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AHigh-StrengthAluminumAlloyFinStockforCABProducedUsingaTwin-RollStripCaster

50　 Furukawa-Sky Review No.4 �008

1.　INTRODUCTION

Since aluminum alloys have high specific strengthand good thermal conductivity , they are usedextensively in heat exchangers for automobiles.Recently,thedemandformakingaluminumalloyheatexchangers small and light has been strong becauseitwill improve the fuel efficiencyandperformanceofautomobiles. Tomeet the demand, it is essential tomakecomponentsofheatexchangersthinner.

Tomakethinnerfinswhicharethemaincomponentof heat exchangers, several properties of finsmustbe improved. The first ismechanical strength.Whenheat exchangers are used, tubes expand as a resultof internal pressure. If the fin stocks have insufficientmechanical strength, the finwill buckle and break,leading to breaking of tubes. The second is thermalconductivity. Tomaintain enough performance ofsmallerand lighterheatexchangers, it isnecessarytoincrease the cooling efficiency per unit of volume ofthe exchangers.Thus, the thermal conductivity of thefinstockmustbeincreased.Thethirdisthecorrosionresistance.When tubes are corroded to thepoint that

holes occur, the heat exchanger breaks down.Thus,the fin stock is designed to have a sacrificial effectso that the fins corrode first. However, corroded finslowerthecoolingefficiencyof theheatexchangerandexcessively corroded fins lead to the break down ofheat exchangers, because excessive corrosion lowerthemechanical strength of the fins. Therefore, finstocksthemselvesmusthavegoodcorrosionresistance.The fourth is brazeability. Themajority of heatexchangers aremanufactured using the controlledatmosphere brazing（CAB） process. The above-mentioned are related to post-brazing properties.But in-process properties, such as sag resistance anderosion resistance againstmolten fillermetals, areimportant too. Because parts of heat exchangers aretiedstronglybywiresduringthebrazingprocess,poorsagresistance leadstothebuckleof fins.Poorerosionresistancecausesdiffusionofthefillermetalsinsidethefins,andthediffusiondegradesthemechanicalstrengthof the fin stock.Generally, these in-processpropertiesaredegradedclearlywhenfinstocksaremadethinner.This paper explains the development concept

and properties of the new fin stock ofwhich above

技術論文

Go Kimura Akira Kawahara Akio Niikura Takeyoshi DokoTakahiro Shinoda Jonggag Kim Dinos Mantis

Abstract：We developed a new fin stock of an Al-Si-Fe-Mn-Zn alloy for automotive heatexchangers. The new stock has good properties suitable formaking thinner fins than the onesusedatpresent. Itshowsanultimatetensilestrengthofabout130MPaatathicknessof0.05mmafterbrazingandelectricalconductivityofabout50％IACS.Incontrast,conventionalAl-Mn（3000-series）alloy finstocksshowthestrengthofabout110MPaandelectricalconductivitybelow40％IACS.Therefore,thestrengthofthenewalloyfinstockismuchhigherthanthatoftypical3000-seriesalloy finstocks,while the thermalconductivityof the former iscomparabletothatofcommercialpurealuminumstocks.Inaddition,corrosionresistanceofthenewalloyfinstockisthesameasthatofthe3000-seriesalloyfinstocks.Thesepropertiesareattributabletoconstituentparticlesdispersedfinelyanddenselyinthenewalloyfinstocks.Thisalloyismanu-facturedbyusingatwin-rollcontinuouscaster.

A High-Strength Aluminum Alloy Fin Stock for CAB Produced Using a Twin-Roll Strip Caster



propertiesaregood.

�.　GENERAL FIN STOCKS

Genera l ly , commerc ia l pure a luminum and3000-series al loys are used in the bare fins ofautomotive heat exchangers. In thinner fin stocks,however , commerc ia l pure a luminum i s no tmechanically strong enough, and a 3000-series alloyshowinsufficientthermalconductivity（Table 1）.

Alloys that have both highmechanical strength andhigh thermal conductivity are not extensively usedas fin stocks. This is becausemany heat exchangersare produced using CAB process. In this brazingprocess, heat exchangers are heated up to about 600degrees C andmaterialswith amelting temperaturelower than 600degreesC areunusable.Materialswithhighmagnesium contents are also unusable, becausemagnesium decreases the effect of the non-corrosivefluxwhich plays an important role in CAB.Therefore,materials for fin stocks are limited. Since the brazingtemperatureishighasabove,propertiesoffinstocksareaffected during brazing process in severalways. First,the dissolution of alloy elements such asmanganese inthe aluminummatrix takes place and it lower thermalconductivity.Second,thestrengthofwork-hardenedfinsisdecreasedbyrecoveryandrecrystallizationduringthebrazing process. Third, penetration of themolten filleralloy along grain boundaries decreases the brazeabilityof fine grained fin stocks. Thus, it is rather difficultto obtain fin stocks having highmechanical strengthtogetherwithhighthermalconductivity.

