Mechanical Properties of Diffusion Bonded Joints Between Titanium and Stainless Steel With Nickel Interlayer
By Kundu, S Chatterjee, S
Solid state diffusion bonded joints of commercially pure titanium and 304 stainless steel using a 300 [mu]m nickel interlayer were prepared at 850[degrees]C for 1.8-10.2 ks under 3 MPa load in vacuum. The diffusion bonds were characterised by light and scanning electron microscopy. The SEM-BSE images show the existence of different reaction layers in the diffusion zone. The composition of these layers was determined using an electroprobe microanalyser, indicating that TiNi^sub 3^, TiNi and Ti^sub 2^Ni are formed at the nickel/titanium interface and the stainless steel/nickel interface is free from intermetallics. The presence of intermetallics was confirmed by the X-ray diffraction technique. Nickel can inhibit the diffusion of Ti to the stainless steel side for all bonding times. A maximum tensile strength of ~312 MPa (~98% of Ti) and a shear strength of ~236 MPa (~81% of Ti) along with ~9.1% ductility were obtained for diffusion couple processed for 7.2 ks. Observation of fracture surfaces by SEM using EDS demonstrates that failure takes place mainly through the nickel/titanium interface. Keywords: Diffusion bonding, Interlayer, Intermetallics, X-ray diffraction technique
Titanium and titanium alloys have high specific strength and excellent corrosion resistance, which make them suitable for use in aerospace and nuclear industries.1’2 On the other hand, austenitic stainless steel is widely used in nuclear, chemical and aerospace industries due to its high strength and corrosion resistance. Diffusion bonded joints formed between these two materials have applications in vessels and reactors of chemical processing and nuclear industries respectively.3,4 The conventional fusion welding of these materials results in chemical, mechanical and structural heterogeneities due to the difference in linier co-efficiency of thermal expansion of the two materials.5
Literature reports that the direct bonding between titanium/ titanium alloy and stainless steel promotes the formation of various Fe-Ti and Fe-Cr-Ti base intermetallic compounds in the diffusion zone, because the solid solubility of Fe, Cr, Ni and Ti is limited in each other and these intermetallics weaken the mechanical properties of the transition joint.6_8 Formation of brittle intermetallics during direct bonding of these materials can be minimised by using appropriate intermediate materials. Nickel can also be considered as a useful intermediate material due to its excellent corrosion resistance for application at high temperature as compared to the bonded joints with copper interlayer. Kamat9 reported that a nickel-stainless steel diffusion couple is free from intermetallic. The Ni-Ti binary phase diagram shows that various intermetallics are formed with increasing Ni content.10 He et al.11 reported that Ni-Ti intermetallic phases have higher plasticity than that of Fe-Ti and Fe-Ti-Cr base intermetallics. Ghosh et al.12 reported that solid state diffusion bonding was carried out of titanium to stainless steel and a maximum tensile strength of ~242 MPa has been achieved at 800[degrees]C for 120 min. They also observed that the bond strength drops with a rise in the bonding temperature due to the increase in the thickness of intermetallics. In an earlier investigation, the present authors carried out solid state bonding with a nickel interlayer in the 800-950[degrees]C temperature range for 30 min under 3 MPa uniaxial pressure and achieved a maximum tensile strength of -281 MPa and a shear strength of -202 MPa along with 7-2% elongation at 900[degrees]C processing temperature.13
In the present investigation, diffusion bonded joints were produced between commercially pure titanium and 304 stainless steel using nickel as an interlayer and with a focus on mechanical properties and interface microstructure of the bonded joints with varying bonding time.
Commercially pure titanium (Ti) and 304 stainless steel (SS) used in the present investigation have dimensions of 25 x 400 mm (diameter x length) and 150 x 150 x 900 mm respectively. The chemical compositions are Ti-0-02C-0-10Fe-0-15O-0-02N-0-0011H and Fe- 0-06C-l-38Mn-0-37Si-0-013S-0-03P-18-15Cr-8-50Ni-0-005N (wt-%) respectively. The room temperature mechanical properties are given in Table 1.
Cylindrical specimens (phi 15 x 30 mm) were machined from the base metals. The mating surfaces of cylinders were prepared by conventional grinding and polishing techniques with final polishing on 1 [mu]m diamond paste. A 300 [mu]m thick nickel foil (purity 99- 5 at.-%) was used as an intermediate material and both the surfaces were polished in the same fashion. Mating surfaces were cleaned in acetone and dried in air. The Ti-Ni-SS assembly was placed in a fixture under 3 MPa uniaxial load applied along the longitudinal direction of the specimen and was inserted in a vacuum chamber. Diffusion bonding was carried out at 850[degrees]C for 1.8-10.2 ks in steps of 1.8 ks in (3-5) x 10^sup -3^Pa vacuum. During processing, the constant heating rate was 0-24 K s^sup -1^ and after the joining operation, the samples were cooled in vacuum.
