Functionally Graded Stainless Steel Fabricated by Direct Laser Deposition: Anisotropy of Mechanical Properties and Hardness

doi:10.1007/s40195-017-0668-2

Abstract

Abstract:

Direct laser metal deposition is a kind of advanced rapid manufacturing technology, which can produce near net shape parts by depositing metal powders layer by layer. This study demonstrates fabrication, the anisotropy of mechanical properties and hardness of a graded steel. The characteristics of constituent phases, microstructure, mechanical anisotropy, and microhardness were investigated using electron backscatter diffraction, optical microscopy, tensile test machine, and microhardness tester. It was found that the graded steel is dense and free of cracks. The crystal structures of the as-built samples evolved in three grades from fcc structures to fcc + bcc structures and then to bcc + fcc structures. Samples in x and z directions showed obvious mechanical anisotropy. The samples machined in x direction showed higher strength and lower elongation than those machined in z direction due to the presence of lack-of-fusion pores and the higher metallurgical bonding between layers in the x direction. The microhardness of the as-built samples increased along the cross section from the substrate (159.7 HV) to the top surface (545.4 HV).

Key words: Graded steel, Direct laser metal deposition, Electron backscatter diffraction (EBSD), Anisotropy of mechanical properties

Qiang Wang, Song Zhang, Chun-Hua Zhang, Chen-Liang Wu, Ling Ren, Jian-Qiang Wang, Jiang Chen. Functionally Graded Stainless Steel Fabricated by Direct Laser Deposition: Anisotropy of Mechanical Properties and Hardness[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(1): 19-26.

Figures/Tables 8

Table 1 Chemical composition of steel powders used in DLMD (wt%)

	C	Si	B	Cr	Ni	Mo	Mn	Fe
Alloy powder 1	≤ 0.07	0.61	0.4	17.89	13.1	2.49	≤ 0.49	Bal.
Alloy powder 2	≤ 0.07	0.83	0.4	17.33	8.54	1.89	≤ 0.49	Bal.
Alloy powder 3	≤ 0.14	1.04	1	16.78	3.98	1.30	≤ 0.50	Bal.

Fig. 1 Schematic of the DLMD process a, mechanical test samples before b and after c fracture

Fig. 2 Phase distributions of the as-built samples by EBSD: a the first grade, b the second grade, c the third grade. Red: fcc; green: bcc; yellow: C6Cr23; blue: C3Fe7

Fig. 3 Macroscopic morphologies of the graded steel by DLMD

Fig. 4 Typical tensile stress-strain curves a the samples in the first grade; b the samples in the second grade; c the samples in the third grade

Table 2 Mechanical properties of the samples

Materials	Yield strength, σ _0.2 (ΜPa)	Tensile strength, σ _b (ΜPa)	Elongation (%)	Reduction of area, ψ (%)
First grade-x	529 ± 30	951 ± 79	6.8 ± 1.5	8.4 ± 3.9
First grade-z	501 ± 19	765 ± 20	12.0 ± 0.17	10 ± 1.7
Second grade-x	485 ± 40	1050 ± 91	0.67 ± 0.39	1.9 ± 1.4
Second grade-z	469 ± 38	761 ± 49	10 ± 0.57	8.1 ± 1.0
Third grade-x	675 ± 114	1056 ± 207	0.87 ± 0.55	1.1 ± 0.99
Third grade-z	507 ± 23	721 ± 30	12 ± 0.77	8.9 ± 0.42
LENS SS16L [33] (x direction)	576	776	33	-
LENS SS16L [33] (z direction)	479	703	46	-
LMDS part [35] (z direction)	558	639	21	-
LMDS part [35] (x direction)	352	536	46	-
DLD/Single-Built 316L(as-built) [39]	405-415	620-660	32-40	-
DLD/Single-Built 316L(heat-treated) [39]	325-355	600-620	42-43	-
DLD/Nine-Built (as-built) [39]	465-485	660-685	12-20	-

Fig. 5 Schematic diagram of the mechanical anisotropy

Fig. 6 SEM micrographs of the fracture surfaces of the samples: a and b the samples in x direction in the first grade; c and d the samples in the z direction in the first grade; e and f the samples in x direction in the second grade; g and h the samples in z direction in the second grade; i the samples in x direction in the third grade; j the samples in z direction in the third grade

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