technological problems of equal channel angular pressing

26 R.A. Parshikov, A.I. Rudskoy, A.M. Zolotov and O.V. Tolochko

© 2013 Advanced Study Center C_. Ltd.

Rev. Adv. Mater. Sci. 34 (2013) 26-36

Corresponding author: O.V. Tolochko, e-mail: [email protected]

TECHNOLOGICAL PROBLEMS OF EQUAL CHANNELANGULAR PRESSING

R.A. Parshikov, A.I. Rudskoy, A.M. Zolotov and O.V. Tolochko

Saint-Petersburg State Polytechnic University, St. Petersburg, Russia

Received: July 27, 2012

Abstract. Equal channel angular pressing (ECAP) is an effective process to produce materialswith increased mechanical properties and no change of billet size. In the present paper theexperimental and finite element analysis results of ECAP investigation were obtained. The influ-ence of die geometry and friction condition on irregularity of shear strain field in the cross sectionof the billet and therefore on mechanical properties distribution was studied. ECAP was shownto be always characterized by irregular shear strain distribution. Depending on die geometry andfriction condition that irregularity could be reduced.

1. INTRODUCTION

Equal Channel Angular Pressing (ECAP) which isknown as one of the most promising material pro-cessing techniques involves severe plastic deforma-tion. In contrast to rolling, drawing, extrusion themain purpose of ECAP is to accumulate deforma-tion in material without any reduction in workpiececross-section.

There is a uniform grain structure refinement ina bulk material due to simple shear scheme. Mul-tiple pressing provides ultra-fine grain structure. Ul-tra-fine grain materials typically exhibit a grain sizein the range of hundreds of nanometers. This is atransition region between the coarse grained mate-rials and nanostructured metals, where the grainboundaries play a decisive role during plastic defor-mation. The first few ECAP passes result in an ef-fective grain refinement taking place in successivestages: homogeneous dislocation distribution, for-mation of elongated sub-cells, formation of elongatedsubgrains and their following break-up into equiaxedunits. On the contrary, the microstructure tends tobe more equiaxed as the number of passes in-creases. Later on, the sharpening of grain bound-

aries and final equiaxed ultrafine grain structuredevelops.

The ultra-fine grain structure produced by ECAPis sensitive to the technological details of the pro-cess, lubrication, deformation rate, dimensions ofthe die, etc. With no doubt, these factors influencethe microstructure and finally the properties of ul-tra-fine grain material.

The structure received is characterized by un-usual effects such as ideal plastic behavior, higherstrength, enhanced ductility and toughness, and lowtemperature superplasticity [1-5].

Nowadays an application of ECAP to industry isthe issue of the day. By the moment the only appli-cation of this method was performed by its authorV.M. Segal at Honeywell Electronic Materials (USA)[6]. The method was used during processing of tar-get billets for magnetron sputtering of thin film.

ECAP is not widely used as the researchers fo-cus mainly on material structure. They are inter-ested in formation of unusual structures and prop-erties of material after ECAP while structure homo-geneity, fraction of high angle grain boundaries de-pends on mechanics of ECAP. The last is defined

27Technological problems of equal channel angular pressing

Fig. 1. Scheme of ECAP process.

by external kinematics and outside force to deformedworkpiece (deformation scheme, die geometry, fric-tion conditions). The mechanics also specifies somebasic process parameters: homogeneity ofstressedly-deformed state, pressing load, billetshape change, accumulative strain value and strainrate in each point.

ECAP research is performed only in laborato-ries, the tools used for this process consist of multi-component parts [7,8]. For example, a special dieset with backpressure under computer control andmeasurement was designed and manufactured inMonash University [9].

In paper [10] ECAP process was carried out onINSTRON 8502 machine. The die had the exit chan-nel closed.

There are other schemes to realize the process:the use of movable exit channel wall for friction re-duction [7,11] and polishing of billet surface beforeeach pass [12] but these approaches are compli-cated as well.

