A l u m i n u m :

T h e C o r r o s i o n R e s i s t a n t

A u t o m o t i v e M a t e r i a l

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Publication AT7 – May, 2001

The Aluminum Association and its member companies assume no responsibility or liability for the use of information containedherein. The Aluminum Association and its member companies assume no responsibility for its use. No warranties, express orimplied, by The Aluminum Association or its member companies accompany this information.

© Copyright 2001 The Aluminum Association, Inc.

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Tableof Contents

Chapter Title Page

1. Introduction 2

2. Aluminum in Automobiles - A Brief History 3

3. Aluminum Parts in the Cars of Today 6

4. Key Characteristics of Aluminum 10

5. Designing for Durability 12

6. Anti-corrosion Design Tips 15

7. References 17

Appendix Properties of Commonly Used Automotive Aluminum Alloys 18

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1.Introduction

Automotive aluminum use hasbeen growing for years (from anaverage of 87 pounds per car in1976 to 248 pounds in 1999),mainly to reduce weight andimprove fuel economy. Eachpound of aluminum used canreduce vehicle weight as much as1.5 pounds. Automotive framesand bodies can make even furtheruse of aluminum’s uniquecombination of strength, lightweight, crash-energy absorption,corrosion resistance, and thermaland electrical conductivity.

As new car prices increase (theyroughly quadrupled between 1978and 1999), durability and corrosionresistance take on new importance.Buyers want vehicles that willretain their appearance and keep ahigh resale value. That issomething that aluminum canprovide, as automakers offer longerwarranties against componentfailure and body rust-out.

Aluminum — even unpainted anduncoated — resists corrosion bywater and road salt and, in non-cosmetically critical parts, its usecan avoid the substantial extracosts of galvanizing, coating andpainting required for steel.Aluminum does not rust like steelif the paint is scratched orchipped. Nor is it weakened orembrittled, as some plastics maybe, by desert heat, northern cold,or the ultraviolet radiation insunlight. For its new deliveryvans, the U.S. Postal Servicespecified aluminum bodiesdesigned to last 24 years!

F i n a l l y, when a car must bescrapped aluminum is readilyrecycled with a high residualscrap value, providing botheconomic and environmentalb e n e f i t s .

Aluminum, with its wide choiceof alloys and tempers, offers a

wealth of advantages toautomotive engineers developingnew car designs of the future.

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2.Aluminum in Automobiles

A Brief History

During the past 25 years, the useof aluminum in automobiles hasincreased steadily, both inabsolute quantity per car and as apercentage of vehicle weight.H o w e v e r, aluminum is hardly a

newcomer to the automobile; infact, it has a long and successfulhistory in automotive applications.Aluminum crankcases were usedon the 1897 Clark (a three-wheeler) and the 1898 De Dion

Bouton. (Figure 1) Substantialuse of aluminum in automobileswas reported in 1900 in bothFrance and the United States, twincradles of the modern aluminumi n d u s t r y. (Figure 2)

FIGURE 1 –– Three-wheeler with aluminum crankcase.

FIGURE 2 –– Aluminum twin cradle.

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A number of aluminum partsturned up on cars exhibited in thesecond New York automobiles h o w, in 1901. Aluminum bodypanels were replacing wood, thetraditional coach-body material, inautomobiles around the sametime. By 1903, the GordonBennett Napier had an aluminumcylinder block; the 1904L a n c h e s t e r’s rear axle housingwas made of aluminum. A n d ,automotive uses of aluminummultiplied during the early 1900s,showing up in gear housings, fancowls, oil pans, water pumps,steering boxes, steering wheels,radiators, dashboards andother parts.

Before World War I, the autoindustry was aluminum’s biggestsingle market, absorbing up tohalf of the aluminum produced.

The popular Ford Model T u s e daluminum in its transmission and

hood. (Figure 3) In 1913, W. O .Bentley pioneered the use ofaluminum pistons in racing cars.

What may have been the first“AIV” (aluminum-intensivevehicle) was designed and builtin 1923 by L.H. Pomeroy, a

famous British engineer. T h ePomeroy car weighed only abouttwo-thirds as much as a standardautomobile, and proved to beextremely durable.

In mass-production cars steelbecame predominant, largely foreconomic reasons. However, theadvantages of aluminumcontinued to give it a prominentrole in transportation —particularly in aircraft, railroadcars, trucks and buses wherea l u m i n u m ’s combination oflight weight, strength, andc o r r o s i o n-resistant durability werehighly valued.

Those qualities were also highlyvalued in racing and luxury cars,such as the aluminum-bodiedRolls Royce “Silver Ghost” andthe classic 1930 Duesenberg .(Figure 4) Their value for

FIGURE 3 –– H e n ry Ford with the last and first of his Model T Ford s .

FIGURE 4 –– 1930 Duesenberg .

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standard production cars would berediscovered after World War II,and particularly after the suddenrise in gasoline prices that beganin 1973.

The U.S. aluminum industryexpanded rapidly during World Wa rII to meet the nation’s need for

military aircraft; after the war, thisexpanded production capacity madealuminum available for new andrenewed markets, includinga u t o m o b i l e s .

The first U.S. fluid drivetransmission, in 1948, had analuminum housing. A l u m i n u m

pistons have been standard on U.S.-made automobiles since 1955.Aluminum trim, virtually unknownin the early 1950s, was inwidespread use by the end of thatdecade; by the mid-60s, most U.S.cars had aluminum grilles.

Since then, automotive applicationshave multiplied: aluminumbumpers since the early 1970s,aluminum intake manifolds since1977; aluminum engine heads,engine blocks, wheels, radiators,driveshafts, and in recent years, asignificant number of auto bodyclosure panels. More than ahundred types of auto parts aremade of aluminum and the listkeeps growing.

