Automotive Mg & Al: Curb weight, $ and CO2

Economic and Environmental Issues
in Automotive Magnesium
Applications

Carlos H. Cáceres
School of Engineering
The University of Queensland
Brisbane, Qld. Australia

Invited Lecture to ICAA-10 Vancouver, October 2006
Metall. Mater. Mater. Trans. A38,(2007) 1649-1662
Metall. Trans. A 38, (2007) 1649-1662. 1/42

What drives the current interest in Al and
Mg automotive applications?

Metall. Mater. Trans. A 38, (2007) 1649-1662 2/42

l/100 km

Gasoline
vehicles

Diesel
vehicles

Weight
Using Al and Mg lightens up the car and
cuts gasoline consumption and emissions.
http://www.innovaltec.com/iom3_dt/scamans_cans_to_lowco2_cars.pdf

Issues Regarding Automotive Applications

• What is the cost penalty of using Al and Mg in
cars?
• Is the cost penalty related to the mechanical
function?
• Is the gasoline saved by a lighter car enough to
off-set the cost penalty of using light alloys?
• Does the gasoline saved by a lighter car off-set
the environmental burden of producing Al and
Mg?

Light Alloy substitutions for
Cast Iron and Steel in cars

• Same volume (Castings): Engine blocks, Valve covers
• Stiff Beams (bending): Steering wheels, Space frames
• Stiff Panels (bending) : Instrument panels, Door panels


Approach

Material Indices and Exchange
Constants
MF Ashby et al. 1992, 1996, 1999, 2003


Material Indices for Minimum Mass

Function Index
Same Volume
1/ρ
minimise mass (castings)

E1/2/ρ
Beam (bending)
minimise mass for given
Panel (bending)
stiffness E1/3 / ρ

Minimise Select a material that maximises
mass? this Material Index !

How good are Al and Mg when it comes
to reducing mass?
A 10 kgf component made of Steel…
ρ Equal
Beam Panel
E
Volume
(GPa) (Mg/m3) E1 / 2 E1 / 3 1
ρ ρ ρ

Steel 210 7.8 10 10 10

Al 75 2.7 5.9 4.9 3.5

Mg 44 1.7 5.1 3.9 2.2


Material Indices for Minimum Cost?
Material Indices for Minimum Mass

cost [c ] = $/kg

Function Index
Same Volume
minimise cost per unit vol
1/c ρ
(castings)

E1/2/c ρ
Beam (bending)
minimise cost for given stiffness
E1/3/c ρ
Panel (bending)

Use a material that maximises
Want to minimise
Metall. Mater. Trans. A 38, (2007) Material Index !
this 1649-1662
cost? 9/42

Designing with conflicting goals

cheap car (Iron and Steel: cheap but heavy)
Conflicting
light car (Al, Mg: light but expensive)
Goals

Use trade-off plot
(Ashby)


Plot all viable materials according to
Trade-off plot their material indices

expensive
ρc/E 1/2
Cost
for given
What is the meaning of α ?
stiffness

Materials substitution:
join candidates by a Mg
Z1
linear function
Fe
P2
(cost)
B

A
Slope α = exchange constant
α heavier
α is the cost penalty of substituting P1 (mass)
material B for A ($/kg) Mass
1/2
ρ/E for given
stiffness
Paretto, 1906; Ashby, 2005

Trade-off plot on log scales

Log scales
Linear scales

A
A

P2
P2
(cost)
(cost) B
B

P1 (mass)
P1 (mass)


How much does it cost to put Mg or Al in a car ?

•Cast Iron: about $0.5 US/kg
•Steel: about $0.8 US/kg
•Aluminium: about $2.5 US/kg
•Magnesium: about $3.4 US/kg


http://www.fuelgaugereport.com/
How much does it cost to drive a car ?
U.S. Gasoline Fuel Price, September 2005
1 gallon = 3.78 lg => @ 3.1 $ per gallon => 0.8 $ per liter
$US per
gallon

2003

month-year


Curb weight and fuel economy

How much gasoline can we save per kg of mass reduction?

