Vacuum brazing of dissimilar tubular component of AA2219 and AISI 304 by a low melting Al-18Ag-20Cu-5Si-0.2Zn braze alloy

Dissimilar and tubular components, AA2219 and AISI 304, were joined using a conventional threaded joint with a metallic sealing in the inter thread regions using a low melting Al-18Ag-20Cu-5Si-0.2Zn alloy (melting range 494-515 °C). The braze alloy was cast into ingot and processed into foil form, wherein the melting range of braze foil further dropped and narrowed down to 491-506 °C. Preliminary vacuum brazing experiments were carried out with the braze foil on uncoated and Cu coated specimens of 2219 and 304, of which Cu coated assembly showed improved wettability and joining feasibility. Vacuum brazing was then performed on the Cu coated 2219-304 component assembly and successful braze joint was formed due to capillary action of the braze filler into the inter-thread regions of 2219 and 304 providing a complete metallic sealing. Microstructural characterization of braze alloy and dissimilar brazed joint were carried out using optical microscopy, SEM, EPMA and XRD techniques. The braze alloy formed continuous and crack free diffusion reaction layers of Al-Fe-Si-Cu-Cr and Al-Cu-Fe-Ni at 304 interface and Al-Ag layer at the 2219 interface, indicating a sound and leak-proof joint. The brazed zone comprised of α-Al matrix, intermetallic phases Al2Cu, Ag2Al (fine particles as well as plate-like Widmanstätten structure), Si particles and minor amounts of Ag3Al and Ag5Al. Instrumented and spherical indentation technique in continuous multi-cycle mode was employed to estimate the hardness and strength properties across the brazed joint. The average true stress of brazed region was calculated to be 629 MPa at 8% true strain, the equivalent of which is ∼606 MPa for AISI 304. Also, the brazed component has been successfully leak tested at room temperature with designated conditions.