1-s2.0-S0261306910000889-main.pdf

Short CommunicationEffect of Fe-rich intermetallics on the wear behavior of eutectic AlSipiston alloy (LM13)V. Aboueia, H. Saghaana, S.G. Shabestaria,*, M. ZarghamibaCenter of Excellence for Advanced Materials Processing (CEAMP), School of Metallurgy and Materials Engineering, Iran University of Science & Technology (IUST),Narmak, Tehran 16846-13114, IranbSupplying Automotive Parts Co., Km 12, Karaj Road, Tehran, Iranarti cle i nfoArticle history:Received 25 November 2009Accepted 8 February 2010Available online 11 February 2010abstractIn the present study, the effect of Fe-rich intermetallics has been investigated on the wear behavior ofeutecticAlSialloy(LM13). Dryslidingweartestshavebeenconductedusingapin-on-diskmachineunder different normal loads of 18, 51, 74 and 100 N at a constant sliding speed of 0.3 m/s. Addition of1.2% Fe to the LM13 alloy leads to the formation of the ake like b-intermetallic compounds. These hardcompounds initiate micro cracks and can reduce the wear resistance of the alloy. The addition of Mn con-verts the ake like b-intermetallic compounds to the star-like a-intermetallics and decreases the detri-mental effectof iron. Applyinghighcoolingrateduringsolidicationof thealloycontainingFeandMn, resultedto theformation of ner a-intermetalliccompoundsand improved thewear behavior ofthe alloy to a great extent. 2010 Elsevier Ltd. All rights reserved.1. IntroductionWear is one of the most commonlyencounteredindustrialproblemsleadingtothereplacementofcomponentsandassem-blies in engineering [1]. Therefore, many efforts have been madeto produce more durable materials and techniques to reduce thewear of tools and engineering components. These include modi-cation of bulk properties of the materials, surface treatments andapplication of coating. Over the last few years, many efforts havebeen made to understand the wear behavior of the surfaces in slid-ing contact and the mechanism, which leads to wear [2]. The appli-cations of aluminiumalloys for the machine parts are widelyincreasingintheindustry. However, littlehasbeenreportedonthewearbehavior ofaluminium and itsalloys withtheadditionof grain rener and modier [3]. Amongst the commercial alumin-ium casting alloys, AlSi alloys are the most common alloys havingsome attractive characteristics such as high strength to weight ra-tio, excellent castability and pressure tightness, low coefcient ofthermal expansion, goodthermal conductivity, goodmechanicalproperties, andcorrosionresistance[4]. AlSi alloys ndwiderange of applications in marine castings, motor cars and lorry t-tings/pistons and engine parts, cylinder blocks and heads, cylinderliners, axles and wheels, rocker arms, automotive transmission cas-ings, water-cooledmanifoldsandjackets, pistonfortheinternalcombustionengines, pumpparts, highspeedrotatingpartsandimpellers [4,5].The in-service performance of the AlSi alloy castings primar-ilydependsontheirmicrostructures, chemical compositionandtheamountsofimpurities involved[68]. Ironisalwayspresentincommercial Al alloys and has consistently emergedas themain impurity element and perhaps the most detrimental tothe mechanical properties of these alloys. This is due mainlyto the precipitation of brittleb-Al5FeSi intermetallics that appearasneedlesorplate-likemorphologiesinthemicrostructure[912].Recently, Taghiabadi et al. [13] have shown that the addition of0.7 wt.% Fe increased the hardness and improved the wear resis-tance of the alloy. Addition of iron up to 2.5 wt.