j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204j our nal homepage: www. el sevi er . com/ l ocat e/ j mat pr ot ecMachining of LM13 and LM28 cast aluminium alloys: Part ID.K. Dwivedia,, A. Sharmab, T.V. RajanbaDepartment of Mechanical and Industrial Engineering, Indian Institute of Technology, Roorkee 247667, IndiabDepartment of Metallurgy, MNIT, Jaipur 302017, Indiaarti cle i nfoArticle history:Received 22 April 2006Received in revised form 4 May 2007Accepted 18 May 2007Keywords:Cast Al alloysMachining behaviourMelt treatmentHeat treatmentMicrostructureCutting forceSurface nishCutting tool temperatureabstractIn present paper, the inuence of melt treatment (grain renement and modication) andheat treatment (T6) of cast LM13 and LM28 aluminium alloys on machining behaviour hasbeen reported. Alloys under investigation were prepared by controlled melting and castingfollowed by heat treatment (T6). As-cast, melt-treated and heat-treated alloys were inves-tigated for machining characteristic at different cutting speeds. Melt treatment of both thealloys(LM13andLM28)reducedthecuttingforceandcuttingtemperaturewhereasheattreatment increased both cutting force and cutting temperature. Cutting temperature wasfound higher in machining of LM28 alloy than the LM13 alloy. Maximum cutting tempera-ture was found for both the alloys in heat-treated conditions. Heat treatment of LM28 alloyreducedthesurfaceroughnesswhereasheattreatmentofLM13alloyincreasedit. Melttreatment and heat treatment of LM13 alloy increased the average number of chips per gm.LM28 alloy produced higher number of chips per gm than the LM13 alloy. 2007 Published by Elsevier B.V.1. IntroductionSelection of any casting alloy is dependent on wide variety offactors such as service requirement and economy of process-ing (weldability, castability and machinability). Machinabilityof materials plays an important role in its selection of mate-rial for commercial exploitation. In general, more than 80% ofmanufacturedpartsaremachinedbeforetheyarereadyforuse (Pathak and Tiwari, 1995). Thus, machinability of a mate-rialdeterminesitseconomyinvariousapplications.Oneormore of the following criterias may be used to assess machin-abilityofamaterial. However, relativeimportanceoftheseparametersforevaluationofmachinabilityvariesaccordingto the requirement (Trent and Wright, 2000; Dwivedi, 2000a): Tool life. The amount of material removed by a tool,under standardized condition, before tool performanceCorresponding author. Tel.: +91 1332 285826; fax: +91 1332 285665.E-mail address: dkd04fme@iitr.ernet.in (D.K. Dwivedi).becomesunacceptableortool iswornoutbyastandardamount. Limiting rate of metal removal. The maximum rate at whichmaterial can be machined for a standard tool life. Cutting force. The forces acting on tool during the machiningunder specied condition. Surfacenish. Thesurfacenishachievedunderspeciedcutting condition. Chip. The chip shape and size produced under standardizedcondition as this can affect chip disposability.Pure aluminiumadheres to the tool andforms built upedgeapart from producing long chips that are too ductile to break.Addition of some alloying elements improves the machinabil-itybyreducingadherencetothetoolandmakingthechipsmorebrittle. Constituentswhicharepresent insolidsolu-tionsuchascopper, silicon, magnesiumandzincincrease0924-0136/$ see front matter 2007 Published by Elsevier B.V.doi:10.1016/j.jmatprotec.2007.05.032198 j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204the hardness of aluminiummatrix thereby reducing the metalpick up from tool, formation of burrs and tearing of metal sur-face (Van Horn, 1967; Tay and Lee, 1992). Constituents out ofsolution,whencombinewithaluminiumpromotebreakupof chips. If constituents such as CuAl2, FeAl3and Mg2Si, arenot extremely hard, machinability is improved. If constituentssuchassiliconorthoseformedbyboron, chromium, man-ganese and titanium, are extremely