Examining the Oxide Disruption Mechanism of a Nickel PVD Coating on Anodized Aluminum Braze Sheets
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INTRODUCTION
NICKEL aluminides are the subject of study in various synthesis methods for their beneficial properties; low density, high strength and corrosion/oxidation resistance.[1] These methods include reactive solidification, explosive bonding, and self-propagating high-temperature synthesis (SHS).[1–3] The formation of these intermetallics results in large exothermic reactions which initiates more intermetallic formation further from the heat source. In situ intermetallic formation has been used as a braze promoter to join automotive aluminum (Al) braze sheet.[4–7] In this process, a nickel layer is placed on the surface of aluminum brazing sheet which consists of an AA3xxx core and an AA4xxx cladding. Upon heating the material to simulated brazing temperatures, the presence of the nickel acts to breakdown the oxide film on the Al braze sheet so that a the molten clad would flow via capillary action and a sound metallurgical joint
COLIN A. TADGELL and MARY A. WELLS are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada. STEPHEN F. CORBIN is with the Department of Materials Engineering, Dalhousie University, Halifax, NS B3J 1Z1, Canada. Contact e-mail: [email protected] BRIAN CHEADLE and SOOKY WINKLER are with the Dana Canada Corporation, Oakville, ON L6K 3E4, Canada. Manuscript submitted January 22, 2019. Article published online May 29, 2019 3816—VOLUME 50A, AUGUST 2019
can be formed. These studies were conducted with relatively low concentrations of nickel and silicon at the surface resulting in the ternary reaction (Al) + (Si) + Al3Ni.[4] The heat of formation was measured using Differential Scanning Calorimetry (DSC) and showed this reaction occurring prior to, as well as during, the melting of the Al-Si eutectic.[5,6] A former investigation of this intermetallic formation on the braze sheet surface thought the reaction to be of a mechanical nature where the oxide was broken up during in situ intermetallic growth.[7] Further work examined the effect of the surface aluminum oxide layer and its effect on aluminum nickel interaction.[6] It was found that a sufficiently thick oxide (roughly 60 nm) can inhibit intermetallic formation allowing for a more delayed reaction which could be studied. The novelty of this current work is to provide a more in-depth study into the nature of the intermetallic formation and braze-promoting mechanisms in the presence of a thick oxide film using X-ray photoelectron spectroscopy (XPS) and high-resolution scanning electron microscopy (HRSEM).
II.
MATERIALS AND METHODS
A. Material Brazing sheets consisting of a modified AA3003 (low Si content) core with an AA4045 (high Si content) clad layer on one side was utilized in this study. These composite sheets had a total gage thickness of 0.2 mm METALLURGICAL AND MATERIALS TRANSACTIONS A
and approximately 10 pct clad ratio. These sheets were cut into approximately 200 9 100 mm strips, and then certain samples were surface treated. Following these surface treatments, s
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