EDM for machining hard metals

EDM for machining hard metals

FL_hartmetal_E 15.9.2005 10:39 Pagina 2

In modern cutting, punching and met-al forming technology, ever-greaterdemands are made on the tool. Some-times, high precision die clearances of1 µm need to be achieved in punching,blanking, embossing, and progressivedies. In some applications, the toolmust be unchanging in quality, andhave a life of between 1 and 10 millionstrokes. Sometimes a total operatinglife of 500 million strokes is aimed for.20 years ago, these requirementswould have been simply inconceivable.Today, heavy-duty presses can achieveover 2000 strokes a minute. It is easyto see that the demands made on toolmaterials have sharply increased.The economic viability

of hard metal

A punching strip runs into

a heavy-duty press and is punched,

bent, embossed and cut

by the progressive

die’s hard-metal inserts.

ED wire-cut die made of hard metal

for tool lives of over 5 million press strokes.


3

ED machined

carbide insert

for various tool uses.

A good tool material displays manyproperties, not only hardness and lowfriction, but also:- High compressive strength- Low thermal expansion- Good adhesive behaviour- Sufficient toughness- Good machinability.Hard metals (HM) based on tungstencarbide and cobalt have proved to bethe most suitable materials.

Profitability increases with thegrowing number of stampings.Costs comparisons show that from afew million stampings upwards, greatsavings are associated with the useof HM. In the case of a steel tool, thewear increases continuously. With aHM tool, after reaching a certain ba-sic roughness, it remains almost un-changed over a long period. There-fore unchanging quality of stampedor cut products can be guaranteedfor long periods.

In the past economic machiningmethods of HM, where non-existentmeaning that the use was prohibitedin certain applications. It is only withthe advances made with Electric Dis-charge Machining that the manufac-ture and the use of these tools havebeen made a financial reality.


4The term “hard metal” describes ageneral group of materials which dis-play great hardness and high wearresistance whilst at the same timenot foregoing the toughness typicalfor tooling metals. Cobalt (Co) is thealloying metal and makes up only toa very small part (3 to 20% of totalmass) and is found in over 95% of allhard metals. It is used to cement theceramic components (the carbides) ina kind of baking process. In English-speaking countries hard metals aregenerally known as “cemented car-bides”, or more simply as “carbides”.

HM materials are composed mainlyof carbides (ceramic-like materials)which give them their extreme hard-ness and wear resistance – even athigh temperatures. The metals arebonded together with the elementcarbon (C) in a precise ratio, the finalproduct been a covalent bond.

0 250 500 750 1000 1250 1500 1750 2000 2250

WC-Co

AI2O3

Stellit

Cast Iron

Steel

Vickers - Hardness (Hv 30)

1

2

3

4 5

6 7 8

9 10 12

11 13

The schematic diagram

shows the various steps to the

manufacture of hard metal.

1 WC + Ni/Cr + TiC/TaC

2 Mixing and milling

3 Drying

4 Linear pressing

5 Isostatic pressing

6 Presintering

7 Forming

8 Sintering

9 HIP - sintering

10 Physical control

11 Raw materials

12 Eroding

13 End control

A Vickers hard metal

hardness comparison.

hard metalThe material

The most-used carbide is tungstenmonocarbide (WC) it may be mixedwith small portions of other carbides,such as titanium carbide (TiC), tanta-lum carbide (TaC), niobium carbide(NbC), vanadium carbide (VC), molyb-denum carbide (Mo2C) or chromiumcarbide (Cr2C3). The carbides maybein prismatic or cubic grain form,which have a typical size between 1and 5 µm (.00004’’-.0001’’).

HM is thus composed mainly of car-bides (ceramic-like materials) whichgive it its extreme hardness andresistance – even at high tempera-tures. The covalent bond is producedby bringing together a metal with the element carbon (C) in a clearlydefined ratio.