hard, rapid wear of tooltake place. Machinability of hypereutectic AlSi alloys is poormainly because of coarse primary silicon crystals, which haveextremely high hardness and tends to wear out tool rapidly.Addition of phosphorous renes the primary silicon particlesand improves the machinability. Modication of hypoeutecticalloys with sodium improves machinability (Wang et al., 1995;Mondolfo, 1979). Damodaran et al. (Damodaran, 1991) studiedthe inuence of rare earth additions on machining behaviourofLM28alloyandfoundthattheadditionofelementslikecesium and lanthanum reduces the specic power consump-tion. Low silicon alloys with copper after heat treatment mayhave equal or better machinability thanthose withhighsiliconand copper (Hatch, 1998; Sadasivan and Sarthy, 2000). Someimprovement inmachinabilityisobtainedbytheadditionofzinc,magnesium,titanium,bismuthandlead(Mondolfo,1979).During the manufacturing of engineering componentssuch as piston, cylinder head, which are rst processed by diecasting frequently need machining for obtaining the desireddimensions and surface nish. Renement and modicationof micro-constituents of these alloys are common industrialpracticesbeforecasting. Thesetreatmentsinturnchangetheir microstructure and so mechanical properties. Thus, it isexpected that thermal and melt treatment would also affectmachinability of these alloys. Literature survey did not revealanysystematic studyonthe inuence of melt treatment(grain renement and modication) and heat treatment (T6)onmachiningbehaviour of thesepistonalloys(LM13andLM28).Therefore,inthepresentwork,anattempthasbeenmadetostudytheeffect of grainrenement, modicationandheat treatment onmicrostructure, mechanical proper-ties and machining behaviour. The specic objectives of thepresentinvestigationare: (1) tostudytheinuenceofmelttreatment and heat treatment of alloys under investigation onmachining characteristics such as cutting forces, cutting tem-perature, number of chip per gm and surface roughness and(2) to study the inuence of metal cutting parameters such ascutting speed and feed rates on machining characteristics ofLM13 and LM28 aluminium alloy.2. Experimental procedure2.1. MaterialThe two base alloys developed for the investigations are (i) anear eutectic AlSi alloy (LM13) and (ii) a hypereutectic AlSialloy(LM28). Experimental alloyswerepreparedbycarefulmelting of master alloys such as Al28%Si, Al30%Cu, Al10%Ni and Al10% Mg in appropriate quantities with aluminiumof 99.99% purity in an electric resistance furnace. NecessaryallowancesformeltinglosseswerealsotakenintoaccountTable 1 Nominal composition of AlSi alloysAlloy Element (wt.%)Si Ni Cu Mg AlEutectic alloy (LM13) 12.0 1.0 0.80 0.60 BalanceHypereutectic alloy (LM28) 17.0 1.0 0.80 0.60 Balanceincomputationofcharges.Afterpropermixing,themoltenalloys were cast inmetallic mould(25mm37mm150mm).The nominal compositions of experimental alloys are as givenin Table 1.Molten LM13 alloy was treated for grain renement of alu-minium crystals and modication of eutectic silicon whereasLM28alloywasmelt-treatedforrenementofprimarysili-con crystals only. Alloys in melt-treated conditions have beenreferred as melt-treated alloys in the forgoing sections.Grain renement of LM13 alloy was carried out by adding0.2% of Al5% Ti1% B master alloy. Modication was done byaddition of Al10% Sr master alloy. The charge was melted ina preheated graphite crucible using an electric resistance fur-nace. The melt was covered with ux to avoid the oxidation.Degassingwascarriedout withhelpof hexachloroethane.After uxing anddegassing, calculatedamount of Al5%Ti1%BwrappedinaluminiumfoilwasaddedtoLM13alloywithconstant stirring of the melt at 7205C. This was followed(after 20min of addition of grain rener) by addition of 0.06%ofstrontiuminformofAl10%Srmasteralloytothemelt.After holding for 10min dross was removed and subsequentlymolten alloy was poured in cylindrical metal moulds (diame-ter 