

Fig. 5. Electron back-scattered diffraction (EBSD) inverse pole figure (IPF) relative to the building direction + IQ in (a) the lateral surface; (b) the top surface and (c,d) the corresponding pole figures.

Fig. 6. TEM bright field micrographs of the typical microstructure of the as-built 316L: (a) cellular structure with arrays of dislocations pilling up at cell boundaries; (b) evidence of stacking faults and inclusions.

Fig. 7. (a) STEM-HAADF micrograph of the as-built 316L and the corresponding STEM-EDS chemical maps revealing segregation at cell boundaries; (b) evidence of Si-oxides and transition-metal-rich precipitates.

Fig. 8. Stress-strain curves of the as-built 316L strained at 2.5 μm s-1 at room temperature.

Table 3 Tensile properties of the as-built 316L in building and scanning directions.

	Yield strength (MPa)	Elongation to failure (%)
Building direction	378 ± 3	54.5 ± 1
Scanning direction	440 ± 5	51.5 ± 0.8

Fig. 9. The yield strength and ductility data of the DLD-LENS and conventional 316L SS from literature (the elongation to failure (elongation at break) was used).

Fig. 10. SEM fractographs of the LENS 316L after tensile test along building direction (a) low magnification view; (b) typical ductile dimple fracture; (c) evidence of secondary cracking.

Fig. 11. (a) SEM micrograph of the cross section of the broken specimen; (b,c) SEM image of the area where EBSD analyzes were carried out with the corresponding IPF along the building direction + IQ; (d) misorientation profile along the black line in (c).

Fig. 12. (a) TEM bright-field micrograph revealing a high density of twins in the as-built sample strained to fracture; (b) the dark-field micrograph imaged using the diffraction spot outlined by the dashed circle in (c); (c) the corresponding electron diffraction pattern EDP (subscripts “T’’ and “M’’ refer to “Twin’’ and “Matrix’’ respectively).

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