3. FIN STOCK PRODUCTION USING A　CONTINUOUS CASTER

We deduced that dispersion strengthening usingconstituent particles and precipitates is themost

effectivemethod for giving both highmechanicalstrengthandhighthermalconductivitytothefinstocks.Moreover, adding alloy elements ofwhichmaximumsolid solubility in the aluminummatrix at 600 degreesC is smallmay contribute to alleviate the lowering ofthermal conductivity during the brazing process: itis because thermal conductivity of 3000-series alloysdecreaseduring thebrazingprocessdue todissolutionofmanganeseofwhichthemaximumsolidsolubility israther high.We noted that themaximum solubility ofironinbinaryAl-Fealloysislessthan0.05％1）andthesolubilityofnickelinbinaryAl-Nialloysisalsosmall2）.Therefore,we first selected anAl-Si-Fe-Ni-Zn alloyandinvestigateditsproperties3）,4）.However,whenwe used conventional direct chill

（DC）casting,additionofacertainamountofalloyingelements such as iron and nickel brought about asparse distribution of coarse constituent particlesunexpectedly. These coarse dispersoids hardlycontribute to dispersion strengthening, but theydeteriorate theworkability and sometimes causebreakingoffinstocks,andthisproblembecomesmoreseriousasfinstocksbecomethinner.

For this reason,we adopted a twin-roll continuous（TRC）casting.ThecoolingrateofDCcastingisfrom0.5 to 10 degrees C/sec, and that of TRC casting,from100 to 700degreesC/sec. Since the cooling rateof TRC casting is higher than that of DC casting,constituent particles becomesmuch finer and denserwhenusedTRCcasting.Fig.1 showsschematicviewsof the direct chill castingmachine-（a） and twin-rollcontinuous castingmachine-（b）, respectively. UsingTRCcasting,wedevelopedanAl-Si-Fe-Ni-Znalloyfinstockthathasbothhighmechanicalstrengthandhighthermal conductivity 5）. Themechanical strength ofthisfinstockismuchhigherthanthatofconventional3000-series al loy f in stocks, while the thermalconductivity of the former is comparable to that of

Table1Comparisonofchemicalcompositionsandpropertiesoffinstocks.Here,Al-Si-Fe-Ni-Znalloywaspreviouslydevelopedandthenewalloywasdevelopedlater.

Alloy Chemical composition（wt.％） Properties

Si Fe Cu Mn Ni ZnMechanical

strengthThermal

conductivityCorrosion resistance

AA1050 ＜0.25 ＜0.4 ＜0.05 ＜0.05 －＜0.05 Not good Good Good

AA3003＋Zn ＜0.6 ＜0.70.05

－0.21.0

－1.5－ 1.0

－2.5Good Not good Good

Al-Si-Fe-Ni-Zn 0.5 1.7 －－ 1.2 0.55 Good Good Not good

The new alloy 0.9 1.5 － 0.8 － 0.55 Good Good Good


5�　 Furukawa-Sky Review No.4 �008

commercial pureAl fin stocks. TheAl-Si-Fe-Ni-Znfinstockwithathicknessof0.06mm,hasbeenusedinradiatorssince1999 inJapan.Asfarasweknow,thisfin stock is the first onemanufactured by usingTRCcastingandappliedforradiatorsintheworld.

Unfortunately,thecorrosionresistanceoftheAl-Si-Fe-Ni alloy is inferior to that ofAl-Mn alloys. Thisloweredcorrosionresistance isdue to theconstituentparticleswhich are dispersed densely and finely intheAl-Si-Fe-Nialloy.Baseduponthenoblecorrosionpotentials ofAl-Fe,Al-Ni and Si compounds 6）, theparticles inAl-Si-Fe-Ni alloy are supposed to havemuch nobler corrosion potential than the aluminummatrix. The surfaces of the noble compounds shouldact as strong cathode for thematrix surface aroundthem and the anodic dissolution of thematrix shouldbe accelerated. Thinning of this fin stock furtherdegrades the reliability of radiator cores. In addition,nickel is hardly recyclable. Therefore, a new alloywhich does not contain nickel is desirable, thoughAl-Si-Fe-Ni-Zn alloy fin stock has highmechanicalstrengthandhighthermalconductivity.

