Deformations and structure of metal during cold butt-seam welding of copper blanks

ISSN 1067�8212, Russian Journal of Non�Ferrous Metals, 2012, Vol. 53, No. 1, pp. 45–53. © Allerton Press, Inc., 2012.Original Russian Text © Yu.N. Loginov, A.G. Illarionov, S.Yu. Klyueva, M.A. Ivanova, 2012, published in Izvestiya VUZ. Tsvetnaya Metallurgiya, 2012, No. 1, pp. 37–44.

45

PROBLEM TOPICALITY AND WORK OBJECTIVE

It is hard to imagine the modern process of wire�drawing without the technology of welding. Joiningwire ends makes it possible to achieve drawing withoutcontinuous procedures of sharpening the forward endsand threading the drawing dies and draw pulleys ofdrawing machines, as well as periods of accelerationand deceleration of drawing machines.

The process of cold welding is based on the appli�cation of a strong reduction of metal in the placewhere two wire blanks are joined. This wire part willpredictably have properties different than the blank asa whole. Knowledge of these properties is necessaryfor the consumer of this production to take intoaccount to ensure heterogeneity in the finished prod�uct (wire or cable). It is of importance for a technolo�gist of the drawing industry to evaluate the probabilityand causes of the break in the shut during the followingsteps of drawing. Therefore, determining the charac�teristics of metal in the shut is a topical problem.

In this work we consider a variant of cold weldingwire blanks, which is often used in the production ofelectrotechnical copper wire (Fig. 1).

Different information sources give various hypoth�eses of the appearance of the metal joint in the modeof pressing two blanks by outside forces [1–5]. Cur�rently, new interpretations of the phenomena accom�panying the processes of metal setting are appearing.For example, the authors of [6] assert that the presenceof surface cracks, through which the metals that are

joined are extruded into each other with the formationof diffusion compositions, plays an important role inwelding.

The objective of our work is to determine the defor�mation mode and reveal the features of structure for�mation in the place where two blanks are cold welded.

STATEMENT OF THE PROBLEM

Most authors of hypotheses of the cold weldingmode agree that the decisive role in the processbelongs to the increased level of shear deformationsand to the presence of a sufficient level of compressivestresses. It should be noted that shear deformationscan be evaluated in various ways.

If the deformation on shear surfaces is neglected,the relative reduction can be evaluated by the formula

ε = 100(H0 – H1)/H0 = 100ΔH/H0,

where H0, H1, and ΔH are the distances betweenclamps in the first and last instants and the absolutereduction, respectively.

It is not always convenient to apply parameter ε incalculations because the additivity principle is not ful�filled. Therefore, the concepts of the degree of defor�mation (sometimes called logarithmic)

or the shear degree of deformation

are often used.

ε H0/H1( )ln=

Λ 3ε 3 H0/H1( ).ln= =

Deformations and Structure of Metal during Cold Butt�Seam Welding of Copper Blanks

Yu. N. Loginov*, A. G. Illarionov**, S. Yu. Klyueva***, and M. A. Ivanova****Ural Federal University, pr. Mira 19, Yekaterinburg, 620002 Russia

*e�mail: [email protected]**e�mail: [email protected]***e�mail: [email protected]

****e�mail: [email protected]

Abstract—Calculations by the finite element method are carried out for the deformation mode of copperblanks as they are being joined by cold welding. It is found that the degree of deformation in a shut is distrib�uted nonuniformly and reaches 3–5. A metallographic analysis revealed the appearance of a zone of abruptgrain�size refinement in the region of a weld seam. A study of the distribution of microhardness showed anincrease in this parameter up to 1000 MPa in the arrangement region of the weld seam with respect to790 MPa in the parent metal. The experimental data confirmed the computed characteristics.

Keywords: copper, welding, deformation mode, structure.

DOI: 10.3103/S1067821212010154

PRESSURE TREATMENT OF METALS

46

RUSSIAN JOURNAL OF NON�FERROUS METALS Vol. 53 No. 1 2012

LOGINOV et al.

It should be noted that the nonuniformity in thedeformation distribution, which is inherited from themethod of holding the blank in clamps of a machine,is not taken into account in these formulas. Therefore,we applied the RAPID�2D bundled software, whichwas developed by researchers at the Ural State Techni�cal University (UPI) [7], for the following modeling.This software is meant mainly for the mathematicalmodeling of hot forging and stamping. The calculationis based on the use of the finite element method, andwe successfully applied it to solve the problem of draw�ing the copper rod [8].

