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Development of Manufacturing Technology for Precision Compressor Wheel Castings for Turbochargers

by Takayuki Sotome and Shoichi Sakoda*

Shaft

Compressor wheel Turbine

Intake Exhaust

Figure 1 Cross section of typical turbocharger.

25

20

15

10

0

2004 2005 2006 2007 2008Year

Millions of vehicles per year

Figure 2 Forecast of diesel vehicle production. * Casting&ForgingDiv.,Furukawa-SkyAluminum

1. INTRODUCTION

Analuminumalloycastcompressorwheel,alsoknownasacompressorimpeller,isthemaincomponentofaturbo-charger. Since 1965 we have manufactured compressorwheelsatourcastingplantatOyama,TochigiPrefecture,delivering high-quality products that meet the needs ofourcustomers.

Figure1showsacrosssectionofatypicalturbocharg-er,comprisingacompressorwheel,aturbineandahous-ing,with thecompressorwheeland the turbinecoupledtoasingleshaft.Exhaustgas from theengine turns theturbine, causing the compressor wheel mounted at theengine intakealso to turnandsupplycompressedair totheengine.

Because their exhaust is black, diesel vehicles have

tended inJapan tobesaddledwith the imageofhavinganadverseeffectontheenvironment,butinEuropeandothercountries themanyadvantages in termsofecono-my,includingthecostoffuel,andlowlevelsofCO2emis-sionhave led tosteadygrowth in thedemand fordieselvehicles,asshowninFigure2 1).

In addition, with stricter environmental regulations, ithasbecomenecessaryfordieselvehiclestobefittedwithturbochargers,leadingtoastrongincreaseindemandforthem.

On the other hand, as turbocharger performance hasimproved,customerexpectationsintermsoflevelsofquali-tyhave risenenormously.Because they turnat speeds inexcess of 150,000 rpm, there is a need not only for highdurability,butalsoforhighdimensionalaccuracy,andpar-ticularlyforproductcoaxiality,alsoknownasinitialbalance.

Toaddresstheseuserneeds,FurukawaSky-Aluminum

Recently,thedemandforturbochargersforautomobilesisincreasingworldwidebecauseof theemission regulations inEuropeand theexpansionof theAsian

carmarket.Thecompressorwheelisthemainunitofaturbocharger,andnotonlyhighdurabili-ty,butalsohighdimensionalaccuracy,isrequiredofitbycustomers.Themostimportantprop-erty required is the initial balance (i.e., coaxiality). We have developed a new manufacturingtechnologyatourcastingplanttoimprovetheinitialbalance.

ABSTRACT

Furukawa Review, No. 32 2007 56

Furukawa-Sky Aluminum

2.3 Rubber Die FabricationArubberdiehavingashapeidentical tothecompressorwheel is fabricated by filling the female mold formed inthe previous process with a silicone rubber that sets atroomtemperature.

2.4 Plaster Die FabricationPlaster is an inorganic substance which, when water isadded and stirred, is simply poured in, and hardens by ahydratedcementationreaction.Therubbermoldisplacedinaframeandfilledwithaslurryofwaterandplaster,andtherubbermoldisremovedtoproduceaplastercastingdie.

2.5 Drying of Plaster ModelAs shown in Table 1, cemented plaster can be used as acastingdieforthecastingprocessbyheatinganddryingtoremove the water of crystallization, transforming it fromplaster dihydrate to anhydrous plaster. The final tempera-ture of a plaster drying furnace is generally between 220and230°C 2), a temperatureatwhich theprocessofdehy-drationtoproduceanhydrousplasterisdefinitelycomplete.

2.6 Casting ProcessFigure4shows theapparatus for the low-pressurecast-ing process that we use. Air pressure is applied to thesurface of the melt that is contained and heated in asealed vessel in the lower portion of the plaster castingmold, forcing the melt through a stalk connected to themoldandinsertedintothemeltsothatitflowsupintothe

has developed new manufacturing technology thatachievesdramaticimprovementsininitialbalance.

2. PROCESS FOR MANUFACTURE OF COMPRESSOR WHEELS

Figure3showsthevarietyofcompressorwheelsthatwemanufacture.Theseproductsarecharacterizedbybladeshaving a spiral configuration and a smooth product sur-face.

Wemanufacture thecompressorwheelcastingsusingplaster dies and a low-pressure casting method. This isknownas thePPCmethod,standing forprecision (low-)pressurecasting,usingplasterdies.

