centrifugal precision cast tial turbocharger wheel using ceramic mold

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journal of materials processing technology 204 ( 2 0 0 8 ) 492–497 journal homepage: www.elsevier.com/locate/jmatprotec Short technical note Centrifugal precision cast TiAl turbocharger wheel using ceramic mold Wang Shouren , Guo Peiquan, Yang Liying School of Mechanical Engineering, University of Jinan, Jinan 250022, China article info Article history: Received 9 August 2007 Received in revised form 13 January 2008 Accepted 29 January 2008 Keywords: Ceramic mold Mold multi-piece design Cast Ti–47Al–2Cr–2Nb alloys Microstructure Mechanical properties abstract It is the potential candidate for gamma TiAl-based alloys that would replace Ni-based superalloys, which are being used in fabrication of the turbocharger wheel currently. Cast Ti–47Al–2Cr–2Nb alloys due to their specific performance requirement are now on the verge of a commercial application. A novel precision casting technique, which combines the ceramic mold casting with centrifugal casting, was described in this work. Multiple-section ceramic mold was designed and fabricated successfully. All the mold sections fit together to form a cavity and then open in many directions to eject the molded part. Microstructures and mechanical properties of cast Ti–47Al–2Cr–2Nb alloys under the action of centrifugal force are discussed. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Turbocharger is one of the most effective devices, which can result in an increase in the performances of diesel engines and a reduction in their fuel consumption, environmental pol- lutants and CO 2 emissions (Galindo et al., 2007). In order to improve the engine’s work efficiency, improved turbocharg- ers response has been put forth as a priority in recent years (Tetsui, 2002). The simplest way to solve the fatal draw- back called “turbo-lag” is to make rotating parts lighter in weight. Application of lightweight structural materials in fab- ricating turbocharger rotor is the most effective approach to solve the above problem. Ni-based superalloys (Inconel 713C) is most commonly used for turbocharger turbine wheels yet faced with large challenge due to their relatively high Corresponding author. Tel.: +86 531 82765476. E-mail address: [email protected] (W. Shouren). density (8 g/cm 3 ). TiAl-based alloy is about 4 g/cm 3 which is about half of that of commonly used Ni-based superalloys, and therefore has attracted broad attention as potential can- didates for high-temperature structural application in the fields of turbocharger manufacture (Jovanovic et al., 2005; Tetsui and Ono, 1999; Nakagawa et al., 1992). Due to low density (3.8 g/cm 3 ), high specific strength, high Young’s mod- ulus and excellent oxidation resistance at high temperatures, they represent a good alternative for nickel-based superal- loys (Zollinger et al., 2007; Qu and Wang, 2007; Cao et al., 2007). Moreover, TiAl-based alloys consist of the major - phase and minor 2 -phase, are nearly equal to Ni-based superalloys for turbine blades in specific tensile strength and specific creep strength but slightly inferior to superalloys in oxidation resistance above 700 C(Jovanovic et al., 2005; 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.01.062

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Page 1: Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 492–497

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

Short technical note

Centrifugal precision cast TiAl turbochargerwheel using ceramic mold

Wang Shouren ∗, Guo Peiquan, Yang LiyingSchool of Mechanical Engineering, University of Jinan, Jinan 250022, China

a r t i c l e i n f o

Article history:

Received 9 August 2007

Received in revised form

13 January 2008

Accepted 29 January 2008

Keywords:

a b s t r a c t

It is the potential candidate for gamma TiAl-based alloys that would replace Ni-based

superalloys, which are being used in fabrication of the turbocharger wheel currently. Cast

Ti–47Al–2Cr–2Nb alloys due to their specific performance requirement are now on the verge

of a commercial application. A novel precision casting technique, which combines the

ceramic mold casting with centrifugal casting, was described in this work. Multiple-section

ceramic mold was designed and fabricated successfully. All the mold sections fit together

to form a cavity and then open in many directions to eject the molded part. Microstructures

and mechanical properties of cast Ti–47Al–2Cr–2Nb alloys under the action of centrifugal

Ceramic mold

Mold multi-piece design

Cast Ti–47Al–2Cr–2Nb alloys

Microstructure

force are discussed.