The finite difference representation of the region ofdeformation and welding in the case of working a cop�

per rod 8 mm in diameter for initial and end distancesbetween clamps of 10 and 0.3 mm, respectively, isshown in Fig. 2. The relative reduction is 97% in thiscase, which corresponds to a logarithmic degree ofdeformation of 3.5 (Λ = 6). However, these values areaveraged by the volume of the deformation place, andtheir coordinate distribution should be refined.

In the formulation of the problem, it was assumedthat the joined faces of blanks behave as a single wholefrom the viewpoint of the deformation mechanics.Therefore, their interface can be omitted and the asso�ciation of two blanks can be presented as a single blankcompressed by clamps and deformed by them.

ANALYSIS OF THE DISTRIBUTION OF DEFORMATION RATES

The formation of the deformation region duringthe stages of welding is shown in Fig. 3. To track upchanges of the pattern of the deformation region, weused data on the calculation of the intensity of sheardeformation rates:

where ξij are the components of the deformation�ratedeviator and i, j = r, z, ϕ are cylindrical coordinates.

The application of the rate parameter allows us toevaluate the instantaneous state of the deformedmedium and the distribution scheme for deformationincrements in each current instant.

To analyze the first stage of the process (Fig. 3a), weused the coordinate grid formed in the undeformedstate by rectangular cells. In this case the shape of the

H 2ξijξi j,=

1

2

3

45

H1

(a) (b) (c)

H0

Fig. 1. Schematic of cold butt�seam welding. (a) Blank clamping, (b) compression with flash formation, and (c) jointed blankswith flash. Arrows show the direction of force effect on a tool. (1) Clamps, (2, 3) blank edges, (4) interface, and (5) flash.

12

3

4

2

Fig. 2. Schematic of the statement of the problem.(1) Blank, (2) clamp contour, (3) coordinate grid, and(4) finite element grid.


DEFORMATIONS AND STRUCTURE OF METAL DURING COLD BUTT�SEAM WELDING 47

deformation region after the stage completion corre�sponds to the scheme of depression for a cylinder ofmoderate height on plane heads. The zone of intensedeformation is observed in the center. There is directevidence of the phenomenon of formation of the tubbyside face; correspondingly, the lines of the coordinategrid are convex in the direction of the free cylinder sur�face. In the case of common upsetting, this phenome�non is correlated with the effect of friction stresses atthe surface contacting with heads. However, boundaryconditions of welding are different; in contrast toheads, clamp edges prevent the displacement of thematerial deformed in the radial direction, which canbe compared with the presence of full adhesion duringcommon upsetting. However, even such a comparisonis not quite correct because the region of intense flowof the metal (usually called shear) appears in theregion of clamp edges during welding.

During an analysis of the second and followingstages (Figs. 3b–3d), the corresponding coordinategrid was not used because its cells were deformed toostrongly and the finite element grid is shown, thethickenings of which usually correspond to regionswith large gradients of deformations or stresses.

During the second stage of welding, the region ofintense formation in the center of the deformationregion disappears. The shape of the deformationregion approaches a low cylinder, but it is in the pres�ence of the region of metal flow along external conesurfaces of clamps with the formation of the expandedconical volume of metal. In the interval between thefirst and second stages, the metal slips over the conicalsurfaces of clamps, meeting the resistance of friction

stresses acting in this place. In this way the displace�ment resistance and the increased level of compressivestresses are formed. It is seen in Fig. 3b that the metalbegins to move away from the conical surfaces ofclamps with the formation of a subsequently increas�ing clearance. The zones of the intense deformationwere completely displaced into the angular zones ofthe deformation region. Here the metal is subjected tomaximum peening under conditions similar to shear.

Passage to the third step (Fig. 3c) is accompaniedby a decrease in the height of the deformation regionand the integration of angular zones of intense forma�tion into a single one. The zone also continues itsexistence during the fourth finishing stage (Fig. 3d).

ANALYSIS OF THE STRESSED STATE

The distribution of average stresses (σ) over the defor�mation and welding region was evaluated by the deforma�tion mode index σ/T, where T = (1/2sijsij)

1/2 is the shearstress intensity and sij are the components of the stressdeviator. Figure 4 shows two characteristic states, namely,in the start of the process and close to its termination. Forthe first state, the presence of two regions with anincreased deformation mode index is characteristic in theblank center at the level of clamp edges and in its periph�eral part. During cold welding, the flash is pressed out. Itis the waste of the treated metal; therefore, the state ofperipheral metal layers is of no interest for the technolo�gist. The appearance of a zone with increased index σ/Tin the blank center is fraught with the interior metal fail�ures. However, the shear degree of deformation in thiszone is not too large; therefore, the risk of metal breakage

(a)

(c)

(b)

(d)

Fig. 3. Localization of the deformation and welding region for various blank reductions. Zones with an increased shear deforma�tion rate are light segments marked by circles. ε: (a) 30, (b) 67, (c) 91, and (d) 97%.

