Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

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  • j ournal of mater ials process ing technology 2 0 4 ( 2 0 0 8 ) 492497

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

    Short te

    Centr chwhee

    Wang SSchool of Me

    a r t i c

    Article history:

    Received 9 August 2007

    Received in revised form

    13 January 2008

    Accepted 29 January 2008

    Keywords:

    Ceramic mo

    Mold multi-

    Cast Ti47A

    Microstruct

    Mechanical

    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

    Ti47Al2Cr2Nb alloys due to their specic 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

    1. In

    Turbochargresult in anand a reduclutants andimprove thers respons(Tetsui, 20back calledweight. Apricating tuto solve th713C) ismoyet faced w

    CorresponE-mail a

    0924-0136/$doi:10.1016/ld

    piece design

    l2Cr2Nb alloys

    ure

    properties

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

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

    and mechanical properties of cast Ti47Al2Cr2Nb alloys under the action of centrifugal

    force are discussed.

    2008 Elsevier B.V. All rights reserved.

    troduction

    er is one of the most effective devices, which canincrease in the performances of diesel engines

    tion in their fuel consumption, environmental pol-CO2 emissions (Galindo et al., 2007). In order to

    e engines work efciency, improved turbocharg-e has been put forth as a priority in recent years02). The simplest way to solve the fatal draw-turbo-lag is to make rotating parts lighter in

    plication of lightweight structural materials in fab-rbocharger rotor is the most effective approache above problem. Ni-based superalloys (Inconelst commonly used for turbocharger turbinewheelsith large challenge due to their relatively high

    ding author. Tel.: +86 531 82765476.ddress: sherman0158@tom.com (W. Shouren).

    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 theelds of turbocharger manufacture (Jovanovic et al., 2005;Tetsui and Ono, 1999; Nakagawa et al., 1992). Due to lowdensity (3.8 g/cm3), high specic strength, high Youngs 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-basedsuperalloys for turbine blades in specic tensile strength andspecic creep strength but slightly inferior to superalloysin oxidation resistance above 700 C (Jovanovic et al., 2005;

    see front matter 2008 Elsevier B.V. All rights reserved.j.jmatprotec.2008.01.062chnical note

    ifugal precision cast TiAl turbol using ceramic mold

    houren , Guo Peiquan, Yang Liyingchanical Engineering, University of Jinan, Jinan 250022, China

    l e i n f o a b s t r a c targer

  • j ournal of mater ials process ing technology 2 0 4 ( 2 0 0 8 ) 492497 493

    Yamaguchi et al., 2000). However, in spite of above good prop-erties of them, the low temperature elongation and harshenvironmeperature exaerospacemuch contrin their wa2005). Receminide (Tia remarkab(Liu and Wa

    Differencasting (Johwax castinWrzesinskicentrifugaling (Yangetextrusion ccasting of Ttion of shosmooth suthin wallsfore, IC is thmethodof tHowever, th(i) signicasumption o(iii) increasin tool weadust formeage of brokto solve thcombines tdie castinging, calledceramic mthe restrictparting dirogy are eliunder centgood dimencorrosion rbetter creepnique suchinstead ofmanufactu

    In our pmold for fabochargertechnologycussed.

    2. Ex

    2.1. Cer

    Multi-pieceworks) in wDesigning

    Schharg

    cavitireic moembectiopuzzd totricad thoducic motricadetaps. Fic Mafor

    ilicatllingly, the slurry was allowed to gel and harden for 30minstripping the pattern and then the gelled preform wasachined to form each piece of ceramic mold. Thirdly,

    lled mould pieces were then immersed into an alcoholr 3h followedbyburning-out and further redat 1650 Ch as well cooled in the furnace to room temperature., taking into the very chemical reactivity of TiAl-basedwith ZrO2, each piece of ceramic mold surface wasby plasma sprayed Y2O3. The coating thickness is notan 2mm to endure the pressure of chemically verymolten metal. The schematic illustration of ceramicieces was shown in Fig. 2.

    CMCC technology

    ost important issue is the inhibition of casting defects incast state. In order to avoid their occurrence in perma-ulti-piece ceramic mold, centrifugal casting is highlyary. Thus, CMCC technology is quickly developed anden completely successful in preventing casting defects.nt (subjected to long-term exposure to high tem-haust gases) limits the development in vehicle andindustries. Alloying additions to TiAl-based alloyibuted to overcome the obstacle, which once stoody for the practical application (Li and Taniguchi,ntly, a second-generation gamma titanium alu-47Al2Cr2Nb alloy (TACN, at.%)) was evaluated asle interesting material in turbocharger fabricationng, 2006).t casting methods such as conventional sandnson et al., 1998), investment casting (IC) (lostg) (Kuang et al., 2002), die casting (Rawers and, 1990), low pressure casting (Zhang et al., 2007),casting (Wen-bin SHENG, 2006), shell mould cast-al., 2003a,b),metalmold casting (Sahin et al., 2006),asting (Vijayaram et al., 2006) and, etc., are used forACN alloys. IC methods are used for the produc-rt series of premium quality components havingrface nish, near-net-shape, complex shapes and(Zhang et al., 2006; Sung and Kim, 2005). There-e optimum and preferred but only manufacturingurbocharger turbinewheel from thepast until now.is process showed several disadvantages such as

