PAN
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Archiwa uof Foundry,
Year 2004, Volume 4, Nr 11
Archiwum Odlewnictwa
Rok 2004, Rwzni k 4, Nr I E
Katowice PI, lSSN 1642-5308
ANALYSIS OF SILICON INFLUENCE ON MICROSTRUCTURE
OF HIGH-ALLOY BRASS Cu70Mn7A15Fe2Si2Pbl
D.A. KOTOV, R.K. MYSIK, A.V. TROPOTOV,
S.V. BRUSNITSYN, V.A. DEMAKOV,
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SUMMARY
Products from rnultialloy brasses are used in conditions both wear and impact
load. Particular structure i s formed. which provide required level both stress-strain
properties and wear resistance due to high alloying of brasses. In this connection high
requirements of phase ratio quantification and morphology of in tennetdlic inclt~sionare
produced to products from mu1tinlloy brasses.
In the scientific work were studied such subjecls as:
-
influencc of fermm and siliciu~nwhich arc main intermct;lllicforming
componcnts on rnicrostmclure of mu1tinlloy brasses;
stress-strain propertics both cast cendi tion and extruded condition.
The investigation dlowed to correct the percentage of intertnetallicforrning
components in the multiaEloy brasses.
Special alloy brasses we widely applied in many industries of nowadays Rilssia.
particularly in motor-car industry. shipbuilding. rnechanica1 enginwring, and so on.
These brasses contain, in addition to copper and zinc, such alloy clcrnenrs as silicon,
manganese. iron, aluminum, nickel. lead, ctc. Because of the high alloying degrcc, there
is a definife microstructure has been shaped in brasses of this kind that provides thc goal
not only of ~ncchanical properties but special ones eithcr. such as wcar resishncc,
corrosion rcsislancc. cavitation resistance, and the li kc. For examplc, such si !iconmanganese alloy brasses as Cu58Mn3Si 1.5Ni 1.5Pb1, Cu58MnZPbZSi 1 All,
Cn70Mn7A15Fc2SiZPh 1 have high anti friction properties due to a cornplicatcd
microstructure consisting of a-n P'-base phases, and also of intermcrnllic (or silicidc)
compounds and of lead particles; consequently these brasses arc used basically for
production details being operated in intensive friction conditions. The durability of
details of such kind depends on the following microstructure parameters: firstly, ratio
between both base phases of the alloy, secondly; quantity, number. s i x and
eqtlipartition of intermetdlic compounds; thirdly, quality of blanks, which the details
have been made of. It is necessary to notice, one of the main problem relating to the
brass Cu70Mn7 AEFe2Si2PbI (indicated as CuZnMnAl FeSiPb in the lexl further). is
the instability of the alloy rnicrosmcture (namely the volume ratio between a-and P'phases, the volume ratio and size of intennetallic compoundsl, which one ha$ hcen
formed as a rcsul t of crystal Iization conditions, and by the complexity of the chemical
composition as well.
The CnZnMnAIFeSiPb-alloy is used now for manufacturing of the details "the
blocking sing of thc synchronization unit of thc gear box" for cars of models VAZ,
Russia. It has been established, the highest wear resistance is given to these details by
needle-shaped silicide compounds with the length from 40 to 120 pms, the width
from1 0 to 25 p s , and aIso by spherical (or petalled) silicide compounds with the
diameter not more than 40 pms. However, we do not still manage to produce stably the
alloy with required silicide size, because of the number of objective reasons, main of
which is the lack of an established information about mechanisms of fonnation the
silicide compounds during crystallization of melt. The existing technology for semicontinuous casting creates such crystallization conditions of ingots that promote
formation of separate silicide compounds with the large more than 500 pms. The
situation is made worse by the fact that these particles can often form rather large
congestions. thereby upsetting their equipartition inside the base phases. The drawbacks
of the cast structure are usually saved in extruded semi-finished itcms (tubes). and i t
affects the technological and opera tion properties of finished products very adversely.
Thus the problem of searching for ways to grow smaller siliclde particles during
the ingo~scasting directly is extremely relevant and actual now.
1. INFLUENCING SILICON CONTENTS ON PROPERTIFS OF RRASSFS.
According to the specifications TY 184550-106-033-97, there is in the chemical
composition of the CuZnMnAIEeSiPb-alloy 1.7 to 2.5 % (weight) of silicon. The main
part of silicon has been fixed in form of intennetallic compounds during the
crystallization; therefore thcse particyes get consolidated by the large silicon contents,
reaching undesirable large sizc.