Let us consider ECAP more detailed. Its schemeis performed in Fig. 1. A metal billet is pressedthough a die containing two channels, equal in cross-section, intersecting at an angle . During the press-ing, the billet undergoes severe shear deformationbut retains the same cross-sectional geometry sothat it is possible to repeat the pressings for a num-ber of passes, each one refining grain size.

Basing on slide lines the mechanics of this pro-cess was described by V.M. Segal. The analyticalsolution was obtained for rigid-perfectly plastic ma-terial model assuming the channel is filled com-pletely up (stationary process), the material cannotlose contact with channel walls, frictionless sliding.As that stressed and deformed states arehomogenic, stress fields were obtained with the helpof discontinuous solution.

Because of much scientific interest to submi-cron-grained and nanostructured materials duringthe last ten years ECAP appeared to be in the cen-ter _f researchers’ attenti_n. The atte]pts _f its in-dustrial application revealed significant disadvan-tages of analytical solution obtained by V.M. Segal.

Modern approaches used to solve problems con-nected with plastic deformation of metals are basedon numerical (finite-element) method (FEM) and al-low to get rid of some assumptions and simplifica-tions. There appeared information in works statingthe possible corner gap during ECAP. For example,P.B. Prangnell et al. [13] used elastic-plastic mate-rial model to describe friction and die angle influ-ence on shear strain inhomogeneity in bulk mate-rial. The constant of friction was equal to: = 0 and

= 0.25. It was noticed that under frictionless con-ditions there was a corner gap. Under the frictioninfluence the channel intersection is fully filled withmetal and shear strain inhomogeneity increases inbulk material. Besides with reduction of die anglefrom 100 to 90°the inhomogeneity also increasesand can result in dead zone developing in the diecorner. However such situation was not analyzeddue to additional computational expenses.

The dead zone effect was also faced by the au-thors [14]. They prepared alloy ingots ofBi

1.9Sb

0.1Te

2.7Se

0.3 by melting a mixture of each met-

als (>99.99%) in a sealed evacuated silica tube.Rapidly solidified foils ware prepared from alloy in-gots using the single-roller melt-spinning techniquein an argon atmosphere. These foils were then pul-verized and sifted through 1 mm2 sieve holes. Thesepulverized foils were stacked into an ECAP die madefrom tool steel. The die channel was a rectangularshape _f 30 × 6 ]]2. The angle ( ) between theinlet channel and outlet channel was 90°. The ECAPprocess was implemented in an argon atmosphereat three separate temperatures of 693K, 733K, and773K. Repetitive pressing was performed up to 6passes. The repetitive extruded specimens wererotated 180°at each pass in order to apply a homo-geneous and cumulative strain.

All specimens were fully densified (R.D. >98%)after passing through the ECAP process. Here, atheoretical density of 7.78 g/cm3 is utilized for theBi

1.9Sb

0.1Te

2.7Se

0.3 specimen. An inhomogeneous

deformation caused by a frictional affect betweenthe specimen and the die wall was observed lyingalong the portion adjacent to the inner corner in thecross-section of the single pass specimen. How-ever, this inhomogeneity was provided in the speci-men processed with 2 passes via route C. A dead-


metal zone was formed in the outer corner of thespecimen after ECAP processing. A metal slip ap-peared along the surface of the dead metal zone.The canting angle ( ) from the extrusion direction ofdeformed elements was plotted versus the numberof passes (N). Under ideal ECAP processing, isgiven by Eq. (1) with a shear strain ( ).

1tan .

1 (1)

The shear strain was also derived by inEq. (2). In their study, is 90°.

N2 cot .2

(2)

Due to the dead-metal zone and the frictionaleffect, the experimental value was larger than thetheoretical value. Here, the amount of equivalentstrain (

eq) imposed by the ECAP processing can

be estimated by Eq. (1). Typically, an amount eq

isgiven by Eq. (3) under the ideal condition of

= 90°.

eq

N2.