    ntly prolonged process of ring causing high con-f electricity; (ii) the heavy ceramic shell assembly;ing difculty in clearing up ceramic shell resultingr; (iv) severe environmental problem (hazardousd during shell breaking) together with the stor-en ceramic parts (Jovanovic et al., 2005). In orderese problems, a novel casting technique, whichhe traditional die casting, gravity permanent mold(pre-prepared ceramic mold) with centrifugal cast-CMCC, was developed and adopted. A multi-pieceold is used in this technology, which overcomesions imposed by traditional molds by having manyections. The major advantages of CMCC technol-mination of porosity and shrinkage in productrifugal force accompanying good surface nish,sional accuracy, improved wear resistance, higheresistance, higher hardness, improved fatigue andstrength. In addition, some attributes of this tech-

    as low cost and green product are themain reasonsIC technique in the elds of turbocharger wheelring.resent work, a permanent multi-piece ceramic

    bricating turbocharger wheel was designed. A tur-wheel with TACN alloys was fabricated by CMCCand that of microstructure and properties was dis-

    periments

    amic mold design and fabrication

    ceramicmoldwas designed by 3D soft ware (Solid-hich 3D schematic illustration was shown in Fig. 1.the mold is a challenging task, because complex

    Fig. 1 turboc

    moldsthe enceramsubassing dirjigsawsemblegeomeexpannew prceramgeome

    Theing steCeramrials toethyl sand geSecondbeforeCNC mthe gebath fofor 12Finallyalloyscoatedless thactivemold p

    2.2.

    Themthe as-nent mnecesshas beematic illustration about 3D subassembly ofer ceramic mold.

    ty often require very complex undercuts to realize3D geometry (Gyger et al., 2007). The multi-pieceld consists of 10 mold pieces and more than twolies. Each of these mold pieces has a different part-n. All the mold pieces can be visualized as a 3Dle to t together to form a cavity and then disas-eject the molded part. The ability to manufacturelly complex objects economically will signicantlye design space and will allow development ofts in many different areas. Therefore, multi-piecelding technology is an ideal candidate for makinglly complex product such as turbocharger wheel.il manufacturing processes ofmold are the follow-irst, ZrO2 powders (diameter 100m, Shanghaiterials Plant, China) were chosen as startingmate-m mixed slurry using a binder as prehydrolysede (with a solid SiO2 content of approximately 20%)agent as ammonium carbonate aqueous solution.

  • 494 j ournal of mater ials process ing technology 2 0 4 ( 2 0 0 8 ) 492497

    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 4.7O 0.05N 0.006Ti

    TACN alnique. Thecruciblemaing. Testedpouring attem, which(which wasmold piecewaswater-system waThe ceramialloys are pimmediatetrifugal for1015min tdirections aat room temwith 105m

    Fig. 3 Schmold.

    Fig.

    Turbocharger wheel fabricated by CMCC technique.

    nsisting of 10 twisted blades with thin (1mm) leadingwhich was shown in Fig. 5.Balance

    loys were fabricated by vacuum-arc melting tech-ir chemical compositions are listed in Table 1. Ade of ZrO2-stabilizedY2O3 (YSZ)was used formelt-alloy was melted and overheated to 1560 C before1520 C. The melt was poured into a gating sys-was shown in Fig. 3. Application of a steel jointshown in Fig. 4) restricts the motion of ceramic

    s and prevents leakage of molten metal. The moldcooled sometal leakage is not a problem. The entires lled with high purity argon (99.99%) up to 1kPa.cmoldwas pre-heated to 400600 C.Whenmoltenoured into the mold, the steel joint is water-cooledly; then, molten alloys are solidied under cen-ce with speed of mold rotation as 260 rpm. Afterhe mold pieces were removed from many differentnd the casting product was then taken out to cool

    Fig. 5

    tion coedges,perature. The turbine turbocharger wheel castingm in diameter has a rather complicated congura-

    ematic illustration of turbocharger ceramic

    2.3. Ch

    The morphmission eledistributionyses (EDS,with Cu Kization. Spthe centraature Vickload and 1Specimens20mm in gat room tefracture toomnipotenwere loadeThe notcha geometrmal width34mm.4 Steel joint of multi-piece ceramic mold.aracterizations

    ologies and microstructure are observed by trans-ctron microscopy (TEM, H-800). Chemical elements were examined by the energy spectrum anal-OXFOED INCA). X-ray diffraction (XRD) analysisradiation was used for microstructural character-

    ecimens for these examinations are cut out froml sprue of the wheel casting. The room temper-ers hardness (HV10) was measured with a 10N5s indentation time, averaging at least ve tests.for tensile tests were 4mm in diameter and

    auge length. Unaxial tensile tests were performedmperature at a strain rate e=1.3103 s1. Theughness tests were carried out on the electronce testing machine (Instron 5569). The specimensd at a constant crosshead speed of 102 cms1., cut through electro-discharge machining, hasy characteristic of root radius of 200m, nor-of 2mm and a normal length of approximately

