4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

A STUDY ON DIFFUSION BONDING BETWEEN Ti-6Al-4V ALLOY AND INTERSTITIAL FREE STEEL

Musa Kılıç1, *Mehmet Kaya2, İhsan Kırık3, , Eyyüp Murat Karakurt2

1Batman University, Technology Faculty, Mechanical and Manufacturing Engineering Department, Turkey 2,4Adıyaman University, Engineering Faculty, Metallurgy and Material Engineering Department, Turkey3Batman University, Engineering Faculty, Metallurgy and Material Engineering Department, Turkeymusa_kilic21@hotmail.com, *mehmetkaya75@hotmail.com, 3alihsankirik@gmail.com, 4ekarakurt@adiyaman.edu.

Abstract

In this study, Ti–6Al–4V alloy was diffusion bonded to Interstitial Free Steel at different temperatures, under a pressure of 5 MPa for 30 min. The effect of bonding temperature on the microstructural development across the joint region was investigated using an optic microscopy, a scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometer (EDS), and shear strength of bonded specimens. The results showed that intermetallic phases such as FeTi and Fe2Ti were occurred interface of bonded specimens. In addition to, it was seen that shear strength of bonded specimens were decreased with increasing temperature due to growing intermetallic.

Keywords: Diffusion bonding, Interstitial Free Steel, Ti–6Al–4V, Interface characterization.

1. Introduction

Diffusion bonding is a solid-state coalescence process between two materials, similar or dissimilar, through the application of pressure at a temperature below the melting points of the materials to be joined. This coalescence technique is generally used to join materials for special purpose where relatively large contact areas are involved. It produces joints with a minimum macroscopic deformation and without deterioration in the mechanical properties of the base metals [1, 2]. Microstructural changes in bonded interface of base metals are depended on atomic combustion of metals, and also bonding temperature, holding time and bonding pressure during procedure. Microstructure of a material is affects its mechanical properties. So, thickness of interface region and intermetallic phases formed at the interface region are important factor for mechanical properties and to determine joint quality.

Ti–6Al–4V alloy is a well-known alloy for aerospace and implant applications because of its low density, high specific strength, excellent corrosion resistance, excellent biocompatibility, high melting point, low thermal conductivity, high toughness and good high temperature durability. In addition, this alloy shows superplastic properties that allow for large plastic deformation under certain conditions [3, 4]. In addition to titanium alloys, stainless steel (SS) and cobalt-based alloys are commonly used for implant applications [5]. Titanium alloy and stainless steel joints are used in space and nuclear industries due to their excellent mechanical behavior and good corrosion resistance [6].

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

Interstitial-free (IF) steels constitute an important class of steels having carbon content less than 0.01 wt.%. These steels are extensively used in automotive industries for making car bodies owing to the high formability that they possess. In recent years, efforts have been made to improve the strength of this class of steels by means of grain refinement mostly through SPD procedures [7]. The term ‘Interstitial Free steel or IF steel’ refers to the fact that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio, high strain rate sensitivity and good formability.

Conventional IF steels which were developed commercially in Japan during 1970s following the introduction of vacuum degassing technology contained carbon (C) in the range of 40–70 ppm and nitrogen (N) in the range of 30-50 ppm. Later, niobium (Nb) and/or titanium (Ti) were added to these steels to stabilize the interstitial C and N atoms [8, 9].

Diffusion bonding studies between titanium alloy and stainless steel have presented by different authors [1, 2, 6, 10, 11]. Earlier works reports that some intermetallic phases like Fe2Ti, FeTi, Fe2Ti4O and TiC in the interface region are formed, and also they affect joint strength [1]. Titanium alloy is chemically reactive, so it is very difficult to be welded. Titanium alloys can easily pick up nitrogen and oxygen from the atmosphere. Brittle intermetallic compounds are easily formed when diffusion welding method is used to join titanium alloy and stainless steel. So, diffusion bonding is recommended [6]. According to our research literature, any study on diffusion bonding between Ti–6Al–4V alloy and Interstitial-free (IF) steels is not seen yet. Therefore, Ti–6Al–4V alloy was diffusion bonded to Interstitial Free Steel, and it was investigated joint quality.