48


LOGINOV et al.

is minimal. As the flash is forced out outside the deforma�tion region, the level of tensile stresses increases within theextruded metal and the deformation mode index becomeshigh (Fig. 4b), which results in a decrease in plasticity; themetal subjected to strong peening is concentrated at thesame place. The applied calculation program does notfollow the character of metal failure; however, in prac�tice, because of the effect of the two abovementionedfactors, cracks appear in the flash and then the metal isfragmented by single petals along them (Fig. 5).

ANALYSIS OF THE DISTRIBUTION OF THE ACCUMULATED DEFORMATION

Wire peening in each point of the deformation andwelding region were evaluated by the degree of sheardeformation determined by the formula

where t is the time it takes the particle to pass along thetrajectory of its displacement.

The final result of calculations is shown in Fig. 6,where zones with equal levels of Λ are shown.

Λ H τ,d

0

t

∫=

It is evident from Fig. 6 that the maximum defor�mation levels are localized within the layer at the inter�face of two blanks; in this case, the largest deformationis characteristic of the metal pressed out outside thelimits of the deformation and the welding region.Thus, we succeeded in establishing that the maximallydeformed metal takes no part in welding and trans�forms into the flash. The zone of intensely deformedmetal as a function of the height coordinate is local�ized within a thin layer in which a decrease in thepeening level occurs by its two sides. The thickness ofthis layer is determined by the extension of the zone ofthe peened metal during drawing. Because of anincrease in the cross section of the wire, its length dur�ing the stages of drawing increases and subsequentlyan increase in the length of the metal segment peenedto a high degree of deformation occurs.

As is evident from our solutions, the degree of sheardeformation indeed achieves great values at a level of5–7, which was established by a calculation accordingto formulas. However, the place where the metal hadsuch a large degree of deformation was found outsidethe blank interface. In general, this metal was pressedout in the flash. Its place was taken by other metal fromthe undeformed zone. The exception was a smallregion close to the blank center and the metal partadjacent to the flash.

(a) (b)

Fig. 4. Zones with an increased deformation mode index σ/T (light segments marked by circles) for blank reduction of (a) 30%and (b) 91%.

Fig. 5. Flash fragmentation in actual welding.



Cold welding often uses alternating reductions withthe strangulation of the blank by clamps. It is naturalto ask the question where the metal reduced in the pre�vious step of deformation falls, i.e., if it remains in theblank or displaces into the flash. In the second case,the effect of the first welding must insignificantly affectthe strength of junction because the peened metal isreplaced by a new portion of metal from the non�peened part of the blank.

The RAPID�2D bundled software allows the blankto be transferred into new deformation conditions,and the deformation distribution at the previous stepcan be fixed. The sequence of operations was as fol�lows:

(i) The flash of the virtual blank after the first weld�ing was removed due to the elimination of peripheralfinite elements; this operation allows us to avoid theaccumulation of errors in the following iteration com�

putations (the influence of the flash at the secondwelding on the stress level is weak because this is asmall�dimension formation making little resistance);

(ii) The finite element grid was extended to theundeformed blank part;

(iii) The clearance between the clamps wasincreased up to a primary value of 10 mm;

(iv) The blank with the inherited deformation levelwas transported into a new arrangement of tools.

The comparison of solutions for the first and sec�ond reductions of the blank shows that the deforma�tion distribution patterns are almost identical, i.e., theprocess is repeated in the same mode.

The distribution of the degree of shear deformationalong two sides from the interface is shown in Fig. 7. Itis evident from the diagram that, as it moves from theinterface, the value of Λ decreases according to thehyperbolic law.

6

5

4

3

2

1

03.02.41.8–2.4–3.0–3.6 –1.8 –1.2 –0.6 0.6 1.20 3.6

Distance to interface, mm

Λ

Fig. 7. Distribution of the shear deformation on both sides of the interface between two blanks.

Color Range

min

max

0.0032565

0

1

2

3

4

5

5.5646

0

Fig. 6. Distribution of the degree of shear deformation in the final instant. The table of correspondence between tonality and Λ ison the right.

