automotive mg & al: curb weight, $ and co2

Metall. Mater. Trans. A 38, (2007) 1649-1662 1/42

Carlos H. CáceresSchool of Engineering

The University of QueenslandBrisbane, Qld. Australia

Economic and Environmental Issues in Automotive Magnesium

Applications

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


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

Metall. Mater. Trans. A 38, (2007) 1649-1662 3/42http://www.innovaltec.com/iom3_dt/scamans_cans_to_lowco2_cars.pdf

Gasoline vehicles

Diesel vehicles

l/100 km

Weight Using Al and Mg lightens up the car and cuts gasoline consumption and emissions.


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


Material Indices and Exchange Constants

MF Ashby et al. 1992, 1996, 1999, 2003

Approach


Material Indices for Minimum Mass

Function Index

Same Volume (castings)

Beam (bending)

Panel (bending)/ρE1/2

ρE1/3 /

1/ρ

minimise mass for given stiffness

minimise mass

Minimise mass?

Select a material that maximises this Material Index !


How good are Al and Mg when it comes to reducing mass?

E(GPa)

ρ(Mg/m3)

Beam Panel Equal Volume

Steel 210 7.8 10 10

4.9

3.9

10

Al 75 2.7 5.9 3.5

Mg 44 1.7 5.1 2.2

ρ

3/1E

A 10 kgf component made of Steel…

ρ

2/1E

1ρ


Material Indices for Minimum Mass

Function Index Same Volume (castings)

Beam (bending)

Panel (bending)

Material Indices for Minimum Cost?

cost [c ] = $/kg

minimise cost per unit vol

ρ/c E1/2

ρE1/3 /c

1/cρ

minimise cost for given stiffness

Want to minimise cost?

Use a material that maximises this Material Index !


cheap car (Iron and Steel: cheap but heavy)

light car (Al, Mg: light but expensive)Conflicting

Goals

Designing with conflicting goals

Use trade-off plot(Ashby)


P1 (mass)

B

P2

(cost)

Trade-off plot

1/2/Eρ

Plot all viable materials according to their material indices

Costfor givenstiffness

Massfor givenstiffnessParetto, 1906; Ashby, 2005

1/2c/Eρ

Materials substitution: join candidates by a

linear functionZ1

αSlope α = exchange constant

α is the cost penalty of substituting material B for A ($/kg)

What is the meaning of α ?

A

heavier

expe

nsiv

e

Mg

Fe


Trade-off plot on log scales

Log scales

P1 (mass)

A

B

P2

(cost)

Linear scales

P1 (mass)

A

B

P2

(cost)


•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

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


How much does it cost to drive a car ? http://www.fuelgaugereport.com/U.S. Gasoline Fuel Price, September 2005

1 gallon = 3.78 lg => @ 3.1 $ per gallon => 0.8 $ per liter

2003

$US per gallon

month-year

http://www.fuelgaugereport.com/


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

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

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

Curb weight and fuel economy

litres of gasoline


CAFE Penalty: 0.5 $/kg

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).

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


Manufacturer's upper bound: 0.5 $/kg (CAFE penalty)

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

If the substitution costs you more than this, it is

not worth doing

When is a material substitution worth doing?


Possible substitutions (2)

•Incumbent materials: Aluminium alloys

•Replaced by Magnesium alloys

Possible substitutions (1)

•Incumbent materials: Cast Iron and Steel

Replaced by

•Aluminium alloys

•Magnesium alloys


0.1 1mass relative to steel beams

0.1

1

10

100

co

st re

lativ

e to

ste

el b

eam

s

AZ91

A356Steel Cast Fe

ρc/E

1/2

(103 $

m-3 G

Pa-1

/2)

0.1 1ρ/E1/2 (Mg m-3 GPa-1/2)

0.1

1

10

αAM = 7.6

αFA = 1.2

beams

αFM = 2.4

2

The cost penalty of Al or Mg Beams substitutions for steelMaterial

Costfor givenstiffness

Massfor givenstiffness

α FeAl= 1.2 $/kg

α FeMg = 2.4 $/kg

CAFE: α < 0.5 $/kg

Driver's savings

α < 6 $/kg

This is what a lighter vehicle will save you

(upper bounds)

