ROLE OF SiCp ON THE SOLIDIFICATION RATE AND ......addition of magnesium with the liquid aluminum and...

International Journal of Scientific & Engineering Research, Volume 2, Issue 1, January-2011 1ISSN 2229-5518

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ROLE OF SiCp ON THE SOLIDIFICATIONRATE AND FORGEABILITY OF STIR

CAST LM6/SiCp MMCs

Rabindra Behera, A.Datta, D.Chatterjee, G.Sutradhar

ABSTRACT- The present paper aims to investigate the solidification behavior and the forgeability of Aluminumalloy (LM6)-SiCp composites at different section of three-stepped composite castings. The temperature of the castcomposites during solidification has measured by putting K-type thermocouples at the center of the eachstep/section, from which the solidification curves were constructed. The forgeability of the as cast MMCs were alsomeasured at different section of the casting. The results show that the forgeability of cast metal matrix composites atthe middle section of the casting is minimum compared to both end section of a three-step casting. Experimentswere carried out over range of particle weight percentage of 7.5 -12.5 wt% in steps of 2.5wt%. The solidificationcurves of Aluminum alloy (LM6)-SiCp composites compared with the unreinforced alloy (LM6) and the resultsreveal that significant increase in solidification time with the addition of SiC particles. The curves also show that therate cooling and the solidification time are different at different section of the castings. This practical researchanalysis and test results on solidification behavior and the forgeability of Al/SiC-MMC will provide usefulguidelines to the present day manufacturing engineers.

Key words: Metal matrix composites (MMCs), Solidification, Cooling curve, Forgeability.

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1. INTRODUCTIONMetal–matrix composites (MMCs) have been one ofthe key research subjects in materials science duringthe past two decades [1].MMCs have emerged aspotential alternatives to conventional alloys and arewidely used in aerospace and automobile industriesbecause of their excellent physical, mechanical anddevelopment properties. However, the difficulties inproduction and the high manufacturing cost restrictstheir wider application in modern industry, althoughpotential benefits in weight saving, improvedmechanical properties and increased component life.Now a day, even in those terms, MMCs are stillsignificantly more expensive than their competitors.Only simple production methods, higher productionvolumes, and cheaper reinforcements [2,3] canachieve the cost reductions. The search for cheaper,easily available reinforcement has led to the wideruse of SiC and Al2O3 particles [3]. Therefore, theapplication of particle reinforced MMCs are nowdominating the MMC market. There are severalmethods are used for the manufacturing of MMCs, ofwhich, stir casting method is quite popular due to itsunique advantages [4-7]. In this casting method, thereinforcing particles has introduced into the melt and

stirred thoroughly to ensure their proper mixing withthe matrix alloy. The properties of particle-reinforcedmetal matrix composites produced by stir castmethod has influenced by various parameters such astype, size & weight fraction of reinforcementparticles and its distribution in cast matrix metal. Italso depends on their solidification behavior duringcasting. The rate of solidification has a significanteffect on the microstructure of cast composites,which in turn affects their mechanical properties.From the moment of crystallization and solidificationcommencement, the crystalline phase begins to grow.Its growth proceeds in a direction opposite to theparticles’ movement. Thus, apart from the geometricfactor, i.e. the type, volume fraction and size ofreinforcing particles, it is the crystallization rate andthe casting’s solidification time that determine thestructure obtained and particles’ distribution in thematrix.Particulate metal matrix composites have producedeconomically by conventional casting techniques.However, the stiffness, hardness and strength toweight ratio of cast MMCs are increased, but asubstantial decrease in ductility has obtained. It has

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observed that some improvements in strength andductility has found with the application of plasticforming processes i.e. forging to the cast composites.The forged MMCs having better mechanicalproperties compared to cast MMCs, such as itimproves density, hardness and tensile strength etc.the forging process also avoids the use of secondaryoperation like machining. The forgeability is one ofthe important parameter, which gives informationregarding the limitation of forging [8-11].The study aims at determining and comparing thecooling curves obtained at different section of thecastings (three-stepped casting) in sand mould for thematrix (LM6) and for its composites containingvarying weight percentage of SiCp. The forgeabilityof as cast MMCs at different section of the castingshave also studied.