25mm and length 100mm).Primary silicon particles in LM28 alloy were rened using0.05% red phosphorus. The molten metal was kept at 900C.Beforeaddingphosphorusbasedrener,degassingwascar-ried out withhexachloroethane. After this rener was plungeddeep into the melt till the reactioncompleted. The melt so pre-pared was poured at 900C into metallic mould. A lter waskept at the top of metallic mould to trap inclusions and drossparticles, if any, during pouring in both the cases.Alloysunderinvestigationweresubjectedtoheattreat-ment cycle (T6); which consisted solutionizing, quenching andarticial agehardening. Melt-treatedLM13andLM28alloywereheat-treatedtofurtherenhancethemechanicalprop-erties. Both the alloys were solution treated at 5105C for6h followed by quenching in water at 30C and articial agingat 1705C for 12h. These alloys have been referred as heat-treated LM13 or LM28 alloy in the forgoing sections.2.2. Mechanical testingSamplesrequiredforvarioustestsweremachinedfromas-cast, melt-treated and heat-treated LM13 and LM28 alloy. Eachtest was repeated three times. Average value of properties wastaken for study. Tensile properties (tensile strength and duc-tility in terms of percentage elongation) were measured usingHounseeldcomputerizedtensiletestingmachine(20kN).Tensiletestswerecarriedout onroundsampleshavinga5.05mm gauge diameter and 25.2mm gauge length. The sam-plesweretestedatconstantstrainrateof1.0mm/min.Theultimate tensile strength (UTS) and ductility in terms of per-j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204 199Table 2 Cutting parameters used to study themachining behaviourParameters (1) (2) (3) (4) (5)Feed rate (mm/rev) 0.046 0.046 0.046 0.046 0.046Cutting speed (m/min) 11 16 24 30 50Depth of cut (mm) 2 2 2 2 2centage elongationwas calculated. Samples for hardnessmeasurement were polishedwithemery paper upto 2/0 grade.Thesamplesweredegreased, washedanddriedbeforethetest. Hardness at ve different locations over the entire cross-section was taken on Vickers hardness testing machine usinga load of 5kgf.2.3. Machining behaviourMachinability tests were conducted on all geared head stocklathe without coolant. These tests were conducted under dif-ferent cutting conditions (Table 2). During machining cuttingforces, number of chips per gm, cutting temperature and sur-faceroughnessweremeasuredusingtheproceduregivenbelow.2.3.1. Cutting forceCuttingforces(FX, FYandFZ) generatedontool duringthemachining were measuredusing lathe tool dynamometer(IEICOS,Bangalore).Thetangentialforce(FZ)isbyfargreat-est of the three components of forces as it does most of thework and is therefore responsible for most of the power con-sumption. Unless otherwise specied, cutting force indicatesthe tangential cutting force (FZ). Cutting tool was mounted onlathe tool dynamometer. High-speed steel tool (10, 12, 13, 15,16, 17 and 0.6) was used for machining in all the experiments.Cutting tool was overhanging from dynamometer and there-fore it did not allow heavy cutting conditions, i.e. high cuttingspeed and feed rate. Therefore, all machining tests were con-ductedincomparativelylowcuttingspeed(1050m/min)tostudytheeffect of melt treatment andheat treatment onmachining behaviour of alloys under investigation. Varia-tionincuttingforcewithtimeduring2minmachiningwasrecordedatinterval of 5sfordurationof 2min. Meanandstandarddeviationwere calculatedusing 24 data points. Arep-resentative cutting force was obtained corresponding to eachcondition, which was used for comparative study in the work.2.3.2. Chip formChipsproducedduringmachiningwerecollectedtostudytheirsize.Chipsizewasstudiedusingnumberofchipspergm criteria.2.3.3. Cutting temperatureVariation of cutting tool temperature with time duringmachiningofexperimentalalloyswasmeasuredbyembed-ding a thermocouple on rake face of the tool at 3mm distanceaway from cutting edge. Tool temperature was recorded dur-ing 2min of machining. Steady-state