Preparation of white alumina spherical composite magnetic abrasive by gas atomization and rapid solidification

4 Pages·0.528 MB·English

by  Zhang G. (et al.)

Available online at www.sciencedirect.com
ScriptaMaterialia65(2011)416–419
www.elsevier.com/locate/scriptamat
Preparation of white alumina spherical composite magnetic abrasive
by gas atomization and rapid solidification
Zhang Gui-xiang,a,b Zhao Yu-gang,b,⇑ Zhao Dong-biao,a YinFeng-shi,b and
Zhao Zeng-dian,c
aCollege ofMechanical andElectrical Engineering, NanjingUniversity ofAeronautics andAstronautics,
Nanjing210016,People’s Republicof China
bSchool ofMechanical Engineering, ShandongUniversity of Technology,Zibo 255049,People’s Republicof China
cCollegeof ChemicalEngineering, ShandongUniversity of Technology,Zibo, 255049,People’s Republicof China
Received9May2011;accepted17May2011
Availableonline23May2011
Whitealumina(WA)sphericalcompositemagneticabrasivecanbeprepareddirectlybyaprocessthatcombinesgasatomization
andrapidsolidification.Thestructureandphasecompositionofthismaterialwerecharacterizedbyscanningelectronmicroscopy
andX-raydiffractionanalysis.Theresultsshowthatthecompositemagneticabrasivehasgoodsphericity,andtheWAgrainsare
tightlyembedded uniformly anddensely into the surface layer ofmatrix which consists ofFe–Si–Al–Ni softmagnetic alloy.
(cid:2)2011Acta MaterialiaInc. Published byElsevier Ltd. Allrightsreserved.
Keywords:Gasatomization;Rapidsolidification;Powderprocessing;Sphericalcompositemagneticabrasive
Magnetic abrasive finishing (MAF) is an excel- Moreover, the shape of the magnetic abrasive is irregu-
lentmethodoffinishingandhasattractedwideattention lar with weak processing capacity and low processing
in the field of aeronautics, astronautics, industrial pro- efficiency. Owing to these limitations, existing methods
duction,etc.,inwhichthefinishingoperationisthecrit- cannot meet the demands of commercial application of
ical and most expensive phase of the production of mass production. In the present study, an attempt has
metal and alloy parts [1–5]. Magnetic abrasive is the been made to prepare spherical composite magnetic
mostcriticalmaterialintheMAFprocess[6,7].Theper- abrasive using ferromagnetic Fe–Si–Al–Ni–Cr alloy
formance requirements of magnetic abrasive for MAF raw materials and white alumina (WA) micropowder
are as follows [8,9]: good soft magnetic properties; directly through a process that combines gas atomiza-
strongprocessingcapacity;andstrongbondingbetween tion and rapid solidification. In order to prepare spher-
the ferromagnetic matrix phase and hard abrasive cera- icalcompositemagneticabrasive,theapparatususedfor
mic grains, i.e. long service life. In addition, a simple the gas atomization and rapid solidification process is
preparation process, low cost and a spherical shape modified and the matrix composition of the magnetic
are desired. The ideal structural model of a single mag- abrasive is optimized according to the theory of
netic abrasive grain is shown in Figure 1. It is a kind of atomization and rapid solidification [13–15] as well as
compositepowder,generallyconsistingofferromagnetic the performance requirements of magnetic abrasives
matrix phase and hard abrasive ceramic grains. and the wettability of metal-ceramic. Scanning electron
Althoughseveralmethodstopreparemagneticabrasives microscopy (SEM) (Sirion 200, FEI, The Netherlands)
havebeenreportedintheopenliterature—sintering[10], and X-ray diffraction (XRD) (D8 Advance, Bruker,
bonding [11], plasma spraying [9,12], etc.—these meth- Germany) were used to characterize the morphology
ods are usually time consuming, complex and costly. and phase composition of the spherical composite
magnetic abrasive.
⇑
Figure2showsatwo-dimensionalschematicdiagram
Corresponding author at: School of Mechanical Engineering,
ShandongUniversityofTechnology,Zibo255049,People’sRepublic of the modified gas atomization and rapid solidification
of China. Tel./fax: [phone];e-mail addresses: system. A device to mix the high-pressure gas and cera-
[email protected];[email protected] mic hard abrasive powder, together with pipelines,
1359-6462/$-seefrontmatter(cid:2)2011ActaMaterialiaInc.PublishedbyElsevierLtd.Allrightsreserved.
doi:10.1016/j.scriptamat.2011.05.021
G.-x.Zhangetal./ScriptaMaterialia65(2011)416–419 417
abrasive phase atomizingchambercontrolledintheatomizingchamber
to 104–105Ks(cid:2)1) and solidified, a spherical composite
magnetic abrasive powder with bonded WA grains was
obtained.
