Secondary foundry alloy damage and particle fracture

The damage mechanisms of a hypoeutectic Al–Si–Cu foundry alloy, obtained through die casting, have been studied. Observations were made during tensile tests using a tensile test machine that was inserted into an scanning electron microscope. The microstructure features that are critical for damage were determined. The cleavage fracture of intermetallic particles was found to be the main process that limited the alloy ductility. Shrinkage cavities mainly play a role in the final fracture stage; the reduction in cross-section area induced by shrinkage cavities was assumed to be the main contribution of these defects to the fracture of the specimens. Attempts were made to determine the cleavage stress that initiated the fracture of the particles. Three-dimensional finite element computations were performed considering particular locations at the free surface of the observed fractured particles. The maximum principal stress was calculated for different locations of the particle with reference to the free surface. Different shapes and different elastic behaviour were studied. It was shown that the influence of a free surface on the stress level in a particle depends on its shape. The results were compared with those found in literature concerning the fracture strength of particles in Al alloys.