| 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 amount of research being performed on magnesium castings must also increase. Little research has been performed on magnesium alloys compared to aluminum castings, however, this has changed during recent years.;Most research performed on magnesium castings generally consists of tensile, fatigue, and bending tests. Before performing these tests the cast skin is usually removed because it may contain defects which may reduce the mechanical properties and lead to premature failure. The purpose of this investigation was to examine to fatigue properties of a die cast AM60B magnesium auto component. Specifically, to leave the cast skin intact and examine the effects of the cast skin on fatigue properties (i.e. do fatigue cracks initiate from defects in the cast skin?) Fatigue samples were removed from the component, machined edges polished and cast surfaces left untouched.;It was found that the majority of fatigue cracks initiated from sub-surface voids underneath machined surfaces, not the cast skins. The results show that a cast skin increases the tensile and fatigue properties. Two reasons for this are because there are fewer voids in the cast skin compared to the center and the skins may contain residual compressive stresses from the casting and cooling process. It was also determined that sub-surface voids cause a higher stress concentration than internal voids. Machining a specimen out of a casting causes internal voids to become sub-surface voids which causes premature fracture. |
