Improvement in mechanical properties of hypereutectic Al-Si-Cu alloys through sono-solidiifed slurry

Special ReportCHINA FoUNdRYVol.11 No.4 July 2014 ..Celebrating the 1 G h Anniversray 2004-2014Improvement in mechanical properties ofhy pereutectic Al-Si-Cu alloys through sono-solidified slurry*Yoshiki Tsunekawa', Shinpei Suetsugu', Masahiro Okumiya', Naoki Nishikawa', and Yoshikazu Genma?1. Toyota Technological Institute, Tenpaku, Nagoya 468-8511, Japan;2. Toyota Motor Corporation, Toyota 471-8571, JapanAbstract: For the wider applications, it is necessary to improve theductility as well as the strength and wear-resistance of hypereutectic AI-Si-Cu alloys, which are typical light-weight wear-resistant materials. Anincrease in the amounts of primary silicon particles causes the modifiedwear-resistance of hypereutectic Al-Si-Cu alloys, but leads to the poorstrength and ductility. It is known that dual phase steels composed ofhetero-structure have succeeded in bringing contradictory mechanicalproperties of high strength and dutility concurrntly. In order to applythe idea of hetero-structure to hypereutectic Al-Si-Cu alloys for theachievement of high strength and ductility along with wear resistance,ultrasonic iradiation of the molten metal during the solidification, whichis called sono-solidification, was carried out from its molten state to justabove the eutectic temperature. The sono-solidified AI-17Si4Cu alloy iscomposed of hetero-structure, which are, hard primary silicon particles,soft non-equilibrium a-AI phase and the eutectic region. Rheo-castingwas performed at just above the eutectic temperature with son-olidifedslurry to shape a disk specimen. After the rheo-casting with modified sono-* Yoshiki Tsunekawasolidified slury held for 45 s at 570 °C, the quantitative optical microscopee, borm in 1945, is currentlyobservation exhibits that the microstructure is composed of 18area% ofhard primary silicon particles and 57area% of soft c-AI phase. In contrast,Institute. He has been working inthere exist only 5 area% of primary silicon particles and no a-AI phase inthe fields of casting processes withrheo-cast specimen with normally solidified slurry. Hence the tensile testsultrasonic vibration, wettability inof T6 treated rheo-cast specimens with modified sono-solidified sluryexhibit improved strength and 5% of elongation, regardless of havingphenomenon at the vibrating endmore than 3 times higher amounts of primary silion particles compared tosurface and sono- solidification, asthat of rheo-cast specimen with normally solidified slurry.well as reactive thermal spraying.Key words: semisolid casting; slurry; ultrasonic vibration; sono-publication of more than 150 peer-solidification; hetero-structure; hypereutectic aluminumreviewed papers in interationallyalloy; primary silion; a-Al solid solutionpublished journals. From 2003CLC numbers: TG146.21Document code:APresident of the Japan ThermalArticle ID: 1672-6421(201404-396-06Spray Sociey. He has been a Fellowof ASM International since 2009and President of the Japan FoundryPhe wider applications of aluminum alloy components are expected to achieveEngineeing Society from 2010 toweight reduction of transport equipment such as automobiles. Since hypereutectic2012.Al-Si alloys exhibit high strength at elevated temperatures, good wear resistance and中国煤化Ioyta-ti.acjpsmall thermal expansion cofficient, they have been applied to compressor and engicomponents ". 21. These outstanding material characteristics are mainly owing to hCNM H25-25primary silicon particles crystallized during the solidification. It is known that highSpecial ReportCelebrating the 16 th Anniversray 2004-2014CHINA FOUNDRYamounts of crystallized primary silicon particles are required temperature and degassed with argon bubbling for 15 min. Afterto improve the wear resistance 8, finer and granular primaryimmersing the ultrasonic radiator in the molten aluminum alloy,silicon particles are moreover necessary to assure high strength the sample was cooled down to a given semisolid temperatureand ductility [9.10. However, the liquidus temperature of with ultrasonic vibration. Additionally, normal solidification washypereutectic Al-Si alloys rapidly increases with an increasealso carried out without ultrasonic vibration to compare with thein silicon content, so that the addition of the alloying element sono-solidified microstructure. Semisolid slurries cooled downsilicon extends the semisolid temperature range composed ofto a given temperature with or without ultrasonic vibration wereliquid alloy and solid silicon . Consequently, the primaryrheo-cast immediately after pouring into a sleeve.silicon particles are ready to grow coarsely during thesolidification, which causes the mechanical properties, plasticdeformability and machinability to deteriorate. As a counterTable 1: Chemical compositions of original ingots formeasure, the generation of heterogeneous nucleus of AlP isrheo-casting with sono-solidified slurryknown to be quite effective in refining primary silicon particlesAlloy No. SiCuFeMnPAIin hyereutectic AI-Si aloys 'Si7.020.0070.14 0.002 0.0004 Bal.Hard primary silicon particles are made finer by ultrasonic0.09 0.010vibration of the molten hypereutectic A1-Si alloys during25Si25.0 0.002 0.047 0.002 0.0002 Bal.the solidification, which is called sono-solidification7Cu0.0727.070.140.001< 0.001Bal.The authors have reported that non-equilibrium a-Al phaseappears in hypereutectic Al-Si alloys in sono-solidification“, in addition to the refinement of primary silicon particles.The rheo-casting apparatus shown in the bottom of Fig. 1In contrast, the usually contradictory mechanical properties of consisted of plunger tip, sleeve and mold. The plunger tip wasstrength and ductility are concurrently improved by creatingdirectly connected with a 20-ton oil cylinder, whose moving ratehetero-structure composed of hard and soft dual phases in steels was set at 0.05 ms' without loading. The preheat temperature ofn. Then the expression of characteristics arising from hetero-the mold was 200。C in all rheo-castings. The semisolid slurrystructure can be also expected to sono- solidified hypereutecticwas shaped to a disc-like rheo-cast sample 45 mm in diameterAl-Si alloys consisted of hard primary silicon particles and soft and 15 to 20 mm in thickness. Rectangular tensile specimens ofa-Al phase.8 mm width and 3 mm thickness were formed from as cast disc-In the present study, the focus is on leading light-weight anti- like samples by wire-cut electro-discharge machining (WEDM).wear material of hypereutectic Al-17wt.%Si- 4wt.%Cu alloy, that Some of the Al-17Si-4Cu and Al-7Si-4Cu alloy samples wereis, fundamental composition of A390. The hetero-structure ofheat-treated for 5 h at 500。C as solution treatment and for 5 h athard primary silicon particles and soft a-A1 phase was prepared 160 °C as aging (T6 treatment). The cross-head speed was fixedby the sono-solidifcation, in which slurry was shaped by rheo- at 0.2 mm:min' in all tensile tests. The tensile test specimenscasting at the semisolid temperature. Through the measurementprepared by WEDM were found to exhibit nearly the sameof tensile strength and elongation of rheo-cast Al-Si-Cu alloy strength and elongation as those without re-melted surface layerwith hetero-structure, the effect of hetero-structure composed by WEDM.of hard silicon particles and soft a-Al phase on the usualcontradictory mechanical properties are elucidated.UrtrasonicThermocouple「 Dataradiator sLogger1 Experimental procedureTemperatureCOPtrollerWith precisely compounding ingots listed in Table 1,- conthypereutectic Al-17Si-4Cu and Al-17Si, hypoeutectic Al-7Si-、MoltenFurnace4Cu and Al-7Si alloys were prepared without the addition十Crucibleof grain refining agents to primary and/or eutectic silicon.ThermocoupleSemisolid slurries were prepared with and without applying+ Holderultrasonic vibration during the solidification by using theapparatus shown in Fig.1, in which the rheo-casting apparatusSemisolid.was also shown at the bottom. The sono-solidification apparatusslurry |Plunger的for slurry preparation was composed of an ultrasonic generatorActuator[ Calcium silicateand radiator, electric furmace, sample holder, BN coated SUS304Moldcontainer and sample lifter. The output power of the ultrasonic中国煤化工generator was 600 W, the resonant frequency was 19 kHzand the vibration peak to peak amplitude was 20 um withoutFig.1: Schemawith sono-souulied siurryTHC N M H Gapparatusloading. Each alloy was heated up to 50 °C above the liquidus397Special ReportCHINA FoUNdRYVol.11 No.4 July 2014 .Celebrating the . h Anniversray 2004-20142 Results and discussionFig. 