1 kg mass reduction, over 200 000 km vehicle life
Johnson, 2002 ; IPAI, 2000

litres of
gasoline

Driver’s (10 years) savings = 7 lg/kg = 6 $/kg


What are the incentives for substituting
Al or Mg for steel in automobiles?

If a manufacturer does not meet the Corporate
Average Fuel Economy [C.A.F.E.] standard, it is
liable for a civil penalty of $5 for each 0.1 mpg (40
m/l) its fleet falls below the standard of 22.2 mpg
(9.4 km/l) (as of 2007).

CAFE Penalty: 0.5 $/kg


When is a material substitution worth doing?

If the substitution costs
you more than this, it is
not worth doing

Manufacturer's upper bound: 0.5 $/kg (CAFE penalty)
Driver’s upper bound: 7 lg/kg = 6 $/kg (Driver's savings)


Possible substitutions (1)
•Incumbent materials: Cast Iron and Steel
Replaced by
•Aluminium alloys
•Magnesium alloys
Possible substitutions (2)
•Incumbent materials: Aluminium alloys
•Replaced by Magnesium alloys


The cost penalty of Al or Mg Beams substitutions for steel
Material
ρ/E1/2 (Mg m-3 GPa-1/2)
Cost This is what a lighter
0.1 1
for given 100 vehicle will save you
stiffness (upper bounds)
CAFE:
α < 0.5 $/kg
ρc/E1/2 10
Driver's savings
α < 6 $/kg
cost relative to steel beams

α AlMg = 9.9 $/kg

ρc/E1/2 (103 $ m-3 GPa-1/2)
10
αAM = 7.6
α FeMg = 2.4 $/kg
αFM = 2.4 AZ91

α FeAl= 1.2 $/kg 1
A356
αFA = 1.2 Steel Cast Fe

1
This is what it costs you to
lighten up your vehicle ($/kg)
0.1
Mass
for given
beams
0.1 stiffness
2
0.1 1

ρ/E1/2
mass relative to steel beams

Cost of Mg and Al Castings substitutions for cast iron
ρ (Mg m-3)
1 10
100

Cost
per m3
100
cost relative to cast iron

10

ρc (103 $ m-3)
α FeMg= 0.7 $/kg
αFM = 0.7 10
AZ91 Steel
α FeAl= 0.6 $/kg A356
Cast Fe
αFA = 0.6
1

1
αAM = 1.3
volume
0.1
density
2 20/42
Metall. Mater. Trans. A 38, (2007) 1649-1662
0.1 1
mass relative to cast iron

Cost analysis Substitutions below this
line are OK for Driver's
Al=>Mg
Driver's savings
savings
10 Beam Al-Mg
6 $/kg
lifespan savings (6$/kg)

Panel Al-Mg
Fe=>Mg
α ($/kg)
Beam Fe-Mg
($/kg) Fe=>Al
α

Cast Al-Mg
Beam Fe-Al
Panel Fe-Mg
1

Cast Fe-Mg
CAFE penalty (0.5$/kg)
Panel Fe-Al
Cast Fe-Al

CAFE 0.3
penalty
Substitutions below this
0.5 $/kg
line are OK for CAFE

How about environmental
(greenhouse gas, CO2)
effects?


Define: h = CO2 footprint: kg of CO2 per kg of alloy

material h (kg of CO2 / kg)

Iron/Steel 1 ~ 2 kg/kg
Al ~12 kg/kg
(45% hydro electricity, 55% fossil world
avge.)

Electrolytic Mg (30% ~23 kg/kg
of world production) (45% hydro electricity, 55% fossil world
avge.)

Pidgeon Mg (70% of
~42 kg/kg
world production)
Sources: IPAI (2000); Koltun et al. 2005; CES, 2006

Material Indices to minimise CO2 creation?

CO2 footprint equivalent [hq ] = lg/kg

Function Index
minimise CO2 per unit vol 1/ ρ hq
Same Volume
(castings)

E1/2/ρhq
minimise CO2 footprint for Beam (bending)

given stiffness E1/3/ρ hq
Panel (bending)

Maximise
these !

