Effect of interlayer addition on microstructure and mechanical properties of NiTi/stainless steel joint by electron beam welding

doi:10.1016/j.jmst.2020.05.043

Abstract

Abstract:

NiTi/Stainless Steel (SS) sheets have been welded via a vacuum electron beam welding process, with three methods (offsetting electron beam to SS side without interlayer, adding Ni interlayer and adding FeNi interlayer), to promote mechanical properties of the NiTi/SS joints. The joints with different interlayers are all fractured in the weld zone near the NiTi side, which is attributed to the enrichment of intermetallic compounds including Fe₂Ti and Ni₃Ti. The fracture mechanisms of different joints are strongly dependent on the types of interlayers, and the joints without interlayer, adding Ni interlayer and adding FeNi interlayer exhibit cleavage fracture, intergranular fracture and mixed fracture composed of cleavage and tearing ridge, respectively. Compared with the brittle laves phase Fe₂Ti, Ni₃Ti phase can exhibit certain plasticity, block the crack propagation and change the direction of crack propagation. The composite structure of Ni₃Ti and Fe₂Ti will be formed when the FeNi alloy is taken as the interlayer, which provides the joint excellent mechanical properties, with rupture strength of 343 MPa.

Key words: NiTi, Stainless steel, Electron beam welding, Interlayer, Mechanical property

H. Niu, H.C. Jiang, M.J. Zhao, L.J. Rong. Effect of interlayer addition on microstructure and mechanical properties of NiTi/stainless steel joint by electron beam welding[J]. J. Mater. Sci. Technol., 2021, 61: 16-24.

Figures/Tables 18

Table 1 Chemical compositions (wt.%) and mechanical properties of base materials.

Material	Ni	Ti	Fe	Cr	Mn	C + Si + P+S	R_p0.2(MPa)	R_m(MPa)
NiTi	55.86	44.14	-	-	-	-	339	663
AISI 304	8.06	-	72.22	18.01	1.01	0.70	316	779

Fig. 1. The schematic diagram of electron beam welding. (a) without interlayer, (b) with interlayer.

Table 2 Welding parameters used in the study.

Welding Method	Acceleration Voltage (KV)	Beam Current (mA)	Focusing Current (mA)	Welding Speed (mm/min)	Focal Length (mm)
EBW	60	8-12	2325	1000	260

Fig. 2. The schematic diagram of tensile sample.

Fig. 3. (a) Face and back image of NiTi/SS electron beam welded sample. (b) X-ray inspection image of the three samples: 1-without interlayer, 2-with Ni interlayer, 3-with FeNi interlayer.(c) Cross section microstructure of the electron beam welded joint with FeNi interlayer.

Fig. 4. EBSD phase distribution of NiTi/SS electron beam welding joints with different interlayers. (a) without interlayer, (b) Ni interlayer, (c) FeNi interlayer. The yellow part represents γ-(Fe,Ni) phase, the red part represents intermetallic compounds contain Fe2Ti phase and Ni3Ti phase.

Table 3 Volume fraction of phases in the weld zone.

	No interlayer	Ni interlayer	FeNi interlayer
Austenite	53 %	57 %	91 %
Intermetallic compounds	47 %	43 %	9 %

Fig. 5. SEM micrographs showing intermetallic compounds region in NiTi/SS electron beam welding joint with different interlayer. (a) without interlayer, (b) Ni interlayer, (c) FeNi interlayer.

Fig. 6. Vickers microhardness distribution of NiTi/SS electron beam welding joints with different interlayer. (a) without interlayer, (b) Ni interlayer, (c) FeNi interlayer. Weld seams were outlined by white lines from Fig.4.

Fig. 7. Engineering stress-strain curves of electron beam welding joints with different interlayer addition and two base materials.

Fig. 8. Lateral micrographs of NiTi/SS electron beam welded joints. (a,d) without interlayer, (b,e) Ni interlayer, (c,f) FeNi interlayer, (d-f) show the higher magnification of red box in (a-c). The interface of the weld zone and NiTi base material was marked by white dotted line.

Fig. 9. Fracture surface morphologies of welded joints. (a) without interlayer, (b) Ni interlayer, (c) FeNi interlayer.

Fig. 10. Fe-Ni-Ti ternary phase diagram, isothermal cut at 1000℃.

Fig. 11. SEM and EBSD figures show microcracks near the fracture in intermetallic compounds region. (a, d) Without interlayer (b, e) Ni interlayer (c, f) FeNi interlayer. Blue color represents Fe2Ti phase and red color represents Ni3Ti phase. By reason of the internal stress inside the intermetallic compounds region, a few regions cannot be distinguished effectively, these regions show grey color. The cracks were marked by white dotted line.

Fig. 12. Schematic drawing of microcracks propagation in intermetallic compound region: (a) Without interlayer, (b) Ni interlayer, (c) FeNi interlayer.

Fig. 13. Fracture section microstructure of the joint with FeNi interlayer addition.

Fig. 14. SEM and EBSD figures of the main crack deflections in FeNi interlayer addition weld. (a, d) present a region in Fig. 13. (b, e) present b region in Fig. 13. (c, f) present c region in Fig. 13.

Fig. 15. Schematic drawing of main crack deflection in the joint with FeNi interlayer. Intermetallic compounds region is on the right side of dotted line, and austenite region with simplified representation is on the left side of dotted line. The columnar and exquiaxed dendrites of Fe2Ti phase are shown in the figure. Ni3Ti phase is distributed around Fe2Ti dendrites. Green lines represent model-I cracks and red lines represent model-II cracks.

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