Abstract
Due to economic and environmental issues, the components weight and so the material thicknesses have been reduced which led to fatigue problems. In this study, a prototype dye was created to perform the fatigue test directly on aluminium mock-ups (tube and header of car thermal exchanger). The Wöhler curves at the crack initiation compared with that of the mock-ups complete torn-off and the temperature influence are shown. A metallographic analysis coupled with EBSD analyses is proposed to understand the crack propagation as a function of the grains misorientation and a SEM study shows the propagation mode on a tube.
The system studied is composed of: a tube which is a 3 co-rolled aluminium alloy (4XXX, 7XXX, 3XXX, thickness 0,27mm), and a header which is a 2 co-rolled aluminium alloy (4XXX, 3XXX, total thickness 1,5mm).The Wöhler curves have been performed during constant stress amplitude with a sinus signal, and at different temperatures to understand the influence of this parameter (R=0, 1; f=30Hz; T= -30, 20, 120°C). Wöhler curves showed that an increase in temperature significantly decreases the fatigue resistance. There is a relationship between the crack initiation and the tube total tearing-off: initiation appears at 40% of the total rupture not depending on the temperature.
The EBSD analyses show that a higher misorientation between grains on either side of the crack is observed. This misorientation is higher than that of adjacent grain along one crack side. The higher the misorientation between two grains, the higher the probability of the crack propagation.
SEM showed three areas: stable propagation characterised by fatigue bands, mixed area composed of microvoids and fatigue bands perpendicular to propagation direction and final brutal rupture with necking and microvoids.
KEYWORDS – Aluminium, Heat-exchanger, Fatigue, Wöhler curves, EBSD