Materials Science and Engineering A 435436 (2006) 521529

Mechanical and wear properties of rhe56h bedpulogy,ly 20

Abstract

A356 allo ing hand tribolog t samin rheocast s ablywear loss an conby ploughin 2006 Else

Keywords: R ar pr

1. Introdu

It is repmany advantages over the conventional casting process. Themorphology of growing solidliquid interface in conventionalcasting processes is typically dendritic. The conventional cast-ing often contains internal structural defects such as oxide andgas entrapmcal propertitrend to promotive comcasting techbeen develsemi-solidadvantagescess complfraction tai

In the rhliquid sateduring the sing/injectinand the evo

CorresponE-mail ad

teded bys [1,

explored to produce complex-shaped Al-alloy components [8].For wide application of this process in various engineering fields,the mechanical and wear behaviour of rheocast products needto be well understood.

0921-5093/$doi:10.1016/jent, shrinkage porosity that leads to poor mechani-es. To achieve better properties, there is an increasingduce common Al-alloys (e.g., A356 and A357) auto-ponents by semi-solid processing route [3,4]. Twonologies, namely, thixocasting and rheocasting, haveoped for the production of metal components byprocessing route. Rheocasting process offers severalthan the thixocasting process including reduced pro-

exity, increases shot size flexibility and effective solidloring [5,6].eocasting process, molten alloy is cooled from theto the mushy state followed by stirring the alloyolidification to produce semi-solid slurry, then pour-g the slurry directly into the die. The formationlution of non-dendritic rheocast microstructure are

ding author. Tel.: +91 657 2271709; fax: +91 657 2270527.dress: klsah@nmlindia.org (K.L. Sahoo).

The objective of this study was to examine the mechanicaland wear behaviour of rheocast A356 alloy, which is a popu-lar Al-alloy for automotive components. These properties werecompared with those of conventional gravity die cast counter-part.

2. Experimental

In this study, commercial A356 aluminium alloy was used.The composition of the A356 alloy is given in Table 1. Theliquidus and the solidus temperature of the alloy were found tobe 615 C and 538.5 C, respectively.

The melting of the alloy was carried out in an electric resis-tance furnace in a clay bonded graphite crucible coated withalumina paints. After melting sufficient time was given for com-plete homogenisation of the melt. The melt was then degassedwith dry argon.

In the rheocasting process, the melt was continuously cooledand stirred under a constant rotational speed of 320 rpm using a

see front matter 2006 Elsevier B.V. All rights reserved..msea.2006.07.148gravity die cast A3A.K. Dey a, P. Poddar a, K.K. Sing

a National Metallurgical Laboratory, Jamshb National Institute of Foundry and Forge Techno

Received 8 May 2006; received in revised form 10 Ju

y produced by means of conventional gravity die casting and rheocastical properties were compared. The microstructure of conventional casample. The mechanical properties of the rheocast samples are considerd coefficient of friction in rheocast samples are always less than those ing mechanism.vier B.V. All rights reserved.

heocasting; Conventional die casting; Microstructure; Mechanical property; We

ction

orted [1,2] that semi-solid casting processes posses

associafollowclusterocast and conventionalalloy

, K.L. Sahoo a,r 831007, IndiaRanchi 834003, India06; accepted 28 July 2006

as been investigated and their microstructure, mechanicalple is fully dendritic in contrast to spheroidal morphologyhigher than the conventional cast samples. The volumetricventional cast samples at all loads. The wear occurs mainly

operty

with the breakup of dendrite arms into small piecesagglomeration and sintering of these pieces to form

7]. However, the rheocasting process has been largely

522 A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529

Table 1Composition of A356 alloy used for the present investigation

Ti Zn Sr Al

wt.% 0.13 0.04 0.01 Balance

mechanicament. The4 min1. Wrheocast sluof dimensi

Conventhe liquiducomparisonof the rectainvestigatioKellers remicrostruc

The plasamples we(300 C) rowith reversrolled sheeThe specimThe fractuscanning econvention5.0 kg.

