j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335

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

Effect croof hyp

H.K. Fena Key Labora n, anJilin Universb School of M 4, PR

a r t i c

Article histor

Received 17

Received in

26 October 2007

Accepted 20 December 2007

Keywords:

Hypereutec

Ultrasonic t

Microstruct

y wa

exp

trea

distribution of the microstructures of the AlSi alloy were observed by means of an opti-

cal microscope. The results show that after the hypereutectic Al23%Si alloy was treated

by ultrasonic wave, the hydrogen bubbles in the alloy melt were removed, the primary Si

phase was rened, the morphology of the primary -Al dendritic crystal was changed to

equiaxial crystal, and the eutectic lamellar spacing increased. Themechanisms of ultrasonic

1. In

AlSi alloysmilitary indability, therand MaleqAbu-DheirHypereutectribologicalvided by thductility ofproperty oand distribmechanicaSi phase orformly in mit is effectivin order to

CorresponE-mail a

0924-0136/$doi:10.1016/tic AlSi alloy

reatment

ure

treatment improving the microstructures of the hypereutectic AlSi alloy were discussed.

2008 Elsevier B.V. All rights reserved.

troduction

are widely used in the automotive, aerospace, andustries because of their excellent castability, weld-mal conductivity and corrosion resistance (Haqueue, 1998; Criado et al., 1997; Tomida et al., 2003;et al., 2005; Liao et al., 2002; Wang et al., 2003).ticAlSi cast alloys are especially suitable touse forparts owing to their excellent wear resistance pro-e primary Si phase. However, themachinability andhypereutectic AlSi alloys are low. The mechanicalf AlSi alloys depends mainly on the size, shapeution of Si phases. AlSi alloys exhibit excellentl property as long as Si phases, either the eutecticthe primary Si phase, are ne and distribute uni-atrix (Abu-Dheir et al., 2005; Liao et al., 2002). Soe to change the morphology and size of Si phasesdecrease the bad effect of Si phases on matrix and

ding author. Tel.: +86 431 85095862; fax: +86 431 85095876.ddress: yusr@jlu.edu.cn (S.R. Yu).

improve the property of AlSi alloys (Tomida et al., 2003). Sev-eral practical techniques, such as rapid solidication of themelt, adding nucleating agent and rheocasting, are currentlyused to form the ne microstructure (Haque and Maleque,1998; Criado et al., 1997; Tomida et al., 2003; Abu-Dheir etal., 2005; Liao et al., 2002; Wang et al., 2003). In recent years,mechanical modication, including electromagnetic mixingand mechanical vibration to induce forced convection in themelt, has been successfully developed and applied to renegrains in the industry (Wang et al., 2003), and other new tech-niques for rening grains are also being developed. Ultrasonictreatment, as a new method for improving the solidicationstructure andmechanical property of alloys, is obtainingmoreand more attention. The injection of ultrasonic energy intomolten alloys can bring about some nonlinear effects, suchas cavitation, acoustic stream, emulsication, and radiationpressure, which can be used to rene microstructure, reducesegregation and degas (Jian et al., 2005; Eskin, 2001). However,

see front matter 2008 Elsevier B.V. All rights reserved.j.jmatprotec.2007.12.121of ultrasonic treatment on miereutectic AlSi alloy

ga, S.R. Yua,, Y.L. Lib, L.Y. Gongb

tory of Automobile Materials (Jilin University), Ministry of Educatioity, No. 5988 Renmin Street, Changchun 130025, PR Chinaaterials and Metallurgy, Northeastern University, Shenyang 11000

l e i n f o

y:

July 2007

revised form

a b s t r a c t

The hypereutectic Al23%Si allo

was designed specially for this

horn, and the molten alloy wasstructures

d College of Materials Science and Engineering,

China

s treated with ultrasonic wave. A novel horn crucible

eriment. The horn crucible was a part of ultrasonic

ted direct in this crucible. The morphology, size, and

j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335 331

Table 1 Composition of the AlSi alloy (wt.%)

Si 23Mn 0.01Cu 0.08Ti 0.02Fe 0.5Zn 0.005Mg 0.101Al Balance

the mechanisms about the effect of ultrasonic treatment onalloy melt, until now, are not yet clear, so it is necessary tocarry out more researches on this subject.

