Available online at ScienceDirect

ScienceDirectJ. Mater. Sci. Technol., 2014, -(-), 1e7

Microstructure and Mechanical Properties of a CuCrZr Welding Joint After

Continuous Extrusion

Hui Feng, Haichang Jiang, Desheng Yan, Lijian Rong*

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China[Manuscript received September 26, 2013, in revised form November 25, 2013, Available online xxx]

* Corresp24 23971005-03JournalLimited.http://dx

Please

The effect of continuous extrusion forming (CEF) process on the microstructure and mechanical properties of aCuCrZr welding joint was investigated. The experimental results showed that after the CEF process the grainswere refined to submicron-scale through dynamic recrystallization, which improved the mechanical propertiesof the welding joint as well as the base material. Meanwhile, the micron-scale precipitates aggregated at thegrain boundaries in the welding process were broken down to smaller ones and recrystallized grains of severalmicrometers formed around the precipitates after CEF process, which could alleviate the negative effectinduced by the micron-scale precipitates during plastic deforming process. Finer grains and smaller micron-scale precipitates made contributions to improve the properties of a CuCrZr alloy with a welding joint.

KEY WORDS: Continuous extrusion; Dynamic recrystallization; Flash butt welding; Precipitate

1. Introduction

CuCrZr alloys are being considered as the primary candidatematerial for the trolley contact wire for high-speed railway owingto their excellent mechanical properties, above 80% electricalconductivity and high thermal conductivity[1e3]. However,CuCrZr alloys are difficult to be manufactured by up-drawncontinuous casting to produce long contact wires due to thenecessity of vacuum melting. Welding is regarded as an effectiveway to solve the problem, yet relative research reveals that themechanical and electrical properties of CuCrZr alloys show anotable decrease with the introduction of the traditional weldingprocess as a result of the weld stress and defects, as well as thesignificant difference of microstructure between the weldingjoint and the base material, such as the grain size and theprecipitates[4,5].Our previous study has demonstrated that ultra-fine (sub)

grains less than 1 mm could be obtained by continuous dynamicrecrystallization after the continuous extrusion forming (CEF)process[6], which is regarded as an effective way to improve thestrength of CuCrZr alloy while presenting a high electricalconductivity[7]. Besides, with the introduction of CEF processafter welding, both the weld and base material are severely

onding author. Prof., Ph.D.; Tel.: þ86 24 23971979; Fax: þ868883; E-mail address: ljrong@imr.ac.cn (L. Rong).02/$e see front matter Copyright� 2014, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2014.03.025

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deformed at the elevated temperature, which could diminish theweld stress and the microstructural difference.In the present study, the CuCrZr rods were first flash butt

welded (FBW) and then extruded on the TLJ400 equipment. Themicrostructures, mechanical properties and electrical conductiv-ity of the welds and the base material were investigated.

2. Experimental

The Cue0.36Cre0.15Zr alloys were produced in a vacuuminduction furnace by adding electrolytic copper, CuCr interme-diate alloy and pure Zr. The billets were hot extruded into adiameter of 20 mm.The as-extruded rods were solution treated and then subjected

to the following processing and heat treatment: flash buttwelding (FBW), solution annealing (1233 K for 1 h), continuousextrusion forming, cold rolling (60%) and aging (723 K for 2 h).For the flash butt welding, the specimens with a diameter of

20 mm were cleaned first, mounted and clamped in the dies. Thefollowing sequence was heating, flashing and upsetting process.The relative welding parameters were as follows: the extensionlength was 20 mm, the flashing time was 2 s, the upsetting stresswas 200 MPa and the upsetting speed was 150 mm/s.The continuous extrusion process was carried out by using the

TLJ400 copper continuous extrusion machine. The wheel speedwas 4 r/min and the pre-heating temperature was 723 K. Moredetailed parameters were illustrated in literature[7].Element surface distribution was detected by electron probe

X-ray micro-analyzer (EPMA, 1610) with a resolution of 6 nm.By using the Schlumberger 7081 precision digital voltmeter with

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Fig. 1 Microstructure of the welding interface (a), weld center (b), transition zone (c) and base material (d) corresponding to the “b”, “c” and “d” in (a).