cutting temperature wasused for comparative study.2.3.4. Surface roughnessRoughness of machined samples of various alloys under dif-ferentconditionswasevaluatedusingRasurfaceroughnessparameter with the help of surface roughness tester (MitutoyoSJ-301, Japan) under the following conditions:Standard: ISO 1999Prole: RCut-off length: 0.8mmNumber of samples: 5Range: Auto (020,000 vertical and02000 horizontal)Speed: 0.25mm/s2.4. MicroscopySamples for micro-structural studies were cut fromingotcastings(as-cast, melt-treatedandheat-treatedcondition).Specimens were polished by standard metallographic proce-dureusingaseriesof emerypapersfrom1/0to4/0gradeandnallypolishedonsylvetclothusingnealumina.Pol-ishedsampleswereetchedwithKellersreagent.AReichertJung (MEF-3) optical microscope was used for examination ofmicrostructureofthesamples. Someofthemetallographicsamples were observed under scanning electron microscope(Leo-435-VP-England). SEMstudieswerealsocarriedoutontensile fractured surfaces and machined surface.3. Results3.1. MicrostructureThe optical micrographs of LM13 alloy in as-cast, melt-treatedandheat-treatedconditionareshowninFig. 1(a)(c). Itcanbe seen that the primary aluminium dendrites (light etched)are embedded in eutectic matrix of as-cast alloy (Fig. 1(b)). Theeutectic (dark etched) is largely present in the inter-dendriticregion of aluminium. It appears that copper is present as solidsolution in the matrix as its quantity is well within the solubil-ity of copper inaluminium. The microstructure of as-cast alloyrevealsprimaryaluminiumdendriteswithanaverageden-drite arm spacing in range of 4060m. Area fraction of-Alis about 51.3% and that of eutectic is around 47.6% and poros-ityabout 2%. Melt treatment renestheeutecticstructureandaluminiumgrains. Inter-dendriticarmspacingof alu-minium in melt-treated condition is in the range of 2535m(Fig. 1(b)). Heat treatment signicantly changes the morphol-ogy of eutectic silicon. Heat treatment causes spheroidizationof eutectic silicon crystals (Fig. 1(c)). Spheroidization of siliconpredominantly takes place along the grainboundaries. Opticalmicrophotographs of LM28 alloy in as-cast, rened and heat-treated condition are shown in Fig. 2(a)(c). It can be seen thatas-cast alloy contains coarse polyhedral shaped primary sili-con crystals in matrix of eutectic (Fig. 2(a)). Melt treatment ofLM28 alloy using red phosphorous results in renement of pri-mary silicon (Fig. 2(b)). Heat treatment of LM28 alloy changesthe morphology of primary and eutectic silicon crystals signif-icantly (Fig. 2(c)). Spheroidization of eutectic silicon particlestakes place, while sharp edges of primary silicon particles areroundedoff. Roundmorphologyof eutecticsiliconcrystals200 j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204Fig. 1 Microphotograph of LM13 alloy in (a) as-cast and (b)rened condition and (c) heat-treated condition (200).reduces the stress concentration at particlematrix interface.Heat treatment does not affect the size of primary silicon par-ticles.3.2. Machining behaviourMachinability studies were carried out to investigate the inu-ence of melt treatment and heat treatment of LM13 and LM28alloyonmachiningbehaviour. Themachiningbehaviourofexperimentalalloyswasstudiedinrespectofcuttingforce,cuttingtemperature, number of chipsper gmandsurfaceroughness under different machining conditions.LM13 andLM28 alloys were subjectedtomachinability testsover a range of cutting speeds (1050m/min), while feed rateFig. 2 Microphotograph of Al17% Si1% Ni0.8% Cu0.6%Mg alloy in (a) as-cast and (b) rened condition and (c)heat-treated condition (200).