  • j ournal of mater ials process ing technology 2 0 4 ( 2 0 0 8 ) 492497 495

    3. Result and discussion

    3.1. Microstructure

    XRD for the alloys (Fig. 6) identied that -TiAl (tetragonallattice, a=0.4016nm and c=0.4073nm) is the major phasewith small amounts of 2-Ti3Al (close-packed-hexagonal lat-tice, a=0.5753nm and c=0.4644nm). Fig. 7 shows the typicalmicrostructure from the surface TANC alloy. The phaseboundaries can be clearly distinguished with no evident inter-diffusion. The matrix is -TiAl and Ti3Al (2 point) phase. Thewhite area (3 point) clear contrast to matrix is Nb3Al phaseand the greywhite area surrounding the Nb3Al phase is (Al,Ti)3Nb phase; themassive grey area (1 point) is chromium alu-minide. So it is indicated that the matrix phase is TiAl +Ti3Al(+2), the reinforcement phases are chromium and niobiumaluminide.

    The TEM microstructure of the specimen is presented inFig. 8. Microstructure transformation is dependent upon notonly cooling rate but also composition of the alloy. Cooling rateis a predominant factor inuencing the grain size, while addi-tions of elements, which slow down the various solid-statetransformations are another factor (Godfrey et al., 1997). Inthis work, due to the high cooling rate, TACN alloys exhibit

    Fig. 6 XRD analysis of TACN alloys.

    Fig. DS analyses: (b) 1 point; (c) 2 point; (d) 3 point.7 SEM micrographs of the surfaces of TANC alloy (a) and E

  • 496 j ournal of mater ials process ing technology 2 0 4 ( 2 0 0 8 ) 492497

    ric s

    Table 2

    Alloy (at.% rdne

    K418a 310Ti48Ala 250Ti47Al5N Ti48Al1V 360Ti48Al1V 330Ti48Al2N TACNa 300

    a As-cast.b As-cast +

    ne microsdiameter (Fin a fully laphase intercolony, thetion, and thdistinct intThe EDS anTi concentrAl concentsubstitutes

    3.2. Me

    Room temshown inaverage valerties depeconditions,variables. Lity but be beattributesrate. At theing rate anand hardncan be incrboundary dmicrostrucFig. 8 TEM photomicrograph of TACN alloys (a) isomet

    Room temperature mechanical properties of TACN alloys

    ) YS (MPa) UTS (MPa) Elongation (%) Ha

    557 627 3.34.6430 500 0.32.1

    ba 480 510 0.5a (100200m) 430 500 1.2a (300500m) 400 475 1.8b2Mnb 392 460 0.9

    56010 65910 1.60.3

    HIP.tructure with grain size as an average of 200nm inig. 8a). In addition, cooling rate and alloying resultmellar structure where the lamellae are mostly -mixed with 2 lamellae (Fig. 8b). Within a lamellarlamellar laths align themselves in the same direc-e spacing of laths varies from10 to 20nm.There areerfaces between the brighter and the darker laths.alysis showed that the darker regions have higherations, whereas the brighter regions have higherrations. It has been found that Nb preferentiallyfor Ti, whereas Cr occupies the Al sublattices.

    chanical properties

    perature mechanical properties of sample wereTable 2. The results of this works represent theue of ve tests. It is shown that mechanical prop-nd on not only microstructure but also castingalloy composition and a host of process-relatedarger lamellar structure could result in low ductil-necial to creepproperties,while lower elongation

    to solidication conditions such as high coolingsame time, solidication processes such as cool-d centrifugal force also result in higher strengthess. The fracture toughness (KC) of TACN alloyseased under centrifugal force with extensive grainiffusion and sliding. It also intensively depends onture of alloys, i.e. lamellar microstructure exhibit

    superior frature. Grainhomogeneipared withto manufacbased alloystrength (Ubecome this advantaTACN alloyceramic msuccessfulturbocharg

    In conclshown instructure:ture: 1520400600 C,atmospher

    4. Co

    The procesusing multfabricatingexhibits nin diametetructure and (b) lamellae structure.

    ss (HV10) KC (MPam1/2) Reference

    21.2 Huanming et al. (2002)14.5 Kim (1989)18.5 Yang et al. (2003a,b) Jovanovic et al. (2005) Jovanovic et al. (2005) Kuang et al....

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