2. Both dark gray contrast of primary silicon particlesand white area of granular a-Al phase recognized in Fig. 2(a)2.1 Sono-solidification of Al-17Si-4Cu alloyare thought to be solid just before the rheo-casting, and theThe liquidus temperature and the eutectic temperature of AI- remaining gray area of the eutectic region to be crsallied from17Si-4Cu alloy were calculated to be 655。C and 570。Cthe liquid state by rapid cooling. In contrast, coarse primaryrespectively using Thermo-Calc. Then immediately after the silicon particles with a complex facetted shape appear as shownsonsoldification of molten alloy from 700。C to 570。C，in Fig. 2(b) rheo-cast with nomally sldifid sury, and thererheo-casting was crried out at 570 °C. Typical microstructures exist no non-equilibriuimn granular a-Al phase, which is difentof disc samples rheo-cast with sono-solidified and normally to that in Fig.2(a). .solidified slury (without ultrasonic vibration) are shown ina)(b200 μmFig. 2: Typical microstructures of rheo-cast AI-17Si-4Cu alloy: sono-solidified (a) and normally solidified slurry (b)Tensile tests of rheo-cast samples were carried out in as-determined to be 595 °C in Al-7Si alloy and 580 °C in Al-7Si-cast condition. Their tensile strength and elongation are 1934Cu alloy based on the Thermo-Calc calculation. Then rheo-MPa and 0.2% for sono-solidifed slurry, and 162 MPa and casting with sono-soldifed slurry was performed immediately0.6% for normally solidified slurry. The tensile strength after the termination of sosoldidicationr..increases because of the refinement and granulation of primaryTypical microstructures of tensile specimens after fracturesilicon particles by sono- solidification. However, there is are shown in Fig.3. The shapes of the a-AI phase are modifiedno improvement in elongaion, nevertheless soft granular from drndritetpe to ganlartype in Al-7Si and A1-7Si4Cua-Al phase, which is expected to contibute high duetilit, is alloys by applying son-oidifed slurry to theo-casting. Thecrystallized to nearly 30 area% by sono-solidification.amount of crystallized a-Al phase is measured to be 60area% inAl-7Si alloy and 58area% in Al-7Si-4Cu alloy, through imageanalysis. Based upon the microstructure shown in Fig. 3(a) close2.2 Effects of alloying element copper onto the fractured surface, nodules of c-Al phase in Al-7Si alloymechanical propertiesare prolonged in the tensile direction, which contributes to theAlthough the hetero-structure was achieved in rheo-castmodified ductility. However, the a-Al phase is not distinctlysamples of Al-17Si-4Cu alloy with sono-olidifed slurry, the deformed in Al-7Si-4Cu alloy shown in Fig. 3(b), which meanselongation was not improved in all tensile tests. In order to that it does not contribute to the ductility. Black dots of CuAl2elucidate the reason, the effect of alloying element copper onphase are recognized in the eutectic region of Fig. 3(b) whichelongation was studied in rtheo-cast samples with sono-olidified has been fnally solidified during the rheo-casting. T6 treatmentslurries of hypoeutectic Al-7Si and Al-7Si-4Cu aloy, which are(solution treatment and aging) was carried out to make thecharacterized by good ductility. The primary silicon particles are CuAl2 phase disappear from the eutectic region.not crystallized in hypoeutectic Al-7Si alloy, in which more thanThe tensile strength and elongation of as-cast Al-7Si rheo-cast50area% of a-Al phase is crstallized in equilibrium conditionsamples with sono-solidified slurry, as-cast and T6 treated