Furukawa-Sky Aluminum

Development of Manufacturing Technology for Precision
Compressor Wheel Castings for Turbochargers
by Takayuki Sotome and Shoichi Sakoda*

Recently, the demand for turbo chargers for automobiles is increasing worldwide
because of the emission regulations in Europe and the expansion of the Asiancar market. The compressor wheel is the main unit of a turbo charger, and not only high durability, but also high dimensional accuracy, is required of it by customers. The most important property required is the initial balance (i.e., coaxiality). We have developed a new manufacturingtechnology at our casting plant to improve the initial balance.

ABSTRACTINTRODUCTION

An aluminum alloy cast compressor wheel, also known asa compressor impeller, is the main component of a turbocharger. Since 1965 we have manufactured compressor
wheels at our casting plant at Oyama, Tochigi Prefecture,delivering high-quality products that meet the needs ofour customers.
Figure 1 shows a cross section of a typical turbocharger, comprising a compressor wheel, a turbine and a housing, with the compressor wheel and the turbine coupled
to a single shaft. Exhaust gas from the engine turns theturbine, causing the compressor wheel mounted at theengine intake also to turn and supply compressed air tothe engine.
Because their exhaust is black, diesel vehicles haveCompressor wheelTurbine

ExhaustIntake

Shaft

Figure 1tended in Japan to be saddled with the image of having
an adverse effect on the environment, but in Europe andother countries the many advantages in terms of economy, including the cost of fuel, and low levels of CO2 emission have led to steady growth in the demand for dieselvehicles, as shown in Figure 2 1).
In addition, with stricter environmental regulations, it
has become necessary for diesel vehicles to be fitted withturbochargers, leading to a strong increase in demand forthem.
On the other hand, as turbocharger performance has
improved, customer expectations in terms of levels of quality have risen enormously. Because they turn at speeds inexcess of 150,000 rpm, there is a need not only for highdurability, but also for high dimensional accuracy, and particularly for product coaxiality, also known as initial balance.
To address these user needs, Furukawa Sky-Aluminum

Cross section of typical turbocharger.

* Casting & Forging Div., Furukawa-Sky Aluminum

25

Millions of vehicles per year

1.

20

15

100

2004

Figure 2

2005

2006

2007

2008
Year

Forecast of diesel vehicle production.

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Development of Manufacturing Technology for Precision Compressor Wheel Castings for Turbochargers

has developed new manufacturing technology thatachieves dramatic improvements in initial balance.

2.

PROCESS FOR MANUFACTURE OF
COMPRESSOR WHEELS

Figure 3 shows the variety of compressor wheels that wemanufacture. These products are characterized by bladeshaving a spiral configuration and a smooth product surface.
We manufacture the compressor wheel castings using
plaster dies and a low-pressure casting method. This isknown as the PPC method, standing for precision (low-)pressure casting, using plaster dies.
2.1 Outline of Manufacturing Process
The process for the manufacture of compressor wheels isshown below. Except in certain cases, we deliver to ourcustomer (the turbocharger manufacturer) the productblanks prior to final machining.
Die design(1) Die design(2) Fabrication of aluminum alloy master model(3) Master model reversal(4) Rubber mold forming(5) Plaster die fabrication
Product manufacture(6) Drying of plaster die(7) Casting process(8) Finishing processes (gate cutting, separating C/W
casting from plaster die)(9) Heat treatment process (dissolving)(10) Blade correction(11) Heat treatment process (artificial aging)(12) Inspection process: shipment of product blanks(13) Final machining and balance cutting
2.2 Master Model Reversal
A rubber female mold is formed based on a master model
having a shape identical to the product.