4.　DESIGN OF A NEW ALLOY FIN STOCK

We investigated the effects of alloying elementshaving smallmaximum solubility on the corrosionresistance. In contrast to the compounds ofAl-Si-Fe-Ni alloy, the compounds ofAl-Mn alloys haveless noble corrosion potentials than the aluminummatrix7）anddon’ tacceleratethecorrosion.Moreover,manganese addition is reported to suppress thecorrosionaccelerationbytheironcontent8）.Therefore,we focused on the simultaneous addition of iron andmanganese. Themaximum solubility ofmanganese inthebinaryAl-Mnalloyisnearly1.0％at600degreesC,butthemaximumsolubilityofmanganeseinternary

Al-Fe-Mn alloys is reducedwhen the iron contentis increased 9）.We therefore investigatedAl-Si-Fe-Mn-Zn alloyswith high iron contents.As a result,we successfully developed a new alloy fin stock in2000 10）. Other fin stockmanufactured by usingTRCcasting has been reported in 2001 11）. Constituentparticles are dispersed finely and densely in the newalloywhenmanufactured by usingTRC casting, andthey improved both themechanical strength andthe thermal conductivity. Furthermore, the fin stockshowedgoodcorrosionresistance.

5. PROPERTIES OF THE NEW ALLOY FIN STOCK

We deve loped th i s new a l l oy f in s tock byinvestigating the effect of the chemical composition,casting and process conditions on properties ofthe alloy fin stocks. Since the developed fin stockmanufactured by using TRC casting has goodproperties,we used it tomake a thinner fin stock.Anexampleof chemical compositionof thenewalloyfin stock is shown inTable �.Wewill describe thepropertiesofthisfinstockinthefollowingsections.

5.1 Mechanical Strength and Thermal Conductivity

Fig.�showsanultimatetensilestrength（UTS）andelectricalconductivityof thenewalloy finstockafterbrazingprocessincomparisonwiththoseoffinstocksof 3000-series alloy（AA3003+1.5Zn）, commercialpure aluminum（AA1050） andAl-Si-Fe-Mn-Zn alloymanufacturedbyusingDCcasting,ofwhichchemical

b）a）

Mould

Roll

TipWater

Water Spray

Buttom block

Liquid Metal

Liquid Metal Solid Metal

Ingot

Solidification front

Solidification front

Fig.1 Schematicviewof （a）thedirectchillcastingmachineand （b）thetwin-rollcontinuouscastingmachine.

Table2Exampleofchemicalcompositionofthenewalloyfinstock（wt.％）.

Si Fe Mn Zn Al

0.9 1.5 0.8 0.55 Rem.

35

40

45

50

55

1401301201101009080Tensile strength（MPa）

1000-series alloy

3000-series alloy

Al-Si-Fe-Mn-Zn alloyproduced using a DC caster

Al-Si-Fe-Ni-Zn alloy

New alloy

Electrical conductivity（% IACS）

Fig.2 Relationshipbetweenultimatetensilestrengthandelectricalconductivityafterbrazing.



compositionisthesameasthenewalloy.Inthisfigure,weusedelectricalconductivityasanindexofthermalconductivity,because the twovaluesareproportional.The thickness of the new alloy fin stock is 0.05mmandthatoftheotherfinstockis0.06mm.UTS of the new alloy fin stockmanufactured by

usingTRCcastingisabout130MPa,andissuperiortothatof3000-seriesalloyfinstockandAl-Si-Fe-Mn-Znalloy fin stockmanufactured by usingDC casting. Inaddition,theelectricalconductivityofthenewalloyfinstockisabout50％IACSandnearlyequivalenttothatof the commercial pure aluminum fin stock.Thenewalloy finstockhasequivalentproperties to theAl-Si-Fe-Ni-Znalloyfinstockthatwedevelopedpreviously.The reasonwhy the new alloy fin stock has both

high tensile strength and high electrical conductivityisshownintheSEMimages（Fig.3）andTEMimages

（Fig.4）of fin stocks after brazing. Fine and denseconstituentparticlesareobservedinthenewalloyfinstock andAl-Si-Fe-Ni-Zn alloy fin stock. Dispersionstrengthening is increased as constituent particles

becomefineranddenser.Thus,thelevelofdispersionstrengthening in these fin stocks ishigher than thoseof 3000-series alloy fin stock and of the same alloyproducedbyDCcasting.Inaddition,thedegradationofelectricalconductivityissmaller,becausethemajorityof the alloy elements are contained in constituentparticlesand theamountofelementsdissolved in thealuminummatrixisrathersmall.In the 3000-series alloy fin stock, smaller amounts

of constituent particles are observed in Fig.3（d） andFig.4（c）: thismeans that the effect of dispersionstrengthening is small andmanganese atomswhicharenotcontainedinconstituentparticlesaredissolvedin aluminummatrix, leading to reduction of electricalconductivity of the alloy. In theAl-Si-Fe-Mn-Znalloy fin stockmanufactured by usingDC casting,coarse and sparse constituent particles are observedbecausethecoolingrateofDCcasting isslowerthanthat ofTRC casting.When the effect of dispersionstrengthening is smaller and dissolved amountsofmanganese and silicon increase, bothUTS andelectricalconductivitydecrease.

