Interlayer engineering for dissimilar bonding of titanium to stainless steel

Materials Letters 64 (2010) 1105–1108

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Interlayer engineering for dissimilar bonding of titanium to stainless steel M.K. Lee a,⁎, J.G. Lee a, Y.H. Choi a, D.W. Kim a, C.K. Rhee a, Y.B. Lee a, S.J. Hong b a b

Nuclear Materials Research Division, Korea Atomic Energy Research Institute, Yuseong, Daejeon, 305-353, South Korea Division of Advanced Materials Engineering, Kongju National University, Cheonan, 330-717, South Korea

a r t i c l e

i n f o

Article history: Received 22 January 2010 Accepted 9 February 2010 Available online 14 February 2010 Keywords: Intermetallic alloys and compounds Mechanical properties Dissimilar joining Titanium Stainless steels Interlayer

a b s t r a c t Strong bonding of titanium to stainless steel, even with strengths more than that of the base metal, is challenging, but has never been achieved owing to its inherent brittleness. Here we report a strong bonding of Ti Gr. 2 to STS by using engineered interlayer structure of V–Cr–Ni and Ti-base amorphous alloy filler. Under optimum conditions, the Ti(base)/Ti solid solution/V/Cr/Ni/STS(base) joints were reliably produced without any detrimental brittle phases, exhibiting a remarkably high strength exceeding that of the Ti Gr. 2. © 2010 Elsevier B.V. All rights reserved.

1. Introduction With the increasing use of Ti and its alloys in aerospace, transportation, power generation, and chemical industries because of their high strength to weight ratio and excellent corrosion resistance, there is an obvious requirement to join them to other dissimilar materials [1]. In particular, the joining of Ti to structural steels with high strength and toughness is of considerable importance for the integration and fabrication of Ti-base components, although a conventional fusion welding technique has not yet been technically usable [2]. Of the processes available to joining these dissimilar metals, solid-state diffusion bonding and brazing have been most heavily investigated. In particular, efforts have been made to produce sound joints by using various multi-component alloy fillers (Ag-base, Ti-base, etc.) [3–5] and metallic interlayers (Cu, Ni, Al, etc.) [6–8]. Unfortunately, most joints exhibited low strengths with a fracture in a joint due to the presence of brittle intermetallic phases, indicating that their brittleness are inherent and thus difficult to avoid. A feasible way to solve such fundamental brittleness can be an application of a proper combination of interlayer metals that are highly soluble with each other and with both the adjacent filler and base metal, thereby realizing a joint free from any detrimental brittle phases. In this study, dissimilar joining between Ti and STS was performed by employing the engineered interlayer structure of V–Cr– Ni and the interfacial microstructure and bonding strength properties ⁎ Corresponding author. Nuclear Materials Research Division, Korea Atomic Energy Research Institute, Dukjin-dong 150, Yuseong-Gu, Daejeon, 305-353, Republic of Korea. Tel.: +82 42 868 8565; fax: +82 42 868 4847. E-mail address: [email protected] (M.K. Lee). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.024

of the joints were investigated. A Ti–Zr–Ni amorphous alloy was used as a filler material for the joining, as it is representative of the Ti-based fillers with excellent melting and wetting characteristics for the brazing of Ti [1]. 2. Experimental The base materials used for the dissimilar joining were commercially pure Ti Gr. 2 (ASTM grade 2, 0.01N–0.01C–0.0001H–0.1O–0.06Fe–bal. Ti, wt.%) and super stainless steel (UNS S31254; 20Cr–18Ni–6Mo, wt.%). The specimens for a joining were rectangular cubes with dimensions of 10 mm×13 mm×26 mm, and the surfaces to be joined were polished and cleaned ultrasonically with acetone. A Ti–Zr–Ni amorphous alloy foil (58Ti–16Zr–26Ni, at.%) with a thickness of 100 μm was used as a filler, which is very close to the Ti–Zr–Ni equilibrium eutectic one. Its solidus and liquidus temperatures were 844 °C and 857 °C, respectively, based on a differential thermal analysis (DTA; data not shown). Metallic layers of Ni, Cr, and V were employed as an interlayer by a sputtering technique with thicknesses of 30, 10, and 20 μm, respectively. Infrared heating was used to join Ti and STS, and details on the conditions were described elsewhere [9]. The microstructure and quantitative chemical analyses of the joints were performed by a scanning electron microscopy (SEM; JEOL 6300) equipped with an energy dispersive spectroscope (EDS). The bonding strengths were evaluated by means of a tensile testing machine (INSTRON MODEL 4465) at a strain rate of 8.3×10− 4 s− 1 at room temperature. 3. Results and discussion Based on our experimental investigations, it was possible to join pure Ti with stainless steel directly by using a Ti–Zr–Ni alloy filler, but the