On basis of results of the X-ray spectral analysis, the supposition has been, made
that was possible to obtain more small-sized intermetallic particles by means of
decreasing of silicon contcnts in comparison with the said limi~s.However, in this case
there is a danger of decreasing the volume ratio of intemttallic compounds, and as a
consequence of it. of dwrcase the wear resistance as well. Therefore, a number of
laboratory cxpcrirnents was conducted with the purpose to endorse or disprove the
strpposition.
The different chemical structures of CuZnMoAIFeSiPb-alloy were me1 ted in an
indl~ctivehigh frqucncy assay crucible furnace from fresh burden rnaterids, and the
experimental ingots (specimens) were casted with the weight of 500 grams each one
(table 1 ) . The silicon contents were increawd gradualilly in she experimental specimens
from 0.13 to 2.7 % (weight);, while fie rest factors were supponed stable, such as the
contents of other alloy elements, the ternpetahre of charging hot-metal, tempcraturc of
m l d . into which the rnetal of investigated specimens was filled up, etc.
Table 1. Chemical composition of experimental specimens.
2.
ANALYSIS OF BRASS SPECIMENS
The qualitative metallographic analysis has shown that the regular change of thc
shape and si7x: of interrnetailic particles takes place by the consccutivc transition from
the specimen with the minimum silicon contcnts to onc with thc maximurn contents. So
therc are small-sizcd globular silicide particles with thc avcrngc size of 1 pm or less in
the specimen with the silicon contents of 0.13 %. Thc silicide compounds reach 2 to 3
prns in s i 7 ~in lthe spccirncn with 0.5 % of silicon, whilc all of these particles have the
globular rnorpho!agy only. The size of globular particlcs has risen a little by increasing
of thc silicon conients up to 1.6 %, and some stnall needle-shaped particles has began to
occur. The specimens with the silicon contents from 2.2 fo 2.7 % contain silicide
compounds with almost identical morphology: needle-shaped, hexahedral and smallsized globular. The small-sized globular particles are ususlly collected into
accurnnlations ("chains").
Presumptively. the needle- and hexahedral-shaped particles have I hc sntnc
morphology, but they get to a plane of a micro-section under difi'crcnt anglcs. Both
needles and hcxahedrons have incErrsions of a-phasc inside thern.
Fig. 1. Influence of the silicon contcnts on the silicides
volume ratio
20
01
5
i
-
15
10
5
0
1
0
2
7
3
Sl, %
Fig. 2. The avemge size o f silicidc particles depending on
the silicon contents
i'O:o
2
Si, %
Fig. 3. The average distance betwecn silicide parliclcs as a
function of thc silicon contents in the alloy
The outcotnes of the qualitative analysis of alloys melted in the labomtory arc
shown in figs. 1 - 3. I1 is obvious from fig.1 that [he volu~mratio of sificidc particlcs
grows proportional to the silicon conlcnts in the alloy inside !he interval from 0 up to
2.2 %. It can bc exprained by the fact that silicon is the main silicidc-forming chcmical
clernent. The subseqaen! increasing the silicon contents from 2.2 up !o 2.7 % results in
some saluratien on volumc ratio of intermcltallic compounds, so the subsequent input of
silicon does not result in increase of voPume ratio of thesc particlcs. Pmhahly. i t is
explained hy thc fact that silicon fixes all the manganese and iron into form of silicidcs
with the s toichiomctric conszmcrion of (Mn,Fc),S i,.
3. METALLOGMFIC RE!EAKCH OF BRASS SPECIMENS
Fig.2 shows that the average s i x of siIicidc particles is not more than 5 pms
insidc thc interval of the silicon contents from 0.5 up to 2.2 %, hut ?hereis obviolls thc
nhnlpt increasing their sizc with increase thc silicon contents higher than 2.2 9%.
It is expcsimntally established, that the distance between silicidc particlcs
increases with increase llhe silicon contents in the hmss (fig.3). It must be connccrcd
with the fact that atoms of silicon and iron form structures with the short-range ordcr
during ~ h ccrystallization neat the Eiquidus tcrnperamre. and silicon hinds nucleation
centers (atoms of iron). As a rcstrlt of it. the pxtielcs get to be enlarged, and rhc necd!es
grow, some of thcm can reach the length up to 1 to 2 rnrns.
