AISI 304 steel brazing using a flexible brazing foil fabricated by tape casting method

The authors report the fabrication of a flexible nickel brazing foil using powdered nickel alloy filler (BNi-2) mixed with an organic binder. The organic binder was composed of a polyacrylic acid polymer, glycol, carbon tetrachloride, and water as a dispersion medium for the powder. The brazing paste so formed was then tape-cast on a polymer foil with different polymer to paste (dispersant) ratios, of≈ 9:1, 8:2 and 7:3 by weight, and was dried in a low-temperature oven. The filler pyrolysis temperature, paste spreading, permeability, and wettability were examined. The thermal analysis results showed that the filler paste decomposition temperature was in the range of 463-498 °C, while the tape-cast brazing foil had a pyrolysis temperature of ≈334.36 °C. The resulting flexible self-adhesive foil was used to braze steel foils. The AISI 304 steel joint microstructure and joint tensile shear tests were also performed. It was found that the brazing foil produced ≈1 wt% of residue after melting. The microstructural analysis showed a uniform distribution of a Cr-rich in Ni-rich matrix at a polymer to dispersant ratio of 8;2. It is suggested that the wettability of the brazing foil on AISI 304 steel will be maximum, and a higher joint strength can be obtained when the polymer to dispersant ratio is kept at 8:2.