'v1et-

iardietal

retail

994)

vfater

T, 82

)996)

eft, 14

(Asia

ts,2nd

IndiafI Journal of Engineering & Materials SciencesVol. 1.1,December 2004, pp. 481-486

Filing brass swarf gives a fine sinter-able gradepowder. Thus, the engineering components can bemanufactured directly without melting with 100%yield using P/M technique.

The heat treatment or sintering of powder compactstakes place in three phases. Initially, neck growthbetween particles develops rapidly, but powderparticles retain their identity. In the second phase, thestructure is recrystallized. However, the powderparticles lose their separate identities and diffuse intoeach other. Most of the densification occurs at thisstage. In the third phase, isolated pores begin tospheroidize and densification progresses at a slowerrate. The sintered parts exhibiting residual porositypossess poorer physico-mechanical properties -Icompared to the similar parts produced by casting. Itis therefore, cannot be employed in electronicequipments, where variations in the properties of,materials can affect the output signal.

A marked improvement in the properties of PIMcomponents is attained with porosities less than 3-4%(ref. 3). With the help of single pressing andconventional sintering process with muffle furnaceheating (MFH), such a low level of porosity cannot beachieved. However, it is possible to attain relativedensities of 90-93% in pressing.'. This is achieved bythe use of hydrogen-containing gases as sinteringatmospheres, Hydrogen diffuses into copper andcombines with the oxygen of oxides and forms watervapour'. When the density of a compact is high andclosed pores predominate, high pressure builds-up inpores. This pressure leads to the rupture of inter-

Stabilising dimensions of brass powder components

Akhter H Ansari", M Hameedullah" & M S J Asghar""University Polytechnic, bMechanical Engineering Department, "Electrical Engineering Department,

Aligarh Muslim University, Aligarh 202 002, India

Received 13 January 2004; accepted 4 August 2004

Powder metallurgy components are manufactured by compacting metal powders. Hence, compaction pressure affectsthe properties of the components. In the present case, pre-alloyed cartridge brass (70:30) powder is employed forinvestigation and the effect of compaction pressure on dimensional properties of components is studied. A volumetricshrinkage is observed at various levels of compaction pressure. The shrinkage reduces with an increase in compactionpressure. It is 1.0% at 242 MPa, which reduces further to 0.4% at 725 MPa. At high level of compaction pressure, theshrinkage is found along the diameter. A swelling of 1.0% in diameter is exhibited at low level.

IPC Code: Int. Cl.7 B22F 3/02

The use of copper in powder metallurgy (PIM)industry dates back to the 1920s, when porous bronzebecame commercialised. General Motors Corporationand Bound-Brook oil-less Bearing Co! manufacturedthe self-lubricating bearings. It is the oldestapplication of PIM components. The copper alloyshave special physico-mechanical properties, e.g., highelectrical and thermal conductivities, good corrosionresistance, low coefficient of friction, suitability forbrazing and electro-deposition. Added to it, P/Mcomponents have better tribology, toughness andvibration properties compared to the forgedcomponents with consequent saving in" cost'.Moreover, the processes like melting, casting andsubsequent machining operations can be avoided ifP/M technique is feasible. Therefore, it is extensivelyused for the manufacture of components used inmachines and instruments. Extensive applications ofbrass powder metal include automotive, building,coins, lIIledals, medallions, electrical and electronic

).. . . .components, hardware, office equipment, ordinances,personal products and printing materials. There is acontinuous increase in applications of PIM brasscomponents. Its current boom can be directly relatedto the accelerated demand for PIM products from theauto industry.

On the other hand, a large quantity of brass scrap inthe form of chips or filing swarf is available indifferent cities of India like Aligarh and Moradabad.But these scraps are not being reutilised properly".