© 2008 Elsevier B.V. All rights reserved.

phase and minor � -phase, are nearly equal to Ni-based

Mechanical properties

1. Introduction

Turbocharger is one of the most effective devices, which canresult in an increase in the performances of diesel enginesand a reduction in their fuel consumption, environmental pol-lutants and CO2 emissions (Galindo et al., 2007). In order toimprove the engine’s work efficiency, improved turbocharg-ers response has been put forth as a priority in recent years(Tetsui, 2002). The simplest way to solve the fatal draw-back called “turbo-lag” is to make rotating parts lighter inweight. Application of lightweight structural materials in fab-ricating turbocharger rotor is the most effective approach

to solve the above problem. Ni-based superalloys (Inconel713C) is most commonly used for turbocharger turbine wheelsyet faced with large challenge due to their relatively high

∗ Corresponding author. Tel.: +86 531 82765476.E-mail address: [email protected] (W. Shouren).

0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2008.01.062

density (8 g/cm3). TiAl-based alloy is about 4 g/cm3 which isabout half of that of commonly used Ni-based superalloys,and therefore has attracted broad attention as potential can-didates for high-temperature structural application in thefields of turbocharger manufacture (Jovanovic et al., 2005;Tetsui and Ono, 1999; Nakagawa et al., 1992). Due to lowdensity (3.8 g/cm3), high specific strength, high Young’s mod-ulus and excellent oxidation resistance at high temperatures,they represent a good alternative for nickel-based superal-loys (Zollinger et al., 2007; Qu and Wang, 2007; Cao et al.,2007). Moreover, TiAl-based alloys consist of the major �-

2

superalloys for turbine blades in specific tensile strength andspecific creep strength but slightly inferior to superalloysin oxidation resistance above 700 ◦C (Jovanovic et al., 2005;

Page 2: Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 492–497 493

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amaguchi et al., 2000). However, in spite of above good prop-rties of them, the low temperature elongation and harshnvironment (subjected to long-term exposure to high tem-erature exhaust gases) limits the development in vehicle anderospace industries. Alloying additions to TiAl-based alloyuch contributed to overcome the obstacle, which once stood

n their way for the practical application (Li and Taniguchi,005). Recently, a second-generation gamma titanium alu-inide (Ti–47Al–2Cr–2Nb alloy (TACN, at.%)) was evaluated asremarkable interesting material in turbocharger fabrication

Liu and Wang, 2006).Different casting methods such as conventional sand

asting (Johnson et al., 1998), investment casting (IC) (lostax casting) (Kuang et al., 2002), die casting (Rawers andrzesinski, 1990), low pressure casting (Zhang et al., 2007),

entrifugal casting (Wen-bin SHENG, 2006), shell mould cast-ng (Yang et al., 2003a,b), metal mold casting (Sahin et al., 2006),xtrusion casting (Vijayaram et al., 2006) and, etc., are used forasting of TACN alloys. IC methods are used for the produc-ion of short series of premium quality components havingmooth surface finish, near-net-shape, complex shapes andhin walls (Zhang et al., 2006; Sung and Kim, 2005). There-ore, IC is the optimum and preferred but only manufacturing

ethod of turbocharger turbine wheel from the past until now.owever, this process showed several disadvantages such as

i) significantly prolonged process of firing causing high con-umption of electricity; (ii) the heavy ceramic shell assembly;iii) increasing difficulty in clearing up ceramic shell resultingn tool wear; (iv) severe environmental problem (hazardousust formed during shell breaking) together with the stor-ge of broken ceramic parts (Jovanovic et al., 2005). In ordero solve these problems, a novel casting technique, whichombines the traditional die casting, gravity permanent moldie casting (pre-prepared ceramic mold) with centrifugal cast-

