Abstract
The continuous Ti-Cu compound layer produced in brazing of graphite to copper with Ti foil is found to be seriously detrimental to joint properties due to its brittleness. In this work, a transient liquid phase (TLP) bonding method with a diffusion process below melting point is developed to realize a Ti-Cu compound layer free joint. The degradation of Ti-Cu compound layer depends on two simultaneously occurring processes, namely flow of titanium atoms to copper substrate and that to TiC layer on graphite. The latter is determined by growth kinetics of TiC layer based on carbon diffusion process. A degradation model is proposed and applied to optimize the TLP bonding. The improved graphite/copper joints without Ti-Cu compound layer show 20.8% higher in shear strength compared with that of brazing joints.
Highlights
Carbon based materials (CBMs) including graphite and carbon fiber reinforced carbon composites have been widely used as structural materials and electronic components owing to their unique combination of high melting point, high thermal/electrical conductivity, excellent thermal fatigue and plasma compatibility[1,2,3,4]
Because copper is inert to carbon characterized by neither with mutual solubility nor formation of stable carbides[9], some strong carbide-forming element like titanium is usually added in filler alloy to facilitate reactive wetting
Since residual stress caused by the mismatch of coefficient of thermal expansion (CTE) between CBMs and copper is created in the joint, the brittle intermetallic compounds (IMCs) dramatically increase the possibility of joint failure in service[12,13,14]
Summary
Carbon based materials (CBMs) including graphite and carbon fiber reinforced carbon composites have been widely used as structural materials and electronic components owing to their unique combination of high melting point, high thermal/electrical conductivity, excellent thermal fatigue and plasma compatibility[1,2,3,4]. Various active brazing alloys (such as Cu50TiH210, Cu-ABA11, TiCuNi6) have been used to join CBMs and copper. It’s known that typical TLP bonding involves long isothermal solidification process at joining temperature to make sufficient diffusion of the interlayer element (or a constituent of an alloy interlayer) into substrate materials[15,16]. The contact reaction between Ti and Cu is first carried out at 920 °C to promote wetting, isothermal process (at 860 °C) is set below the melting point so as to prevent sever infiltration of liquid alloy into graphite. The degradation model proposed by this work can be used to predict the residual IMCs layer in seam and is helpful to improve the joining technique. The microstructure and mechanical properties of graphite/copper joint joined by an optimized TLP bonding are presented
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