Automotive Engineering - December 2022 - 20

FUTURE-TECH
FLAGSHIP
Celestiq helped
kick GM's additive
manufacturing
operation into
overdrive. Shown
is the 3D printed
aluminum steering
wheel aperture
in unfinished and
finished form.
supplier is renowned for its innovative engineering and success in
balancing high-volume programs with niche and performance customers.
Multimatic famously handled the complete build of the 2018
Ford GT. Roma praised his Ford contacts for " graciously getting us in
the door there. "
Casting a new architecture
Celestiq exemplifies how engineering helped deliver a design breakthrough:
achieving a sleek, low-roof body with luxury seating for
four, with the Ultium battery pack and electric propulsion system underneath.
The learnings will be useful for GM's future EV programs.
" In a lot of luxury cars, you have to sit in the back seat to get the best
seat in the house, " Loveday observed. " We wanted all four of Celestiq's
seats to be top-notch - 20-way adjustable with heat, vent, cool, massage
and memory in all four places. But we recognized the challenge of
getting the body structure to 'mold' around the battery and occupants. "
Roma recalls a question he and others asked in the first team
meeting: Can we use the battery pack from the Lyriq? " It's flat,
space-efficient and power dense, but it's also fairly tall, " he said. " We
realized to meet the design vision, the Lyric battery wasn't going to
work. " Even with Celestiq's Ultium cells oriented horizontally in the
pack, the solution involved a vehicle structure using precision megacastings
as a foundational element, enabling an appropriate rear
chair height to accomodate the super-low roofline.
" The megacastings really lent the engineers and the artists a common
ground. They were a natural fit, " Loveday said. " We use
6000-series aluminum for mass efficiency, put the strength where we
need it. It proved to be very investment-friendly for such a low volume
car, but also very package-friendly. "
Roma explained that GM's megacastings approach is different from
that of Tesla (see https://www.sae.org/news/2020/06/tesla-model-ybig-castings).
Mainstream media have failed to accept that GM isn't
" following Tesla. " It's been moving methodically to large structural
castings; the 2017 Cadillac CT6 body structure used 13 Magna20
December 2022
produced castings, for example. But where die casting
is economically feasible for high volumes, Celestiq production
uses a gravity-sandcast process. " It delivers
die-cast material properties from the aluminum alloys
used, the way the molten metal is introduced into the
mold, and from the heat-treat process, " he said.
Celestiq's entire lower structure combines six of these
precision sand castings, as shown in the accompanying
illustration. The front and rear structures are connected
longitudinally with two 8-ft-long (2.4-m) floorpan castings
that are spot-welded together then machined.
According to the engineers, each casting reduces
the bill of material by 30 to 40 components, compared
to typical fabricated construction. Improved package
efficiency, rigidity and reduced cost are other benefits.
" We've saved conservatively $100 million between
manufacturing and traditional vendor tooling with our
entire lower structure, " Roma noted. " This was our
'epiphany' moment for making our business case. "
The megacastings strategy has another upside: dimensional
stability. " We got incredible stability metrics
in our very first prototype body, " Roma reported, " data
that's as good as what we'd normally find exiting the
prototype phase and entering the production phase.
And that was the first body that we made! It's only gotten
better from there, and that's blown us all away. "
Observed Sandy Munro, CEO of competitive analysis
firm Munro & Associates: " The future is megacastings.
We've done the math. lnvestment cost compared with
fabrications is less. Bill of material is less. Rigidity is
higher. Assembly is improved. Anyone not moving in
this direction needs to find new consultants or talk to
their internal financial people. "
While GM has been a leader in mixed-materials architectures,
Celestiq body-in-white is 99% aluminum,
AUTOMOTIVE ENGINEERING
CADILLAC
https://www.sae.org/news/2020/06/tesla-model-y-big-castings https://www.sae.org/news/2020/06/tesla-model-y-big-castings
Automotive Engineering - December 2022

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