Bolt-Load Retention Behavior of a Die Cast Magnesium-Rare Earth Alloy

The need for improved understanding of new magnesium alloys for the automotive industry continues to grow as the application for these lightweight alloys expands to more demanding environments, particularly in drivetrain components. Their use at elevated temperatures, such as in transmission cases, presents a challenge because magnesium alloys generally have lower creep resistance than aluminum alloys currently employed for such applications. In this study, a new die cast magnesium alloy, MEZ, containing rare earth (RE) elements and zinc as principal alloying constituents, was examined for its bolt-load retention (BLR) properties. Preloads varied from 14 to 28 kN and test temperatures ranged from 125 to 175°C. At all test temperatures and preloads, MEZ retained the greatest fraction of the initial imposed preload when compared to the magnesium alloys AZ91D, AE42, AM50, and the AM50+Ca series alloys. The BLR behavior of MEZ did not show significant sensitivity to temperature within the range examined, whereas the other alloys displayed a clear decrease in bolt-load retention with increased temperature at a given preload. Retained bolt-load decreased for MEZ with increasing preload in a manner similar to the behavior of other alloys. The higher BLR can be attributed to the greater resistance to creep and arises mainly from the Mg-RE phases present at cell and grain boundaries and the relatively high solidus temperature (T_s) of MEZ. Additional means of improving BLR by varying geometrical dimensions in the bolted assembly for AZ91D and AM50 was investigated and no significant improvement were observed in the limited studies that were performed.