microstructure and mechanical properties of precision cast tial turbocharger wheel

Journal of Materials Processing Technology 167 (2005) 14–21

Microstructure and mechanical properties of precisioncast TiAl turbocharger wheel

M.T. Jovanovic∗, B. Dimcic, I. Bobic, S. Zec, V. MaksimovicDepartment of Materials Science, Institute of Nuclear Sciences “Vinˇca”, 11001 Belgrade, P.O. Box 522, Serbia and Montenegro

Received 24 August 2004; accepted 11 March 2005

Abstract

From a technical perspective, many of the unique properties of TiAl-based aluminides making them attractive for high-temperature structuralapplication also assign these alloys a challenge to process into useful products. Cast TiAl alloys due to their relatively low production costare on the verge of a commercial application, especially in vehicle industry. The results concerning technology of precision casting togetherwith microstructural and mechanical tests examinations of a turbocharger TiAl-based wheel prototype have been described. The processingtechnology of “self-supporting” ceramic shell molds was successfully verified during precision casting of turbocharger wheels. According toall results, a processing window for precision casting was established.©

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2005 Elsevier B.V. All rights reserved.

eywords:TiAl; “Self-supporting” ceramic mold; Precision casting; Microstructure; Tensile properties; Processing window

. Introduction

TiAl-based aluminides (“� alloys”) represent a goodxample of how fundamental and applied research can leado a new class of advanced engineering materials. Theselloys with two-phase structure consisting of the major�hase (ordered TiAl face-centred-tetragonal with L10 crystaltructure) and minor�2 phase (ordered Ti3Al close-packed-exagonal with DO19 structure) were intensively studiedithin the last decade. Due to their attractive properties,alloys are considered for high-temperature application in

he aerospace and automotive industries. These propertiesnclude low density (3.8 g/cm3), less than half of that ofuperalloys and comparable to 3.2 g/cm3 of heat resistanteramics, such as Si3N4 and SiC. They are nearly equal toi-based superalloys for turbine blades (Inconel 713C) inpecific tensile strength (tensile strength versus density) andpecific creep strength (creep strength versus density) overemperatures from 20 to 1000◦C, but they are slightly infe-ior to superalloys in oxidation resistance above 700◦C. Itas reported that the commercial diesel-truck-size gamma

wheels have more than doubled fatigue life than the cmercial Inconel 713LC superalloy[1]. However, TiAl alloysdo not show appreciable room temperature elongation wmaximum 3% for cast alloys[2–5]. There were some efforto introduce TiAl castings for the fourth stage compressobine blade in jet engines, but further development was coff due to insufficient reliability resulting from low tempeture elongation, in spite of other good attributes[3].

In order to improve acceleration and to reduce the amof harmful substances in the exhaust gases of passengecles the improvement of turbocharger properties is hieffective. The simplest way to do this is the applicaof lightweight materials for the turbine wheel. Howevas turbines are subjected to long-term exposure totemperature exhaust gases (at least at 850◦C), heat resistancis an essential prerequisite for turbine wheel material. Tainto account these facts, conventional lightweight matesuch as aluminum and titanium alloys, cannot be used fobocharger production. In contrast to the tremendously crrequirements for jet engines, it would be far easier to contrate efforts on technology and application of TiAl alloys

∗ Corresponding author. Tel.: +38 111 2439 454.E-mail address:[email protected] (M.T. Jovanovic).

passenger vehicles. A turbocharger is a part of the enginein which heat energy from engine exhaust gas turns a tur-bine along with a compressor on the same axis, such that

924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
oi:10.1016/j.jmatprotec.2005.03.019

M.T. Jovanovi´c et al. / Journal of Materials Processing Technology 167 (2005) 14–21 15

inflow air is pressurized by the compressor and supplied tothe engine, thereby improving the engine’s combustion effi-ciency[6]. Advantages to be obtained through the applicationof TiAl turbocharger wheel to a passenger vehicle are asfollows:

- specific power increase which improves vehicle perfor-mance;

- improves fuel economy;- broad engine application range.