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an accuracy of 10 nV, the electrical resistance was determined bymeasuring the voltage of samples with a constant current of50 mA. The electrical conductivity was deduced from electricalresistance according to literature[8]. It should be pointed out thatthe electrical conductivity of the weld in this study means theconductivity of samples containing the welding joint rather thanthe whole part in the joint.For the tensile tests of samples with welding joints, the plates

were polished and etched firstly, and then the samples for testwere prepared with the welding joint in the gage length portion.The preparation of tensile specimens referred to GB/T 228.1-2010 and the gage dimension was 22 mm � 2.5 mm � 3.8 mm.Microstructural characterization was performed along the rollingdirection by optical microscopy (OM) and field emission gun-scanning electron microscopy (FEG-SEM, JEOL 7001F).Transmission electron microscopy (TEM) observation was car-ried on a JEM 2010 transmission electron microscope with anoperating voltage of 200 kV.

3. Results and Discussion

3.1. Microstructure before and after CEF process

Fig. 1 shows the microstructure of CuCrZr alloy after flashbutt welding. The microstructure of the welding joint consistingof the weld center, the transition zone and the base material isdemonstrated in Fig. 1(a). The weld center (Fig. 1(b)) iscomposed of recrystallized grain of about 50 mm, which may beattributed to the upsetting process at a high temperature. The

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transition zone (Fig. 1(c)) shows a mixed crystal morphology,and part of the grains remelt leading to a segregation of micron-scale precipitates (as shown in the inset of Fig. 1(c)). Themorphology is characteristic of divorced eutectic, which couldbe attributed to the non-equilibrium solidification process and therelatively small fraction of the precipitates[9]. Similar resultshave been reported in literature[10]. Fig. 1(d) is the morphologyof the base material with developed annealing twins, and theaverage grain size is about 200 mm.The microstructure of the welding joint after CEF process is

shown in Fig. 2. The welding interface is largely bended alongthe extrusion direction (Fig. 2(a)), and the weld center, transitionzone and the base material can still be distinguished. It can benoticed that the grains of weld center and the base material arerefined to a submicron-scale (Fig. 2(b) and (d)), whereas thetransition zone demonstrates the morphology of micron-scaleprecipitates and recrystallized grains with size of about 10 mm(Fig. 2(c)).

3.2. Grain refinement at the weld center and base material

Fig. 3(a) shows the TEM image of the base material after CEFprocess. It can be seen that the subgrains less than 1 mm are thedominating morphology, and the selected area electron diffrac-tion (SAED) pattern shows small misorientations betweenadjacent grains. The TEM result demonstrates that the CuCrZrrod is incompletely dynamically recrystallized during thecontinuous extrusion process, which has been illustrated in ourprevious study[7]. CuCrZr alloys possess a relatively low SFE,which implies that cross slip will be very difficult and even

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Fig. 2 Microstructure of the welding joint after CEF process (a), weld center (b), transition zone (c) and base material (d) corresponding to the “b”, “c”and “d” in (a).

Fig. 3 TEM images of the base material (a)[7] and weld center (b) after CEF process. The insets are the corresponding SAED patterns, respectively.

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forbidden in some cases[11]. Meanwhile, precipitation took placeduring the continuous extrusion process. The fine and homoge-neously distributed precipitates have a strong pin effect on thesubgrain boundaries, which prohibits the transition from low-

Fig. 4 EPMA mapping images showing the distribution of Cr after FBW a

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angle boundaries to high-angle boundaries, leading to a sub-grain dominating morphology.The microstructure of the weld center after CEF process is

similar to that of the base material, shown in Fig. 3(b). In fact,

t the weld center (a), the transition zone (b) and the base material (c).

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Fig. 5 SEM morphology showing micron-scale precipitates along thegrain boundaries at the transition zone.

