Mechanical behaviour and interface correlative microscopic analysis of vacuum diffusion bonded dissimilar stainless steel/α-Ti alloy joint for aerospace applications

Abstract

In the present study, dissimilar metal joints between stainless steel (SS321) and α-Ti alloy are produced through diffusion bonding under vacuum at 850, 900, and 950 °C temperature under 10 MPa pressure for 10–30 min by using a thermo-mechanical simulator. A correlative microscopic approach involving Transmission electron microscopy (TEM), Scanning TEM, and Transmission Kikuchi Diffraction (TKD) is employed for examining the resultant interface microstructure, chemistry, and structural information at finer length scales. The detailed microstructural investigation revealed that the joint interface possessed a series of five layers of intermetallics phases viz. σ phase (layer 1), χ phase (layer 2), (Fe,Cr)2Ti (layer 3’), FeTi (layer 3”) and β-Ti (layer 4). The joint produced at 850 °C for 30 min exhibited a maximum tensile failure strength of ∼247 MPa (∼40 % of σ(SS321)). The produced joints have poor strength under tensile loading as compared to under shear loading because of the difference in load sharing mechanism of intermetallic phases. The fracture surfaces of the tensile tested sample and interface failure regions analysis showed (Fe,Cr)2Ti and FeTi phases as the weakest and most embrittling phases. The importance of the type of loading while designing the dissimilar metal joint configuration has been established.