Transport Phenomena Involved in Controlled Atmosphere Brazing of Microchannel Aluminum Heat Exchanger

Controlled atmosphere brazing (CAB) is a state-of-the-art joining technology associated with high production rates of many products, including microchannel aluminum 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, many state-of-the-art manufacturing operations lack predictability and clear description of the phenomena involved. The related phenomena are to a large extent only partially explored, owing the progress mostly to the lack of a detailed knowledge of involved physical-chemical processes. This paper elaborates on the comprehensive research and development activities involved in manufacturing new generation Al heat exchangers using brazing technology. Our current activities are aimed at a set of goals that significantly contribute to the optimization of bonding processes, including material selection and process conditions. In this paper, an overview of the CAB aluminum brazing process will be introduced first. Transport phenomena related to molten metal flow behavior and joint formation during microchannel heat exchanger brazing process is the major objective of this study. The in-situ real time monitoring facilities in our brazing lab are used for direct observation of wetting kinetics of surface tension governed reactive flows of molten aluminum filler metal during brazing. Interesting phenomena such as flow of liquid metal through capillary grooves and its relation with Al heat exchanger brazing will be explored. Examples of in-house designed and brazed Al compact heat exchangers will be illustrated. The examination of brazing results using metallographic procedure will also be elaborated.