To enhance the compressive strength of an extruded flat pipe of aluminum pipe which is formed into a flat shape with a passage hole by extrusion and is fitted up with fins, header tubes and so on by brazing for composing a heat exchanger.
The extruded flat pipe of aluminum alloy for the heat exchanger is subjected to a low strain process at a distortion rate of 2-15% prior to brazing. The flat pipe having recrystallized structure due to extrusion is changed, by heating brazing, in its microstructure such that the surface layer is composed of the recrystallized structure and the internal layer has a non-recrystallized structure area. The distortion rate, for example, is indicated as (1-H/H0)×100%, where H0 is the height of the flat pipe before the low strain process and H is the height of the flat pipe after the low strain process. The recrystallized structure of the surface layer 1a and the surface layer 2a of the passage hole suppresses erosion by brazing so that the deterioration of the strength due to the erosion is prevented. The non-recrystallized structure of the internal layer 1b keeps the high compressive strength of the flat pipe so as to enable the heat exchanger to have excellent durability.
KATSUMATA MASAYA
WATABE AKIRA
