COMPARATIVE INVESTIGATION INTO THE BRUSHELECTRODISCHARGE ALLOYING WITH THE

ELECTRODES OF ALLOY STEEL AND TUNGSTEN

Bogdan Nowicki*, Robert Pierzynowski*, Sławomir Spadło*** - Warsaw University of Technology, Warsaw, Poland

** - Kielce University of Technology, Kielce, Poland

Summary The electric discharge alloying by a brush electrode (BEDMA) is a newmachining method invented by the authors. It is effected using rotating brush as anelectrode being made of a material which is to be alloyed on the part surface. The rotatingelectrode is elastically pressed against the alloyed surface and that way free form surfacesand surface of varying geometry can be alloyed.The thickness of layers obtained by this method is several mm up to several hundred mm.The alloyed layer contains from 50 up to 70% of the anode material. The alloying withtungsten makes a special case for which in spite of the mass decrement of the alloyed part,the layer of large tungsten share is still constituted.

INTRODUCTION

The specific area of interest in the field of superficial layer engineering is generatinglayers of thickness beginning from several tens of mm and introduction of the alloyingelements enhancing the hardness, resistance to wear, corrosion, etc. such as tungsten (W),molybdenum (Mo), vanadium (V), chrome (Cr), nickel (Ni) and others.

These layers can currently be obtained by the following ways [1]:• Facing by welding,• Laser alloying,• ED hardening.Electrodischarge alloying (EDA), also known as ED hardening, has already been used for

fourty years as a method used mostly for hardening cutting tool tips [2,3] of high-speed steelor alloy steel. The working electrode (anode), which is used as a source of material to bemoved onto the machined surface, is mostly made of the cemented carbides. The introductionof up-to-date tools of the cemented carbides, ceramic or ceramet tool inserts, cubic boronnitride, etc. and also the expanding application of PCD covers of the high-speed tools hasresulted in shrinkage of ED hardening.

According to bibliographic sources, there are promising prospects for the electrodischargealloying of the pressing tools and swedges which can be effected directly on the sinkelectrodischarge machines with the reversed polarity [5, 6, 7]. Titanium is typically used insuch cases, and the superficial layer constitution is also assisted by carbon coming from thedielectric. The most significant limitations restricting the wider use of the electrodischargealloying used to be connected with inadequate productivity of this process, difficulties with itsautomation and small thickness of the obtained layers.

THE ELECTRIC DISCHARGE ALLOYING WITH A DISCRETE ELECTRODE

The authors have been looking for a chance of enhancing the productivity of the electricdischarge alloying and they proposed a new way of the EDA process as based on the knownprocesses of conventional hardening by the vibrating electrode and the electric

Fig. 1. Origins of brush electrodischarge mechanical alloying (BEDMA).

discharge decremental machining by the rotating brush electrodes [9,10].This method wasdenoted as BEDMA. In the BEDMA method (fig.1) the working electrode has the form of arotating brush, which is elastically pressed against machined surfaces using constant force.Such a system enables atypical complex-shape surfaces to be machined. The elastic wires aremade of a material which is to be alloyed on the part surface. The brush rotates at high speedand contact of the individual wires with the machined surface is momentary so that touches ofthe machined surface by random brush elements initiate electrical discharges resulting inmelting and evaporating of the anode (wire) and cathode (workpiece). The conditions of theheat transfer are much worse for the brush wires and it implies that the case of a several mmthick rod electrode used in traditional electrodischarge hardening.

The electric discharges taking place in the inter-electrode gap have been researched formore than fifty years but no general model of this phenomenon has been developed until now.The differing views on the subject concern such basic matters as temperature of plasma in thespark channel, structure of this channel, discharge initiation, the role of impurities (dischargedebris) in the inter-electrode gap during the discharge, erosion of the non-melted metal, etc.[2,3, 5, 8, 11]. The authors of these publications are mostly preoccupied with the liquid dielectricbut they argue that the results can be also applied as general for the discharges in gases.

According to the classical model, the discharge begins as so called cold emission of theelectrons from the cathode, taking place in the area of the highest electric field intensity.Because of the electron collision with the inter-electrode medium, ionization of the medium istaking place resulting in the rapid increase in the flux intensity of the electrons and ionizedmolecules; more collisions occur, temperatures rise and the formation of the plasma channel[2, 3, 8, 11].

