The Effect of Anodic Oxide Films on the Nickel-Aluminum Reaction in Aluminum Braze Sheet
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OVER the past few decades, the desire for a more lightweight, energy efficient automobile has become apparent. Through the use of lighter metals in areas such as the frame, body, and internal components, improvements have been made to fuel economy and overall emissions.[1] One such improvement lies in the use of aluminum alloys for heat exchangers in automotive components. Aluminum is lightweight, easily machinable, and offers excellent thermal conductivity for thermal applications.[2] The primary method to manufacture automotive heat exchangers, such as engine and transmission oil coolers, is through furnace brazing of a stacked assembly of multi-layered aluminum sheet. In the case of automotive furnace brazing, each individual aluminum brazing sheet is a multi-layered structure consisting of a core and clad. Typically, the clad is on one or both sides of the braze sheet surface and consists of an Al-Si alloy at or below the eutectic composition of 12.6 wt pct Si (e.g., AA4343, AA4045, COLIN A. TADGELL is with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, Canada and also with the Department of Mechanical Engineering, Dalhousie University, 1360 Barrington Street, Box 15000, Halifax, NS, B3H 4R2, Canada. MARY A. WELLS is with the Department of Mechanical and Mechatronics Engineering, University of Waterloo. STEPHEN F. CORBIN is with the Department of Materials Engineering, Dalhousie University, Halifax, NS, B3H 4R2, Canada and also with the Department of Mechanical Engineering, Dalhousie University. Contact e-mail: [email protected] LEO COLLEY, BRIAN CHEADLE, and SOOKY WINKLER are with the Dana Canada Corporation, Oakville, ON, L6K 3E4, Canada. Manuscript submitted May 5, 2016. Article published online January 5, 2017 1236—VOLUME 48A, MARCH 2017
AA4047). The core is usually an alloy with an Al-Mn composition (e.g., AA3003). Owing to its higher silicon content, the clad has a lower melting temperature than the core of the sheet.[3] The manufacture of aluminum brazing sheet is done primarily through roll bonding of a clad ‘sandwiched’ 3XXX series core. This process creates new surface area through tension in the rolling direction, which exposes the underlying surface metal as aluminum oxide is fractured.[4,5] Aluminum naturally passivates in open air forming an ever-present adherent thin oxide layer. This oxide layer protects the metal from corrosive environments by acting as a barrier between the two. However, the oxide can also act as a barrier to flow of the molten clad metal during the brazing process.[6,2] Therefore, it is necessary to remove the surface oxide layer at areas of joining prior to brazing. One method of oxide removal is via conventional flux brazing methods which use inorganic KAlF6 compounds to chemically remove the oxide layer before melting of the clad.[6,7] A second method of oxide removal is vacuum brazing, which utilizes aluminum brazing sheet materials with magnesium additions in a high vacuum environment to disrupt the oxide l
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