A transverse section of bonded assemblies was taken and surfaces were prepared by conventional metallographical techniques. The samples were observed in a light microscope (Correct SDME TR5) to reveal the structural changes due to diffusion. The polished surface of the bonded couples was also examined in a scanning electron microscope (Leica S440) in backscattered mode (SEM-BSE) to obtain finer structural details in the diffusion zone. An electron probe microanalyser (Cameca Sx 100) was used to obtain the elemental concentration profiles across the diffusion interfaces. The k^sub alpha^ lines of Ti, Fe, Ni and Cr are generated at an operating voltage of 15 kV and a specimen current of 12 x 10^sup -8^ A. The LiF crystal was used to diffract the corresponding characteristic X- ray radiation. The presence of the intermetallic phases on the fracture surfaces was confirmed by X-ray diffraction (Philips PW 1840) using a copper target. The 20-80[degrees] scanning range with a step size of 0.01[degrees] (26) was used during the diffraction study.
Tensile properties of the transition joints were evaluated in a universal testing machine (Instron 4204) at a crosshead speed of 8.33 x 10^sup -4^ m s^sup -1^ at room temperature. Cylindrical tensile specimens were machined as per ASTM specification E8M-96 with a gauge diameter of 4 mm and a length of 20 mm. The interlayer was at the centre of the gauge length. The shear strength of the bonded joints was evaluated at room temperature using a screw tensile testing machine set at a crosshead speed of 8.33 x 10^sup – 3^ m s^sup -1^. The shear test specimens were machined to a diameter of 10 mm. Four samples were tested at each process parameter to check the reproducibility of results. Fracture surfaces of bonded samples were observed in the secondary electron mode of SEM (Leica S440) using energy dispersive spectroscopy (Oxford 5431) to reveal the nature and location of failure under loading.
Table 1 Mechanical properties of base metals at room temperature
Results and discussion
The microstructures of the bonded joints are shown in Fig. 1. It is observed that a certain amount of diffusion occurs between the interlayer and the two substrates. The stainless steel/nickel (SS/ Ni) interface is planar in nature and a thin diffusion zone was observed for all processing times. However, the nickel/titanium (Ni/ Ti) interface is characterised by the presence of a light shaded reaction zone and the Widmanstatten a-/? Ti structure. Nickel is a strong beta stabilising element; after migration in titanium lattices, it lowers the eutectoid transformation temperature of titanium and the acicular alpha-beta Ti occurs from the decomposition of /?-Ti during cooling.1314 It is also be observed that the total diffusion zone at the SS/Ni interface is thinner than that at the Ni/ Ti interface. At 850[degrees]C processing temperature, nickel and stainless steel both have fee crystallography and being the close packed structure, the extent of diffusion of chemical species is limited. However, Ti has a two phase structure, i.e. alpha(hcp) + beta(bcc) at that temperature and it transforms to beta(bcc) at 882[degrees]C; therefore, Ni atoms can travel a longer distance in the titanium matrix due to more open crystallography of bec structure. Moreover, the intrinsic diffusion co-efficient of Ti (D^sub Ti^=5.5 x 10^sup -14^ m^sup 2^s^sup -1^ at 900[degrees]C and D^sub Ti^=9 x 10^sup -14^ m^sup 2^ s^sup -1^ at 800[degrees]C) is greater than those of alpha-Fe (D^sub alpha- Fe^=5 x 10^sup -15^ m^sup 2^ s^sup -1^ at 900[degrees]C), gamma- Fe (D^sub gamma-Fe^=3 x 10^sup -17^ m^sup 2^ s^sup -1^ at 900[degrees]C) and Ni (D^sub Ni^=3 x 10^sup -17^ m^sup 2^ s^sup – 1^ at 800[degrees]C).7,15
The electron micrographs and concentration profiles of the elements obtained from electron probe microanalysis (EPMA) across the diffusion interfaces are given in Figs. 2 and 3 respectively. At the SS/Ni interface, the composition changes gradually for Fe, Cr and Ni. The diffusion zone is free from reaction products for all processing time. The presence of Fe (-4.8-13-7 at.-%) and Cr (-2-5- 3-9 at.-%) in the Ni side indicates substantial diffusion of the two alloying elements to the nickel side. On the other hand, Ni also migrates to the stainless steel substrate in adequate quantity (-21- 4-32-4 at.-%). At the Ni/Ti interface, layerwise reaction products are observed. The faint reaction layer has been observed at the nickel side. This area contains Ni (-69-5-72-4) and Ti (bal.); hence, the Ni-Ti binary phase diagram indicates the formation of TiNi^sub 3^ intermetallic. Close to the titanium base metal, the deep shaded area has been observed, which is enriched with Ti (-67- 5-68-3) and depleted in Ni (bal.), hence indicating the formation of Ti^sub 2^Ni intermetallic compound.5 In between TiNi^sub 3^ and Ti^sub 2^Ni reaction layers, the shaded diffusion band is enriched with Ti (-50-1-52-2) and Ni (bal.); hence, the composition indicates the layer as TiNi intermetallic compound.15
From EPMA concentration profiles and electron micrographs, it is evident that a rise in the joining time promotes the profuse diffusion of chemical species across the bond line. For all the bonding times, the nickel interlayer completely blocks the diffusion of titanium to stainless steel and vise versa. The width of Ni-Ti base intermetallics at the Ni/Ti interface varies with the change in bonding time (Fig. 4). Thicknesses of Ti^sub 2^Ni, TiNi and TiNi^sub 3^ are increased with increasing bonding time. At the SS/Ni interface, the same trend is also observed as Fe and Cr are present in the nickel matrix and enrichment of nickel in the stainless steel side increases with increasing bonding time.
