Study on requirement of nickel electroplating in OFE copper-316L stainless steel brazed joints

Abstract

The paper describes an experimental study to evaluate the need of nickel electroplating on stainless steel part in vacuum brazed oxygen-free electronic (OFE) copper-type 316L stainless steel transition joints with silver copper eutectic braze filler metal (BVAg-8 of AWS A5.8). The results of the study demonstrated that nickel electroplating is not required to obtain hermetically sealed sound and strong OFE copper-316L stainless steel transition vacuum brazed joints. The only pre-treatment requirement for stainless steel part is simple ultrasonic cleaning, although pickling treatment has been found to have a favorable effect in improving its wettability for the braze filler metal. Brazed joints made with unplated stainless steel (SS) not only displayed (i) required level of hermeticity (helium leak rate ≤2 × 10−10 mbar·l/s) and bakeablility at 523 K for ultra-high vacuum application and (ii) reasonably good tensile strength comparable to those made with Ni-plated SS, but also exhibited significantly suppressed extent of intergranular penetration of braze filler metal into copper. Important factors responsible for the development of sound OFE copper-316L stainless steel brazed joints with ultrasonically cleaned stainless steel part are excellent wettability of OFE copper which compensated for poor wettability of stainless steel surface and presence of hydrocarbon-free high vacuum environment during brazing.