In 1960, the average U.S. carcontained about 54 pounds ofaluminum — 1.4 percent of its totalweight. Twenty-seven years later,average aluminum content hadclimbed to nearly 250 pounds, orabout eight percent of total weight.And further opportunities lie open,as auto designers choose aluminumto satisfy drivers who want it all:performance, comfort, fuele c o n o m y, safety, and durability.

In recent years, the Audi A8 and theFord A I V have highlighted theperformance and benefitsachievable by using all aluminumbody structures. The fieldexperience that is being gained withthese vehicles continues to confirmthe excellent corrosion performanceand durability of aluminum inautomotive applications.

Audi A8

F o rd AIV

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3.Versatile, Tough, Durable...

Aluminum Parts in the Cars of Today

Since the mid-1970s, thepercentage of aluminum use inautomobiles has increased almostthree-fold. To d a y, more than ahundred different auto parts aremade of engineered alloyaluminum and the list is stillgrowing. While lighter weightand efficient function were theprimary reasons for selectingaluminum, extended life throughbetter corrosion resistanceprovided an added benefit that ishighly important in achieving thedesired useful life of the vehicle.

A sampling of those wide-rangingapplications is depicted onthese pages.

Air Conditioners — Aluminum isan excellent conductor of heat andis widely used in automotive airconditioner condensers,evaporators, liquid lines, andcompressor housings.

Body Panels — Aluminum hasbeen successfully used in hoods,deck lids and other exterior parts

in large production volumemodels of passenger cars, pickuptrucks, vans and sport utilityv e h i c l e s .

Brackets — A l u m i n u m ’scombination of strength,resilience, and durability makes itan excellent material for enginemounting and accessory brackets.It is widely used for powersteering brackets, pump-mountingbrackets, air conditioner mountingbrackets, steering column bracketsand similar applications.

Brake Cylinders and Pistons —Light weight, corrosion resistance,e c o n o m y, and reliability explainthe choice of aluminum in thisimportant application.

Brake Drums — Strength anddurability under exposure tow a t e r, road salt and dirt areamong the advantages ofaluminum in this application. Inaddition, the heat-transfercapability of aluminum helps tokeep brake linings from

overheating and so reduces brake“fading” in severe use, animportant safety factor.

Bumper Reinforcements — T h e s esafety-related parts have highstrength, light weight, goodforming characteristics, andresistance to corrosiveenvironments.

C h a rge Air Coolers — In additionto its good heat exchange andcorrosion resistance characteristics,aluminum can be readily formed,cast or extruded into complexhollow shapes, as required for thisa p p l i c a t i o n .

Complete Bodies — A l u m i n u mhas been used successfully forcomplete auto bodies,demonstrating strength, lightweight, durability, and excellentcrashworthiness. It is thepreferred body material for larg etrucks, buses and other utilityvehicles and was selected for thecurrent U.S. Postal Service van,with a projected body life of 24y e a r s .

Driveshafts — This relatively newapplication of aluminum wasprompted by the metal’scombination of high strength,light weight, and corrosionresistance in a severely exposedlocation. A l u m i n u m ’s lightweight not only improves generalvehicle performance and

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e c o n o m y, but also reducesdriveline vibration and noise.

Engine Heads and Blocks — T h eengine is one of the heaviestsingle units in an automobile ando ffers one of the greatestopportunities for weight savingthrough the use of aluminum.Many car engines have aluminumheads and some have aluminumengine blocks as well.

Fuel Injection Systems —Aluminum offers weight savings,corrosion resistance, machinabilityand extrudability, as manufacturerscontinue to make fuel injectionsystems smaller and lighter.Aluminum is used for pumphousings, tubing and cylinderp a r t s .

Heater Cores — Aluminum is anappropriate material for heatercore applications, since it is anexcellent conductor of heat and isformable, and can be brazed,soldered or welded.

Intake Manifolds — A l u m i n u mallows the production of intakemanifolds in more advancedshapes and with thinner walls thanare practical in iron. In addition,aluminum engine parts of allkinds present an attractive “high-tech” appearance under the hoodwhich effectively conveys a senseof the vehicle’s quality topotential purchasers. As a result ofall these factors, aluminum has

become the material of choice forthese parts.

Load Floors — This applicationdemonstrates the versatility ofaluminum. Its combination oflight weight, strength, andcorrosion resistance provides apart that can take contact withvarious materials, weights andimpacts, without specialprotection or maintenance.

Luggage Racks and Air Deflectors —In these parts, aluminum combinesesthetic appearance and stylingwith function and durability inenvironmental exposure withoutpainting or coatings.

Oil Coolers — Auxiliary engine oilcoolers and transmission oilcoolers make use of aluminum fore fficient heat exchange, durability,and light weight.

Pistons — These moving partsmust last for the life of the vehiclein a demanding environment ofhigh heat, stress, and potentiallycorrosive compounds. A l u m i n u mmeets these demands, with theadded advantage that its lightweight makes engines moreresponsive and efficient inconverting fuel energy intovehicle performance. A l u m i n u mhas been the standard material forautomobile pistons since the 1 9 5 0 s .

Radiators — T h r o u g h o u tautomobile history, aluminum has

been used in the radiators ofselected cars. Now, with newproduction techniques,automakers are equipping mostmodels with aluminum radiatorsto take advantage of their lightweight, heat-transfer capacity, andcorrosion resistance. Aluminum isformable, machinable, and can bebrazed, soldered or welded.

Seat Tracks, Shells and Headrests —The mechanical properties oflightweight aluminum alloys andtheir ease of fabrication makethem an advantageous choice forthese safety-sensitive parts.

Spare Tire Carrier Parts — T h e s eparts are both functional and styledfor appearance. A l u m i n u mprovides both the necessaryfunctional strength and durabilityplus the desired styling.