In order to achieve optimum compo-sition and homogeneity, these pow-ders are ground, mixed, and thencompacted into the required form attemperatures of between 1350 and1500° C / 2462 and 2732 F. This ther-mal process is called sintering. Thetemperature chosen must be highenough to melt the metallic phase ofthe filler material (usually Cobalt)and, by means of capillary diffusion,close all the pores between the car-bides, combining them into a com-pact block. At the end of sintering,densities of over 99.8% are achieved.

In order to attain an ideal density, theblock can also be pressed (at a pres-sure of 1000 to 1500 bar/ 7,200 to10,800 psi) post sintering using a hotisostatic process. In this way it ispossible to produce nearly a non-porous material that is known as HIP(Hot Isostatic Pressing).

5

Structure of a coarse-grained

WC-10 Co alloy, a mixed

carbide-content alloy

(9% cobalt, 7% titanium carbide,

10.5% tantalum carbide and 73.5%

tungsten carbide),

as well as a fine-grained WC-10 Co alloy,

in 1500-fold magnification in each case.

Traditionally due to the extreme hard-ness of HM, parts are sintered to theirfinal form. However, as sintering leadsto considerable volume shrinkage (10-15%), it has been impossible to manu-facture high-precision complex tools,punches and dies without the use ofextremely costly finishing operations.

It is possible to manufacture HMblocks by conventional methods suchas milling and grinding and some toolmanufacturers do use these conven-tional machining processes. However,these operations require diamondtools and long machining times. Con-sequently complicated forms can onlybe produced with a great deal of ex-pense, if at all. With its more recentadvances in technology, EDM is gain-ing ever more acceptance. Thanks toits versatility, great precision, and highdegree of automation EDM is nowyielding very advantageous productioncost savings.

Nowadays, it is possible to combinethese two processes in special sin-tering-HIP furnaces. As a result, in-creased homogeneity in the distribu-tion of the metallic bonding agent isachieved which, consequently, givesbetter mechanical properties.

As WC is unstable at high tempera-tures, it is not possible to produceHM using the conventional metallur-gical processes, such smelting, al-loying or casting. Powder metallurgyis therefore used, but in order to doso a mixture of chemical and me-chanical processes are first used tocreate a highly pure carbide metalpowder to the required grain size.


6

Sequence of eroded material

from full cut to the third trim cut as metallografic sections

and as roughness measuring protocol.

The ED machining of hard metalED machining is a thermal processand depending on the energy exertedthe process may effect the structureof the surface being EDM’ed. In high-energy situations, surface tensioncan occur, leading to microcracks.Great attention must therefore bepaid to the current pulses, especiallyin the case of trim cuts (final finish-ing passes similar to “sparking out”where power and size of cut is re-duced). The trim cuts should be car-ried out with constantly diminishingenergy, in order to limit the thermalinfluences. It is imperative therefore,to remove, the thermally influencedarea from the previous machining cutuntil there is little or no thermally af-fected area. From this it can be seenthat it is wrong to try to reduce thenumber of trim cuts on carbide, justto save machining time.

ED wire cutting hard metal as standardAll Agie ED wire-cutting systemsnowadays have standard technologies(machine parameter settings) avail-able guaranteeing a final roughnessof Ra 0.2 µm / 7 RMS with cuttingheights up to 100 mm / 3.93’’ inches.They make it possible to manufacturedies, punches and other tools withcomplex geometries and high preci-sion. Depending on the wire used,cutting speeds can be between 10and 20 mm2/min – 9.44 and 18.89 sqin. hr. It should be noted that highercutting speeds are quite possible,however, this increases the risk ofcausing microcracks.

Special cutting technologiesIn addition to the standard technolo-gies (machine parameter settings),special technologies for finest sur-faces (Super Finish) have been devel-oped by Agie in order to minimise thethermal effects on the material. Newgenerator technologies and improvedprocess controls can produce finalroughness of Ra 0.05 µm / 2 RMSwith the absolutely highest contouraccuracy. With the use of specialtechnologies, both higher workinglives for the ED machined tool insertsare realised and sharper corners andedges are produced. The advent ofsuper fine wires means dies can besplit up into part dies. Depending onthe Agiecut model, these perform-ance features can also be offered asan option.