ng, called CMCC, was developed and adopted. A multi-pieceeramic mold is used in this technology, which overcomeshe restrictions imposed by traditional molds by having manyarting directions. The major advantages of CMCC technol-gy are elimination of porosity and shrinkage in productnder centrifugal force accompanying good surface finish,ood dimensional accuracy, improved wear resistance, higherorrosion resistance, higher hardness, improved fatigue andetter creep strength. In addition, some attributes of this tech-ique such as low cost and green product are the main reasons

nstead of IC technique in the fields of turbocharger wheelanufacturing.In our present work, a permanent multi-piece ceramic

old for fabricating turbocharger wheel was designed. A tur-ocharger wheel with TACN alloys was fabricated by CMCCechnology and that of microstructure and properties was dis-ussed.

. Experiments

.1. Ceramic mold design and fabrication

ulti-piece ceramic mold was designed by 3D soft ware (Solid-orks) in which 3D schematic illustration was shown in Fig. 1.esigning the mold is a challenging task, because complex

Fig. 1 – Schematic illustration about 3D subassembly ofturbocharger ceramic mold.

molds cavity often require very complex undercuts to realizethe entire 3D geometry (Gyger et al., 2007). The multi-piececeramic mold consists of 10 mold pieces and more than twosubassemblies. Each of these mold pieces has a different part-ing direction. All the mold pieces can be visualized as a 3Djigsaw puzzle to fit together to form a cavity and then disas-sembled to eject the molded part. The ability to manufacturegeometrically complex objects economically will significantlyexpand the design space and will allow development ofnew products in many different areas. Therefore, multi-piececeramic molding technology is an ideal candidate for makinggeometrically complex product such as turbocharger wheel.

The detail manufacturing processes of mold are the follow-ing steps. First, ZrO2 powders (diameter ≤100 �m, ShanghaiCeramic Materials Plant, China) were chosen as starting mate-rials to form mixed slurry using a binder as prehydrolysedethyl silicate (with a solid SiO2 content of approximately 20%)and gelling agent as ammonium carbonate aqueous solution.Secondly, the slurry was allowed to gel and harden for 30 minbefore stripping the pattern and then the gelled preform wasCNC machined to form each piece of ceramic mold. Thirdly,the gelled mould pieces were then immersed into an alcoholbath for 3 h followed by burning-out and further fired at 1650 ◦Cfor 12 h as well cooled in the furnace to room temperature.Finally, taking into the very chemical reactivity of TiAl-basedalloys with ZrO2, each piece of ceramic mold surface wascoated by plasma sprayed Y2O3. The coating thickness is notless than 2 mm to endure the pressure of chemically veryactive molten metal. The schematic illustration of ceramicmold pieces was shown in Fig. 2.

2.2. CMCC technology

The most important issue is the inhibition of casting defects in

the as-cast state. In order to avoid their occurrence in perma-nent multi-piece ceramic mold, centrifugal casting is highlynecessary. Thus, CMCC technology is quickly developed andhas been completely successful in preventing casting defects.
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494 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 492–497

Fig. 2 – Schematic illustration of ceramic mold pieces.

Table 1 – Chemical compositions of the TACN alloysspecimens (mass%)

Alloy TACN

C 0.007Al 33.5Cr 2.6Fe 0.01Nb 4.7

Fig. 4 – Steel joint of multi-piece ceramic mold.

Fig. 5 – Turbocharger wheel fabricated by CMCC technique.