Splash and Heat Shields —A l u m i n u m ’s resistance to water,road salt, hydrocarbons and dirt,and its ability to reflect andconduct away heat provides fordurable shields to protect autoparts made of more vulnerablem a t e r i a l s .

Suspension Parts — A l u m i n u mhas proven its value for suspensionparts, where strength, lightweight, and corrosion resistanceare vital, in a popular “high-performance” car. It has been usedin such parts as the upper andlower control arms, front and rear

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steering knuckles, trailing arms,wheel spindle control rods, tie rodsockets, drive line support, wheelshafts and axle cover beams.

Transmission Housings — In apart requiring strength, corrosionresistance, ease of fabrication ande c o n o m y, aluminum meets all ofthe requirements, whilesubstantially reducing vehicleweight. Transmission housingswere one of the earliestapplications of aluminum inautomobiles, for those very samer e a s o n s .

Trim Moldings — Aluminum trimmoldings have solved corrosion

problems and provided anattractive and durable appearancefor several generations ofautomobile designers and owners.Anodized aluminum exterior trimhas been used for more than thirtyyears, with excellent outdoord u r a b i l i t y, corrosion performanceand a bright finish.

Wheels — Aluminum wheelsgreatly reduce a car’s unsprungweight, improving ride andhandling. They are not susceptibleto rusting. Aluminum wheels wereintroduced as optional equipmentfor styling reasons. Produced ascastings, forgings, fabricated sheetand hybrid cast and wrought

configurations, aluminum wheelsnow have become standardequipment on many makes andm o d e l s .

Wheel Covers — These visuallyattractive parts must be lightweight and formable, and mustretain their good appearance overthe expected life of the vehicle.Aluminum is an excellent materialfor this application. Its naturalcorrosion resistance ensures thatthe esthetic styling given to thepart will last.

Aluminum in To d a y ’s Automobile

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Aluminum in To d a y ’s Automobile

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4.Key Characteristics of Aluminum

Aluminum offers a wide range ofproperties that can be engineeredprecisely to the demands ofspecific automotive applicationsthrough the choice of alloy,temper and fabrication process.To name a few of its advantages,aluminum off e r s :

S t rength — Some aluminumalloys and tempers approach orsurpass the strength of commonlyused automotive steels. To cite afew examples, automotivealuminum alloys achieve tensilestrengths of 310 MPa (45 ksi) foralloy 6061-T6; 290 (42 ksi) for6 111-T4; and 430 MPa (62 ksi) foralloy 7029-T6. Some aluminumalloys are heat treated to strengthsapproaching 700 MPa (100 ksi),although these are primarily usedin the aircraft industry.

Light weight — A l u m i n u mweighs about 35 percent as muchas steel by volume: 170 poundsper cubic foot of aluminum, versus490 pounds per cubic foot of steel.Aluminum auto parts save weightdirectly as well as indirectlythrough redesign of other parts.

High strength-to-weight ratio —A l u m i n u m ’s strength-to-weightratio is much greater than that ofsteel: often double, or more. T h i sproperty of aluminum has been akey factor in development of theaerospace industry, and it offers thesame advantages to auto designersseeking improved performance andhigher fuel eff i c i e n c y.

Resilience — Aluminum alloyswill deflect under load and springback, providing flexible strengthand shape retention. A l u m i n u malloys can also be used to meet thes t i ffness and crash energ yabsorption requirements forautomotive vehicle structures,while providing up to 50 percentweight savings compared withother materials.

C o r rosion resistance —Aluminum does not “rust away” onexposure to the environment likesteel; its natural oxide coatingblocks further oxidation. The riskof galvanic corrosion can be

minimized by the appropriatechoice of alloy, component design,and protective measures.

Forming and fabricating —Aluminum can be formed andfabricated by all commonmetalworking methods includingcasting, stamping, forg i n g ,bending, extruding, cutting,drilling, punching, machining andf i n i s h i n g .

Joining — Aluminum can bejoined by all common methodsincluding: welding, soldering,brazing, bolting, riveting, adhesive-bonding, weld bonding, clipping,clinching, and slide-on, snap-together or interlocking joints.

C r a s h w o rthiness — A l u m i n u mabsorbs more crash energy per unitmass than steel or plastic. Also, itis non-combustible and it does notstrike sparks.

C o l d - resistance — At lowtemperatures, aluminum does notembrittle; it has higher strengthAND ductility at subzerotemperatures, and is often used forcryogenic applications down toabsolute zero (-273°C, -459°F).

Recyclability — Aluminum hassubstantial scrap value and a well-established market for recycling,providing both economic andenvironmental benefits.

Thermal conductivity —

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Aluminum conducts heat about1.8 times better than copper,pound for pound and more thanthree times better than steel. T h i smakes aluminum an excellentmaterial for heat exchangers.Aluminum heat exchangers arewidely used in automotiveradiators, air conditioning systemsand similar types of equipment.

Reflectivity — S m o o t haluminum is highly reflective ofthe electro-magnetic spectrum,from radio waves through visiblelight and on into the infrared andthermal range. Aluminum bouncesaway about 80 percent of thevisible light and 90 percent of the

radiant heat striking its surface. Itshigh reflectivity gives aluminum adecorative appearance; it alsomakes aluminum a very eff e c t i v ebarrier against thermal radiation,suitable for use in automotive heats h i e l d s .

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5.Designing for Durability

5.1 - Uncoated Aluminum

Nature has provided aluminumwith a highly protective “skin” inthe form of a clear barrier oxide onits surface that forms quickly andis tough enough to hinder thedeeper intrusion of oxygen andother gases and liquids to thesubsurface aluminum atoms. T h i soxide is tightly chemically boundto the underlying surface, and ifdamaged, reforms immediately inmost environments. On a freshlyabraded surface, the barrier oxidefilm is only 1 nm (10 angstroms)thick, but is highly effective inprotecting the aluminum fromc o r r o s i o n .

