Microstructural and corrosion issues of embossed and welded aluminium heat exchanger tubes

Aluminium alloys are widely used in Heat, Ventilation, Air Conditioning and Refrigeration (HVAC&R) industry due to their excellent thermal conductivity in addition to tailorable corrosion resistance and ease of formability. Recent efforts in improving the performance of all Aluminium tube and fin-based heat exchangers has led to the development of increasingly complex inner surface patterns that has given the possibility to nearly triplicate the heat exchanging coefficients for these systems. Inner surface enhanced Aluminium tubes are produced by embossing of the required pattern onto Aluminium sheet, followed by sheets forming and welding into tubes. This two-stage process calls for understanding and optimization of the Aluminium metallurgy and also the effect on microstructure and electrochemistry of the produced embossed welded tube materials, with or without a brazing/sacrificial clad. The current work studies the effect of embossing and high frequency welding on a 3xxx series commercially available Al alloy sheet of interest for this application, cladded with a sacrificial AA7072 on the outer side. The microstructure of the embossed and welded tubes was studied using polarized light microscopy, SEM coupled to an EDS, FIB. The electrochemical behavior of the finished welded tubes was assessed using Seawater Acidified Tests and pH indicator gel tests. Results showed modification of the microstructure of the core alloy near the weld region and related changes to the electrochemical behavior near the heat affected region, because of which the weld region remained resistant to corrosion during exposure tests, thus influencing final corrosion performances of welded tube. The underlying microstructural and electrochemical phenomenon are presented and discussed.