Al-instead. Al-7Si alloy is quite suitable for the study on the effect 7Si-4Cu alloy, are shown in Fig. 4. In the as-cast condition, theof soft a-Al phase on ductility, since a-Al phase granulates tensile strength is improved by the adition of copper to Al-7Siin the sono-solidification 4. The onset temperature of sono-alloy. The elongation of Al-7Si alloy is 27%; however, it dropssolidification was determined to be 50 °C above the liquidus abruptly to 3% in中国煤化工ontaining CuAl2 .temperature, that is, 617 °C in Al-7Si alloy and 603 °C in AI-phase in the euteYH中国保化二he CuAlphase7Si-4Cu alloy. In order to adjust an amount of a-AI phase atdisappear in the (c N M H G, the eongationsolid fraction of 0.3, the termination of sono-solidifcation was is increased to 10% along with a rise in tensile strength.98 ISpecial ReportCelebrating the 16 th Anniversray 2004-2014.. Vol.11 No.4 July 2014CHINA FOUNDRYa)b)1200 μmFig. 3: Typical cross-section microstructures perpendicular to fracture surface after tensile test of rheo-castAl-7Si alloy (a) and Al-7Si-4Cu alloy (b) with sono-solidified slurry (note: TD denotes tensile direction)。300n 3(of eutectic silicon in Al-7Si alloy and Al-17Si alloy, which。0were produced by normal liquid casting and rtheo-casting, are20 e200亿Rshown in Fig. 5 and Fig. 6, respectively. The eutectic siliconin hypoeutectic Al-7Si alloy of Fig. 5(b) is finer after rheo-5 100AS一10casting compared with that after conventional casting. Incontrast, that of hypereutectic Al-1 7Si alloy is the same or evencoarser after rheo-casting than after conventional liquid casting.Al-7SiAl-7Si-4Cu A-7Si-4CuWhen the solidification proceed to just above the eutectic(as cast)(T6)temperature, the latent heat is emitted with the crstallizationFig. 4: Tensile strength and elongation of rheo-cast Al-7Siof primary a-Al phase in Al-7Si and primary silicon particlesand AI-7Si-4 Cu alloys with sono-solidified slurryin Al-17Si alloy. The cooling curves of molten hypoeutecticafter T6 treatmentAl-7Si and hypereutectic Al-17Si alloys are shown in Fig. 7.Hypoeutectic Al-7Si alloy containing a great deal of granularIn the hypoeutectic AlI-7Si alloy of Fig. 7(a), the cooling rateis reduced below the liquidus temperature of 617。C; hence,a-Al phase exhibits the extreme elongation of 27%.the crack initiation and propagation preferentially occur inthe eutectic silicon becomes finer, because the solidificationthe eutectic region containing CuAl2 phase as shown in Fig.rate in rheo-casting becomes faster compared with that in3(b). Hence, the granular a-Al phase seems to lose the effectconventional casting. In contrast, the decrease in cooling rate isregarding the improvement in ductility. In the case of AI-Si-Cunot recognized below the liquidus temperature of 645 °C in thealloy rheo-castings, the disappearance of the CuAl2 phase by T6hypereutectic Al-17Si alloy of Fig. 7(b). When the molten AI-Si alloy is cooled down to the eutectic temperature of 577 °C intreatment is expected to make the ductility partially recover.the equilibrium condition, the solid fraction of primary a-Al is2.3 Refinement of eutectic silicon by rheo-48.3wt.% in the hypoeutectic Al-7Si alloy, and that of primarycastingsilicon particles is calculated to be 5.7wt.% in the hypereutecticThe morphology of eutectic silicon in Al-Si alloys is expectedAl-17Si alloy from the equilibrium phase diagram . Thusto afet the tensile strength and elongation. The morphology the latet heat of solifed primary silicon particles has lttle(a(b20 um中国煤化工MHCNMH G .Fig. 5: Typical microstructure of conventional die-castings (a) and rheo-castings (6) of hypoeutectic Al-7Sialloy with normally solidified slurry399Special ReportCHINA FoUNdRYVol.11 No.4 July 2014 ..Celebrating the . h Anniversray 2004-2014(a)b)20 umFig. 6: Typical microstructures of conventional die-castings (a) and rheo-castings (b) ofhypereutectic Al-17Si alloy with normally solidified slurryT6 treatment, are shown in Fig. 8. The area fraction of granular(ea-Al phase increases from 30area% with non-held sono-solidified卜65slurry at 570。