Analysis of Fluxless, Reactive Brazing of Al Alloys Using Differential Scanning Calorimetry
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ALUMINUM and its alloys are extensively used in the aerospace and automotive industries due to their low density, high corrosion resistance, and high specific strength.[1–3] Brazing is an important step in the manufacturing of many aluminum industrial components, including automotive heat exchangers.[1–4] Automotive heat exchangers are commonly produced by joining together an assembly of composite aluminum brazing sheets each having a clad/core-layered structure. The core layer material, which provides strength and the lifecycle requirements of the assembly, typically consists of a AA3000 Al-Mn type alloy. The clad layer uniformly covers both surfaces of the core layer (constituting ~20 pct of the overall braze sheet thickness, 10 pct on each side) and consists of a lower melting point AA4000 Al-Si hypo-eutectic type alloy.[4] During the brazing process, the clad surface acts as the filler metal, thus melting and joining the core sheets together where they are in physical contact. A principal challenge in Al brazing is the presence of a tenacious oxide film on the faying surface of the braze sheet which can prevent wetting, free flow of the clad layer, and the formation of a defect free metallurgical bond.[3,5] A common way to remove this oxide layer is STEPHEN FRANCIS CORBIN, Professor, is with the Department of Civil and Resource Engineering, Dalhousie University, 1360 Barrington Street, P.O. Box 15,000, Halifax, NS B3H 4R2, Canada. Contact e-mail: [email protected] SOOKY WINKLER, Manager, is with the Materials & Joining Technology, Dana Canada Corporation, 656 Kerr St., Oakville, ON L6K 3E4, Canada. DENNIS R. TURRIFF, Senior Engineer, is with the MEA Forensic Engineers and Scientists, 226 Britannia Road East, Mississauga, ON L4Z 1S6, Canada. MARK KOZDRAS, Manager, is with the Canmet MATERIALS, Natural Resources Canada, 183 Longwood Road South, Hamilton, ON L8P 0A5, Canada. Manuscript submitted July 3, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
through the use of controlled atmosphere brazing (CAB) (e.g., the use of a high-purity N2 atmosphere) and the use of a chemical flux such as potassium fluoroaluminate commonly known as NOCOLOK.[3,5,6] An alternative method that avoids the need for flux is Dana Canada Corporation’s proprietary Ni-based fluxless brazing process. CAB, with either the use of NOCOLOK flux or a Ni-based fluxless process, are well established, mass production, commercial methods to produce brazed aluminum components.[3–7] In the fluxless process, a Ni-based braze promoter is electrolytically deposited onto the surface of the Al brazing sheet. Upon heating, oxide penetration is achieved when the Ni reacts exothermically with the underlying Al-rich surface during the brazing process.[7] Qualitatively, the expected mechanisms of the exothermic reaction between an Al brazing sheet and Ni plating have been outlined in Reference 7. According to the Al-rich corner of the ternary Al-NiSi phase diagram (see Figure 1), the presence of Ni at the clad surface and the Si content within the Al-rich clad l
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