O 0.05N 0.006Ti Balance

TACN alloys were fabricated by vacuum-arc melting tech-nique. Their chemical compositions are listed in Table 1. Acrucible made of ZrO2-stabilized Y2O3 (YSZ) was used for melt-ing. Tested alloy was melted and overheated to 1560 ◦C beforepouring at 1520 ◦C. The melt was poured into a gating sys-tem, which was shown in Fig. 3. Application of a steel joint(which was shown in Fig. 4) restricts the motion of ceramicmold pieces and prevents leakage of molten metal. The moldwas water-cooled so metal leakage is not a problem. The entiresystem was filled with high purity argon (99.99%) up to 1 kPa.The ceramic mold was pre-heated to 400–600 ◦C. When moltenalloys are poured into the mold, the steel joint is water-cooledimmediately; then, molten alloys are solidified under cen-trifugal force with speed of mold rotation as 260 rpm. After

10–15 min the mold pieces were removed from many differentdirections and the casting product was then taken out to coolat room temperature. The turbine turbocharger wheel castingwith 105 mm in diameter has a rather complicated configura-

Fig. 3 – Schematic illustration of turbocharger ceramicmold.

tion consisting of 10 twisted blades with thin (1 mm) leadingedges, which was shown in Fig. 5.

2.3. Characterizations

The morphologies and microstructure are observed by trans-mission electron microscopy (TEM, H-800). Chemical elementdistributions were examined by the energy spectrum anal-yses (EDS, OXFOED INCA). X-ray diffraction (XRD) analysiswith Cu K� radiation was used for microstructural character-ization. Specimens for these examinations are cut out fromthe central sprue of the wheel casting. The room temper-ature Vickers hardness (HV10) was measured with a 10 Nload and 15 s indentation time, averaging at least five tests.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 × 10−3 s−1. Thefracture toughness tests were carried out on the electronomnipotence testing machine (Instron 5569). The specimenswere loaded at a constant crosshead speed of 10−2 cm s−1.The notch, cut through electro-discharge machining, hasa geometry characteristic of root radius of 200 �m, nor-

mal width of 2 mm and a normal length of approximately3–4 mm.
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t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 492–497 495

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Fig. 6 – XRD analysis of TACN alloys.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

. Result and discussion

.1. Microstructure

RD for the alloys (Fig. 6) identified that �-TiAl (tetragonalattice, a = 0.4016 nm and c = 0.4073 nm) is the major phaseith small amounts of �2-Ti3Al (close-packed-hexagonal lat-

ice, a = 0.5753 nm and c = 0.4644 nm). Fig. 7 shows the typicalicrostructure from the surface TANC alloy. The phase

oundaries can be clearly distinguished with no evident inter-iffusion. The matrix is �-TiAl and Ti3Al (2 point) phase. Thehite area (3 point) clear contrast to matrix is Nb3Al phase

nd the grey–white area surrounding the Nb3Al phase is (Al,i)3Nb phase; the massive grey area (1 point) is chromium alu-inide. So it is indicated that the matrix phase is TiAl + Ti3Al

� + �2), the reinforcement phases are chromium and niobiumluminide.

The TEM microstructure of the specimen is presented inig. 8. Microstructure transformation is dependent upon notnly cooling rate but also composition of the alloy. Cooling rate

s a predominant factor influencing the grain size, while addi-

ions of elements, which slow down the various solid-stateransformations are another factor (Godfrey et al., 1997). Inhis work, due to the high cooling rate, TACN alloys exhibit

Fig. 7 – SEM micrographs of the surfaces of TANC alloy (a)

and EDS analyses: (b) 1 point; (c) 2 point; (d) 3 point.
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496 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 4 ( 2 0 0 8 ) 492–497

Fig. 8 – TEM photomicrograph of TACN alloys (a) isometric structure and (b) lamellae structure.