Many authors maintain that it is impossible to generate a breakdown at the voltage lowerthan 100 V. Basing on the investigations carried out during alloying steel with the anodemade of the cemented carbide and using the fast photo-cameras for recording, it has beendetermined that the initiation of the discharge is contact-dependent, i.e. the discharge isreleased during opening out the contact between the electrodes [2]. It has been determined

that after a prolonged time of the alloying process (more than two minutes per squarecentimetre) and high temperature of the working electrodes, there always exist the erosionproducts (produced during the previous discharges) in the inter-electrode gap. It results fromthe fact that in the beginning of the EDA process, the discharges are likely to occur duringopening out the contact between the electrodes whereas in the subsequent discharges they canbe additionally initiated due to their approach or the reaction of the impurities.

THE INVESTIGATIONS ON THE BEDMA WITH A DISCRETE ELECTRODE

The BEDMA alloying is realized using the rotating brush (similar to the brush used forcleaning the corroded or dirty mechanical parts); the brush diameter D = 80 ÷120 mm and itswidth B = 20 mm. The elastic elements are usually 0.1÷0.3 mm in diameter and they are madeof high-alloy steel such as 1H18N9 or tungsten, molybdenum, etc.

The brush electrode is pressed against the surface with a little force (of several newton)and it rotates with a speed from several hundred to several thousand rev/min. The sample issubjected to a feed motion with respect to the rotary brush. The alloying is carried out in theinert gas environment (argon, nitrogen or carbon dioxide) in order to prevent the formation ofthe oxide layers, blocking the electric discharges on the part surface.

The machining circuit is powered by the direct current generator or the RC generator. Thebrush electrode is denoted as anode and the machined part as cathode.

The BEDMA process is governed mainly by the mechanical reaction of the elastic brushelements and the electric discharges. The time of contact between the single elastic elementsand the alloyed surface is very short because of the high speed of the brush rotation.

Fig. 2. Diagrams voltage vs. time for a) Fn = 4 N, b) Fn = 1 N, c) Fn = 2 N.

a)

b)

c)

The following are the most important effects of the mechanical reaction:• mechanical contact between the brush elastic elements and the machined surface,• initiation of the electric discharge in the wake of the opening out,• removal or smoothing of the highest roughness peaks,• plastic deformation of the superficial layer (SL) and the stress generation in SL.The electric discharge is associated by the formation of the plasma channel of concentrated

energy affecting both electrodes:• local melting and evaporating of the anode surface,• ejecting the evaporated and the melted particles from the anode,• local melting and evaporating of the cathode surface and ejection of the cathode

particles into the interelectrode gap,• transfer of the melted metal from the anode onto the cathode,• mixing the melted particles of the anode with the locally melted cathode

material(convection, diffusion), local heating of the material which is adherent to thedischarge zone,

• rapid cooling of the alloyed layer due to heat transfer through the part core.Some wires affect the surface by electrical discharges and by the above-described process

of mass transfer from the anode to the cathode and thermochemical modification of thesuperficial layer. The mechanical contact of the wires with the machined surface isaccompanied by an electric current without any discharges. This electromechanical influenceresults in smoothing the roughness peaks created by the electric discharges and in thetemperature increase of the machined surface.

Throughout the alloying process, the following conditions can be observed: occasionaldischarges (low loads) (fig. 2a), shortcuts (high loads)(fig. 2b) or the spark discharges (2c).

a)

Fig. 3. Principle of investigation into discharge withsingle wire electrode (a) and SEM picture of thealloying surface: with a visible trace of a single

discharges �(b); peripheral speed is parallel to themachining traces, anode- steel 1H18N9, cathode-

steel 45 (× 500), c- craters with the bottoms underthe level of the alloyed surface (x 4000)

b)

c)

THE INVESTIGATIONS ON THE BEDMA WITH DISCRETE ELECTRODE

The aim of these investigations was the assessment of the reaction of the individualdischarges or discharge series in the environment which is close to that existing duringalloying by the brush electrode:

• the electrode was made of a single wire (fig.2a) of diameter Φ = 0.2 mm and lengthl = 20 mm; it was made of a material which is typical of the alloying process,

• the longitudinal feed with respect to the workpiece was applied,• the wire was deflected due to the pressure against the machined surface.As a result of the reaction of a single elastic electrode on the machined surface, the traces

of the single discharges or the discharge series paths can be obtained.The SEM scan made of several parallel �paths� resulting from the spark discharges taking

place on the ground surface along the grinding traces have been presented on fig. 3b. Thepaths are continuous, well melted into the regular base.

The craters account for the considerable share of the discharge traces (craters � pits withthe bottoms placed under the level of the alloyed surface). This is a proof that mass transportis bi-directional.