3 (3)

Measurement of the value reveals that boththe presence of a dead-metal zone and the frictionaleffect may lead to shear strains which are signifi-cantly lower than their ideal values.

Sometimes there was a material destroy in thechannel. Such a conclusion was drawn by the au-thors [15]. They studied ECAP and modified pro-cess: Cross-ECAP, T-ECAP, U-ECAP, and S-ECAP.An isothermal two-dimensional plane strain finiteelement simulations of the ECAP and modifiedECAP processes were carried out by using com-mercial rigid-plastic software DEFORM 2D. All simu-lations were realized under ideal conditions (rigid-perfect, plastic material, with frictionless contactbetween the die and the w_rkpiece _f 10 × 10 ]]2

in cross sectional area.The deformation characteristics of the various

modified ECAP processes were compared with thatof the conventional ECAP process in the followingaspects: overall deformation/flow behavior, stress-strain variations, and strain homogeneity. The stressstate of the modified deformation process was ana-lyzed through maximum principal stress. More se-vere tensile stresses were generated in the Cross-ECAP and T-ECAP processes in comparison toECAP and U-ECAP and S-ECAP processes. Themore severe tensile stresses generated in the modi-

fied ECAP processes may generate cracks on thesurface of the workpiece during multiple passes.

The deformation is simple almost shear and thestrain rate is higher at the corner where two chan-nels meet. In Cross-ECAP the deformation is highlylocalized along the shear line of 45°. The strain rateis higher at four corners and central area where fourchannels meet. In T-ECAP the deformation is local-ized along the shear line of 60°. The strain rate ishigher at the corner regions where two cannels meetand at the bottom region. In U-ECAP and S-ECAPthe deformation is almost simple shear. However,the strain rate is very high at the inner corners andlow at the outer corners with U-ECAP. In S-ECAPthe strain rate is higher at both corners where twochannels meet, similar to ECAP.

The deformation behavior is more complicatedin modified ECAP processes than that of the ECAPprocess. The induced strain is also higher in modi-fied ECAP processes. However, the strain homoge-neity is possible only through ECAP and S-ECAPprocesses.

In another work [16], the plastic deformationbehavior of forward extrusion, ECAP and a combi-nation of the forward extrusion and ECAP processeswere analyzed by the finite element method. TheFEM simulations were carried out similarly: isother-mal two-dimensional plane-strain, commercial rigit-plasic finite element code, DEFORM 2D. The strainhardening material properties of aluminum and fric-tion factor were considered for these simulations. Aw_rkpiece ]easuring 45 × 13.3 × 13.3 (L × B × T mm3 and an ECAP die with a channel angle of

= 0° and an outer corner angle of = 0° was usedin the simulations. An extrusion ratio of 7 : 1 alongwith a radius of curvature of 3 mm was also used.

The flow and strain homogeneity of the forwardextrusion and ECAP combined processes wereanalyzed through the stress-strain contours and theload history. The values were then compared withthose of the individual forward extrusion and ECAPprocesses.

The deformation behavior is more complicatedin the forward extrusion + ECAP and ECAP + for-ward extrusion processes in comparison to the for-ward extrusion and ECAP processes. The predomi-nant formation of a corner gap was not observed inthe forward extrusion + ECAP and ECAP + forwardextrusion processes. Nonetheless, it is impossibleto achieve strain homogeneity in these processes.For strain homogeneity forward extrusion is a de-termining process.

To solve the problem of material destroy duringECAP the authors of [9,17] suggest using back pres-


sure. Brittle cast alloy Al-5%Fe destroying after twoECAP passes without back pressure was success-fully deformed after 16 passes with a back pressureof 275 MPa.

Stressedly-deformed state during ECAP withdifferent back pressures and frictionless conditionfor aluminum alloy 2024 was analyzed using themetal forming code Q-form. It is noticed the hydro-static stress along the internal channel wall changesits value fr_] “-” t_ “+” after def_r]ati_n z_ne. Thisfact is the reason of surface cracks on the billet.

Thus strain homogeneity is an important param-eter of ECAP analysis.