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the initial microstructure before CEF process of the weld center,equiaxed grains of about 50 mm, is different from that of the basematerial, grains of about 200 mm with developed twins. How-ever, the difference was eliminated by the continuous extrusionwith introducing severe plastic deformation.

3.3. Precipitates and recrystallization at the transition zone

3.3.1. Precipitates. It is shown in Fig. 1(c) that part of the grainsremelt leading to a segregation of micron-scale precipitation,which is harmful to the properties of the welding joint. Fig. 4

Fig. 6 Micron-scale and nano-scale precipitates at the transition zone after CEmorphology of the micron-scale precipitates broken down in the CEFprecipitates inside the subgrains nearby the micron-scale precipitates.

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shows an EPMA analysis of the Cr distribution at the weldcenter, transition zone and the base material, respectively.Enrichment of Cr along the grain boundaries at the transitionzone could be noticed, while at the weld center and the basematerial Cr is distributed relatively homogeneously. An SEMobservation shows that micron-scale precipitates exist at thegrain boundary (Fig. 5). These precipitates are probably pure Cror Cr rich phases based on the phase diagram[9,12,13] and theenergy dispersive spectroscopy (EDS) results showing that themeasured compositions of Cu (3e60 at.%), Cr (43e98 at.%) andZr (0e3 at.%). These micron-scale precipitates at the grainboundaries not only result in a loss of strengthening elements butalso introduce stress concentration during plastic deformation,which is harmful to the weld.These precipitates demonstrate a much different morphology

after CEF process. It can be seen in Fig. 6(a) that precipitates ofabout several-hundred-nanometers are aggregated, marked byarrows. In our previous study, precipitates would maintain a fineand disperse morphology and would not grow up to micron-scaleafter CEF process[7]. Thus, it can be deduced that the several-hundred-nanometers precipitates are mainly from the segre-gated precipitates in the flash butt welding process, which wouldbe broken down during the CEF process (Fig. 6(b)). Thisdeduction could also be verified by the similar EDS results of theprecipitates in Fig. 5.Besides the micron-scale precipitates, nano-scale precipitates

are recognized inside the subgrains nearby, as shown in Fig. 6(c)and (d). It is reasonable that dynamic precipitation took placeduring continuous extrusion where the temperature would be

F process: (a) TEM morphology of the micron-scale precipitates, (b) SEMprocess; (c) and (d) the bright filed and dark filed images of nano-scale

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Fig. 7 Schematic microstructure of CuCrZr welding joint after FBW (a and b), after CEF (c and d) and after cold deformation and aging (e and f); (b),(d) and (f) are magnified schematic microstructure of the transition zone corresponding to (a), (c) and (e), respectively.

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above 1000 K. It can be seen in Fig. 6(c) and (d) that the pre-cipitates are homogeneously distributed with an average size ofless than 10 nm. According to the literature[13e15], the pre-cipitates could be indexed to be Cr, CrCu2Zr and Cu4Zr, and arecoherent with the matrix.The fine and coherent precipitates have a great effect on the

dynamic recrystallization process by pinning dislocations andsubgrain boundaries effectively, which results in the subgraindominating morphology in Fig. 6(a). Besides, these nano-scaleprecipitates have a significant strengthening effect as discussedin literature[7].

3.3.2. Recrystallization. It can be seen in Fig. 2(c) that recrys-tallized grains of about 10 mm with developed annealing twinsinside are located between the aggregated precipitates regions. Afew researches have reported the developed annealing twin

Fig. 8 Micro hardness of the FBW joint and the welding joint after CEFprocess.