The ED machining of hard metal


7Agiecut Vertex is ideal

for machining hard metal.

The performance features

of Ra 0.05 µm / 2 RMS surface quality,

1 µm / .00004’’ positioning accuracy

and 17 µm / .0006’’ corner radii

with 0.02 mm / .0008’’

wire make it the most accurate

ED wire-cutting machine in the world.

Microscopic examinations

in Agie's Laboratory

for Metallography and Analysis

keep EDM technologies

always state-of-the-art.

Hard metals with identical chemicalcomposition, but with wildly differentmechanical properties, are to befound on the market. This fact is at-tributable to the tricky process of sin-tering, and in particular the difficultyin maintaining a suitable, neutral fur-nace atmosphere during sintering. Inorder to investigate the behaviour ofthe various HM types, regular EDMtests are carried out at Agie with asubsequent metallographic analysisand, if necessary, SEM examinations(Scanning Electron Microscope).

Laboratory examinations show that alltypes of HM behave similarly towardsEDM, it is confirmed that the cobaltcontent of an HM influences the EDMspeed and the roughness of the EDmachined surface. With an increasingratio of metallic bonding agent, theremoval speed deteriorates, as doesalso the final roughness, as larger re-fluxes of metal resolidify on the EDmachined surface. On the other hand,with a diminishing ratio of cobalt, thesensitivity of the hard metal to ther-mal shocks increases, with a corre-sponding risk of microcrack forma-tion.

The grain size has a considerable in-fluence on the hard metal's mechani-cal properties but its influence on theremoval speed and final roughnessplays a subordinate role comparedwith the cobalt content. It is thereforepossible that very good results can beachieved, even without so-called «mi-crograins». Small ratios of mixed car-bides, such as TiC and TaC do not in-fluence ED machining in any way.

controlUnder constant


8

Although the quality of HM availableon the market today is noticeablybetter than in the past, great atten-tion should always be paid to ensuregood HM quality. Possible faults inthe course of HM production andtheir effects on ED machining areshown below.

The decarburisation of the hard met-al promotes the formation of a veryhard and brittle phase, the so-calledeta-phase, the effects of which onthe hard metal's mechanical proper-ties are catastrophic. Carburisationof the hard metal leads to the forma-tion of small inclusions of free car-bon (graphite) when ED machiningthe surface. These inclusions behavelike porosity retaining dielectric thatmay lead to electrochemical reac-tions, and to undesirable corrosion.In addition, they also have a negativeeffect on mechanical properties asthey provide starting points for theformation of cracks.

The material's porosity and homo-geneity are two important factorsinfluencing hard metals' resistanceto corrosion. Experience shows thatcorrosive actions are not always justattributable to reactions with freecarbon or porosity. More likely theinhomogeneity of certain materialsalso plays a decisive role. Only im-peccably conducted sintering canprovide the guarantee for an opti-mum nearly porous free and ho-mogenous material.

In addition to visual and

physical examinations,

chemical tests are also carried out

in Agie's Laboratory for

Metallography and Analysis in order,

among other things,

to analyse hard metal's reaction

to acids and caustic solutions.

Hard metal'squality


9

Recently, Agie has paid particular at-tention to corrosion on HM. Corrosivephenomena can occur during or afterthe machining of HM, as when EDwire cutting, the dielectric used inthe process (basically water) plays aninteresting role. Undesirable reac-tions may happen such as the releas-ing (also known as “washing out”) ofthe metallic phase, which sometimesgenerally called “corrosion”. Specialtechnologies with “neutral mediumpotentials” were developed whichhelp greatly reduce this effect. How-ever it is interesting to note that lab-oratory tests have shown that the EDmachining as such does not causeany corrosion on the workpiece.