5.� Corrosion Resistance

Fig.5 shows the results of a copper acceleratedaceticacidsaltspray（CASS）testforaluminumsheetspecimens after 3 days or 7 days. In the CASS test,highweightlossmeanslowcorrosionresistanceoffinstocks.Corrosionresistanceofthenewalloyfinstockis approximately the same as that of the 3000-seriesalloyfinstock.Ontheotherhand,corrosionresistanceof Al-Si-Fe-Ni-Zn alloy fin stock, whose tensilestrengthandelectricalconductivityisthesameasthatofthenewalloyfinstock, ismuch lessthanother finstocks.Onlythenewalloyfinstockcanbeusedinthemass-productionof finstockswithathicknessof0.05

d）

b）a）

c）

10 µm

Fig.3 SEMimagesoffinstocksafterbrazing. a）thenewalloyfinstock. b）theAl-Si-Fe-Mn-Znalloyfinstockmadeusing

DCcasting. c）theAl-Si-Fe-Ni-Znalloyfinstock. d）the3000-seriesalloyfinstock.

b） c）

1 µm

a）

Fig.4 TEMimagesoffinstocksafterbrazing. a）thenewalloyfinstock. b）theAl-Si-Fe-Ni-Znalloyfinstock. c）the3000-seriesalloyfinstock.

0

5

10

15

20

25

30

35

40

45

50

new alloy fin stock

3000-series alloy fin stock

Al-Si-Fe-Ni-Zn alloy fin stock

Weight loss（％）

CASS 3 days

CASS 7 daysmore than 50 %

Fig.5 WeightlossoffinstocksafterCASStest.



mmandstillmaintainthereliabilityofradiatorcores.Zinc has been added to the new alloy fin stock to

createasacrificialanodeeffect.Theadditionalamountofzincwasdeterminedbythecorrosionpotentialofthetubes and plates thatwere brazedwith the fin stock.ThecorrosionresistanceofheatexchangersisthoughttobehighbasedontheresultoftheCASStest.

5.3 Brazeability

Fig.6 showsmacro structures of fin stocks of thenewalloy-（a）,theAl-Si-Fe-Ni-Znalloy-（b）,andofthe3000-seriesalloy-（c）afterbrazing.Thenewalloyfinstockhasrecrystallizedgrainsofwhichsizerangesfrom 0.3 to 2mm in diameter.Fig.7 shows the crosssectionofafin-tubejointafterbrazingat610degreesCforthreeminutes.Theconditionofthefin-tubejointsremained good in every fin stock. During the brazingprocessofaluminumalloyfinstocks,fillermetalerodesthe fin stock along the grain boundaries and the finstocks are segmented at the fin-tube joints. Thisphenomenon is explained by the diffusion ofmoltenfiller alloys along grain boundaries, and this degradesthe strength of heat exchangers. If the recrystallized

grains are too small, area fraction of grain boundaryincreases: asa resultgrainboundariesactasdiffusionpathsofthe filleralloysanderosionoccurseasily. Ifafin stock is thin,molten filler alloymaypenetrate thefinstockeasily.Thoughthethicknessofthenewalloyfin stock is only 0.05mm, the erosion of the fin stockwasnotobserved.Thismeanstherecrystallizedgrainsof the new alloy fin stock are sufficiently larger forpreventingtheerosionofmoltenmetal.

We then evaluated the sag resistance of the finstocks bymeasuring sag distance. To perform suchmeasurements, fin stockswere overhung by 50mmas shown inFig.8（a）, heated to 600 degrees C andheld at that temperature for 5minutes.A small sagdistance of a fin stockmeans that the fin stockhas afavorable sag resistance. Evaluated sag distances ofevery fin stockare shown inFig.8（b）.Generally, sagresistancedegradesasthethicknessdecreasesandtherecrystallizedgrainbecomessmaller12）.However, sagresistance of the new alloy fin stock is comparable tothat of the other fin stocks, though the new alloy finstockisthinnerthantheotherfinstocks.

