Liquid Film Migration in Warm Formed Aluminum Brazing Sheet
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TRODUCTION
ONE method of fabricating automotive heat exchangers is by forming aluminum (Al) alloy brazing sheet into a desired geometry, assembling the components in the required configuration, and brazing the assembly together in a controlled atmosphere. The brazing sheet used in heat exchanger production is comprised of at least two Al alloy layers, depending on the application.[1] The core layer, usually an AA3xxx alloy, remains solid during brazing and provides strength for the assembly, whereas the clad layer melts during brazing to provide the brazed joint filler metal, due to the melting point reduction from the high silicon (Si) content of these AA4xxx alloys. Al alloys possess many desirable properties such as low density, high thermal conductivity, and good corrosion performance;[2] however, the room temperature (RT)
M.J. BENOIT, M.A. WHITNEY and M.A. WELLS are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave. W., Waterloo, ON, N2L 3G1 Canada. Contact e-mail:[email protected] H. JIN is with the CanmetMATERIALS, 183 Longwood Rd S, Hamilton, ON, L8P 0A5 Canada. S. WINKLER is with the Dana Canada Corporation, 656 Kerr St., Oakville, ON, L6K 3E4 Canada. Manuscript submitted January 10, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
formability of Al alloys is relatively poor compared to conventional automotive materials such as sheet steel, which can lead to issues in forming components with complex geometries.[3] Warm forming has previously been investigated as a method to improve formability of Al alloys, although most of this work has focused on AA5xxx or AA6xxx series.[4,5] Recently, however, improvements in forming limits and springback reduction of Al brazing sheets have also been demonstrated through warm forming.[6–9] To date, no research has been conducted to investigate the potential impacts of warm forming on subsequent brazing. A number of different interactions during brazing between the liquid clad and solid core alloys have previously been identified,[10–14] but the most important for strained Al brazing sheet is the phenomenon of liquid film migration (LFM). Woods first noted the occurrence of LFM in Al brazing sheet,[15] although it had been observed in a number of material systems as noted by Yoon.[16] As the liquid film migrates through the core alloy, a dislocation free solid solution consistent with the solidus composition at the brazing temperature is re-precipitated in its wake. While there are cases where LFM may be beneficial, its occurrence in Al brazing sheet is considered to be detrimental. The severity of LFM in Al brazing sheet has previously been shown to depend on the level of cold work imparted to the sheet prior to brazing, and consequently
the post-braze core alloy microstructure.[15,17–22] A non-linear relationship between the extent of LFM and applied strain was observed; LFM first increased with applied strain, but upon reaching a critical strain, the extent of LFM decreased with further increases in applied strai
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