50


LOGINOV et al.

MICROSTRUCTURAL AND MICRODUROMETRIC ANALYSES

To analyze the microstructure and microhardnessover the section of the welded joint, the followingmethods were used:

(i) optical metallography (an OLYMPUS GX�51microscope);

(ii) microhardness measurement (an auxiliarymicrodurometry device for a Neophot�21 opticalmicroscope);

(iii) a study of backward scattered electron diffrac�tion (a Nordlys HKL Channel 5® auxiliary firmwaredevice for a JSM 6490 scanning electron microscope).

Samples of seams of the copper rod and the wireproduced by ZAO SP Katur�Invest enterprise in theUral Mining and Smelting Company were investi�gated. The wire material was M00�grade copper(GOST (State Standard) 859–2001; the oxygen con�tent was no higher than 0.035%).

Figure 8 shows the structures of the copper rod inthe region of the welded joint (Fig. 8a) and the seamzone itself with the use of the orientational contrast(Fig. 8b). The characteristic feature of the seam zoneis a very fine�grained structure in comparison with theparent metal (PM); i.e. the grain size decreases more

than by a factor of ten from 30 to 2.5 μm, the fiberstructure in the deformation effect zone (DEZ)(Fig. 8a) is almost absent, and the grains are mainlyequiaxial (Fig. 8b). Most likely the formation of equi�axial grains is caused by the shattering of grains, whichis observed during the intense plastic deformation.The diminishment of precipitates of copper oxides,which predominantly take the round shape, is alsorevealed.

According to previous computations, the seamzone has the highest degree of deformation ( > 3),which can be considered intense. It is well known that,during intense deformation, the refinement processesof the grain structure can occur, which was observedearlier in copper [9].

In addition, one of the effects of intense plasticdeformation is the possibility that oxide precipitateswill dissolve, which was observed earlier for iron [10].This effect also probably exists in regards to inclusionsof copper oxide Cu2O. An indirect confirmation ofthis fact may be the significant increase in microhard�ness within the seam zone in comparison with theadjacent DEZs (ΔHV to 50 MPa) (Fig. 9). Thisincrease in microhardness can be a result of the addi�tional solid solution strengthening of copper becauseof the transfer of oxygen particles from oxides into the

ε

200 μm

(a) (b)

5 μm

Fig. 8. Structure of the weld�seam zone (vertical dark strip; the wire axis is horizontal) and adjacent zones of the deformationeffect. (a) Microstructure and (b) orientational contrast within the seam zone.



solid solution; however, this fact undoubtedly requiresadditional investigations by finer methods.

Thus, during cold pressure welding, the structure ofthe rod significantly changes because of grain reorien�tation; the precipitation of copper oxides; and, possi�bly, the fragmentation and partial dissolution of oxidesas a result of intense plastic deformation within theseam zone.

It was already mentioned that the properties of therod within the seam zone differ from the other zone;therefore, the microhardness was measured withinthree zones: the parent metal, the deformation effect,and the seam itself. Figure 9 represents the values ofthis parameter acquired along the axis of the rodnearby the welded joint. It is evident from the graphthat the microhardness varies nonmonotonically.

The statistical averaging of the results showed thatthe average microhardness of the parent metal is at alevel of 790 MPa.

We can distinguish three characteristic segments inthe zone of the deformation effect. The first is adjacentto the PM and has a size of about 0.6 mm. An abruptincrease in microhardness by about 40–50 MPa incomparison with the PM is observed for this segment.In the second part close to the seam, which has alength of ~0.5 mm, microhardness increases lessintensively by about 10–20 MPa. Finally, microhard�ness increases abruptly by 30–50 MPa within a narrowzone ~0.2 mm wide near the seam.

In the seam region, microhardness is maximal andexceeds 900 MPa.

This character of varying the microhardness can beexplained based on the computational data for defor�mations during the cold welding of the rod. The activehardening of the zones adjacent to the PM is associ�ated with a gradual increase in the degree of deforma�tion, which manifests itself in a change the micro�structure, namely, the reorientation and flattening ofthe grains and twins; it is known that this promoteshardening activation [11]. The increased rate of

microhardness within the DEZ decreases uponapproaching the seam apparently because of a certainsaturation of hardening (the transverse slip causing thepartial annihilation of dislocations can be developedmore actively) and the formation of a similar fiberstructure within this zone. It was already mentionedthat an abrupt increase in microhardness within theseam zone is caused by the effect of the most intenseplastic deformation, which results in the significantdecomposition of the grained structure and the partialdissolution of oxide particles, which was found bymicroscopic investigation.