2. Experimental procedure

The compositions of materials (IF steel and Ti-6Al-4V) used in this investigation are seen in Table 1. For diffusion bonding, IF steel and Ti–6Al–4V were cut into 12 mmx10mmx10 mm thickness, and cut into cylinders with a diameter of 10 mm and a height of 12 mm, respectively. Prior to joining, one face of each specimen was ground on SiC abrasive papers up to 1000 mesh, and then polished using diamond pastes. Polished samples were cleaned ultrasonically in acetone to eliminate impurities and then dried by air blowing. Diffusion bonding procedure was carry out under an argon atmosphere in a bonding chamber. The specimen couples were heated up to the bonding temperature using induction heating with a heating rate of 30 ºC, and then the specimens were hold for 30 min under a pressure of 5 MPa at two different temperatures (900 ºC and 950 ºC). The bonded specimens were cooled to room temperature in air when the bonding process was completed.

Table 1. Chemical composition (wt.%) of ultra-low carbon IF steel and Ti-Al-V used.Alloy Fe Mn Cr S Si P N Ni C O Ti Al VIF Steel bal 0.1

70.04

0.08

0.01

0.012

0.0027

0.002

0.002

- 0.072

0.041

0.005

Ti.6Al.4V

0.22

- - - - - - - - 0.18

bal 6.08 4.02

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

For metallographic examination, the specimens were cut through the bond, and the surface of specimen was ground on grinding water-papers and polished using diamond of 1 μm. The surface of Ti–6Al–4V alloy was etched in Keller’s reagent for 40 s, and the surface of IF steel was etched in 2% Nital solution for 20 s. The phases in the microstructure and composition of the specimens were investigated by scanning electron (SEM) equipped with X-ray energy dispersive spectrometer (EDS). Shear strength measurements of bonded couples were made using an Instron tensile testing machine, at a crosshead speed of 0.5 mm/min.

3. Results and Discussion

Fig. 1 shows optic micrographs of specimens bonded at 900 ᵒC and 950 ᵒC. Big grains are seen in microstructure of IF steel for both of temperatures, it is like ferritic phase because of it has ultra-low carbon. The grain size a little increased with increasing temperature. IF steel is termed as ‘clean steel’ as the total volume fraction of precipitates is very less. In spite of this, the precipitates appear to have a very significant effect on the properties of IF steels. Liquid steel is processed to reduce C and N to levels low enough that the remainder can be ‘stabilized’ by small additions of Ti and Nb. Ti and Nb are strong carbide/nitride formers, taking the remaining C and N out of solution in liquid iron, after which these latter two elements are no longer available to reside in the interstices between solidified iron atoms. IF steel has ultra-low carbon content, achieved by removing carbon monoxide, hydrogen, nitrogen, and other gasses during steelmaking through a vacuum degassing process. Interstitial elements like nitrogen or carbon are also in the form of nitrides and carbides due to the alloying elements such as Nb and/or Ti used for the stabilization of the residual interstitials. Therefore, IF steel possess typically non aging properties. Because of their non-ageing properties, IF steels are the standard base for hot dipped galvanized products [7-9].

(a) (b)

Figure 1. Optic images of IF steel sides of specimens bonded; (a) 900 ᵒC, and (b) 950 ᵒC.