2. Experimental procedureLM6, is a well-known alloy of aluminum, is used asthe base/matrix metal in the experiments for thefabrication of the composites that has been reinforcedwith 7.5 to 12.5 wt% in steps of 2.5% of SiCp ofaverage 400 mesh size. The chemical composition ofthe matrix material (LM6) and the thermo physicalproperties of aluminum alloy, SiCp & sand havegiven in the table-1 & table-2. The composites arefabricated by the liquid metal stir casting technique.The aluminum alloy is melted in an electricresistance furnace and 3wt.% Mg has been addedwith the liquid metal, in order to achieve a strongbonding by decreasing the surface energy (wettingangle) between the matrix alloy and the

reinforcement particles. The addition of puremagnesium has also enhanced the fluidity of themolten metal. The pre-heated silicon carbideparticles (about 850-9000C) added with the liquidmetal and then mechanically stirred by using animpeller. The processing of the composite has carriedout at a temperature of 7500C with a stirring speed of400-500 rpm. The melt has poured at a temperatureof 745 0C into a stepped silica sand mould. Three(i.e.T1, T2 & T3) K-type thermocouples of 0.3 mmsize has used at the centre of the different section ofthe mould to measure the temperature variation in thecasting during solidification has shown in Fig.1. Onethermocouple has inserted into the sand to measurethe temperature variation of the molding sand afterpouring of molten metal and during solidification ofthe castings. The solidification curves of the castingsand the variation of temperatures at different sectionsin the mould are recorded with the help of a computeraided data acquisition system, the schematic sketchof the computer aided temperature data acquisitionset up has shown in Fig.2. The figure of compositecasting with thermocouples has shown below inFig.3. Experiments carried out for a wide range ofparticle weight percentage varying from 7.5% to12.5% in steps of 2.5%. Finally, the solidificationcurves of LM6-SiCp composites have compared withthe unreinforced LM6 matrix alloy at differentsection of the casting. The micro structuralcharacteristics of the alloys and composites atdifferent section of the castings have also evaluated.

TABLE-1Chemical Composition (LM6)

Elements Si Cu Mg Fe Mn Ni Zn Pb Sb Ti AlPercentage (%) 10-13.0 0.1 0.1 0.6 0.5 0.1 0.1 0.1 0.05 0.2 Remaining

TABLE-2Thermo physical properties of the matrix, reinforcement particle and sand

Properties LM6 SiC particulates SandDensity(gm/cm3) 2.66 3.2 1.6

Average particle size ( mesh) ------- 400 -----Thermal conductivity(W/m-K) 155 100 0.52

Specific heat (J/Kg-K) 960 1300 1170




Fig1.The geometry of mould cavity withK-thermocouples (T1,T2&T3).(All dimensions are in mm)

Fig2.Schematic sketch of the computer aided temperature data acquisition set up. Thermocouples (T1, T2 & T3 } connected with different section of casting and T4 inserted into the sand

Fig3. Composite casting with thermocouples3. Addition of alloying elements and particletreatmentThe addition of reactive elements with the liquidmetal during the production of composites by liquidmetallurgy techniques promotes excellent bondingbetween reinforcement particles and molten matrix[12]. For examples, the addition of magnesium,calcium, titanium, or zirconium with the liquid metalmay promotes its wettability by reducing the surfacetension of the melt, decreasing the solid- liquidinterfacial energy of the melt, or by chemicalreaction. It has been observed that in case aluminumbased composites, magnesium has a greater effectincorporating reinforcement particles in the melt andimproving their distribution than other elements suchas cerium, lanthanum, zirconium, titanium, bismuth,lead, zinc and copper [13-15].Magnesium is a powerful surfactant element, theaddition of magnesium with the liquid aluminum andits alloy melt improves its wettability because of itslower surface tension (0.599 Nm-1) compared withthat of aluminum (0.760 Nm-1) or aluminum-11.8wt% Si (0.817 Nm-1) [16]. The addition of 3 wt%magnesium to melt reduces its surface tension from

0.760 to 0.620 Nm-1 at 720oC. In the work ofSukumaran et al.[16] they concluded that the additionof magnesium is necessary during the synthesis ofA356-SiCp composites by a stir casting route, andfound the optimum addition of magnesium forobtaining the best distribution and maximummechanical to be 1 wt%. the addition of magnesiumlower than the optimum value results in the formationof agglomerates of reinforcement particles and theirnon-uniform dispersion in the melt.Magnesium is also acts as a powerful scavenger ofoxygen, it reacts with the oxygen present on thesurface of particles, thinning the gas layer, and thusimproving wetting and reducing the agglomerationtendency. A composite prepared by the liquidmetallurgy route with SiCp in A356 alloy matrix,shown that the addition of magnesium helped inthinning the gas layer, which was present over theSiC particles [17]. So, it has concluded that thepresence of magnesium in aluminum alloy matrixduring manufacturing of composites, not onlyenhance the fluidity of the liquid matrix, but alsoscavenges the oxygen from the surface of theparticles, leading to an increase in the surface energyof the particles. But, the excess addition ofmagnesium in an aluminum melt will alter themicrostructure of the matrix alloy and deteriorates themechanical properties.The heat treatment of particles before dispersion inthe liquid metal aids their transfer by causingdescription of adsorbed gases from the particlesurface. Agarwala and dixit [18] observed that theimportance of preheating in the incorporation ofgraphite particles in an aluminum alloy. There was noretention when the graphite particles were notpreheated, whereas the particles were retained when