In order to be able to avoid any possi-ble corrosion, it is advantageous tounderstand the mechanisms of cor-rosion. As already mentioned, cobalthas proved to be an ideal metallicbonding agent for the production ofHM with excellent mechanical prop-erties. Like most metals, cobalt iselectrochemically very active in aque-ous media; i.e. it shows the tendencyto dissolve into ionic form in water. If a metal of another type, which is

jointly earthed (e.g. the EDM headmade of steel) is brought into thevicinity of the HM workpiece, then anelectrochemical potential drop oc-curs that generates an ion current inthe dielectric. As a result, the cobaltis continuously released from thesurface and corrosion occurs in theform of small pits that cannot alwaysbe detected with the naked eye. Thisis known as “pitting”.

It is crucial therefore that workpieces should be removed from thework tank immediately after machin-ing and dried. If this is not possible,it is recommended that the EDMhead should be removed as far aspossible from the workpiece after theED machining.

1Co++

3

3

5

2 4

Electrochemical potential drop that generates an ion current in the dielectric.

1 Workpiece

2 Water dielectric

3 Currant pickups

4 Generator

5 Grounding

Corrosionon hard metal

Enlargement of a structure

of hard metal with free carbon

that can lead to pitting

which for its part

is the starting point

for the formation of cracks.


In addition, the presence of ions thatare particularly aggressive towardsHM (chlorides, sulphates, amines,etc.) can considerably accelerate cor-rosion. Chlorides, even in very smallconcentrations, can play the mostimportant role in this situation. It istherefore extremely important to havea good water quality at your disposal.In order to supply the dielectric unitwith an absolutely clean and chlo-ride-free dielectric, the water is to bepre-treated by means of deionisingresin or reverse osmosis.

10A workpiece with multiple

machining clamped

in the work area

of an Agiecut Vertex.

All the negative influences,

that would have been

detrimental to the EDM result,

were eliminated here in advance.

Parts of the EDM system, which arein direct contact with the dielectric,must not be treated with cleansingagents containing ammonia oramines as these substances formvery stable and water-soluble com-plexes with the cobalt. Acids are alsoto be kept away from the work asthey may produce corrosion withinthe dielectric. Before clamping work-pieces to the system, they must becarefully cleaned in order to preventresidue from the abrasive materials(which in part contain amines thattransform metals into water-solublesalts) from being brought in. De-posits of EDM particles on the work-piece surfaces lead to the formationof so-called local elements that canalso trigger corrosion. The work areais to be kept as absolutely clean aspossible.

water dielectric on hard metal

The influence of

The current effects described aboveare further intensified by high waterconductivity. The ideal conductivity ofthe water dielectric should thereforelie below 5 µS/cm. Another importantproperty of the dielectric is the hy-drogen ion concentration (H+) whichis also called pH value. The optimumpH value lies between 7 and 8, mean-ing ideally only neutral water may beused. With a pH value < 7 the solu-tion behaves very aggressively to-wards the cobalt in the HM, and in-deed generally towards metals. ThepH value is very greatly dependent onthe type of deionising resins beingused. In order to avoid a drop in thepH value, you are recommended toalways replace the deionising resin ingood time.


11

The three most important pointsMaterial: Only hard metal of certifiedquality should be used. Hard metaltypes, which have been machined upto now without problem using tradi-tional grinding, milling etc. is notnecessarily suitable for ED machin-ing. The dimensions of hard metalblanks should be kept as small aspossible.

EDM: Work should be carried out onlywith EDM systems that have absolute-ly optimised technologies (machineparameter settings) for hard metalmachining. As far as possible, work-pieces made of hard metal should beED machined without interruption - inone go and then removed from thework tank immediately.

Dielectric: The correct processing ofthe water into a dielectric should beensured. Attention is to be paid toensuring the greatest cleanliness inthe dielectric unit. The correct appli-cation of the machining technologyin the correct environment must beguaranteed.

With nearly inexhaustible possibilities

of combination of material,

wire type and workpiece height,

ED wire-cutting systems from Agie

have the largest selection

of standard machining technologies

on the market.


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All rights to changes reserved.© AGIE.

500.029.108September 2005Printed in Switzerland.

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EDM for machining hard metals

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