Gasoline equivalent to the CO2 footprint ?

Cars create ~ 2.85 kg of CO2 per litre of gasoline

hq = (equivalent) litres of gasoline
burnt producing 1 kg of alloy

Define: hq = (h / 2.85) lg/kg

β = exchange constants involving CO2

Gasoline equivalent to the CO2 footprint ?
A lighter vehicle saves 7 lg/kg over 200x103 km

material hq
Iron/Steel ~ 0.5 lg/kg
Al ~ 4 lg/kg
Electrolytic Mg ~ 8 lg/kg
Pidgeon Mg ~ 15 lg/kg

Sources: IPAI (2000); Koltun et al. 2005; CES, 2006

CO2 creation: exchange constants for Same
Volume substitutions (castings)
hqρ
CO2
This is what a lighter vehicle
footprint
saves (per kg) over 2x105 km
Driver's savings
Pidgeon Mg
β = 7 lg/kg
Electrolytic Mg

Al

gasoline burnt producing
the materials to achieve
one kg of mass reduction mass
ρ 27/42

Electrolytic Mg substitutions for
CO2 creation analysis Al are viable for castings.
Pidgeon Mg is out of bounds.

Al=>Mg
Pidgeon Mg’s
Substitutions below CO2-footprint is
the line are OK excessive for
beams and Beam*
Panel*
panels, OK for
30
castings
Beam

Cast*
Beam*
Driver's Panel
10
savings
7 lg/kg βCO2
Panel*
Beam
(7 lg /kg)

Cast*
Beam Panel
Cast
(lg/kg)

Fe=>Mg*
Panel
Cast
β

Cast
Fe=>Mg
Al substitutions
for Fe are OK
1
Fe<Al Fe<Mg Fe<Mg* Al<Mg*
Al<Mg
Fe=>Al
CAFE liability Electrolytic Mg substitutions
for Fe: castings & panels
(0.6 lg /kg)
OK beams are off

Simultaneous selection by Cost and CO2 footprint
Distance to
break-even distance, dα (x103 km)
break-even
10 100
(x103 km)
β (lg/kg)
Aluminum replaced
by Pidgeon Mg
1000

break-even distance, dβ (x103 km)
Iron and steel
replaced by
Pidgeon Mg 200x103 km
β (lg/kg)

Iron and steel
replaced by Mg

10
βCO2
Iron and steel
replaced by Al
(7 lg /kg) Aluminum
replaced by Mg

100
Driver's
savings
(7lg/kg)
Substitutions inside
the box are OK
α ($/kg)
30
1
CAFE liability
(0.5$/kg) 1 10
αCAFE (0.5 substitutions are economically not viable. αS (6 $/kg)
α ($/kg)
•Only two $/kg)
Driver's
Metall. 14 substitutions are environmentally1649-1662
• 8 out of Mater. Trans. A 38, (2007) not viable 29/42
savings (6
(primary alloys).
$/kg)

Analysis so far assumed primary alloys

Effect of recycling ?

• Recycling Al or Mg uses only about 5%
of the energy required to produce
primary metal.

• The exchange constants decrease in
proportion to the recycled fraction.


Effect of recycling (post-consumers scrap)

• Al and Mg wrought alloys are nearly 100% refined
metal. (Al: up to 8% is recycled metal)

• Al castings: as much as 60% is recycled metal.

• Diecast Mg : up to 20~ 35% is recycled metal.


Effect of recycling on the driving distances
to break even?


Driving distances to break even
A cast Fe engine replaced by an Al or Mg engine
Primary electrolytic Mg An engine block cast on a
=> 70x103 km primary Al alloy (A356)
Primary Pidgeon Mg requires 55x103 km to
break-even
=> 130x103 km

Cast on alloy A319 (60%
recycled) cuts the driving
With 35% recycled Mg:
Electrolytic Mg => 35x103 km ~10x103 km
distance to
Pidgeon Mg => 75x103 km


Driving distance to break even for Al
or Mg space frames replacing steel
An extruded Al beam
A rolled Al panel requires
requires 130x103 km
75x103 km to break-even
to break-even

An extruded
electrolytic Mg beam
A rolled electrolytic Mg panel requires 210x103 km
requires 125x103 km to to break-even
break-even

Wrought alloys are made of primary stock (Al: ~8%
max old scrap), little benefit from recycling.

