Process Development for Vacuum Brazed Niobium–316L Stainless Steel Transition Joints for Superconducting Cavities

Abstract

The paper describes process development for producing sound, strong, and ductile Nb pipe–316L stainless steel (SS) flange brazed joint suitable for application in superconducting radiofrequency (SRF) cavities. The developed transition joints, made with BVAg-8 braze filler metal (BFM), were free of brittle intermetallic compounds, in contrast to the existing global brazing practice of using oxygen-free electronic copper as BFM which results in the formation of a continuous layer of Fe–Nb brittle intermetallic compound at Nb–braze interface. In view of the large difference in the mean thermal expansion coefficients between niobium and 316L stainless steel, a new design for manufacturing and assembly (DFMA) has been developed to ensure achievement of desired joint thickness with uniformity in circumferential and longitudinal directions. An environment-friendly prebraze cleaning procedure has been qualified and implemented. DFMA has resulted in (i) significant reduction of the out-of-roundness errors (≤10 μm) while machining of the niobium pipe, (ii) simplified clearance fit prebraze assembly at room temperature (RT), and (iii) uniformity of joint thickness. A process flow chart has been developed to ensure repeatability of joint characteristics. The brazed joint, of niobium pipe and 100CF knife edge 316L SS flange made by standardized practice, displayed helium leak tightness better than 5 × 10−10 mbar l/s at RT and at liquid helium temperature (LHT). The braze-joint sustained 873 K/10 h postbraze hydrogen degassing treatment and thermal cycling between RT and LHT without any loss in hermeticity.

The Effect of Ultrasonic Cleaning on the Secondary Electron Yield, Surface Topography, and Surface Chemistry of Laser Treated Aluminum Alloy

“…A variety of solvents, such as perchloroethylene, acetone, absolute ethyl alcohol, etc., are usually adopted for the cleaning of vacuum products made of stainless steel, copper, and aluminum alloy [11][12][13]. Ultrasonic cleaning combined with appropriate solvents is usually used for surface cleaning of different UHV components in accelerators, such as thin film coatings [14,15], SRF cavities [16,17], laser engineered surfaces [18], high-heat-load absorbers [19], etc.…”

Laser welding of niobium to AISI 304 steel using a copper interlayer

“…The joining of Nb to austenitic stainless steel requires the use of a particle accelerator [1][2][3]. During the operation of a particle accelerator, the cooling of superconducting Nb by liquid He is accommodated by an austenitic stainless steel container [1].…”

“…The joining of Nb to austenitic stainless steel requires the use of a particle accelerator [1][2][3]. During the operation of a particle accelerator, the cooling of superconducting Nb by liquid He is accommodated by an austenitic stainless steel container [1]. Therefore, a reliable and cryogenic leak-tight seal is necessary between the Nb cavity and the austenitic stainless steel container.…”

“…However, the extensive differences in the physical and chemical properties between Nb and stainless steel have caused many problems in their direct joining [3]. The most obvious result is the formation of brittle intermetallics (i.e., Fe2Nb and Fe7Nb6) attributable to the crystallographic mismatch between Nb and stainless steel, which could lead to solidification cracks in the joint and therefore brittle failure [1,3]. Existing solid-state welding methods such as brazing-welding and friction stir welding can inhibit the formation of some intermetallic compounds, so these techniques have become the most common welding methods for joining Nb to dissimilar metals.…”

“…Nb-1Zr alloy and 1Cr18Ni9Ti steel using a Ni-based solder (BNi-2, BNi-5, and BNi-7) were joined through high-temperature vacuum brazing, and the brazed joint exhibited excellent airtightness and good thermal shock resistance [8]. Kumar et al [1] studied the vacuum brazing of Nb-AlSI 316L steel with a Ag-28% Cu filler metal; at the temperature of liquid He, the brazed joint displayed improved hermeticity. An improved vacuum brazing method was developed by Fuerst et al [2], and a good cryogenic leak-tight joint was obtained via vacuum brazing Nb to stainless steel using a Cu filler metal.…”