preheated. Heating of SiC particles to 900oC assist inremoving surface impurities, desorption of gases, andaltering the surface composition due to the formationof an oxide layer on the surface[19]. The ability ofthis particle oxide layer to improve the wettability ofSiC particles by an alloy melt has been suggested byseveral investigators [20,21]. In the presentexperiment the SiC particles were heated for 2 hoursat 850-900oC in a electrical resistance muffle furnaceand then introduced into the liquid metal within ashort period of time of about 1-2 minutes duringstirring of the melt. After stirring the liquid metal for10-15 min., the metal tapped immediately and pouredinto the mold cavity.

4. Results and Discussion4.1. Microstructural AnalysisSamples of as cast MMCs for metallographicexamination were prepared by grinding through 320,400, 600, 800, 1200 and 1500 grit papers followed bypolishing with 6 µm diamond paste. Then thesamples were etched with the etchant i.e. Keller’sreagent. The microstructure of etched and driedsample has observed by using optical microscope(Olympus, CK40M). The microstructure of the ascast LM6 MMCs are shown in Fig.4-6 at differentsection of the casting. The micrograph of MMCcastings at different section shows that thedistributions of SiC particles are not uniformthroughout the casting and segregation of particlesare more in the eutectic region. This tendency may beattributed to the fact that the rate of cooling is notuniform throughout the casting due to change inthickness of the casting and slower rate of cooling inthe sand mold.

a. Section-I

b. Section-II

c. Section-III Fig4. Microstructure of LM6/7.5wt% SiCp as cast MMC at different section of the casting.




a. Section-I

b. Section-II

c. Section-IIIFig5. Microstructure of LM6/10wt% SiCp as cast

MMC at different section of the casting.

a. Section-I

b. Section-II

c. Section-IIIFig6. Microstructure of LM6/12.5wt% SiCp as cast

MMC at different section of the casting.




4.2. Solidification curves and their analysisAddition of an alloying element or second phaseparticles into a matrix alloy usually affects thevarious time and temperature parameters of itssolidification curve. The variation in the nature of thecooling curve always has a significant impact on themicrostructure and mechanical behavior of thematerial. Fig.7-10 shows the cooling curve of the Alalloy (LM6) and LM6 reinforced with 7.5wt%,10wt% and 12.5 wt% of SiCp metal matrixcomposites. The cooling curves for different sectionof castings at different weight fraction of SiCpindicates that the rate of cooling decreasing onincreasing the weight percentage of SiCp in the castMMCs. The cooling rate from the cooling curves ithas observed that the eutectic solidification time (i.e.the time interval between the start and the end of theeutectic phase solidification) increases on increasingthe weight percentage of SiC particles in thealuminum alloy matrix. It has also observed that theintroduction of SiC particles in the matrix metallowered the liquidus temperature when comparedwith the unreinforced alloy. Fig.11 shows thevariation of liquidus temperature with increase inweight percentage of SiC particle. This can beattributed to the unfavorable primary aluminumnucleation condition prevailing at the reinforcementsurface and the depression in the freezing point dueto the presence of reinforcement, which is consideredas an impurity. Studies by Gowri and Samuel [22]have also shown that addition of particles lowers theliquidus temperature by about 10 oC. The similartrend has also observed by T.P.D.Rajan et al. [23].Itis observed that the start of eutectic solidification ofthe matrix alloy (LM6) at a temperature of 574oCwith the solidification ending at 572 oC. Afteraddition of reinforcement particles i.e. SiCp in matrixalloy, the start and end temperature of eutecticsolidification changes.

The addition of SiCp, increasing the eutecticsolidification time as compared with the coolingcurve of aluminum alloy (LM6). The eutecticsolidification time also changed with the modulus ofthe casting, the cooling curve indicates that ondecreasing the section modulus of the MMC castingsthe eutectic solidification time decreases at differentweight fraction of SiC particles i.e. in case of lowestmodulus the eutectic solidification time is lesscompared to highest modulus. This validates that theChvorinov’s rule still applies to the solidificationprocess, irrespective of what additives are added tothe molten metal [24,25]. The cooling curve showsthat the eutectic solidification time enhanced onincreasing the weight fraction of reinforcementparticles compared to unreinforced matrix alloy.Fig.12 shows the variation in eutectic solidificationtime with respect to different weight percentage ofSiCp and at different section of the castings. It hasobserved that on increasing the weight percentage ofSiCp in the cast Al alloy (LM6) metal matrixcomposites the eutectic solidification time increasesat different modulus of the castings. That means thetotal solidification time (i.e. the time interval betweenthe start of primary aluminum phase nucleation andthe end of the eutectic phase solidification) increaseson increasing wt% of SiCp. This trend may beattributed to the fact that the amount of heatextraction reduced on increasing the weightpercentage of SiC particles in the liquid matrix metalas the presence of SiC particles in the matrix metalreduced the thermal conductivity and thermaldiffusivity[23].