C to 57area% with sono-solidifed slurry held forLiquidus temperature.45 s at 570 °C. The area fraction of the eutectic region drastically(6170C)罾600decreases with increasing a-Al phase, since the maximumEutectic temperature色577solubility of silicon in a-Al phase is only 1.6wt.% at the eutectic50l-temperature in the Al-Si phase diagram. The increase in theamount of crystallized a-Al phase also leads to the higher amountof primary silicon particles due to the ejection of silicon from thea-AI phase. The primary silicon is l3area% in theo-cast sampleswith non-held sono-solidifed slurry. It then rises to 18area% when0ot"Eutectic temperature |held for 45 s at 570。C with ultrasonic vibration. The amounts ofV77C)]primary silicon particles, that is, the area percentage of primary50300400silicon, increase greatly in rheo-cast samples with sono-solidifedTime t (S)slury, because the primary silicon is only 5area% in that withFig. 7: Typical cooling curves of AI-7Si alloy (a) andnormally solidifed slurry. The hardness of primary silicon particlesAl-17Si alloy (b) in normal solidificationwas measured to be approximately 1000 HVooso and that of a-AIphase is 95 HVoo25 after the T6 treatment.effect on the cooling rate below the liquidus temperature in theThe tensile strength and elongation of rheo-cast samples afterhypereutectic Al-17Si alloy. The cooling rate in theo-casting ofT6 treatment, whose microstructures are shown in Fig. 8, areAl-17Si alloy is expected to be nearly the same or even slowershown in Fig. 9. The improvement in the tensile strength of thecompared with that in conventional casting from the heatrheo -cast sample with non-held sono solidified slurry at 570。Ctransfer point of view, so that it is difficult to refine the eutecticis recognized. The ductility is also slightly improved by applyingsilicon in AI-17Si-4Cu alloy by rtheo casting.sono-solidified slurry compared with that of rheo-cast sampleswith normally solidified slurry. However, the tensile strength2.4 Modified hetero-structure for higherof rheo-cast samples with sono-solidified slurry held for 45 s atductility570 °C declines from that with non-held sono-solidified slurry. ButThe improvement in tensile strength and ductility based on the the ductility is remarkably improved owing to a great amount ofhetero-structure of hypereutectic AlI-17Si-4Cu alloy is disturbed crystallized granular a-Al phase. The decrease in tensile strengthby both CuAl2 phase crystallized in the eutectic region andin the samples with ultrasonic iradiation for 45 s at 570。C iscoarse eutectic silicon. When the CuAl2 phase is removed by caused by higher amounts of crystallized primary silicon particlesT6 treatment the ductility is better due to the soft a-Al phase. and soft a-Al phase, occupying more than 50area% in the wholeMoreover, the eutectic region is considerably reduced by both amicrostructure, created by additional ultrasonic vibration for 45 sathigher amount of crystallized non-equilibrium a-Al phase and 570 °C. Although the amount of primary silicon particles increases,primary silicon to restrict the crack initiation and propagation.the decrease in ductility of hypereutectic Al-17Si-4Cu alloy withRheo-casting at 570 °C was carried out with sono- solidified AI-the modified hetero-structure is caused by the large amount of17Si- 4Cu alloy slurry which was iradiated with ultrasound from crystallized soft a-Al phase. The hetero-structure produced by700。C to 570。C, as mentioned previously. The other type of sono-odification中国煤化Isistance becauserheo-casting was also carried out with sono-solidified slurry which of larger amountsHCNMHGesascomparedwas given additional ultrasonic vibration for 45 s at just above with those rheo-casl salnprcs wul liialiy sudified slurry 38.the eutectic temperature of 570。C. Both microstructures, after00 ISpecial ReportCelebrating the 16 th Anniversray 2004-2014vol.11 No.4 July 2014CHINA FOUNDRY(a)200 um200 μmFig. 