Microstructure and mechanical properties of diffusion bonded joints between titanium and stainless steel with copper interlayer

Abstract

The solid state joining of titanium to stainless steel with copper interlayer was carried out in the temperature range of 850–950°C for 7·2 ks in vacuum. The interface microstructures and reaction products of the transition joints were investigated with an optical microscope and a scanning electron microscope. The elemental concentration of reaction products at the diffusion interfaces was evaluated by electron probe microanalysis. The occurrence of difference in intermetallics at both interfaces (SS/Cu and Cu/Ti) such as CuTi₂, CuTi, Cu₄Ti₃, χ, FeTi, Fe₂Ti, Cr₂Ti, α-Fe, α-Ti, β-Ti, T₂(Ti₄₀Cu_60?xFe_x; 5<x<17), T₄(Ti₃₇Cu_63?xFe_x; 5<x<7) and T₅(Ti₄₅Cu_55?xFe_x; 4<x<5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were detected by X-ray diffraction technique. The maximum tensile strength of ～91% of Ti strength and shear strength of ～74% of Ti strength along with ～ 7·2% ductility were obtained for the joint bonded at 900°C due to better coalescence of mating surfaces. At a lower joining temperature of 850° C, bond strength is poor due to incomplete coalescence of the mating surfaces. With an increase in the joining temperature to 950°C, a decrease in bond strength occurred due to an increase in the volume fraction of brittle Fe–Ti base intermetallics.