2.1 Outline of Manufacturing ProcessTheprocessforthemanufactureofcompressorwheelsisshownbelow.Except in certain cases,wedeliver toourcustomer (the turbocharger manufacturer) the productblankspriortofinalmachining.

Diedesign (1) Diedesign (2) Fabricationofaluminumalloymastermodel (3) Mastermodelreversal (4) Rubbermoldforming (5) Plasterdiefabrication

Productmanufacture (6) Dryingofplasterdie (7) Castingprocess (8) Finishingprocesses(gatecutting,separatingC/W

castingfromplasterdie) (9) Heattreatmentprocess(dissolving)(10) Bladecorrection(11) Heattreatmentprocess(artificialaging)(12) Inspectionprocess:shipmentofproductblanks(13) Finalmachiningandbalancecutting

2.2 Master Model ReversalArubberfemalemoldisformedbasedonamastermodelhavingashapeidenticaltotheproduct.

Figure 3 Selected compressor wheels.

Table 1 Plaster transformation by water mixing and heating.

Temperature FormofplasterMolecularformula

Note

Rm.temp. Hemihydrateplaster CaSO4・1/2H2OPowderbefore

mixingwithwater

Rm.temp.Dihydrateplasterincluding

surpluswater CaSO4・2H2OCementationaftermixingwithwater

100°C Evaporationofsurpluswater CaSO4・2H2O

Approx.120°C

Dehydrationfromdihydratetohemihydrateplaster CaSO4・1/2H2O

Approx.200°C

Dehydrationfromhemihy-dratetoanhydrousplaster

CaSO4Castcasting

mold

Lid

Air Stalk

Flow of melt

Plastercasting mold

Figure 4 Apparatus for low-pressure casting.



mold.Comparedtogravitycasting,thepressuremethodhas the effect of reducing shrinkage cavities and otherinternalvoids.

To balance strength and ease of casting, the materialused for the compressor wheel is normally an Al-Si-Cu-Mgalloy.

2.7 Final Machining and Balance Cut ProcessThecompressorwheelblanksthatwehavemanufacturedarefurthersubjectedbyourcustomerstofinalmachiningandbalancecutting,and they thenassemble themontotheshaft.

2.7.1 FinalMachiningIn order to satisfy extremely stringent dimensional toler-ances,thecompressorwheelsaremachinedtotheirfinalconfiguration.

AsFigure5shows,thisprocessinvolvesthemachiningof the shaft hole and the cutting of the top, bottom andlateralsurfaces.

2.7.2 BalanceCuttingBalance cutting is an extremely important process for aproductlikethis,whichoperatesathighrotationalspeeds,andinvolvesremovingmaterialfromtheupperportionofthecompressorwheel(referredtoastheboss)andfromthelowersurfacetoachieverotationalbalance.

Dependingontheproductthereisanupperlimittothe

weight, position and number, depth and length of thematerialtoberemoved,andifthisupperlimitisexceededtheproductmustberejected.Balancecuttingisgenerallycarriedoutinaseriesofiterations.

3. INITIAL BALANCE OF COMPRESSOR WHEEL BLANKS

Theaforesaidbalancecuttingprocessinproductsforfinalmachining requires not only a good machining methodand machining accuracy, but also good balance in thecompressorwheelblank(i.e.,initialbalance).Thatistosayiftheblankhasgoodinitialbalance,theamountofmaterialtoberemovedinbalancecuttingislessandthenumberofiterationsofthebalancecuttingprocesscanbereduced.

3.1 Evaluating Initial BalanceTheinitialbalanceofacompressorwheelisevaluatedintermsofcoaxiality,measuringtheamountofrunoutintheproduct using a 3-dimensional tester. As expressed inEquation(1), theamountof runoutrepresents thedevia-tionwithrespecttothecenterofthebossoftheproductwith reference to the center of the lower surface of theproduct.CoaxialityAistherelationshipexpressedby:

xA= 2 y2

where: xisrunout(x-axisdeviationinmm)

yisrunout(y-axisdeviationinmm)

Figure6showsasimplifiedcrosssectionof theprod-uct,wheretheamountofrunoutasdescribedaboveisanindicator forevaluatingtheeccentricitybetweenthecen-terof the lowersurfaceof theproductand thecenteroftheboss(thearrowinthefigure).

3.2 Initial Balance before ImprovementFigure7showsthedistributionofrunoutandthecoaxiali-tyofthecompressorwheelblankbeforeimprovement.It

Lower surface cutting

Top surface cutting andshaft hole machining

Lateral surfacecutting

Figure 5 Schematic of final machining of compressor wheel.

