Mechanism of intergranular penetration of liquid filler metal into oxygen-free copper

Intergranular penetration (IGP) of liquid filler metal (FM) into oxygen-free copper (OFC) occurred when OFC and Ni-plated materials were brazed with AgCu28 filler. In order to analyze the mechanism of IGP, nine different joints were brazed, and the diffusion behavior of Ni and Ag and the concentration distribution of Ni in liquid were studied. Results show that IGP occurs only when Ni element exists in the joints. In the brazing process, Ni element tends to diffuse from the liquid region with low Cu content to the liquid region with high Cu content, and then to solid. Therefore, the concentration distribution of Ni in liquid is uneven. Ni is enriched at the solid–liquid interface. After adding Ni in the joints, a saturated Cu solid solution layer is easy to form on the surface of base metal (BM), which hinders the interdiffusion between solid and liquid. But, a large number of Ni atoms in liquid are unstable due to the high chemical potential, and Ag concentration in liquid is still higher than the equilibrium concentration. Grain boundaries (GBs) become the channel for rapid diffusion of Ni and Ag atoms in liquid into BM. In addition, the saturated solubility of Ag in Cu solid solution decreases after adding Ni in the joints, making GBs easier to be liquefied. IGP further increases the area of solid–liquid interface.