A NOVEL BRAZING TECHNIQUE FOR ALUMINUM

This paper describes a novel brazing technique for aluminum, in which at least one of the contacting aluminum surfaces is coated with a powder-mixture consisting of silicon and a potassium fluoroaluminate flux. Brazing is carried out by heating the joint to approximately 600-degrees-C in nitrogen gas at near-atmospheric pressure over a time interval of a few minutes. During heating, the flux melts at 562-degrees-C and dissolves the surface oxide layers on the aluminum, thus allowing the silicon particles to come into intimate contact with the bare metal. At temperatures exceeding 577-degrees-C, the silicon dissolves rapidly into the aluminum and generates in-situ a layer of Al-Si liquid alloy of eutectic composition. The filler metal forms a fillet around the joint by capillary action. A metallurgical bond is formed on cooling. The addition of Zn powder to the Si/flux powder mixture allows diffusion of Zn during brazing and modifies the resistance to corrosion of the brazed assembly. The novel brazing technique may be exploited using elements other than silicon for generating filler metal. The only requirement on these elements, such as Cu, Ge, Zn, etc., is that they form a relatively low-temperature eutectic alloy with aluminum. The technique has been used successfully for brazing aluminum/Cu, Cu/Cu and Cu/brass in addition to aluminum/aluminum joints.