Magnesium steering wheel replacing a
steel steering wheel
At 35% recycling rate
electrolytic Mg requires
A steering wheel of primary
~130x103 km electrolytic Mg alloy requires
210x103 km to break-even

At 35% recycling rate A steering wheel of primary
Pidgeon Mg requires Pidgeon Mg alloy requires
~260x103 km 390x103 km to break-even

Tzabari and Reich, 2000

Al engine replaced by a Mg engine?
Primary electrolytic Mg
=> 323x103 km
Al engine block cast on
Primary Pidgeon Mg
alloy A319 (60% recycled)
=> 724x103 km

Mg is not a good
replacement for existing
With 35% recycled metal:
cast Al components
electrolytic Mg => 160x103 km
Pidgeon Mg => 430x103 km


Are current cars any lighter than back in 1970?
Light trucks & USV

cars


Conclusions

• Cost and environmental penalties of light
alloy applications strongly depend on the
mechanical function.
• Penalty in decreasing order: castings, panels,
beams.
• The cost penalty can be off-set by the savings
of gasoline in most cases.

CO2 - creation

• The high recyclability of Al casting alloys
gives them a leading edge over both Al and
Mg wrought alloys and Mg casting alloys.

• Pidgeon Mg is environmentally unsuitable
for most automotive applications.


CO2-footprint according to the source of energy
(Al and electrolytic Mg)

Energy 100% 55% fossil fuels 100% fossil
source hydro 45% hydro/ fuels
/nuclear nuclear
Al 6.2 12 16.7
Mg
7.5 20 30
(no SF6)
Mg
10.6 23 33.1
(with
SF6)
present analysis

Magnesium’s safest bet?
• E.U. imposing a tax on CO2 footprint should
mark the end of Pidgeon Mg.

• Increased use of Hydro (or Nuclear Power)
electricity to reduce Mg’s (and Al’s) CO2
footprint.

• Increasing the recycling rate of Mg castings.


The End


Extra slides:


Titanium

β FeTi = 28 lg/kg


CO2-footprint according to the source of energy
Al and electrolytic Mg
present analysis: ~55%
100% fossil fuels
fossil fuels

Clean sources of energy are
100% hydro/nuclear/other
essential
for clean Mg or Al
renewables

Green: α-values of the order of the CAFE liability
Blue: within the savings over a Driver's of 2x105 km
Red: economically or environmentally not viable
Brackets: distance to break even
Metrics Gasoline equivalent footprint
Cost penalty
(lg/kg) primary alloys
($/kg)
αFA αFM αAM
Function βFA βFM β*FM βAM β*AM

Beam 1.2 2.2 9.9 4.6 7.4 13.6 25 76
(40) (73) (330) (132) (212) (389)* (715) (2200)
*
Panel 0.5 1.1 4.6 2.6 4.4 8.8 12 40
(17) (37) (153) (74) (126) (252)* (343) (1144)
*
Casting 0.4 0.6 1.3 1.9 2.3 4.7 3.8 18
(13) (20) (43) (54) (66) (134)* (109) (514)*

Critical Recycling Rates to make
the exchange constants = 0 ?
At these recycling rates Al and
Mg create the same amount of
CO2/kg as Fe

volume panels beams
Al 82% 69% 75%
Mg 87% 82% 87%
Mg* 95% 93% 96%

Pidgeon Mg
~95%! 47/42

Finding α: Ashby, 2005

Exchange Constants for Transport Systems

α ($US per kg)
Transport System: mass saving
Family car (based on C.A.F.E. penalty) ~0.5
Family car (based on Driver's savings) ~6
Truck (based on payload) 5 to 20
Civil aircraft (based on payload) 100 to 500
Space vehicle (based on payload) 3000 to 10000


Automotive Mg & Al: Curb weight, $ and CO2

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