New approach in microstructural analysis of a modified AA4XXX/AA3XXX brazing sheet before and after brazing

Aluminium brazing sheet is a sandwich material made out of two aluminium alloys (AA4XXX/AA3XXX) and is widely used in automotive heat exchangers. Re-solidification of an AA4XXX cladding as well as the related particle formation and distribution in a modified AA3XXX core material before and after brazing has been investigated with a new microstructural analysis approach. Optical microscope (OM), scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX) and thermo-dynamical calculations (JmatPro software) have been used. The particles size, distribution and chemical composition have been analyzed. The equilibrium phases, their weight percentages and the temperature ranges through which various phases are stable have been calculated. The results were used to identify the particles in the clad and the core material. Analysis of the structure before brazing showed that besides silicon (which is the most dominant particle), β-AlFeSi and α-Al(Fe-Mn)Si are the other existing primary particles in the clad material. α-Al(Fe-Mn)Si and Al2Cu were found to be the primary dominant particles that exist in the core structure. Accumulation of copper on the surface of the re-solidified clad material, the reduction of the copper content of the particles and the increase of silicon concentration in the diffusion zone were found to be the major changes that occurred due to brazing process. Low concentrations of some transition elements (Cu, Cr) in the phase that is expected to be α-Al(Fe,Mn)Si particle were detected.