Figure 3a shows the raw morphology of WA micro-
powder (W7) used in this experiment; Figure 3b and c
shows, at low and high magnification, respectively,
SEM micrographs of the WA spherical composite mag-
netic abrasive prepared by the gas atomization and ra-
pid solidification process. It can be clearly seen that
ferromagnetic
the magnetic abrasive prepared has good sphericity
phase
shape and the WA grains are distributed uniformly
Figure1. Idealstructuralmodelofasinglemagneticabrasive. and densely on the surface of the magnetic abrasive
(Fig.3bandc).NoagglomerationofWAmicropowder
grainsisfound.Figure3calsoindicatesthatthereisde-
nozzles and other supporting devices, was developed.
gree of wettability of the WA abrasive grains by the li-
The WA micropowder can be mixed uniformly with
quid ferromagnetic Fe–Si–Al–Ni–Cr alloy matrix
high-pressure atomizing gas in this device, and then in-
phase, and that the WA grains are well embedded into
jected into the molten ferromagnetic alloy matrix at
the ferromagnetic Fe–Si–Al–Ni–Cr alloy matrix, where
the atomizing focal point. The ferromagnetic alloy ma-
they form strong bonds. Compared with the morphol-
trix of the spherical composite magnetic abrasive has
ogy shown in Figure 3a, the WA abrasive grain phase
the following composition (in wt.%): 9.6Si, 5.4Al,
inFigure3cmaintainsitsoriginalshapeduringthispro-
5.0Ni, 0.5Cr, balance Fe; this has been optimized based
cess, and thus its processing capability persists. The
on the performance requirements of magnetic abrasive
structure and morphology of magnetic abrasive pre-
and the wettability of metal-ceramic [16–18]. A particu-
pared by the present method conforms to the ideal one
lar type of WA micropodwer, W7, was selected for the
as shown in Figure 1. Many other techniques, such as
abrasive grains of the magnetic abrasive. N gas was
2 sintering [10], bonding [11] and plasma spraying [9,12],
used as the atomizing gas in order to prevent oxidation
have also been used to prepare magnetic abrasives, but
of the molten ferromagnetic matrix alloy during the
these methods are usually time consuming, more com-
atomizingprocess.Thepreparationprocessofthespher-
plicatedandtheshapeofmagneticabrasiveisirregular.
icalcompositemagneticabrasivewasasfollows:theraw
Inourstudy,suitablealloymaterialscanbeeasilyadded
materials of the ferromagnetic alloy matrix were melted
duringmelting,resultinginthechemicalcompositionof
ina medium-frequency inductionfurnace andheated to
the magnetic abrasive matrix being uniform. Only the
an appropriate temperature (1580(cid:3)C). The atomizing
binding force between the atoms of the liquid molten
pressure was adjusted up to an appropriate value
metal needs to be overcome in order to disperse these
(2.2MPa). The molten matrix alloy was then poured
duringtheatomizationprocess,sotheexternalforcesre-
intoaheatingleakcrucible,andflowedtotheatomizing
quired are much smaller than those involved in the
focal point by gravity, where it was atomized into tiny
mechanical crushing method of sintering and bonding.
droplets by high-pressure mixed-gas consisting of high-
The chemical composition of the matrix of magnetic
pressure N and WA micropowder (W7). When the
2 abrasive shows no segregation. The powder particles
droplets were rapidly cooled down (cooling rate in the
produced by this rapid solidification process are fine
grained, have high strength and density with good soft
magnetic properties, and are corrosion resistant. This
medium frequency heat insulation high-pressure
induction melting furnace leakage crucible mixing gas processissimpleandlowcost,andhenceisverysuitable
for large-scale commercial manufacturing.
abrasive powder
and high-pressure The successful production of this magnetic abrasive,
atomizing nozzle gas mixing device which conforms to the ideal structural model, is mainly
valve 6valve 5
due to the following reasons. First, appropriate control
valve 2 oftheprocessparametersforgasatomizationandrapid
atomizing chamber valve 4
solidification.Thehigh-pressuregasatomizationprocess
cooling water valve 3 pressure gauge disperses and atomizes the molten liquid ferromagnetic
circulation system valve 1 alloy into tiny droplets; the rapid solidification process
resultsinthetinydropletsformingsphericalpowderpar-
reducing valve ticles possessing high strength, high density and corro-
fine powder
sion resistance. Secondly, alloying of the ferromagnetic
collecting vessel
matrix. Appropriate amounts of alloying elements, i.e.