The slidmachine. Tfabricatedalloy steellent to AIStrack radiu350 rpm, re2.0 m/s. Thapplied forcoefficienttronic sensforce in kgthe sensor oout for a totof the pins,fered by thwere calcucarried outthe complesurfaces ofworn surfaunderstand

3. Results

Fig. 1aingots (bot

the cast iron plate mould (average cooling rate 10 C/min).t microstructure of both castings show that phases are

mly distributed. The figures clearly show a morphologi-nge in the microstructures. In conventional cast sample,crostructure is fully dendritic whereas in rheocast sam-e primary dendrites are fragmented due to mechanical. Fig. 1b clearly shows that some grains are nearly spher-

d some are agglomerated together to form a bigger grain.imary Al dendrites are plastically deformed during rheo-

processing. However, with the continued stirring, thestrains within the fragmented grains would be consider-ss and process of coarsening or ripening will start. Sincearsening/ripening is driven by interfacial energy [12,13],cess will lead to a reduction in the surface area and even-spheroidal morphology is obtained. It was observed thatElement

Si Mg Mn Fe Ni

7.22 0.45 0.01 0.15 0.016

l stirrer type rheocaster with bottom pouring arrange-average cooling rate during stirring was abouthen the temperature was reached about 580 C, the

rry was poured in a cast iron rectangular type mouldon 20 mm 100 mm 300 mm.tional gravity die casting, poured from 30 C aboves temperature, was also made in the same mould, for

purposes. The samples were cut from the middlengular plate for structural and mechanical propertiesns. The polished samples were etched with modified

agent (2 cm3 HF, 3 cm3 HCl, in 175 cm3 H2O) forture examination.te type castings of both conventional and rheocastre cut into thickness of approximately 10 mm for hotlling. The rolling was carried out in a four high mille rolling facilities. Tensile specimen from as cast andt were prepared as per sketch given elsewhere [9].ens were tested at a strain rate of 8.3 104 s1.

red surfaces of the specimen were examined underlectron microscopy (SEM). The Vickers hardness ofal and rheocast samples were taken using a load of

ing wear tests were carried out using a pin-on-dische pins of 8 mm diameter and 40 mm length were

from 20 mm plate and made to slide against a lowdisc (material: 103 Cri-Eng-31HRS60W61, equiva-I 4340) of diameter 215 mm and hardness 62 Rc. Thes and the disc speed were maintained at 55 mm andspectively, to maintain at constant sliding velocity ofree loads, namely, 19.6 N, 29.4 N and 49.0 N wereeach test materials. Tangential force and hence, the

of friction were measured continuously with an elec-or attached to the machine and recorded. Frictionaland cumulative wear loss inm were measured fromutput as a function of time. The wear test was carriedal sliding distance of about 2.4 km. The specific weardefined as the cumulative volumetric wear loss suf-

e pin per unit sliding distance per unit load [10,11],lated from the cumulative wear data. Wear tests wereat room temperature without any lubrication. Aftertion of wear test, the Vickers hardness of the worneach pin was measured under a load of 5 kg. The

ces of pin samples were examined under SEM tothe wear mechanism.

fied inAs casuniforcal chathe miple, thstirringical anThe prcastingplasticably lethe cothe protuallyand discussion

and b shows the optical microstructure of as casth conventional gravity die cast and rheocast) solidi-

Fig. 1. OpticA356 sampleal microscopy of as cast: (a) conventional cast and (b) rheocasts.