In this work, a novel ultrasonic treatment equipment wasdeveloped, and the inuence of the ultrasonic treatment onthe microstructures of hypereutectic Al23%Si alloy usingthis equipment was investigated, and the mechanisms ofultrasonic treatment for improving the microstructures werediscussed.

2. Experimental

2.1. Material and equipment

Commerciathe raw mTable 1.Alcontrolled e

The ultrment consiThe schemHorn is an iThe ordinathe linearhorn (Lalwas design

Fig. 1 Schequipment

end. So, the special horn was named horn crucible (Fig. 1).The length and cross-section area of the horn crucible weredeterminedcontinuous1996; Lin, 2a 20kHz trwas smalleof the ultrawave emittacoustic homelt. Thethe actioncontrast wnovel horntor was inradiator waby this neform, andavoided.

The temmelt was m

2.2. Exp

Si altemuredated700

as 4raturater

milymwer

etallo

Re

Eff

y ishenicao thicrosltrasmeniculdl hypereutectic Al23%Si alloy ingot was used asaterial. Its chemical composition was listed inSi alloywasmeltedusing an intelligentlynumericallectric resistance furnace.asonic treatment equipment used in this experi-sts of the temperature and power control systems.atic diagram of this apparatus is shown in Fig. 1.mportant part in ultrasonic treatment equipment.ry horn includes four styles, i.e., the stepped horn,horn, the exponential horn, and the catenoidaland White, 1996). In this work, a special horned and fabricated, and there was a crucible at its

Al23%at thisand powas treof 680tudewtempeinto w

Theusing Ophasestive m

3.

3.1.

Porositings. Tmechaings. SThe mwith uthat soultrasobles coematic diagram of ultrasonic treatment.

of coolinggravity casthe specimultrasonicis in accordsonic degasAn ultrasoate alternathe rarefacaccording to the theory that the stress passesly through the length of the horn (Lal and White,005). The horn crucible was vertically bolted withansducer, and the diameter of the horn crucibler than that of the transducer. During the coursesonic treatment of a metal melt, the ultrasoniced from the transducer and passed through thern (horn crucible) is propagated direct into thealloy melt becomes a part of acoustic horn, soof energy on the melt is raised remarkably. Inith ordinary ultrasonic treatment, because thecrucible was invented and applied, no radia-

serted into the melt, and the impurity from thes avoided. The microstructures of alloys treatedw ultrasonic treatment system were more uni-the segregation of the chemical composition was

perature of the ultrasonic treatment of the metalonitored with a temperature sensor.

erimental procedures

loymelt of 200 gwas heated to 800 Cand thenheldperature for 30min. The melt was stirred slightlyinto a preheated horn crucible (680 C), and then itwithultrasonicwave for 10minat the temperatureC. The ultrasonic power was 50W, and the ampli-m. After that, the horn cruciblewasmoved out thee control system and was immediately quenchedof 25 C.crostructures of the samples were investigatedpus GX51 optical microscope. The sizes of variouse measured statistically by means of the quantita-graphy analysis method.

sults and discussion

ect of ultrasonic treatment on degassing

one of the major defects in aluminum alloy cast-presence of porosity can be detrimental to thel properties and corrosion resistance of the cast-e degassing of aluminum alloy is very important.tructures of as-cast Al23%Si alloy without andonic treatmentwere shown in Fig. 2. It can be foundgas holes existed in the specimen not treated bywave (Fig. 2(a)). This indicates that the gas bub-not oated out the AlSi alloy melt in the courseand solidication under the condition of normalting. However, the gas holes were not found inen treated with ultrasonic wave (Fig. 2(b)), i.e., thewave has the function of degassing. This resultance with Eskin (2001). The mechanism of ultra-sing is closely related to the cavitation in the melt.nic wave propagating through a melt can gener-te regions of the compression and rarefaction. Intion region of the melt, a large number of small

332 j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335

Fig. 2 Mictreated bywave.

cavities arethese cavitulated andmelt.

