High mechanical strengths and ductility of stainless steel 304L fabricated using selective laser melting

doi:10.1016/j.jmst.2018.10.013

Abstract

Abstract:

Achieving not only high mechanical strengths but also high ductility is recently established using an additive manufacturing technique called selective laser melting. In the present study, stainless steel 304L fully dense samples were successfully printed using the 3D systems - ProX 300 printing machine. The ductility and tensile yield strength were almost two and three times higher compared to those of ASTM cast’s alloy. Honey comb like nano-cellular structure with different orientation was observed in the fine grains (～4 μm) due to fast cooling rate. In addition, the formation of martensite phase in random grains is also a contributor to the strengths. Furthermore, negative residual stresses in the build and horizontal directions were detected and assisted further increase in the tensile strength. Fractography revealed the ductile feature of plastic deformation and the crack openings at unmelted particles or pores.

Key words: Additive manufacturing, Stainless steel, Powder, Selective laser melting

Q.B. Nguyen, Z. Zhu, F.L. Ng, B.W. Chua, S.M.L. Nai, J. Wei. High mechanical strengths and ductility of stainless steel 304L fabricated using selective laser melting[J]. J. Mater. Sci. Technol., 2019, 35(2): 388-394.

Figures/Tables 14

Table 1 Chemical composition of stainless steel 304L (wt%).

Cr	Ni	Si	Mn	P	S	C	O	N	Fe
18.02	8.83	0.41	0.80	0.02	<0.01	<0.01	0.005	0.002	Balance

Fig. 1. Surface morphology and cross-section of stainless steel 304L powder.

Fig. 2. Rheological characteristics of stainless steel 304L powder.

Table 2 Particle size distribution, density and rheological characteristics of stainless steel 304L.

Material	D₁₀	D₅₀	D₉₀	Hall flow rate (s/50 g)
Material	(μm)	(μm)	(μm)	Hall flow rate (s/50 g)
SS304L	27.26 ± 1.04	40.32 ± 1.12	58.02 ± 1.03	19.25 ± 0.34
Apparent density	Tapped density	True density	Packing at apparent density (%)	Packing at tapped density
(g/cm³)	(g/cm³)	(g/cm³)	Packing at apparent density (%)	(%)
4.29 ± 0.09	5.15 ± 0.07	7.97 ± 0.06	53.82 ± 0.6	64.61 ± 0.08
BFE	SI	FRI	SE	CBD
(mJ)	SI	FRI	(mJ/g)	(g/ml)
751 ± 5	1.03 ± 0.05	1.19 ± 0.04	3.03 ± 0.06	4.61 ± 0.02

Fig. 3. Process parameters optimization for stainless steel 304L.

Fig. 4. X-ray diffractography showing the gamma (fcc) matrix and the martensite phase (bcc).

Fig. 5. X-ray diffractography showing negative residual stress in build and transverse direction.

Fig. 6. Micrograph of optimized parameters sample showing near fully dense condition.

Fig. 7. Optical micrograph showing 3D view microstructure, heat flow effect in the build direction and nearly equi-axed grains in the transverse direction.

Fig. 8. EBSD images showing elongated grains in the build direction and equi-axed grains in the transverse direction along with their weak texture.

Fig. 9. EBSD image showing the presence of martensite phase (needle shape) within a random grain.

Fig. 10. Formation of very fine honey comb like nano-cellular structure within grains and the formation of very fine nano-carbides.

Fig. 11. Fractography showing plastic deformation feature with void coarsening at the defect region (unmelted-submicron particles).

Fig. 12. Tensile and ductility data of stainless steel 304L produced by SLM versus other manufacturing methods.

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