The Case for Aluminum in Body-in-White Design, by Dr. Alex Graf, Constellium

1. LIGHT METAL AGE, OCTOBER 201538
A
s the design cycle for the 2025 model year ve-
hicles approaches, OEMs are making technol-
ogy choices to ready their models to meet the
fuel economy and emission targets set by vari-
ous regulatory entities. One of the fuel economy drivers
is the mass of the vehicle, so OEMs consider lightweight
technologies as one approach to reduce fuel consump-
tion. In North America, the fuel economy targets set by
the U.S. National Highway Traffic Safety Administration
are based on the footprint of the vehicle (wheel base x
average track). The standard was written to discourage
vehicle downsizing1
and to address the safety concerns
attributed to smaller vehicles.2
To achieve the 2025 fuel economy target of 50+ mpg,
many technologies will have to be adopted. However, it
has been demonstrated that no cost-efficient technology
can achieve the proposed 2025 target without significant
weight reduction and that weight reduction is a long-term
sustainable technology platform for other advanced fuel
economy improvement technologies.3
Therefore, car
makers are directing weight reduction efforts to more ef-
fective designs, including more efficient joining methods,
lightweight materials, and the resulting powertrain down-
sizing and secondary weight savings afforded.
Regarding lightweight materials, there has never been
a better time for aluminum in the automotive industry.
The combination of high strength-to-weight and stiffness-
to-weight ratios and wide availability in multiple wrought
and cast product forms, is proving to be a game-changer
in car design to achieve the desired fuel economy (and
CO2
emissions) targets. Today, both premium and mass-
market car manufacturers have aluminum as an integral
part of their design cycle to achieve those targets. Many
OEMs are no longer limiting aluminum to hang-on and
bolt-on parts, but are incorporating it into the structure of
the vehicle, from a few choice pieces (roof or front end) to
hybrid structures,4
to full aluminum body-in-white (BIW),
as is the case of the 2015 Ford F-150 pickup truck.5
Designing with Aluminum
New developments in aluminum alloys are making
it possible to design parts and structures with higher
strength and energy absorption, and with increased man-
ufacturing process windows, while maintaining already
established corrosion resistance and surface quality attri-
butes. However, before addressing new developments, it
is important to establish how the intrinsic physical prop-
erties of aluminum are enablers for lightweight designs
when replacing mainstream steel. The issue can be sum-
marized as follows: aluminum density is about one third
of that of steel, but its elastic modulus is also one third, so
how can the use of aluminum result in weight savings in a
stiffness dominated structure like a car body?
The answer is quite simple: the effect of through-thick-
ness bending on stiffness. Figure 1 shows the calculated
elastic distortion, via finite element analysis (FEA), of a
car hood in cantilever loading (both hinges attached,
two supports at the bumper stops, loaded at the latch
point). The original steel design, with the original thick-
ness, resulted in a deflection of about 26 mm. The same
steel design up-gauged and down-gauged by a factor of
two resulted in deflections of 8 mm and 112 mm respec-
tively. Repeating the same calculations, but using alumi-
The Case for Aluminum in Body-in-White Design
By Dr. Alex Graf, Constellium
num elastic properties, results in the previous deflection
numbers increased by a factor of three. It is easy to ob-
serve that to match the elastic deflection of the original
steel hood, the aluminum design needs to be up-gauged
by a factor of two, thus resulting in a weight savings of
without any redesign. Further topology optimizations
of any structure increase the weight savings to the usual
range of 35-50%. Thus, to increase the weight savings af-
forded by the use of aluminum, the designer needs to
strive for the maximization of through-thickness bending
in structures. The thickness influence on performance is
also relevant for crash absorbing members. In these cases,
the energy absorbed at every localized bend that results
from the crash event grows exponentially with thickness.
Two of the main trends in aluminum car design, and
particularly in BIW structures, are the introduction of
extrusions in the structure of the vehicle and the migra-
tion toward higher strength alloys. The former allows the
designer to increase the overall stiffness of a particular
component by avoiding local hinge points and other lo-
calized deformation modes, since continuous joint lines
and internal walls to provide added stiffness are intrinsic
characteristics of multi-chamber extrusions. The latter is
the logical consequence of having locations in the struc-
ture of a car that are not stiffness constrained but heavily
stressed, either during continuous service or event load-
ing, which can benefit from higher strength materials for
added weight reduction.
Component Design with Extrusions vs. Sheet
Traditionally most aluminum applications in a vehicle
have been sheet stampings, but with the higher penetra-
tion of aluminum in structures, a number of parts are
being re-evaluated in the light of advances in extrusion
materials and fabrication methods. Whether a structural
member should be designed as a sheet stamping or a fab-
ricated extrusion will depend on the balance of several
design considerations (Table I).
From a deeper analysis of Table I, it can be inferred
that sheet and extrusion components are complemen-
tary, each one bringing its own strengths and weaknesses.
