Abstract
The erosion of aluminium-base alloys by molten brazing flux, which is injurious to the brazing of thin plates and fins used for the heat-exchanger or the evaporator, was investigated in regard to aluminium brazing. Effects of flux composition, additional fluoride to flux, brazing temperature and brazing time on the erosion of aluminium-base alloys were examined. The prepared aluminium-base alloys were 1100, 3003, 5052 and high-purity aluminium (99.99%), and the employed brazing flux was a mixture of LiCl, KCl and a small amount of fluoride. The degree of erosion by molten brazing flux was examined by the amounts of wieght loss of aluminium-base alloys. The eroded surface of aluminium-base alloys was observed by an electron microscope and a scanning electron microscope.
The results obtained are summarized as follows:
(1) The erosion of aluminium-base alloys was affected by the kinds of fluoride added to the LiCl-KCl flux, especially by the addition of LiF or AlF3.
(2) The erosion of aluminium-base alloys may follow an activation process of the Arrhenius type, and the activation energies were obtained as 27.8 kcal/mol for 1100, 32.9 kcal/mol for 3003, 36.6 kcal/mol for 5052 and 38.9 kcal/mol for high-purity aluminium (99.99%).
(3) The commercial-grade aluminium 1100 was more eroded by the brazing flux than 3003 or 5052 alloy. The degree of erosion of aluminium-base alloys probably depends on the electrochemical potential difference between the matrix and the intermetallic compounds precipitated as secondary phases. The matrix of 1100 or 3003 and the intermetallic compound of 5052 were preferentially eroded.
