Magnesium castings are experiencing increased use in the transportation industry because of their high strength to weight ratio, high stiffness, and excellent castability. The casting characteristics allow complicated shapes to be cast to close tolerances with high reproducibility. As the use of magnesium castings increases, the need for research on this alloy is becomes more prevalent. Indeed, little research has been performed on magnesium alloys compared to aluminum castings.
In the present study, the influence of the microstructure in a high pressure die casting AM60B magnesium auto component is investigated. Fatigue samples were removed from the component. Staircase fatigue testing was conducted first and the fracture surfaces were then examined with the scanning electron microscope.
It was found that the majority of fatigue cracks initiated from sub-surface voids underneath machined surfaces, not the cast skins. Machining a specimen out of a casting causes internal voids to become sub-surface voids, which in turn causes premature fracture. In the region which is close to the fracture initiation site, extremely flat areas covered with fine fatigue striations can be easily observed. However, numerous randomly oriented serrated surfaces, indicative of rough fatigue crack growth were found near the regions farther away from the initiation site. Finally, the final fracture zone is characterized by a high level of porosity and dimples surrounding second phase particles.