THE INFLUENCE OF NICKEL INTERLAYER FOR DIFFUSION WELDING OF TITANIUM AND AUSTENITIC STAINESS STEEL

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1 THE INFLUENCE OF NICKEL INTERLAYER FOR DIFFUSION WELDING OF TITANIUM AND AUSTENITIC STAINESS STEEL Ladislav KOLAŘÍK, Marie KOLAŘÍKOVÁ, Pavel NOVÁK, Martin SAHUL, Petr VONDROUŠ CTU in Prague, Faculty of Mechanical Engineering, Department of Manufacturing Technology, Technická 4, Praha, EU Abstract The diffusion welding of titanium alloy Ti6Al4V and austenitic stainless steel was produced at Laboratory of Education of Welding Technologies on CTU in Prague. The welds were done without and with Nickel interlayer (thickness 20 μm) and are evaluated by metallography, SEM, EDX and nanohardness measurement. There was mapped chemical composition across the joint interface by EDX analysis. Mechanical characteristics (reduced modulus E r, indent hardness H IT ) were measured by system Hysitron TI 950 TriboIndenter. Results show that using of Ni interlayer leads up to creation of NiTi phase with shape memory. This phase is not present at sample welded without Ni interlayer. The presence of NiTi phase has resulted in slower transition of reduced modulus from one material to another. At sample without Ni Interlayer fluctuations of measured hardness values was noted, so mechanical properties of the joint with Ni interlayer would be better. Keywords: Diffusion Welding; Titanium; Austenitic Stainless Steel; Nickel Interlayer; Heterogeneous Joints 1. INTRODUCTION Titanium and its alloys are attracting much attention lately from many different industrial areas. Its properties, low density, strength at higher temperature, oxidation resistance make it feasible for chemical industry, aircraft industry etc. Thanks to its biocompatibility medical science is using it widely for implants (screws, prosthetic joints). Titanium is expensive material, so structures combining titanium with other metal is often desirable, Ti welding or/and cladding. However titanium has very low solubility with other metals. Welding with other materials by fusion welding is problematic, because of creation of brittle intermetallic phases in the weld (with Fe - Fe 2 Ti, FeTi). High reactivity and affinity to gasses, e.g. nitrogen, oxygen, is also problematic. Welding of titanium results difficult, but as very important and prospective, it is in the close focus of research. One of the areas of development are e.g. laser welding [1] and diffusion welding [2] [6]. This article focuses on diffusion welding. Welded structures combining steel and titanium are of great importance in industry. Often special intermediary layers are used as being transition area well connecting to steel and titanium material. Mutual diffusion, plastic properties and face contacts are improved by interlayer. These interlayers can be deposited by electrolysis, thermal spraying, thin foils etc. Most often Ni and Cu are used, because of plasticity properties [2] [3] [5]. This article focuses on heterogeneous diffusion welds of steel and titanium when using and non-using Nickel interlayer. 2. MATERIALS The most widely used titanium alloy Ti6Al4V (type α+β) and austenitic stainless steel (according to EN ) were used. Chemical composition and material properties are at Tab. 1 and Tab. 2. Base metal structure of Ti6Al4V is shown at Fig. 1a. Structure of steel is at Fig. 1b. As interlayer, Nickel coating of thickness 20 μm was electroplated on steel sample.

2 Titanium alloy Ti6Al4V has high strength, ductility and great corrosion resistance. At higher temperatures due to high affinity to oxygen, stable oxides are created. Oxygen well dissolves in titanium and during welding it creates weld embrittlement so certainly inert atmosphere needs to be used for full protection of weld. Most often it is Argon shielding of the weld face and root. Cr-Ni alloyed austenitic stainless steel is the most widely used in food industry, because it is corrosion resistant in urban areas, to water, weak alkali and acids. It is well weldable by many fusion welding technologies. Welding of austenitic steels should control heat input, to prevent grain coarsening, BM purity and phase composition should be checked to prevent hot cracking. Tab. 1 Chemical composition and mechanical properties of alloy Ti6Al4V Tensile strength R m [MPa] Ti [%] Al [%] V [%] Fe [%] Mo [%] Yield Young Melting Hardness Density strength modulus HV [kg/m 3 point ] R p0.2 [MPa] E [GPa] T T [ C] Thermal expansivity [10-8 K -1 ] Tab. 2 Chemical composition and mechanical properties of steel Fe [%] Cr [%] Ni [%] Mn [%] Si [%] Cu [%] Mo [%] Co [%] Tensile Yield Young Melting Thermal Hardness Density strength strength modulus HV [kg/m 3 point expansivity ] Rm [MPa] R p0.2 [MPa] E [GPa] T T [ C] [10-8 K -1 ] μm 20 μm 1a 1b Fig. 1 Metallography of Base Metals titanium alloy Ti6Al4V (1a), steel (1b), mag. 200x 3. EQUIPMENT Special diffusion welding equipment Indutherm SU 450 was used, shown at Fig. 2a. Heating of welded parts is done by induction in closed chamber. Before the welding, the chamber is evacuated and inert Argon atmosphere is inserted. Heating is enabled by use of high frequency inductor and graphitic crucible. Welding samples are round with hole inside and ground contact faces. Central hole (min. diameter 5 mm) is used for placement and fastening the samples, as at Fig 2b, Heating cycle is fully programmable, controlled by