*For correspondence (Email: akhter@bharatmail.com)

482 INDIAN J. ENG. MATER. SCL, DECEMBER 2004

Items Parameters

Table I-Details of the parameters used in the present investigation

Properties

Powder TypeParticle sizeChemical compositionApparent densityFlow rateLower punchUpper punchDieJacketShapeDiameterThicknessMassCompaction pressureTime of passing current

Die and punchassembly

Compact

Variables

Pre-alloyed atomized brass powder-100 urnCopper: 70.1 % :: Zinc: 29.9%3.31 glcc19.4 s/50 g08mmxlOmm08 mmx50mmOD = 50 mm; ID = 8 mmOD = 75 mm; ID = 50 mm; Length = 60 mmCylindrical8.0+ 0.3 mm10-13 mm3-5 g242 MPa to 725 MPa30-90 s

p,article contacts and formation of micro-cracks(hydrogen blight). The dimensions of componentsincreases and their mechanical properties getadversely affected. If this phenomenon is to beprevented the porosity of parts after pressing shouldbe sufficient (15-20%) (ref. 3). However, it is notpossible to produce components from copper alloypowders with porosities less than 7-10%4.

The diffusion porosity is another obstacle inattaining the high density. During sintering ofcompacts pressed from mixtures of powders (e.g.mixtures of copper powder and tin powder ormixtures of copper powder and zinc powder) fluxes ofdifferent intensities are set-up. It is due to thedifference in the diffusion coefficients of theelements. As a result, pores are formed and thedimensions grow". There is 2% increase in size, and6% decrease in density for the mixtures of copperand tin powders. The increase in size of sinteredcompact is directly proportional to the green density.To prevent this, it is necessary to employ alloypowder and ensure that the porosity of compacts isnot less than 15-20% (ref. 5). To obtain improvedproperties, they must be subjected to additionaldensification (i.e. re-pressing and re-sintering).

Evaporation of zinc takes place during sintering ofbrass. This phenomenon is also observed in high-temperature annealing of cast brass". The pressedcomponents are porous and have large specificsurface area. It is more pronounced in PIM materials.It can lead to variations in chemical composition andphysico-mechanical properties. The use of pre-alloyedpowders appreciably reduces the heterogeneity ofP/M brass.

The diffusion impregnation of mixtures and that ofsintering are highly critical'". In the former case, with'

an insufficiently homogeneous powder equalization ofzinc concentration may bring about uneven shrinkageor dimensional growth. While in the latter case; thereare fluctuations in the temperature and humidity ofthe protective atmosphere'". The fluctuations caused asubstantial scatter in values of physico-mechanicalcharacteristics. A certain rate of evaporation of zinc isa critical one!'. During the evaporation at a sub-critical rate, the diffusion porosity does not appeareven when a substantial amount of zinc (up to 25%) isremoved". Thus, both shrinkage of specimenslO•12 andtheir growth under the action of gases trapped inclosed pores':' may take place. The evaporation ofzinc results in sintering shrinkage of specimensamounts to 1.2 to 2.6%. The evaporation of zinc andthe magnitude of volume shrinkage were linearlyrelated to each other".

There is a need to develop an economically viablesimple sintering process for manufacturing brassproducts through PIM route. Since most of theengineering applications require strict dimensionaltolerances, a dimensional control is important. In thepresent work the effect of compaction pressure on thedimensional properties (diameter and volume) ofatomized brass powder components is explored.

Experimental ProceduresDetails of various parameters with relevant features

used in the present investigations are provided inTable 1. Figure 1 shows the details of die and punchassembly used to produce atomized brass powdercomponents. The walls of dies and punches werecleaned with balls of cotton, rinsed with acetone andlubricated-with borax. The cavity of the die and punchassembly was filled with 4.0 g of brass powder. Afterplacing outer jacket, the whole assembly was pressed

(

Fi;

uphydiloadpres:wasshovmairperietreat,condcornjrecor

.correestimdeviaanalyof coI

ResulAn

size 0

zation ofhrinkagese, thereiidity ofcaused a.chanicalif zinc ist a sub-It appear) 25%) isS10,12 andapped inration ofpecimenszinc andlinearly

lly viableng brasst of thenensionalnt. In theIre on thelume) ofed.

It features.rvided inmd punchs powder:hes wereetone andand punchder. After1S pressed

ANSARI et al.: ST ABILISING DIMENSIONS OF BRASS POWDER COMPONENTS 483

@lr@' oo.}1.Lowerpunch 2. Upperpunch

60

@:lb---d3. Die

70

o [=:e..•4. Jacket

Fig. I-Details of die and punch assembly with jacket.