% further increasedthe hardness, but decreased the wear resistance of the alloy. In AlSi piston alloys, iron is a desirable element that enhances the hightemperature properties and thermal stability of the alloy [14,15].Attemptsshouldbemadetomodifythenegativeeffectsofironintermetallics, therefore, byreningandmodifyingthemtotheless deleterious morphologies.Alloy chemistry is one of the most important factors that inu-ences the formation of theb-intermetallics. It is well known thattrace addition of suitable neutralizer elements like Mn, Cr, Be, Co,andSrcanmodifytheb-phasemorphologytothelessharmfulforms[912]. Amongthese, Mnisaneffectivemodierof nee-dle-like intermetallic compounds [16,17]. It has been shown thatmanganeseadditionuptothehalfoftheamountofironresultsto the formation of some Chinese-script or star-like intermetalliccompounds in the matrix such as Al15(Fe,Mn)3Si2 [16], which haveless detrimental effects on the mechanical properties of the alloy[17].0261-3069/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2010.02.015*Corresponding author.E-mail address: [email protected] (S.G. Shabestari).Materials and Design 31 (2010) 35183524ContentslistsavailableatScienceDirectMaterials and Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat desThis investigation has been focused on the modication of theseiron-rich intermetallics and to study their effects on the dry slidingwear behavior of AlSi piston alloy.2. Experimental procedureCommercial ingots of the Al12Si alloy were used. The chemicalcomposition of them is given in Table 1. In order to investigate theTable 1Designations and chemical compositions of the alloys (wt.%).Alloy code Si Cu Ni Mg Zn Fe Mn AlBase 12.64 1.01 1.10 0.98 0.018 0.41 Balance1.2Fe 12.82 0.96 0.99 0.95 0.016 1.12 Balance1.2FeMn 12.94 1.08 1.09 0.91 0.016 1.19 0.62 Balance1.2FeMn-CMa12.85 0.98 1.01 0.93 0.016 1.15 0.66 BalanceaThe alloy prepared in a water-cooled copper mold are designated by CM.Fig. 1. Microstructures of the (a) base alloy, (b) 1.2Fe alloy, (c) 1.2FeMn alloy and (d) 1.2FeMn-CM alloy.Table 2Chemical composition of the phases shown in the micrographs of Fig. 1 (at.%).Alloy code Phases Morphology Atomic percentageAl Si Fe Mn Cu NiBase alloy A 60.64 22.04 13.23 4.09B 76.72 12.18 10.29 12.511.2Fe C Needle-like 67.04 15.48 16.88 1.2FeMn D Stare-like 72.44 10.67 10.09 6.71 0.391.2FeMn-CM E Stare-like 73.21 10.67 9.27 6.34 V. Abouei et al. / Materials and Design 31 (2010) 35183524 3519effect of Fe-rich intermetallics on the wear behavior of the alloy,iron and manganese were added to the base alloy in order to obtain1.2Fealloycontaining1.2 wt.%Feand1.2FeMnalloycontaining1.2 wt.% Fe and 0.6 wt.% Mn (Table 1).Iron and Manganese were added to the melt at 750 C using AL-TABFeCompact(75 wt.%Fe, 15 wt.%Al and10 wt.%nonhygro-scopicNa-freeux) andMncompact (75 wt.%Mn, 15 wt.%Al,and 10 wt.% nonhygroscopic Na-free ux), respectively.After addition of Fe and Mn, The temperature of the melt wasraised to 800 C, held for 15 min to homogenize the liquid and thencooled in the furnace to 750 C. The melt was stirred and degassedusingFoseco600tabletfor10 minbeforepouring. FinalpouringTable 3Hardness,volume fraction,and the average maximum size of Fe-rich intermetallicsphases.Alloy code Hardness(HB)Volume fraction ofphase (%)Average maximum sizeof phase (lm)Base alloy 83 0.91 54.62 23.11 1.2Fe 91 0.65 6.07 4.131.2FeMn 91.5 0.71 6.8 3.53 30.42 13.951.2FeMn-CM 115 0.93 5.8 2.68 18.49 10.62024681018 51 74 100Load, NWear Rate, mm3. m-1. 