A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529 523

Fig. 2. Optica(b) rheocast.

not only thalso eutectcast samplestructure coa range ofprocessed.rolling in oand therebshows optition) for boprimary -The Al gra

Fig. 3asamples inThe rheocathe conven

Table 2Mechanical p

Sample and c

ConventionalRheocast, as cConventionalRheocast, 70%l microstructure of 70% reduced sample: (a) conventional cast and

e primary Al grains (white portions in Fig. 1) butic Si particles (black portions) are globular in rheo-s in comparison to conventional cast samples. As thentains good amount of eutectic phases it should givemechanical properties when thermo-mechanicallyThe plate castings are subsequently processed byrder to examine its improvement in microstructurey, any escalation in mechanical properties. Fig. 2cal microstructures of rolled products (70% reduc-th the samples. It was observed that in both cases

Al grains are elongated in the direction of rolling.ins are softer than the eutectic Si phases.and b shows the ultimate tensile strength (UTS) ofas cast and in rolled condition (70% reduction).

st sample shows better strength and elongation thantional cast samples. Table 2 shows the 0.2% proof

Fig. 3. Comp(70% reductio

stress (YS)ples. Rheothe convenciated porooccurs at asolid structfavourablecomplex fecess [14]. Tmay effecti

roperties of conventional cast and rheocast samples

ondition YS (MPa) UTS (MPa), as cast 79.0 143.0ast 117.0 203.5

, 70% reduction 179.0 196.5reduction 217.0 232.2arative plots of stressstrain: (a) as cast and (b) rolled samplesn).

, UTS and percent elongation (f) of both the sam-casting shows better strength and elongation thantional cast samples because less shrinkage and asso-sity are expected in the rheocastings as pouring

temperature below the liquidus. The globular primaryure in the mushy rheocast material would be moreto liquid penetration for feeding, in comparison to theeding through dendritic structure in conventional pro-he globular shaped Si particles in the rheocast samplevely reduce the stress concentration at the boundary

Elongation (%) Hardness (VPN)4.12 83.36.30 88.62.20 87.22.60 94.0

524 A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529

Fig. 4. SEMconventional

between Sifore, the glwell as ducof the rheoconventionplates of thfor rolling.particles anthe micro-pThe rollingbring downment in stre

Fig. 4a afaces for bopaths tendethe shearincast samplethe tensileinitiation amicrostructure showing fractured surface of tensile specimen: (a)cast and (b) rheocast.

particles and matrix under an applied stress. There-obular shape would improve the tensile strength astility in the rheocast products. The Vickers hardnesscast samples at 5.0 kg load is also higher than theal cast sample (Table 2). The samples in the form ofickness 10 mm are homogenised at 400 C for 2 hThe object of hot rolling was to fragment the largerd to disperse them in the matrix as well as to sealorosity/voids formed due to interdendritic shrinkage.operation was carried out in several steps in order tothe reduction level of 70%. A considerable improve-ngth was observed for both the samples after rolling.nd b shows the SEM microscopy of the fractured sur-th the samples. In both samples, the tensile fractured to follow the primary -phase boundaries. Also

g of primary -phase is observed in the conventionals. From the fractographic view, it is also evident thatfracture occurred by dimpled rupture with the voidt the eutectic Si particles for both samples.

Fig. 5. The vaand rheocast A

The platto 8 mm diaThe microsformly distoptical micloss as a furiation of cumulative wear loss vs. sliding distance of conventional356 alloy under the load of: (a) 19.6 N, (b) 29.4 N and (c) 49 N.

e castings of 20 mm thickness were machined downmeter for wear behaviour study by pin-on-disc tests.tructure of both samples show that phases are uni-ributed in Al matrix. No porosity was observed byroscope. Fig. 5ac represents the cumulative wearnction of sliding distance at different loads, namely,

A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529 525

19.6 N, 29.4 N and 49.0 N, respectively. After a transient period,the wear loss was found to increase linearly with increase insliding distance. The volumetric wear loss also increases rapidlywith increasing load. The wear loss in case of conventional castsamples is distinctly higher than that of rheocast alloys at allloads. The wear test at the highest load (49.0 N) in conventionalcast alloy could not be carried out for longer duration due tohigh rate of wear. It suffered a wear loss of above 2 mm in trav-elling around 1 km distance. The sudden jump in wear rate in the

Fig. 6. Specifisliding distan

Fig. 7. The variation of cumulative wear loss vs. sliding distance of conventionaland rheocast A356 alloy under the load of 19.6 N.

