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INTRODUCTION

TITANIUM aluminides are one of the most promising intermetallic compounds, and a lot of attempts have been made to develop their applications in the past decades.[1] In particular, Ti3Al-based alloy is a structural material with a potential for applications in aerospace field due to its relatively low density, high specific strength, excellent creep behavior, and good oxidation stability at high temperature.[2,3] To realize its practical application, developing joining technologies of Ti3Albased alloy to itself or to other materials such as TiAl intermetallics has become an important issue. Fusion welding, brazing, and diffusion bonding have been used to join Ti3Al-based alloys. For instance, Wu et al. studied the joining of Ti-24Al-17Nb (at. pct) alloy using laser beam welding technology and the joint strength at room temperature was equivalent to that of the base material.[4] The dissimilar joining of Ti-22Al-25Nb (at. pct) to Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (wt pct) alloys was conducted using electron beam welding technique in Reference 5, and the joint strength

H.S. REN, Ph.D. Candidate, is with the Department of Material Science and Engineering, Beihang University, Beijing 100191, China, and also with the Welding and Plastic Forming Division, Beijing Institute of Aeronautical Materials, Beijing 100095, China. H.P. XIONG, Professor, B. CHEN and X. WU, Senior Engineers, Y.Y. CHENG, Professor, and B.Q. CHEN, Doctor, are with the Welding and Plastic Forming Division, Beijing Institute of Aeronautical Materials. Contact e-mails: [email protected] and xionghuaping69@ sina.cn S.J. PANG, Professor, is with the Department of Material Science and Engineering, Beihang University. Manuscript submitted August 15, 2015 Article published online January 11, 2016 1668—VOLUME 47A, APRIL 2016

was even higher than that of base material. Moreover, Cadden et al.[6] investigated the brazing of Ti-13.4Al-21.2Nb (at. pct) alloy using Ti-Cu-Ni system fillers. Commercial laminated Ti-15Cu-25Ni (wt pct) foil produced joints with high nickel segregation and caused the formation of high amount of centerline alpha-two phase that contributed to the poor tensile behavior. In Reference 7, the brazing of dissimilar materials of Ti3Al and Ti-6Al-4V (wt pct) alloys was carried out with Ti-Cu-Ni system fillers, and it was found that the presence of continuous Ti2Ni phase deteriorated the joint strength. Furthermore, the joining of Ti2AlNb and TiAl intermetallic alloys was conducted using diffusion bonding method,[8] and a new phase of Al(Nb, Ti)2 was formed within the joint through a eutectoid-like reaction: Ti2 AlNbðB2Þ þ Al ! AlðNb; TiÞ2 þa2  Ti3 Al. The presence of Al(Nb, Ti)2 with AlNb2 structure deteriorated the mechanical properties of the joints. It should be noted that in this study, no interlayer was used. Although the maximum shear strength of 260 MPa for the joint bonded at 1273 K (1000 °C) was obtained at room temperature, a high bonding pressure of 20 MPa and a long dwell time of 60 min were needed in the experiment.[8] On the ot