10-3base1.2Fe1.2FeMn1.2FeMn-CMFig. 2. Variation of wear rate versus applied load for different alloys.Fig. 3. SEM micrographs of worn surfaces (a) base alloy at applied loads of 18 N, (b) 1.2FeMn alloy at applied loads of 18 N, and (c) 1.2FeMn-CM alloy at 100 N.3520 V. Abouei et al. / Materials and Design 31 (2010) 35183524temperaturewasalways720 5 C. Themoltenalloyswerecastintoacastironmoldwhichwaspreheatedto250 Chavingtheaverage cooling rate of 3 C s1. To investigationthe effect ofcoolingrateonthestructureandwearproperties, the1.2FeMnalloys were also cast into a water-cooled copper mold to achievethe average cooling rate of 15 C s1.The hardness of all samples was measured using a Brinell hard-ness tester with the load of 31.25 kgf. The effect of alloy chemistryon the microstructure was studied using a scanning electronmicroscope equipped with an energy dispersive X-ray spectrome-ter (EDS). The volume fraction of the iron-rich intermetallic was re-lated to the area fraction which was measured by the quantitativemetallography using a computer-assisted Buhler Omnimet imageanalysis system. Dry sliding was carried out at a relative humidityof 40 2% at room temperature (25 C) against the counterface of ahardenedandpolishedsteeldiskwithHRC6265hardness. Thepins, 5 mm 5 mm, wereinaconformal contactwiththedisk.The wear tests were conducted under nominal loads of 18, 51, 74and 100 N, at a constant sliding speed of 0.3 ms1for a sliding dis-tance of 1000 m. Each test was repeated three times at a given loadand sliding velocity.Table 4Chemical composition of the worn surfaces of the base alloy and 1.2FeMn alloy shownin Fig. 3 (at.%).Alloy code Atomic percentageO Al Si Fe Ni CuBase alloy 41.93 48.19 3.57 5.03 1.2FeMn 34.48 43.28 5.70 16.21 Fig. 4. Longitudinal cross-section of the worn surface of 1.2FeMn alloy at an applied load of (a) 18 N, (b) 100 N, and (c) enlarged view of the marked region in the micrograph(b).V. Abouei et al. / Materials and Design 31 (2010) 35183524 35213. Results and discussion3.1. Microstructure and hardnessFig. 1 shows the microstructural features of the LM13 piston al-loys having different amounts of iron and manganese (Table 1). Thebase alloy (Fig. 1a) contains some Fe-rich, Ni and Cu-rich interme-tallics due to the presence of Fe, Cu and Ni in the composition ofalloy. Eachintermetallicphasehasbeenanalyzedthreetimesinthe sample and the average chemical composition of the interme-tallics is given in Table 2.The addition of iron to the LM13 piston alloy led to the precip-itation of needle-like intermetallic phases in the matrix as shownin Fig. 1b. The average atomic concentrations of Al, Fe and Si wereingood agreements withthe concentrations obtained for theb-Al5FeSi needles by others [1820]. Fig. 1c shows the micro struc-tureof 1.2FeMnalloy. Theadditionof Mnuptothehalf of Feamountcausesthereplacementofb-needle-likeintermetallicbythe star-like and polygonal morphologies. The average atomic per-centage of the elements in these intermetallics indicates that theyare Al15(Fe,Mn)3Si2 phases (Table 2).Fig. 1d shows the effect of cooling rate on the micro structuresof Fe, Mn containing LM13 alloy. As can be observed, high coolingrate results to the rening of the alloy micro structure, particularlythe a-intermetallic phase.Table 3 presents the hardness of the alloys. It is observed thatthe hardness of the as-cast alloy shows an enhancement asthe iron content of the alloy increased. As can be noticed, higherFig. 