Different casting processes are used for casting of TiAlalloys. They include processes, such as induction-skull melt-ing (ISM), vacuum-arc remelting (VAR), counter-gravity lowpressure atmosphere melting (CLIM), plasma-arc melting(PAM), etc. The majority of these processes produce cast-ings of high quality, but the cost of products is rather high.Centrifugal casting, as practiced for titanium and TiAl alloys,usually involves spinning one or more molds on a turntable.Compared to other processes, centrifugal casting is lessexpensive although the highly turbulent nature of the alloyunder pressure may lead to gas porosity and ceramic shellinclusions. “Ald Vacuum Technologies GmbH” developed acentrifugal permanent mold casting process to produce auto-motive valves[7]. All these processes, as can be expected,have their own advantages and limitations, and some are bet-ter suited than others for certain products.

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frequently occurs because of the high superheat tempera-tures (up to 1700◦C) and high concentration of very reactivetitanium and aluminum. For these reasons, the conventionalceramic molds of silica and zirconia (zirconium silicate) areunsuitable.

In the present work, a conventional “lost wax” procedurewas used to fabricate ceramic shell molds. First attemptswere made according to a commercial instruction which waswidely accepted for polycrystalline nickel-based superalloys,except that a ZrO2-flour-ethyl silicate slurry was used as acolloidal suspension, whereas ZrO2 powder was used for the“primary coating”. Each mold was placed in a steel can filledwith the refractory mixture composed of SiO2, MgO and CaOsands suspended in water and the assembly was heated at900◦C for at least 6 h. However, this process showed severaldisadvantages, such as:

- very slow and programmable heating rate significantly pro-longed the process of firing causing high consumption ofelectricity in the same time;

- the whole shell assembly was heavy (several kilograms fora casting of a few hundred grams);

- the ceramic material after casting exhibited very high hard-ness and pneumatic tools had to be applied in order toremove casting from the shell causing sometimes the dam-age of the relatively brittle castings (Fig. 1);

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Some vehicle manufacturers have reported tests on c�-iAl alloy turbocharger turbine wheels developed for “Arown Bovery” (ABB) and “Toyota”[8]. Very recently, Tetui and Miura[9,10]published results on the developmenhe new TiAl alloy for “Mitsubishi Heavy Industries” (MHurbocharger wheels. These wheels range in size from 5n diameter for (MHI) gasoline engine to 250 mm forndustrial diesel turbocharger turbine for (ABB). In all ocions, SIM or VAR processes have been applied.

The goal of the results described in this paper waevelop a prototype of a�-TiAl turbocharger wheel via vacum centrifugal casting process with cost that could benough to be applicable in automobile industry. In this centrifugal force was utilized with the idea to minimize porty, i.e. to reduce the necessity for hot isostatic pressing (Hust lowering the cost of the product. In view of these fahe results concerning technology of precision castingell as microstructural and mechanical properties of a

urbocharger wheel prototype developed at “Vinca” Instituteere described.

. Experimental

.1. Ceramic shell mold processing technology

Considering the very high chemical reactivity of titanind its alloys, special attention was paid to the developf ceramic shell molds. During solidification of titaniuluminides, a severe molten metal/ceramic mold rea

hazardous dust formed during shell breaking presentenvironmental problem together with the storage of broceramic parts.

Taking into account these problems, the process of fabion of shell molds was changed and the procedure of a “upporting” ceramic shell molds was developed and ado11]. Shell molds were processed by dipping wax patf the turbocharger wheel into a colloidal suspensionisting of titanium-acetyl-acetonate in isopropyl/n-butanoliquid) and ZrO2 powder. A wet pattern was then pouith ZrO2 powder (particle size approximately 200�m). This

Fig. 1. Damaged blade caused by breaking of hard shell mold.