The oxide film develops slowly innormal atmospheres to greaterthicknesses, and when corrosiveenvironments are present, theoxide may both thicken anddarken. However, it generallyretains its protective character.

Thus, in normal environmentalexposure, aluminum does notcorrode (rust) away as does steel.

Aluminum surfaces do oxidizewhen exposed to air, but thisd i ffers from the oxidation of steelin two important ways:

Aluminum oxide is effectively transparent and invisible to the unaided eye.

Aluminum oxide clings tightly to the surface of aluminum and forms a protective film that blocks progressive deteri-oration. It does not flake off, thereby exposing fresh surfaces to further oxidation. When damaged, it quickly reforms again, providing continuing p r o t e c t i o n .

With this natural corrosionresistance, the aluminum bodiesof many commercial motorvehicles, rail cars and aircraft areunpainted; aluminum has provendurability in such applications.

5.2 - Coatings

Although aluminum componentsgenerally perform well without

coatings, aluminum is an excellentsubstrate for paints and othercoatings, often applied for estheticreasons as well as for additionalcorrosion protection.

Adhesion can be maximized withthe appropriate pretreatments orundercoats which are compatiblewith other components of thecoating system.

A complete coating systemincludes the following:

C l e a n e r ;

Conversion coating( p r e t r e a t m e n t ) ;

Electrocoat primer;

Primer/surfacer; and

Top coat.

5.2.1 - Anodic Coatings

Anodic coatings are among themost useful for many applicationsbecause they:

Increase corrosion resistance;

Increase paint adhesion;

Increase adhesive bondd u r a b i l i t y ;

Improve decorativeappearance; and

Increase abrasion resistance.

The basic approach in anodizingis to increase the thickness of thenatural oxide coating on aluminumby converting more of theunderlying aluminum surface to

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aluminum oxide while the partbeing anodized is the anode in anelectrolytic cell.

The basic process steps toaccomplish anodizing are:

1 ) Chemical cleaning of thesurface to remove soils andc o n t a m i n a n t s ;

2 ) Etching to remove the existingo x i d e ;

3 ) Electrolytically treating thepart in chromic acid, sulfuricacid or another appropriatesolution to build a thick newoxide coating; and

4 ) Sealing the resultant coating inhot water, a hot dichromatesolution, or some othersuitable agent.

Such anodic treatments provideboth corrosion resistant surfacesand surfaces amenable toadditional protective finishes ifthey are needed.

5.2.2 - ChemicalConversion Coatings

Chemical conversion coatings areadherent surface layers of low-solubility metal oxide, phosphate,or chromate compounds producedby the reaction of suitable reagentswith the metal surface. They diff e rfrom anodic coatings in thatconversion coatings are formed bya chemical oxidation-reductionreaction at the aluminum surface,whereas anodic coatings areformed by an electrolytic reaction.

Chemical conversion coatings areexcellent for:

Improved adhesion of org a n i cc o a t i n g s ;

Mild wear resistance;

Enhanced drawing or formingo p e r a t i o n s ;

Decorative purposes whencolored or dyed;

Improved corrosion resistanceunder supplementary org a n i cfinishes or films of oil orwax; and

Adhesive bonding.

The sequence of operations forapplying satisfactory conversioncoatings includes:

1 ) Removal of org a n i ccontaminants and oxide orcorrosion products;

2) Conditioning the surface withacid or alkaline solutions;

3) Conversion coating with oxide-type, phosphate or chromateprocesses; and

4) Rinsing followed bysupplemental coating ifrequired. The final step can beomitted if no-rinse conversioncoatings are applied.

5.2.3 – Painting

The only difference betweenpainting aluminum and steel is thesurface preparation. Aluminum isan excellent substrate for org a n i ccoating if the surface has beenproperly cleaned and prepared.

For many applications, such asinterior decorative parts, thecoating may be applied directly toa clean surface. However, asuitable wash primer or zincchromate primer usually improvesthe performance of the finish coat.(Note that chrome-free primers arenow recommended and arereplacing the chromate primers).

For applications involving exteriorexposure, surface treatments suchas anodizing or chemicalconversion coating are requiredprior to the application of a primeror finish coat. As noted earlier,sulfuric acid or chromic acidanodic coatings provide excellentsurfaces for organic coatings.Usually only thin anodic coatingsare required as a pre-paintt r e a t m e n t .

Conversion coatings are lessexpensive pretreatments thananodic coatings, provide a goodbase for paint, and improve thelife of the paint by retardingcorrosion of the substrate.Adequate coating of the entiresurface is very important forpaint bonding. The conventionalautomotive finishing systemconsisting of a) cleaning with adilute alkali, b) followed by zincphosphate as the pretreatment, andc) the cathodic electrocoat whichprovides excellent corrosionr e s i s t a n c e .

It is useful to note that manyvehicles now have aluminumclosure panels made from alloys

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6016 and 6111 which haveexhibited excellent corrosionresistance and paint adhesionperformance in service.

5.3 - Anti-corro s i o nE n h a n c e m e n t

In automotive applications,appropriate designs andprecautions can protect aluminumagainst the most likely forms ofcorrosive attack: galvanic, crevice,filiform, poultice and interg r a n u l a rstress corrosion.

Galvanic corrosion — W h e ndissimilar metals are held incontact in the presence ofmoisture, galvanic corrosion ispossible. Aluminum is anodic(i.e., has a more negative solutionpotential) to steel and many othercommon metals, except zinc andmagnesium, and so is vulnerableto galvanic corrosion because themore anodic material corrodespreferentially to the other.Protection is afforded by: a)keeping bimetallic junctions dryand, b) separating dissimilarmetals with coatings or otherinsulators. Anodizing also helpscombat galvanic corrosion bythickening the protectivealuminum oxide film.