5. Longitudinal cross-section of the worn surfaces at the applied load of 100 N, showing mechanical mixed layer, (a) base alloy and (b) 1.2FeMn alloy.Fig. 6. (a) SEM micrographs of wear debris of base alloy at applied loads of 18 N, and (b) enlarged view of the marked region A in the micrograph (a).3522 V. Abouei et al. / Materials and Design 31 (2010) 35183524cooling rates results to an increment in the hardness of 1.2FeMn-CM alloy.The image analysis results of the volume fraction and the aver-age of the maximum size of the intermetallics are presented in Ta-ble 3.Comparingthealloy1.2FeMn-CMwiththealloy1.2FeMn, itindicates that the size and the volume fraction of iron-rich inter-metallics were decreased about 39%and 14%respectively, byincreasing cooling rate.3.2. Wear characterizationsThe wear rate of the base alloy,1.2Fe, 1.2FeMn and 1.2FeMn-CMalloysatdifferentappliedloadsof18, 51, 74and100 NarecomparedwitheachotherinFig. 2. Itcanbeobservedthattheaddition of about 1.2%Fe to the base alloy creates a detrimental ef-fect on the wear behavior of the alloy. Also, the 1.2Fe alloy has thehighest wear rate compared to the base alloy at all applied loads.Based on Fig 2, the addition of Mn to the 1.2Fe alloy declines thedetrimental effects of iron and improves the wear rate of 1.2FeMnalloy compared to that of 1.2Fe alloy. Applying high cooling rate tothe 1.2FeMn alloy leads to the reduction in the wear rate. Thus, the1.2FeMn-CM alloy displays the highest wear resistance among thealloys.The SEM micrographs of the worn surfaces of the alloys underapplied loads of 18 and 100 N are shown inFig. 3. Fig. 3a showsthat the worn surface was mostly covered by oxide particles underappliedloadof 18 N. Theoxideparticlesformedontheoverallworn surface of the pin contained a certain amount of iron, alumin-ium and oxygen as examined by EDS (Table 4). These debris couldentrapped between the sliding surfaces and gets compacted due totherepetitiveslidingandformsatribolayeroverthesurface, asshown in Fig 3b and d. The composition of the tribolayer formedon the overall worn surface of the pin has been presented in Table4. Fig. 4 shows the subsurface micrographs of 1.2FeMn alloy sub-jected to an applied load of 100 N. Sliding high tangential stressesthat occur on and below the sliding surface, result in nucleation ofcrackswithintheplasticallydeformedmaterialbeneaththesur-faceasshowninFig4a. Thecrackscanbepropagatedandtheirconnection to each other can lead to fracture of metallic and inter-metallic particles fromthe surface [21,22]. These fragmentedmetallicparticlescouldbemechanicallymixedwiththeoxidesin the contact zone and form a tribolayer (MML) as shown in Fig5a and b. The tensions derived on the surface during sliding, canweakenthetribolayer andleadtothedelamination andfractureofoxidelmgeneratedthroughtheweardebris(Figs. 6and7).According to Table 5, the composition of the wear debris containeda certain amount of iron, aluminium and oxygen that is similar towhat can be observed from the worn surfaces in Fig. 3.Decrease in the wear properties of 1.2Fe alloy compared to thebase alloy, as shown in Fig. 2, can be explained based on the micro-structural features of the alloys. Fig. 1b shows that addition of ironto the LM13 alloy led to the precipitation b-phase intermetallic inthe matrix. b-Al5FeSi needle-like intermetallics are hard and brittlephases. They exist as discrete particles with a highly faceted natureinthealloymatrix[23]. Accordingly, ithasrelativelylowbondstrength with the matrix and the interfacial regions between