Abstract
Higher strength and better corrosion resistance are necessary to enable the thickness reduction of the aluminum alloy brazing sheets. In this work, an age-hardenable Al-Mn-Cu-Mg-Si core alloy was developed by the addition of Mg, Si and Cu. The corrosion behavior of 4004/Al-Mn-Cu-Mg-Si/4004 aluminum alloy brazing sheet in sea water acidified accelerated test (SWAAT) was investigated and compared with the traditional 4004/3003/4004 aluminum alloy brazing sheet. The microstructure was characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). After brazing, it was found that the microstructures of two aluminum alloy brazing sheets from outer to the center were precipitate-free zone (PFZ), the band of dense precipitates (BDP) and core material, respectively. In particular, both Cu and Mg concentration gradients were formed in the BDP region of 4004/Al-Mn-Cu-Mg-Si/4004. Moreover, the SWAAT results showed exfoliation corrosion in 4004/Al-Mn-Cu-Mg-Si/4004, while in 4004/3003/4004, severe inter-granular corrosion in core material was observed after corrosion for 584 h. The theoretical electric potential distribution showed that the presence of Cu and Mg concentration gradient promoted the formation of the electric potential trough in the BDP region, which may be the reason for the occurrence of exfoliation corrosion in 4004/Al-Mn-Cu-Mg-Si/4004.
Highlights
Aluminum alloy brazing sheets are commonly used for heat exchangers in the automotive industry, due to their high thermal conductivity, excellent mechanical properties, good corrosion resistance combined with low specific weight [1-3]
Due to the fact that the brazing process is usually operated at elevated temperatures (~600 °C) for a short time, in principle, it is possible to develop age-hardening AA3xxx core alloy to enhance the strength of aluminum alloy brazing sheets
It is found that large precipitation-free α-Al grains, i.e. precipitate-free zone (PFZ), are formed on the surface of two aluminum alloy brazing sheets due to the liquid film migration [11, 12]
Summary
Aluminum alloy brazing sheets are commonly used for heat exchangers in the automotive industry, due to their high thermal conductivity, excellent mechanical properties, good corrosion resistance combined with low specific weight [1-3]. In order to improve corrosion performance of aluminum alloy brazing sheets, long life alloys with a Cu-containing AA3xxx core alloy, have been developed and used for many years [10-12]. In such sheets, Si atoms diffuse from the cladding into the core during the brazing process, promoting the precipitation of Mn out of solid solution in the core adjacent to the cladding and subsequently the formation of the band of dense precipitates (BDP). We intend to contribute to the understanding of the corrosion mechanism involved
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