Corrosion of AA 6061 Brazed Al-Si-(Cu) Type Fillers: Metallurgical and Corrosion Behavior of Novel Tapered Wedge Coupons

Naval applications may benefit from the use of cellular metal structures due to dynamic and static mechanical, acoustic, and/or thermal properties combined with light weight and low effective density. One attractive fabrication method in complex sandwich materials involves transient liquid metal bonding with a braze alloy.  Brazing easily joins complex core topographies containing many connection sites with planar face sheet materials. However, one challenge is the susceptibility to corrosion. The corrosion susceptibility of AA 6061-T6 (Al-Mg-Si) to pitting and intergranular corrosion in T4, T6, and two overaged conditions was investigated.  Additionally, AA 6061 T6 samples were taken through the thermal cycle of a typical dip brazing process without applying filler metal. Other samples of base AA 6061 T6 were dip brazed with either paste form (Al balance, 0.15%wt max Mg, 9.3-10.7%wt Si, 3.3-4.7%wt Cu, 0.8%wt Fe) or foil form (Al balance, 0.1%wt max Mg, 11-13%wt Si, 0.3%wt max Cu, 0.8%wt max Fe)filler materials.  These were placed on the surface of the base material and reheat treated to T6 temper per ASTM B918.  Isolated filler foil in stock sheet, as well as isolated foil treated thermally and left in the T4 condition, were examined.  Anodic polarization scans on all materials were run in deaerated 0.6 M NaCl solution giving the pitting, open circuit, and repassivation potentials.  Intergranular corrosion studies were conducted in accordance with BS ISO 11846 (24 hr immersion in 30 g/L NaCl and 10mL+-1mL of concentrated HCl at room temperature) along with SEM imaging and EDS of the characteristic regions. Additional investigations involve the use of a tapered model braze joint with variable clearance. Braze microstructure and composition, were characterized as a function of diffusion length and processing parameters.   The metallurgy characterization was guided by thermochemistry calculations.