2.3 Rubber Die Fabrication
A rubber die having a shape identical to the compressor
wheel is fabricated by filling the female mold formed in
the previous process with a silicone rubber that sets atroom temperature.
2.4 Plaster Die Fabrication
Plaster is an inorganic substance which, when water isadded and stirred, is simply poured in, and hardens by ahydrated cementation reaction. The rubber mold is placed ina frame and filled with a slurry of water and plaster, and therubber mold is removed to produce a plaster casting die.
2.5 Drying of Plaster ModelAs shown in Table 1, cemented plaster can be used as acasting die for the casting process by heating and drying toremove the water of crystallization, transforming it fromplaster dihydrate to anhydrous plaster. The final temperature of a plaster drying furnace is generally between 220and 230°C 2), a temperature at which the process of dehydration to produce anhydrous plaster is definitely complete.
2.6 Casting Process
Figure 4 shows the apparatus for the low-pressure casting 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 casting
mold, forcing the melt through a stalk connected to themold and inserted into the melt so that it flows up into theTable 1

Plaster transformation by water mixing and heating.

Temperature

Form of plaster

MolecularformulaNote

Rm. temp.

Hemihydrate plaster

CaSO4・1/2H2O

Powder before
mixing with water

Rm. temp.

Dihydrate plaster including
surplus water

CaSO4・2H2O

Cementation
after mixing
with water

100°C

Evaporation of surplus water CaSO4・2H2O

Approx.
120°C

Dehydration from dihydrate
CaSO4・1/2H2O
to hemihydrate plaster

Approx.
200°C

Dehydration from hemihydrate to anhydrous plasterCast casting
moldCaSO4

Plastercasting mold

Air

LidStalk

Flow of melt

Figure 3

Selected compressor wheels.

Figure 4

Apparatus for low-pressure casting.

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Development of Manufacturing Technology for Precision Compressor Wheel Castings for Turbochargers

mold. Compared to gravity casting, the pressure method
has the effect of reducing shrinkage cavities and otherinternal voids.
To balance strength and ease of casting, the material
used for the compressor wheel is normally an Al-Si-CuMg alloy.
2.7 Final Machining and Balance Cut Process
The compressor wheel blanks that we have manufactured
are further subjected by our customers to final machining
and balance cutting, and they then assemble them onto
the shaft.
2.7.1 Final Machining
In order to satisfy extremely stringent dimensional tolerances, the compressor wheels are machined to their final
configuration.
As Figure 5 shows, this process involves the machiningof the shaft hole and the cutting of the top, bottom andlateral surfaces.
2.7.2 Balance Cutting
Balance cutting is an extremely important process for a
product like this, which operates at high rotational speeds,
and involves removing material from the upper portion of
the compressor wheel (referred to as the boss) and fromthe lower surface to achieve rotational balance.
Depending on the product there is an upper limit to theweight, position and number, depth and length of thematerial to be removed, and if this upper limit is exceeded
the product must be rejected. Balance cutting is generally
carried out in a series of iterations.

3.

INITIAL BALANCE OF COMPRESSORWHEEL BLANKS

The aforesaid balance cutting process in products for finalmachining requires not only a good machining method
and machining accuracy, but also good balance in thecompressor wheel blank (i.e., initial balance). That is to say
if the blank has good initial balance, the amount of material
to be removed in balance cutting is less and the number of
iterations of the balance cutting process can be reduced.
3.1 Evaluating Initial Balance
The initial balance of a compressor wheel is evaluated interms of coaxiality, measuring the amount of runout in theproduct using a 3-dimensional tester. As expressed inEquation (1), the amount of runout represents the deviation with respect to the center of the boss of the productwith reference to the center of the lower surface of theproduct. Coaxiality A is the relationship expressed by:
A= x 2 y 2
where: x is runout (x-axis deviation in mm)
y is runout (y-axis deviation in mm)

Figure 6 shows a simplified cross section of the product, where the amount of runout as described above is anindicator for evaluating the eccentricity between the center of the lower surface of the product and the center ofthe boss (the arrow in the figure).

Top surface cutting andshaft hole machining

Lateral surface
cutting

3.2 Initial Balance before Improvement
Figure 7 shows the distribution of runout and the coaxiality of the compressor wheel blank before improvement. ItLower surface cutting

Figure 5

Schematic of final machining of compressor wheel.