C revice corrosion —Unprotected crevices at matingsurfaces can collect and retainmoisture that may form a pathwayfor corrosive electric currents.Measures that eliminate or sealcrevices, and designs that shield

them from splash greatly reducethe risk of corrosion.

Filiform corrosion — F i l i f o r mcorrosion can occur on paintedsurfaces where a defect or scratchin the coating occurs allowsaccess. This type of corrosionmanifests itself as thin filamentsthat grow under the coating fromscratch lines. The filaments arefine tunnels of corrosion producttrailing the active cell. Using anappropriate conversion coatingand ensuring the consistency andquality of coatings best preventsfiliform corrosion. Filiformcorrosion is really only of concernfor painted exterior panels, andthe alloys now used for theseapplications have been developedto minimize their susceptibility tothis type of corrosion.

Poultice corrosion — S u r f a c eaccumulations (“poultices”) thatretain moisture promote corrosionin much the same way as crevices.The design of metal componentsand their surfaces should be suchas to shed dirt and liquids;permanent contact between metalsurfaces and absorptive materialsshould be avoided. If thesemeasures are insufficient orimpossible, the metal may begiven a protective coating.

I n t e r g r a n u l a r and stre s sc o r rosion cracking — S t r e s scorrosion cracking (scc) is unlikelywith the combinations of alloysand products in most automotiveapplications. Most 5xxx and 6xxx

alloys are resistant to stresscorrosion cracking. However,aluminum alloys containing morethan three percent of magnesium(Mg) may become sensitized(susceptible) to stress corrosioncracking if exposed for longperiods at temperatures aboveabout 75°C (150°F). T h e r e f o r etheir use in exposed structuralapplications, where there iscontinuous or intermittentexposure to engine heat or otherhigh temperatures, should beavoided. If the advantages of the5xxx (Al-Mg) alloys are needed insuch situations, the selection ofalloys such as 5454 and 5754 withlower Mg levels is recommended.It should be noted that paint-bakecycle aging has no deleteriouse ffect upon the corrosionresistance of 5xxx alloys. Heattreatable 2xxx and 6xxx alloysmay show some minorsusceptibility to interg r a n u l a rcorrosion when partially aged (asin the paint-bake condition) butthis is of no concern after thepaint coating is applied.

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6.Anti-corrosion Design Tips

6.1 - Pre f e rred DesignF e a t u res for Joints andFaying Surf a c e s

To minimize corrosion attack inbutt welded and lap joints, theweld material (or rivet or bolt)should be less active than thel a rger area metals being joined.

In lap joints, use of fillet welds,insulating material, or a seamsealer is recommended.

Metallic fasteners which joinaluminum to a dissimilar metalshould be made of an alloycathodic to aluminum. Forexample, use steel bolts in an

aluminum-steel joint, notaluminum bolts; aluminized steelbolts are even better. Sacrificialprotective coatings, typicallyformed by epoxy resins containingzinc, applied to steel fasteners arevery eff e c t i v e .

Entrapment sites in offset lapwelds and standing seams shouldbe eliminated with a sealer or abead weld.

Coatings should be applied toboth the anode and the cathode orto the cathode only (e.g., to thesteel in an aluminum-to-steeljoint), but never to the anode only(e.g., to the aluminum only insuch a joint). Damage to thecoating on the anode would resultin serious corrosion due to smalla n o d e - l a rge cathode combination.Coating the faying surfaces of the

dissimilar metals as well canincrease protection. Sealantsshould be applied to crevices forbest results.

Flanges should protect jointsexposed to direct splash. T h e s emay have to be angled to protectwithout creating entrapment sites.

6.2 - Avoiding EntrapmentA re a s

Orientation of floor panel and sidepanel lap joints is important inavoiding entrapment areas.

Design, and use of sealer,minimizes entrapment areas.

Flange orientation and designprevents entrapment of moistureand debris.

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6.3 - Contro l l i n gEntrapment Are a s

The proper location of theopening in lower doors canminimize chances of plugging andcan enhance drainage. The designabove right tends to plug withdebris more easily than the designat left. Sealants in tight jointsfurther improve corrosionr e s i s t a n c e .

The horizontal catchment areas, asin fender at left, should be avoided.The hood section, at right, requiresprotective coating and drainage.

6.4 - Other DesignF e a t u re s

The vertical rise of components inthe path of airborne solids shouldbe minimized.

Sharp contours and certaindirectional design features shouldbe minimized. (Arrows indicateareas of concern).

6.5 - Design andOrientation of Stru c t u r a lMembers andR e i n f o rc e m e n t s

Hat section and H- or I-beamreinforcements are good designsbut the hat section should be openat the bottom for easy drainage.

If not inverted, channels requiredrain holes to avoid entrapmentareas; angle sections should haverounded corners, smooth tapers,and drain holes as indicated.

When joining dissimilar metalsdesign for a large anode/cathoderatio, and insulate the entirecontact area with a protective

coating as shown in previouscolumn. If possible, the steelplate should be galvanized orpainted and sealants should beapplied to joints. Steel rivets arebetter than aluminum in such ajoint; coated steel or stainlesssteel rivets are preferred.

Drain openings should be properlylocated to enhance drainage and toprevent entry of road contaminants.Sealant in joint crevices enhancescorrosion resistance.

When box sections must be used,provide sufficient openings for theapplication and the drainage ofprotective coatings. Drain flutesand louvered holes should pointdown and to the rear of thevehicle. Crevices should bepainted or sealed.

Use open construction wherepossible. In a severe corrosionenvironment, box sections andenclosed areas should be avoidedor treated with a protective coating.

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7.References

Automotive Design

Aluminum for Automotive BodySheet Panels, AT3, T h eAluminum Association, 1998,Washington, DC

Automotive Aluminum Extru s i o n ,AT5, The Aluminum A s s o c i a t i o n ,Washington, DC, 1998.

