powder Is added, reachlng a value of around 12 tlmes thatobtalned wtth no yowder Men operatlng wlth aconcentratlon Of 40 gr . me authors quote an Increase Inworkplece surface hardness from 200 to 400 HV Ath nocracklng, uslng modest / low dlscharge energles. Afurther polnt made Is the decrease In surface roughnesswlth an Increase In powder concentratlon.Much the same plcture Is detalled by Masul et a1 [23] andKruth et a1 [24], both groups recommendlng the need toutlllse low power to awld Instablllty. The process Is notwlthout its drawbacks, however, whlch Include dllAculty Inensurlng that the powder Is held In suspenslon. Thls Iseasler Ath more dscous dlelectrlcs but at the cost ofreduced flushing emclency.Durlng normal EDM operatlon wlth a standard Cuelectrode and preferred polarlty, workplece surfacealloylng Is largely unwoldable, albeit at a very low level4% [24]. Use of reverse polartty, together wlth hlghdlscharge energy and a hydrocarbon oll dlelectrlcproduces Increased alloylng [26], as wlll the use of Rnemetalllc wlre electrodes (10&600 pm 0). due to easlermeltlng [27. It Is generally acknoAedged however thatdellberate alloylng can be promoted by uslng partlallyslntered / green compact PM electrodes wlth IDwar blndlngenergy than for solld tools [24, 28361. The relevantltterature Is falrly extensive, atthough specltlc alloyrecommendatlons and operatlng parameters for partlcularappllcatlons hwe not been adequately formallsed.
In general, PM surface alloylng Is assotlafed Mth thlcker
recast layers contalnlng more cracks than wHh suspended
powders. Compacts detalled Include WC/Co. TVAI. CrN,
NVCo, NVSI, NFe. CUM, CulMn, FeMIC. TICMI, CulSn,
TICMlCpCo and W/CrC/Cu, produced at pressures -200-
5r10 MPa wlth temperatures ranglng from -1,00&1,30ooC.
It Is suggested that the Incluslon of elements such as SI,
Co and Mn reduce cracklng due to a reductlon In
solldltlcatlon shrlnkage. Detalls of partlcle she are less
forthcomlng, a few references gMng values <2 pm,
another 44 pm. Several papers lndlcate use of a
composite electrode structure wlth the PM element
bonded to a carrler substrate of Cu or Al. As wlth
suspended powders, an Inverse or negative tool electrode
polartty Is generally recommended, together Ath kerosene
as a source of pyroiytlc C. Quoted dlscharge energles are
generally hlgh wlth recast layer thlckness vaMng from 10-
100pm. Hlgher levels of alloylng. by up to 40%, are
quoted when uslng PM electrodes over suspended
powders [24]. Almost a11 researchers quote slgnltlcantly
hlgher workplece surface hardness. Mohrl et a1 [30] detall
a two stage operatlon wlth a green compact PM electrode
used to produce an Inttlal accretlon. followd by a sparklng
operatlon wlth a standard Cu electrode In order to ensure
good adheslon to the workplece. make et a1 [32] detall
the alloylng / coatlng of WC end mllls uslng a composite TI
electrode. The resuttlng recast lyer contalnlng TIC Is
quoted as hdng a hardness of 2,5OOHV, compared to
1,300 HV for the WC substrate, wlth a 4 fold Improvement
In tool We.
The research reported here WEIS undertaken as part of trm
UK Government / Industry projects to establlsh the
worklng envelope Men EDM surface alloylng hardened
steel rolls / moulds / dles and TI alloy aeroenglne
components wlth PM tool electrodes. Dle slnklng,
scannlng, tedurlng and Mre cuttlng operatlons were
perform ed .
3 EXPERIMENTAL WORK
3.1 Equipment and workplece I tool materlals
An electrlcal dlscharge grlndlng (EDG) machlne Rtted wlth
a vertlcally mounted mlnlature DC servo untt WEIS
employed to perform a dle slnklng operatlon on hardened
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