Phase Transformation And Mechanical Performance Of Interface Between Ti/Ni Diffusion Bonding Couples Activated By Electric Field

Titanium and its alloys, characterized by high specific strength, excellent heat-resisting, corrosion resistance and fracture toughness property, are widely used in aviation, aerospace, vehicle manufacturing and chemical industry. Due to high price, titanium and its alloys are always served after bonding with other metals such as stainless steel, in which brittle intermetallic compounds forms and leads to poor bonding strength consequently. Nickle is commonly used as an intermediate layer to improve the bonding mechanical property of dissimilar metals and meet the requirement in service. It’s guiding significance to determine the phase transformation and properties of the Ti/Ni interface for bonding between titanium and other metals.In this work, Ti/Ni diffusion bonding has been achieved by Field Activated Diffusion Bonding technology (FADB), and the new phase formation and interface bonding strength has been studied. The microstructure, phase component and element distribution across interfaces have also been determined by metallographic microscope, scanning electron microscope, and energy spectrometer were used to observe and analyze. The shearing properties of bonded interface have been evaluated by universal testing.The results indicate that the formation intermetallic compound is Ni3Ti、 NiTi2and NiTi in order during diffusion bonding. The diffusion temperature, dwelling time and electric current are important factors during diffusion bonding. With the rise of diffusion temperature, the thickness of NiTi and NiTi2increases gradually while that of Ni3Ti firstly increases then decreases afterwards, but the total thickness of the interface increases continuously. When diffusion temperature is650℃, with increase of dwelling time, the thicknesses of all Ni3Ti、 NiTi2and NiTi increases, and that of Ni3Ti is always bigger than that of NiTi. At750℃, the thickness of NiTi quickly increases with the extension of time; the growth velocity of NiTi2increases at first and then keeps as a constant afterwards. When diffusion time reaches to30min, Ni3Ti begins to transfer into columnar structure, as time extension, more and more columnar structure forms till Ni3Ti disappears completely. Electric current can not only accelerate the growth of diffusion layer but also promotes the formation of columnar organization. The thicknesses of Ni3Ti、NiTi2and NiTi also increase with the current. When it is bigger than1000A, the black long strip β-Ti begins to form in Ni3Ti layer, and gradually expands with the current till Ni3Ti disappears, as a result, a Ti-rich layer forms between NiTi and Ni(Ti). Meanwhile, with the increase of current, dendritic NiTi layer becomes more and more obvious. The current direction has no any influence on diffusion process.The fracture surface of Ti/Ni diffusion couple shows brittle characteristics, which takes place in Ni3Ti layer ajacent Ni substrate. Ni3Ti layer disappears and forms a new Ti-rich layer when the diffusion temperature is750℃. The Ti-rich layer improves the shear strength of the joints, and the weak link shifts to the interface between NiTi2and NiTi.