Calculated-experimental research described inpaper [12] is devoted to the influence of die geom-etry, friction condition between the die and workpieceon the distribution of plastic deformation duringECAP for Al-alloys. Five typical zones were chosento investigate plastic strain distribution irregularity.Basing on equivalent plastic strain value in eachzone distribution of the deformation in the workpiececross-section was analyzed. The analysis showedthat scatter of equivalent plastic strain values be-comes wider for the zones chosen when frictioncoefficient increases from 0.01 to 0.14. Besides themaximal deformation irregularity occurs in the nar-row zone located near the billet center. This effectis more intense with die angle decrease.

The influence of many parameters on the stateof material strain was estimated in the work [18]during ECAP. Three values of die angle ( = 90°,105°, 120°), different outer radius of the channel andfriction conditions ( = 0.1; 0.3; 0.5) were studied.For frictionless conditions the equivalent plasticstrain values were obtained both by finite elementcode MARK and analytical solution. The results hada good agreement. The authors came to the con-clusi_n: “fricti_n c_nditi_n d_es n_t affect the re-sulting strain distributi_n”.

Another work was carried out to predict defor-mation behavior and to investigate the deformationflow for pure copper along the die during ECAP us-ing ANSYS V12 [19]. The die for ECAP was de-signed with three different angles of 90°, 110°, and120°for five different hydrostatic pressure conditions.It was observed that deformation along the die dur-ing pressing is inhomogeneous for various channelangles under different hydrostatic pressure condi-tions. Total deformation of sample during pressingdecreases with increase of channel angle.

Some mechanical aspects of ECAP for a chan-nel with die angle 90° were analyzed in the paper[20]. Finite element analysis was carried out for twohypothetical materials: elastic-perfectly plastic and

elastic-plastic. The influence of die corner angle,strain hardening and friction on metal deformationbehavior, plastic deformation zone and working loadwas estimated. The possible gap formation for eachmaterial was studied. This gap always appears forfrictionless pressing and small corner angle irrespec-tive of material properties. The gap can also occurin the exit part of the channel. For elastic-plasticmaterials the billet looses less contact with chan-nel surface than in case of perfectly-plastic mate-rial. Deformation zone form depends more on cor-ner angle than on friction. At the same time the de-formation inhomogeneity index increases with cor-ner angle, strain hardening and friction coefficient.

Thus the application of ECAP to industry is de-layed because the problems connected with me-chanical aspect of the process are not solvedenough. However the studies differ from one anotherand even c_ntradict. That’s why there are n_ s_undrecommendations for tools design and lubrication.

To solve the problem mentioned above it wasnecessary to carry out a complex study of ECAPmechanics: estimate the influence of basic techno-logical parameters on flow nature and stressedly-deformed state of metals during ECAP. Die geom-etry and contact friction are the basic technologicalparameters of ECAP. The length of the billets wasassigned to be limited. The billets were put into thechannel _ne after an_ther. It’s assu]ed the backpressure is created only due to the previous billet.

2. FINITE ELEMENT ANALYSIS OFECAP PROCESS

Numerical solution of lateral metal pressing task forsquare section channel was obtained using FEM[21].

The following admissions were made: material]_del – elast_-plastic hardenable b_dy, def_r]a-ti_n ]_del – plain strain c_nditi_n. The ch_ice _fmaterial model was explained by deformation con-dition: small part of plastic deformation zone andthe rest – resilient z_ne.

The basic geometry parameters of the channelare: die angle , corner angle defined by outercoupling radius R, inner coupling radius r and chan-nel width b (Fig. 2).

The angle varied in the range 90°-120° at theinterval 15°. This choice was determined by thepossibility to realize the process on the one handand its best efficiency on the other hand. The ratiosof outer radius to channel width R/b were the follow-ing: 1:4, 1:2, 3:4, 1:1. The inner radius r was equalto 5 mm and the channel width was 20 mm. Fric-


tion characteristic values on the surface of contactbetween die and workpiece were as follows: = 0;0.1; 0.2; 0.25; 0.3. Commercial Al was used as amodel material.