3.2. Effof the prim

The distribAlSi alloyand no agg(Fig. 2(a)). Tcoarse owtal, and thecrystal withSi phase wSi phase w

After thultrasonichomogeneocomparedrostructures of as-cast Al23%Si alloy: (a) notultrasonic wave and (b) treated by ultrasonic

created. Therefore, the hydrogen diffused towardies, and the hydrogen bubbles were formed, coag-oated, which resulted in the degassing of the

ect of ultrasonic treatment on the morphologyary Si phase

ution of the primary Si phase in the hypereutecticnot treated by ultrasonic wave was homogeneous,regation was found over the section of the sampleshe shapes of the primary Si phase included mostlyer crystal, polygon or blocky crystal, and ne crys-ower crystalwas commonly composedof the club56 petals. The edges and corners of the primary

ere clear (Fig. 3(a)). The largest size of the primaryas up to 500m.e hypereutectic AlSi alloy was treated by thewave, most of the primary Si phase distributedusly in the section of the specimen (Fig. 2(b)). As

with the microstructures without ultrasonic treat-

Fig. 3 Micprimary Sithe primar

ment, theand the sizwas aboutunconspicucould be foucoarse andbecame sm

The reaSi phase mthe comprewave havephase in thSecondly, tstrong enouperse themimpact coutime couldwere the ctation acceas diffusion1996).rostructures of as-cast Al23%Si alloy: (a) thephase not treated with ultrasonic wave and (b)y Si phase treated with ultrasonic wave.

large and irregular primary Si phase disappeared,e of the primary Si phase decreased obviously and180m. The morphology of six petal ower wasous, and just slightly aggregative primary Si phasend. Themorphology ofmost primary Si phasewasblocky rod, and the edges of the primary Si phaseooth (Fig. 3(b)).sons producing the modication of the primaryentioned above include several aspects. Firstly,ssion and relaxation of high frequency ultrasoniceffect on the melt, so the edge of the primary Sie melt would be scoured and form a circle surface.he transient cavitation could produce an impactgh to break up the clustered ne particles and dis-more uniformly in the melt. Thirdly, the strong

pled with locally high temperatures in a very shortalso remelt the primary Si phase, so their edges

ircle (Yang et al., 2004). In addition, acoustic cavi-lerated the heat and mass transfer processes such, dispersion, emulsication, etc. (Abramov et al.,

j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335 333

3.3. Effect of ultrasonic treatment on the morphologyof -Al

Under the condition of non-equilibrium solidication, the pri-mary -Al phase can also produce in hypereutectic AlSi alloy(Tomida et al., 2003). Fig. 4 shows the morphology of the pri-mary -Al phase in as-cast Al23%Si alloy. It can be seen thatthe primary -Al phase, not treated by ultrasonic wave, wasdeveloped obviously into dendritic crystal and the length ofits primary arm was even up to 120m (Fig. 4(a)). After thealloy was treated by ultrasonic wave, the morphology of theprimary -Al dendritic crystal was changed to equiaxial crys-tal (Fig. 4(b)), and the size of the primary -Al phase reducedobviously from 120m to about 40m.

Themodication of the primary-Al phase resultedmainlyfrom the action of the cavitation and acoustic stream of ultra-sonic wave. The transient caviation produced strong impactand broke up the primary -Al dendritic crystal, and thenacoustic stream dispersed uniformly them to the melt. So thegrowth of the primary -Al dendritic crystal was restrainedand resulted in the formation of the primary -Al equiaxialcrystal.

Fig. 4 Micprimary -primary -

Fig. 5 Microstructures of as-cast Al23%Si alloy: (a) thegrowing correlation of the primary Si and -Al phases nottreated with ultrasonic wave and (b) the growing correlationof the primary Si and -Al phases treated with ultrasonicwave.