thisphase and the matrix become quite prone to microcracking[9,24,17]. Moreover, sharp edges of the b-needles introduce severestress concentration effect into the alloys matrix [10].According to Fig. 2, the enhancement in the wear properties of1.2FeMnalloy compared to 1.2Fe canbe originatedfromthereplacement of b-ake-like intermetallics by the modied a-inter-metallic compounds.Since the a-intermetallics have a modied morphology ratherthan the b phase, they have little effect on the formation of surfaceand subsurface microcracks. Also, the a-intermetallics formaroughinterfacewiththematrixandtheir better bondingwiththe matrix declines the possibility of crack formation in the inter-face of the intermetallic compounds with the matrix.AsobservedinFig. 1d, increasingthecoolingrateledtotherenement of microstructural features mainly the eutectic siliconFig. 7. (a) SEM micrographs of wear debris of 1.2FeMn-CM alloy at applied loads of 51 N, and (b) enlarged view of the marked region B in the micrograph (a).Table 5Chemical compositionoftheweardebrisofthebasealloyand1.2FeMn-CMalloyshown in Figs. 6 and 7 (at.%).Alloy code Atomic percentageO Al Si Fe Ni CuBase alloy (region A) 48.52 39.50 4.37 5.59 1.2FeMn (region B) 36.91 43.00 6.47 5.41 V. Abouei et al. / Materials and Design 31 (2010) 35183524 3523particles and the Fe-richintermetallics. The highcooling ratedecreases the a-phase formationtemperature andrestricts itsgrowthtime[25,26]. Increasingthecoolingratealsodecreasesthesizeof thesecondarydendritearmspacingand, therefore,the a-phases that form in the interdendritic spaces become ner[27,28]. Table 3 conrms that the size and the volume fraction ofFe-rich compounds in 1.2FeMn-CM alloy compared to 1.2FeMn al-loy, were decreased by about 39% and 14%, respectively.Solidication at high cooling rate also increased the hardness ofthe alloys about 20%. This increase in hardness resulted in a higherwear resistance than expected by Archard wear law [29]. In addi-tion to solid solution strengthening effect of the high cooling ratesolidication, thesuperiorwearbehaviorobservedin1.2FeMn-CM alloy (Fig. 2), can be attributed to the ne distribution of thehard a-intermetallicsandeutecticsiliconparticlesinthematrixthat decreasedthesusceptibilityof thealloytoembrittlementand microcracking.4. Conclusion(1) The addition of iron to AlSi alloys resulted in the formationof needle-like iron-rich intermetallics in the matrix.(2) Flake-like intermetallics due to the higher tendency tomicrocrackingleadtothereduction ofthewearresistanceof the alloy.(3) The Mn addition to the alloy results to the reduction of thedetrimental effect of iron due to the formation of the modi-ed a-intermetallic phases.(4) Solidication at high cooling rate results to the renement oftheintermetallicparticlesandcausestheimprovementofthe alloy wear resistance.References[1] Eyre TS. Wear characteristics of metals. Tribol Int 1976;9:110.[2] HaqueMM, SharifA. Studyonwearpropertiesofaluminiumsiliconpistonalloy. J Mater Process Technol 2001;118:6973.[3] Katsuta M, Oodoshi K, Kohara S. Wear resistance of AlSi alloys. In:Proceedings of the sixth international conference on aluminiumalloys(ICAA-6); 1998. p. 194550.[4] SahebN, Laoui T, DaudAR, HarunM, RadimanS, YahayaR. InuenceofTiaddition on wear properties of AlSi eutectic alloys. Wear 2001;249:65662.[5] Gruzleski JE, Closet BM. The treatment of liquid aluminiumsiliconalloys. Illions: AFS; 1990. p. 1254.[6] Huran M, Talib IA, Daud AR. Effect of element additions on wear property ofeutectic aluminiumsilicon alloys. Wear 1996;194:549.