1 Optical microstructures of diffusion bonded assemblies processed at 850 C for a 18 ks, b 3 6 ks, c 7 2 ks and d 10.2 ks
2 Images (SEM-BSE) of transition joints processed at 850[degrees]C for a 1.8 ks (SS/Ni interface), b 1.8 ks (Ni/Ti interface), c 3.6 ks (SS/Ni interface), d 3.6 ks (Ni/Ti interface), e 10.2 ks (SS/Ti interface) and f 10 2 ks (Ni/Ti interface)
3 Concentration profiles of EPMA across bond interfaces of specimens bonded at 850 C for 7-2 ks
The intermetallic compounds in the diffusion zone have been confirmed by the X-ray diffraction (Fig. 5). The X-ray diffraction study manifests the formation of intermetallic compounds such as Ti^sub 2^Ni, TiNi, TiNi^sub 3^, alpha-Ti and beta-Ti phases in the reaction zone. From the X-ray diffraction pattern, it is confirmed that the bonded assemblies have been fractured at the Ti/Ni interface for all processing times.
4 Width of intermetallics formed at Ti/Ni interface for varying time at constant temperature
The room temperature mechanical properties of the bonded joints with change in bonding time are shown in Fig. 6. It can be seen that when processing time is 1 -8 ks, both the tensile strength and the shear strength are minimum due to the lack of contact between the mating surfaces. The fracture morphology of the diffusion bonded joints are shown in Fig. 7. The dark regions indicate the presence of voids (Fig. la). These voids occur due to the lack of coalescence of the mating surfaces and promote the failure under loading. The volume fraction of the dark area reduces with increasing bonding time. This indicates that the plastic collapse of mating surfaces increases with increasing bonding time and the bond strength gradually increases up to 7.2 ks. The dark and bright areas indicate the presence of Ni (-28.5-30.2 at.-%) and Ti (bal.). Hence, the composition is Ti^sub 2^Ni phase.14 A processing time of 7.2 ks leads to an increase in the width of the intermetallic phases at the Ni/Ti interface (Fig. 4); however, sufficient holding time promotes the deformation of the surface asperities near bond interfaces and strong chemical bonding takes place, which dominates the embrittlement effect of the intermetallics. The maximum tensile strength and shear strength of diffusion bonds are 311-6 and 236-1 MPa respectively.
5 X-ray diffraction analysis of fracture surfaces of bonded couples processed at 850[degrees]C
6 Effect of bonding time on mechanical properties of transition joints processed at 850[degrees]C
With a further rise in the bonding time, the width of intermetallics increases considerably (Fig. 4) and the embrittlement effect overbalances the positive gain obtained from the increase in the coalescence of mating surfaces. Therefore, the bond strength drops gradually and attains the lowest value for 10-8 ks processing time (the tensile and shear strength are 262-6 and 192-2 MPa respectively). The total width of intermetallic compounds formed at the Ni/Ti interface for 1-8 ks processing time is five times smaller than that of the intermetallic compounds for 10-8 ks and three times smaller than that of the sample processed for 7-2 ks. The fracture surfaces of the higher joining time clearly indicate the brittle nature of the transition joint by the presence of cleavage planes (Fig. lb). The bright and shaded regions are TiNi with the composition of Ti (-51-1-52-2) and Ni (bal.).5
Summary and conclusions
The solid state diffusion bonding of commercially pure titanium and 304 stainless steel with a 300 [mu]m nickel interlayer has been produced at 850[degrees]C for 1.8-10.8 ks under 3 MPa uniaxial load in vacuum. Characterisation of the transition joints reveals the following.
Optical micrographs indicate that nickel can diffuse greater distance in titanium than titanium in nickel with increasing bonding time.
At the Ni/Ti interface, interdiffusion between Ni and Ti encourages layerwise formation of TiNi^sub 3^, TiNi and Ti^sub 2^Ni, and their widths increase with increasing bonding time; however, the SS/Ni interface is free from reaction product for all bonding times.
7 Fracture interface at Ti side processed for a 5 4 and b 10.2 ks
A maximum tensile strength of 311 -6 MPa along with 9.1% ductility and a shear strength of 236-1 MPa have been obtained for the transition joint processed for 7.2 ks owing to the better coalescence of mating surfaces. With increasing bonding time, the volume fraction of Ti-Ni base intermetallics is promoted and hence the bond strength drops gradually. At a lower joining time, the bond strength is also poor due to the incomplete coalescence of the mating surfaces.
The fracture surface at low joining time is featureless; however at higher joining time, the existence of cleavage pattern indicates brittle nature and for all processing times, failure takes place through the Ti/Ni interface.
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S. Kundu and S. Chatterjee*
Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India
* Corresponding author, email firstname.lastname@example.org
Copyright Institute of Materials Oct 2007
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