| Abstract: |
To promote long-range interplanetary missions, ISAS/JAXA is developing a new hybrid Ti-6Al-4V/Si3N4 spacecraft thruster capable of producing higher thrust. The combustion chamber (high-temperature section) will be made of Si3N4 ceramics, whereas the nozzle skirt will be made of Ti-6Al-4V alloy (low-temperature section). Brazing non-weldable and poor-workable ceramics to metal enables the production of complex structural assembly for high-temperature applications. Since the operating temperature around the joining section of thruster is engineered to be ~873 K, two different Ag-based fillers having good oxidation resistance, CUSIL-ABA® (63Ag-35.25Cu-1.75Ti mass%) and SILVER-ABA® (92.75Ag-5Cu-1Al-1.25Ti mass%), with solidus temperatures of 1053 K and 1133 K, respectively, were evaluated. The as-received CUSIL-ABA® filler consists of Ag-Cu solid solutions (s.s.) and Cu4Ti intermetallics (IMCs), whereas SILVER-ABA® filler consists of Ag s.s. and AlCu2Ti IMC. Prior to brazing the Nb/Si3N4 side, transient-liquid-phase bonding of Ti-6Al-4V/Nb side using pure Cu and Ni foils as fillers was conducted at 1213K for 180 min to eliminate brittle IMCs for reliability enhancement. Cu and Ni were alloyed with the Ti-6Al-4V substrate and the TLP-bonded Ti-6Al-4V/Nb side mainly consisted of (?+?)-Ti. In the brazing zone of Nb/Si3N4 side, the microstructures had phases similar to the as-received fillers. Despite the higher solidus temperature of SILVER-ABA® filler, finite element analysis incorporating the as-received fillers’ elasto-plastic properties showed that residual stress in the Si3N4 ceramics is lower attributed to its better plastic deformability. Experimental joints brazed with SILVER-ABA® filler were prone to failure from the reaction layer along Si3N4 ceramics, whereas those brazed with CUSIL-ABA® filler had fracture initiated from the Si3N4 ceramics. While SILVER-ABA® filler better accommodates residual stress, brazing had to be optimized with longer holding time and temperature to promote sound interfacial bonding with the Si3N4 ceramics due to its lower Ti content.
|
|
