Ti/Al Multilayers Diffusion Bonding γ-TiAl Based Alloy

With the rapid development of the aerospace and automobile industry, There are new requests in materials, which drives the upsurge of studying related materials and its processing technology. TiAl based alloy becomes the most potential candidate material due to its high specific strength , specific modulus and good high temperature performance and oxidation resistance. However, those shortcomings, poor ductility at room temperature, high melting point, susceptible to generate brittle phase and difficult to process into complex structural components, are major problems in welding processes.Alternating nanometric layers of titanium and aluminium were used as interlayers in diffusion bonding.On the one hand, alternating nanometric layers could make exothermic reaction in comparatively lower temperature,the released heat of reaction promotes the diffusion among the multilayer itself and base material.Interlayers themselves can react into various TiAl alloys. Joints with microstructures close to that of the parent material can be achieved by controlling of Ti/Al ratio and bonding processes.Ti/Al multilayer thin films were deposited onto the polished surface of theγ-TiAl based alloy samples by dc-magnetron sputtering. The thin films are alternating nanometric layers of titanium and aluminium with a total thickness of 3 to 5.5 m and 100~400nm period. The Diffusion bonding processes are studied.Adding bonding presure can enlarge physical contact area,and then diffusion reaction is promoted effectively.A certain extant of increaseing temperature could accelerate diffusion rate and sufficient diffusion reaction could obtained in a relatively short time.Prolong the holding time can also make diffusion reaction sufficient.The excess of temperature (1000℃) or holding time (5h) may cause coarsing of the joint microstructure and generation of pores,which make the property of joints decreased. Sound joints without voids and fracture were obtained by diffusion bonding at 900℃, at a pressure of 30MPa for 3h. 100nm period multilayer produced the joint with the highest shear strength 230.84MPa, the 200nm period multilayer produced the joint with the highest shear strength 254.38MPa. Based on the research of bonding pocess, the joining mechanism have been studied, diffusion reaction between Ti/Al multilayers occurs through a single lamellar grain boundaries, interface reaction between interlayers and substrate takes place by Ti atom diffusing into substrate, while joining between the two matching surfaces is achieved by atomic thermal motion.