16 M.T. Jovanovi´c et al. / Journal of Materials Processing Technology 167 (2005) 14–21

Fig. 2. “Self-supporting” ceramic shell mold. (a) Turbocharger wheel and (b) tensile test specimens.

process was repeated several times enabling “the primarycoating” consisting of three layers to be formed. The sur-face of this coating must be smooth, chemically stable andwith sufficient strength to sustain the pressure of the chem-ically very active molten metal. In the next procedure, thecoated pattern was covered with several “secondary coat-ings” of mullite (a compound 3Al2O3·2SiO2) with particlesize ranging between 200 and 500�m. “Secondary coat-ings” (five to seven layers) should enhance the strength ofthe shell mold. At the end of this process, the total thick-ness of the mold was 5–7 mm. The mold was dewaxed andfired at 900◦C for 3 h to induce sufficient strength for subse-quent handling.Fig. 2a shows the “self-supporting” ceramicmold after firing. The porosity of these molds was between20 and 25%. The same procedure was performed for shellmolds of tensile test specimens (Fig. 2b). The cross-sectionof a mold showing wax pattern inside the mold together with“primary coating” and “secondary coating” is presented inFig. 3.

2.2. Precision casting

A “master alloy” in the shape of small ingots (buttonsof 50 g) was manufactured by vacuum-arc melting ofhigh-purity aluminum (99.5%) and a commercial Ti–6 wt.%Al–4 wt.% V alloy (Ti64). To ensure homogeneous chemicalcomposition buttons were remelted for several times in anargon protective atmosphere. The chemical composition(note: in further text all chemical compositions are given inatomic percent) of buttons was: 48Al, 1V and balance Ti.The amount of oxygen was approximately 0.2. The chargedstock of “master alloy” was first remelted in a “Linn” vacuumcentrifugal furnace. Graphite crucibles, which inside surfacewere coated by plasma sprayed Y2O3, were used for melting.Vacuum pumps were switched off before pouring and theentire system was filled with high-purity (99.99%) argon upto 1 kPa pressure and the melt was poured into a previouslypreheated ceramic mold. Conditions during melting andcasting were as follows: pouring temperature was varied inthe range between 1550 and 1650◦C, preheated temperatureof ceramic shell mold was varied between 400 and 800◦C,speed of mold rotation: 200 rpm, vacuum during processing:1 Pa. Approximately the same melting and casting procedurewas performed for the processing of turbocharger wheelsand specimens for tensile tests. “Self-supporting” ceramicmolds demonstrated excellent chemical stability, very goodt stainp ime,i singa

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Fig. 3. Cross-section of ceramic mold.
hermal shock resistance coupled with strength to suressure of the melt during solidification. In the same t

t was possible to break molds quite easily without impony damage to castings.

.3. Characterization of microstructure and mechanicalroperties

X-ray diffraction analysis with Cu K� radiation, lighicroscopy and scanning electron microscopy (SEM) w


used for microstructural characterization. Specimens forthese examinations were cut out from the central sprue of thewheel casting. Kroll’s reagent (a mixture of 6 ml nitric acid,3 ml of 40% hydrofluoric acid and 100 ml of distilled water)was used as an etchant for light microscopy and SEM. Screw-type specimens for tensile tests were 4 mm in diameter and20 mm in gauge length. Unaxial tensile tests were performedat room temperature at a strain rate ˙e = 1.3 × 10−3s−1.Vickers hardness (HV10) was measured applying a loadof 10 kg.

3. Results and discussion

3.1. The effect of melting and casting parameters on thequality of castings

The turbine turbocharger wheel casting with 75 mm indiameter has a rather complicated configuration consisting of10 twisted blades with thin (∼1 mm) leading edges. There-fore, the precision casting with ceramic investment mold wasas a near net shape processing technique. Some attempts weremade with conventional gravity vacuum induction casting,but the quality of castings was unacceptable because theysuffered from defects, such as misrun, small surface cracks

and macroporosity. The most promising technique was foundto be centrifugal vacuum casting.

