A Novel Brazing Technique for Aluminum and Other Metals
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A NOVEL BRAZING TECHNIQUE FOR ALUMINUM AND OTHER METALS
ROLAND S. TIMSIT* AND B.J. JANEWAY* *Alcan International Ltd., Kingston R&D Centre, Kingston, ON, Canada K7L 5L9
ABSTRACT In the novel brazing technique, the aluminum components in the joint assembly are coated with a powder mix consisting of elemental Si and a potassium fluoroaluminate flux. During brazing at -6001C in nitrogen gas, the flux melts and removes the native AI20 3 surface film from the coated aluminum components. This action allows the silicon to diffuse into the aluminum to generate in-situ a layer of Al-Si filler metal of eutectic composition.The liquid metal then flows into thejoint and yields a metallurgical bond on cooling. This brazing technique maybe exploited with aluminum using intermediary elements other than Si. The technique may also be used for joining other metals. INTRODUCTION Recent years have witnessed an increased reliance on nitrogen-furnace brazing for the joining of aluminum heat-exchangers and air-conditioningcondensers [1,2]. This brazing technique requires that one of thealuminum components in ajoint beclad with filler material consistingof an Al-Si alloy of near-eutectic composition such as AA4045 or AA4343 [3]. These alloys are characterized by a melting temperature (-577,C) [4] considerably lower than that of the core alloy (-660'C). Joining is carried out at approximately 6001C in the presence of a non-corrosive flux, such as potassium fl uoroaluminate [1], to remove native oxide films from the aluminum surfaces. At that temperature, the filler metal melts and flows into thejoint to yield a metallurgical bond on cooling. The present work was motiva ted by the need to develop a brazing technique that obviates the use of clad aluminum sheet. The novel technique reported in this paper fulfills this requirement and has been used for brazing aluminum/Cu, Cu/Cu and Cu/brass in addition to aluminum/aluminum joints. THE BRAZING PROCESS In the novel brazing technique, at least one of the aluminum surfaces is coated with a layer of a powdermix consisting of Si and a flux capable of dissolving surface oxide films [5], as illustrated in Fig. 1(a). A noncorrosive flux compatible with aluminum [1,31 consists of a mixture of KAIF4 and K2AIF 5.H2 0 powders in a molar ratio of approximately 13:1 with a particle dimension of the order of 1 pm. Typical surface coverages by Si powder range from a few to several grams per square metre, depending on the joining application. Considerably larger Si surface coverages may be used. The weight ratio of Si to flux-powder varies typically from -1:1 to 1:3, also depending on the application. The Si powder-particle dimensions may range from -1 to 100 plm. Brazing is carried out by heating the joint at approximately 6001C in nitrogen gas at nearatmospheric pressure for a time interval of the order of one minute. During temperature ramp-up, the flux melts at -562°C and dissolves the surface oxide layers on aluminum [6] as illustrated in Fig. 1(b). Oxide removal allows the silicon particles to come in
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