Automotive Engineering - April 2021 - 10

TECHNOLOGY REPORT

The majority
of long-range
EVs in current
production
worldwide use
aluminum as the
main material
for the battery
enclosure.

" Soon, it may no longer be
economically beneficial to use
aluminum, especially for the
small cars that have moderate
range and target the lowestpossible price point. "

or if a vehicle catches fire. " Aluminum
has very high thermal conductivity and
the melting point is 630°C, " Afseth said.
" A battery fire can reach 1200°C or more
and the aluminum casing will last only a
short time before the metal melts. So,
for the top cover either a heavier steel
sheet or a fire-retardant loaded polymer
molding will resist longer and give the
passengers more time to evacuate. "
Heat generated by the battery cells
also can be a concern for aluminum enclosures, especially for parts that are in
direct contact with the cells or other parts
of the high-voltage system that gets
heated during charging or discharging.
" My main concern would be with alloys
like 5182, which has more than 3.5 wt%
Mg, as these may over time develop a
film of beta-phase precipitates at the
grain boundaries which can result in degraded properties, " Afseth explained.
For parts of the enclosure that are
away from the cells, such as the bottom
10 April 2021

plate located below the cooling plate,
heat is not a concern. Afseth said he
does not see any issues regarding immersion cooling: " Aluminum alloys of
the 3000, 5000 and 6000 series are
very well compatible and completely
resistant to common coolant liquids. "

Move to multi-materials

Justification for the over-cost of aluminum structures is found in the secondary mass and cost savings tied to being
able to downsize the battery and the
powertrain, Afseth stressed. But, as battery costs continue to drop, the value
equation for aluminum may dissipate.
In the past decade, battery cost has
fallen by almost a factor of ten, he noted, from about $1,000 kWh in 2010 to
less than $150 kWh last year. Energy
density has almost tripled over the
same period, so batteries also weigh
much less than before.
" Why we see all the long-range EVs

today using aluminum is because at the
time they were making their engineering
and material choices, this equation was
super simple: You spend a few hundred
dollars more on the body structure and
you save thousands of dollars on downsizing the battery, " Afseth explained. " Today,
at the current prices, it's still strongly in
favor of aluminum designs, especially for
the larger vehicles like SUVs and trucks
that target long range. But soon, it may no
longer be economically beneficial to use
aluminum, especially for the small cars
that have moderate range and target the
lowest possible price point. "
Afseth said he sees a transition to
more mixed materials for battery enclosures in the coming years - but " very
little " carbon fiber, which makes more
sense in motorsports or ultra-luxury
sports cars where cost is not an issue.
" Other, cheaper fiber-reinforced plastics
may grow more, " he added.
Ryan Gehm

AUTOMOTIVE ENGINEERING

BOTH IMAGES: CONSTELLIUM

A dual-frame prototype illustrated by Constellium employs two different advanced, extruded
6000-series alloys.

- Dr. Andreas Afseth,
technical director for
Constellium North America
operations

Automotive Engineering - April 2021

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