For example, the use of extrusions for straight and con-
Figure 1. Effect of material and thickness multiplier on the cantilever
bending distortion of a car hood.
1 −
2.7
7.8
∗ 2 = 31%
Reprinted with permission for Constellium, ©2015 Light Metal Age

2. LIGHT METAL AGE, OCTOBER 2015 39
stant cross section parts like the lower sill (rocker) seems
the evident choice, since its linear aspect coupled with
the need of complex cross sections for the part are all at-
tributes of extruded products. On the other side, center
tunnel sections or floor pans fall squarely in the sheet
stamping column, as they require a variable section along
the part and a number of added fasteners.
Several of the parts in the aluminum-intensive 2015
Ford F-150 are formed from extrusions, where the bound-
aries of design and manufacturing have been extended
to incorporate the attributes of extrusions into what have
historically been sheet components. This transformation
resulted in the consolidation of several traditionally steel
stampings into single aluminum parts,6
such as the wind-
shield header, roof bow, and rocker, with exceptional
stiffness and strength.
High Strength Aluminum Sheet vs. Steel
As mentioned, there are areas in the structure of a ve-
hicle that are strength rather than stiffness dominated.
In those cases, increasing the material strength will result
in weight savings, as it will allow a thinner component to
handle the same load. Although aluminum has a high
strength-to-density ratio, the parts normally produced with
ultra-high-strength steels in current steel structures do not
find a corresponding aluminum offering in the current
market. As indicated in Figure 2, alloys with strength in
excess of 450 MPa are needed to cover that range.
To close that gap, Constellium is working to develop
high strength alloys of the 7xxx series, specifically de-
signed for stampings that are currently produced via the
hot forming process. Figure 3 is a schematic representa-
tion of the production processes involved, from the sheet
mill to the OEM, to produce and assemble hot formed
parts. Logically, since 7xxx series alloys are heat treatable,
the high temperature processes are key to successfully
manufacturing the finished part.
This process yields parts of very high strength, excellent
ductility, and robust corrosion resistance. The mechani-
cal properties were measured from actual parts, such as
B-pillar stampings (Figure 4), produced at Constellium’s
Technical Center. The strength-to-density ratio resulting
from the combination of alloy and stamping processes, is
equivalent to those of hot stamped steels. This develop-
ment should result in significant thickness reduction in
high stress areas of a car body, achieving not only weight
reduction but also packaging improvements and overall
section optimization.
References
1. “Corporate Average Fuel Economy for MY 2017-MY
2025 Passenger Cars and Light Trucks,” NHTSA, August
2012, p. 1,014, www.nhtsa.gov/staticfiles/rulemaking/
pdf/cafe/FRIA_2017-2025.pdf
2. Lund, A., “The Relative Safety of Large and Small
Passenger Vehicles,” NHTSA Workshop on Mass-Size-
Safety Symposium, Washington, DC, February 2011.
3. “VENZA Aluminum BIW Concept Study,” Sce-
naria, Inc. for the Aluminum Association, 2012,
www.drivealuminum.org/research-resources/PDF/
Research/2013/venza-biw-full-study.
4. Peckham, R., “Reviewing Challenges and Identifying
Opportunities with Integration of Lightweight Materi-
als into High Volume Automotive Production – an OEM
Perspective,” Global Automotive Lightweight Materials
conference, Detroit, MI, 2015.
5. Pope, Byron, “Ford No Stranger to Aluminum,”
WardsAuto, March 19, 2015.
6. “Aluminum Extrusions and the 2015 Ford F-150,
Chief Engineer Dr. Bruno Barthelemy” (video), The
Aluminum Channel, April 2015, www.thealuminumchan
nel.com/videos/view/aluminum-extrusions-and-the-
2015-ford-f-150.
Raw Material Form Extrusion Sheet
Cross Sections
Easy to implement
closed sections
Open, close sections
need subassembly
Cross Section Type
Complex, multiple
chambers
Multi-piece sections
necessary
Longitudinal Section Constant along part Variable along part
Component
Thickness
Variable Constant
Features and
Fasteners
Additional cost Easy to implement
Cycle Time 2-10 spm 5-25 spm
Table I. Decision making factors when considering raw material forms
for aluminum components.
Figure 2. Strength and ductility of aluminum alloys compared with
equivalent strength-to-weight ratio in steel.
Sheet manufacturing process
Part manufacturing process OEM assembly
Process
Temperature
Hot rolling Cold Rolling
Blanking
Blanking
Solution Heat
Treatment
Stamping and
press-quenching
Ageing
~500 °C ~180 °C
Assembly Paint-bake
Figure 3. Schematic of the manufacturing route for hot formed parts.
Figure 4. B-pillar trial parts produced at Constellium’s Technical Center.
Dr. Alex Graf is director of Engineering at Constellium, a world-
wide manufacturer of aluminum products that utilizes cutting-
edge technologies to supply innovative aluminum material and
components required by today’s car manufacturers.