Analysis of Liquid Film Migration in Aluminum Silicon Alloys

Aluminum-silicon alloys are used for brazing of aluminum heat exchangers such as automotive radiators, air conditioning condensers, and fuel/oil coolers. The Al-Si alloys are typically roll-bonded to the surface of other aluminum alloy sheets that have a higher melting temperature. The sheet in between them (core alloy and braze alloy on the surfaces) are then stamped to a desired shape, assembled, and processed through a high-temperature furnace for brazing. In the brazing stage, the Al-Si alloy will melt and join adjacent sheets together. A processing defect called liquid film migration (LFM) can occur where excessive grain growth during brazing will create a microstructure that is prone to corrosion attack and early degradation. LFM happens only in certain alloys and in cases where the material was cold worked to a certain extent prior to the brazing operation.The objective of this project is to use differential scanning calorimetry (DSC) and optical microscopy to investigate the LFM phenomonea in an industrial Al-Si alloy system. Sheets of material pre-stresses to certain levels are available as initial samples. From those samples, DSC specimens werecut and tested to measure the calorimetric behavior of the alloy during simulated brazing cycles. The same samples were thenmounted and polished after DSC testing so that the brazed microstructure can be analyzed via optical microscopy to measure degree of LFM. The final goal is to determine if a relationship exists between cold work prior to brazing and liquid film migration