Transport phenomena involving molten clad during aluminum brazing process.

摘要

Controlled atmosphere brazing (CAB) is a state-of-the-art joining technology associated with high production rates of many products, most notably aluminum compact heat exchangers. The complex designs of highly augmented compact heat transfer surfaces impose very difficult requirements for a brazing process. Regardless of the fact that brazing is a highly developed engineering art, the state-of-the-art manufacturing operations lack predictability and clear description of the phenomena involved. The related phenomena, in particular transport phenomena, are to a large extent only partially explored.; In this dissertation, an overview of the CAB process was elaborated first. A study of thermal integrity of brazed joint has illustrated further how joint quality could influence a heat exchanger performance. Transport phenomena related to molten metal flow behavior prior to solidification of molten clad during CAB process was the major objective of this study. A non-equilibrium model for clad melting and residuum formation was proposed and verified by experimental results. The hot stage microscopy technique was extensively used for a direct observation of wetting kinetics of surface tension governed reactive flows of molten metal during brazing. Spreading rate of sessile drop on horizontal surface fits the power law format, however slower than predicted by Tanner's law. It was also found that spreading rate of molten clad through grooves was much faster than over smooth surfaces. A novel method of joining mating surfaces by delivering filler metal through capillary grooves was proposed. Due to a reactive nature of molten clad spreading, the interaction between liquid and solid during brazing (before solidification of molten clad) was studied as well.