First it was studied how the metal filled the chan-nel turning part (unsteady stage of the process).Friction condition on the surface of contact betweendie and workpiece: = 0.1.

In the inlet part of the channel there is a com-pression of the billet and forming of its forepart asshown in Fig. 3, h means ram displacement.

The forepart shape depends on die angle . Forrectangular channel ( = 90°) the shape is close toinitial, the more die angle is, the more complicatedit becomes. This stage is characterized by intenserise of pressing load.

As the metal fills plastic deformation zone thehydrostatic pressure distribution becomes irregu-lar. By the end of unsteady stage of the process thenormal contact stress increases and reaches itsmaximal value (Fig. 4). At that the channel hasheavily loaded places: at the exit from plastic defor-

Fig. 2. Die geometry.

Fig. 3. Position of the billet in the channel at = 105°: a) h = 0 mm; b) h = 8 mm; c) h = 13 mm;d) h = 18 mm.

(a) (b) (c) (d)

mation zone along the outer channel wall and be-fore the entry to plastic deformation zone.

In both cases it is the place where the motionpath of contact point changes. In mechanics it is astress concentrator. When the process becomessteady the normal contact stress values decreasein the indicated points.

On the outer channel wall before the entry toplastic deformation zone the metal looses contactwith tool surface and a gap forms. The less outercoupling radius is, the more gap size forms.

The strained state of the billet forepart does notcorrespond to simple shear.

As while the practical realization of ECAP pro-cess the billets were put one after another into theinlet part of the channel it was necessary to esti-mate the influence of the previous workpiece. Obvi-ously this influence is caused by the force of fric-tion in the channel. So numerical modeling of ECAPof two billets was carried out for different frictionconditions in the channel. Fig. 5 illustrates the po-sition of billets at the final stage of the process.

ECAP process was observed on 60 forming stepswhere the ram displacement was h = 1 mm foreach step. By this moment in both cases the form-ing of forepart and back part of the first billet andforepart of the second billet had finished. To solvecontact task a layer of elements (see the shadedlayer in Fig. 5) was assigned between the billets.The properties of the layer differed from those of thebillets. From Fig. 5 it results that forming of the con-tact surfaces between the billets (forepart and backpart shape) depends on friction conditions. For lessfricti_n (Fig. 5b the previ_us w_rkpiece d_esn’t prac-tically impede forming of the forepart of the next


Fig. 4. Normal contact stress distribution alongchannel walls at the moment of its maximum for

= 105°, R/b = 1:2.

Fig. 5. The position of billets in the channel during ECAP for different friction conditions: a) = 0.3;b) = 0.15.

(a) (b)

Fig. 6. Distribution of flow velocity along the longitudinal section of rectangular channel during ECAP fordifferent ratios R/b: a) 1:4; b) 1:1.

(a) (b)

one. There is a free turning of the billet in the chan-nel. With more friction the contact area of the neigh-boring billets increases (Fig. 5a) and the nextworkpiece has to push the previous one out. Con-tact surfaces form according to the direction ofmaterial plastic flow.

To analyze the steady stage of the process themodel with the turning part of the channel filled pre-viously with metal was taken. Validity of this as-sumption was confirmed by corresponding compara-tive analyses of strain fields. Due to the fact thatcontact friction stress (at = 0.1) was not enoughto fill the channel completely the numerical analy-sis was carried out with back pressure. ECAP pro-cess was observed also on 60 forming steps. As aresult metal filled the exit part of the channel andthe process came to steady (quasistationary) stage.

Basing on calculation results for all die geom-etry options the diagrams of flow velocity duringECAP along the longitudinal section of theworkpiece were obtained (Fig. 6).