Eccentricity

Boss

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

Figure 6

Cross section of compressor wheel.
Figure 7

Distribution of product runout before improvement ofbalance.

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Development of Manufacturing Technology for Precision Compressor Wheel Castings for Turbochargers

can be seen that there are cases in which maximum concentricity exceeds 0.1 mm, and that there are large deviations.

4.

TECHNIQUES FOR IMPROVING INITIALBALANCEThe keys to improving initial balance are increasing die
reversing accuracy and reducing the variation in thedimensions of the die itself. Here we describe the resultsof investigation into the master model reversing techniques that yielded the greatest improvement.
4.1

Investigation of Methods of Master Model
Reversal
We have developed a new reversal method to contrast
with the method that we have used in the past. Table 2
compares the two methods.
4.1.1 Existing Reversing Method: Split Mold
The technology we have been using to fabricate thefemale mold for forming the rubber die made use of a
split mold. As can be seen from Figure 8 this is the rubber
female mold for which portions corresponding to eachindividual blade were machined and assembled.
This technology required a very high degree of skill inthe machining and assembly of the molds, and sufferedfrom the problem that the joins of the divided mold weretransferred to the product, adversely affecting its surface
quality. For these reasons the dimensional accuracy ofthis method was strongly influenced by the skill of theoperator, and deviations in initial balance were also large.
The distribution of product runout using this reversing
method is as shown in Figure 8.

4.1.2

Investigation of Improved Reversing Method:
Electroplate Reversal
As shown in Figure 9, this method involves plating themaster model, after which, by dissolving the mastermodel, a rubber female mold is formed. This method
offers the highest reversing accuracy, but requires anextremely high technological level.
Specifically the plating technology requires joint
research to be conducted with the plating shop andrepeated trial and error. During electroplating, current
flows preferentially in the leading end, so there was a
problem of inability to plate the master model. However
by determining appropriate plating conditions it has beenpossible to establish technology for the fabrication of arubber female mold that can stand up to rubber die casting.
Figure 10 shows the coaxiality and the runout distribution for a casting fabricated by electroplating reversal. It
can be seen that coaxiality and standard deviation are
greatly improved in comparison with Figure 7.
4.2

The Effect of Products with Improved InitialBalance
We thus see that by investigating a reversing method that(electroplating of the master model, followed by dissolving of the master)

Table 2Comparison of two methods of master model reversing.
Split mold(existing)Electroplate reversing(improved)

Master model

Remains

Dissolved away

Reversing accuracy

Depends on skill of operator

ExcellentTechnological levelHighHigh

Quality of productsurface

Inferior

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

Figure 8

Split mold reversing method (existing).

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.

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remains true to the master model we have developed a
new electroplating reversal method and have been able
to improve the initial balance of compressor wheels.
We have received information that compared to theexisting product, this improved product has dramaticallylower rejection rates for balance of the finally machined
product, and smaller variations from production lot to production lot in terms of initial balance.
5.
CONCLUSION
With the ongoing trends to smaller size and higher powerfor passenger and commercial vehicle superchargers,
there will be a need for compressor wheels to withstandhigher speeds and temperatures than at present.
In addition to the initial balance discussed in this paper,
there are problems for the future in terms of improved
fatigue strength and improved high temperature strength.

Meanwhile on the supply side, in an effort to respond tostrong consumer demand for increased production, wehave established a plant in Vietnam and are in the process of shifting the focus of manufacture from Japan tothat country.
We assure you that both in quality and volume we willcontinue to respond to customers’ needs, and will contribute to improving the performance of compressor
wheels.

REFERENCES
1) Toyo Keizai Weekly, August 27, 2005, Toyo Keizai Shimbunsha. (inJapanese)
2) Precision Casting, 234, 1981, Japan Foundry Society. (in Japanese)

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