In addition to mold material, primary interest for thesuccessful casting of TiAl alloy requires careful attention tocasting parameters, such as pouring temperature and moldpreheat temperature. Mold preheat temperature is one of themost important casting parameters. Higher preheat not onlyimproves fill and feeding but also reduces thermal gradientand cooling rates. However, higher preheating may lead tosevere mold/metal reaction and may increase the propensityfor surface-connected porosity. Slower cooling rates mayinduce coarser grain size and inferior mechanical properties.Therefore, between these parameters a balance must befound in order to achieve properties according to designrequirements.

It should be noted that the first castings showed defects,such as a misrun (Fig. 4a), when the preheat temperature wasbelow 500◦C. In the case when the preheating was too high(above 800◦C), rough surface and macroporosity (Fig. 4b)as well as microporosity (Fig. 4c) were observed. Pouringtemperature above 1600◦C also causes a significant extentof microporosity (Fig. 4d). Applying higher preheat temper-ature (between 750 and 800◦C), many of these defects weresuccessfully eliminated and a smooth surface together withthin and sharp blade edges were obtained (Fig. 5a and b).

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ig. 4. Macro and micro defects caused by inadequate mold preheat or picropores and (d) extensive microporosity.

ouring temperature. (a) Misrun, (b) rough surface and macropores (arrows), (c)


Fig. 5. Correctly cast turbocharger wheel and assembly of specimens for tensile testing. (a) General look of wheel, (b) detail showing blade edge and (c) tensilespecimens.

The same conditions enabled a good quality of specimensfor tensile tests (Fig. 5c).

3.2. Microstructural characterization

X-ray diffraction analysis (Fig. 6) proved the existence of�2 ordered Ti3Al phase with close-packed-hexagonal lattice(a= 0.5753 nm andc= 0.4644 nm) and� ordered TiAl phasewith tetragonal lattice (a= 0.4016 nm andc= 0.4073 nm)with c/a ratio of 1.014. This ratio is close to 1.02 whichcorresponds to the equiatomic TiAl composition, whereastetragonality increases up toc/a= 1.03 with increasing alu-minum concentration[5]. A few peaks of retained ordered� (B2) phase, which crystallizes in the CsCl structure, havealso been detected. Contrary to castings of a commercial alloy(Ti64) when a peak of TiC and/or TiCN was detected due toreaction of melt and carbon crucible[12], no presence ofcarbon in castings was detected in this case.

When the turbocharger wheel of the chemical compo-sition Ti–48Al–1V is cast from around 1600◦C, in addi- Fig. 6. X-ray diffraction pattern of casting.


Fig. 7. Light microscope. As-cast microstructure. (a) Mold preheated at 500◦C and (b) mold preheated at 800◦C.

tion to cooling rate the as-cast microstructure depends onthe chemical composition. Some literature data show thatgrains are becoming smaller with decreasing aluminum con-tent and with small additions of vanadium, manganese andchromium [9,13]. However, in the case of this work thecooling rate during solidification was a predominant factorinfluencing the grain size. Higher cooling rate (mold was pre-heated at 500◦C) yielded grains between 100 and 300�m insize (Fig. 7a), whereas lower cooling rate (mold tempera-ture was around 800◦C) produced rather coarse (between300 and 600�m) grains (Fig. 7b). Higher magnificationclearly reveals that coarse grains possess the fully lamellarmicrostructure where the lamellae are mostly� intermixedwith dark lamellae of the�2 phase (Fig. 8). Individual rod-like particles (having light contrast) of the� (B2) phase maybe also seen in the same micrograph. A similar morphology ofthis phase was reported in ingots of Ti–47Al–4(Cr, Nb, Mo,B) [14]. Although a number of experiments were performedapplying variations of pouring temperature and preheatingtemperature of the ceramic shell, the appearance of someporosity could not be avoided. To preserve this lamellar struc-ture that might be lost during HIP[10], no experiment withHIP has been performed in order to eliminate microporos-

ity. In general, lamellar structure could result in low ductility(even less than 1%)[15], but with regard to creep properties afully lamellar structure is desirable[8,16]. Lamellar spacingwas found to depend on cooling rate (Fig. 9a and b), i.e. in

Fig. 8. Light microscope. Higher magnification of specimen shown inFig. 7b showing lamellar structure. Light contrast corresponds to the� (B2)phase.