The character of flow velocity diagram is definedby the ratio R/b. The main deformation zone has aretarded flow area for the channel with the least ra-tio R/b (Fig. 6a). Under back pressure conditionsthe metal fills the outer corner of the channel. Withincrease of the ratio R/b the retarded flow area dis-appears. In the radial channel (Fig. 6b) along theouter coupling radius the metal layers move at morevelocity compared to others. At the same time alongthe inner radius the metal moves slower. Such be-havior is explained by the difference in distancesthe neighboring metal layers have to pass. The ini-tial regular field of velocities becomes irregular sincethe condition of constant metal volume per secondalong the cross-section is fulfilled. With die angledecrease the velocity gradient rises.

Fig. 7. Shear strain intensity distributions in the cross-section of the billet for different die geometry:a) = 90°; b) = 105°; c) = 120°.

Due to the fact that during ECAP strained stateis provided mainly by intense shear (mixed) com-ponent of tensor deformation, the invariant charac-teristic – shear strain intensity was used to esti-mate that state.

Shear strain intensity distributions received fordifferent die geometry has revealed an irregularity ofthis characteristic. It’s advisable t_ investigate value

distribution irregularity behavior in the cross-sec-tion of the billet under steady stage of the process.Fig. 7 shows shear strain intensity distributions inthe cross-section of the billet for different die geom-etry at the steady stage of ECAP process.

Irregularity of shear strain intensity distributionis the consequence of irregularity of metal flow inthe main deformation zone.

(a)

(b)

(c)


(a) (b)

Fig. 8. Distribution of flow velocity during ECAP (a) and shear strain intensity (b) along the longitudinalsection of the workpiece for = 90°, R/b = 1:4, = 0.25.

R/b q

900 1:4 0.0741:2 0.1163:4 0.2021:1 0.232

1050 1:4 0.0591:2 0.0873:4 0.1991:1 0.227

1200 1:4 0.0581:2 0.0763:4 0.1241:1 0.181

Table 1. Values of irregularity parameter q for differ-ent die geometry.

For the numerical estimation of the level of ir-regularity a parameter q was used [22]:

n

ave i i

i

ave

p

q

2

1 ,100

(4)

where ave – is the average value _f shear strain in-

tensity in the workpiece cross-section; i – is the

mean strain of the ith area of the workpiece whichbelongs to the given strain range; p

i– is the per-

centage of the mentioned area. When the strain ofthe workpiece is regular the irregularity parameter qis zero. In other cases more value of parametermeans more irregularity of the strain fields. Table 1contains parameters of the dies and correspondingvalues of q.

The closer channel shape to the radial is, themore irregular shear strain intensity distribution inthe workpiece cross-section appears. With die angleincrease the shear strain field becomes more regu-lar.

At the same time the velocity field analysisshowed the possible appearance of retarded flowarea of metal. Between this area and the main flowthere is a very localizated zone of intense shearstrain. So in spite of small value of irregularity pa-rameter for low-plastic material it can result in itsdiscontinuity.

Finite element analysis of friction conditions in-fluence on stressedly-deformed state of metal wascarried out for three die angles = 90°, 105°, 120°.Irregularity parameter had the least value atR/b = 1:4 so friction influence was investigated onlyfor this case.

The model was assumed to have the turning partof the channel filled previously with metal. ECAPprocess was observed on 35 forming steps.

As a result metal filled the exit part of the chan-nel and the process came to steady(quasistationary) stage. Basing on calculation re-sults the diagrams of flow velocity during ECAPalong the longitudinal section of the workpiece wereobtained (Fig. 8).

For frictionless condition the channel was notfilled completely and metal passed the turning rela-tively easily. With friction increase metal had to fillthe channel and retarded flow area mentioned aboveappeared. At the border with this area there was


greater difference of flow velocity with more frictionthat could result in significant irregularity of the maindeformation zone.

In the rectangular channel flow velocity valuesfor various metal layers differ ten times more

Fig. 9. Shear strain intensity distribution in the cross-section of the channel for different die angles andfriction conditions: a) = 90°; b) = 105°; c) = 120°.

