Effect of Temperature and Sheet Temper on Isothermal Solidification Kinetics in Clad Aluminum Brazing Sheet
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WITH increasingly stringent government regulations, virtually every major automaker is actively working to design and commercialize a range of electric and hybrid vehicles that depend on advanced batteries or hybrid battery and engine propulsion solutions, which are low or zero greenhouse gas-emission vehicles. The success of the lithium-ion battery technology being adopted by most automakers for use in these vehicles is largely dependent on the integration of advanced thermal management and control, since battery life and efficiency depends on the ability to control the battery’s operating temperature. Aluminum (Al) alloys are extensively used in automotive thermal management devices, such as battery coolers for electric and hybrid vehicles, due to their low density, good thermal conductivity, and corrosion resistance.[1] The primary method to manufacture heat-exchange devices is through furnace brazing of a stacked assembly of multilayered aluminum sheet consisting of a core and clad layer. The clad layer is typically an AA4xxx-series Al alloy, which is rich in MICHAEL J. BENOIT, Ph.D. Candidate, MARK A. WHITNEY, Research Associate, and MARY A. WELLS, Professor, are with Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Canada. Contact e-mail: [email protected] SOOKY WINKLER, Manager, is with Materials and Processing Engineering Group, Dana Canada Corporation, Oakville, Canada. Manuscript submitted November 9, 2015. Article published online July 8, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
silicon (Si) and has a relatively low melting point and good flowability in the liquid state, while the core layer is typically an AA3xxx-series Al alloy, which is rich in manganese (Mn) and serves as the structural component of the sheet.[2] The clad alloy is of hypoeutectic to eutectic composition to ensure that the melting point of the clad is sufficiently lower than that of the core. Upon heating of the clad sheet assembly, the lower melting point clad melts and wets the surfaces of the adjacent core layers; upon cooling, the liquid clad solidifies to create the joints. It has been documented in a number of previous studies that upon formation of liquid clad, interactions can occur between the clad and the solid core, reducing the amount of clad available for joining and altering the properties of the core alloy due to penetration of clad metal into the core.[3,4] One phenomenon that is of particular importance for clad Al brazing sheets is isothermal solidification (IS). When the brazing sheet is heated to the peak brazing temperature and the clad layer becomes molten, the solute that depresses the melting point (Si in the case of AA4xxx-series Al alloys) diffuses into the solid core. As a result of diffusion, the amount of liquid with sufficient Si content to remain liquid decreases, and solidification occurs at the solid/ liquid (S/L) interface in the absence of cooling (i.e., isothermally).[5,6] Given sufficient time at peak temperature, significant diffusion can occur until the S/L interface has prog
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