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morphology in copper alloys or even pure Cu after severe plasticdeformation, such as equal channel angular pressing (ECAP),high pressure torsion (HPT), etc., in which only deformationtwins could be recognized by TEM[16e18]. In the present study,this phenomenon may be due to the precipitate segregationduring the welding process shown in Fig. 1(c). The remeltingregions in the welding process cool down rapidly due to a highthermal conductivity coefficient, resulting in the microstructureof aggregated precipitates and alloying elements depleted ma-trix[10]. Then, during the subsequent CEF process, nano-scaleprecipitates in the matrix are insufficient to pin dislocationsand subgrain boundaries, thus the transition from low-angleboundaries to high-angle boundaries is favored, forming themorphology of recrystallized grains. Besides, the above 1000 Kextruding temperature makes great contribution to the recrys-tallized morphology by promoting the transition from low-angleboundaries to high-angle boundaries and the grain growth aswell.

3.4. Summary of microstructural evolution

To facilitate understanding, the microstructural evolution ofthe main process is schematically illustrated in Fig. 7. Fig. 7(a)demonstrates the weld center, the transition zone and the basematerial of the CuCrZr alloy after FBW process, the transitionzone of which is the weak area where micron-scale precipitatesaggregate at the remelted grain boundaries (shown in Fig. 7(b));after CEF process, these micron-scale precipitates are brokendown into smaller ones, as indicated in Fig. 7(d). Meanwhile,incomplete dynamic recrystallization occurs, forming the sub-grain morphology with nano-scale precipitates insides. Besides,the alloying elements depleted grains resulted from the pre-cipitates aggregation in FBW process recrystallize, which makes

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Fig. 9 Fracture morphology of the base material (a) and the welding joint (b) after CEF process.

Fig. 10 Mechanical and electrical properties of the weld and the base material after CEF process (a) and after CEF process, cold deformation andsubsequent aging (b).

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the transition zone clearly recognized, elongated and bended,shown in Fig. 7(c); after cold deformation and aging process, thetransition zone is elongated further and could be hardly recog-nized, reducing the microstructural difference between thewelding joint and the base material.

3.5. Mechanical properties

The micro hardness is investigated along the longitudinalsection of the welding joint, as shown in Fig. 8. The hardness ofthe welding joint is illustrated in line and triangle symbol. It canbe seen that the micro hardness is about 70 HV, and the hardnessof the weld center is slightly higher due to the relatively finergrains. The hardness of the welding joint is improved to above110 HVafter CEF process (illustrated in line and square symbol),where severe deformation makes significant contribution. In thisdiagram, two hardness drops of about 15 HV corresponding tothe transition zones of the welding joint after CEF process aremainly due to the recrystallized microstructure, which indicatesthat the recrystallized grains are weak regions of the weldingjoint. Fig. 9 shows the fracture surface of the base material andthe welding joint, where fracture morphology of the base ma-terial shows a typical ductile fracture in the form of small dim-ples, while the welding joint shows a more inhomogeneousdimple size and the bigger dimples may be originated from therecrystallized grains.Nevertheless, the recrystallized grains and the subgrain with

aggregated precipitates form the sandwich structure as indicated

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in Fig. 7(d). The subgrain with aggregated precipitates acts asstrengthening part, which reduces the negative effect of recrys-tallized grains on the strength. This explanation is supported bythe results in Fig. 10(a) where the strength, elongation and theelectrical conductivity of the weld are close to those of the basematerial, illustrating that the micron-scale precipitates after CEFprocess have no much negative effects on the mechanical andelectrical properties.The CuCrZr alloy after CEF process was subjected to a 60%

cold rolling and an aging treatment at 723 K for 2 h. Theresultant mechanical and electrical properties are shown inFig. 10(b). Similar to the results in Fig. 10(a), both the strengthand the elongation of the weld are close to those of the basematerial. It is generally accepted that recrystallized grains havemuch lower dislocation density, which would alleviate the stressconcentration induced by the aggregated precipitates in thefollowing cold deformation process[19]. Besides, after colddeformation, the sandwich structure was elongated along thedeformation direction and became more intense, which wouldimprove the mechanical properties of the welding joint[20,21].However, it could be noticed that the electrical conductivity ofthe welding joint is about 5% IACS lower than that of the basematerial, which is 72.8% and 77.6% IACS, respectively. Besidesthe residual stress and impurities introduced by the flash buttwelding process, the microstructural difference between thewelding joint and the base material in grain size, precipitates, etc.makes contribution to the decrease of conductivity though theyhave a weak effect on the electrical conductivity[22,23].

