Selective laser melting (SLM) of CX stainless steel: Theoretical calculation, process optimization and strengthening mechanism

doi:10.1016/j.jmst.2020.09.031

Abstract

Abstract:

In the present work, selective laser melting (SLM) technology was utilized for manufacturing CX stainless steel samples under a series of laser parameters. The effect of laser linear energy density on the microstructure characteristics, phase distribution, crystallographic orientation and mechanical properties of these CX stainless steel samples were investigated theoretically and experimentally via scanning electron microscope (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). Based on the systematic study, the SLM CX stainless steel sample with best surface roughness (R_a = 4.05 ± 1.8 μm) and relative density (Rd = 99.72 %±0.22 %) under the optimal linear density (η = 245 J/m) can be obtained. SLM CX stainless steel was primarily constituted by a large number of fine martensite (α’ phase) structures (i.e., cell structures, cellular dendrites and blocky grains) and a small quantity of austenite (γ phase) structures. The preferred crystallographic orientation (i.e., <111> direction) can be determined in the XZ plane of the SLM CX sample. Furthermore, under the optimal linear energy density, the good combinations with the highest ultimate tensile strength (UTS = 1068.0 %±5.9 %) and the best total elongation (TE = 15.70 %±0.26 %) of the SLM CX sample can be attained. Dislocation strengthening dominates the strengthening mechanism of the SLM CX sample in as-built state.

Key words: Selective laser melting, CX stainless steel, Forming quality, Mechanical property, Strengthening, Mechanism

Dongdong Dong, Cheng Chang, Hao Wang, Xingchen Yan, Wenyou Ma, Min Liu, Sihao Deng, Julien Gardan, Rodolphe Bolot, Hanlin Liao. Selective laser melting (SLM) of CX stainless steel: Theoretical calculation, process optimization and strengthening mechanism[J]. J. Mater. Sci. Technol., 2021, 73: 151-164.

Figures/Tables 19

Fig. 1. (a) SEM micrograph of the CX powders utilized in this study and the etched cross-sectional figure of a single particle; (b) powder particle size distribution; (c) scanning method of the SLM manufacturing and the CX steel parts fabricated applying SLM technology.

Table 1 Chemical composition of CX steel powders provided by EOS GmbH [24].

	Composition (wt.%)
	Fe	Cr	Ni	Mo	Al	Mn	Si	C
Min.	Bal.	11.0	8.4	1.1	1.2	/	/	/
Max.	/	13.0	10.0	1.7	2.0	0.4	0.4	0.05

Table 2 SLM parameters adopted in this work.

Laser parameters	Value
Laser spot diameter	100 μm
Overlaps stripes	0.12 mm
Stripe width	9.75 mm
Hatch distance	100 μm
Layer thickness	30 μm
Laser power	200, 250, 260, 300, 350 W
Scanning speed	800, 900, 960, 1000, 1060, 1100, 1160, 1200 mm/s

Table 3 Physical properties and manufacturing parameters of the raw materials for theoretical calculation.

Physical parameters	Symbol	Value
Density	ρ	7.7 × 10³ kg/m³ [24]
Specific heat capacity	C_p	470 J/(kg K) [52]
Melting point	T_m	1723 K [52]
Initial temperature	T_i	353 K
Latent heat of fusion	L_l	2.8 × 10⁵ J/kg [52]
The absorptivity of stainless steel powders	η	0.6 [53]
Laser spot radius	r_l	5 × 10^-5 m
Radius of the equivalent sphere (as shown in Fig. 2(b))	R	1.6 × 10^-4 m
Effective depth of the melting zone (as shown in Fig. 2(b))	H	4.5 × 10^-5 m

Fig. 2. (a) Schematic diagram of the heat transfer process of the molten pool in the SLM fabrication; (b) the enlarged view of (a) and the macro appearance of the SLM single track of CX steel (XY plane and XZ plane, respectively).

Fig. 3. Surface roughness of the SLM CX samples manufactured under different linear energy density using large-scale processing parameters.

Fig. 4. (a) Ra of the SLM CX specimens under different linear energy density obtained in further experimental optimization; (b) relationship between Ra and the linear energy density.

Fig. 5. Microstructures in the unetched state of the SLM CX samples manufactured under different linear energy density using large-scale processing parameters.

Fig. 6. (a) Rd of the SLM CX specimens under different linear energy density obtained in further experimental optimization; (b) relationship between Rd and the linear energy density.

Fig. 7. (a) XRD patterns of the SLM CX samples under different linear energy density; (b) enlarged view of α-Fe phase (110) crystal plane; phase distribution of the SLM CX samples under η = 245 J/m detected using EBSD method in (c) XY plane and (d) XZ plane.

Fig. 8. Microstructures of the cross-sections in XY plane of the SLM CX samples under different linear energy density: (a, b, c) microstructures of the SLM CX sample under η = 182 J/m; (d, e, f) microstructures of the SLM CX sample under η = 245 J/m; (g, h, i) microstructures of the SLM CX sample under η = 333 J/m. First row: SEM micrographs before polishing. Second row: OM micrographs after etching. Third row: SEM micrographs after etching.

Fig. 9. EBSD analysis of the SLM CX samples under η = 245 J/m: (a) IPF-Z of the cross-sections in XY plane; (b) grain size distribution in XY plane; (c) IPF-Y of the cross-sections in XZ plane; (d) grain size distribution in XZ plane.

Fig. 10. (a) KAM of the cross-sections in XY plane of the SLM CX samples; (b) local disorientation map plotted according to (a); (c) KAM of the cross-sections in XZ plane of the SLM CX samples; (d) local disorientation map plotted according to (c).

Fig. 11. (a) Slip system of the cross-sections in XY plane of the SLM CX samples; (b) SF map plotted according to (a); (c) slip map of the cross-sections in XZ plane of the SLM CX samples; (d) SF map plotted according to (c).

Fig. 12. Mechanical properties of the SLM CX samples: (a) microhardness and (b) stress-strain curves with macro fracture surface of the SLM CX specimens under different linear energy density.

Table 4 Mechanical properties of the SLM CX samples.

Samples	Microhardness (HV_0.05)	YS (MPa)	UTS (MPa)	TE (%)
η = 182 J/m	325 ± 3.5	892 ± 10.7	1033 ± 4.4	11.1 ± 0.55
η = 245 J/m	351 ± 4.8	889 ± 7.3	1068 ± 5.9	15.7 ± 0.26
η = 333 J/m	342 ± 4.2	886 ± 12.3	1059 ± 2.8	14.7 ± 0.26

Fig. 13. (a) Fractographies of the SLM CX samples fabricated under different linear energy densities after tensile test: (a, b, c) η = 182 J/m; (d, e, f) η = 245 J/m; (g, h, i) η = 333 J/m. Left column: low magnification images. Middle column: high magnification images. Right column: enlarged views.

Fig. 14. Bright field image (BFI) of the SLM sample in (a) XY plane [25] and (b) XZ plane.

Table 5 Strength contributions of various strengthening mechanisms for SLM CX sample in as-built state.

Strengthening contribution (MPa)	SLM CX sample
σ_gb	236.50
σ_d	274.37
σ_t	205.72
σ_p	164.53
Calculated σ_YS	881.12
Experimental σ_YS	888.50

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