Brazing of Additively Manufactured Metals

The additive manufacture (AM) of metals promises geometrical freedom, but there are limitations with repeatability and cost which hinder its adoption in industry. Within nuclear fusion, AM is seen as a route to create parts from hard-to-manufacture materials and to optimise part geometries, but a joining method is required due to the mismatched material properties that multi-material AM is not capable of printing yet. Coupling AM with brazing, which uses a molten filler metal to form a joint above 450◦C, could be the solution.

This PhD explores air furnace brazing, using AgCuZnSn alloys of variable flow properties, and vacuum furnace brazing, using a CuGeNi alloy in rod and foil form, of three build orientations of AM 316LSS and examines a case study of joining AM W-10%Ta to Cu. Whilst industrial brazing practices recommend a roughness (Ra) of 0.6 - 1.6 μm, the surface roughness (Sa) of as-built AM parts are significantly rougher, at 10.7 ± 2.0 μm for horizontal, 7.8 ± 1.0 μm for vertical, and 19.0 ± 2.4 μm for a 45◦ build angle. A combination of surface parameters should be used to fully differentiate between the surfaces. In utilising the native AM surface, joints with fill greater than 80% and thicknesses within industrial tolerances are possible to manufacture via air and vacuum furnace brazing.

The AM non-equilibrium microstructure influences the interactions between the 316LSS and CuGeNi. A BCC-Fe interfacial layer forms upon brazing machined 316LSS with CuGeNi, due to the enrichment of Ge and depletion of Ni. The phase change is suppressed when brazing AM 316LSS due to the AM substructure of solidification cells resulting in the retained FCC-Fe phase and more prevalent intergranular penetration of Cu into the steel. When brazing with 45Ag27Cu25.5Zn2.5Sn, an interfacial BCC-Fe layer is also suppressed in AM 316LSS, but forms at the interface with machined 316LSS. The shear strength of the AM joints is lower for all orientation types compared to the machined control, and correlates with surface roughness and joint fill. Even when 100% filled, the machined joints are 11.2% stronger than the horizontal AM joints due to their ability to form a thinner joint. The fractography shows the BCC-Fe layer does not influence the failure location at 20◦C, but high temperature mechanical testing reveals interfacial failure prevails from 250◦C for 316LSS joints brazed with 55Ag21Cu22Zn2Sn.

The AM W-10%Ta to Cu brazed joints form successfully for the partially polished surfaces (Sa = 7.5 ± 0.8 μm), but voids are present in the native AM W-10%Ta joints (Sa = 17.5 ± 2.5 μm). The immiscibility between Cu and W results in a sharp brazing interface which is seemingly not influenced by the AM processing route. However, build
defects in the AM base material result in the CuGeNi filler metal flowing through cracks, deep into the W-10%Ta, showing the AM process itself must be improved ahead of its use in nuclear fusion.