Regular Article

Exhibition of veiled features in diffusion bonding of titanium alloy and stainless steel via copper

¹ Department of Applied Science and Humanities, Guru Nanak Institute of Technology, Panihati, 700114 Kolkata, West Bengal, India
² Department of Metallurgy and Materials Engineering, Indian Institute of Engineering Science and Technology, Shibpur, 711103 Howrah, West Bengal, India
³ Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, Kharagpur, 721302 Paschim Medinipur, West Bengal, India
⁴ Product Technology, Tata Steel Limited, 831007 Jamshedpur, Jharkhand, India

^* e-mail: gopinath@metal.iiests.ac.in; gopinath23t@gmail.com

Received: 13 March 2017
Accepted: 20 September 2017

Abstract

An investigation was carried out to know the extent of influence of bonding-time on the interface structure and mechanical properties of diffusion bonding (DB) of TiA|Cu|SS. DB of Ti6Al4V (TiA) and 304 stainless steel (SS) using pure copper (Cu) of 200-μm thickness were processed in vacuum using 4-MPa bonding-pressure at 1123 K from 15 to 120 min in steps of 15 min. Preparation of DB was not possible when bonding-time was less than 60 min as the bonding at Cu|SS interface was unsuccessful in spite of effective bonding at TiA|Cu interface; however, successful DB were produced when the bonding-time was 60 min and beyond. DB processed for 60 and 75 min (classified as shorter bonding-time interval) showed distinctive characteristics (structural, mechanical, and fractural) as compared to the DB processed for 90, 105, and 120 min (classified as longer bonding-time interval). DB processed for 60 and 75 min exhibited layer-wise Cu–Ti-based intermetallics at TiA|Cu interface, whereas Cu|SS interface was completely free from reaction products. The layer-wise structure of Cu–Ti-based intermetallics were not observed at TiA|Cu interface in the DB processed for longer bonding-time; however, the Cu|SS interface had layer-wise ternary intermetallic compounds (T₁, T₂, and T₃) of Cu–Fe–Ti-based along with σ phase depending upon the bonding-time chosen. Diffusivity of Ti-atoms in Cu-layer (D_{Ti in Cu-layer}) was much greater than the diffusivity of Fe-atoms in Cu-layer (D_{Fe in Cu-layer}). Ti-atoms reached Cu|SS interface but Fe-atoms were unable to reach TiA|Cu interface. It was observed that DB fractured at Cu|SS interface when processed for shorter bonding-time interval, whereas the DB processed for longer bonding-time interval fractured apparently at the middle of Cu-foil region predominantly due to the existence of brittle Cu–Fe–Ti-based intermetallics.