Aluminum, Vol. 1-Pro p e rties andPhysical Metallurg y Edited byJohn E. Hatch. American Societyfor Metals, 1984, Metals Park,O h i o .

Schlitt, R.P and R.K. Eschebach.“Material Selection andC o rrosion protection for Bi-Metallic Systems in A u t o m o t i v eE n v i ro n m e n t s . ” S.A.E. Paper831831, 1983.

Rowe, L.C. “The Application ofC o rrosion Principles toEngineering Design.” S . A . E .Paper 770292, 1977.

Aluminum: Te c h n o l o g y,Applications and Enviro n m e n t , b yDietrich Altenpohl, T h eAluminum A s s o c i a t i o n ,Washington, DC, 1998.

C o rrosion Resistance

Guidelines for the Use of A l u m i n u mwith Food and Chemicals, CFC-6 0 ,The Aluminum A s s o c i a t i o n ,Washington, DC, 1984.

Mozelewski, EA. S u m m a ry ofC o rrosion Testing of A l u m i n u mABS A l l o y s , Alcoa LaboratoriesReport No. XC-5 2 8 , 1 9 8 0 .

Godard, H.P. The Corrosion ofLight Metals, John Wiley &Sons, Inc, 1967.

Handbook of Corrosion Data,Bruce D. Craig, Editor, A S MInternational, 1989.

Aluminum, Pro p e rties andC h a r a c t e r i s t i c s

Aluminum Standards and Data,The Aluminum A s s o c i a t i o n ,Washington, DC, 2000.

The Aluminum Design Manual,The Aluminum A s s o c i a t i o n ,Washington, DC, 2000.

Aluminum and Aluminum A l l o y s ,E d i t o r, J. R. Davis, A S MInternational, 1993.

18

A p p endix AProperties of Commonly UsedAluminum Automotive Alloys

The Aluminum Association haspublished several comprehensivemanuals describing thecompositions, properties andapplications for both thealuminum sheet and extrusionalloys that have been developed oroptimized for automotiveapplications. Respectively, theseare entitled “Aluminum forAutomotive Body Sheet Panels” -AT3, and “Automotive A l u m i n u mExtrusion Manual” - AT5. T h ereader is therefore stronglyadvised to obtain copies of thesetwo documents as well as“Aluminum Standards and Data”for detailed information.H o w e v e r, to aid the reader, thefollowing basic information isprovided to give guidance on thecomposition and typical propertiesof the materials most commonlyused in vehicle structures.

A1 - Aluminum SheetA l l o y s

Various non-heat treatable and heattreatable aluminum alloys havebeen successfully utilized infabricating prototype unibodystructures in sheet metal

stampings. The compositions,typical mechanical properties,typical physical properties, andcomparative characteristics of themost commonly used sheet alloysare presented in Tables 1 through 4.

The 5xxx (Al-Mg) alloys are non-heat treatable. Their formabilitygenerally increases with increasing magnesium content.H o w e v e r, 5xxx alloys with nominal magnesium contentsgreater than about three weightpercent are subject to“sensitization”, whereby, with acombination of cold work (as instamping) and long-term elevatedtemperature exposure (as wouldarise in proximity to the enginecompartment), precipitation occurs at grain boundaries.Consequently the material maybecome susceptible to i n t e rgranular forms of corrosion,including stress corrosion cracking. Although the highmagnesium alloy 5182-O has beensuccessfully used in a productionapplication (with an appropriatepretreatment and a protective paint coating, e.g., chromating

followed by a baked electro-coating), the lower magnesiumalloys such as 5454-O and 5754-Oare considered the leading choicesfor structural stampings. A l l o y5754-O is the material that hasbeen almost exclusively used foradhesively bonded unibody sheet structures.

Heat treatable alloys 6009, 6111 ,and 6022 have been developedprimarily for closure panels. T h e yare characterized by high ductilityin the T4 temper in which they areformed, and high strength in thefinished application because theystrengthen during the paint-b a k ecycle. There are also certainapplications where they may beused advantageously in vehiclestructures. However, it isinadvisable to use them where they will be continuously exposedto elevated temperatures duringvehicle service since this willcontinue the age hardening process and potentially lead to loss of ductility which maycompromise the energy absorption capability.

19

TABLE 1 CHEMICAL COMPOSITION LIMITSOF ALUMINUM BODY SHEET ALLOYS(1,2)

AAAlloyDesig-nation

5182

5454

5754

6009

6022

6111

0.20

0.25

0.40

0.60 -1.0

0.8 -1.5

0.6 -1.1

0.35

0.40

0.40

0.50

0.05--0.20

0.40

0.15

0.10

0.10

0.15-0.60

0.01--0.11

0.5-0.9

0.20-0.50

0.50-1.0

0.50(3)

0.2-0.8

0.02--0.10

0.10-0.45

4.0-5.0

2.4 -3.0

2.6-3.6

0.4-0.8

0.45--0.7

0.50 -1.0

0.10

0.05-0.20

0.30(3)

0.10

0.10

0.10

0.25

0.25

0.20

0.25

0.25

0.15

0.10

0.20

0.15

0.10

0.15

0.10

0.05

0.05

0.05

0.05

0.05

0.05

0.15

0.15

0.15

0.15

0.15

0.15

––

––

––

––

––

––

Si Fe Cu Mn Mg Cr Zn Ti OthersEach

OthersTotal

Note

Notes:(1) Maximum limit unless a range is shown(2) Shown as a percent by weight(3) Mn + Cr = 0.10-0.6

TABLE 2 TENTATIVE MECHANICAL PROPERTIESOF ALUMINUM BODY SHEET ALLOYS(1)

Alloy &Temper

MPa (ksi) MPa (ksi) % MPa (ksi) GPa (ksi) 103

5182-0

5454-0(2)

5754-0

6009-T4

6009-T62(3)

6111-T4

6111-T62(4)

6022-T4

6022-T62(4)

275

250

220

220

300

280

360

255

325

40

36

32

32

43

42

52

37

47

130

115

100

125

260

150

320

150

290

19

17

14

18

38

22

46

22

42

24

22

26

25

11

26

11

26

12

165

160

130

130

180

170

215

155

195

24

23

19

19

26

25

31

22

28

71

70

71

69

69

69

69

69

69

10.3

10.2

10.3

10.0

10.0

10.0

10.0

10.0

10.0

UltimateTensile

Strength

Tensile YieldStrength

(0.2% offset)

Elongationin 50 mnor 2 in.