(0.046mm/rev) and depth of cut (2.0mm) were kept constant.Fig. 3(a) and(b) showsthevariationsof cuttingforcewithincrease in cutting speed during machining of LM13 and LM28alloy. It can be observed that the cutting force is not affectedsignicantly with increase in cutting speed during machiningofboth thealloysirrespectiveoftheirconditions.LM13 andLM28 alloy in heat-treated condition generate higher cuttingforce ontool during machining thanthose inas-cast and melt-treated condition. Both alloys in as-cast condition generatedhigher cutting force than that in melt-treated condition. Boththe alloys in melt-treated condition needed minimum cuttingforce for machining. Acareful observationof Fig. 3 showedthatcutting force for LM28 alloy was higher thanthat for LM13 alloyirrespective of alloy condition.j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204 201Fig. 3 Cutting force vs. cutting speed relationship (at0.046mm/rev feed rate and 2.0mm depth of cut) underdifferent conditions in machining of (a) LM13 and (b) LM28alloy.Fig. 4(a) and (b) shows the variations of number of chips pergm with increase in cutting speed during machining of LM13andLM28alloy. ItcanbeobservedthatmachiningofLM13alloyproducedlessnumberof chipspergmwithincreaseincuttingspeed. However, increaseincuttingspeedduringmachining of LM28 alloy had insignicant effect on number ofchips per gm. Melt treatment and heat treatment of LM13 alloydecreasedthe number of chips per gm. Melt treatment of LM28alloyalsodecreasedthenumberofchipspergmwhiletheheat treatment increases the number of chips per gm. Num-ber of chips per gm produced in machining of LM13 alloy wasfewer than the LM28 alloy under identical conditions. Size ofchips is important from their disposability point of view espe-ciallyinautomaticmachining. Theseresultsshowthatthealloy composition, condition and cutting parameter inuencethe disposability of chips.Fig. 5(a)and(b)showthevariationofsurfaceroughnesswith increase in cutting speed during machining of LM13 andLM28 alloy. It can be observed that surface roughness (Ra) ofmachinedsurface of boththe alloys decreasedwithincrease incutting speed. Melt treatment of LM13 and LM28 alloy reducedthe surface roughness. Heat treatment of LM13 alloy adverselyaffected surface nish and while heat treatment of LM28 alloyimproved nish of machined surface.Cuttingtemperatureisanimportantfactortobeconsid-ered for the evaluation of machinability of a material besidesFig. 4 Number of chips per gm vs. cutting speedrelationship (at 0.046mm/rev feed rate and 2.0mm depth ofcut) in different conditions during machining of (a) LM13and (b) LM28 alloy.surfacequality,magnitudeofcuttingforceandchipdispos-ability; as it affects the tool life to a large extent. Fig. 6(a) and(b)showthevariationofcuttingtemperaturewithincreasein cutting speed during machining of LM13 and LM28 alloy. Itcan be observed that cutting temperature during the machin-ing of both the alloys increased with increase in cutting speedand there is linear relationship between two. Heat treatmentof LM13andLM28alloyincreasedthecuttingtemperaturewhereasmelt treatment reducedit. Comparisonof cuttingtemperaturefortwoalloysrevealedthatlowercuttingtem-perature was generated in machining of LM13 alloy than LM28alloy under similar conditions.4. Discussion4.1. Cutting speedCutting force generated ontool during the machining isgoverned by work material characteristic and machiningparameters such as cutting speed and dimensions of cut, i.e.feedrateanddepthof cut. Cuttingspeedaffects(1) builtupedgeformationtendency, (2) frictionat chiptool inter-face and (3) work hardening characteristics (Wang et al., 1995;SadasivanandSarthy, 2000; Dwivedi, 2002a; Dwivedi etal.,2005; Dwivedi, 2000b). Thesefactorsinturnaffect cuttingforce. Hence the extent to which cutting speed affects the cut-ting force depends on how far the above factors are affected202 j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204Fig. 5 Surface roughness vs. cutting speed relationship (at0.046mm/rev feed rate and 2.0mm depth of cut) fordifferent conditions of (a) LM13 and (b) LM28 alloy.bycuttingspeed. Cuttingforceremainsmoreor lesscon-stant withincreasingcuttingspeedinmachiningof bothalloys (Fig. 3). Owing to high