DC supply

Switch

Fig. 2-Circuit diagram to pass an electric current through anspecimen.

up to a predetermined load by a single actinghydraulic compacting machine. To avoid impactloading, compaction was done at a slow speed and thepressing was ejected out. An electric current (d.c.)was passed axially. The relevant circuit diagram isshown in Fig, 2. A predetermined current wasmaintained in the component for a predeterminedperiod. Likewise, 5 to 6 samples of compacts weretreated for each set of predetermined experimentalconditions. The diameter and thickness of thecomponent before and after passing the current wererecorded, The change in diameter and thecorresponding change in volume of component wereestimated. For each experimental condition, standarddeviation and mean were estimated and used foranalysis. Experiments were performed at three levelsof compaction pressure.

Results and DiscussionsAn increase in compaction pressure decreases the

size of pores, Since an electric current generates heat,

§Q)

E:5 r==:=1j -+-T1~30"e ~ 0 2 " - --- - 12= 60~'-' . • \ s"0 ' 1- -0- -n~90,

_ __ __ \~ __ J.-----,-,,.,-~---~;;--------_ _ .• ~ 8~0

Q)Olcrox:U

200-0.2 -.

-0.4 J ICorrpaction pressure (rv'pa).

Fig. 3-Effect of compaction pressure on diameteral change ofproduct, current density = low.

*Ero'5tl:>'0"":'

e'#.a. ~.sQ)encess:U

1411.2 • I~T1 =30s I1 ' --O--12=60s

0.8 ' - -e- - T3 = 90 s0.60.40.2

o I ..,.~.-0.2-0.4 I I

.•.,,

200 8~0

Compaction pressure (Nlpa).

Fig. 4-Effect of compaction pressure on diameteral change ofproduct, current density = medium.

Q;4iE'"'0tl"'0 ....:.e?fl.a. ~.S

CI>OlC

'"s:U

1.5 rl -------------------,

1t. n·3(I,--o--T2=60s

0.5 j... -.~:=90s

-,

-0.5 •...400200 8l!i0

-1

Corrpactionpressure (Wpa).

Fig. 5-Effect of compaction pressure on diameteral change ofproduct, current density = high.

therefore, the trapped ~ir in the pores also gets heatedup. Thus the volatile zinc may vapourize due toheating. They result in an increase in the partialpressure in pores. The properties of P/M brasscomponent are affected by compaction pressure aswell as the magnitude of an electric current.Figures 3-8 show the effects of compaction pressureon the dimensional properties of PIM brasscomponent.

Effects on change in diametersFigure 3 shows effects of compaction pressure on

the change in diameter of brass powder component ata low level of current density. For a specimencompacted at 242 MPa, a gain (0.25% to 0.58%) is

484 INDIAN J. ENG. MATER. SCI., DECEMBER 2004

O-r-------~------~------~------~400 6002001----------------1I---+-- T1= 30 s

1-* -T2=60s1- -~ - T3=~~

o -0.2Q)en .c ~ro -.:. -04ro~ .ii~o U -0.6".5 -5~ ~ -0.8:J~ -1

200 400 600 • #..

•-+--T1 =30s- "'*- - T2 = 60 s- -- -T3=90s . ,;-.. --'~:-.-•... ---~

-1.2Corrpaction pressure (Wpa).

Fig. 7-Effect of compaction pressure on volumetric change ofproduct, current density = medium

1.5 '1--------__-- ---------------------,---+-- T1= 30 s- •.•....-1'2=60s

0.5 - .. - - - T3 = 90 s

oQ)

en .~~'5~UU.c :J0)"8§ 0.~ -0.5

.-..

o f- --~-----~----~:_~---_l

~.-----200 400 80

-1 ~----------------------------~

Corroactlon pressure (l'Ipa).

Fig. 8-Effect of compaction pressure on volumetric change ofproduct, current density = high.

found in its diameter. Traces of zinc-white were alsoobserved on the surfaces of the components. It isevident that some of the vapour of zinc has escaped tothe surfaces of component, from its interior. The flowof electric current has caused zinc to evaporate. It hasalso heated the trapped air. The swelling attributes anincrease in partial pressures in pores. Swelling ofcomponents were also reported in variousinvestigationsS-8,13-I6. Kutty'" and Ansari et ai.IS havereported swelling of components sintered throughconventional MFH technique. Akhter et al." hasreported a swelling of product diameter to a minimumof 0.439% after passing an electric current.