UltimateShear

Strength

Modulus ofElasticity,

Average forTension andCompression

Notes:(1) Not for design; represents typical for all products of these alloys(2) Typical per Aluminum Standards & Data, 1997(3) Artificially aged 1 hr. at 200-210°C (392-410°F) from the T4 temper(4) Artificially aged 1/2 hr. at 200-210°C (392-410°F) from the T4 temper

20

TABLE 4 COMPARATIVE CHARACTERISTICS OF ALUMINUM BODY SHEET ALLOYS(1)

TABLE 3 TENTATIVE TYPICAL PHYSICAL PROPERTIESOF ALUMINUM BODY SHEET ALLOYS

Alloy

20° to 100°, per °C(68° to 212°, per °F)

°C (°F) W/M•k(BTU in/ft2•hr) °F

EqualVolume

EqualWeight

103 kg/m3

(lb/in3)

5182-0

5454-0(2)

5754-0

6009-T4

6111-T4

6022-T4

24.1 (13.4)

23.6 (13.1)

23.8 (13.2)

23.4 (13.0)

23.4 (13.0)

23.4 (13.0)

575-640(1070-1185)

600-645(1115-1195)

590-645(1095-1195)

605-650(1120-1205)

585-650(1090-1200)

580-650(1075-1205)

121 (840)

134 (930)

132 (916)

167 (1160)

––

––

18 (31)

20 (34)

19 (33)

26 (44)

23 (40)

––

64 (110)

66 (113)

66 (113)

84 (144)

76 (131)

––

2.65 (0.096)

2.69 (0.097)

2.67 (0.097)

2.71 (0.098)

2.71 (0.098)

2.69 (0.097)

AverageCoefficient of

ThermalExpansion

x10-6

Melting RangeApprox.(1)

ThermalConductivity at

25°C.

ElectricalConductivity at

20°C (68°F),MS/m (%)

(Percent of Int’lAnnealed Copper

Standard)

Density

Notes:(1) Eutectic melting may be eliminated by homogenization(2) Typical per Aluminum Standards & Data, 1997

Alloy

5182-O

5454-O

5754-O

6009-T4

6111-T4

6022-T4

Resistance toGeneral

Corrosion

A

A

A

A

A

A

Formability

A

B

A

B

B

B

Fusion Weldability

A

A

A

B

B

B

Spot Weldability

C

B

C

A

A

A

A=Best B=Better C=GoodNotes:(1) Ratings are for original bare aluminum alloy sheet; ratings may vary dependent upon combination of forming and paint bake cycle.

21

A2 - Aluminum Extru s i o nA l l o y s

Aluminum extrusions in both the6xxx and 7xxx alloy series areroutinely used today in a widerange of automotive applications.

The compositions, typicalmechanical properties, typicalphysical properties, andcomparative characteristics of themost commonly used sheet alloysare presented in Tables 5 through 8.

For automotive space framestructures, however, the 6xxx (Al-Mg-Si) alloys are the preferredones due to ease of extrusion, good formability, excellentcorrosion resistance and goodw e l d a b i l i t y. These alloys provide

TABLE 5 CHEMICAL COMPOSITION LIMITSOF ALUMINUM EXTRUSION ALLOYS(1,2)

AAAlloyDesig-nation

6005

6005A

6061

6063

7004

7005

7029

7116

7129

0.60-0.9

0.50-0.9

0.40-0.8

0.20 -0.6

0.25

0.35

0.10

0.15

0.15

0.35

0.35

0.7

0.35

0.35

0.40

0.12

0.30

0.30

0.10

0.30

.015-0.40

0.10

0.05

0.10

0.50-0.9

0.50 -1.1

0.50-0.9

0.10

0.50

0.15

0.10

0.20-0.7

0.20-0.7

0.03

0.05

0.10

0.40-0.6

0.40-0.7

0.8-1.2

0.45-0.9

1.0-2.0

1.0-1.8

1.3-2.0

0.8-1.4

1.3-2.0

0.10

0.30

0.04-0.35

0.10

0.05

0.06-0.20

—

––

0.10

0.10

0.20

0.25

0.10

3.8-4.6

4.0-5.0

4.2-5.2

4.2-5.2

4.2-5.2

0.10

0.10

0.15

0.10

0.05

0.01-0.06

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.03

0.05

0.05

0.15

0.15

0.15

0.15

0.15

0.15

0.10

0.15

0.15

––

(4)

––

––

(5)

(6)

(7)

(7,8)

(7,8)

Si Fe Cu Mn Mg Cr Zn Ti OthersEach

OthersTotal

Note

Notes:(1) Maximum limit unless a range is shown(2) Shown as a percent; remainder is aluminum.(3) The sum of those “others” metallic elements: expressed to the second decimal place before determining sum.(4) Mn + Cr = 0.12-0.50(5) Zr = 0.10-0.20(6) Zr = 0.08-0.20(7) V = 0.05 max.(8) Ga = 0.03 max.