hardness and low ductility, thesealuminiumalloys(LM13andLM28)showlittletendencyforthebuilt upedgeformation. Effect of increaseinaveragechiplengthwithincreaseincuttingspeedonfrictionforce(increaseinchiptool contact lengthincreasesthefrictionforce) iscounteractedbydecreaseinfrictioncoefcient atchiptool interface. Therefore, inuence of increasing cuttingspeed on cutting force is negligible (Dwivedi, 2000b; Dwivediet al., 2000; Dwivedi, 2001a).Improved ow of chips on rake face of tool with increaseincuttingspeedmightreducethetendencyforbreakingofchips. Increase in speed will improve the chip ow action thatinturnwouldreducethechip-breakingtendency(Dwivedi,Fig. 6 Cutting temperature vs. cutting speed relationship(at 0.046mm/rev feed rate and 2.0mm depth of cut) indifferent conditions of (a) LM13 and (b) LM28 alloy.2001a), whichmayincreasetheaveragechiplengthunderidenticalconditions(Fig.5).LM13alloyparticularlyinheat-treated condition generates longer chips (few number of chipper gm) even at low speed. This may be due to increased duc-tility of LM13 alloy after heat treatment (Table 3). Chips fromLM28alloyarebasicallyverysmallandfragmented. Hence,they are not inuenced much by improved chip ow action athigh cutting speed.Probably increase incutting speedreduces the built upedgeformation tendency and therefore reduces the surface rough-ness of boththe alloys irrespective of their condition(Fig. 5). Atvery low speed the cutting temperature is low and duration ofcontact between chip and tool would be large. So there will besufcient time for necessary plastic owto occur andestablishatomic bonding between chip and tool (Sadasivan and Sarthy,2000; Dwivedi et al., 2000). This phenomenon leads to built upTable 3 Mechanical properties of alloys under investigationAlloy LM13 alloy LM28 alloyAs-castaMelt-treatedaHeat-treatedaAs-castaMelt-treatedaHeat-treatedaTensile strength (N/mm2) 208 224 252 144 152 183Hardness (VHN) 105 110 124 117 124 145Ductility (% age elongation) 1.3 1.8 2.5 1 1.08 1.28aAlloy condition.j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204 203edge formation. When built up edge is broken, part of it goeswith the chip and remaining on to the work material surfacewhich deteriorates surface nish. At high speed, tendency ofbuiltupedgeformationreducesbecauseoflessdurationofcontact that is required to establish atomic bond at elevatedtemperature.Increaseincuttingspeedincreasesthecuttingtempera-ture and there is linear relationship between the two (Fig. 6).Work done in machining is converted into heat that eventu-ally leads to increase in cutting temperature. Increase in workdone per unit time with increasing speed would raise the heatconcentrationincuttingzone.Higherheatconcentrationincutting zone causes greater rise incutting temperature. Reduc-tion in number of chip per gm (conversely increase in averagechip length) with increase in cutting speed will increase thefrictional heatwhichinturnwill increasethecuttingtem-perature.Cuttingtemperature(Tc)duringmachiningcanbeexpressed asTc = Vbwhere V is the cutting speed in m/min and b is the materialdependent exponent.4.2. Effect of microstructureAs-cast LM13 and LM28 alloy generated higher cutting forceontool duringmachiningthanthat inmelt-treatedcondi-tion(Fig. 3). MelttreatmentoperationofLM13alloyrenestheeutecticsiliconandthatofLM28alloyreducesthesizeof primary silicon particles. Fracture of comparatively coarse,hardsiliconparticleswouldbemoredifcult thanthat ofnesiliconcrystal present inmelt-treatedalloys(Dwivedi,2002b; Singhetal., 2003; Dwivedi, 2000c, 2001b). Therefore,melttreatmentofboththealloysreducesthecuttingforcegeneratedontool duringthemachining. Itisanalogoustodifcultyexperiencedinmachiningofthickhardcementitelayer inhypereutectoidsteel. Thinner thecementitelayerlesser is the resistance to cutting. It shows that the