magneziului, în materialele compozite Al --Si/grafit. Analiza microstructuralÄ (SEM) a arÄtat în mod clar cÄ eutecticul Al-Si este prezent în formÄ finÄ Ã®n imediata ...
OF ALT]MINIUM-GNAPHITE CO]HPOSITES MICNOSTRT]CTUNB MATERIALS FOR TRIBOI,OGICALAPPI,ICATIONS P. MOLDOVAN,
N. PANAI'f,
G. POPESCU'
G. GI-IICA*
A fost studiat potenlialul de germinarc a particulclor dc gralit ln prezen{a magncziulul, In materialele compozite Al -Si/gratit. (SEl\{) a ardtat in mod clar ci eutecticul A1 -Si Analiza micfostncluralir este prezent ln fornri finir In irncdiata veciniitatc a particulelor de glafit. l\,Iicrografiile electrotrice gi analiza enelgiei dispersive pe tl-ricroprobe o concentralie ridicatd (EDX) a niaterialllui compozit LNI 13 -grafit,.arati -Si. r l e m a g n e z i l l a i n t e r f a ! a p n r t i c u l : r r : ' rg r : r f i t / n r a t r i c e , c o r n p a r a t t i c l l I n a t r i c c a A l Prczen[a nragneziului asiguri olier]tarea convenal)ilI a particulelor de grafit
.
ln raport cu siliciul. Energia sciizutli a intcrfcJei erea eterogena'
G/Si ac!ioncazi
ca suport pentru nucle-
The nucleation potency of graphite substrates in presence of nragnesium composites werc studied. in Al -St/Graphite anal-vsis (SEI\I) clearly shorred that eutectic silicon is I{icrostructural present in t1c finer fortn in the irnmediate vicinity of the graphite particles. Scanning electron micrographs (SENI) and energy dispersive mlcroprobe' composite showcd a higher magnesium conanalysis (EDX) of LMl3-Graphite comparcd to AI -Si matrix' centiation at the interface graphite particle/matrix In the presence of rnagnesiunr therc is a relatively high probability that the graphlte farticle surfnces will bc suitably orletrted, with respect to silicon' to fdrm a Iorv cncrgy irrtcrtacc tNith it :lnd act as substralcs lor hetcrogeneous nucleation. I(EYWORIJS Piston, composites, alurninium,
magnesiurn, silicon,
graphite,
vortex,
lvotUng, modi-
llcatlon.
Introrluelion Al-Si alloys are extensively used in-tribologicat appll'catiols such as pistons and" ii some cases as cylinder liners in internal combustion
engftnes.
trn a,normal enEinemechanicallossesdue to friction in the piston assemblycompriseOb"7oof the total mechanicallosses.fhus the selecmaterial is very important. The use_of Al-qi/ tion (,f il suitalblepistriii ^compo.site piston_results in reduction !n !h9 specitic Graphite particle fuel-consrinption, a reductioir in the wear of piston anal piston rings.
i i{ , i1 i{
i*
r"* ittr i.il
* prof. dr. ing. : Prof. dr. ing. ; $ef lucrdri ing. ; ]\'IetallurgyDept.' Polyr'echnic Insti' tute ol Eucbarcst ; IDg.' I.N.T.E O'-S.4., Bucharest
15?,
P. i\.Ioldovan, et al.
These leductions appear to be due to increased lubrication from the graphite particles. Under boundary lutrrication conditions these composites shon ecl muoh improverl seizure and galling resistance comparecl u'ith conventional bcaring allo;'s. Several attcmpts hal'e been nr:rdc to study the microstructure of a_luminium allo.r s in fhc prcsen(re of dispersed graphite particles f1l. Krishnan and rtohatgi ha,vc sho'wn that the presence of dispersectgrapl:ite particlcs in qunntil,ies of 3 wt.)i produces partial modificaiion of eutectic silicon,in the culcctic Ai-Si alloy r,1\I12,and parbial modification arid re{irrclnent of eutectic ancl plimiry silicon in'the rryper-eutectic Al-Si alloy Lli28. Repg1ted observation have con{irmed that a-Al dentlrites in hypoeutectic AI-Si alloys tend to avoicl the dispcrsed graphite partibles, and do not nucleate on their surfaces. At present, hou'ever, there is lirnited understar,rdingof the phenongrena occurring during solidification of aluminium-matrix particulate composites. Nucleation and refinement of crystalline phases, physical and chemical interactions betrvcen clispersedgraphitc parl,icle anrl solidif.ving interfaccs are arcils rvherc a linorvlcdgo liase is beginning ,to be formed.' The objective of the prcsent investigations ri'as to examine the role of graphite particlcs itt inflncncing l;hc modification microstructures of eutectic Al-Si allovs. Bxperimeutnl Dct:rils The proceclure userl is basocl on graphite particles dispersion classified. ancl "activated." in the cone created in the aluminium melt at temperatures 1023-1L23 K.by its intcnse mechanic stirring [8.1. F,or g:xperiments wo used eutectic,alloy for piston.s LM13 (LL.5% Si; 1.2o/o Cu; 1.0o/oNi; 1.0o/oMg; bal. Al) and graphite particles 6O-1SO gin in size. Aluminium alloys has been melt ancl overheated at 1070 K into a graphite cylindric crucible of 1 Kg capacity, introcluced into a Kanthal resistor electric furnace. Temperature has bccn rncrasuredwil,h a Pt-Pt-Rh 10 thermocouple and acljusted with S K error. Metal bath treatment has been realized witlq degassing flux (based. on C2Cl6), deoxidation flux (Coveral 65). The modification has been realized. with elementary sodium (0.25 wt.o/) before introducing graphite particles. The classified graphite has been '(activated" by calcination at 81q K iqto ag electric chamber furnace with siliter resistors {SiC) during _120n!rn, in_tho aim to remove tho volatile substancos and gases. pr6heated graphite_particles have been mixod. with magnesium powder, in portions, into the "vortey" created by mechaniial stining.-After the gr?phite is assimilated. in tho melt, then suspensions viscoSity became.' sensible to increase, wo performed the clie mould casting of pistons.' The samples.have been submitted. to chemical analysis to.deter-eseaping mine the graphite'eontent (by measurement methocl of OO2.
