Ductility Anisotropy Induced by Ferrite in Direct Laser Deposited 17-4 PH Steel: Combined Microstructure and Dislocation Density Simulation

doi:10.1007/s40195-020-01172-3

Abstract

Abstract:

The anisotropic ductility of a direct laser deposited 17-4 PH cubic part was investigated. Anisotropic elongations in the specimens from varied surfaces of the part were obtained: ~ 6.2%, ~ 1.5%, and ~ 4.5% in XY, YZ, and XZ samples, respectively. Furthermore, various orientations of ferrite were found in different specimens, taking the loading direction as reference. A finite element analysis depending on actual microstructures and dislocation density revealed that the orientation of ferrite caused the ductility anisotropy. The orientation of ferrite affected its plastic deformability and the deformation compatibility between phases during the uniaxial loading. The ferrite parallel to the tensile direction in the YZ sample had the worst deformability and induced severe strain localization and stress triaxiality, which resulted in inferior ductility. The ferrite perpendicular to the tensile direction showed the best deformability, whereas strain localization remained intense in the XZ sample owing to the unmatched deformability of martensite. The inclined ferrite in the XY sample exhibited moderate deformability and was found to enhance the plastic flow of martensite, leading to the best deformation compatibility and ductility.

Key words: Direct laser deposition, Stainless steel, Ductility, Anisotropy, Microstructure, Finite element analysis

Chuanfeng Wu, Junmei Chen, Zhiyuan Yu, Hao Lu, Chun Yu, Jijin Xu. Ductility Anisotropy Induced by Ferrite in Direct Laser Deposited 17-4 PH Steel: Combined Microstructure and Dislocation Density Simulation[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(6): 765-776.

Figures/Tables 16

Table 1 Chemical composition (wt %) of 17-4 PH powder

C	Mn	P	S	Si	Cr	Ni	Cu
0.049	0.48	0.019	0.003	0.52	16.02	4.00	3.35

Fig. 1 a Alternating cross scan strategy used in the DLD process; b extraction scheme of the tensile specimens from the integrated 17-4 PH part; building direction was along the Z-axis, and scanning surfaces were parallel to the XY plane; c geometry of tensile specimens; the blue area of the sample was cut for OM and EBSD characterization after tensile tests

Table 2 Summary of tensile properties in different surfaces of the DLD 17-4 PH part

	Yield stress (MPa)	Ultimate stress (MPa)	Uniform elongation (%)
XY	624 (+10.75, - 10.05)	1209.72 (+8.09, - 9.02)	6.23 (+1.69, - 1.25)
YZ	1001.2 (+72.65, - 58.42)	1232.68 (+16.57, - 8.30)	1.45 (+0.78, - 0.97)
XZ	685.63 (+58.37, - 69.04)	1171.39 (+8.07, - 11.29)	4.45 (+0.57, - 0.31)

Fig. 2 Engineering stress-strain curves of extracted 17-4 PH specimens in different surfaces

Fig. 3 Real microstructures of the samples: a, d XY sample; b, e YZ sample; c, f XZ sample. White dash lines in d-f show the prevalent orientations of ferrite in the current sample: inclined, vertical and horizontal ferrite, respectively

Fig. 4 EBSD phase maps of a XY, b YZ, c XZ samples. The analyzed area for each sample was 200 μm?×?150 μm

Fig. 5 EBSD inverse pole figures of a XY, c YZ, e XZ; EBSD pole figures of b XY, d YZ, f XZ

Fig. 6 Microstructural model (based on Fig. 3d-f) of a XY sample, b YZ sample, c XZ sample. The red area in a-c stands for the ferrite phase, and the blue area is the martensite phase. The enlarged figure in a shows details of the meshed model; LD represents the loading direction in the simulations

Table 3 Calibrated parameters used in the dislocation density model of martensite and ferrite

	XY		YZ		XZ
	Martensite	Ferrite	Martensite	Ferrite	Martensite	Ferrite
σ₀ (MPa)	605.7	404.2	956.5	646	614.5	413
U (m^-2)	9.5 × 10¹⁶	6.9 × 10¹⁵	7.5 × 10¹⁶	5.7 × 10¹⁵	6.7 × 10¹⁶	8.8 × 10¹⁵
Ω	48.4	3.7	43.2	31.6	17.2	1.8
ρ₀ (m^-2)	3.4 × 10¹³	1.75 × 10¹³	3.8 × 10¹³	1.95 × 10¹³	3.1 × 10¹³	1.05 × 10¹³

Fig. 7 Comparisons of stress-strain curves between simulations and experiments

Fig. 8 Distributions of stress (×?106 MPa), equivalent plastic strain at the global strain of 0.02: a, d XY sample; b, e YZ sample; c, f XZ sample

Fig. 9 Effect of ferrite orientations on degree of plastic deformation in ferrite at the global strain of 0.02. a-c Representative area of the XY, YZ, and XZ samples; d distributions of equivalent plastic strain along the corresponding arrows in a-c

Fig. 10 Evolutions of the rate of dislocation density in ferrite from different samples

Fig. 11 a Discrepancy (Δεp) in average equivalent plastic strain (PEEQ) between ferrite and martensite; b evolutions of strain localization factor in different samples

Fig. 12 a Relative frequency of kernel average misorientation (KAM) from deformed samples; b relative frequency of simulated equivalent plastic strain

Fig. 13 Stress triaxiality distributions of a XY sample, b YZ sample, c XZ sample, at the global strain of 0.02; d XY sample, e XZ sample at the global strain of 0.045

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