Vacuum Brazing Of Titanium/316L Stainless Steel Transition Joint For Application In Helium Vessel Of Superconducting RF Cavities

Abstract

Superconducting radiofrequency (SRF) cavities would form important part of the proposed Indian Spallation Neutron Source and Accelerator Driven System. The high beta 650 MHz SRF cavity would be enclosed in a cylindrical vessel to hold liquid helium (LHe). Titanium (Ti) is being considered as the material of construction of helium vessel. The LHe inlet supply line and return helium gas pipe line of the helium vessel would be made of 316L stainless steel (SS). This requires a bi-metallic tubular transition joint between Ti and 316LSS, operating at LHe temperature. Vacuum brazing and explosive welding are potential processes for joining these two dissimilar metals, as fusion welding leads to extensive cracking. In the present experimental study, vacuum brazing was performed to join Ti-pipe/316LSS flange with BVAg8 braze filler metal (BFM). Notable features of the process include use of (i) 5-10 μm nickel electro-plating as a diffusion barrier on SS part for preventing possible iron migration towards titanium while improving surface wettability for BFM and (ii) use of 304 SS plug, shrunk fit into Ti-pipe, for achieving dimensional and profile accuracy of Ti-pipe during machining while also controlling joint gap during brazing process. The brazed joint displayed uniform joint thickness and acceptable level of hermeticity (helium leak rate 2 x 10-10mbar.lit/s) and also sustained six thermal cycles between 293 K and 77 K. Shear strength of Ti-SS brazed specimens, made in sandwich configuration, was found to be in the range of 50-60 MPa, with failure occurring at Ti/braze interface.