Combustion joining of carbon-carbon composites to tial intermetallics

To meet the requirements of energy-efficiency and high-quality joining between C/C composite and TiAl intermetallics in the aerospace field, a novel combustion joining method was developed for joining these two materials. In this method, a composite interlayer that consisted of a Ti-Al-C powder mixture and an Ag-Cu-Ti filler foil was employed. The effect of the reactant compositions and preheating temperature on the adiabatic temperature was calculated. Based on the calculated results, the composition of the Ti-Al-C reactants was selected. The joint microstructure was characterized using scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. The results showed the presence of a TiC layer on the carbon-carbon composite side and Ti-Al-Cu layers on the TiAi side. The shear strength of the joint was tested to evaluate the joint quality. The joint strength reached a maximum value of 17.6 Mpa when the interlayer thickness was 500 um and m=1.1.

Carbon-carbon composites have attracted great interest due to their excellent ratio of mechanical properties to density at high temperature, small thermal expansion and remarkable thermal-stability. These advantages of C/C composites make them important high-temperature structural materials, which have been widely used in fusion reactors, the space shuttles, rocket nozzles and aircraft brakes. The nozzle applications of C/C composites require them to be joined to metals, such as TiAl intermetallics, with high efficiency. The achieved joint should occupy high quality, and good high temperatures performance. Therefore, there is considerable interest in developing novel technologies capable of joining C/C composites to TiAl intermetallics.

However, the joining of C/C composites to TiAl is still challenging due to the poor wettability of C/C composites and the coefficient of thermal expansion mismatch between the two materials. The techniques that could be used to join C/C composites mainly include diffusion bonding, adhesive bonding and brazing. Adhesive joining could be used to join C/C composites, but the joint would not hold up to harsh environments. The diffusion bonding process requires a relatively long holding time and high temperature. The total heating of the joining couples often leads to a loss in desirable properties of the substrates. The C/C composite joint made using reactive metal brazing can be utilized only at relatively low temperature. Hence, developing a method for joining C/C composites to TiAl that is rapid, only creates local heating and is energy-efficient is still a challenging and demanding task.

Combustion joining, which involves the use of a powder or multilayer interlayer as a heat source by self-propagating high-temperature synthesis reaction, is an attractive local heating and energy-efficient method for joining C/C composite. The Ti-C, Si-C, Ti+Ni-Al +Ti interlayers have been successfully used to join C/C composites. However, our previous studies revealed that interfacial residual stresses had a strong influence on the quality of the C/C-TiAl joint, and a defect-free joint could only be obtained with a specially designed assembly. The normal reaction products of the reactive interlayer are usually brittle and not suitable for releasing the residual stresses. In addition, the direct joining of C/C composites to TiAl intermetallics using combustion joining and the corresponding joint properties has still not been reported. Therefore, taking advantage of the brazing technology that releases the residual stress and improves the interfacial reaction, a novel combustion joining method was developed to join C/C composites to TiAl intermetllics.

In this study, C/C composites were joined to TiAl using a Ti-Al-C powder interlayer and an Ag-Cu-Ti filler foil. The adiabatic temperature was calculated to design the interlayer composition, and the real combustion temperature was measured. The microstructure of the reaction products in the C/C-TiAl joint was examined. In addition, the effects of the joining parameters on the microstructure and the joint properties were systematically investigated.



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