Interfacial characteristics and mechanical properties of additive manufacturing martensite stainless steel on the Cu-Cr alloy substrate by directed energy deposition

doi:10.1016/j.jmst.2021.03.008

Abstract

Abstract:

Copper/steel is a typical bimetal functional material, combining the excellent electrical and thermal conductivity of copper alloy and the high strength and hardness of stainless steel. There has been recent interest in manufacturing copper/steel bimetal by directed energy deposition (DED) due to its layer-by-layer method. However, cracks tend to form on the copper/steel interface because of the great difference in thermal expansion coefficient and crystal structure between copper and steel. In this work, interfacial characteristics and mechanical properties of the copper/steel bimetal were studied from one layer to multilayers. The laser power has a great influence on the Cu element distribution of the molten pool, affecting the crack formation dramatically on the solidification stage. Cracks tend to form along columnar grain boundaries because of the Cu-rich liquid films and spherical particles in the cracks. Crack-free and good metallurgical bonding copper/steel interface is formed at a scanning velocity of 800 mm/min and the laser power of 3000 W. The ultimate tensile strength (UTS) and the break elongation (EL) of the vertically combined crack-free copper/steel bimetal are 238.2 ± 4.4 MPa and 20.6 ± 0.7%, respectively. The fracture occurs on the copper side instead of the copper/steel interface, indicating that the bonding strength is higher than that of the Cu-Cr alloy. The UTS of the horizontally combined crack-free copper/steel bimetal is 746.7 ± 22.6 MPa, which is 200% higher than that of the Cu-Cr alloy substrate. The microhardness is 398.6 ± 5.4 HV at the steel side and is 235.3 ± 64.1 HV at the interface, which is 400% higher than that of the Cu-Cr alloy substrate. This paper advances the understanding of the interfacial characteristics of heterogeneous materials and provides guidance and reference for the fabrication of multi-material components by DED.

Key words: Directed energy deposition, Additive manufacturing, Bimetal, Interfacial characteristics, Crack

Wenqi Zhang, Hailong Liao, Zhiheng Hu, Shasha Zhang, Baijin Chen, Huanqing Yang, Yun Wang, Haihong Zhu. Interfacial characteristics and mechanical properties of additive manufacturing martensite stainless steel on the Cu-Cr alloy substrate by directed energy deposition[J]. J. Mater. Sci. Technol., 2021, 90: 121-132.

Figures/Tables 19

Fig. 1. Schematic diagram of the DED process.

Fig. 2. The starting stainless steel powder: (a) SEM image and (b) particle distribution.

Table 1 Chemical compositions of the starting powder.

Elements	C	Cr	Ni	Mo	Fe
wt.%	0.05-0.08	13.5-15.00	5.20-5.70	0.80-1.00	balance

Table 2 Thermophysical properties of Cu and Fe at room temperature [6, 32].

Parameters	Cu	Fe
Density (kg·m^-3)	8900	7780
Melting point (K)	1358	1811
Thermal conductivity (W·m^-1·K^-1)	400	80
Thermal expansion coefficient (10^-5 K^-1)	1.67	1.23
Laser beam absorptivity [33], [34]	0.02 ~ 0.2	0.2 ~ 0.6

Table 3 Processing parameters used for the Cu-Cr substrate.

Parameters	Values
Laser power (P, kW) Scanning velocity (V, mm/min) Hatching space (H, mm) Powder feeding rate (g/min) Z-axis increment (mm) Laser spot diameter (mm)	2.0, 2.5, 3.0, 3.5, 4.0, 4.5 800 0.3 12 0.3 1

Table 4 Processing parameters used for the steel substrate.

Parameters	Values
Laser power (P, kW) Scanning velocity (V, mm/min) Hatching space (H, mm) Powder feeding rate(g/min) Z-axis increment (mm) Laser spot diameter (mm)	1.5 1000 0.4 12 0.3 1

Fig. 3. Details of the DED (Directed energy deposition) experiments: (a) schematic of the copper/steel bimetal specimen fabrication and the cross-section specimen; (b) configuration of the horizontally combined bimetal tensile specimens; (c) configuration of the vertically combined bimetal tensile specimens; (d) dimension feature of the tensile specimen.

Fig. 4. OM images of the cross-section of the one-layer copper/steel specimens at different laser power of (I-1) 2000 W, (I-2) 2500 W, (I-3) 3000 W, (I-4) 3500 W, (I-5) 4000 W, and (I-6) 4500 W.

Fig. 5. OM images of the cross-section of the two-layer copper/steel specimens at different laser powers of (II-1) 2500 W, (II-2) 3000 W, (II-3) 3500 W, and (II-4) 4000 W.

Fig. 6. OM images of the cross-section of the five-layer copper/steel specimens at different laser powers of (V-1) 2500 W, (V-2) 3000 W, (V-3) 3500 W, (V-4a) 4000 W at the bottom area, and (V-4b) 4000 W at the top area.

Fig. 7. The cross-section microstructures of the specimen II-3 (P = 3500 W): (a) Low-magnification SEM overview of the interface, (b) zoom-in image of the columnar grain corresponding to region b marked in (a), and (c) zoom-in image of the crack morphology corresponding to region c marked in (b).

Fig. 8. EDS map of individual Fe, Cr, Cu, and Ni elements in the cracked specimen.

Fig. 9. Quantitative chemical maps of the copper/steel interface by EPMA: (a) specimen II-2 (P = 3000 W), (b) specimen II-3 (P = 3500 W), (c) specimen II-4 (P = 4000 W).

Fig. 10. The Fe-Cu phase diagram [41].

Fig. 11. Schematic diagram of the formation mechanism of the DED fabricated copper/steel interface: (a1-a3) specimen II-2 (P = 3000 W), (b1-b3) specimen II-3 (P = 3500 W), and (c1-c3) specimen II-4 (P = 4000 W).

Table 5 The processing parameters for fabricating steel on the Cu-Cr substrate with multilayers.

Parameters	Specimen V-2 and LX-2
Parameters	1 - 2 layer	3rd layer and above
Laser power (P, kW) Scanning velocity (V, mm/min) Hatching space (H, mm) Powder feeding rate (g/min) Z-axis increment (mm) Laser spot diameter (mm)	3.0 800 0.3 12 0.3 1.0	1.5 1000 0.4 12 0.3 1.0

Fig. 12. Microhardness profile of the specimen II-2 and II-3 along the copper/steel interface.

Fig. 13. Stress-strain curves of tensile specimens: (a) vertically combined crack-free copper/steel bimetal (specimen LX-2); (b) Cu-Cr alloy substrate; (c) horizontally combined crack-free copper/steel bimetal (specimen V-2); (d) DED fabricated steel on the Cu-Cr substrate.

Fig. 14. SEM images showing the fracture morphologies of the horizontally combined copper/steel bimetal (specimen V-2): (a) steel side; (b) copper/steel interface; (c) copper side.

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