above sample indicates a change in wear mechanism from mildto severe mode. The specific wear (understood as the wear lossdivided bytemperaturat differentsignificantlples due tonature of gobserved toing distancload of 49.and that ofwear rate aof wear whvalue of co is presenThe averag

istogram showing the average coefficient of friction at different loads.

c wear of conventional cast and rheocast alloys as a function of

ce at a load of: (a) 19.6 N, (b) 29.4 N and (c) 49.0 N. Fig. 8. Hthe normal force and the sliding distance) at roome is plotted in Fig. 6 as a function of sliding distanceloads. The specific wear rate of rheocast sample is

y lower as compared to that of conventional cast sam-combined influence of lower porosity and globular

rains. The wear rates at 19.6 N and 29.4 N loads arebe nearly constant or slightly increase after a slid-

e of 1300 m for both samples whereas at the applied0 N, the wear rate of rheocasting sample decreasesconventional cast sample rapidly increases. The hight the initial stage (Fig. 6) is due to the adhesive natureich is associated with high material loss and high

efficient of friction (). The representative graph forted in Fig. 7 for both the alloys at a load of 19.6 N.e value of at different loads is shown in Fig. 8. The

526 A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529

Fig. 9. Hardn

coefficientthat of conof frictionsamples isbecause dutogether (shThe hardneboth the alness of theincreasingThus, extenHowever, tis always habove resuhas a greatcasting offthan the co

The SEMdepicted somagnificatirials at 19.6respectivelload are sh

The wographical fstrips, themarks. Theend of thethe conventhe rheocasentrapped dhard asperiwear by scvolume offine groovealloy at 19.tinuous groess of as cast and worn surfaces of the samples at a load of 5 kg.

of friction of rheocast samples is always less thanventional cast samples. The variation of coefficientfrom its mean values in case of conventional castcomparatively larger than that of rheocast samplesring wear run the large particles piled up and forgedown later). This gives some obstruction to wear run.ss (VPN) of the worn surfaces was determined for

loys at three different wear loads (Fig. 9). The hard-worn surfaces of the pin materials increased with

load. This is due to work hardening of the materials.t of work hardening is more with increase in load.

he hardness of the worn surfaces of the rheocast alloyigher than that of conventional cast alloy. From thelts and discussion, it can be said that casting methodinfluence on the properties of the alloys. The rheo-

ers better mechanical properties and wear resistancenventional one.

micrograph of the worn surfaces of the samples hasme feature to understand the wear mechanism. Low

on micrographs of the worn surfaces of the pin mate-N and 49.0 N load are presented in Figs. 10 and 11,

y. The higher magnification micrographs at 49.0 Nown in Fig. 12.rn surfaces of the pin materials show diverse topo-eatures. Most of the worn surfaces consist of smoothsurfaces of which are characterized by fine scoringse smooth strips were extend uninterrupted from onepin specimen to the other. The worn surfaces of

tional cast samples were more heavily scored thant counterparts. Scoring may be due to abrasion byebris, work-hardened deposits on the counterface orties on the hardened steel counterface [1518]. Theoring mechanism, however, may not lead to a largewear since the amount of material removed from ais small. The worn surfaces of the conventional cast

6 N load (Fig. 10a) also shows extensive and long con-oves on the surfaces parallel to the sliding direction

Fig. 10. Low(a) convention

whereas thlesser grootional castsamples. Asignificantlindicatinging wear ruseen in theing directisliding direerally, occunear to thesure actingmagnification SEM micrograph of worn surface at 19.6 N load:al cast and (b) rheocast.

e rheocast alloy at the same condition shows muchves. The width of the continuous grooves in conven-samples is larger than that of the grooves on rheocasts the load increases, the width of the grooves enlargesy. Irregular plastic flow lines can be seen (Fig. 12)the occurrence of extensive plastic deformation dur-n. Cracks and strips of roughened surfaces are alsosame micrograph. The cracks propagate in the slid-

on (Fig. 11a). A number of cracks inclined to thection are shown in Fig. 12a. Crack nucleation, gen-rs at some depth below the surface rather than very

surface, owing to high hydrostatic compressive pres-near the asperity contact [19]. Cracks may initiate

A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529 527

Fig. 11. Low(a) convention

in the highface regionof metal isFig. 12a shmode of weobserved.