F erature (cooling rate) on�2/� lamellar spacing. (a) Slower cooling rate and (b) higherc

ig. 9. SEM. Secondary electrons. The effect of preheat mold tempooling rate.


Table 1Room temperature mechanical properties of some TiAl-based alloys

Alloy, at.% Strength, MPa Elongation, % Hardness, HV10 Reference

Yield Strength Ultimate tensile strength

Ti–48Al–1Va (100–200�m) 430 500 1.2 360 This workTi–48Al–1Va (300–500�m) 400 475 1.8 330 This workTi–48Ala 390 483 0.3–2.1 250 [3]Ti–48Al–1V 400 507 2.3 – [3]Ti–44Al–1V 754 – 0.6 – [18]Ti–48Al(1–3)V 520 – 1.5–3.5 – [3]Ti–48Al–2Nb–2Mn (as-cast + HIPed) 392 460 0.9 – [19]Ti–47Al–5Nba 480 510 0.5 – [20]

a As-cast.

coarse grains (slower cooling rate,Fig. 9a) this spacing wassomewhat larger than in smaller grains (higher cooling rate,Fig. 9b). This is in accordance with the results of Kim[17]who reported the effect of cooling rate on�2/� spacing.

3.3. Mechanical properties

Room temperature mechanical properties of TiAl-basedalloys (with a chemical composition similar to alloy of thiswork) are compared inTable 1. The results of this paperrepresent the average value of four tests. The effect ofsmaller grain size on mechanical properties is evident, i.e.higher strength and hardness, but lower elongation showedspecimens solidified under higher cooling rate. Although inmost alloys the previous “thermal history” was not specified,it is obvious that the results of this work agree quite well withthe literature data. Considering the results of mechanicaltesting presented in this paper, one should be cautious sincethese values refer not to the turbocharger wheel itself, but totensile specimens solidified under the conditions intendedto be similar during solidification of casting with differentgeometry.

Based on inspection of casting, microstructural anal-ysis and mechanical properties, a processing window forprecision casting of a prototype of the turbocharger wheelm 1580a een7 ndv eatt gthw osend esep heelh gth:4 rains

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cision casting of the prototype of a turbocharger wheelmade of Ti–48Al–1V alloy. “Self-supporting” ceramicmolds demonstrated excellent chemical stability togetherwith strength to sustain pressure during solidification.“Self-supporting” molds exhibit a number of advantagesover molds commercially recommended.

It was demonstrated that applying centrifugal vacuum fur-nace as a casting facility complicated shapes of a turbochargerwheel may be cast without surface defects, provided thatthe pouring temperature and preheat temperature of ceramicmolds were correctly maintained.

The microstructure of the precision cast turbochargerwp int ctsg werc

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ay be established, i.e. pouring temperature: betweennd 1600◦C, shell mold preheating temperature: betw50 and 800◦C, speed of mold rotation: 200 rpm aacuum during melting: 1 Pa. Although with mold prehemperature at 500◦C smaller grains and higher strenere achieved, higher preheat temperature was chue to better quality of surface castings. Applying tharameters, the prototype of the TiAl turbocharger waving yield strength: 400 MPa, ultimate tensile stren75 MPa, elongation: 1.8%, hardness: 330 HV and gize: between 300 and 500�m was produced.

. Conclusions

The processing technology of a “self-supportieramic shell mold was successfully verified during

heel is fully lamellar consisting of alternate�2 and �hase lamellae. Some� (B2) phase was also detected

he microstructure. Cooling rate during solidification afferain size and lamellar spacing—both increase with sloooling rate.

