Effect of Scandium on the Interaction of Concurrent Precipitation and Recrystallization in Commercial AA3003 Aluminum Al
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INTRODUCTION
FOR decades now, aluminum-based heat exchangers have made moderate inroad into the automotive industry, owing to their relatively low cost, low weight, high thermal conductivity, and good corrosion resistance. Furthermore, with the advent of highly automated assembling technologies such as brazing, large-scale production of aluminum microchannel heat exchangers began to take a great leap. Typically, brazed aluminum heat exchangers are produced by assembling aluminum brazing sheets with multiport Al tubes. The aluminum brazing sheets have a clad/core-layered structure typically composed of Al-Mn alloy (e.g., AA3003) core, and a lower melting point Al-Si hypoeutectic alloy (e.g., 4343) clad that uniformly covers the surface on the both sides.[1–3] During the brazing process, there occurs simultaneous recrystallization of the core layer and melting of the clad layer. The clad layer essentially acts as the filler metal, creating a metallurgical bond of the core sheets. In addition, the melting clad layer tends to penetrate into the core layer along the grain boundaries, resulting in an undesirable degradation of brazeability.[4, 5] This could be circumvented by the presence of large pancake-like recrystallized grains on the core layer of
YIYOU TU, Associate Professor, XUEFENG ZHOU, Lecturer, 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, Nanjing 211189, P.R. China. Contact e-mail: [email protected] HUAN QIAN, Ph.D. Student, is with the School of Materials Science and Engineering, Southeast University. Manuscript submitted July 13, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
multilayered aluminum brazing sheet, which may restrain the penetration of melting clad layer, thereby providing a high resistance to corrosion and sagging.[2,3,6] It is well known that precipitation has a strong effect on the recrystallization kinetics and the final microstructure.[7–11] In case of deformed and supersaturated alloys, such as Al-Mn alloy, where precipitation occurs before or simultaneously with the recovery, the recrystallization process is commonly referred to as concurrent precipitation. The effect of concurrent precipitation depends on the critical temperature, TC. Greater than TC, recrystallization is slightly affected by concurrent precipitation, while at temperatures less than TC, concurrent precipitation tends to retard recovery and recrystallization. This results in the formation of large and pancake-like grains observed as elongated grain structures in the longitudinal section along the RD/ND plane.[7,12] For Al 4343/3003/4343 multilayer aluminum brazing sheets, the brazing temperature is typically ranging from 853 K to 883 K (580 C to 610 C). This temperature range is much higher than the critical temperature of the core layer alloy, AA3003, which varies in the range from 573 K to 723 K (300 C to 450 C) with the level
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