1/2/Eρ

1/2c/Eρ

This is what it costs you to lighten up your vehicle ($/kg)

α AlMg = 9.9 $/kg

Metall. Mater. Trans. A 38, (2007) 1649-1662 20/420.1 1mass relative to cast iron

1 10ρ (Mg m-3)

1

10

100

ρc

(103 $

m-3)

AZ91 A356 SteelCast Fe

0.1

1

10

100

cost

rela

tive

to c

ast i

ron

αFA = 0.6

volume

αFM = 0.7

αAM = 1.3

2

Cost of Mg and Al Castings substitutions for cast iron

Costper m3

density

α FeAl= 0.6 $/kg

α FeMg= 0.7 $/kg


1

10

α (

$/kg

)

Cast Fe-AlPanel Fe-Al

Beam Fe-Al

Cast Fe-Mg

Panel Fe-Mg

Beam Fe-Mg

Cast Al-Mg

Panel Al-Mg

Beam Al-Mg

CAFE penalty (0.5$/kg)

lifespan savings (6$/kg)

0.3

Driver's savings 6 $/kg

Cost analysis

Fe=>Al

Fe=>Mg

CAFE penalty 0.5 $/kg

α($/kg)

Substitutions below this line are OK for CAFE

Substitutions below this line are OK for Driver's

savingsAl=>Mg


How about environmental (greenhouse gas, CO2)

effects?


material h (kg of CO2 / kg)

Iron/Steel 1 ~ 2 kg/kg

Al ~12 kg/kg(45% hydro electricity, 55% fossil world

avge.)

~23 kg/kg (45% hydro electricity, 55% fossil world

avge.)

~42 kg/kg

Electrolytic Mg (30% of world production)

Pidgeon Mg (70% of world production)

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

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


Function Index Same Volume (castings)

Beam (bending)

Panel (bending)

Material Indices to minimise CO2 creation?

CO2 footprint equivalent [hq ] = lg/kg

minimise CO2 per unit vol

qhρE /1/2

q /ρE h1/3

qhρ1/

minimise CO2 footprint for given stiffness

Maximise these !


Gasoline equivalent to the CO2 footprint ?

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

Cars create ~ 2.85 kg of CO2 per litre of gasoline

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

β = exchange constants involving CO2


material hq

Iron/Steel ~ 0.5 lg/kgAl ~ 4 lg/kg

~ 8 lg/kg

~ 15 lg/kg

Electrolytic Mg

Pidgeon Mg

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

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


CO2 creation: exchange constants for Same Volume substitutions (castings)

CO2footprint

mass

Electrolytic Mg

Driver's savings

β = 7 lg/kgPidgeon Mg

Al

gasoline burnt producing the materials to achieve one kg of mass reduction

This is what a lighter vehicle saves (per kg) over 2x105 km

ρ

ρhq


Cast

Panel

Beam

Cast

Panel

Beam

Cast

Panel

Beam

Cast*

Panel*

Beam*

Cast*

Panel*Beam*

1

10

β (

lg/k

g)

CAFE liability(0.6 lg /kg)

βCO2

(7 lg /kg)

30

Al<MgFe<Al Fe<Mg Fe<Mg* Al<Mg*

CO2 creation analysis

Fe=>Al

Fe=>MgFe=>Mg*

Substitutions below the line are OK

Driver's savings 7 lg/kg

Al=>Mg

Al substitutions for Fe are OK

Electrolytic Mg substitutions for Fe: castings & panels

OK beams are off

Pidgeon Mg’s CO2-footprint is

excessive for beams and

panels, OK for castings

Electrolytic Mg substitutions for Al are viable for castings.

Pidgeon Mg is out of bounds.


1 10 αCAFE (0.5 $/kg) α ($/kg) αS (6 $/kg)

1

10

β (l

g/kg

)

10 100break-even distance, dα (x103 km)

100

1000

brea

k-ev

en d

ista

nce,

dβ

(x10

3 km

)

βCO2

(7 lg /kg)

30

Simultaneous selection by Cost and CO2 footprint

β (lg/kg)

α ($/kg)CAFE liability

(0.5$/kg)

Driver's savings (7lg/kg)

Driver's savings (6

$/kg)

Distance to break-even (x103 km)

200x103 km

Substitutions inside the box are OK

Iron and steel replaced by Al

Iron and steel replaced by Mg

Iron and steel replaced by Pidgeon Mg

Aluminumreplaced by Mg

Aluminum replaced by Pidgeon Mg

•Only two substitutions are economically not viable.

