Dissimilar metal joints for the APT superconducting cavity's cryogenic plumbing systems

Titanium (Ti) is the material of choice for the helium (He) vessel surrounding the accelerator for the accelerator production of tritium (APT) superconducting cavities. The Ti helium vessel must be joined to a stainless steel (SS) cryogenic plumbing system in the cryomodule. In addition, a niobium (Nb) to SS joint must be developed that can replace a braze connection currently used to attach the stainless-steel Conflat flange to the Nb beam tube of the cavity. Inertia friction welding (IFRW) was chosen to ensure that sound joints were formed for both applications. IFRW is typically well suited for joining dissimilar metals. However, even this process has its limitations, particularly when the base metals are not physically or metallurgically compatible. Such incompatibilities can lead to the formation of brittle intermetallics at the interface; consequently, mechanical properties suffer. To remedy this, interlayer materials are used that are both structurally and microstructurally compatible with both base metals. The interlayer metal, while ideally at least as strong as the weakest base metal, is kept thin so that constraint effects will raise its apparent yield strength to match that of the base-metal yield strength. With this in mind, the main objective for the current study was to develop tube-to-tube parameters for joining 316L, SS to Nb, and to commercially pure Ti using a Nb interlayer. In the present study, welding parameters were developed and this paper highlights salient the microstructural features associated with the interface regions between the base metals and the interlayer metal, and reports on the mechanical properties of various joint combinations. To accomplish this, specimens were metallurgically prepared from as-welded and tested joints, and analyzed using light, scanning electron and transmission electron microscopy.