together. This can elevate up- front costs, as tools must be built for each section of the welded assembly. Part consolidation through metalcasting eliminates the need for multiple tools, so a one-piece magnesium casting can replace a steel design at a reduced startup cost. In the case of the liftgate inner panel, a six-piece weldment was consolidated into one piece while also achieving a 21.2-lb. weight reduction, not only because magnesium is lighter than steel, but because the casting process allowed the designers to produce varying wall thicknesses and place the metal only where it was needed. The metalcasting process also al-
lowed the part to include a number of cast-in features that previously had to be added to the part in post- processing, such as attachment points for several other components included in the liftgate assembly. “You need to have a structural component that can absorb crash
The single piece magnesium liftgate inner panel is the first diecast magnesium closure ever to satisfy 55 mph rear crash requirements.
requirements, with a variety of other components hemmed to it,” Moyer said. “You need that base.” The final casting, which weighs
17.6 lbs. and is 54.3 x 51.8 in., in- cludes six structural reinforcements cast-in that would have had to be welded onto steel. The final benefit of the casting process
was that the component took up less space than its steel predecessors. “The process allowed space for
more components, increased head room [in the vehicle] and provided another window opening,” Berk- mortel said. “With magnesium, we
were able to push thinner walls than with other cast- ing [alloys], with less draft, and at a greater strength- to-weight ratio in a smaller volume space. With mag- nesium, you can achieve the same strength with a lot less material.”
Designer and Designee Producing a component
that constituted so many firsts
in the automotive industry was no easy task. With such a large, thin casting, the design team was concerned with the endproduct’s structural integrity, stiffness and di- mensional tolerances. But according to Moyer and Berk-
mortel, the group was able to ac- complish all of its goals by working together from early on in the process of product design. “Once their initial business case
was defined for this type of vehicle with this type of volume, that allowed us to come to them with a proposal that said, here is a way to get there,”
32 MetAl CAsting Design AnD PurChAsing
July/August 2010
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