Duringstick to theof a conventhe wear ruis quite apsticking ofincreases afacilitate thmagnification SEM micrograph of worn surface at 49.0 N load:al cast and (b) rheocast.

ly work-hardened layer, particularly in the subsur-. When cracks grow and get interconnected, a layerremoved leaving a crater. This is delamination wear.ows such evidence. At the highest load, the mixedar, i.e., ploughing with local delamination wear were

wear run the worn particle debris agglomerated andgrooved surfaces. This is very prominent in case

tional cast alloy. Fig. 13 shows such areas. Duringn, with increasing load the frictional heat generatedpreciable. The localized heating actually facilitatesthe pin to the disc surface. The interface temperatures the wearing couples slide against each other thate sticking of wear debris at the grooved surfaces

Fig. 12. High(a) convention

[20]. The inhas also be[2123]. Tmay be attthe pin and

Archardtion of slidwas basedmaterials reasperities. Tation and tWith the aradius as cer magnification SEM micrograph of worn surfaces at 49.0 N load:al cast and (b) rheocast.

crease of friction coefficient at elevated temperatureen reported particle-reinforced cast alloy compositeshe rise in friction coefficient with increase in loadributed to enhanced accumulation of wear debris atdisc surface.[24] defined adhesive wear volume loss as a func-

ing speed, normal load and materials hardness. Thison a mechanism of adhesion at the asperities and themoval process was related to the cohesive failure ofhe initial microstructure, the process of crack nucle-

he subsequent growth were ignored in this theory.ssumption of hemispherical wear particles of sameontact area, Archard [24] developed an equation of

528 A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529

Fig. 13. SEM(a) convention

wear rate a

W = kLP3H

where W isL the slidinness of theapplied loa

Table 3Results of fitt

Materials

ConventionalRheocastmicrograph showing the particle agglomeration on the wear track:al cast and (b) rheocast.

s:

(1)

volume of materials worn out, k the wear coefficient,g distance, P the applied load and H is bulk hard-materials. The proposed proportionality between thed and the wear rate, and the inverse proportionality

Fig. 14. Corr(a) convention

between hain sliding w

The expis used toof Archardrelation pro

W = b0Lb

where W isL the slidinhardness (BThese valucorrelationof wear losimental dashown in Fgeneral agwith an errobe fitted to

ing data for the conventional cast and rheocast alloys

Coefficients

b0 b1 b2 b3

cast 0.49 0.6 1.29 1.350.49 0.9 0.8 1.4elation between experimental and theoretical values of wear loss:al cast and (b) rheocast alloy.rdness and the wear rate were not always observedear system.erimental wear data obtained in the present study

develop a generalized equation (a modified versions equation) to estimate wear loss of materials. Theposed is

1Pb2Hb3 (2)the volume wear loss (mm3), b0 the wear coefficient,g distance (m), P the applied load (N), H the bulkHN) and b1, b2 and b3 are the fitting coefficients.

es for both the alloys are tabulated in Table 3. The(R) for the fitting is very good (0.9937). The values

s predicted by using Eq. (2) are compared with exper-ta for both conventional cast and rheocast alloys asig. 14a and b, respectively. The graphs indicate a goodreement between predicted and experimental datar range of 8%. The experimental data obtained canan exponential equation W = 0.49L0.6P1.2H1.3 for

R2 Final loss (observed predicted)2

0.98 1460.98 96

A.K. Dey et al. / Materials Science and Engineering A 435436 (2006) 521529 529

conventional cast alloy and W = 0.49L0.9P0.8H1.4 for rheocastalloy with good correlation and high coefficient of determina-tion. As expected, the exponent of hardness is negative. All theexponents deviate from unity. The reason for this deviation is theignorance of the initial microstructure. The variation of load andhardness exponents clearly indicates that the initial microstruc-ture plays an important role to the wear behaviour. The loadexponent deviates much from unity in conventional cast sam-ples because wide variation of rate wear with increasing load.

