aluminum heat exchanger, surface morphology, brazing
The market demand for aluminum brazed exchangers in air conditioning and refrigeration industry is continually growing owing mostly to the cost and weight savings of aluminum materials, with undisputed compactness and low charge. Manufacturing of aluminum heat exchangers requires reliable metal bonding technology to join together various components, such as tubes, fins and header manifolds. Controlled atmosphere brazing (CAB) of aluminum is the state-of-the-art technology for mass production of compact aluminum heat exchangers. Traditionally, the development of a brazing process relies on trial and error in practice. However, modern manufacturing process requires good understanding of scientific principles involved in the brazing operations. It is well known that a brazing process is assisted by the capillary flow of liquid filler metal on the base surfaces of various heat exchanger components. Therefore a good understanding of the molten flow behavior on aluminum surfaces is critical in quality assurance of the brazed heat exchangers. Capillary flow occurring at high temperature level during brazing is usually much more complicated than the wetting in an inert system with a smooth solid surface. For example, the flow of liquid is not only controlled by surface tension force, but also influenced by interactions between liquid and base metal materials. In addition, the metal surface of a heat exchanger component to be brazed is usually not smooth due to the fabrication process such as extrusion or hot/cold rolling. In this paper, the phenomena closely related to liquid filler metal wetting behaviors on base metal with different surface morphologies are presented. Experimental facilities such as heating stage microscopy system and transparent furnace are used to visualize the capillary flow of molten filler metal under typical CAB brazing conditions. The influence of the base metal surface morphology on wetting behavior as well as brazed joint quality are examined, assisted by metallographic analysis of re-solidified brazing joints. The physical and chemical phenomena in the interaction between liquid (molten flux and filler metal) and solid (base metal) are described. The results from this study provide useful insights on how material selection and surface treatment can affect the brazing process and brazed heat exchanger quality.