high-pressure gas 9.6Si,5.4Al,5.0Ni,0.5Cr(wt.%),wereaddedtotheiron
valve 7
substratebasedonthemetal-ceramicwettabilityandthe
performance requirements of the magnetic abrasive.
magnetic powder
composite powder collecting vessel Appropriate amounts of Si can significantly reduce the
meltingpointoftheironmatrix,decreasethesurfaceten-
Figure 2. Schematic diagram of the modified gas atomization and sion,assistthewettingofWAabrasiveandresistoxida-
rapidsolidificationsystem. tion simultaneously. Active elements, Al and Cr, can
418 G.-x.Zhangetal./ScriptaMaterialia65(2011)416–419
Figure3. SEMmicrographsofWAmicropowderandsphericalcompositemagneticabrasive:(a)WAmicropowder;(b)low-magnificationSEM
micrograph;(c)high-magnificationSEMmicrograph.
decreasethewettinganglebetweenmoltenferromagnetic a-Fe(Si–Al–Ni)andFe Si.Comparisonofthecross-sec-
3
alloymatrixandtheWAabrasive,improvethebondbe- tional SEM micrograph with the XRD pattern of this
tween them, and also resist oxidation. Ni is an excellent spherical composite magnetic abrasive demonstrates
alloyingelement:itcanenhancethestrengthandtough- that the WA is the abrasive phase, while a two-phase
ness of ferromagnetic alloy, reduce its brittleness, and alloyofa-Fe(Si–Al–Ni)andFe Si,whichisanexcellent
3
thus improve the alloy’s mechanical and soft magnetic soft magnetic material, is the matrix phase. In addition,
properties. Finally, owing to the hydrodynamic effect, Figure 4a also indicates that the ferromagnetic matrix
whentheWAabrasivegrainsareinjectedintothemolten phase has a typical microcrystalline structure, and has
ferromagnetic alloy matrix together with the high- a degree of wettability to the WA abrasive grains. They
pressureatomizinggas,thesegrainscanbedisperseduni- form strong bonds. Therefore, the WA spherical com-
formly into the ferromagnetic alloy matrix, which has posite magnetic abrasive prepared by the present meth-
originally weak wettability to WA abrasive grains, and od possesses good mechanical properties, soft magnetic
thusimprovethewettability,andalsoincreasethebind- properties and long service life.
ing strength between grains and matrix. Hence, due to In contrast to Figure 3a, the cross-sectional SEM
the above-mentioned three factors, an ideal magnetic micrograph shown in Figure 4a and the external mor-
abrasive with high strength and toughness, good soft phologySEMmicrographsinFigure3bandcbothdem-
magnetic properties, strong processing capacity and onstrate that the raw morphology of the WA grains
spherical shape was obtained. persistsintheWAsphericalcompositemagneticabrasive
Figure4ashowsthecross-sectionalSEMmicrostruc- preparedbythepresentmethod.Thisisbecausethetem-
ture of WA spherical composite magnetic abrasive pre- perature (1580(cid:3)C) of molten ferromagnetic matrix be-
pared after their particles were molded with phenolic fore atomization is lower than the melting point of WA
resin and then ground, polished, corroded and metal abrasives (2020(cid:3)C), and the WA abrasives are stable,
sprayed. The WA abrasive grains are tightly embedded so this process has no effect on the morphology of WA
into ferromagnetic a-Fe(Si–Al–Ni) and Fe Si matrix as abrasivesandtheWAabrasivesthereforeretaintheirori-
3
shown in Figure 4a. Figure 4b shows the XRD pattern ginalshapeandcuttingability.Hence,thisprocessavoids
of WA spherical composite magnetic abrasive prepared the disadvantages of the plasma spray process in which
bygasatomizationandrapid solidificationinwhichthe the sharp cutting edges of abrasive phase of magnetic
(200)superstructurepeakoftheorderedFe Siphasecan abrasive are blunted, reducing the grinding efficiency.
3
be clearly seen, and also indicates that this spherical At the same time, compared with the irregular shape of
composite magnetic abrasive mainly consists of WA, the magnetic abrasives prepared by sintering and
Figure4. Cross-sectionalSEMmicrographandXRDpatternofsphericalcompositemagneticabrasivepreparedbythegasatomizationandrapid
solidificationprocess.
G.-x.Zhangetal./ScriptaMaterialia65(2011)416–419 419
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