Friction stir welding of SS 316 LN and Nitronic 50 jacket sections for application in superconducting fusion magnet systems

Abstract

This study explored the possibility of using friction stir welding (FSW) to join jacket web sections of two nitrogen-containing stainless steels for housing internally cooled superconducting cables which are utilized to generate magnetic fields in tokamak type fusion reactor systems. The two candidate materials chosen for the jacket are SS 316 LN and Nitronic – 50 owing to their desirable mechanical and physical properties at cryogenic service temperature. The current manufacturing techniques to fabricate the jackets or conduits include fusion butt welding. There are some inherent disadvantages of utilizing the fusion joining process such as the possibility of sensitization and the evolution of undesirable phases detrimental to the application. An attempt has been made to fabricate the jackets with FSW to evaluate its feasibility to obtain the desired mechanical and physical properties critical to the application. The welding parameters optimization, workpiece clamping approach, microstructure evolution, hardness line profiles, tensile properties, and magnetic properties of the jacket welds corresponding to both the materials have been discussed in the paper. It has been shown that the FSW fabricated SS 316 LN jackets possessed the required strength and magnetic properties critical to this application.