morphol-ogyofsiliconcrystalsaffectsthecuttingforcesignicantly.Heattreatmentofalloysunderinvestigationsincreasesthehardness, ductilityandtensilestrength(Table3). Increaseinhardness,ductilityandtensilestrengthofalloysinheat-treated condition may be attributed to increase in its cuttingresistance in respect of cutting force and cutting temperature(Figs. 3 and 6). Greater the strength of alloy higher the cuttingforce needed during machining because specic cutting pres-sureincreaseswithincreaseintensilestrength. Therefore,increase in hardness and tensile strength of LM13 and LM28alloy after heat treatment may be attributed to higher cuttingforce in heat-treated condition than that in as-cast and melt-treatedcondition(Singhetal.,2003;Dwivedi,2000c,2001b).Increase in ductility of heat-treated LM13 alloy increases theaverage chip length that would also raise the frictional heat atchiptool interface and hence the cutting temperature. Higherbuilt up edge formation tendency with increase in ductility ofheat-treated of LM13 alloy than the as-cast and melt-treatedalloy may be attributed to increase in surface roughness. Heattreatment of LM28 alloy increases the hardness signicantlyas comparedtoductilitythereforeit produces verysmalland fragmented chips. Heat treatment of LM28 alloy reducessurfaceroughnessduetoincreaseinhardnesssincehardmaterialgivesbetternishthanthesoftmaterial(Fig.7)astheyhave lower tendencyfor built upedge formation(Dwivedi,2001a).Higher cutting force in machining of LM28 alloy than theLM13alloyunder identical conditioncanbeattributedtopresenceofcoarsepolyhedral shapedprimarysiliconcrys-tals, whichoffer greater resistancetomachiningbecauseFig. 7 SEM images of machined (machined at 24m/min cutting speed, 2mm depth of cut and 0.036mm/rev feed) surfacesof heat-treated (a) LM13 at 50, (b) closer look of (a) at 200 and (c) LM28 alloy and (d) closer look of (c) at 200.204 j ournal of materi als processi ngtechnology 1 9 6 ( 2 0 0 8 ) 197204oftheirhigherhardness. Highercuttingforcegeneratedontool duringmachiningof LM28alloythantheLM13alloywouldalsoincreasetheheatconcentrationincuttingzonethat will increasethecuttingtemperature. Higher cuttingtemperature generated in machining of LM28 alloy may alsobe attributed to higher silicon content, which has com-parativelylowthermal conductivitythanaluminium. Workmaterialhavinglowerthermalconductivitydoesnottrans-fer the heat generated during machining process away fromcuttingzoneeffectively, whichinturnwouldincreasethetool and work temperature. Presence of hard and brittlecoarsenon-metallic siliconcrystals inLM28alloyacts asstress raiser and discontinuity in metallic matrix. Therefore,fractureofsuchhardandbrittlesiliconparticlespromotesformationof large number of small fragmentedchips inmachiningof LM28ascomparedtoLM13alloy. Compara-tivelypoor surfaceroughness andmorefragmentationofchips while machining of LM28 alloy vis- ` a-vis LM13 alloy maybe attributed to brittle fracture of coarse non-metallic siliconcrystals.5. Conclusion1. Increaseincuttingspeedincreasedthecuttingtempera-ture and linear relationship was found between them andthe same is primarily attributed to increased localizationof heat. Surface roughness decreased with increase in cut-tingspeedduetoreductioninbuild-up-edgeformationtendency.2. Melt treatment of both the alloys (LM13 and LM28) reducedthecuttingforceandcuttingtemperatureduetorene-ment of hardandbrittlesiliconparticleswhereasheattreatmentincreasedbothcuttingforceandcuttingtem-peratureandthesamewasattributedtoincreaseinthehardness and strength of both the alloys after heat treat-ment. Cutting temperature was found higher in machiningofLM28alloythanLM13alloyduetohigherhardnessofLM28.3. Heat treatment of LM28 alloy reduced the surface rough-ness whereas heat treatment of LM13 alloy increased.Reductioninsurfaceroughnessof LM28alloyafter theheat treatment is attributed to signicant increase in hard-nesswhichinturnlowersthebuilt upedgeformationtendency.4. 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