.il ':J
, dj
\ l i c l o s tr r r t ' tr r r r . o f , \ l r r n r ri r i r r n r(-i r a p l r i t t ( j o t r r t r o s i t t , s
quan;itv fomr 1-2 g r,otnl)osit:o rc,Ict,iolr with hot solution of aciclified. potassiunl lrir,hrrrrnrl c). illhc sarnplcs \\'cre polisltcrl zrccot,(liug to st tndarcl nretoaller:lphic procedurcs ctcrllcrl l'it,lr Iit'llcl.'s rt'ast'rrl anrl obselletl in opl,ical and. filcanning clectron rr,ir.rosr.opc. ctluippctl rvit,h rrir,r'c-length clispersir-e X-ra1' spectroscopc.
'
Ilcsulls antl l)iscussirlrr.s
Thel 1-rrocetlnleof el:rlrot'afiotr of a,luminiurn - graphite composites supposgs the rnclt cornpler trca,trnent ns graphite partlcles befoie casting-- The assimr'lat,iotr of t hc ulaphitr: prrltic,les in liqrrid alloy depencls on the follorving fir,ct,ot.s ; - the morphologl' oli gr.aphite particles; : thg speetl of _stirring-anclholdirrg tirne before casting. The disper"siondegrec of graphite particles in i,he melt, i"s conditionecl by thcir flotatiotr. spectl, heal, of solirlification and transfer heat parametet's as b}' thc coagulation phetromellonof finest graphite particles. L. In.fl uetrt,e rtf ttt titrt,Jrtt.t, QsllqlitittttsoJ ltrc Xtelt'in Weilitt,g, tLirluid alurniniurn rloes not rr.et solicl graphite particle at tempera. . tures bclory 137;l ri, it"iyas, separated turn suspenslon, ' by flot:rtidn at tlrat one snrface, rluc 1,o trrore-rerhrcetltlensit.v: 'Il1e increasing otl l'el,ting tleglct'is lcalizeclby decr.easing0 contact atrgle belos g0o: B - ttrccosl{o,* -
o.r)ioru]
(L)
whcre osEl_6-"1 and o,- a,r'ethe solicl-ga.s,solicl-liquid, respecti'vely liquidgas intcrphase tcnsirtn t,ocfficients. If the data rif l'orli ar e used [-1]: o . u. : 1 . 2 J l N / n r l , o " r : 1 . 7 0 ? [ N / m ] , o r s : 0 . 8 0 4 l N l r n l it results 0 : 12?'. Ilclting :rssirnilat,ion degree of inactivatcci graphite is reclucetl, being of about, J0')/o. Thc incrcase of, n-etting degree has been realized experirnentally b). tt*"tttuting" graphite p:rrtir:les beforc l,ransfering then-in the me[t ancl bv addition of 1o7oMg mixed rvith graphite powder. - I'lagnesiurn addition (rvith o 1\{g: 552.10-3 N/m) in aluminium (oAl : - 82i'r.t0-3 NAn) leacls to an increasing of magnesium concentration at the melting surface. l'his n'ay the rn'etting degree of, graphite particles by liquid alumiuium is increased due to decreasing contact angle and so the graphite assimilation dcgree into the rnelt increases. Due to rnagnesiurn va,llors large enough tension, its partial evapciration is also protlucecl as the clcir,ning of graphite particles which,
r $j
* 6r
$i,
fr # r,
ft; ffi H tr m
l ) . , \ l o l r l r , r ' r r r rr ',t a l .
154
this 'wa-v, a better u-cttetl b1' t|e liqlitl from 'ivortel" sut'face. Ifagne' sium vanor tension has llecn detetrrrittctl fi'ont fhe rela,tion ; /.)\ I o g p - = - 7 5 5 0 / T+ 1 2 . 7 9- 1 . 4 1 l o g T , i n u n H g l Tireretore thc aclditiott of reacrtivc elertrents such as Mg to the matrix improve rvet,ting ch:rlactelistics of rnctal-ceranric system through : (i) a reduction of thc surface tension s1 the nrelt, atrd (ii\ a i'eduction of the solid-liquid intet'facial energY.
a4
w s ffi $ $
$ k
E '
E
* !t F
2. Microstructure ,f
Solitlified Composites
ft is generalll' belio'r'c
