Study On Transient Liquid Phase Bonding Of Ti₃Al Based Alloy

Ti₃Al intermetallic has attracted particular attention as suitable materials in aerospace industry. Compared with titanium alloys, Ti₃Al-based alloys can be used at higher temperaturedue to their low density and high temperature properties. Their low ductility and poor formability at ambient temperature are the major disadvantages for practical applications, which have been solved by the addition of elements such as Nb, V, and Mo stabilize the Ti₃Al in order to acquire dual microstructure withα2+β. Thus, Ti₃Al-based alloys with better comprehensive mechanical properties have been developed in aerospace industry. TC4 has a dual microstructure ofα+βand it covers 60% of the full output and 95% of the finished production of titanium alloy. The engine blade is almost fabricated with TC4 alloy due to its high strength and low density. Recently, TC4 alloy has become a new application material with the development of new application filed.It is necessary to solve the problem for joining Ti₃Al based alloy and TC4 alloy when they are used as structural materials. Joining technology is a major factor limiting the application of intermetallics. However, it is difficult to join these two alloys by fusion welding technology due to the difference of crystal structure, melting point, thermal conduction and linear expansion coefficients between them. Transient liquid phase (TLP) bonding has been successfully applied for joining some metallic and ceramic systems owing to a number of technological and economical advantages. TLP bonding has more and more comprehesive application to join high temperature alloy, dissimilliar alloys and particularly alloys such as intermellalics, singl crystal material, composite material and ceramics, which can not be joined with traditional joining technology.Pure copper, pure aluminum, TiZrNiCu alloy and Al/Cu composit alloys were used as interlayer to join Ti₃Al based alloy in order to reveal TLP joint microstructure and mechanical properties, and the effection of TLP bonding parameters envolving bonding time and bonding temperature on them. Moreover, composition,microstructural evolution and micro-hardness of TLP bonded Ti₃Al based alloy/Ti-6Al-4V joints have been studied using a TiZrNiCu alloy as interlayer. This can give information for understanding the welding behavior of Ti₃Al-based alloy.TLP bonded Ti₃Al-based alloy joints composed of intermetallic compound layers were produced with a 20μm thickness pure copper interlayer. Microstructural evolution of joints depended on both bonding time and bonding temperature. With increasing bonding temperature and bonding time, the joint width increased and amount of compounds in the joint decreased. The joint microstructure changed from Ti-rich + Ti3Cu+Ti2Cu+Ti3Cu4+Ti(Ni,Cu ) phase at 850℃×1min to Ti+Ti₃Al +Ti2Cu phase at 900℃×60min. The joint shear strength of 307.5 MPa can be obtained at 950℃for 60min. Ti₃Al-based alloy can be joined by TLP bonding using pure Ni as interlayer. With increasing the bonding time, the bonding zone width increased and the TLP process require long time at 960℃. The joint microstructure was mainly Ti₃Al+Ti solid solution, Ti-Ni compounds with a higher Al and Ti-Ni compounds. When the bonding temperature was 960℃and the bonding time was 30min, the maximum hardness of joint was 603(HV). However, the joint hardness had a relatively smooth distribution curve, which helped to enhance joint property.The results show that integrated joints were produced by TLP bonding using the TiZrNiCu alloy interlayer. The high bonding temperature and long bonding time help to obtain the joints with uniform composition and microstructure. With increasing bonding temperature and bonding time, the width of the bonding zone increased and reaction zones decreased. When bonding temperature was 900℃for bonding time of 60min, the joint microstructure consisted of Ti solid solution, Ti₃Al andTi2Cu and the joint strength of 420 MPa was acquired.Composition,microstructure evolution and micro-hardness of TLP bonded Ti₃Al based alloy/Ti-6Al-4V joints have been studied using a TiZrNiCu alloy as interlayer. Experimental results showed that bonding temperature and bonding time have a great effect on joint composition and microstructure. With increasing bonding temperature and bonding time, composition distribution of joint had a uniform tendency and width of the bond zone increased. When bonding temperature were 850℃and 900℃, Ti-Cu intermetallics were found in the joint due to the existence of residual liquid. When bonding temperature was 950℃and bonding time was 30min, Ti-Cu intermetallics disappeared from the joint owing to completion of isothermal solidification. Micro-hardness of bond zone decreased as bonding temperature and bonding time were increased. When bonding temperature was 950℃and bonding time was 30min, the joint acquired a relatively uniform micro-hardness distribution.