Fatigue mechanisms of brazed Al-Mn alloys used in heat exchangers

Abstract

The ratio of aluminium alloys used in the automotive industry tends to increase as a consequence of the enforcement of tougher environmental regulation (minimization of vehicles weight). For example, thanks to their good thermal, corrosion and mechanical properties, aluminium alloys have steadily replaced copper alloys and brass for manufacturing heat exchangers in cars or trucks. Such components have been constantly optimized in terms of exchange surface area and, nowadays, this has led to Al components in heat exchangers with a typical thickness of the order of 0.2 to 1.5 mm. With such small thicknesses, the load levels experienced by heat exchangers components has drastically increased leading to an important research effort in order to improve the resistance to damage development during service life. This paper focuses on the resistance to fatigue damage of thin sheets of brazed co-rolled aluminium alloys used for manufacturing heat exchangers and particularly on the mechanisms of fatigue cracks initiation. Digital Image Correlation (DIC) has been used to monitor damage development during constant amplitude fatigue tests of thin (0.27 mm) samples. Fatigue cracks have been found to initiate from deformation bands which presence can be correlated with solidification drops at the sample's surface resulting from the brazing process. X-ray tomography has been used to obtain the spatial distribution of drops as well as their characteristics (height, surface...), on the sample gauge length. Those 3D data have been used to produce finite element meshes of the samples in order to assess the influence of the drops on fatigue crack initiation.