An increase in compaction pressure to 483 MPa hascaused a sharp decrease in swelling of diameter. Infact it was shrinkage of 0.2%. It has also reduced thetraces of zinc-white depicted on the surfaces. Thereduction in the size of zinc-white indicates that theincrease in compaction pressure restricts themovement of the vapours of zinc. The increase incompaction pressure might have suppressed theevaporation of volatile zinc. Alternatively, it mighthave caused the vapour to segregate on the particleinterfaces. The segregation prevents the formation ofinter-particle bonds resulting in weakening." of thecomponent. A further increase in compaction pressurehas practically no effect on diametric change.

Figure 4 shows the effect of compaction pressureon the change in diameter at medium level of currentdensity. At 242 MPa, the figure shows a gain indiameter from 0.55% to 1.2%. At this level ofcompaction pressure, the swelling in the diameterincreases for high level of current density. Theincrease in current density has increased the rate ofheating. It has thus increased the internal pressure inpores resulting in an increase in swelling of theproduct. An increase in compaction pressure to 483MPa causes shrinkage in diameter (up to 0.2%). Afurther increase in compaction pressure haspractically no effect on diametric losses.

Figure 5 shows the effect of compaction pressureon the change in diameter at high level of currentdensity. The increase in current density has increasedthe rate of heating which might have developed veryhigh pressure on pore walls. The rate of evaporationof zinc might have also intensified. Hence, when anelectric current at this level was passed, thecomponent disintegrated. A component, whencompacted at 483 MPa, depicts a gain in its diameterof -0.08% to 1.1%. The increase in compactionpressure increases the interlocking force between theparticles. The passage of current through thecomponent attributes the interlocking force to besufficiently strong to withstand the partial pressuredeveloped in the pores. Traces of zinc oxide depictedon the surfaces of the component attribute formationof zinc into vapour. An increase in compactionpressures to 725 MPa has caused a little effect on theproperty.

8 0

Effects on change in volumesFigure 6 depicts the effect of compaction on the

change in volume of a product at low, level of current

dealtspa InovofinActht

nomezin10

CatdelIt,thereccarpresiz:I

precurvolthicbetraehiglpmfIm(72:

Fon tthisdisi:(48~furt!of tl

Ingrailzincpres:andpres:com]conecom]

tIPa haseter. InIced thees. Thethat thets the'ease in.ed thet mightparticleation of'of theiressure

iressurecurrentgain inevel ofliameter:y. Therate of

ssure inof theto 483

2%). Are has

pressurecurrent

icreasedled veryoorationvhen aned, the

whenliameter .ipaction/een thegh the~ to bepressure;Iepictedmnationipaction:t on the

1 on thecurrent

ANSARI et al.: ST ABILISING DIMENSIONS OF BRASS POWDER COMPONENTS 485

density. At compaction pressure of 242 MPa,although there is a gain in diametric change of aspecimen (Fig. 3), yet at this experimental condition,a loss in thickness is found (-l.5% to -l.0%, which isnot shown in figure). It causes a net loss in itsvolumetric change (-0.9% to -0.65%). It validates thefi di f li h 101215-16m ings 0 some ear ier researc ers " .According to them, the shrinkage is due to escape ofthe vapour of zinc.

An increase in compaction pressure to 483 MPa hasno effect on the volumetric shrinkage. However, asmentioned earlier, it reduced the size of the traces ofzinc-white. Therefore, in the present case, an increasein compaction pressure traps the vapour of zinc andcauses the shrinkage in the component. This is ad .. f h f di f h li klO 12-16eviation rom t e m mgs 0 t e ear rer work" .It attributes inter-particle fusion. A further increase inthe compaction pressure to 725 MPa causes somerecovery in volumetric loss. The volumetric recoverycan be attributed to the increase in the internalpressure of arrested vapour and the expansion of poresize.

