Microstructure and mechanical response of dissimilar joint of ferritic interstitial-free steel to austenitic low-density steel produced by diffusion bonding

Abstract

Lightweight Fe–Mn–Al–C steels, also referred as low-density steels (LDS), have gained significant importance in the recent past owing to their reduced density, high strength and ductility, along with good oxidation resistance at elevated temperatures. The present study is aimed at exploring the feasibility of solid-state joining of LDS to the other prominent ferrous alloys, for example, interstitial-free steel (IFS), in order to extend their applications. In the present work, diffusion bonding has been attempted to produce IFS-to-LDS joints with and without the application of nickel (Ni) interlayer. Diffusion bonding is performed at two different temperatures of 850 °C and 950 °C for 30 min at the bonding pressures of 10, 20 and 30 MPa under high vacuum conditions. The interfacial microstructures of the joints are examined using an optical microscope and scanning electron microscope in conjunction with energy dispersive spectroscopy and electron backscattered diffraction. Microhardness and nanoindentation measurements across the interface and shear tests are performed to evaluate the interfacial bond strength of the joints. The joint produced with the application of Ni interlayer at 950 °C under 30 MPa bond pressure exhibited superior shear properties with a maximum shear strength of ∼340 MPa. The formation of stable oxides resulting, especially by diffusion of aluminum from LDS to the faying surfaces is found to be detrimental to establish a good interface at lower bonding parameters. The application of higher bonding pressure and Ni interlayer along with enhanced diffusional activities from LDS to IFS within the austenite phase are found to be effective in achieving a sound joint. The metallurgical aspects involved in the formation of the reaction products and subsequently, the influence of the evolved interface microstructure on the mechanical response of the joints are discussed in-detail.