8: Typical microstructures of rheo-cast AI-17Si-4Cu alloy with sono-solidified slurryat 570 °C (a) and after holding for45 sat 570 °C (b)毫00rn3References00 t------- .1] Kurita H, Yamagata H, Arai H, et al. Hypereutectic Al-20%Si alloyengine block using high-pressure die-casting. SAE Paper, 2004-01-1028. ，t 10[2] Jorstad J L. ASM Handbook, VOL.15, Castin. ASM International,A中2008: 263-265.[3] Sharma R, Anesh B, and Dwivedi D K. Influence of silicon and heatNormalSoSono-teatment on abrasive wear behaviour of cast AI-Si-Mg aloys. Mater.solidifficationsolidification solidifcaticSci. Eng.A, 2005, 408: 274-280.(570C 0s) (570 C 45 s)[4] Birol Y and Birol F. Wear properties of high-pressure die cast andthixo-formed aluminum alloys for connecting rod applications inFig. 9: Tensile strength and elongation of rheo-cast Al-compressors. Wear, 2008, 265: 590 597.17Si-4Cu alloy with and without sono-solidified[5] Chen M and Alpas A T. UItra-mild wear of a hypereutectic Al-slurry after T6 treatment18.5wt.%Si lloy. Wear, 2008, 265: 186-195.[6] EImadagli M, Perry T, and Alpas A T. A parametric study of therelationship between microstructure and wear resistance of AI-Si alloys.Wear, 2007, 262: 79-92.3 Conclusions[7] Barekar N S, Dhindaw B K, and Fan Z. Improvement in siliconWith the application of ultrasonic vibration to molten Al-17Si-morphology and mechanical properties of AI-17Si alloy by melt4Cu alloy, the creation of hetero-structure composed of hardconditioning shear technology. Intemational J Cast Metals Research,primary silicon particles and soft a -AI phase was atempted for2010, 23: 225 -230.rheo-casting. Through the tensile tests of rheo-cast samples with[8] Reddy T V S, Dwivedi D K, and Jain N K. Adhesive wear of stir casthypereutectic Al-Si-Mg alloy under reciprocating sliding conditions.sono-solidified slurry, the following results were obtained:(1) The hetero-structure of hard primary silicon particles [9)] Oda K, Komarov s, and Ishiwata Y. Refinement of primary sion ofwith hardness of 1000 HVao50 and soft non-equilibrium a-Alhypereutectic Al-Si alloys by ultrasonic radiation and its application fordie castings. J. JILM, 2011, 61: 149-154.phase with 95 HVo.o25 (after T6 treatment) is achieved in sono-[10] KitaokaS. Al-SiAlloys. J. JILM., 2011, 61: 485- 503.solidified AI-17Si-4Cu alloy. However, the tensile testsof [1 Okamoto H. Phase Diagrams for Binary Aly. ASM Ieational,rheo-cast samples show that ductility is not improved due to2010: 44.the existence of CuAl2 phase in the eutectic region and coarse12] Eskin G I. Ultrasonic Treatment of Light Alloy Melts. Gordon andBreach Sci. Pub.: Amsterdam, 1998: 240 -244.eutectic silicon. .13] Osawa Y, Takamori S, Arakane G, et al. Changes in primary crystal(2) Large amounts of refined primary silicon particles andmorphalogy of AI-Si lloys at sld-liquid coexisting temperatures bygranular a-Al phase can be created in Al- 17Si-4Cu alloy byultrasonicbration. J. JFS.,. 2000, 72: 187-192aditional ultrasonic vibration of sonosolidifed slurry for 45s [14] Taga K, Fukui Y, Tsunekawa Y, et al. Crslalzaion of nguilbiuimaluminum solid solution in AI- 18mass %Si alloy through acousticat 570 °C.cavitation. J. JFS., 2009, 81: 469-474.()Through the removal of CuAl2 phase by T6 teatment and [15] Komarov s, Ihwa v, and oda K. Reinement of pimay slion inthe modified hetero-structure composed of large amounts of finecasting aluminum alys via aliation of ultrasonic vibration to DCprimary silicon particles of 18area% and granular a -Al phase ofprocess. In: Proc. 12th International Conf. Aluminum Alloys, 2010:652- 657.57area%, both tensile strength and ductility are simultaneously16] Fukui Y, Tsunekawa Y, and Okumiya M. Grain refinement of primaryimproved by the creation of modified hetero-structure, which issilion in hypereutectic Al-Ssi aloy by ultrasonic radiation. J. JFS, 2010,produced by the use of additional ultrasonic vibration to sono-82:618-624.solidified slurry and T6 treatment.[17] Rashid MS.:中国煤化iae so., 181.1245-266.This study was financially supported by the Grantin Aid for Scientific Research (2356MHCNMHGouppuii rivyian for FormingStrategic Research Infrastructure from the Ministry of Education, Science, Sports and Culture, Japan.I 40