Development of hyper-interfacial bonding technique for ultra-fine grained steels and microstructural analysis of bonded joints

Abstract

A new conceptual bonding technique, ‘hyper-interfacial bonding’ has been proposed as the most effective bonding technique for ultra-fine grained steels. The hyper-interfacial bonding process was characterised by instantaneous surface-melted pressure bonding which involved a series of steps, namely surface heating by high frequency induction, rapid removal of a heating coil and simultaneously pressing specimens together with an oil pressure system. All sequences were typically completed within 1 s under a vacuum/gas atmosphere. Ultra-fine grained high strength steels with an average grain size of 1·0–1·5 μm were used for bonding. The surface of the specimen could be rapidly heated up and melted within about 0·2 s. No voids existed on the bond interface and primary ferritic phase was formed at the bond interface. A bainitic structure and martensite–austenite constituents were confirmed in the heat affected zone (HAZ), while the base metal indicated a fine ferrite–pearlite texture. The hardened area was limited to the higher temperature region in the HAZ (the maximum hardness was 320–400 HV) and softening did not occur in the HAZ. Prior austenitic grains were coarsened in the vicinity of the bond interface, however, grain growth in the bond layer could be depressed below about 11–16 μm. It could be concluded that the hyper-interfacial bonding technique is a feasible joining method for ultra finegrained steels.