Fig. 2 shows optic micrographs of joint regions of diffusion bonded specimens at different temperatures. Two different structures are seen in microstructure of Ti-6Al-4V alloy. These phases are α-phase and β-phase matrix. Literature suggests that microstructure and grain morphology affects the tensile and mechanical properties of Ti-6Al-4V [6, 10]. Controlling

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

morphology of the sample can be done through heat treatment and cooling rates. As suggested low to intermediate cooling rates create an α-β lamellar structure with α-phase lamellae in a β-phase matrix. The formation of α-lamellae is a diffusion controlled nucleation and growth mechanism of the α-lamellae into the β-grains. As cooling rates increase the length and thickness of the α-lamellae decrease which leads to higher yield strengths [6, 11]. In this study, both of microstructure are similar because of cooling rates are the same for both of specimens.

Figure 2. Optic micrographs of the bond interfaces of the specimens bonded; (a) 900 ᵒC, and (b) 950 ᵒC.

Figure 3. SEM micrographs of the bond interfaces of the specimens bonded; (a) 900 ᵒC, and (b) 950 ᵒC.

Fig. 3 shows SEM images of diffusion bonded specimens at 900 ᵒC and 950 ᵒC, respectively. EDS analysis in some regions were done. In generally, the compositions of the main phases in far from bonding regions not change after diffusion bonding. Fe diffused from IF steel to Ti alloy side, close to bonding regions. It was seen that Fe2Ti and FeTi intermetallic phases occur out in bonding regions. These phases are also seen in Fe-Ti phase diagram in Fig. 4. The similar phases

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

were seen in previous studies [1, 6, 10]. Diffusivity or diffusion coefficient is proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). Diffusivity is encountered in Fick's law and numerous other equations of physical chemistry. Atomic diffusion is depended on concentration in regions, temperature, time, pressure and atomic radius. Atomic radius of Fe is smaller than those of the other elements (Ti, V, Al). Atomic radius of these elements are 126 pm, 134 pm, 140 pm and 143 pm for Fe, V, Ti and Al, respectively. That is, atomic radius of Fe is smaller than those of the other elements. So, Fe diffused to Ti alloy side, and V diffused from Ti alloy to IF steel side.

Figure 4. Phase diagram of Ti-Fe alloys [12].

The bond shear strengths of the specimens are 215 MPa and 205 MPa for 900 ºC and 950 ºC, respectively. It is seen that shear strength decreases with increasing bonding temperature. It is assumed that the intermetallic phases grow when the processing temperature increases, hence bond strength drops due to growing intermetallic phases. Because brittle intermetallic compounds in the reaction zone are mainly responsible for lowering the strength. The similar results were determined for Ti-6Al-4V alloy and stainless steel [1, 6, 10, 11].

4. Conclusions

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

The effect of bonding temperature on the microstructure and shear strength of diffusion bonds between IF steel and Ti–6Al–4V alloy was investigated. Fe2Ti and FeTi intermetallic phases occurred in region of diffusion bonding. The shear strength of the specimens decrease due to intermetallic phases occurred. The bonding temperature a little affects grain morphology (grain size a little increasing with increasing bonding temperature) of the main phases. So, the tensile and mechanical properties of the main phases don’t change. To determine of effects bonding temperature, pressure and time, more studies about these alloys should be done.

5. Acknowledgement

The authors are grateful to Professor Dr. Ibrahim Karaman at Texas A&M University, USA, for the alloys.

6. References

[1] B. Kurt, N. Orhan and M. Kaya: Interface characterisation of diffusion bonded Ti–6Al–4V alloy and austenitic stainless steel couple, Materials Science and Technology 25 (2009), pp. 556-560.

[2] A. Elrefaey, W. Tillmann: Solid state diffusion bonding of titanium to steel using a copper base alloy as interlayer, Journal of Materials Processing Technology, 209 (2009), pp. 2746–2752.

[3] H. S. Lee, J. H. Yoon, C. H. Park, Y. G. Ko, D. H. Shin, C. S. Lee: A study on diffusion bonding of superplastic Ti–6Al–4V ELI grade Journal of Materials Processing Technology, 187–188 (2007), pp. 526–529.

