Research On Brazing Process And Joints Properties Of High Nb Containing TiAl Alloy

In the present research, high Nb containing Ti Al alloy was successfully brazed using Ti-28Ni(wt.%) eutectic brazing alloy. The effects of brazing temperature and holding time on the interfacial microstructure and joining properties were investigated in details. In particular, the best brazing process was summarized, i.e., brazed at 1100°C for 15 min. Simultaneously, part of the as-prepared joints(brazed at 1100°C for 15min)was thermally exposed at 800°C for durability test. The emphases were placed on clarifying the effects of high-temperature and oxidation corrosion on the interfacial microstructure. Meanwhile, the microstructure evolution of the joint end portion and the base metal surface was analyzed comprehensively. In addition, fracture analyses were carried out to investigate the relationship between joining properties and interfacial microstructure. Especially, the formation and evolution of interfacial microstructure were summarized comprehensively. Furthermore, the growth kinetics of the reaction layers was also studied in details.The Ti-28 Ni brazing alloy prepared by vacuum arc remelting consisted of two kinds of phases(Ti(s, s) and Ti2Ni(δ)). The solidus and liquidus temperatures were about940°C and 980°C, respectively. The interfacial microstructure presented a symmetrical characteristic, which could be divided into two diffusion zones(I) and a brazed seam(II). It is determined that four kinds of phases including α2-Ti3 Al, δ-Ti2 Ni, τ3-Al3 Ni Ti2,B2 were detected during brazing reactions. The brazing temperature and holding time had a great influence on the interfacial microstructure. The diffusion of Ni from molten brazing alloy to Ti Al substrate played an important role during the interfacial microstructure evolution, which resulted in an increase of diffusion zone and a reduction of brazed seam with increasing the brazing temperature or prolonging the holding time. However, the continuous α2-Ti3 Al layer in the brazed seam was insensitive to the change of brazing conditions. Shear test indicated that the maximum shear strength at room and high temperature reached 248.6MPa and 166.4MPa respectively when the joints were brazed at at 1100°C for 15 min. Fracture analyses indicated that brittle fracture preferred to initiate and propagate in the continuous intermetallic layers(δ-Ti2 Ni and α2-Ti3Al) during shear test. The high temperature or long holding time was detrimental to the joints properties, which caused a lot of microcracks appeared in diffusion zones.The durability test showed that the microstructure morphology, phase constitution and element content had not changed significantly after thermally exposure for 300 h.When the holding time was over 55 h, the surface of joint end portion began to form amixed oxide reaction layer, and a porous structure was formed beneath the oxide film.When the holding time reached 300 h, the oxidation corrosion depth would increase to about 25μm. With the prolongation of high temperature storage time, an increasing number of oxide film produced on the surface of base metal. When the time reached300 h, the thickness of surface oxide layer could increase to about 5μm. The minimum shear strength at room temperature was 147.8MPa, which was decreased by 40.5%compared with the maximum one(248.6MPa). High temperature and oxidizing environment could embrittle the interfacial microstructure. Especially, the porous structure was formed on the joint end portion after the joints were subjected to oxidation corrosion for a long time. It was noted that cracks preferred to initiate and propagate in the porous structure, which led to the rapid reduction of shear strength.The formation process of the joint brazed at 1100°C for 15 min could be divided into six stages. The joints brazed at different brazing conditions had a similar formation process, but there were some differences between them. Based on the diffusion theory of Fick’s first and second laws and the diffusion of Ni atom in the diffusion zone, the growth behavior of the diffusion zone was described and the dynamic equations for growth behavior of diffusion zone were obtained.