Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting

doi:10.1016/j.jmst.2019.12.020

摘要/Abstract

Abstract:

300M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles, owing to its high strength, excellent transverse plasticity, fracture toughness and fatigue resistance. Herein, low and high power selective laser melting (SLM) of 300M steel and their microstructural evolution and mechanical properties have been reported. The results show that the optimal energy density range with the highest relative density for SLMed 300M steel is between 60 and 160 J/mm³. Furthermore, molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300-600 W but a keyhole mode with “U” shape at a higher laser power of 800-1900 W. The heterogeneous microstructure of as-built samples is characterized by a skin-core structure which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone. The skin-core structure of SLMed 300M steel has the characteristics of hard inside and soft outside. The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900 W. Interestingly, ultimate tensile strength (1156-1193 MPa) and yield tensile strength (1085-1145 MPa) of dense samples fabricated at different laser powers vary marginally. But, the elongation (6.8-9.1%) of SLMed 300M steel is greatly affected by the laser power.

Key words: Selective laser melting (SLM), 300M ultra-high strength steel, Microstructural evolution, Mechanical properties

. [J]. 材料科学与技术, 2020, 48(0): 44-56.

Guanyi Jing, Wenpu Huang, Huihui Yang, Zemin Wang. Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting[J]. J. Mater. Sci. Technol., 2020, 48(0): 44-56.

图/表 21

Table 1 Chemical composition of 300M steel powders.

Elements	C	Si	Mn	Cr	Ni	Mo	Cu	V	Fe
Mass fraction%	0.42	1.79	0.84	0.99	1.68	0.40	0.10	0.09	Bal.

Fig. 1. SEM image of spherical 300M steel powders.

Fig. 2. Laser scanning strategy used in the SLM process.

Fig. 3. Configuration of the tensile specimens.

Fig. 4. Relationship between the volumetric energy density and relative densities of as-deposited samples processed at various laser powers. Optical micrographs reveal lack of fusion voids in the samples with different relative densities.

Table 2 Parameters used to produce the cubic samples with relative density more than 99.9% by SLM.

Laser power, P (W)	Layer thickness, δ (mm)	scanning velocity, v (mm/s)	hatch spacing, h (mm)	P/v (J·mm^-1)	P/v (W·s^0.5·mm^-0.5)
300	0.04	800	0.10	0.375	10.6
600	0.08	600	0.12	1.0	24.5
800	0.08	800	0.14	1.0	28.3
1000	0.12	600	0.16	1.67	40.8
1900	0.16	1000	0.18	1.9	60.1

Fig. 5. Build-up rate and volumetric energy density for the SLM process under various laser powers. δ represents the layer thickness.

Fig. 6. Optical micrographs of XZ section of 300M steel bulk samples under various laser powers: (a) 300 W, (b) 600 W, (c) 800 W, (d) 1000 W, (e) 1900 W. The red two-way arrows measure the depth and half width of the molten pools.

Fig. 7. Measured width, depth and aspect ratio of the molten pools at various laser powers.

Fig. 8. XRD patterns of 300M steel powders and SLMed samples processed with various laser powers: (a) overview, (b) details inside the black dotted bordered rectangle of (a).

Fig. 9. SEM images of the XZ sections of as-deposited 300M steel samples under various laser powers: (a) 300 W, (b) 600 W, (c) 800 W, (d) 1000 W, (e) 1900 W. Zone A and B represent the microstructure in the molten pool and heat-affected zone of as-deposited samples, respectively. Boundaries of columnar prior austenite grains (PAG) are marked by yellow dashed lines. Alpha ferrite (α-Fe) and granular carbides are indicated by the black and pink arrows, respectively.

Fig. 10. Microstructural characteristics such as PAG sizes and the sizes of α-Fe in the molten pools (MP) and heat-affected zones (HAZ) of as-deposited samples at different laser powers.

Fig. 11. Inverse pole figure color maps with high-angle (>10°) boundaries of as-deposited samples under various laser powers: (a) 300 W, (b) 600 W, (c) 800 W, (d) 1000 W, (e) 1900 W.

Fig. 12. Grain size distribution and average grain size of SLMed samples under various laser powers.

Fig. 13. Misorientation of BCC crystals: (a) misorientation distribution of BCC crystals, (b) high and low misorientations of BCC crystals.

Fig. 14. EBSD grain-boundary maps of 300M steel samples fabricated at different laser powers: (a) 300 W, (b) 600 W, (c) 800 W, (d) 1000 W, (e) 1900 W.

Fig. 15. Microhardness distribution and average microhardness of the XZ sections of as-deposited 300M steel samples under various laser powers.

Fig. 16. Tensile properties of samples fabricated at different laser powers.

Fig. 17. Fracture morphologies of SLM-processed 300M steel tensile test pieces under different laser powers: (a) 300 W, (b) 600 W, (c) 800 W, (d) 1000 W, (e) 1900 W.

Fig. 18. Schematic diagram of microstructural evolution. Steps 1 to 4 describes successive variable processes of the microstructure of a bulk deposited by SLM.

Fig. 19. Schematic representation of heat gradient and heat history for the two different melt pool modes: (a) conduction mode and (b) keyhole mode.

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