Boss Eccentricity

Figure 6 Cross section of compressor wheel.

N=30; Ave. concentricity = 0.060 mm; Std. deviation = 0.028mm

Figure 7 Distribution of product runout before improvement of balance.



canbeseenthattherearecasesinwhichmaximumcon-centricityexceeds0.1mm,andthattherearelargedevia-tions.

4. TECHNIQUES FOR IMPROVING INITIAL BALANCE

The keys to improving initial balance are increasing diereversing accuracy and reducing the variation in thedimensionsofthedieitself.Herewedescribetheresultsof investigation into the master model reversing tech-niquesthatyieldedthegreatestimprovement.

4.1 Investigation of Methods of Master Model Reversal

We have developed a new reversal method to contrastwith the method that we have used in the past. Table 2comparesthetwomethods.

4.1.1 ExistingReversingMethod:SplitMoldThe technology we have been using to fabricate thefemale mold for forming the rubber die made use of asplitmold.AscanbeseenfromFigure8thisistherubberfemale mold for which portions corresponding to eachindividualbladeweremachinedandassembled.

This technology requiredaveryhighdegreeofskill inthemachiningandassemblyof themolds,andsufferedfromtheproblemthatthejoinsofthedividedmoldweretransferred to theproduct,adverselyaffecting itssurfacequality. For these reasons the dimensional accuracy ofthis method was strongly influenced by the skill of theoperator,anddeviationsininitialbalancewerealsolarge.The distribution of product runout using this reversingmethodisasshowninFigure8.

4.1.2 InvestigationofImprovedReversingMethod:ElectroplateReversal

As shown in Figure 9, this method involves plating themaster model, after which, by dissolving the mastermodel, a rubber female mold is formed. This methodoffers the highest reversing accuracy, but requires anextremelyhightechnologicallevel.

Specifically the plating technology requires jointresearch to be conducted with the plating shop andrepeated trial and error. During electroplating, currentflows preferentially in the leading end, so there was aproblem of inability to plate the master model. Howeverbydeterminingappropriateplatingconditionsithasbeenpossible to establish technology for the fabrication of arubberfemalemoldthatcanstanduptorubberdiecast-ing.

Figure10showsthecoaxialityandtherunoutdistribu-tion for a casting fabricated by electroplating reversal. Itcan be seen that coaxiality and standard deviation aregreatlyimprovedincomparisonwithFigure7.

4.2 The Effect of Products with Improved Initial Balance

Wethusseethatbyinvestigatingareversingmethodthat

Table 2 Comparison of two methods of master model reversing.

Splitmold(existing)

Electroplatereversing(improved)

Mastermodel Remains Dissolvedaway

Reversingaccuracy Dependsonskillofoperator Excellent

Technologicallevel High High

Qualityofproductsurface

Inferior Excellent

(fabrication by assembling individual blades)（翼一枚ずつを組合せて作製）

Figure 8 Split mold reversing method (existing).

(electroplating of the master model, followed by dissolving of the master)

Figure 9 Electroplate reversing method (improved).

N=30; Ave. concentricity = 0.016 mm; Std. deviation = 0.008mm

Figure 10 Distribution of initial runout of plated product.



remains true to the master model we have developed anew electroplating reversal method and have been abletoimprovetheinitialbalanceofcompressorwheels.

We have received information that compared to theexistingproduct, this improvedproducthasdramaticallylower rejection rates for balance of the finally machinedproduct,andsmallervariationsfromproductionlottopro-ductionlotintermsofinitialbalance.

5. CONCLUSIONWiththeongoingtrendstosmallersizeandhigherpowerfor passenger and commercial vehicle superchargers,therewillbeaneed forcompressorwheels towithstandhigherspeedsandtemperaturesthanatpresent.

Inadditiontotheinitialbalancediscussedinthispaper,there are problems for the future in terms of improvedfatiguestrengthandimprovedhightemperaturestrength.

Meanwhileonthesupplyside,inanefforttorespondtostrong consumer demand for increased production, wehave established a plant in Vietnam and are in the pro-cess of shifting the focus of manufacture from Japan tothatcountry.

Weassureyou thatboth inqualityandvolumewewillcontinue to respond to customers’ needs, and will con-tribute to improving the performance of compressorwheels.

REFERENCES

1) Toyo Keizai Weekly, August 27, 2005, Toyo Keizai Shimbunsha. (in Japanese)

2) Precision Casting, 234, 1981, Japan Foundry Society. (in Japanese)



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