3 thermocouple inside the chamber. Welding parameters were searched in literature [3] and subsequently tested. 2a Fig. 2 Diffusion welder Indutherm SU 450 (2a), stem that serves to mount and fasten welding samples and graphite rings to into the diffusion welder (2b) 2b 4. EXPERIMENTAL Samples of diameter 30 mm and thickness 5 mm were ground and polished to roughness Ra = 0,01 μm. Used welding parameters are as follows: welding temperature 900 C welding time was 900 s welding pressure 2.5 bar Inert gas Argon 4.6 was used during whole heating and welding cycle, when temperature was over 100 C. Welding chamber was preheated to 500 C. Placement of sample on the mounted stem is shown at Fig. 2b. Sample without interlayer and with Ni interlayer were welded. Metallography was done together with analysis, chemical composition, mechanical properties were measured and these samples were compared. Chemical analysis was done by a scanning electron microscope (TESCAN VEGA 3 LMU) equipped with EDS analyser (OXFORD Instruments INCA 350). Micromechanical properties were obtained using the nanomechanical instrument Hysitron TI 950 TriboIndenter. The measurement methodology is in detail described in [4]. From the measured results, it was found out that H IT is more sensitive parameter than reduced modulus E r. so this research work will only present results of nanohardness H IT and E r is not presented.

4 5. RESULTS Metallography is shown at Fig. 3. And chemical analysis results are at Fig. 4, Fig. 6. Mechanical properties are shown at Fig. 5, Fig. 7. Ti 2Ni Ti 2Ni + Ti NiTi TiNi 3 TiNi 3 20 μm 20 μm 3a 3b Fig. 3 Metallography sample without Ni interlayer (3a) and with Ni interlayer (3b) From the metallography and chemical analysis it is visible that weld area composes of many layers with different layers (Fig. 3) with different properties. Sample without Ni interlayer Sample without Ni interlayer (Fig. 4, 5) TiNi 3 intermediary phase thick 4 μm was created. This layer is very brittle, the hardness peak is visible at Fig. 5, hardness of this layer H IT = 14 GPa is much higher than base metal H IT = 5 GPa. Diffusion of Ti, Cr, Al, V into steel part up to depth 2 4 μm and Fe, Ni diffused into Ti alloy up to depth 10 μm. In direction of Ti alloy Ti 2 Ni phase was created, 25 μm thick. On the boundary of TiNi 3 and Ti 2 Ni decrease of hardness was measured. Inside Ti alloy the hardness varies, which is created by existence of different phases α, β inside the alloy. Fig. 4 Chemical composition across weld diffusion joint sample without Ni interlayer

5 Fig. 5 Indentation hardness evolution across the fusion line sample without Ni interlayer Sample with Ni interlayer The sample with Ni interlayer (Fig. 3b) has TiNi 3 phase near the diffusion boundary of Ti alloy and Ni interlayer (thickness 4 μm), this is similar to the sample without Ni interlayer. In direction to steel no intermediary phase was created. Only Cr, Fe diffused into electroplated Ni. In direction to Ti alloy, NiTi phase was created, thick 6 μm, with hardness of H IT = 10 GPa, higher that the surrounding phases with H IT = 5 GPa. Eutectic Ti 2 Ni + Ti adhere to this layer. Fig. 6 Chemical composition across weld diffusion joint sample with Ni interlayer From hardness measurement (Fig. 7) Ni interlayer had positive influence on mechanical properties, by reducing maximum hardness and slowing the gradient of this change. The sample without Ni interlayer had much higher hardness and the change of hardness was steep.

6 Fig. 7 Indentation hardness evolution across the fusion line sample with Ni interlayer 6. CONCLUSION Using Ni interlayer during diffusion welding of stainless steel and Ti alloy resulted in decreasing hardness of created intermediary layers to 10 GPa, compared to sample without Ni interlayer with peak hardness 14 GPa. So expectedly the resulting weld with Ni interlayer would be more ductile, so using of Ni interalayer is advantageous for this kind of diffusion welding. This research would develop in the future research, where new possible interlayers, their thickness, plating methods. Also mechanical properties will be measured, micro charpy, bend test, tensile strength. Furthermore optimization of welding parameters should be researched in closer detail, e.g. temperature, length, shielding gas, contact pressure, sample preparation etc. ACKNOWLEDGEMENT The research was financed by the Czech Ministry of Education, Youth and Sport within the frame of project SGS CVUT SGS13/187/OHK2/3T/12 LITERATURE [1] SAHUL M., SAHUL, M., TURŇA, M.: Zváranie vybraných kombinovaných ocelí diskovým laserom, Sborník konference Zvárania 2012, VUZ, 2012 [2] KOLAŘÍK, L, KOLAŘÍKOVÁ, M., KOVANDA, K., VONDROUŠ, P. Difúzní svařování. MM Průmyslové spektrum [online]. 2012, č. 4 [cit ]. Dostupné z: [3] KAZAKOV. N.F. Diffuzionnaja svarka materialov, Mašinostrojenije, Moskva, 1976 [4] ŠEPITKA, J., LUKEŠ, J., STANĚK, L., FILOVÁ, E., BURDÍKOVÁ, Z., ŘEZNÍČEK, J. Nanoindentation of intervertebral disc tissues localised by SHG imaging, Computer Methods in Biomechanics and Biomedical Engineering. Vol. 15, No S1 (2012), pp [5] TURŇA, M. Špeciálne metódy zvárania, Alfa, Bratislava, 1989 [6] JÁŇA, M., TURŇA, M. KOLAŘÍK, L., KOLAŘÍKOVÁ, M.: Difúzne zváranie kombinovaných kovov, Sborník konference Zvárania 2012, VUZ, 2012