1106

M.K. Lee et al. / Materials Letters 64 (2010) 1105–1108

produced joints were generally dominated by the Ti-based brittle intermetallic compounds (IMCs) such as (Ti,Zr)2(Fe,Ni), TiFe, and Ti2(Fe, Ni) due to a chemical reaction of Ti with other elements in the filler and STS base metal. The intact specimens also showed low joint strengths less than 100 MPa with their brittle nature. According to the investigations on the mutual solubility properties of Ti and Fe with other elements of the periodic system in the temperature range up to 1200 °C [10], the Ti readily reacts with most other elements to form stable and brittle IMCs, except for a few elements in the same or adjacent groups of the periodic system (IVA: Zr, VA: V, Nb, Ta, VIA: Mo, W) (Fig. 1). It also forms the Ti–Fe IMCs with the main element Fe in the STS, which is known as the most brittle among Ti-based IMCs [4]. To make matters worse, none of the elements have a sufficient solubility with both Ti and Fe, in other words, the elements being highly soluble in Ti form IMCs with Fe due to their limited solubility, and vice versa. From both the experiments and theoretical considerations, it was reasonably concluded that the application of a single filler/braze alloy or an insert metal would be insufficient to overcome the brittleness of the Ti–STS dissimilar joints, indicating a need for supplementary suitable intermediate/interlayer metals or their combination. Vanadium (V) was first considered as highly promising for an interlayer metal for the Ti–STS joining, because it exhibited a high compatibility with the Ti base metal. The β Ti formed a complete range of solid solutions with the V element, whereas the behavior of α Ti was more limited in this respect [10]. These promising properties as an interlayer were further enhanced by a negligible difference in thermal expansion coefficients, where a ratio of Ti:V is 8.5:8.3 [11]. With regard to a reaction with the filler, it was also expected to be a suitable interlayer metal. As shown in Fig. 2(a) and (b), isothermal sections of the Ti–V–Zr and Ti–V–Ni ternary alloy phase diagrams [12] indicated that the original joint/filler composition could be shifted toward the Ti-rich solid solution region without producing any brittle phases such as the V–Zr and V–Ni IMCs. For instance, for the Ti–V–Ni system (Fig. 2(b)), the initial filler composition (denoted by A) containing the IMCs like NiTi2 could move to the Ti–V solid solution region, assuming that the Ti and Zr would be

regarded as the same element to have the filler composition with 74Ti– 26Ni in at.%. This could be achieved by inducing a sufficient dissolution of the Ti elements from the Ti Gr. 2 base metal into the molten filler, so as to reduce the Zr and Ni concentrations in the molten filler to less than the solubility limit of Ti, through a proper control of the processing parameters, e.g., isothermal holding temperature and time. It was also expected that the resultant compositional change at the solid/liquid interface and subsequent isothermal solidification of the molten filler would be responsible for the solidification reaction of the joint. To verify the feasibility of V as an interlayer for Ti–STS joining, the joint structures were investigated as a function of isothermal holding temperature and time by using a V interlayer. The structure in Fig. 3 (a) is typical of the joints produced with a V interlayer at 900 °C for 10 min. The observed BEI revealed that the V interlayer blocked a diffusion of the elements out of the STS base metal effectively. In the vicinity of Ti Gr. 2 base metal, the continuous Ti–V solid solution region (A) was formed free from any brittle IMCs, suggesting that there was a sufficient dissolution of the Ti base metal into the molten filler to form Ti-rich solid solution containing the Zr and Ni. In this region, the evolved structure was fully attributed to the compositional change at the solid/liquid interface due to Ti dissolution and the resultant isothermal solidification. This was in good agreement with what was predicted above. However, at the lower temperatures and/ or shorter isothermal holding periods, the joints often contained brittle Ti-base IMCs like Ti2Ni and (Ti,Zr)2Ni that were mostly segregated close to the V layer due to an incomplete isothermal solidification. It was noted that the relatively thick brittle σ phases were continuously formed between the V-rich region (B) and the STS base metal, as denoted by C and D. The observed σ phase basically resulted from the reaction between V and Fe (C), which is known as a D8b-type IMC [10], and a V–Fe–Cr base ternary σ phase (D) was often formed near the STS base metal by a more intensive reaction with the Cr. Since the σ phase generally exhibited deleterious effects on the mechanical

Fig. 1. Solubility properties of Ti and Fe with the other elements in the binary alloy periodic system: the properties were investigated up to 1200 °C by considering the general brazing temperature regime [10].