The following features can be seen on fig.3:• long-sized micro regions covered with the anode material (Fig. 3b) belonging to the

wires, which has been transferred onto the cathode surface as a liquid,• the crater trace with the bottom placed under the level of the alloyed surface (fig. 3c),• material which has been moved to the cathode surface in form of overlapping waves of

the solidified material; the waves are probably caused by the migrating reactions of thedischarge areas of the micro-channels containing the discharge,

• traces of the metal vapour evolving, shaped as the deep micro-orifices.

Fig. 4. A profilogram of a machining trace for the brush electrode equipped with a single wire made ofsteel 1H18N9 (φ = 0.3 mm): a) U = 12 V, b) U = 16 V and tungsten electrode (φ = 0.1 mm): c) U = 9 V, d)

U = 15 V (DC- generator).

a)

b)

c)

d)

The investigations of the machining results for the rotating brush with a single wire haveproved that, due to the discharges, metal can be moved to the discharge craters, where it ismixed with the liquid cathode material or it is present as small metal particles stuck to themachined surface. The height of the metal paths transferred onto the working surface isdependent on the energy parameters of the discharge as well as on thermal properties of thecathode [fig. 4]. The �path� height is higher when the electrodes of 1H18N9 steel areemployed Rt = 5 ÷10 µm than for tungsten Rt = 4 µm - in this case two types of places can beobserved � the ones with the deposited material and the others where the material decrementcan be noticed.

THE INVESTIGATION OF THE SURFACE LAYER CONSTITUTED BY THEBEDMA

The superficial layer constituting in the BEDMA alloying process is created as a result ofseveral interacting processes. The following contributing processes should be noted:

• Thermal processes (melting, solidification, rapid cooling),• Mass transport and the mixing, convection and diffusion processes,• Structural changes connected to changes in the chemical composition and very fast

cooling,• mechanical reaction (plastic deformation and the associated strain-hardening of

material),• stress inducement in the superficial layer,• the formation of the characteristic geometric surface structure.

Machining conditions

ParametersRotating speedU, (RC)Load / deflectionFeed-rate

ObjectShape and volumeMaterial propertiesSurface condition

ToolPhysical propertiesGeometryTexture

MediumGas typePressureCapacity

Disturbances

BEDMA process

Physical processThermal

Diffusion

Chemical

Output parametersjś i

QualitySL chem. compositionMicrostructureRoughnessWavinessσRES= f(h) / µHV= f(h)Shape errorsSL thickness

EconomyCost of energy and devicesMachining timeCost of electrodes

Productivity

Fig. 5. BEDMA process as a subject of the investigation.

The superficial layer condition is dependent on many factors. The following are the mostimportant ones:

• type and features of the electrode material,• the electric discharge parameters (the energy of the single discharge � E, voltage � U,

the discharge time τw and the break time τp ),• features of the brush electrode (the wire diameter � d, the brush diameter � D, the wire

packing rate on the brush surface),• kinematic parameters (the major motion speed - Vc, the feed motion speed - Vf ),• loads FN,• environment.The geometric structure of the surface is created as a result of the electric discharges and

the local mass transfer from the anode onto the cathode in form of micro-craters and micro-buildup; the smoothing influence of the elastic brush elements can also be observed. Thoseinfluences lead to the generation of random, well developed surface roughness with thetruncated peaks and relatively large pitch. This structure is characterized by the �grain� typestructure and high porosity (fig. 5). The typical roughness levels are Ra=2 ÷ 4 mm.

a) b)

Fig. 6. SEM picture of the machined surface (× 200)-(a) and profilogram of a machined surface-(b).

The feed motion rate has no considerable influence on the machining removal rate, but thefeed-rate vector direction is important. The conventional feed-rate setup has been presented infig. 1; the surface is first alloyed (E zone), then it is subjected to the mechanical treatment(brush machining in M zone). The application of such a feed-rate vector direction with regardto the major motion speed vector ensures the shining surface of the graphite-like colour. Themachining removal rate, defined as the mass increment of the cathode in the time unit is lowin this case.

The visual examination has shown that the discharge zone (E) exists in the area, where theelastic elements are just loosing contact with the surface being alloyed. That way theargument can be confirmed that for U < 100 V the initiation of the discharges is triggered byopening out the electric contact. In case of machining with the feedrate vector along the samedirection as the major speed direction, the discharge area E is situated beyond the mechanicalreaction zone. The resultant surface is in this case the random geometric structure, typical of

the electrodischarge machining i.e. it consists of the mutually overlapping micro-craters of thedeep mat look which is connected to the lack of the smoothing influence of the elastic brushelements on the surface being alloyed.