4. Conclusions

The microstructure of conventional cast samples is fully den-dritic whereas in rheocast samples the primary phases are ofnearly spheroidal morphology.

The rheocast alloy showed significant improvements inmechanical properties. After hot rolling, it shows furtherincrease in strength mainly due to plastic deformation and workhardening.

Rheocasting offers superior wear resistance compared toconventional castings. The wear loss increases with increasingload in all samples. Wear occurs by ploughing mechanism. Atthe highest load, in conventional cast samples the wear modechanges to mixed mode of ploughing and delamination wear.Coefficient of friction of the rheocast samples is lesser than theconventional cast samples at all loads. The hardness of the wornsurfaces increases with load indicating work hardening of thematerials.

The expequation WW = 0.49L0

References

[1] M.C. Flemings, Metall. Trans. A 22 (1991) 957.[2] K.P. Young, R. Fitze, Proceedings of the Third International Conference

on Semi-Solid Processing of Alloy and Compounds, Tokyo, Japan, 1994,p. 155.

[3] K.P. Young, Light Metals, The Minerals, Metals and Materials Society,Warrendale, PA, 1996, p. 775.

[4] H.V. Atkinson, Prog. Mater. Sci. 50 (2005) 341.[5] Z. Fan, Int. Mater. Rev. 47 (2002) 1.[6] T. Haga, P. Kapranos, J. Mater. Proc. Technol. 130131 (2002) 594.[7] R.D. Doherty, H.I. Lee, E.A. Feest, Mater. Sci. Eng. A 65 (1984)

181.[8] C. Park, S. Kim, Y. Kwon, Y. Lee, J. Lee, Mater. Sci. Eng. A 391 (2005)

86.[9] K.L. Sahoo, C.S.S. Krishnan, A.K. Chakrabarti, J. Mater. Proc. Technol.

112 (2001) 6.[10] K.L. Sahoo, C.S.S. Krishnan, A.K. Chakrabarti, Wear 239 (2000) 211.[11] M.A. Martinez, A. Martin, J. Llorca, Scr. Metall. Mater. 28 (1993)

207.[12] R.D. Doherty, H.I. Lee, E.A. Feest, Mater. Sci. Eng. A 65 (1984) 181.[13] N. Apaydin, K.V. Prabhakar, R.D. Doherty, Mater. Sci. Eng. A 46 (1980)

145.[14] D. Brabazon, D.J. Browne, A.J. Carr, Mater. Sci. Eng. A 326 (2002)

370.[15] B.N. Pramila Bai, S.K. Biswas, Scr. Metall. 39 (1991) 833.[16] B.N. Pramila Bai, S.K. Biswas, Wear 87 (1983) 237.[17] A.S. Reddy, B.N. Pramila Bai, K.S.S. Murthy, S.K. Biswas, Wear 181183

(1995) 658.[18] S. Wilson, A.T. Alpas, Wear 225229 (1999) 440.[19] S. Jahanmir, N.P. Suh, Wear 44 (1977) 17.[20] R.T. Spurr, Wear 55 (1979) 289.[21] C.-C. Yang, W.-M. Hsu, E. Chang, Mater. Sci. Technol. 13 (1997) 687.[22] M. Roy, B. Venkataraman, V.V. Bhanuprasad, Y.R. Mahajan, G. Sundarara-

, MetMand. Archerimental data obtained can be fitted to an exponential= 0.49L0.6P1.2H1.3 for conventional cast alloy and

.9P0.8H1.4 for rheocast alloy with good correlation.

jan[23] D.[24] J.Fall. Mater. Trans. A 23 (1992) 2833.al, B.K. Dutta, S.C. Panigrahi, Wear 257 (2004) 654.ard, J. Appl. Phys. 24 (1953) 981.

Mechanical and wear properties of rheocast and conventional gravity die cast A356 alloyIntroductionExperimentalResults and discussionConclusionsReferences