I
0
0.5
1
t,5
?
2.5
3
SI, %
Pig. 4. FIardness of the alloy depcndi ng on thc sillicon contents
4. INFI,WENCE SILICON CONTENT O N HAKI)NE'S
OF BRASS SPECITMENS
Measuring of laboratory spwitncns hwdncss has shown that incrcase o f ~ h c
alloying degrce by silicon rcslllts En the wholc in i n c s ~ ~of
s ethe metal hardrless (frg.41.
Thcrc is thc intensive incrcasc of thc hardness at thc silicon contents of 0 20 0.5 96;
apparently, it is directly connected with increase 'of volume ratio of intermetallic
compounds in alloy and to their enlargement. The hardness is augmented insignificantly
inside the interval from 0.5 up to 2.2 %, evidently as thc size of silicide particles and
also the average distance between them are augmented only slightly. The needle-shaped
intennehllic compounds begin appear with the increase of silicon contents from 2.2 up
to 2.7 %. The abrupt increasing of the hardncss i s watched, apparently because of the
large needle-shaped si Sicide particles stand as a peculiar "rcinforcing" structr~ral
constituent.
5. CONCLUSION
The following conclusions can k made as a result of the rcsearcl~:
The large silicide particles are undesirable in the stnrcture of the
CuZnMnAlFeSiPb-alloy. These particles can, crumble getting nndcr a chiscl during
machining, and that can rcduce the wcar resistance of the material.
The quantirative and qualitative analysis of the laboratory spccirncns havc
shown. [hat in the allay with thc silicon contents of 0.13 % there arc small globular
silicide particles with the averagc size of 1 pm or less. The size of silicidc particlcs
rcachs 2 to 3 pms at the 0.5 % of silicon contents, while all of thc~nsave thc gloh~~lm
shape. The size of globular silicide partielcs decreases a little at the increase of the
silicon contents up to 1.6 %, and small nccdlc-shaped particles begin to nppcnr. The
speeirncns with the silicon contents of 2.2 to 2.7 O/o contain silicidc pnrticlcs of
npproximately idcn tical morphology, narnel y needle-sha ped, hexahedral and smal I-sizcd
globular. Whcn t h e needle-shaped particlcs begin to occur in the spccimcns with thc
silicon contents of 2.2 to 2.7 %, thcrc is observed an abnlpt increase of thcir avcrage
size, and also the distance between the siIjcide hcgins to he augrncntcd with this
increasing thc size of particles. some of the intennetallic particles can reach the lcnglh
up to 1 mrn or more. The volume ratio of intermetallic compounds is proportioiiaE 10 the
silicon con tents in the CuZnMnAl FcSiPb-brass. Howcver, thc suhserluent increasing of
the silicon contents above 2.3 % docs not result in incrcasc thc voluinc rn!io of
in tcr~netalliccompounds.
Measurelnent the hardness of laboratory specimens has allowcd to cstahlish, that
the more nigh alloying by silicon results in incrcase thc hardness o f mctal. There is n
small incrcase of hardness inside the interval of silicon conlcnts frorn 0 up 100.5 %I; it is
connected to rising of volume ratio and sizcs or intcrmctallic coa~poundsin ~ h calloy.
Thc hardness is augmented insi~mificantlyinside the interval from 0.5 np to 2.2 %J; it is
conneclcd to rhe fact that the s i 7 ~of intermetallic co~nporlndsand tlac averngc distnncr:
between them arc not essentially augmented, too, The intermclallic compounds of
needle-shaped morphology hegin to occur by increase thc silicon conlents Frorla 2.2 up
to 2.7 %. The abrupt growth of metal hxdt~essbegins as well.
Thc following conclusion can bc t n ~ d eon the ground of the laboratory rescnrch.
Thc sizcs of intemctallic particles depnd directly on thc degree of alloying by silicon.
It i s cxpdicnt to limit the silicon contents in thc interval of 1.4 to 2,2 % (weight) for
stable grinding of these particles (without f
a
t of decrease thcir quantity. and ltherefore
the wear resistance of the alloy). The pmblem of the more cssenrial decrease of the
silicon contents in the CuZnMnAIFeSiPb-brass can k esteemed only after research !he
metal of industrial melting batches, in which the silicon contains is near the mentiond
low limit.
Recenzowsl prof. Jbzef GawrohsW