Residual stress and damage development in the aluminium alloy EN AW-6061 particle reinforced with Al2O3 under thermal fatigue loading

Abstract Thermal fatigue (TF) tests were performed on the aluminium alloy EN AW-6061-T6, non-reinforced and reinforced with 15 and 22 vol.% Al 2 O 3 particles, respectively. The MMCs were produced by stir casting followed by hot extrusion. Thermal fatigue tests were performed using a 6 kW diode laser with a beam area of about 8 mm × 8 mm focused on the centre of one flat side of disc shaped specimens. The reverse side was either attached to a water-cooled aluminium plate or directly cooled by water. The maximum temperature T max of the irradiated side was varied between 573 and 773 K. The heating rate was 50 K/s. Residual stresses in the matrix alloy were measured by X-ray diffraction using the sin 2 ψ -method after T6 heat treatment and after defined temperature cycles. Initial residual compressive stresses between −20 and −65 MPa result from the machining processes before T6 heat treatment. During the first temperature cycle the residual stresses in all materials change to tension at almost all T max . The peak value of the residual stresses reaches 50 up to 65 MPa and is nearly independent from T max . Damage evolution was observed by light optical microscope and SEM after the same cycles as the residual stress measurements. Grain boundary reliefs arise and increase in all materials with increasing number of TF cycles and intergranular damage of EN AW-6061 and the matrix alloy of the MMCs is observed. Close to the particles, damage is more pronounced due to thermal and mechanical mismatch of the phases. Four mechanisms causing damage and residual stress development could be identified: thermally induced global deformation due to inhomogeneous distribution of temperature, thermally induced local deformation due to coefficient of thermal expansion (CTE) mismatch (different α th of both phases), mechanically induced local deformation due to different deformation behaviour of both phases and overaging. In the non-reinforced alloy global deformation is the dominant mechanism while in the MMCs also local mechanisms are significant.