Fig7.Cooling curves of Al (LM6) composites at different section of the casting.

Fig.9 Cooling curves of Al (LM6)- 10wt%SiC composites at different section of casting.

Fig11. Effect of wt% SiC on Liquidus Temperatureof composite casting.

4.3. Forgeability of cast MMCsThe forgeability test of as cast aluminum alloy metalmatrix composites carried out by upset method atroom temperature. The L/D(L-height of thecylindrical sample and D- diameter of the sample)ratio the samples is 1.5. The limit of forgeability hasexpressed as the critical reduction in height, by thefollowing equation:

Fig8. Cooling curves of Al (LM6)-7.5wt%SiC composites at different section of casting.

Fig.10 Cooling curves of Al (LM6)- 12.5wt%SiCcomposites at different section of casting.

% of Critical reduction= 100i

if

HHH

Where, Hi = the initial height of the sample in mm. and Hf is the final height of the sample in mm.

Critical reductions under unlubricated conditionsonly were compared to assess the forgeability of theexperimental materials. The load was applied at roomtemperature on samples of different section of as castMMCs reinforced with 7.5 wt%, 10 wt% &12.5WT%SiCp. At different load, the percentage ofdeformation investigated. These results havepresented in Fig.12. The figure shown the percentageof deformation due to acting load is different atdifferent section of the casting i.e. the percentage ofdeformation is lowest in section –II (middle section)comparison to Section-III & I. The percentage ofdeformation is highest in section-I and the percentageof deformation in section-III are remains in betweensection-I&II. This indicates that the higherpercentages of SiC particles have accumulated at themiddle section of the casting i.e. at section-II, incomparison to the section-III & I. The above result




indicates that the distributions of silicon carbideparticles are not uniform through out the casting. Thishas occurred because of non-uniform rate ofsolidification of liquid metal at different section ofthe casting. It has also observed that on increasing theweight percentage of silicon carbide particles in cast

composites the percentage of deformation decreasesthat means the forgeability of cast compositesdecreases on increasing the reinforcement ratios, asthe presence of very hard SiCp in the cast MMCsdecreases its ductility and enhance its hardness &brittleness.

(a)

(b)

(c)Fig.12. Load Vs % of Deformation curve of as cast MMCs at different section of MMCs casting reinforced with (a) 7.5wt% SiCp ,(b) 10wt% SiC and (c) 12.5 wt%.

5. ConclusionsThe solidification curves have recordedexperimentally for Al alloy (LM6) and its compositeshaving varied percentage of SiC particles from 7.5wt% to 12.5 wt% in steps of 2.5 wt% and comparedwith the unreinforced matrix alloy.i.e.LM6.

The introduction of SiCp in the matrix metaldecreases the cooling rate, as the presence ofSiCp in matrix metal lower heat transferrates during solidification owing to thereduction of thermal conductivity andeffective thermal diffusivity of thecomposite system. The increase in weightpercentage of SiCp in matrix metaldecreases the rate of solidification rate. Thatindicates the solidification rate is faster incase of unreinforced matrix alloy orcontaining low fraction of SiCp in thematrix.Addition of ceramic reinforcement to alloyenhances the eutectic solidification time, asthe presence of insulating dispersoids i.e.SiCp plays a dominant role in reducing thecooling rates. The solidification time hasalso varied with the change in thickness ofcastings. The solidification time is less incase of thinner section in comparison withthicker section, due to rapid cooling ofthinner section. This trend is similar to themonolithic metal and its alloys.Additions of Mg to the composite melt hasmultifunctional role. Apart from its well-known function as a wetting promoter ofceramic particle with the aluminum alloymatrix, it leads to better contact at themetal/mould interface, thereby by enhancingthe heat transfer rate.The forgeability test has carried out at roomtemperature by upset method. Theforgeability i.e. the percentage ofdeformation decreases on increasing thepercentage of SiCp in the matrix metal andthe middle part of the casting (i.e.section–II)shows low forgeability comparison to the




both end sections in the step castingcomponent owing to accumulation of higherpercentage of SiCp. That indicates the

distribution of SiCp is not uniform throughout the casting.

Acknowledgements

Authors thankfully acknowledge the financial supportprovided by U.G.C, New Delhi under MajorResearch Project Grant [F.No.32-88/ 2006 (SR) dated09.03.2007] without which this work could not beattempted.

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ROLE OF SiCp ON THE SOLIDIFICATION RATE AND ......addition of magnesium with the liquid aluminum and...

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