Microhardness in the welded joint varies not onlyalong the copper rod axis but also within the transversesection of its characteristic zones (seam, DEZ). Forexample, it is found within the seam zone that themicrohardness is somewhat higher than for the centerof the rod (it can reach 1000 MPa); it is almost at thesame level (~900 MPa) within the peripheral zones(Fig. 10). This effect can be associated with the moreactive nonsteady metal flow within the center duringcold welding than in the periphery.

A similar character of the variation in microhard�ness is also observed during diametral measurementswithin the zone of the deformation effect (Fig. 11). Inthis case the difference in levels of maximum and min�imum values only decreases, and the maximum in thecentral zone of the rod is less accented.

Thus, cold welding of the rod largely affects thevariation in the microdurometry characteristics of thejoint. Their level is determined by the arrangement ofthe measured zone (the seam, DEZ) in both longitu�dinal and transverse positions with respect to the seamzone.

Additional texture analysis showed that no prevail�ing orientation within the grained structure wasrevealed in the rod. The fraction of orientation ⟨110⟩(texture of deformation) within the seam zone notice�ably increases. It should be noted that the presence oforientation ⟨111⟩ in the wire is more preferable

910

890

870

850

830

810

7900 –0.5–1.0–1.5–2.0–2.5–3.0 –0.5 –1.0 –1.5 –2.0 –2.5 –3.0

PM DEZ Seam DEZ PM

HV

, M

Pa

Distance from the center, mm

Fig. 9. Variation in microhardness of the rod (8 mm in diameter) within the welded joint zone (along the axis of the rod). PM isthe parent metal and DEZ is the deformation effect zone.

52


LOGINOV et al.

because the highest strength would be achieved in theaxial direction in this case. Thus, starting from the dataof the texture analysis, we can conclude that the placeof welding is a weak link.

CONCLUSIONS

The presence of a segment of cold welding withinthe wire with a degree of deformation higher thanalong its length in general is accompanied by the fol�lowing phenomena:

(i) breakage as it passes through dies because themetal has a higher strength in the welding section butlower plasticity. This is not observed if the reserve ofplasticity is sufficient for breakless drawing;

(ii) spontaneous annealing in the place of coldwelding during drawing due to heat generation bydeformation and friction because the recrystallizationtemperature in this zone with increased peening is

reduced. The wire may break due to insufficientstrength;

(iii) grain growth, structure coarsening, and wors�ening of mechanical properties after drawing withsimultaneous annealing because the temperature ofthe onset of recrystallization in the place of welding ismuch lower than the specified annealing temperature;

(iv) an additional increase in the nonuniformitylevel of properties because the modification processesof deformation textures and metal recrystallizationmay develop according to various scenarios due to thedifference in degree of deformation in the place ofwelding and over the wire as a whole.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Educa�tion and Science of the Russian Federation in theframework of the Federal Target Program “Scientific

855

845840

830

820815810

4210 3 5 6 7

Distance along the diameter of the rod, mm

825

835

850

860

HV, MPa

Fig. 11. Variation in microhardness over the diameter of the rod along the cross section of the center of the deformation effectzone.

1100

1000

900

8004210 3 5 6 7

HV, MPa

Distance along the diameter of the rod, mm

Fig. 10. Variation in microhardness over the diameter of the rod along the seam cross section.



and Scientific and Educational Personnel of Innova�tive Russia.”

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3. Sakhatskii, G.P., Tekhnologiya svarki metallov v kholod�nom sostoyanii (Metal Welding Practices in Cold State),Kiev: Naukova Dumka, 1979.

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7. Polishchuk, E.G., Zhirov, D.S., and Vaisburd, R.A.,Kuznech.�Shtamp. Proizv., 1997, no. 8, p. 16.

8. Loginov, Yu.N. and Eremeeva, K.V., Kuznech.�Shtamp.Proizv., 2009, no. 4, p. 3.

9. Valiev, R.Z. and Aleksandrov, I.V., Nanostrukturnyematerialy, poluchennye intensivnoi plasticheskoi defor�matsiei (Nanostructured Materials Obtained by PlasticDeformation), Moscow: Logos, 2000.

10. Korznikov, A.V., Safarov, I.M., Laptionok, D.V., andValiev, R.Z., Acta Metall. Mater., 1991, vol. 39, no. 12,p. 3193.

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Documents

Deformations and structure of metal during cold butt-seam welding of copper blanks