Journal of Materials Processing Technology 187–188 (2007) 526–529 A study on diffusion bonding of superplastic Ti–6Al–4V ELI grade Ho-Sung Lee a,∗ , Jong-Hoon Yoon a , Chan Hee Park b , Young Gun Ko b , Dong Hyuk Shin c , Chong Soo Lee b a Department of Structure and Materials, Korea Aerospace Research Institute, 45 Eoeun-Dong, Yuseong-Gu, Daejeon 305-333, Republic of Korea b Department of Materials Science & Engineering, Pohang University of Science and Technology, San 31, Hyoja-Dong, Nam Gu, Pohang, Gyungbuk 790-784, Republic of Korea c Department of Metallurgy and Materials Science, Hanyang University, 1271 Sa-Dong, Ansan, Kyunggi-Do 425-791, Republic of Korea Abstract Ti–6Al–4V ELI (extra low interstitials) grade alloy provides improved ductility and fracture toughness comparing to grade 5 Ti–6Al–4V alloy. In order to find the optimum superplastic forming and diffusion bonding (SPF/DB) condition, a series of tensile tests was carried out at the strain rate range of 10 -4 to 10 -2 s -1 and temperature range of 1073–1223 K. The maximum elongation of 1898% was obtained at the strain rate of 10 -3 s -1 at 850 ◦ C. It was shown that the ELI grade alloy performs better than the grade 5 alloy in terms of the optimum superplastic condition for Ti–6Al–4V. Based on this result, diffusion bonding process of superplastic Ti–6Al–4V ELI sheet metals was developed. Bonding was completed by means of inert gas pressure applied in a bonding tool at high temperature. The microstructure of the bonding area was investigated and the bonding interface was microscopically undetectable. The evidence of nucleation of new grains and migration of grain boundaries at the interface proves the diffusion bonding process is successful. It is shown that the superplastic forming and diffusion bonding of Ti–6Al–4V ELI grade is possible at the temperature lower than those of conventional Ti–6Al–4V. © 2006 Elsevier B.V. All rights reserved. Keywords: Diffusion bonding; Superplasticity; Ti–6Al–4V ELI; Solid state bonding; Gas pressure 1. Introduction Diffusion bonding is an attractive manufacturing method for aerospace applications where mechanical properties in the bond area and a sound metallurgical bond are important. Diffusion bonding is such a process in which two matched surfaces are held together at a temperature range between 0.5 of the absolute melt- ing temperature of the materials and the room temperature under a low pressure without causing a macroscopic plastic deforma- tion in the materials. This is different from a brazing process in which a third metal with a lower melting point is used to weld similar or dissimilar metals together. In brazing, a thin film of liquid filler metal is penetrated into the bond interface by capil- lary action and therefore, the bonding can be quite weak unless this filler metal is very thin. However, since diffusion bonding is formed from atomic migration across an interface in a solid state, there is no metallurgical discontinuity at the interface and hence mechanical properties and microstructure at the bonded region are not different from those of the base metal. A similar ∗ Corresponding author. Tel.: +82 42 860 2512; fax: +82 42 860 2233. E-mail address: hslee@kari.re.kr (H.-S. Lee). process utilizing liquid phase is transient liquid phase (TLP) dif- fusion bonding in which a suitable interlayer is inserted between the pieces to be joined and interdiffusion between the interlayer and the base material leads to the formation of a low melting point liquid phase at the bonding temperature. The liquid must be a low melting alloy in which one or more elements will dif- fuse rapidly into the base metal. As this element diffuses away, the interfacial region becomes enriched in the non-diffusing ele- ments, which solidify due to loss into the bulk of the diffusing component. In other words, TLP bonding starts as a braze and ends as a diffusion bond and it reduces pressure and time required for bonding. In this study, diffusion bonding is only a solid state process that is defined as one in which the components being joined undergo no more than a few percent macroscopic deformation without a liquid phase. The process is dependent on various parameters, in particular, time, applied pressure, and bonding temperature to promote microscopic atomic movement to ensure complete metallurgical bond. Ti–6Al–4V alloy is well known for aerospace applications because of its low density, high strength, excellent corrosion resistance, and good high temperature durability. In addition, this alloy shows superplastic properties that allow for large 0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.215