Aluminum heat exchanger, low-temperature brazing
The manufacturing of compact aluminum heat exchangers, such as microchannel heat exchangers, involves the controlled atmosphere brazing process. The joining of different parts, e.g., tube to header, tube to fin, is accomplished by molten brazing filler metal with the assistance of fluxes. The most commonly used brazing materials for aluminum heat exchangers are Al-Si alloys which offer many advantages, such as excellent wettability and material compatibility with the base materials. However, some limitations of the traditional Al-Si filler metals also exist: (1) very precise control of the brazing temperature is critical because of the small difference between the melting temperatures of the filler and base metals; (2) the choices of the base metals are limited to certain alloys such as the 3xxx and 1xxx aluminum alloys due to the material compatibility issues. The Zn-Al filler metals have relatively low melting temperatures (<500 >°C) compared to the Al-Si filler metals (> 575 °C). In this paper, experimental brazing trials of header to the extruded aluminum tube using Zn-Al filler metal at a relatively low brazing temperature range (< 550 °C) are performed. Various base materials are selected for wettability study. The baseline tests using the traditional Al-Si filler metal are also performed for comparison purposes. Corrosion tests and metallurgical analysis are performed on the post-brazing assemblies. The experimental results show good joint integrity using Zn-Al filler metal but inferior corrosion resistance. The significantly lower brazing temperature not only offers a potentially lower cost of the manufacturing process (less energy consumption) than the traditional CAB process, but also reduces the risk of manufacturing defects due to overheating. However, the long term heat exchanger reliability, such as the corrosion resistance and mechanical durability using the Zn based filler metal remains to be investigated and explored.