M.K. Lee et al. / Materials Letters 64 (2010) 1105–1108

1107

Fig. 2. Isothermal sections of ternary alloy phase diagrams: (a) Ti–V–Zr at 800 °C and (b) Ti–V–Ni at 1000 °C [12].

behavior due to its brittle nature [13], its formation indicated a need for additional interlayer metal between the V layer and STS base metal. For this, Cr was used as the second interlayer metal, which has not only an unlimited solubility in V [10], but also is structurally compatible with the STS. Despite our efforts to suppress the brittle σ phase by applying the Cr–V double layers, we were unsuccessful in suppressing the Cr–Fe σ phase between the Cr layer and the STS base metal (F), although it was much thinner compared to the V–Fe σ (Fig. 3(b)). To eliminate such brittle σ phases from the joint completely, a third interlayer metal was necessary between the Cr layer and STS base metal, along with the use of the Cr–V double layers. Ni was chosen as the third interlayer metal, as it is highly soluble with the second interlayer Cr [10], and highly compatible without forming the brittle σ phase with the STS [13]. The results for the Ti–STS joint produced at 900 °C for 10 min with the Ni–Cr–V interlayers are

presented in Fig. 3(c). In the vicinity of the Ti Gr. 2 base metal, the Ti solid solution region comprising α + β Ti and β Ti was continuously formed (G and H) and the interfaces among the Ni, Cr, and V layers and the STS base metal were slightly alloyed to each other without any σ phases. Eventually, the Ni–Cr–V interlayer structure produced a highly reliable joint with the Ti(base)/α + β Ti/β Ti/V/Cr/Ni/STS (base). Room-temperature tensile test results are shown in Fig. 4. The average bonding strength for the samples with the V interlayer was measured to be as low as ∼100 MPa. The obtained stress–stain curves also indicated that a fracture occurred at an early stage before yielding. For the samples with the Cr–V interlayer, the bonding strength increased to ∼ 200 MPa, probably owing to the formation of a much thinner σ phase (Fig. 3(b)). Further investigation on the fracture surfaces confirmed that a fracture occurred along the σ phase region. On the contrary, the samples with the Ni–Cr–V interlayer

Fig. 3. SEM back scattered electron images and EDS chemical compositions of the selected locations for the joints produced at 900 °C for 10 min by the applications of the interlayer: (a) V layer, (b) Cr–V double layers and (c) Ni–Cr–V triple layers.

1108

M.K. Lee et al. / Materials Letters 64 (2010) 1105–1108

interlayers of V–Cr–Ni and Ti-base amorphous alloy filler. A joint structure with the Ti(base)/α + β Ti/β Ti/V/Cr/Ni/STS(base) was reliably formed at 900 °C for 10 min without any brittle Ti-base IMCs or σ phases. It was remarkable that the bonding strengths of the joints exceeded the strength of the bulk Ti Gr. 2 (∼ 480 MPa). Multilayers of V–Cr–Ni show promise as interlayer templates for a joining of Ti and its alloys to various steel alloys.

Acknowledgements This research was financially supported by the Ministry of Knowledge Economy (MKE) through the National Mid- and Longterm Atomic Energy R & D Program of the Republic of Korea.

References Fig. 4. Stress–strain curves obtained by a room temperature tensile test for the samples joined at 900 °C for 10 min with the V layer, Cr–V double layers, and Ni–Cr–V triple layers as an interlayer: inset is a photo image showing a fracture in the Ti Gr. 2 base metal for the sample with Ni–Cr–V interlayer.

displayed fully developed stress–strain curves that were almost comparable to that of the bulk Ti Gr. 2. A fracture did not occur in the joint but in the Ti Gr. 2 base metal, as clearly seen in the fracture image of Fig. 4, where the joint area remained very stable, and the Ti base metal fractured with its considerable yielding. It was clear that the bonding strength exceeded the strength of bulk Ti Gr. 2 (∼480 MPa), although it was not possible to evaluate the absolute value in this study. 4. Conclusions In summary, a superior bonding of titanium (Ti Gr. 2) to stainless steel (STS) has been achieved by employing three consecutive

[1] Shapiro A, Rabinkin A. Weld J 2003;82:36–43. [2] Meshram SD, Mohandas T, Madhusudhan Reddy G. J Mater Proc Technol 2007;184:330–7. [3] Elrefaey A, Tillmann W. Metall Mater Trans A 2007;38:2956–62. [4] Shiue RK, Wu SK, Chan CH, Huang CS. Metall Mater Trans A 2006;37:2207–17. [5] Liu CC, Ou CL, Shiue RK. J Mater Sci 2002;37:2225–35. [6] Kundu S, Ghosh M, Laik A, Bhanumurthy K, Kale GB, Chatterjee S. Mat Sci Eng A 2005;407:154–60. [7] Kundu S, Chatterjee S. Mat Sci Eng A 2006;425:107–13. [8] He P, Yue X, Zhang JH. Mat Sci Eng A 2008;486:171–6. [9] Lee MK, Lee JG, Lee JK, Park JJ, Lee GJ, Uhm YR, et al. J Mater Res 2008;23:2254–63. [10] Massalski TB. Binary Alloy Phase Diagrams, Metal Park (OH). ASM International; 1990. [11] Lison R, Stelzer JF. Weld J 1979;58:306s–14s. [12] Villars P, Prince A, Okamoto H. Handbook of Ternary Alloy Phase Diagrams Metal Park (OH). ASM International; 1995. [13] Hall EO, Algie SH. Metall Rev 1966;11:61–88.