• 8 out of 14 substitutions are environmentally not viable (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.

Analysis so far assumed primary alloys


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 evenA cast Fe engine replaced by an Al or Mg engine

An engine block cast on a primary Al alloy (A356) requires 55x103 km to

break-even

Cast on alloy A319 (60% recycled) cuts the driving distance to ~10x103 km

Primary electrolytic Mg => 70x103 km

Primary Pidgeon Mg

=> 130x103 km

With 35% recycled Mg: Electrolytic Mg => 35x103 km Pidgeon Mg => 75x103 km


Driving distance to break even for Al or Mg space frames replacing steel

A rolled Al panel requires

75x103 km to break-even

An extruded Al beam requires 130x103 km

to break-even

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

An extruded electrolytic Mg beam requires 210x103 km

to break-evenA rolled electrolytic Mg panel

requires 125x103 km to break-even


Magnesium steering wheel replacing a steel steering wheel

Tzabari and Reich, 2000

A steering wheel of primaryelectrolytic Mg alloy requires 210x103 km to break-even

A steering wheel of primaryPidgeon Mg alloy requires

390x103 km to break-even

At 35% recycling rate electrolytic Mg requires

~130x103 km

At 35% recycling rate Pidgeon Mg requires

~260x103 km


Al engine replaced by a Mg engine?

Al engine block cast on alloy A319 (60% recycled)

Primary electrolytic Mg => 323x103 km

Primary Pidgeon Mg

=> 724x103 km

With 35% recycled metal: electrolytic Mg => 160x103 km Pidgeon Mg => 430x103 km

Mg is not a good replacement for existing

cast Al components


cars

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


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 unsuitablefor most automotive applications.


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

Energysource

100% hydro

/nuclear

55% fossil fuels 45% hydro/ nuclear

100% fossil fuels

Al 6.2 12 16.7Mg

(no SF6)7.5 20 30

Mg (with

SF6)10.6 23 33.1

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 energyAl and electrolytic Mg

present analysis: ~55% fossil fuels 100% fossil fuels

100% hydro/nuclear/other renewables

Clean sources of energy are essential for clean Mg or Al


Green: α-values of the order of the CAFE liabilityBlue: within the savings over a Driver's of 2x105 km

Red: economically or environmentally not viableBrackets: distance to break even

Metrics Cost penalty($/kg)

Gasoline equivalent footprint (lg/kg) primary alloys

Function αFA αFM αAM βFA βFM β*FM βAM β*AM

Beam 1.2 (40)

2.2(73)

9.9 (330)

4.6 (132)

7.4 (212)

13.6 (389)*

25 (715)

76(2200)

*Panel 0.5

(17)1.1(37)

4.6 (153)

2.6 (74)

4.4 (126)

8.8 (252)*

12 (343)

40 (1144)

*Casting 0.4

(13)0.6(20)

1.3 (43)

1.9 (54)

2.3 (66)

4.7 (134)*

3.8 (109)

18 (514)*


Critical Recycling Rates to make the exchange constants = 0 ?

volume panels beams

Al 82% 69% 75%

Mg 87% 82% 87%

Mg* 95% 93% 96%

Pidgeon Mg ~95%!

At these recycling rates Al and Mg create the same amount of

CO2/kg as Fe


Transport System: mass saving α ($US per kg)

~0.5~6

5 to 20100 to 500

3000 to 10000

Family car (based on C.A.F.E. penalty)

Family car (based on Driver's savings)

Truck (based on payload)Civil aircraft (based on payload)Space vehicle (based on payload)

Finding α: Exchange Constants for Transport Systems

Ashby, 2005

automotive mg & al: curb weight, $ and co2

Business

cost penalty

minimum cost

material index

kgminimise cost

cost c

material b

heavylight car al

lighter car