The effects of fabrication atmosphere condition on the microstructural and mechanical properties of laser direct manufactured stainless steel 17-4 PH

doi:10.1016/j.jmst.2019.03.009

Abstract

Abstract:

The effects of atmosphere conditions on microstructural and mechanical properties of stainless steel 17-4PH components fabricated by laser direct manufacturing (LDM) were investigated through measurements on phase constitution, porosity, tensile strength, fracture morphology, hardness and evolution of substrate temperature. Results showed that the samples produced in air atmosphere condition possessed higher tensile strength and hardness for both as-deposited and heat-treated states than that in Ar chamber condition, due to dispersion strengthening effect of amorphous oxide particles and nitrogen solution strengthening as a result of higher content of oxygen and nitrogen. The temperature of substrate heat accumulation was higher in Ar chamber condition, leading to dramatically lower porosity and more reverse austenite, which also contributed to the lower strength and hardness.

Key words: Additive manufacturing, Laser direct manufacturing, 17-4 PH stainless steel, Fabrication atmosphere, Mechanical properties

D. Wang, C.T. Chi, W.Q. Wang, Y.L. Li, M.S. Wang, X.G. Chen, Z.H. Chen, X.P. Cheng, Y.J. Xie. The effects of fabrication atmosphere condition on the microstructural and mechanical properties of laser direct manufactured stainless steel 17-4 PH[J]. J. Mater. Sci. Technol., 2019, 35(7): 1315-1322.

Figures/Tables 14

Fig. 1. (a) Schematic view of a typical LDM process; and (b)17-4PH samples fabricated by LDM.

Table 1 Composition of stainless steel 17-4PH powder used in the present work (wt.%).

C	Cr	Mn	Ni	Cu	Nb	Si	Fe
0.04	16.70	0.75	5.03	4.28	0.275	0.68	Bal.

Table 2 Experimental conditions of the LDM processes.

Parameters	Values
Laser power P (W)	800
Laser beam diameter D (mm)	2.0
Overlapping ε (%)	50
Scanning speed V_S (mm/min)	600
Powder feeding speed V_F (r/min)	0.5
Flow rate of protection gas (Ar) V_G (L/min)	7-10
Atmosphere conditions (ppm)	O₂<10, H₂O<3 for Ar chamber condition O₂>1000, H₂O>1000 for air atmosphere condition
Chamber pressure p (mbar)	1-3

Fig. 2. Samples for tensile test and microstructure observation.

Fig. 3. Dimensions of tensile samples.

Table 3 Mechanical properties of the LDMed 17-4PH fabricated in different atmosphere conditions.

Conditions	σ_b (MPa)	σ_s/σ_0.2 (MPa)	δ (%)
Air atmosphere (As-deposited)	1043 ± 3	681 ± 31	18.7 ± 0.8
Air atmosphere (Heat-treated)	1145 ± 3	1076 ± 11	16.8 ± 1.2
Ar chamber (As-deposited)	1005 ± 5	574 ± 16	19.4 ± 1.4
Ar chamber (Heat-treated)	1070 ± 20	856 ± 10	17.3 ± 0.3

Fig. 4. Microhardness of the LDMed 17-4PH components fabricated in different atmosphere conditions.

Fig. 5. Porosity of the LDMed 17-4PH components fabricated in different atmosphere conditions.

Fig. 6. XRD patterns of the LDMed 17-4PH samples fabricated in different conditions.

Table 4 Content of oxygen and nitrogen in the LDMed 17-4PH samples in different conditions (wt%).

Conditions	N content	O content
Air AD-800 W	0.076	0.073
Air HT-800 W	0.083	0.035
Ar AD-800 W	0.066	0.035
Ar HT-800 W	0.067	0.035

Fig. 7. Microstructure of the LDMed 17-4PH samples fabricated in different atmosphere conditions: (a) Air AD 800 W; (b) Air HT 800 W; (c) Ar AD 800 W; (d) Ar HT 800 W.

Fig. 8. Dispersed particles in the LDMed 17-4PH samples of Air AD 800 W: (a) SEM image showing two kinds of precipitates in tensile fracture surface; (b) EDS results of the particle in black box shown in (a); (c) TEM/EDX and SADP images of the particle in blue circle shown in (a).

Fig. 9. Tensile fracture morphology of LDMed 17-4PH samples in different conditions: (a, e) Air AD 600 W; (b, f)Air HT 800 W; (c, g)Ar AD 800 W; and (d, h)Ar HT 800 W.

Fig. 10. Substrate heat accumulation temperature curves for different atmosphere conditions.

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