This study investigated the effect of post-brazing cooling rate and Si addition on the intergranular corrosion (IGC) susceptibility of brazed Al-Mn-Cu alloys by electrochemical analysis and microstructure observation. Water-quenched samples after brazing exhibited no IGC susceptibility, whereas slowly-cooled samples were prone to IGC. The results suggest that IGC is caused by precipitation during cooling. In addition, it was observed that IGC susceptibility depended on the Si content. An alloy sample with a low Si-additive content exhibited high IGC susceptibility because Mn/Cu-depleted zone was formed near the grain boundaries as a result of the preferential precipitation of Al 6 (Mn,Fe) and CuAl 2 on the grain boundaries. In contrast, moderate Si addition inhibited IGC because the decrease of the Mn content in the grain interiors due to enhanced precipitation of Al 15 (Mn,Fe) 3 Si 2 in the grain. Additionally, Cu-depleted zone also disappeared because preferential precipitation of CuAl 2 on the grain boundaries was prevented. The excess-Si alloy exhibited high IGC susceptibility because Si-depleted zone formed around the grain boundaries as a result of the preferential precipitation of coarse Si particles on the grain boundaries although the Mn/Cu-depleted zones were not formed.

Al-Mn based alloys with high-manganese content are expected to have improved mechanical properties due to solid solution hardening and/or dispersion hardening. However, the increase of Mn solubility of the alloy is difficult by using the conventional DC casting. In order to solve this problem, in the present study, we focused on the twin-roll casting method which is characterized by high cooling rates. Several kinds of high Mn-containing Al-Mn-Si alloy strips were fabricated by using a vertical-type high-speed twin-roll caster equipped with a pair of water-cooled copper rolls. Direct temperature measurement of the liquid melt during the casting was also performed. The alloy strips of various compositions containing up to 4 Mn and 2 Si (wt%) were successfully obtained. By observing the microstructure of the cross section of the strip, we found the characteristic solidified structure. The solidified structure consisted of three layers. Two solidified shells with a columnar dendrite structure grew from the roll surfaces toward the strip center. In the mid-thickness region, the band structure consisting of equiaxed dendrites and globular grains was observed between the solidified shells. Very fine primary particles were observed in the matrix near the strip surface, while, relatively coarse particles with blocky and needle-like shape were observed in the central band of the as-cast strip. The electric conductivity measurement was performed for the as-cast strips. Mn solubility in Al matrix was estimated from the obtained values. The estimated Mn solubility in the Al-2Mn-xSi strips was between 1.5 ~ 1.8wt% Mn. It was over 1.43wt%Mn for the Al-4Mn-xSi strips. We found that the Mn solubility of the as-cast strips was considerably high. The strips were cold-rolled to the sheets and then annealed at various conditions. They were subjected to the tensile tests, and the effects of solid solution hardening and dispersion hardening are discussed.

Shohei IWAO * and Masami ASANO * SWAAT for 20 days has been carried out on brazing sheets of 1 mm thickness to estimate outside corrosion resistance on the material for aluminum heat exchanger applied at higher temperature. Maximum corrosion depth after the test decreased in brazing sheets both of Al-1%Mn alloy core and of Al-1%Mn-0.5%Si alloy core clad with Al-Si alloy filler while it increased in that of Al-1%Mn-0.5%Si-0.5%Cu alloy core, with increase of heat treatment time at 200°C after brazing. Marked intergranular corrosion in the core was observed in the latter alloy. On the other hand, general corrosion was observed in brazing sheets clad with filler alloy containing Zn, and the attack stopped at near interface between filler and core, independent of core alloys and of heat treatment time. Corrosion characteristics and attack depth on the sheets were discussed in terms of electrochemical properties of filler and core alloys.

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To investigate effect of additional Mn in thin brazing sheet core alloy on corrosion resistance, SWAAT and electrochemical measurements have been carried out with brazed Al-Mn-1.6 mass%Si alloys. Intergranular corrosion clearly occured in the alloys which Mn content is 1 mass% or less. In the case of these alloys precipitates of Si particle were observed at grain boundary, but they were not observed in grain after brazing. As a result, the potential difference between neighborhood of the grain boundaries and grain matrix became bigger. Therefore, selective dissolution occured at neighborhood of the grain boundaries which was lower solid solution of Si. On the other hand, intergranular corrosion were not observed in the alloys which Mn content is 1.6 mass% or more. In the case of these alloys fine precipitates of AlMnSi were observed in grain after brazing. As a result, the formation of Si-SDZ solute depleted zone by Si precipitation at grain boundary seen in the former alloys were inhibited. In conclusion, addition of Mn in thin brazing sheet had improving effect of corrosion resistance after brazing.