Table 2 – Room temperature mechanical properties of TACN alloys

Alloy (at.%) YS (MPa) UTS (MPa) Elongation (%) Hardness (HV10) KC (MPa m1/2) Reference

K418a 557 627 3.3–4.6 310 21.2 Huanming et al. (2002)Ti–48Ala 430 500 0.3–2.1 250 14.5 Kim (1989)Ti–47Al–5Nba 480 510 0.5 – 18.5 Yang et al. (2003a,b)Ti–48Al–1Va (100–200 �m) 430 500 1.2 360 – Jovanovic et al. (2005)Ti–48Al–1Va (300–500 �m) 400 475 1.8 330 – Jovanovic et al. (2005)Ti–48Al–2Nb–2Mnb 392 460 0.9 – – Kuang et al. (2002)TACNa 560 ± 10 659 ± 10 1.6 ± 0.3 300 ± 6 23.1 ± 1.2 This work

a As-cast.b As-cast + HIP.

fine microstructure with grain size as an average of 200 nm indiameter (Fig. 8a). In addition, cooling rate and alloying resultin a fully lamellar structure where the lamellae are mostly �-phase intermixed with �2 lamellae (Fig. 8b). Within a lamellarcolony, the lamellar laths align themselves in the same direc-tion, and the spacing of laths varies from 10 to 20 nm. There aredistinct interfaces between the brighter and the darker laths.The EDS analysis showed that the darker regions have higherTi concentrations, whereas the brighter regions have higherAl concentrations. It has been found that Nb preferentiallysubstitutes for Ti, whereas Cr occupies the Al sublattices.

3.2. Mechanical properties

Room temperature mechanical properties of sample wereshown in Table 2. The results of this works represent theaverage value of five tests. It is shown that mechanical prop-erties depend on not only microstructure but also castingconditions, alloy composition and a host of process-relatedvariables. Larger lamellar structure could result in low ductil-ity but be beneficial to creep properties, while lower elongationattributes to solidification conditions such as high coolingrate. At the same time, solidification processes such as cool-ing rate and centrifugal force also result in higher strength

and hardness. The fracture toughness (KC) of TACN alloyscan be increased under centrifugal force with extensive grainboundary diffusion and sliding. It also intensively depends onmicrostructure of alloys, i.e. lamellar microstructure exhibit

superior fracture toughness relative to the duplex microstruc-ture. Grain refinement leads to improved microstructuralhomogeneity, higher hardness and decreased ductility. Com-pared with K418 alloy (Chinese Ni-based superalloy, usedto manufacture turbocharger wheel) and conventional TiAl-based alloys, the yield strength (YS) and ultimate tensilestrength (UTS) of TACN alloys have an improved value andbecome the new class of advanced materials. CMCC processis advantageous in eliminating casting defects. In general,TACN alloys fills the gap between heat-resistant metal andceramic materials, while CMCC process would become thesuccessful manufacture method in commercial application ofturbocharger wheel.

In conclusion, above mechanical properties of this papershown in Table 2 attribute to the conditions as following,structure: � + �2, grain size: 150–300 �m, pouring tempera-ture: 1520–1560 ◦C, ceramic mold pre-heated temperature:400–600 ◦C, speed of mold rotation: 260 rpm, and protectionatmosphere: Ar.

4. Conclusions

The processing technology of centrifugal precision casting

using multi-piece ceramic mold was successfully verified infabricating turbocharger wheel. Cast Ti–47Al–2Cr–2Nb alloysexhibits fine structure with grain size as an average of 200 nmin diameter. Solidification factors such as cooling rate, cen-
Page 6: Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

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process of A356 aluminum alloy wheels. Mater. Sci. Eng. A 464,

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rifugal force and alloying result in a fully lamellar structurehere the lamellae are mostly �-phase intermixed with �2

amellae. Within a lamellar colony, the lamellar laths alignhemselves in the same direction, and the spacing of lathsaries from 10 to 20 nm. These factors result in highertrength, hardness and increased ductility of materials.

cknowledgements

his work was supported by the Natural Science Foundationf Shandong Province (Y2006F03), Science and Technologyroject of Educational Committee (J07YA18) and Programs forcience and Technology Development of Shandong Province

2007ZG10004013). Part of this research was done at the Insti-ute of Materials Science of Jinan University and School ofechanical Engineering of Jinan University. The authors owedebt of gratitude to the technical staff of institutions for theirelp.

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