Effect of Microstructure on Diffusional Solidification of 4343/3005/4343 Multi-Layer Aluminum Brazing Sheet
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ALUMINUM-BASED heat exchangers have been widely used in air conditioning systems for automobiles because of the low density, high thermal conductivity corrosion resistance of aluminum. Automotive aluminum heat exchangers are generally produced by assembling aluminum brazing sheets with multiport Al tubes via brazing. Aluminum brazing sheets have a clad/core layered structure and typically consist of an Al-Mn alloy core and an Al-Si hypo-eutectic alloy clad on both sides.[1] During the brazing process, while the assemblages consisted of aluminum brazing sheets and tubes are heated up to 853 K to 883 K (580 C to 610 C),[2–4] the clad layer of brazing sheets melt when the temperature is over the Al-Si eutectic temperature (850 ± 1 K).[5] To avoid collapse of the assembly during the brazing cycles, the thin core material must have a high resistance against deformation during the brazing process, a property generally referred to as sagging resistance.[6] With proper fluxing in a controlled atmosphere brazing furnace, the liquated clad layers establish a joint upon cooling and solidification, thereby sealing the heat exchanger. Thus, the brazing ability of the multi-layer clad sheet is dominated by its sagging resistance and the quality of the solder joints. The interactions of the clad (Si-rich) and core (Si-lean), such as the diffusion of Si and diffusional solidification behavior, play important roles in the brazing ability of the multi-layer clad sheet.[2,7] The amount of liquid formed, duration of stability, and characteristics YIYOU TU, Associate Professor and JIANQING JIANG, Professor, are with the School of Materials Science and Engineering, Southeast University, Nanjing 211189, P.R. China and also with the Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University. Contact e-mail: [email protected] ZHEN TONG, PhD Student, is with the School of Materials Science and Engineering, Southeast University. Manuscript submitted October 10, 2011. Article published online December 1, 2012 1760—VOLUME 44A, APRIL 2013
of flow are functions of the clad composition, the brazing temperature, and the clad/core interactions.[3] Insufficient liquid formation can cause incomplete bonds and leakage, whereas excessive heating can cause core dissolution and heat exchanger sagging. Furthermore, excess liquid can flow and pool non-uniformly in fillet regions of the sheet assembly, causing increased local core dissolution and leaving behind large a-Al residue zones.[8] During the various stages of the brazing cycle, Si diffusion from the clad into the core is expected because the clad is relatively solute rich.[1,7,9,10] Solid-state diffusion during the heat-up stage prior to melting can cause Si depletion in the clad and therefore suppress the liquid formation once above the eutectic.[10] Si diffusion can lead to the formation of a band of dense precipitates at the clad/core interface, which consists of fine aAl(Mn,Fe)Si particles, and decrease the amount of liquid present.[4,8,11,12] A novel method using DSC has been develop
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