Effect of ultrasonic treatment on the relationn the primary Si and -Al phases

ecipitation of the primary Si phase in hypereutecticlloy resulted in the lack of Si element in matrix aroundmary Si phase, and promoted the formation of the pri--Al phases around the primary Si phase. The growthonof theprimary armof-Al dendritic crystalwas alongarp tips of the primary Si when the hypereutectic AlSias not treated by ultrasonic wave (Fig. 5(a)).r the hypereutectic AlSi alloy was treated by ultra-ave, the growth orientation of the primary arm of -Al

tic crystal along the tips of the primary Si phase wasned (Fig. 5(b)). The main reason was that the cavita-nd acoustic stream of ultrasonic wave promoted thesion of elements around the primary Si phase, and theution of elements around the primary Si phase wasomogeneous. On the other hand, the tips of the pri-i phase became obtuse under the action of ultrasonic

and the original growth direction of the primary arm ofndritic crystal was changed.rostructures of as-cast Al23%Si alloy: (a) theAl not treated with ultrasonic wave and (b) theAl treated with ultrasonic wave.

3.4.betwee

The prAlSi athe primary directithe shalloy w

Aftesonic wdendriweaketion adisperdistribmore hmary Swave,-Al de

334 j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335

Fig. 6 Miceutectic streutectic str

3.5. Eff

The shapealloy withThe lamel2.2m. Afttic lamella2.8m.

During euniformly,interface, thand the coends of thesolidliquidcave surfacphase of ththe eutectitreatedwitpression anmelt undermix of the mlargely. Therostructures of as-cast Al23%Si alloy: (a)ucture not treated with ultrasonic wave and (b)ucture treated with ultrasonic wave.

ect of ultrasonic treatment on eutectic structure

of the eutectic Si in the hypereutectic AlSiout ultrasonic treatment was aky (Fig. 6(a)).lar spacing of the eutectic structure is abouter being treated by ultrasonic wave, the eutec-r spacing increased (Fig. 6(b)), and it was about

utectic solidication, if the melt cannot be mixedthe solute will enrich in front of the solidliquide growths of the eutectic phaseswill be restrained,ncave surfaces will be formed gradually on theeutectic phases, i.e., the curvature radius of theinterface become negative (Fig. 7(a)). The con-

e will become more and more deep until anothere eutectic forms in the concave position. Therefore,c lamellar spacing decreases. When the melt wash ultrasonicwave, the alternate regions of the com-d rarefaction were continuously generated in thethe action of ultrasonic cavitation, so the uniformelt in front of the solidliquid interface increased

Al andSi elements in front of the solidliquid inter-

Fig. 7 Schwithout (a)

face could bof the solutor avoided.interface cphases grewnot be forreduce but

4. Co

After the hsonic waveremoved, togy of theequiaxial cphase reduincreased.ematic diagram of AlSi eutectic phase growthand with (b) ultrasonic treatment.

e sufciently dispersed. Therefore, the enrichmente in front of the solidliquid interface was reducedThe negative curvature radius of the solidliquidould not be formed (Fig. 7(b)), and the eutecticahead. As a result, the new eutectic lamels could

med, and the eutectic lamellar spacing did notincreased.

nclusion

ypereutectic Al23%Si alloy was treated by ultra-, the hydrogen bubbles in the alloy melt werehe primary Si phase was rened, the morphol-primary -Al dendritic crystal was changed to

rystal, the primary arm size of the primary -Alced obviously, and the eutectic lamellar spacing

j ournal of mater ials process ing technology 2 0 8 ( 2 0 0 8 ) 330335 335

Acknowledgements

This work was supported by The Specialized ScienticResearch Foundation for Doctor Subject in Colleges and Uni-versities by the Ministry of Education of China (Grant no.20030183019), Program for New Century Excellent Talents inUniversity, and 985 project of Jilin University of China.

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Effect of ultrasonic treatment on microstructures of hypereutectic Al-Si alloyIntroductionExperimentalMaterial and equipmentExperimental procedures

Results and discussionEffect of ultrasonic treatment on degassingEffect of ultrasonic treatment on the morphology of the primary Si phaseEffect of ultrasonic treatment on the morphology of alpha-AlEffect of ultrasonic treatment on the relation between the primary Si and alpha-Al phasesEffect of ultrasonic treatment on eutectic structure

ConclusionAcknowledgementsReferences