[7] Lasa L, Rodriguez-Ibabe JM. Scripta mater. Effect of composition andprocessing route on the wear behavior of AlSi alloys 2002;46:47781.[8] Friction, lubrication, and wear technology. ASM handbook, vol. 18. Metals Park(OH): American Society for Metals; 1992.[9] Shabestari SG. The effect of iron and manganese on the formation ofintermetallic compounds in AlSi alloys. Mater Sci Eng A 2004;383:28998.[10] Mbuya TO, Odera BO, Nganga SP. Inuence of iron on castability and propertiesof aluminiumsilicon alloys: literature review. Int J Cast Metal Res 2003;16:115.[11] Shabestari SG, Gruzleski JE. Modication of iron intermetallics by strontium in413 aluminum alloys. AFS Trans 1995;26:28593.[12] KulunkB, Shabestari SG, Gruzleski JE, Zuliani DJ. Thebenecial effects ofstrontium on A380 alloy. AFS Trans 1996;170:118993.[13] Taghiabadi R, Ghasemi HM, Shabestari SG. Effect of iron-rich intermetallics onthe sliding wear behavior of AlSi alloys. Mater Sci Eng A 2008;490:16270.[14] Mondolfo LF. Aluminum alloys: structure and properties. London:Butterworth; 1978.[15] WangL, Makhlouf M, ApelianD. Aluminiumdiecastingalloys, aluminiumcasting research laboratory. Worcester (MA): Worcester Polytechnic Institute;1993. p. 15065.[16] Murali S, RamanKS, MurthyKSS. Theformationof b-FeSiAl5, andBeFephases in Al7Si0.3Mg alloy containing Be. Mater Sci Eng 1995;A190:16572.[17] Mulazimoglu MH, Zaluska A, Gruzleski JE, Paray F. Electron microscope studyof AlFeSi intermetallics in 6201 aluminumalloy. Metal Mater Trans1996;27A:92936.[18] Samuel AM, Samuel FH, Doty HW. Observations on the formation of b-AlFeSiphase in 319 type AlSi alloys. J Mater Sci 1996;31:552939.[19] Tash M, Samuel FH, Mucciardi F, Doty HW. Effect of metallurgical parametersonthehardnessandmicrostructural characterizationof as-cast andheat-treated 356 and 319 aluminum alloys. Mater Sci Eng A 2007;443:185201.[20] Warmuzek M, Ratuszek W, Sek-Sas G. Chemical inhomogeneity ofintermetallic phases precipitates formed during solidication of AlSi alloys.Mater Charact 2005;54:3140.[21] Abouei V, Saghaan H, Kheirandish SH. Effect of microstructure on theoxidative wear behavior of plain carbon Steel. Wear 2007;262:122531.[22] Abouei V, Saghaan H, Kheirandish SH. An investigation of the wear behaviorof 0.2% C dual phase steels. Mater Process Technol 2008;203:10712.[23] Belov NA, Aksenov AA. Iron in aluminumalloys: impurity and alloyingelement. New York: Taylor and Francis; 2002.[24] Ashtari P, Tezuka H, Sato T. Modication of Fe-containing intermetalliccompoundsbyKadditiontoFe-richAA319aluminumalloys. MaterTrans2003;44:26116.[25] NarayananLA, Samuel FH, Gruzleski JE. Crystallizationbehavior or iron-containingintermetallic compounds in319aluminumalloy. Metal MaterTrans 1994;25A:176173.[26] Shabestari SG, Gruzleski JE. The effect of solidication condition andchemistry on the formation and morphology of complex intermetalliccompounds inaluminumsiliconalloys. Int J Cast Metals Res 1994;6(4):21724(ISICited).[27] Dutta B, Rettenmayr M. Effect of cooling rate on the solidication behavior ofAlFeSi alloys. Mater Sci Eng A 2000;283:21824.[28] Li Z,Samuel AM, Samuel FH, RavindranC, Valtierra S, Doty HW. ParameterscontrollingtheperformanceofAA319-type alloys:PartI.Tensileproperties.Mater Sci Eng A 2004;367:96110.[29] Hutching IM. Tribology: friction and wear of engineering materials. Cambridge(London): Edward Arnold; 1992.3524 V. Abouei et al. / Materials and Design 31 (2010) 35183524