(Fig. 8a). As a consequence the value of accumu-lated deformation in various parts of the billet differsmore than 2.5 times (Fig. 8b). This can result indiscontinuity of material. Further increase of friction(up to = 0.3) for rectangular channel is limited by

(a)

(b)

(c)


m q

900 0 0.0690.1 0.0740.2 0.1410.25 0.205

1050 0 0.0580.1 0.0590.2 0.0660.25 0.0720.3 0.099

1200 0 0,0520,1 0,0530.2 0.0580.25 0.0610.3 0.065

Table 2. Values of irregularity parameter q for differ-ent die angle and friction.

additional computational expense. On the oppositeside of the workpiece there is unfavorable situationas well. Along the upper wall of the channel the strainof the billet is non-monotone: material undergoescompression in the inlet part of the channel andtension in the exit part. With friction increase ten-sile stress values rise as well. It can result in crackformation on the surface of the processed material.

Under different friction conditions in the channelan irregularity of shear strain intensity distributionvalue was observed. On the analogy of die geom-etry influence investigati_n let’s c_nsider value distribution irregularity behavior in the cross-sectionof the channel. Fig. 9 shows shear strain intensitydistribution in the cross-section of the channel fordifferent die angles and friction conditions.

With the help of q parameter a numerical esti-mation of irregularity of shear strain intensity distri-bution in the workpiece cross-section depending onfriction conditions for different dies was obtained(Table 2).

Irregularity of shear strain intensity distributionrises with less value of die angle and friction increase.Friction increase results in rise of both hydrostaticpressure in the channel and pressing load. Theanalysis of stress distribution under the ram showedthat with value increase the total level of stressrises 3-3.5 times.

3. EXPERIMENTAL RESEARCH OFECAP PROCESS

Basing on numerical modeling of ECAP process 2variants of experimental tool were made. In the firstvariant the tool consisted of two parts and had ajoint in the symmetry plane of the channel.

During making and operation of this tool the fol-lowing disadvantages were found out: complicatedfinal polishing of the channel work surface, difficulttaking out of un-pressed material.

According to the second variant the tool con-sisted of more parts (Fig.10) and was used for ex-perimental researches. Geometric parameters of thechannel: die angle = 105°, channel width b = 20mm, outer coupling radius R = 10 mm, inner cou-pling radius r = 5 mm.

Commercial cast aluminum A7 (according toRussian Standard) was used for experiment. Thelubricants on basis of: liquid vegetable fat (lubri-cant №1), higher fatty acids salts (lubricant №2),solidified oils (lubricant №3, 4, 5), tallow (lubricant№6, 7) were used. Graphite and zinc stearate wastaken as a stuff. Fig. 10. Experimental tool for ECAP process.

With the lubricant №1 and 2 ECAP processfailed. Squeezing of the lubricant out and stickingof aluminum to the channel walls was observed. Lu-bricants №3-7 showed sufficient results. Fig. 11 il-lustrates the shapes of the deformed billets afterECAP with different lubricants.

The shape of the forepart and back part of thenext billet was different depending on lubricant used.

With friction increase, for example lubricant №3(Fig. 11a), the third billet had to push out its previ-ous one. Contact surfaces formed according to thedirection of material plastic flow. If the friction re-mains practically the same the shape of next bil-lets is the same either (Fig. 11b).


4. CONCLUSION

The homogeneity of structure and metal propertiesis provided by strain distribution regularity in the bulkmaterial.

ECAP process is always characterized by ir-regular shear strain distribution. Depending on diegeometry and friction condition that irregularity couldbe reduced. If there is a given die angle and cer-tain fricti_n c_nditi_ns it’s necessary t_ ch__se theratio R/b in such a way as to minimize the possibleappearance of areas of retarded or accelerated flowof metal.F_r designing _f the t__l f_r ECAP pr_cess it’s

recommended to use demountable construction thatwill allow making the final polishing of channel worksurface easier. For higher reliability of the tool func-ti_n it’s advised to use a bandage.

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technological problems of equal channel angular pressing

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