[4] M.A. Vasylyev, S.P. Chenakin, L.F. Yatsenko: Ultrasonic impact treatment induced oxidation of Ti6Al4V alloy, Acta Materialia, 103 (2016), pp. 761–774.

[5] P. Yi, L. Peng, J. Huang: Multilayered TiAlN films on Ti6Al4V alloy for biomedical applications by closed field unbalanced magnetron sputter ion plating process, Materials Science and Engineering C, 59 (2016), pp. 669–676.

[6] M. Balasubramanian: Development of processing windows for diffusion bonding of Ti−6Al−4V titanium alloy and 304 stainless steel with silver as intermediate layer, Trans. Nonferrous Met. Soc. China 25(2015), pp. 2932−2938.

[7] K. Máthis, T. Krajnák, R. Kuzel, J. Gubicza: Structure and mechanical behaviour of interstitial-free steel processed by equal-channel angular pressing, Journal of Alloys and Compounds 509 (2011), pp. 3522–3525.

[8] B. Yan, 　 S. Jiao, 　 D. H. Zhang: Microstructure and Mechanical Properties of Semi Continuous Equal Channel Angular Extruded Interstitial Free Steel, Journal of Iron and Steel research, International, 23-2 (2016), pp. 160-165.

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4th International Conference on Welding Technologies and Exhibition (ICWET’16)11-13 May 2016, Gaziantep-TURKEY

[9] A. Hasanbaşoğlu, R. Kaçar: Microstructure and property relationships in resistance spot weld between 7114 Interstitial free steel and 304 austenitic stainless steel, Journal of Materials Science&Technology, 22-3 (2006), pp. 375-381.

[10] S. M. Bhola, S. Kundu, R. Bhola, B. Mishra, S. Chatterjee: Electrochemical Study of Diffusion Bonded Joints between Micro-duplex Stainless Steel and Ti6Al4V Alloy, Journal of Material Science Technology, 30-2 (2014), pp. 163-171.

[11] B. Kurt, N. Orhan, E. Evin, A. Çalik: Diffusion bonding between Ti–6Al–4V alloy and ferritic stainless steel, Materials Letters 61 (2007), pp. 1747–1750.

[12] G. Cacciamani, J. De. Keyzer, R. Ferro, U.E. Klotz, J. Lacaze, P. Wollants, Intermetallics, 14 (2006) 1312-1325.

CORRESPONDENCE ADDRESS:

Associate Prof. Dr. Mehmet Kaya, Adıyaman Üniversitesi, Mühendislik Fakültesi, Metalürji ve Malzeme Mühendisliği Bölümü, 02040 Adıyaman-TürkiyeTel: 0553 266 7465, Email: mehmetkaya75@hotmail.com, mkaya@adiyaman.edu.tr

SHORT BIOGRAPHIES

First Author’s Name:

He was born in 1975 from Kilis-Turkey, and he graduated physics department in Fırat University from Turkey, in 1997. After master on Semiconductor Thin Film Materials in physics department, he interested in Smart Materials Shape Memory Alloys, so he started for PhD in Matellurgy Education, about Prous NiTi Shape Memory Alloys, and he get degree of PhD in January 2008. He researched iron-based Shape Memory Alloys in Texas A&M University for a year. He has been working as Associate Prof. Dr. in Adıyaman University since 2013.

Second Author’s Name : Assistant Prof. Dr. İhsan Kırık, he was born in 1975 from Dıyarbakır-Turkey. He researches on welding of stainless steels. He has been working as Assistant Prof. Dr. in Batman University from Turkey.

Third Author’s Name: Assistant Prof. Dr. Musa Kılıç, he was born in 1976 from Dıyarbakır-Turkey. He researches on porous materials and stainless steels. He has been working as Assistant Prof. Dr. in Batman University from Turkey.

Fourth Author’s Name: Research Assistant Eyyüp Murat Karakurt, he is master student in Adıyaman University from Turkey.

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