The machining productivity is dependent on the electric parameters of machining; itamounts up to 80 mg/ min for alloying with 1H18N9 steel (alloy steel containing 18 % ofchrome addition and 7 % of nickel). In case of alloying high quality constructional steel withthe electrodes made of tungsten, mass decrement can be observed

The low productivity of the steel alloying with tungsten is connected to their high metingpoints and consequently, smaller mass of material transferred from the anode onto thecathode, taking place together with the simultaneous loss of the native material due to theelectric discharges. The obtained widths of the superficial layer for alloying the tungsten areonly tens of mm.

The X-ray examination has been performed using the equipment of 2.5 mm resolution. Theexamination has shown that for the whole section of the re-melted layer, the ratio of the nativematerial content to the transferred material content is almost constant.

The analysis of the chemical composition of the alloyed surface has shown that apart fromthe material of the electrodes, there is a lot of oxygen, mainly as Fe2O3 compound; this can bea good explanation of the rapid drop in productivity with time. The oxide structures preventfrom the spark initiation and propagation, and this results with the lower alloyingproductivity. That is why this process should be carried out in the inert gas e.g. argonenvironment.

a)

b)

Fig. 7. a) The image of the metallografic structure after electrodischarge alloying 1H18N9 steel; b) distributionof alloying elements (Cr and Ni) constituting the surface layer.

The chemical composition of the superficial layer after the alloying with 1H8N8 steelelectrodes, for the RC generator (fig. 7) shows that the superficial layer contains 5 ÷10 % ofchrome and about 5 % of nickel and the superficial layer contains 50 % of the cathode as wellas the anode material. Just below the re-melted material layer, there is an extremely fine-grainstructure, being probably a result of the secondary hardening (heating above the temperatureof the phase transition due to the spark discharges and then the rapid cooling due to theintensive heat transfer through the core). There is a distinct, sharply looking border betweenthe re-melted material layer and the thermal influence layer (TIL).

The results of the chemical composition analysis of the re-melted material layer after themachining with 1H18N9 electrodes and the direct current spark generator have shown that thecontents of the chrome (13 %) and nickel (7.8 %) is for this layer higher than in case of thelayer alloyed employing RC generator. The investigation of the superficial layer obtainedusing RC generator have shown that it turns more smoothly into TIL than layers obtainedwith DC generator. According to the opinion of specialists dealing with X-ray examinationsof the surface layer, they have never met the layer as well cemented to the base as in case ofalloying with the RC generator.

The analysis of chrome and nickel share in the re-melted layer shows that the contents ofthe native material in case of allying with the RC generator is up to 30÷50 %. The similarresults of the alloy elements distribution have been obtained in case of alloying steel 45 withmolybdenum electrodes. The smoother transition between the re-melted layer and TIL hasalso been visible in this case for the DC generator when compared to the RC generator.

The smooth transition of the superficial layer into the core material is clearly visible. Onecan also see a thick re-melted layer (white area) and thermal influence layer with the brighterarea adjacent to the re-melted layer which accounts for the secondary hardening and thedarker shadow representing the structures resulting from tempering.

As a result of the BEDMA alloying with the steel 1H18N9 electrodes, the re-melted zoneof micro-hardness µHV04 380 has appeared, followed by thermal influence layer (secondaryhardening) of micro hardness µHV04 580 ÷ 750 (in some areas its hardness drops to 100 units,i.e. µHV04 500 ÷ 600). The alloying with tungsten makes a special case for which in spite of

a)

b)

Fig. 8. a) The image of the metallografic structure after electrodischarge alloying with tungsten, b) distribution ofalloying elements constituting the surface layer (linear analyses).

the mass decrement of the alloyed part, the layer of large tungsten share is still constituted.The re-melted layer for the surface subjected to the alloying with tungsten electrode shown

content of 10 % W and the micro-hardness µHV04 810. The thermal influence layer shows thesimilar hardness as for alloying with the steel 1H18N9 and for molybdenum but one can alsomeet cases when its micro-hardness amounts only to µHV04 450 ÷ 550.

The significant surface layer feature is its good cementing to the material core. The initialtests have been made by the multiple bending, for samples of the standard thicknessb = 3 mm, on the cylinder of the diameter d = 12 mm.

This bending has been continued until the appearance of the fracture and then, the opticalmicroscope has been used for determining the borderline between the superficial layer and thematerial core. The tests have been made for the samples alloyed with the steel 1H18N9 andmolybdenum. The absence of the undesirable foliation of the superficial layer has beencharacteristic for all cases, in spite of the fundamental differences in the chemicalcomposition, hardness and the other features between the superficial layer and the materialcore.

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