Comportement en fatigue de l'aluminium 357 coule par gravite et rheocoule

The primary objective of this work is to explain the fatigue strength difference between a permanent mold and a rheomolded material on the basis of the microstructural differences. The experimental work has been carried out using aluminum 357 alloy which was produced in six specimens characterized by different microstructures. This alloy was chosen because it is frequently used in the industry to produce automotive and aerospace components and because it is easy to rheomold. Also, this alloy has been widely used to develop the SEED technology. First, the microstructural characteristics and mechanical properties of the materials were quantified. This confirmed that the semi-solid molding (SSM) materials have a globular microstructure. The results also showed an increase of the yield strength attributed to rheomolding. However, among the four SSM materials, only those for which the eutectic structure was modified had higher elongation at fracture than the permanent mold materials. The highest measured elongation at fracture is 19% obtained for the modified SSM material, tested in the as-cast condition. The other materials tested all have an elongation at fracture below 10%. For both shaping process, results show that precipitation hardening is an efficient method of increasing the alloy yield strength. However, blisters formed while solutionizing the SSM materials which are detrimental to the metallurgical quality of the components. Consequently, the SSM materials were mostly tested in a T5 condition.;The short cracks behavior is comparable in the PM and SSM microstructures when observed from da/dN-DeltaK graphs. The most significant difference between the propagation of the short cracks in the materials is revealed on graphs of the propagation rate versus the crack depth. This difference is the average depth of a crack at its first deceleration. In the PM specimens the cracks grow deeper before the first deceleration than in the SSM specimens. It was also observed that the crack depth at the first deceleration is comparable to the average grain half diameter which are 155 mum and 57mum for the PM and SSM materials respectively. The conclusion proposed from these observations is that the fatigue strength at 107 cycles is controlled by the nature of the microstructural barrier, DeltaKmb, and its position, d. This conclusion can be expressed by a simplified model where the propagation threshold of the short cracks is given by DeltaKmb. The proposed relationship shows that the stress amplitude at 107 cycles is proportional to 1/D , where D is the average grain size. The rheomolded materials studied in this project are therefore more resistant to fatigue than the permanent mold materials because of their smaller grain size.;In the discussion, the relationship between the fatigue strength and the grain size is presented in a Kitagawa diagram. This clearly shows that grain refinement can only be efficient if the defect size is smaller than D/2. This corresponds to defect diameters of 155mum and 57mum for the PM and SSM materials respectively. Larger defects will affect the fatigue strength resistance of the materials. The most significant practical conclusion of this work is that rheomolding using the SEED process increases the fatigue strength of aluminum 357 alloy by simultaneously decreasing the grain size and the defect size. (Abstract shortened by UMI.);The second portion of the work consisted in the production of S-N curves to study the fatigue strength differences between these materials and to identify the microstructures that are more resistant to high cycle fatigue. The results obtained at R = -1 showed that the SSM materials have a higher resistance to fatigue than the PM materials. Precipitation hardening was found to be less efficient in increasing the fatigue strength when compared to the materials yield strength. The fatigue strength of the SSM materials was essentially the same in the T5 and in the T6 temper. The most fatigue resistant materials were found to be three SSM specimens hardened by precipitation with an average fatigue strength of 113 MPa at 10 7 cycles. One of them is unmodified with a T5 temper, the second one is modified with a T5 temper and the third one is modified with a T6 temper. These results demonstrate that it is not necessary to modify the eutectic structure or to use a solutionizing step to obtain an improvement in the fatigue strength for the SSM materials.