Smaller grains yield higher strength, but lower elongaalues of tensile properties correspond to those of TiAlimilar chemical composition as a turbocharger wheel.

According to all results, a processing window for precisasting of a turbocharger wheel prototype was establisf pouring temperature and preheat temperature of cer

old are maintained between 1580 and 1600◦C and betwee50 and 800◦C, respectively, than castings of a good suruality with ultimate tensile strength of 475 MPa and eation of 1.8% will be obtained.

cknowledgement

The work was financially supported by the Ministrycience and Environment of the Republic of Serbia thro

he Project No. 1966.

eferences

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[2] Y.W. Kim, Ordered intermetallic alloys. Part III, J. Met. 46 (19930–39.


[3] Y.W. Kim, Intermetallic alloys based on titanium aluminides, ibid.,41 (1989) 24–30.

[4] R.V. Ramanujan, Phase transformations in� titanium aluminides,Int. Mater. Rev. 45 (6) (2000) 217–240.

[5] K. Clemens, H. Kestler, Processing and application of intermetaltic�-TiAl based alloys, Adv. Eng. Mater. 2 (9) (2000) 551–570.

[6] Y. Nashiyama, T. Miyashita, S. Isobe, T. Noda, Development of tita-nium aluminide turbocharger rotor, in: S.H. Wang, C.T. Liu, D. Pope,J.O. Stiegler (Eds.), High temperature aluminides and intermetallics,TMS, Indianapolis, 1990, pp. 557–570.

[7] M. Blum, A. Choudhry, H. Sholz, G. Jarczyk, S. Pleier, P. Busse,G. Frommeyer, S. Knippscheer, Casting of gamma TiAl alloys, in:Y.W. Kim, D.M. Dimiduk, M.H. Loretto (Eds.), Gamma TitaniumAluminides, TMS, Warrendale, 1999, pp. 34–42.

[8] P.A. McQuay, V.K. Sikka, Casting, in: J.H. Westbrook, R.L. Fleischer(Eds.), Intermetallic Compounds, vol. 3, Principles and Practice, JohnWiley and Sons, New York, 2000, pp. 592–615.

[9] T. Tetsui, Y. Miura, Heat resistant cast TiAl alloy for passengervehicle turbocharger, Mitsubishi Heavy Ind. Tech. Rev. 39 (2002)592–615.

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[11] A “self-supporting” ceramic shell mold for investment casting oftitanium alloys, JUS Patent No. 449/03 (2003) (patent pending).

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mechanical properties of investment cast Ti–6Al–4V alloy, Mater.Des., in press.

[13] H. Clemens, A. Lorich, N. Eberhardt, W. Glatz, W. Knabl, H. Kestler,Technology, properties and applications of intermetallic�-TiAl basedalloy, Metallkde 90 (1999) 569–580.

[14] H. Clemens, F. Jeglitsch, Intermetallic� titanium aluminide basedalloys from a metallographic point of view, Prakt. Metall. 37 (2000)194–217.

[15] Y.W. Kim, D.M. Dimiduk, Designing gamma-TiAl alloys: fun-damentals, strategy and production, in: M.V. Nathal, R. Darolia,C.T. Liu, P.L. Martin, D.B. Miracle, R. Wagner, M. Yamaguchi(Eds.), Structural Intermetallics, The Minerals, Metals and MaterialsSociety, Warrendale, 1997, pp. 531–543.

[16] D.M. Dimiduk, D.B. Miracle, Y.W. Kim, M.G. Mendiratta, Recentprogress in intermetallic alloys for advanced aerospace systems, ISIJInt. 31 (10) (1991) 1223–1234.

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[18] H.A. Lipsitt, Titanium aluminides—an overview, in: Mater. Res. Soc.Symp. Proc. 39, MRS, Pittsburgh, 1985, pp. 351–360.

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microstructure and mechanical properties of precision cast tial turbocharger wheel

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