In conclusion, the brazeability of the new alloy finstock isgood, thoughthe thicknessof the finstock isonly0.05mm.

6.　CONCLUSIONS

We developed a new fin stock of anAl-Si-Fe-Mn-Zn alloy for automotive heat exchangers.The newalloyismanufactured by using a twin-roll continuous caster.This produced alloys having fine and dense constituentparticles.Using thiscasterenabledus toproduceanewalloyfinstockthathasbothhightensilestrengthandhighelectricalconductivity.Thenewalloyfinstockshowsanultimatetensilestrengthofabout130MPaatathicknessof 0.05mmafter the brazing process , and its electricalconductivityisabout50％IACS.Thestrengthofthenewalloy fin stock ismuch higher than that of conventional3000-ser ies a l loy f in s tocks , whi le the thermalconductivity of the new alloy fin stock is comparableto that of commercial pureAl fin stocks. Corrosionresistanceofthenewalloyfinstockisapproximatelythesameasthatof3000-seriesalloyfinstocks.Thenewalloyfinstockalsoexhibitsgoodin-processpropertiessuchassagresistanceanderosionresistanceagainstmoltenfillermetals.

b）

a）

c）

10 mm

Fig.6 Macrostructureoffinstocksafterbrazing. a）newalloyfinstock. b）Al-Si-Fe-Ni-Znalloyfinstock. c）3000-seriesalloyfinstock.

a） c）b）0.3 mm

Fig.7 Crosssectionofafin-tubejoint. a）thenewalloyfinstock. b）theAl-Si-Fe-Ni-Znalloyfinstock. c）the3000-seriesalloyfinstock.



The above propertiesmake it very suitable formanufacturing thinner fin stocks than the one usedat present. The new alloy fin stockwith a thicknessof 0.05mm has been used for fins of radiators. Thefin stock can be used to produce lighter and smalleraluminum heat exchangers. The casting of the newalloy is performed byChoilAluminumCo., LTD andELVALS.A.（HellenicAluminiumIndustryS.A.）.

Thispaperwasmadebyaddingapostscripttothepaper published at international conferenceVTMS8（Vehicle ThermalManagement Systems, 20-24May2007）.Copyrightwasgiven to the InstitutionofMechanicalEngineers.

REFERENCES

1） J.K.Edger:Trans.Amer.Inst.Min.Met.Eng.,180（1949）,225.2） R.Krsniketal.:SolidStateComun.,12（1973）,891.3） A.Niikura,T.Doko,K.Ishikawa:FurukawaReview,17（1998）,

101（publishedbyTheFurukawaElectricCo.,Ltd.inEnglish）.4） K.Takeuchi, J. Fukuda, T. Shinpo, T. Doko,A.Niikura:

MateriaJapan,39（2000）,81（inJapanese）.5） A.Kawahawa,A.Niikura, T. Doko, J. Kim,K. Negura:

Vehicle Thermal Management System ConferenceProceedings,2003,415.

6） J.R.Davis:“CorrosionofAluminumandAluminumAlloys”,ASM,1999,29.

7） K.Tohma:J.JapanInst.Metals,46（1982）,973.8） M.Zamin:Corrosion,37（1981）,627.9） H.W.L.PHILLIPS, M.A. , F.R.I　.C. , F.Inst .P . , F.I .M. :

“Annotated equilibrium diagrams of some aluminum alloysystem”;Theinstituteofmetals,1959,10,49.

10） FurukawaElectricCo.,Ltd.:Patent,JP02/241910,2002.11） AlcanInternationalLimited:Patent,WO01/53553,2001.12） T.Doko,H.Takeuchi,K.IshikawaandS.Asami:Furukawa

Review,8（1991）,47,（publishedbyTheFurukawaElectricCo.,Ltd.inEnglish）.

0

5

10

15

20

25

30

40

35

New alloyfin stock

Al-Si-Fe-Ni-Znalloy fin stock

3000-seriesalloy fin stock

Sag distance（mm）

a） b）

50 mm

ajig

sag（mm）

Fig.8 a）Sagtestapparatus（arrowaindicatesaspecimen） b）Sagdistanceofthefinstocks.

GoKimuraTechnicalResearchDivision.

AkiraKawaharaTechnicalResearchDivision.

AkioNiikuraTechnicalResearchDivision.

TakeyoshiDokoTechnicalResearchDivision.

TakahiroShinodaDENSOCorporation,Japan

JonggagKimChoilAluminumCo.,LTD.,Korea

DinosMantisELVALS. A.（Hellenic Aluminium Industry S. A.）,Greece