Figure 7 shows the behaviour of compactionpressure on volumetric change at medium level ofcurrent density. At 242 MPa, there is a loss involumetric change (-0.95% to -0.8%). A loss inthickness (-4.7% to -0.7%, not shown in figure) maybe related to the loss in volume. An increase in thetraces of zinc-white on the surfaces was observed athigher electric current. An increase in compactionpressure to 483 MPa keeps the volumetric loss same.However, a further increase in compaction pressure(725 MPa) recovers some of the losses.

Figure 8 depicts the effect of compaction pressureon the volumetric change at high current density. Atthis condition, a component compacted at 242 MPadisintegrates. An increase in compaction pressure(483 MPa) causes a loss in volume (about -0.6%). Afurther increase in compaction pressure recovers someof the volumetric losses.

In brief, a low level of compaction pressure causesgrain growth. It has little to do with evaporation ofzinc .and its escape. An increase in compactionpressure traps vapour of zinc, restricts its movementand even suppresses its evaporation. The internalpressure in pores also increases with an increase incompaction pressure. Therefore, both processes areconcurrent, viz., (i) evaporation with swelling of thecomponent, and (ii) inter-particle fusion with grain

growth, which brings the particles closer andconsequently, the component shrinks.

ConclusionsThis study concludes that the compaction pressure

affects the properties of brass powder components.However, in this case, volumetric shrinkage has beenachieved. An increase in compaction pressure reducesthe shrinkage and controls their dimensionalproperties. Moreover, the evaporation of zinc wasobserved during the flow of current and the vapourescaped towards the surfaces. However, an increase incompaction pressure arrests the escaping vapour.

AcknowledgementsThe authors owe a sense of gratitude for the

ungrudging help of Fuduka Metal Foil & Powder Co.,Japan, to supply atomized brass powder, withoutwhich it was impossible to make this venture. Theauthors sincerely acknowledge Mr. Azhar Jameel,S.M. Lab., C.E.S., University Polytechnic, AligarhMuslim University, Aligarh, for his help inpreparation of powder compacts. Many thanks of Dr.M. Muzammil, Department of Civil Engineering,Faculty of Engineering and Technology, who helpedin giving the current shape to the present work.

References1 Stevenson R W, Metals handbook, (ASM), Vol. 9, 1987,

733-740.

2 Hameedullah M, Optimization of process variables inbrass powder metallurgy, FTR 10(129)/86 EMR II, CSIR,1993,7.

3 Smirygin A P, Smiryagina N A & Belova A V, (in Russian),Metallurgiya, Moscow, (1974).

4 Radomysel'skii I 0, Baglyuk G A & Mazharova G E, SovPowder Metall Metal Cer, 23(3) (1984) 218-225.

5 Eudier M, Powder Metall, 2 (1978) 101-104.

6 Gudkova T I, Diffusion, Structure and Properties of Metals(in Russian), (1964) 172-176.

7 Howat 0 0, Gralik R L & Cranston I P, J Inst. Met, 15(80)(1978) 352-361.

8 Peissker E, Modern Develop Powder Metall, 7 (1974)597-614.

9 Gulyaev A P, Metallurgy (in Russian), Metallurgiya,Moscow, (1978).

10 Terletskii V E, Kalish V S, Bronin S V & Shatskii V A,Sintered Const Mats (In Russian), Inst Probl Materialoved,Akad Nauk Ukr SSR, Kiev, (1972) 17-25.

11 Terletskii V E, Author's Abstract of Candidate'sDissertation, Kiev, (1972),

486 INDIAN J. ENG. MATER. SCI., DECEMBER 2004

12 Palmer E & Grimme D, (Russian Translation), Metallurgiya,Moscow, (1966) 137-146.

13 Terletskii V E, Epshtein I Yu & Branin S V, (in Russian),Inst Probl Materialoved, Akad Nauk Ukr SSR, Kiev, (1976)51-55,

14 Ansari R & Hameedullah M, Trans PMAI, 13 (1986).

15 Kutty Y B, Investigations into reclamation of brass metalcutting chips using powder metallurgy. Ph.D Thesis, lITMadras, India, 1988.

16 Akhter H A, Hameedullah M & Afaq A, Advances in powdermetallurgy and particulate materials._ MPIF. 1 Pt 4 (1995)153-161.

Fopredi(en1manstiifortalfacantchrformemaanerepgerbeilate10\\

canCO!metflasandfilliloss

pindt

*For