22

6005-T5(2)

6005A-T5(2)

6061-T6

6063-T5

6063-T6

7004-T5(2)

7005-T53(2)

7116-T5(2)

7029-T5(2)

7129-T5(2)

TABLE 6 TYPICAL MECHANICAL PROPERTIESALUMINUM EXTRUSION ALLOYS (1)

Alloy &Temper

MPa (ksi) MPa (ksi) % MPa (ksi) GPa (ksi) 103

305

305

310

185

240

400

395

360

430

430

44

44

45

27

35

58

57

52

62

62

270

270

275

145

215

340

350

315

380

380

39

39

40

21

31

49

50

46

55

55

12

12

12

12

12

15

15

14

15

14

200

200

205

115

150

220

225

200

270

270

29

29

30

17

22

32

32

29

39

39

69

69

69

69

69

72

72

70

70

70

10.0

10.0

10.0

10.0

10.0

10.4

10.4

10.2

10.2

10.2

UltimateTensile

Strength

Tensile YieldStrength

(0.2% offset)

Elongationin 50 mnor 2 in.

UltimateShear

Strength

Modulus ofElasticity,

Average forTension andCompression

Notes:(1) Not for design; represents typical for all products of these alloys(2) Tentative

good strength at low cost, arereadily formed in the T4 temperand yet can be aged to the T5 orT6 temper to give quite highstrengths. Of the commonlyproduced alloys, 6063 has thelowest strength, followed by 6005,6 0 0 5 A and 6061.

The most commonly used alloys in space frames for crash energ ymanagement are 6063, 6005A a n d6061. As with the 6xxx sheetmaterials, consideration must begiven to the thermal stability of

the 6xxx extrusions alloys whenused for the crash energ ymanagement structural members in locations where these will besubjected to elevated temperatureduring vehicle service. This canlead to changes in strength and, insome instances, to a tendency todevelop cracking upon impactcollapse. However, this problemcan be overcome by overaging thematerials to the T7 temper (e.g. 8 hr. at 210°C). This reducesthe strength level from the fullyage hardened condition (T6) but

improves the ductility, toughnessand minimizes any tendency tocrack on impact crushing whileproviding stable properties, evenwith long exposure to aboveambient temperatures.

It should be noted that thechemical composition limits forthese alloys are relatively wide and individual suppliers haveversions of these alloys andtempers optimized for automotivestructural applications.

23

TABLE 7 TYPICAL PHYSICAL PROPERTIESALUMINUM EXTRUSION ALLOYS

AlloyTemper

20° to 100°, per °C(68° to 212°, per °F)

°C (°F) W/M•k(BTU in/ft2•hr) °F

EqualVolume

EqualWeight

103 kg/m3

(lb/in3)

6005-T5(2)

6005A-T5(2)

6061-T6

6063-T5

6063-T6

7004-T5(2)

7005-T53(2)

7029-T5(2)

7116-T5(2)

7129-T5(2)

23.4 (13.0)

—

23.6 (13.1)

23.4 (13.0)

23.4 (13.0)

23.8 (13.2)

23.8 (13.2)

22.8 (12.6)

23.4 (13.0)

22.8 (12.6)

605-655(1)

(1125-1205)

—

580-650(1)

(1080-1205)

615-655(1140-1210)

615-655(1140-1210)

—

605-645(1125-1195)

—

—

—

188 (1310)

—

167 (1160)

209 (1450)

201 (1390)

––

—

163 (1130)

—

163 (1130)

28 (49)

—

25 (43)

33 (55)

32 (53)

––

22 (38)

25 (42)

27 (46)

25 (42)

93 (161)

—

82 (142)

105 (181)

102 (175)

––

72 (135)

77 (133)

86 (148)

77 (133)

2.70 (0.097)

2.70 (0.098)

2.70 (0.098)

2.70 (0.097)

2.70 (0.097)

2.77 (0.100)

2.77 (0.100)

2.77 (0.100)

2.78 (0.101)

2.78 (0.100)

AverageCoefficient of

ThermalExpansion

x10-6

Melting RangeApprox.(1)

ThermalConductivity at

25°C

ElectricalConductivity at

20°C (68°F),MS/m(%)

(Percent of Int’lAnnealed Copper

Standard)

Density

Notes:(1) Eutectic melting may be eliminated by homogenization(2) Tentative

24

TABLE 8 COMPARATIVE CHARACTERISTICS OF ALUMINUM EXTRUSION ALLOYS

Alloy Resistance toGeneral

Corrosion(1)

6005-T5(4)

6005A-T5(4)

6061-T6

6063-T5

6063-T6

7005-T53(4)

7029-T5(4)

7116-T5(4)

7129-T5(4)

A

B

B

A

A

C

C

C

C

Formability(2)

B-C

B-C

B-C

A-A

B-B

A-B

A-B

A-B

A-B

Fusion Weldability(3)

A

A

A

A

A

A

(5)

(5)

(5)

A=Best B=Better C=GoodNotes:(1) Ratings are for original bare extrusions; ratings may vary dependent upon combinations of alloy, temper and filler alloy for

welded structures. Alloys with A and B ratings can be used in industrial and seacoast environments without protection. Alloyswith C ratings should be protected.

(2) Ratings are consensus of formability experts from experience in forming extruded shapes, in decreasing order of merit from A toC. First letter compares alloys in their as-extruded temper (F) or immediately after heat treatment (W). The second comparesalloys in their standards hardened temper (T5, T53 or T6). These alloys naturally age harden at room temperature after extru-sion or solution heat treatment, so delay in subsequent forming may be critical.

(3) Ratings are consensus of Aluminum Association Welding & Joining Advisory Panel. Ratings assume use of recommended filleralloys and use of GMAW or GTAW procedures. A = Generally weldable by all commercial procedures and methods.B = Weldable with special technique only.

(4) Preliminary(5) Welding of 7029, 7116, and 7129 is not recommended. Use mechanical fasteners and/or adhesives.