Interface characterisation of diffusion bonded Ti–6Al–4V alloy and austenitic stainless steel couple B. Kurt*, N. Orhan and M. Kaya In this study, the Ti–6Al–4V alloy was diffusion bonded to austenitic stainless steel at temperatures of 820, 885, 930 and 980uC, under a pressure of 5 MPa for 30 min. The effect of temperature on interface formations and microstructure was investigated using a scanning electron microscope (SEM), energy dispersive spectrograph (EDS) microanalyses, X-ray diffraction and shear strength of bonded specimens. The results showed that intermetallic phases and s-phase formed in the interface region. Keywords: Interface characterisation, Diffusion bonding, Ti–6Al–4V, Austenitic stainless steel Introduction Diffusion bonding is an advanced bonding process in which two materials, similar or dissimilar, can be bonded in solid state. 1 It is generally used to join materials for special purpose where relatively large contact areas are involved. 2,3 The joints between Ti and stainless steel find wide applications in nuclear industry. The conventional fusion welding of these two materials results in segregation, stress concentration sites and formation of intermetallics such as Fe 2 Ti and FeTi at the interfaces. All these deleterious effects lead to failure of components in service condition. 3 Earlier works reports that a number of brittle intermetallic phases like Fe 2 Ti, FeTi, Fe 2 Ti 4 O, x, l and TiC in the interface region are formed due to the limited solubility of Fe and Ti in the solid state. 4–8 These intermetallic phases have detrimental effects on the joint strength. 9 In the present work, the Ti–6Al–4V alloy has been bonded to an austenitic stainless steel. The relationship of the bond parameters and shear strength of the joints are discussed. The reason for choosing this couple is the possibility of combining the ductile and corrosion resistant properties of austenitic stainless steel and excellent room temperature or elevated temperature strength, creep, or fracture toughness characteristics of Ti–6Al–4V. Experimental In the study, Ti–6Al–4V and AISI 316 L type austenitic stainless steel were diffusion bonded. The chemical compositions of the samples are given in Table 1. Ti– 6Al–4V and 316 L stainless steel were received in the form of a plate of 16 and 10 mm thickness respectively. For diffusion bonding, specimens were cut into dimension 10610610 mm from the plate materials. Before diffusion bonding, one face of each specimen was processed to achieve a predetermined degree of rough- ness using 1200 mesh grinding paper. The samples were then degreased in an ultrasonic bath using acetone. Diffusion bonds were produced under Ar atmosphere in a bonding chamber. The material couples were heated up to the bonding temperature using induction heating with a heating rate of 40uC min 21 . Diffusion bonding apparatus is shown in Fig. 1. In argon atmosphere, the DB couples were subjected to a predetermined pressure of 5 MPa at 820, 885, 930 and 980uC for 30 min. Once the bonding process had been completed, the samples were cooled to room temperature before removal from the chamber. Metallographic examination involved preparing transverse sections across the interface followed by grinding and polishing to a 3 mm finish using diamond paste. For microstructural examination, Ti–6Al–4V was etched using Keller’s reagent and the 316L stainless steel was etched electrolytically in a solution of 50 mL HNO 3 and 50 mL pure water. Metallographic observations were performed by scanning electron microscopy (SEM) and changes in composition were investigated using energy dispersive spectrometry (EDS). The presence of intermetallic phases in the reaction zone was confirmed by X-ray diffraction study on the fracture surface of the couples. Shear strength measurements of bonded couples were made. For this, the bonded specimens were further machined to produce shear test specimens with dimensions 868610 mm, to eliminate edge effects on test data. An Instron tensile testing machine set at a crosshead speed of 0?5 mm min 21 was used for the shear tests. Results and discussion Scanning electron micrographs (Fig. 2) show bonding zone of the Ti–6Al–4V and 316L stainless steel couple. Metal Department, Technical Education Faculty, Firat University, Elazig 23119, Turkey *Corresponding author, email bkurt@firat.edu.tr 556 ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 29 February 2008; accepted 9 April 2008 DOI 10.1179/174328408X311107 Materials Science and Technology 2009 VOL 25 NO 4