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4. Conclusions

(1) The mechanical properties of a CuCrZr alloy welding jointafter CEF process were close to those of the base materialsubjected to the same processing and heat treatment,while the electrical conductivity dropped from 77.6% to72.8% IACS due to the introduction of the welding process.

(2) After the CEF process, the grains were refined to submicronscale through dynamic recrystallization. Finer grains helpedto improve both the strength and the ductility of the weldingjoint.

(3) The segregated precipitates introduced by the flash buttwelding process were broken down to several hundredsnanometers and the alloying elements depleted matrixrecrystallized during the CEF process. The recrystallizedgrains could alleviate the stress concentration induced bythe aggregated precipitates during plastic deformation,which reduced the negative effect of the welding process.

AcknowledgmentThe authors are grateful for the financial support of The

National Natural Science Foundation Of China (No. 51001100).

REFERENCES

[1] G.M. Kalinin, M.E. Rodin, Y.S. Strebkov, J. Nucl. Mater. 367(2007) 920e924.

[2] S.G. Mu, F.A. Guo, M.T. Tang, Mater. Sci. Eng. A 475 (2008)235e240.

Please cite this article in press as: H. Feng, et al., Journal of Materials Scien

[3] M. Xie, J. Liu, X. Lu, F. Zheng, Mater. Sci. Eng. A 304e306 (2001)529e533.

[4] A. Durocher, D. Ayrault, W. Saikaly, J. Nucl. Mater. 367 (2007)1208e1212.

[5] A. Durocher, M. Lipa, Schedler, J. Nucl. Mater. 307 (2002)1554e1557.

[6] J. Lu, N. Saluja, Y. Zhou, J. Mater. Process. Technol. 79 (1998)200e212.

[7] H. Feng, H. Jiang, D. Yan, L. Rong, Mater. Sci. Eng. A 582 (2013)219e224.

[8] L. Jiang, W. Zong, Chin. J. Nonferr. Metal 20 (2010) 878e884 (inChinese).

[9] K.J. Zeng, K. Lilius, Scr. Metall. Mater. 32 (1995) 2009e2014.[10] F.Y. Tao, L. Lu, S. Liu, Chin. J. Rare Earth 36 (2012) 858e863 (in

Chinese).[11] A. Rohatgi, K.S. Vecchio, G.T. Gray, Metal. Mater. Trans. A 32

(2001) 135e145.[12] T. Takasugi, C. Liu, E. Lee, Mater. Sci. Eng. A 260 (1999) 108e123.[13] U. Holzwarth, H. Stamm, J. Nucl. Mater. 279 (2000) 31e45.[14] J.H. Su, P. Liu, Q. Dong, Mater. Lett. 61 (2007) 4963e4966.[15] I.S. Batra, G.K. Dey, S. Banerjee, Mater. Sci. Eng. A 356 (2003)

32e36.[16] W. Blum, Y. Li, K. Durst, Acta Mater. 57 (2009) 5207e5217.[17] Y. Li, N. Tao, K. Lu, Acta Mater. 56 (2008) 230e241.[18] H. Jiang, Y.T. Zhu, T.C. Lowe, Mater. Sci. Eng. A 290 (2000)

128e138.[19] J. Tullis, R.A. Yund, Geology 13 (1985) 238e241.[20] G.Matsuo, S.Okawa,R.Chatterjee, Iron Steel Eng. 76 (1999) 27e30.[21] D. Wu, Y.M. Song, Iron Steel 37 (2002) 43e46 (in Chinese).[22] D. Raabe, K. Miyake, H. Takahara, Mater. Sci. Eng. A 291 (2000)

186e197.[23] L. Lu, Y. Shen, X. Chen, L. Qian, K. Lu, Science 304 (2004)

422e426.

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